U.S. patent application number 16/998841 was filed with the patent office on 2020-12-03 for biointerface membranes incorporating bioactive agents.
The applicant listed for this patent is DexCom, Inc.. Invention is credited to James H. Brauker, Victoria Carr-Brendel, Dubravka Markovic, Mark Shults, Mark Tapsak.
Application Number | 20200375515 16/998841 |
Document ID | / |
Family ID | 1000005030900 |
Filed Date | 2020-12-03 |
United States Patent
Application |
20200375515 |
Kind Code |
A1 |
Shults; Mark ; et
al. |
December 3, 2020 |
BIOINTERFACE MEMBRANES INCORPORATING BIOACTIVE AGENTS
Abstract
A biointerface membrane for an implantable device including a
nonresorbable solid portion with a plurality of interconnected
cavities therein adapted to support tissue ingrowth in vivo, and a
bioactive agent incorporated into the biointerface membrane and
adapted to modify the tissue response is provided. The bioactive
agents can be chosen to induce vascularization and/or prevent
barrier cell layer formation in vivo, and are advantageous when
used with implantable devices wherein solutes are transported
across the device-tissue interface.
Inventors: |
Shults; Mark; (Madison,
WI) ; Brauker; James H.; (Addison, MI) ;
Carr-Brendel; Victoria; (Edina, MN) ; Tapsak;
Mark; (Orangeville, PA) ; Markovic; Dubravka;
(San Diego, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
DexCom, Inc. |
San Diego |
CA |
US |
|
|
Family ID: |
1000005030900 |
Appl. No.: |
16/998841 |
Filed: |
August 20, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14290842 |
May 29, 2014 |
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16998841 |
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11416734 |
May 3, 2006 |
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14290842 |
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10842716 |
May 10, 2004 |
7875293 |
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11416734 |
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10647065 |
Aug 22, 2003 |
7192450 |
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10842716 |
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60472673 |
May 21, 2003 |
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60544722 |
Feb 12, 2004 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61L 31/16 20130101;
A61L 2300/00 20130101; A61L 31/10 20130101; B33Y 80/00 20141201;
A61L 31/146 20130101; A61B 5/14865 20130101; A61B 5/14532 20130101;
G01N 33/48707 20130101; A61L 31/14 20130101 |
International
Class: |
A61B 5/1486 20060101
A61B005/1486; A61B 5/145 20060101 A61B005/145; A61L 31/10 20060101
A61L031/10; A61L 31/14 20060101 A61L031/14; A61L 31/16 20060101
A61L031/16 |
Claims
1. An implantable device, the device comprising a sensing region
for sensing an analyte and a biointerface membrane adjacent to the
sensing region, wherein the membrane is configured to modify an in
vivo tissue response by a porous architecture and by incorporation
of a bioactive agent in the membrane.
2. The implantable device of claim 1, wherein the biointerface
membrane supports tissue ingrowth and interferes with barrier-cell
layer formation, and wherein the biointerface membrane comprises
cavities of from about 90 .mu.m to about 370 .mu.m in at least one
dimension.
3. The implantable device of claim 1, wherein the biointerface
membrane supports tissue ingrowth and interferes with barrier-cell
layer formation, and wherein the biointerface membrane comprises
cavities of from about 0.6 .mu.m to about 20 .mu.m in at least one
dimension.
4. The implantable device of claim 1, wherein the biointerface
membrane supports tissue ingrowth and interferes with barrier-cell
layer formation, and wherein the biointerface membrane comprises
cavities, wherein the biointerface membrane comprises a
micro-architecture situated within at least some of the cavities of
a macro-architecture, wherein the macro-architecture comprises a
frame comprising a plurality of elongated strands of a material,
wherein the strands are less than about 6 .mu.m in all but the
longest dimension.
5. The implantable device of claim 1, wherein the biointerface
membrane comprises a material selected from the group consisting of
silicone, polytetrafluoroethylene, expanded
polytetrafluoroethylene, polyethylene-co-tetrafluoroethylene,
polyolefin, polyester, polycarbonate, biostable
polytetrafluoroethylene, homopolymers, copolymers, terpolymers of
polyurethanes, polypropylene, polyvinyl alcohol, polyvinylchloride,
polyvinylidene fluoride, polybutylene terephthalate,
polymethylmethacrylate, polyether ether ketone, polyurethanes,
cellulosic polymers, polysulfones, block copolymers thereof, and
mixtures thereof.
6. The implantable device of claim 1, wherein the biointerface
membrane comprises silicone.
7. The implantable device of claim 1, wherein the bioactive agent
is selected from the group consisting of anti-inflammatory agents,
anti-infective agents, anesthetics, inflammatory agents, growth
factors, angiogenic factors, growth factors, immunosuppressive
agents, antiplatelet agents, anticoagulants, ACE inhibitors,
cytotoxic agents, anti-sense molecules, and mixtures thereof.
8. The implantable device of claim 1, wherein the bioactive agent
is selected from the group consisting of Sphingosine-1-phosphate,
monobutyrin, Cyclosporin A, Anti-thrombospondin-2, Rapamycin,
Dexamethasone, and mixtures thereof.
9. The implantable device of claim 1, wherein the bioactive agent
comprises an anti-barrier cell agent that employs a mechanism
configured to speed up a host infiltration of the porous
architecture by inhibiting at least one of foreign body giant cells
and occlusive cell layers.
10. The implantable device of claim 9, wherein the anti-barrier
cell agent comprises Super Oxide Dismutase Mimetic.
11. The implantable device of claim 9, wherein the anti-barrier
cell agent employs an anti-inflammatory mechanism or an
immunosuppressive mechanism configured to modify a wound healing of
a host tissue.
12. The implantable device of claim 9, wherein the anti-barrier
cell agent is selected from the group consisting of cyclosporine,
Dexamethasone, and Rapamycin.
13. The implantable device of claim 1, wherein the bioactive agent
comprises a non-heparin based synthetic coating configured to
improve a performance of a blood-contacting surface.
14. The implantable device of claim 1, wherein the bioactive agent
comprises a vascularization agent.
15. The implantable device of claim 14, wherein the vascularization
agent is selected from the group consisting of an angiogenic agent
configured for stimulating a neovascularization,
Sphingosine-1-Phosphate, Monobutyrin, an anti-sense molecule, Basic
Fibroblast Growth Factor, Acidic Fibroblast Growth Factor, Vascular
Endothelial Growth Factor, Platelet Derived Endothelial Cell Growth
Factor BB, Angiopoietin-1, Transforming Growth Factor Beta,
Transforming Growth Factor Alpha, Hepatocyte Growth Factor, Tumor
Necrosis Factor-Alpha, Angiogenin, Interleukin-8, Hypoxia Inducible
Factor-I, Angiotensin-Converting Enzyme Inhibitor Quinaprilat,
Angiotropin, Thrombospondin, Peptide KGHK, Low Oxygen Tension,
Lactic Acid, Insulin, Growth Hormone, and mixtures thereof.
16. The implantable device of claim 14, wherein the vascularization
agent comprises a pro-inflammatory agent configured for promoting
an inflammation response or an immune response.
17. The implantable device of claim 16, wherein the
pro-inflammatory agent is selected from the group consisting of a
xenogenic carrier, a Lipopolysaccharide, and a protein.
18. The implantable device of claim 1, wherein the bioactive agent
is incorporated into the biointerface membrane via a carrier
matrix.
19. The implantable device of claim 18, wherein the carrier matrix
is selected from the group consisting of collagen, a particulate
matrix, a non-resorbable matrix, resorbable matrix, a
controlled-release matrix, a gel, and mixtures thereof.
20. The implantable device of claim 1, wherein the bioactive agent
is cross-linked with a material that forms the biointerface
membrane.
Description
INCORPORATION BY REFERENCE TO RELATED APPLICATIONS
[0001] Any and all priority claims identified in the Application
Data Sheet, or any correction thereto, are hereby incorporated by
reference under 37 CFR 1.57. This application is a continuation of
U.S. application Ser. No. 14/290,842, filed May 29, 2014, which is
a continuation of U.S. application Ser. No. 11/416,734, filed May
3, 2006, now abandoned, which is a continuation of U.S. application
Ser. No. 10/842,716, filed May 10, 2004, now U.S. Pat. No.
7,875,293, which is a continuation-in-part of U.S. application Ser.
No. 10/647,065, filed Aug. 22, 2003, now U.S. Pat. No. 7,192,450,
which claims the benefit of U.S. Provisional Application No.
60/472,673 filed May 21, 2003. U.S. application Ser. No. 10/842,716
claims the benefit of U.S. Provisional Application 60/544,722 filed
Feb. 12, 2004. Each of the aforementioned applications is
incorporated by reference herein in its entirety, and each is
hereby expressly made a part of this specification.
FIELD OF THE INVENTION
[0002] The present invention relates generally to biointerface
membranes that can be utilized with implantable devices, such as
devices for the detection of analyte concentrations in a biological
sample, cell transplantation devices, drug delivery devices and
electrical signal delivering or measuring devices. The present
invention further relates to methods for determining analyte levels
using implantable devices including these membranes. More
particularly, the invention relates to novel biointerface
membranes, to devices and implantable devices including these
membranes, and to methods for monitoring glucose levels in a
biological fluid sample using an implantable analyte detection
device.
BACKGROUND OF THE INVENTION
[0003] One of the most heavily investigated analyte sensing devices
is the implantable glucose device for detecting glucose levels in
patients with diabetes. Despite the increasing number of
individuals diagnosed with diabetes and recent advances in the
field of implantable glucose monitoring devices, currently used
devices are unable to provide data safely and reliably for long
periods of time (for example, months or years). See Moatti-Sirat et
al., Diabetologia, 35:224-30 (1992). There are two commonly used
types of implantable glucose sensing devices. These types include
those that are implanted intravascularly and those that are
implanted in tissue.
[0004] With reference to conventional devices that can be implanted
in tissue, a disadvantage of these devices is that they tend to
lose their function after the first few days to weeks following
implantation. While not wishing to be bound by any particular
theory, it is believed that this loss of function is due to the
lack of direct contact with circulating blood to deliver sample to
the tip of the probe of the implanted device. Because of these
limitations, it has previously been difficult to obtain continuous
and accurate glucose levels. However, such information is often
extremely important to diabetic patients in ascertaining whether
immediate corrective action is needed in order to adequately manage
their disease.
[0005] Some medical devices, including implantable analyte
measuring-devices, drug delivery devices, and cell transplantation
devices require transport of solutes across the device-tissue
interface for proper function. These devices generally include a
membrane, herein referred to as a "cell-impermeable membrane" which
encases the device or a portion of the device to prevent access by
host inflammatory or immune cells to sensitive regions of the
device.
[0006] A disadvantage of cell-impermeable membranes is that they
often stimulate a local inflammatory response, called the foreign
body response (FBR) that has long been recognized as limiting the
function of implanted devices that require solute transport.
Previous efforts to overcome this problem have been aimed at
increasing local vascularization at the device-tissue interface,
but have achieved only limited success.
[0007] FIG. 1 is a schematic drawing that illustrates a classical
FBR to a conventional cell-impermeable synthetic membrane 10
implanted under the skin. There are three main layers of a FBR. The
innermost FBR layer 12, adjacent to the device, is composed
generally of macrophages and foreign body giant cells 14 (herein
referred to as the "barrier cell layer"). These cells form a
monolayer of closely opposed cells over the entire surface of a
microscopically smooth membrane, a macroscopically smooth (but
microscopically rough) membrane, or a microporous (i.e., average
pore size of less than about 1 .mu.m) membrane. A membrane can be
adhesive or non-adhesive to cells, however, its relatively smooth
surface causes the downward tissue contracture 21 (discussed below)
to translate directly to the cells at the device-tissue interface
26. The intermediate FBR layer 16 (herein referred to as the
"fibrous zone"), lying distal to the first layer with respect to
the device, is a wide zone (about 30 to 100 .mu.m) composed
primarily of fibroblasts 18, fibrous matrixes, and contractile
fibrous tissue 20. The organization of the fibrous zone, and
particularly the contractile fibrous tissue 20, contributes to the
formation of the monolayer of closely opposed cells due to the
contractile forces 21 around the surface of the foreign body (for
example, membrane 10). The outermost FBR layer 22 is loose
connective granular tissue containing new blood vessels 24 (herein
referred to as the "vascular zone"). Over time, this FBR tissue
becomes muscular in nature and contracts around the foreign body so
that the foreign body remains tightly encapsulated. Accordingly,
the downward forces 21 press against the tissue-device interface
26, and without any counteracting forces, aid in the formation of a
barrier cell layer 14 that blocks and/or refracts the transport of
analytes 23 (for example, glucose) across the tissue-device
interface 26.
[0008] A consistent feature of the innermost layers 12, 16 is that
they are devoid of blood vessels. This has led to widely supported
speculation that poor transport of molecules across the
device-tissue interface 26 is due to a lack of vascularization near
the interface. See Scharp et al., World J. Surg., 8:221-229 (1984);
and Colton et al., J. Biomech. Eng., 113:152-170 (1991). Previous
efforts to overcome this problem have been aimed at increasing
local vascularization at the device-tissue interface, but have
achieved only limited success.
[0009] Although local vascularization can aid in sustenance of
local tissue over time, the presence of a barrier cell layer 14
prevents the passage of molecules that cannot diffuse through the
layer. For example, when applied to an implantable
glucose-measuring device, both glucose and its phosphorylated form
do not readily transit the cell membrane. Consequently, little
glucose reaches the implant's membrane through the barrier cell
layer. The known art purports to increase the local vascularization
in order to increase solute availability. See Brauker et al., U.S.
Pat. No. 5,741,330. However, it has been observed by the inventors
that once the monolayer of cells (barrier cell layer) is
established adjacent to a membrane, increasing angiogenesis is not
sufficient to increase transport of molecules such as glucose and
oxygen across the device-tissue interface 26. In fact, the barrier
cell layer blocks and/or refracts the analytes 23 from transport
across the device-tissue interface 26.
SUMMARY OF THE INVENTION
[0010] It has been confirmed through histological examination of
biointerface membranes that the one mechanism for inhibition of
molecular transport across the device-tissue interface is the
barrier cell layer adjacent to the membrane. There is a strong
correlation between the desired device function and the lack of
formation of a barrier cell layer at the device-tissue interface.
For example, glucose-measuring devices that were observed
histologically to have substantial barrier cell layers were
functional only 41% of the time after 12 weeks in vivo. In
contrast, devices that did not have significant barrier cell layers
were functional 86% of the time after 12 weeks in vivo.
[0011] Consequently, there is a need for a membrane that interferes
with the formation of a barrier cell layer and protects the
sensitive regions of the implantable device from host inflammatory
response. The biointerface membranes of the preferred embodiments
interfere with the formation of a monolayer of cells adjacent to
the membrane, henceforth referred to herein as a "barrier cell
layer", which interferes with the transport of oxygen, glucose, or
other analytes or substances, across a device-tissue interface.
[0012] The biointerface membranes, devices including these
membranes, and methods of use of these membranes according to the
preferred embodiments allow for long term protection of implanted
cells or drugs, as well as for obtaining continuous information
regarding, for example, glucose levels of a host over extended
periods of time. Because of these abilities, the biointerface
membranes of the preferred embodiments can be extremely useful in
implantable devices for the management of transplant patients,
diabetic patients, and patients requiring frequent drug
treatment.
[0013] Accordingly, in a first embodiment, a biointerface membrane
including a nonresorbable solid portion and a bioactive agent is
provided, wherein the nonresorbable solid portion includes a
plurality of interconnected cavities adapted to support a tissue
ingrowth in vivo, and wherein the bioactive agent is incorporated
into the biointerface membrane and is adapted to modify a tissue
response.
[0014] In an aspect of the first embodiment, the interconnected
cavities and the solid portion are configured to redirect a fibrous
tissue contracture in vivo, thereby interfering with formation of a
barrier cell layer within or around the membrane.
[0015] In an aspect of the first embodiment, the membrane includes
a micro-architecture situated within at least some of the cavities
of a macro-architecture, wherein the macro-architecture includes a
frame including a plurality of elongated strands of a material,
wherein the strands are less than about 6 .mu.m in all but the
longest dimension.
[0016] In an aspect of the first embodiment, the solid portion is
selected from the group consisting of silicone,
polytetrafluoroethylene, expanded polytetrafluoroethylene,
polyethylene-co-tetrafluoroethylene, polyolefin, polyester,
polycarbonate, biostable polytetrafluoroethylene, homopolymers,
copolymers, terpolymers of polyurethanes, polypropylene, polyvinyl
alcohol, polyvinylchloride, polyvinylidene fluoride, polybutylene
terephthalate, polymethylmethacrylate, polyether ether ketone,
polyurethanes, cellulosic polymers, polysulfones, block copolymers
thereof, and mixtures thereof. In an aspect of the first
embodiment, the solid portion includes silicone.
[0017] In an aspect of the first embodiment, the bioactive agent is
selected from the group consisting of anti-inflammatory agents,
anti-infective agents, anesthetics, inflammatory agents, growth
factors, angiogenic factors, growth factors, immunosuppressive
agents, antiplatelet agents, anticoagulants, ACE inhibitors,
cytotoxic agents, anti-sense molecules, and mixtures thereof. In an
aspect of the first embodiment, the bioactive agent is selected
from the group consisting of Sphingosine-1-phosphate, monobutyrin,
Cyclosporin A, Anti-thrombospondin-2, Rapamycin, and
Dexamethasone.
[0018] In an aspect of the first embodiment, the bioactive agent
includes an anti-barrier cell agent. In an aspect of the first
embodiment, the anti-barrier cell agent is selected from the group
consisting of an anti-inflammatory agent, an anti-infective agent,
an anesthetic. In an aspect of the first embodiment, the
anti-barrier cell agent employs a mechanism configured to speed up
a host infiltration of the interconnected cavities by inhibiting at
least one of foreign body giant cells and occlusive cell layers. In
an aspect of the first embodiment, the anti-barrier cell agent
includes Super Oxide Dismutase Mimetic. In an aspect of the first
embodiment, the anti-barrier cell agent employs an
anti-inflammatory mechanism or an immunosuppressive mechanism
configured to modify a wound healing of a host tissue. In an aspect
of the first embodiment, the anti-barrier cell agent includes
cyclosporine. In an aspect of the first embodiment, the
anti-barrier cell agent includes Dexamethasone. In an aspect of the
first embodiment, the anti-barrier cell agent includes
Rapamycin.
[0019] In an aspect of the first embodiment, the bioactive agent
includes a non-heparin based synthetic coating configured to
improve a performance of blood-contacting surfaces.
[0020] In an aspect of the first embodiment, the bioactive agent
includes a vascularization agent. In an aspect of the first
embodiment, the vascularization agent includes an angiogenic agent
configured for stimulating a neovascularization. In an aspect of
the first embodiment, the vascularization agent includes
Sphingosine-1-Phosphate. In an aspect of the first embodiment, the
vascularization agent includes Monobutyrin. In an aspect of the
first embodiment, the vascularization agent includes an anti-sense
molecule.
[0021] In an aspect of the first embodiment, the vascularization
agent is selected from the group consisting of Basic Fibroblast
Growth Factor, Acidic Fibroblast Growth Factor, Vascular
Endothelial Growth Factor, Platelet Derived Endothelial Cell Growth
Factor BB, Angiopoietin-1, Transforming Growth Factor Beta,
Transforming Growth Factor Alpha, Hepatocyte Growth Factor, Tumor
Necrosis Factor-Alpha, Angiogenin, Interleukin-8, Hypoxia Inducible
Factor-I, Angiotensin-Converting Enzyme Inhibitor Quinaprilat,
Angiotropin, Thrombospondin, Peptide KGHK, Low Oxygen Tension,
Lactic Acid, Insulin, Growth Hormone, and mixtures thereof.
[0022] In an aspect of the first embodiment, the vascularization
agent includes a pro-inflammatory agent configured for promoting an
inflammation response or an immune response. In an aspect of the
first embodiment, the pro-inflammatory agent includes a xenogenic
carrier. In an aspect of the first embodiment, the pro-inflammatory
agent includes a Lipopolysaccharide. In an aspect of the first
embodiment, the pro-inflammatory agent includes a protein.
[0023] In an aspect of the first embodiment, the bioactive agent is
incorporated into the biointerface membrane via a carrier matrix.
In an aspect of the first embodiment, the carrier matrix is
selected from the group consisting of collagen, a particulate
matrix, a non-resorbable matrix, resorbable matrix, a
controlled-release matrix, a gel, and mixtures thereof.
[0024] In an aspect of the first embodiment, the bioactive agent is
cross-linked with a material that forms the biointerface
membrane.
[0025] In an aspect of the first embodiment, the bioactive agent is
sorbed into the biointerface membrane by a process selected from
the group consisting of absorption, adsorption, imbibing, and
combinations thereof.
[0026] In an aspect of the first embodiment, the bioactive agent is
deposited in or on a surface of the biointerface membrane by a
process selected from the group consisting of coating, cavity
filling, solvent casting, and combinations thereof.
[0027] In an aspect of the first embodiment, the bioactive agent is
configured to be released for a time period of from about one day
to about one year. In an aspect of the first embodiment, the
bioactive agent is configured to be released for a time period of
from about one week to about four weeks.
[0028] In a second embodiment, an analyte measuring device is
provided, including a biointerface membrane including a
nonresorbable solid portion and a bioactive agent, wherein the
nonresorbable solid portion includes a plurality of interconnected
cavities adapted to support a tissue ingrowth in vivo, and wherein
the bioactive agent is incorporated into the biointerface membrane
and is adapted to modify a tissue response.
[0029] In a third embodiment, an implantable glucose-measuring
device is provided including a biointerface membrane including a
nonresorbable solid portion and a bioactive agent, wherein the
nonresorbable solid portion includes a plurality of interconnected
cavities adapted to support a tissue ingrowth in vivo, and wherein
the bioactive agent is incorporated into the biointerface membrane
and is adapted to modify a tissue response.
[0030] In a fourth embodiment, a cell transplantation device is
provided including a biointerface membrane including a
nonresorbable solid portion and a bioactive agent, wherein the
nonresorbable solid portion includes a plurality of interconnected
cavities adapted to support a tissue ingrowth in vivo, and wherein
the bioactive agent is incorporated into the biointerface membrane
and is adapted to modify a tissue response.
[0031] In a fifth embodiment, an implantable drug delivery device
is provided including a biointerface membrane including a
nonresorbable solid portion and a bioactive agent, wherein the
nonresorbable solid portion includes a plurality of interconnected
cavities adapted to support a tissue ingrowth in vivo, and wherein
the bioactive agent is incorporated into the biointerface membrane
and is adapted to modify a tissue response. In an aspect of the
fifth embodiment, the drug delivery device is selected from the
group consisting of a pump, a microcapsule, and a macrocapsule.
[0032] In a sixth embodiment, an electrical signal measuring device
is provided including a biointerface membrane including a
nonresorbable solid portion and a bioactive agent, wherein the
nonresorbable solid portion includes a plurality of interconnected
cavities adapted to support a tissue ingrowth in vivo, and wherein
the bioactive agent is incorporated into the biointerface membrane
and is adapted to modify a tissue response.
[0033] In a seventh embodiment, an electrical pulse delivering
device is provided including a biointerface membrane including a
nonresorbable solid portion and a bioactive agent, wherein the
nonresorbable solid portion includes a plurality of interconnected
cavities adapted to support a tissue ingrowth in vivo, and wherein
the bioactive agent is incorporated into the biointerface membrane
and is adapted to modify a tissue response.
[0034] In an eighth embodiment, a biointerface membrane for
implantation in a soft tissue is provided, the membrane including:
a first domain, wherein the first domain includes a plurality of
interconnected cavities and a solid portion, and wherein a
substantial number of the cavities are greater than or equal to
about 0.6 .mu.m in at least one dimension; a second domain that
allows a passage of an analyte and that is impermeable to cells or
cell processes; and a bioactive agent incorporated into the first
domain or the second domain, and which is adapted to modify an in
vivo tissue response.
[0035] In an aspect of the eighth embodiment, the first domain
supports a tissue ingrowth and interferes with barrier-cell layer
formation.
[0036] In an aspect of the eighth embodiment, the interconnected
cavities and the solid portion are configured to redirect a fibrous
tissue contracture in vivo, thereby interfering with barrier cell
layer formation within or around the first domain.
[0037] In an aspect of the eighth embodiment, the cavities are from
about 20 to about 1000 .mu.m in at least one dimension. In an
aspect of the eighth embodiment, the cavities are from about 90 to
about 370 .mu.m in at least one dimension.
[0038] In an aspect of the eighth embodiment, the cavities are from
about 0.6 to about 20 .mu.m in at least one dimension.
[0039] In an aspect of the eighth embodiment, the cavities include
a nominal pore size of between about 0.6 and 20 .mu.m.
[0040] In an aspect of the eighth embodiment, the solid portion
includes frames of elongated strands of material that are less than
about 6 .mu.m in all but the longest dimension.
[0041] In a ninth embodiment, an implantable device is provided,
the device including a sensing region for sensing an analyte and a
biointerface membrane adjacent to the sensing region, wherein the
membrane is configured to modify an in vivo tissue response by a
porous architecture and by incorporation of a bioactive agent in
the membrane.
[0042] In a tenth embodiment, a biointerface membrane suitable for
implantation in a soft tissue is provided, the membrane including a
plurality of interconnected cavities and a solid portion, wherein
the plurality of interconnected cavities and the solid portion are
configured to redirect a fibrous tissue contracture, thereby
interfering with barrier cell layer formation within or around the
first domain, and wherein the biointerface membrane further
includes a bioactive agent adapted to modify a tissue response.
[0043] In an eleventh embodiment, an implantable glucose device,
the device including a nonresorbable biointerface membrane adapted
to modify an in vivo tissue response, the membrane including a
porous membrane architecture and having a bioactive agent
incorporated therein.
[0044] In a twelfth embodiment, a biointerface membrane for use
with an implantable device is provided, the biointerface membrane
including: a first domain distal to the implantable device, wherein
the first domain includes an open-cell configuration; a second
domain proximal to the implantable device, wherein the second
domain is impermeable to cells or cell processes; and a bioactive
agent incorporated within the membrane.
[0045] In an aspect of the twelfth embodiment, the first domain
supports tissue ingrowth and interferes with barrier-cell layer
formation.
[0046] In a thirteenth embodiment, a method of monitoring an
analyte concentration is provided, the method including the steps
of: providing a host; providing an implantable device, the
implantable device including a housing including electronic
circuitry, and at least one sensing region operably connected to
the electronic circuitry of the housing, the sensing region
including a biointerface membrane, the biointerface membrane
including a first domain distal to the implantable device, wherein
the first domain includes an open-cell configuration, the
biointerface membrane including a second domain proximal to the
implantable device, wherein the second domain is impermeable to
cells or cell processes, and wherein the biointerface membrane
includes a bioactive agent incorporated into the biointerface
membrane; implanting the device in the host whereby the bioactive
agent is delivered to the tissue of the host; and measuring an
analyte concentration.
[0047] In an aspect of the thirteenth embodiment, the device is
implanted in a tissue site selected from the group consisting of
subcutaneous, abdominal, peritoneal, brain, and intramedullary.
BRIEF DESCRIPTION OF THE DRAWINGS
[0048] FIG. 1 is an illustration of classical three-layered foreign
body response to a conventional synthetic membrane implanted under
the skin.
[0049] FIG. 2A is a cross-sectional schematic view of a membrane of
a preferred embodiment that facilitates vascularization of the
first domain without barrier cell layer formation.
[0050] FIG. 2B is a cross-sectional schematic view of the membrane
of FIG. 2A showing contractile forces caused by the fibrous tissue
of the FBR.
[0051] FIG. 3 is a graph of sensor output from a glucose sensor
implanted in a human, showing the raw data signal from the sensor
from time of implant up to about 21 days after implant.
[0052] FIG. 4A is a perspective view of an assembled
glucose-measuring device, including sensing and biointerface
membranes incorporated thereon.
[0053] FIG. 4B is an exploded perspective view of the
glucose-measuring device of FIG. 4A, showing the sensing membrane
and the biointerface membrane.
[0054] FIG. 5 is a bar graph that shows average number of vessels
(per high-powered field of vision) of porous silicone materials
embedded with Monobutyrin after three weeks of implantation.
[0055] FIG. 6 is a graph that shows release kinetics over time in
PBS solution for porous silicone with Dexamethasone incorporated
therein.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0056] The following description and examples illustrate a
preferred embodiment of the present invention in detail. Those of
skill in the art will recognize that there are numerous variations
and modifications of this invention that are encompassed by its
scope. Accordingly, the description of a preferred embodiment
should not be deemed to limit the scope of the present
invention.
Definitions
[0057] In order to facilitate an understanding of the preferred
embodiment, a number of terms are defined below.
[0058] The term "biointerface membrane" as used herein is a broad
term and is used in its ordinary sense, including, without
limitation, to refer to a permeable membrane that functions as an
interface between host tissue and an implantable device. In some
embodiments, the biointerface membrane includes both
macro-architectures and micro-architectures.
[0059] The term "barrier cell layer" as used herein is a broad term
and is used in its ordinary sense, including, without limitation,
to refer to a part of a foreign body response that forms a cohesive
monolayer of cells (for example, macrophages and foreign body giant
cells) that substantially block the transport of molecules and
other substances to the implantable device.
[0060] The term "cell processes" as used herein is a broad term and
is used in its ordinary sense, including, without limitation, to
refer to pseudopodia of a cell.
[0061] The term "cellular attachment" as used herein is a broad
term and is used in its ordinary sense, including, without
limitation, to refer to adhesion of cells and/or cell processes to
a material at the molecular level, and/or attachment of cells
and/or cell processes to microporous material surfaces or
macroporous material surfaces. One example of a material used in
the prior art that encourages cellular attachment to its porous
surfaces is the BIOPORE.TM. cell culture support marketed by
Millipore (Bedford, Mass.), and as described in Brauker et al.,
U.S. Pat. No. 5,741,330.
[0062] The term "solid portions" as used herein is a broad term and
is used in its ordinary sense, including, without limitation, to
refer to portions of a membrane's material having a mechanical
structure that demarcates cavities, voids, or other non-solid
portions.
[0063] The term "co-continuous" as used herein is a broad term and
is used in its ordinary sense, including, without limitation, to
describe a solid portion or cavity wherein an unbroken curved line
in three dimensions can be drawn between two sides of a
membrane.
[0064] The term "biostable" as used herein is a broad term and is
used in its ordinary sense, including, without limitation, to
describe materials that are relatively resistant to degradation by
processes that are encountered in vivo.
[0065] The terms "bioresorbable" or "bioabsorbable" as used here
are broad terms and are used in their ordinary sense, including,
without limitation, to describe materials that can be absorbed, or
lose substance, in a biological system.
[0066] The terms "nonbioresorbable" or "nonbioabsorbable" as used
here are broad terms and are used in their ordinary sense,
including, without limitation, to describe materials that are not
substantially absorbed, or do not substantially lose substance, in
a biological system.
[0067] The terms "oxygen antenna domain" or "oxygen reservoir" as
used here are broad terms and are used in their ordinary sense,
including, without limitation, to refer to a domain composed of a
material that has a higher oxygen solubility than an aqueous media
such that it concentrates oxygen from the biological fluid
surrounding a biocompatible membrane. In one embodiment, the
properties of silicone (and/or silicone compositions) enable
domains formed from silicone to act as an oxygen antenna domain.
The oxygen antenna domain enhances function in a glucose-measuring
device by applying a higher flux of oxygen to certain
locations.
[0068] The term "analyte" as used herein is a broad term and is
used in its ordinary sense, including, without limitation, to refer
to a substance or chemical constituent in a biological fluid (for
example, blood, interstitial fluid, cerebral spinal fluid, lymph
fluid or urine) that can be analyzed. Analytes can include
naturally occurring substances, artificial substances, metabolites,
and/or reaction products. In some embodiments, the analyte for
measurement by the sensor heads, devices, and methods is glucose.
However, other analytes are contemplated as well, including but not
limited to acarboxyprothrombin; acylcarnitine; adenine
phosphoribosyl transferase; adenosine deaminase; albumin;
alpha-fetoprotein; amino acid profiles (arginine (Krebs cycle),
histidine/urocanic acid, homocysteine, phenylalanine/tyrosine,
tryptophan); andrenostenedione; antipyrine; arabinitol enantiomers;
arginase; benzoylecgonine (cocaine); biotinidase; biopterin;
c-reactive protein; carnitine; carnosinase; CD4; ceruloplasmin;
chenodeoxycholic acid; chloroquine; cholesterol; cholinesterase;
conjugated 1-.beta. hydroxy-cholic acid; cortisol; creatine kinase;
creatine kinase MM isoenzyme; cyclosporin A; d-penicillamine;
de-ethylchloroquine; dehydroepiandrosterone sulfate; DNA
(acetylator polymorphism, alcohol dehydrogenase, alpha
1-antitrypsin, cystic fibrosis, Duchenne/Becker muscular dystrophy,
glucose-6-phosphate dehydrogenase, hemoglobinopathies, A,S,C,E,
D-Punjab, beta-thalassemia, hepatitis B virus, HCMV, HIV-1, HTLV-1,
Leber hereditary optic neuropathy, MCAD, RNA, PKU, Plasmodium
vivax, sexual differentiation, 21-deoxycortisol);
desbutylhalofantrine; dihydropteridine reductase; diptheria/tetanus
antitoxin; erythrocyte arginase; erythrocyte protoporphyrin;
esterase D; fatty acids/acylglycines; free .beta.-human chorionic
gonadotropin; free erythrocyte porphyrin; free thyroxine (FT4);
free tri-iodothyronine (FT3); fumarylacetoacetase;
galactose/gal-1-phosphate; galactose-1-phosphate uridyltransferase;
gentamicin; glucose-6-phosphate dehydrogenase; glutathione;
glutathione perioxidase; glycocholic acid; glycosylated hemoglobin;
halofantrine; hemoglobin variants; hexosaminidase A; human
erythrocyte carbonic anhydrase I; 17 alpha-hydroxyprogesterone;
hypoxanthine phosphoribosyl transferase; immunoreactive trypsin;
lactate; lead; lipoproteins ((a), B/A-1, .beta.); lysozyme;
mefloquine; netilmicin; phenobarbitone; phenytoin;
phytanic/pristanic acid; progesterone; prolactin; prolidase; purine
nucleoside phosphorylase; quinine; reverse tri-iodothyronine (rT3);
selenium; serum pancreatic lipase; sissomicin; somatomedin C;
specific antibodies (adenovirus, anti-nuclear antibody, anti-zeta
antibody, arbovirus, Aujeszky's disease virus, dengue virus,
Dracunculus medinensis, Echinococcus granulosus, Entamoeba
histolytica, enterovirus, Giardia duodenalisa, Helicobacter pylori,
hepatitis B virus, herpes virus, HIV-1, IgE (atopic disease),
influenza virus, Leishmania donovani, leptospira,
measles/mumps/rubella, Mycobacterium leprae, Mycoplasma pneumoniae,
Myoglobin, Onchocerca volvulus, parainfluenza virus, Plasmodium
falciparum, poliovirus, Pseudomonas aeruginosa, respiratory
syncytial virus, rickettsia (scrub typhus), Schistosoma mansoni,
Toxoplasma gondii, Trepenoma pallidium, Trypanosoma cruzi/rangeli,
vesicular stomatis virus, Wuchereria bancrofti, yellow fever
virus); specific antigens (hepatitis B virus, HIV-1);
succinylacetone; sulfadoxine; theophylline; thyrotropin (TSH);
thyroxine (T4); thyroxine-binding globulin; trace elements;
transferrin; UDP-galactose-4-epimerase; urea; uroporphyrinogen I
synthase; vitamin A; white blood cells; and zinc protoporphyrin.
Salts, sugar, protein, fat, vitamins and hormones naturally
occurring in blood or interstitial fluids can also constitute
analytes in certain embodiments. The analyte can be naturally
present in the biological fluid, for example, a metabolic product,
a hormone, an antigen, an antibody, and the like. Alternatively,
the analyte can be introduced into the body, for example, a
contrast agent for imaging, a radioisotope, a chemical agent, a
fluorocarbon-based synthetic blood, or a drug or pharmaceutical
composition, including but not limited to insulin; ethanol;
cannabis (marijuana, tetrahydrocannabinol, hashish); inhalants
(nitrous oxide, amyl nitrite, butyl nitrite, chlorohydrocarbons,
hydrocarbons); cocaine (crack cocaine); stimulants (amphetamines,
methamphetamines, Ritalin, Cylert, Preludin, Didrex, PreState,
Voranil, Sandrex, Plegine); depressants (barbituates, methaqualone,
tranquilizers such as Valium, Librium, Miltown, Serax, Equanil,
Tranxene); hallucinogens (phencyclidine, lysergic acid, mescaline,
peyote, psilocybin); narcotics (heroin, codeine, morphine, opium,
meperidine, Percocet, Percodan, Tussionex, Fentanyl, Darvon,
Talwin, Lomotil); designer drugs (analogs of fentanyl, meperidine,
amphetamines, methamphetamines, and phencyclidine, for example,
Ecstasy); anabolic steroids; and nicotine. The metabolic products
of drugs and pharmaceutical compositions are also contemplated
analytes. Analytes such as neurochemicals and other chemicals
generated within the body can also be analyzed, such as, for
example, ascorbic acid, uric acid, dopamine, noradrenaline,
3-methoxytyramine (3MT), 3,4-dihydroxyphenylacetic acid (DOPAC),
homovanillic acid (HVA), 5-hydroxytryptamine (5HT), and
5-hydroxyindoleacetic acid (FHIAA).
[0069] The terms "analyte-measuring device," as used herein is a
broad term and is used in its ordinary sense, including, without
limitation, to refer to any mechanism (for example, an enzymatic
mechanism or a non-enzymatic mechanism) by which an analyte can be
quantified. An example is a glucose-measuring device incorporating
a membrane that contains glucose oxidase that catalyzes the
conversion of oxygen and glucose to hydrogen peroxide and
gluconate:
Glucose+O.sub.2.fwdarw.Gluconate+H.sub.2O.sub.2
[0070] In the above reaction, for each glucose molecule consumed,
there is a proportional change in the co-reactant O.sub.2 and the
product H.sub.2O.sub.2. Current change in either the co-reactant or
the product can be monitored to determine glucose
concentration.
[0071] The term "host" as used herein is a broad term and is used
in its ordinary sense, including, without limitation, to refer to
mammals, preferably humans.
[0072] The phrase "continuous analyte sensing" as used herein is a
broad term and is used in its ordinary sense, including, without
limitation, to describe the period in which monitoring of analyte
concentration is continuously, continually, and/or intermittently
(but regularly) performed, for example, from about every 5 seconds
or less to about 10 minutes or more, preferably from about 10, 15,
20, 25, 30, 35, 40, 45, 50, 55, or 60 second to about 1.25, 1.50,
1.75, 2.00, 2.25, 2.50, 2.75, 3.00, 3.25, 3.50, 3.75, 4.00, 4.25,
4.50, 4.75, 5.00, 5.25, 5.50, 5.75, 6.00, 6.25, 6.50, 6.75, 7.00,
7.25, 7.50, 7.75, 8.00, 8.25, 8.50, 8.75, 9.00, 9.25, 9.50 or 9.75
minutes.
[0073] The term "sensing region" as used herein is a broad term and
is used in its ordinary sense, including, without limitation, to
refer to the area of an analyte-monitoring device responsible for
the detection of a particular analyte. For example, the sensing
region can comprise a non-conductive body, a working electrode
(anode), a reference electrode, and a counter electrode (cathode)
passing through and secured within the device body, forming an
electrochemically reactive surface at one location on the body and
an electronic connection at another location on the body, and a
sensing membrane affixed to the body and covering the
electrochemically reactive surface. The counter electrode
preferably has a greater electrochemically reactive surface area
than the working electrode. During general operation of the device,
a biological sample, for example, blood or interstitial fluid, or a
component thereof contacts, either directly or after passage
through one or more membranes, an enzyme, for example, glucose
oxidase. The reaction of the biological sample or component thereof
results in the formation of reaction products that permit a
determination of the analyte level, for example, glucose, in the
biological sample. In some embodiments, the sensing membrane
further comprises an enzyme domain, for example, an enzyme layer,
and an electrolyte phase, for example, a free-flowing liquid phase
comprising an electrolyte-containing fluid described further
below.
[0074] The term "electrochemically reactive surface" as used herein
is a broad term and is used in its ordinary sense, including,
without limitation, to refer to the surface of an electrode where
an electrochemical reaction takes place. In a working electrode,
hydrogen peroxide produced by an enzyme-catalyzed reaction of an
analyte being detected reacts can create a measurable electronic
current. For example, in the detection of glucose, glucose oxidase
produces H.sub.2O.sub.2 peroxide as a byproduct. the H.sub.2O.sub.2
reacts with the surface of the working electrode to produce two
protons (2H.sup.+), two electrons (2e.sup.-) and one molecule of
oxygen (O.sub.2), which produces the electronic current being
detected. In a counter electrode, a reducible species, for example,
O.sub.2 is reduced at the electrode surface so as to balance the
current generated by the working electrode.
[0075] The term "sensing membrane" as used herein is a broad term
and is used in its ordinary sense, including, without limitation,
to refer to a permeable or semi-permeable membrane that can
comprise one or more domains and that is constructed of materials
having a thickness of a few microns or more, and that are permeable
to reactants and/or co-reactants employed in determining the
analyte of interest. As an example, a sensing membrane can comprise
an immobilized glucose oxidase enzyme, which catalyzes an
electrochemical reaction with glucose and oxygen to permit
measurement of a concentration of glucose.
[0076] The term "proximal" as used herein, is a broad term and is
used in its ordinary sense, including, without limitation, to
describe a region near to a point of reference, such as an origin
or a point of attachment.
[0077] The term "distal" as used herein, is a broad term and is
used in its ordinary sense, including, without limitation, to
describe a region spaced relatively far from a point of reference,
such as an origin or a point of attachment.
[0078] The terms "operably connected" and "operably linked" as used
herein are broad terms and are used in their ordinary sense,
including, without limitation, to describe one or more components
linked to another component(s) in a manner that facilitates
transmission of signals between the components. For example, one or
more electrodes can be used to detect an analyte in a sample and
convert that information into a signal; the signal can then be
transmitted to an electronic circuit. In this example, the
electrode is "operably linked" to the electronic circuit.
[0079] The term "bioactive agent" as used herein is a broad term
and is used in its ordinary sense, including, without limitation,
to describe any substance that has an effect on or elicits a
response from living tissue.
[0080] The term "bioerodible" or "biodegradable", as used herein,
is a broad term and is used in its ordinary sense, including,
without limitation, to describe materials that are enzymatically
degraded or chemically degraded in vivo into simpler
components.
Overview
[0081] Devices and probes that are implanted into subcutaneous
tissue conventionally elicit a foreign body response (FBR), which
forms a foreign body capsule (FBC), as part of the body's response
to the introduction of a foreign material. Specifically,
implantation of a device, for example, a glucose sensing device,
can result in an acute inflammatory reaction resolving to chronic
inflammation with concurrent building of fibrotic tissue, such as
is described in detail above. Eventually, a mature FBC including
primarily contractile fibrous tissue forms around the device. See
Shanker and Greisler, Inflammation and Biomaterials in Greco R S,
ed., "Implantation Biology: The Host Response and Biomedical
Devices" pp 68-80, CRC Press (1994).
[0082] The FBC surrounding conventional implanted devices has been
shown to hinder or block the transport of analytes across the
device-tissue interface. Thus, continuous long-term analyte
transport in vivo has been conventionally believed to be unreliable
or impossible. For example, because the formation of a FBC isolates
an implantable device in a capsule containing fluid that does not
mimic the levels of analytes, such as glucose and oxygen, in the
body's vasculature, long-term device function was not believed to
be reliable. Additionally, the composition of a FBC can prevent
stabilization of the implantable device, contributing to motion
artifact that also renders results unreliable.
[0083] In contrast to conventional belief, it has been recognized
that FBC formation is the dominant event surrounding long-term
implantation of any device, and can be managed or manipulated to
support rather than hinder or block analyte transport. It has been
observed that during the early periods following implantation of an
analyte-sensing device, for example a glucose-sensing device,
glucose changes can be tracked in vivo, although significant time
delays are typically incurred. However, after a few days to two or
more weeks of implantation, these devices typically lose their
function. See, for example, U.S. Pat. No. 5,791,344 and Gross et
al. and "Performance Evaluation of the MiniMed Continuous
Monitoring System During Patient home Use," Diabetes Technology and
Therapeutics, (2000) 2(1):49-56, which have reported a glucose
oxidase device, approved for use in humans by the Food and Drug
Administration, that functions well for several days following
implantation but loses function quickly after 3 days. These results
suggest that there is sufficient vascularization and, therefore,
perfusion of oxygen and glucose to support the function of an
implantable glucose-measuring device for the first few days
following implantation. New blood vessel formation is clearly not
needed for the function of a glucose oxidase mediated
electrochemical device implanted in the subcutaneous tissue for at
least several days after implantation.
[0084] After several days, however, it is believed that this lack
of device function is most likely due to cells, such as
polymorphonuclear cells and monocytes, that migrate to the wound
site during the first few days after implantation, for example,
from the wounding of the tissue during implant. These cells consume
local glucose and oxygen. If there is an overabundance of such
cells, they can deplete glucose and/or oxygen before it is able to
reach the device enzyme layer, thereby reducing the sensitivity of
the device or rendering it non-functional. Further inhibition of
device function can be due to inflammatory cells, for example,
macrophages, that associate, for example, align at the interface,
with the implantable device and physically block the transport of
glucose into the device, for example, by formation of a barrier
cell layer.
[0085] Additionally, these inflammatory cells can biodegrade many
artificial biomaterials (some of which were, until recently,
considered non-biodegradable). When activated by a foreign body,
tissue macrophages degranulate, releasing hypochlorite (bleach) and
other oxidative species. Hypochlorite and other oxidative species
are known to break down a variety of polymers.
[0086] In order to overcome the problems associated with
conventional membranes, the preferred embodiments employ
biointerface membrane architectures that promote vascularization
within the membrane and that interfere with barrier cell layer
formation. The biointerface membranes are robust and suitable for
long-term implantation and long-term analyte transport in vivo.
Additionally, the membranes can be used in a variety of implantable
devices, for example, analyte measuring devices, particularly
glucose-measuring devices, cell transplantation devices, drug
delivery devices, and electrical signal delivery and measuring
devices. For example, in some embodiments of a glucose-monitoring
device, the device interface can include a sensing membrane that
has different domains and/or layers that can cover and protect an
underlying enzyme membrane and the electrodes of the
glucose-measuring device.
Biointerface Membranes
[0087] The biointerface membranes of the preferred embodiments
comprise two or more domains, and incorporate a bioactive agent. A
first domain is provided that includes an architecture, including
cavity size, configuration, and/or overall thickness, that
encourages vascular tissue ingrowth, disrupts downward tissue
contracture, and/or discourages barrier cell formation. A second
domain is provided that is impermeable to cells and/or cell
processes. A bioactive agent is provided that is incorporated into
the first and/or second domain, wherein the bioactive agent
includes mechanisms that induce local vascularization and/or resist
barrier cell formation.
[0088] FIG. 2A is a cross-sectional schematic view of a membrane 30
in vivo in one exemplary embodiment, wherein the membrane comprises
a first domain 32 and second domain 34. The architecture of the
membrane provides a robust, long-term implantable membrane that
facilitates the transport of analytes through vascularized tissue
ingrowth without the formation of a barrier cell layer.
[0089] The first domain 32 comprises a solid portion 36 and a
plurality of interconnected three-dimensional cavities 38 formed
therein. The cavities 38 have sufficient size and structure to
allow invasive cells, such as fibroblasts 35, a fibrous matrix 37,
and blood vessels 39 to enter into the apertures 40 that define the
entryway into each cavity 38, and to pass through the
interconnected cavities toward the interface 42 between the first
and second domains. The cavities comprise an architecture that
encourages the ingrowth of vascular tissue in vivo, as indicated by
the blood vessels 39 formed throughout the cavities. Because of the
vascularization within the cavities, solutes 33 (for example,
oxygen, glucose and other analytes) pass through the first domain
with relative ease, and/or the diffusion distance (namely, distance
that the glucose diffuses) is reduced.
[0090] The biointerface membranes of the preferred embodiments
preferably include a bioactive agent, which is incorporated into at
least one of the first and second domains 32, 34 of the
biointerface membrane, or which is incorporated into the device and
adapted to diffuse through the first and/or second domains, in
order to modify the tissue response of the host to the membrane.
The architectures of the first and second domains have been shown
to support vascularized tissue ingrowth, to interfere with and
resist barrier cell layer formation, and to facilitate the
transport of analytes across the membrane. However, the bioactive
agent can further enhance vascularized tissue ingrowth, resistance
to barrier cell layer formation, and thereby facilitate the passage
of analytes 33 across the device-tissue interface 42.
Architecture of the First Domain
[0091] The first domain of the biointerface membrane includes an
architecture that supports tissue ingrowth, disrupts contractile
forces typically found in a foreign body response, encourages
vascularity within the membrane, and disrupts the formation of a
barrier cell layer. The first domain, also referred to as the cell
disruptive domain, comprises an open-celled configuration
comprising interconnected cavities and solid portions. The
distribution of the solid portion and cavities of the first domain
preferably includes a substantially co-continuous solid domain and
includes more than one cavity in three dimensions substantially
throughout the entirety of the first domain. Generally, cells can
enter into the cavities; however, they cannot travel through or
wholly exist within the solid portions. The cavities permit most
substances to pass through, including, for example, cells and
molecules.
[0092] Reference is now made to FIG. 2B, which is an illustration
of the membrane of FIG. 2A, showing contractile forces caused by
the fibrous tissue, for example, from the fibroblasts and fibrous
matrix, of the FBR. Specifically, the architecture of the first
domain, including the cavity interconnectivity and multiple-cavity
depth, (namely, two or more cavities in three dimensions throughout
a substantial portion of the first domain) can affect the tissue
contracture that typically occurs around a foreign body.
[0093] A contraction of the FBC around the device as a whole
produces downward forces on the device can be helpful in reducing
motion artifacts, such as are described in copending U.S. patent
application Ser. No. 10/646,333, filed Aug. 22, 2003 and entitled
"OPTIMIZED DEVICE GEOMETRY FOR AN IMPLANTABLE GLUCOSE DEVICE,"
which is incorporated herein in its entirety by reference. The
architecture of the first domain of the biointerface membrane,
including the interconnected cavities and solid portion, is
advantageous because the contractile forces caused by the downward
tissue contracture that can otherwise cause cells to flatten
against the device and occlude the transport of analytes, is
instead translated to, disrupted by, and/or counteracted by the
forces 41 that contract around the solid portions 36 (for example,
throughout the interconnected cavities 38) away from the device.
That is, the architecture of the solid portions 36 and cavities 38
of the first domain cause contractile forces 41 to disperse away
from the interface between the first domain 32 and second domain
34. Without the organized contracture of fibrous tissue toward the
tissue-device interface 42 typically found in a FBC (FIG. 1),
macrophages and foreign body giant cells do not form a substantial
monolayer of cohesive cells (namely, a barrier cell layer) and
therefore the transport of molecules across the second domain
and/or membrane is not blocked, as indicated by free transport of
analyte 33 through the first and second domains in FIGS. 2A and
2B.
[0094] Various methods are suitable for use in manufacturing the
first domain in order to create an architecture with preferred
dimensions and overall structure. The first domain can be
manufactured by forming particles, for example, sugar granules,
salt granules, and other natural or synthetic uniform or
non-uniform particles, in a mold, wherein the particles have shapes
and sizes substantially corresponding to the desired cavity
dimensions. In some methods, the particles are made to coalesce to
provide the desired interconnectivity between the cavities. The
desired material for the solid portion can be introduced into the
mold using methods common in the art of polymer processing, for
example, injecting, pressing, vacuuming, or pouring. After the
solid portion material is cured or solidified, the coalesced
particles are then dissolved, melted, etched, or otherwise removed,
leaving interconnecting cavities within the solid portion. In such
embodiments, sieving can be used to determine the dimensions of the
particles, which substantially correspond to the dimensions of
resulting cavities. In sieving, also referred to as screening, the
particles are added to the sieve and then shaken to produce overs
and unders. The overs are the particles that remain on the screen
and the unders are the particles that pass through the screen.
Other methods and apparatus known in the art are also suitable for
use in determining particle size, for example, air classifiers,
which apply opposing air flows and centrifugal forces to separate
particles having sizes down to 2 .mu.m, can be used to determine
particle size when particles are smaller than 100 .mu.m.
[0095] In one embodiment, the cavity size of the cavities 38 of the
first domain is substantially defined by the particle size(s) used
in creating the cavities. In some embodiments, the particles used
to form the cavities can be substantially spherical, thus the
dimensions below describe a diameter of the particle and/or a
diameter of the cavity. In some alternative embodiments, the
particles used to form the cavities can be non-spherical (for
example, rectangular, square, diamond, or other geometric or
non-geometric shapes), thus the dimensions below describe one
dimension (for example, shortest, average, or longest) of the
particle and/or cavity.
[0096] In some embodiments, a variety of different particle sizes
can be used in the manufacture of the first domain. In some
embodiments, the dimensions of the particles can be somewhat
smaller or larger than the dimensions of the resulting cavities,
due to dissolution or other precipitation that can occur during the
manufacturing process.
[0097] Although one method of manufacturing porous domains is
described above, a variety of methods known to one of ordinary
skill in the art can be employed to create the structures of
preferred embodiments. For example, molds can be used in the place
of the particles described above, such as coral, self-assembly
beads, etched or broken silicon pieces, glass frit pieces, and the
like. The dimensions of the mold can define the cavity sizes, which
can be determined by measuring the cavities of a model final
product, and/or by other measuring techniques known in the art, for
example, by a bubble point test. In U.S. Pat. No. 3,929,971, Roy
discloses a method of making a synthetic membrane having a porous
microstructure by converting calcium carbonate coral materials to
hydroxyapatite while at the same time retaining the unique
microstructure of the coral material.
[0098] Other methods of forming a three-dimensional first domain
can be used, for example holographic lithography,
stereolithography, and the like, wherein cavity sizes are defined
and precisely formed by the lithographic or other such process to
form a lattice of unit cells, as described in co-pending U.S.
Provisional Patent Application 60/544,722, entitled "Macro-Micro
Architecture for Biointerface Membrane," which is incorporated
herein by reference in its entirety and as described by Pekkarinen
et al. in U.S. Pat. No. 6,520,997, which discloses a
photolithographic process for creating a porous membrane.
[0099] The first domain 32 can be defined using alternative
methods. In an alternative preferred embodiment, fibrous non-woven
or woven materials, or other such materials, such as electrospun,
scattered, or aggregate materials, are manufactured by forming the
solid portions without particularly defining the cavities
therebetween. Accordingly, in these alternative embodiments,
structural elements that provide the three-dimensional conformation
can include fibers, strands, globules, cones, and/or rods of
amorphous or uniform geometry that are smooth or rough. These
elements are hereinafter referred to as "strands." The solid
portion of the first domain can include a plurality of strands,
which generally define apertures formed by a frame of the
interconnected strands. The apertures of the material form a
framework of interconnected cavities. Formed in this manner, the
first domain is defined by a cavity size of about 0.6 to about 1000
.mu.m in at least one dimension.
[0100] Referring to the dimensions and architecture of the first
domain 32, the porous biointerface materials can be loosely
categorized into at least two groups: those having a
micro-architecture and those having a macro-architecture.
[0101] FIGS. 2A and 2B illustrate one preferred embodiment wherein
the biointerface material includes a macro-architecture as defined
herein. In general, the cavity size of a macro-architecture
provides a configuration and overall thickness that encourages
vascular tissue ingrowth and disrupts tissue contracture that is
believed to cause barrier cell formation in vivo (as indicated by
the blood vessels 39 formed throughout the cavities), while
providing a long-term, robust structure. Referring to the
macro-architecture, a substantial number of the cavities 38,
defined using any of the methods described above, are greater than
or equal to about 20 .mu.m in one dimension. In some other
embodiments, a substantial number of the cavities are greater than
or equal to about 30, 40, 50, 60, 70, 80, 90, 100, 120, 140, 160,
180, 200, 240, 280, 320, 360, 400, 500, 600, 700 .mu.m, and
preferably less than about 1000 .mu.m in one dimension. Although
the macro-architecture is associated the numerous advantages as
described above, in some embodiments it can create an opportunity
for foreign body giant cells to flatten against the second domain
and/or implantable device 34 and potentially create a layer of
barrier cells that can block some or all analyte transport. It is
therefore advantageous to incorporate a bioactive agent into the
macro-architecture in order to modify the tissue response of the
host to the membrane.
[0102] The biointerface material can also be formed with a
micro-architecture as defined herein. Generally, at least some of
the cavities of a micro-architecture have a sufficient size and
structure to allow inflammatory cells to partially or completely
enter into the cavities. However, in contrast to the
macro-architecture, the micro-architecture does not allow extensive
ingrowth of vascular and connective tissues within the cavities.
Therefore, in some embodiments, the micro-architecture of preferred
embodiments is defined by the actual size of the cavity, wherein
the cavities are formed from a mold, for example, such as described
in more detail above. However, in the context of the
micro-architecture it is preferable that the majority of the mold
dimensions, whether particles, beads, crystals, coral,
self-assembly beads, etched or broken silicon pieces, glass frit
pieces, or other mold elements that form cavities, are less than
about 20 .mu.m in at least one dimension.
[0103] In some alternative micro-architecture embodiments, wherein
the biointerface material is formed from a substantially fibrous
material, the micro-architecture is defined by a strand size of
less than 6 .mu.m in all but the longest dimension, and a
sufficient number of cavities are provided of a size and structure
to allow inflammatory cells, for example, macrophages, to
completely enter through the apertures that define the cavities,
without extensive ingrowth of vascular and connective tissues.
[0104] In certain embodiments, the micro-architecture is
characterized, or defined, by standard pore size tests, such as the
bubble point test. The micro-architecture is selected with a
nominal pore size of from about 0.6 .mu.m to about 20 .mu.m. In
some embodiments, the nominal pore size from about 1, 2, 3, 4, 5,
6, 7, 8, or 9 .mu.m to about 10, 11, 12, 13, 14, 15, 16, 17, 18, or
19 .mu.m. It has been found that a porous polymer membrane having
an average nominal pore size of about 0.6 to about 20 .mu.m
functions satisfactorily in creating a vascular bed within the
micro-architecture at the device-tissue interface. The term
"nominal pore size" in the context of the micro-architecture 52 in
certain embodiments is derived from methods of analysis common to
membrane, such as the ability of the membrane to filter particles
of a particular size, or the resistance of the membrane to the flow
of fluids. Because of the amorphous, random, and irregular nature
of most of these commercially available membranes, the "nominal
pore size" designation may not actually indicate the size or shape
of the apertures and cavities, which in reality have a high degree
of variability. Accordingly, as used herein with reference to the
micro-architecture, the term "nominal pore size" is a
manufacturer's convention used to identify a particular membrane of
a particular commercial source which has a certain bubble point; as
used herein, the term "pore" does not describe the size of the
cavities of the material in the preferred embodiments. The bubble
point measurement is described in Pharmaceutical Technology, May
1983, pp. 36 to 42.
[0105] While not wishing to be bound by any particular theory, it
is believed that biointerface membranes with a micro-architecture
as defined herein, are advantageous for inducing close vascular
structures, maintaining rounded inflammatory cell morphology,
preventing barrier cell layer formation, and preventing organized
fibroblasts and connective tissue from entering into the membrane.
In some instances, crushing and delamination of a
micro-architecture biointerface material can occur, which allows
foreign body giant cells to flatten against the implantable device
and potentially create a barrier layer of cells that block some or
all analyte transport. It can therefore be advantageous to
incorporate a bioactive agent into the micro-architecture in order
to modify the tissue response of the host to the membrane.
[0106] The optimum dimensions, architecture (for example,
micro-architecture or macro-architecture), and overall structural
integrity of the membrane can be adjusted according to the
parameters of the device that it supports. For example, if the
membrane is employed with a glucose-measuring device, the
mechanical requirements of the membrane can be greater for devices
having greater overall weight and surface area when compared to
those that are relatively smaller.
[0107] With regard to the depth of cavities, improved vascular
tissue ingrowth is observed when the first domain has a thickness
that accommodates a depth of at least two cavities throughout a
substantial portion of the thickness. Improved vascularization
results at least in part from multi-layered interconnectivity of
the cavities, such as in the preferred embodiments, as compared to
a surface topography such as seen in the prior art, for example,
wherein the first domain has a depth of only one cavity throughout
a substantial portion thereof. The multi-layered interconnectivity
of the cavities enables vascularized tissue to grow into various
layers of cavities in a manner that provides mechanical anchoring
of the device with the surrounding tissue. Such anchoring resists
movement that can occur in vivo, which results in reduced sheer
stress and scar tissue formation. The optimum depth or number of
cavities can vary depending upon the parameters of the device that
it supports. For example, if the membrane is employed with a
glucose-measuring device, the anchoring that is required of the
membrane is greater for devices having greater overall weight and
surface area as compared to those that are relatively smaller.
[0108] The thickness of the first domain can be optimized for
decreased time-to-vascularize in vivo, that is, vascular tissue
ingrowth can occur somewhat faster with a membrane that has a thin
first domain as compared to a membrane that has a relatively
thicker first domain. Decreased time-to-vascularize results in
faster stabilization and functionality of the biointerface in vivo.
For example, in a subcutaneous implantable glucose device,
consistent and increasing functionality of the device is at least
in part a function of consistent and stable glucose transport
across the biointerface membrane, which is at least in part a
function of the vascularization thereof. Thus, quicker start-up
time and/or shortened time lag (as when, for example, the diffusion
path of the glucose through the membrane is reduced) can be
achieved by decreasing the thickness of the first domain.
[0109] The thickness of the first domain is typically from about 20
.mu.m to about 2000 .mu.m, preferably from about 30, 40, 50, 60,
70, 80, 90, or 100 .mu.m to about 800, 900, 1000, 1100, 1200, 1300,
1400, 1500, 1600, 1700, 1800, or 1900 .mu.m, and most preferably
from about 150, 200, 250, 300, 350, or 400 .mu.m to about 450, 500,
550, 600, 650, 700, or 750 .mu.m. However, in some alternative
embodiments a thinner or thicker cell disruptive domain (first
domain) can be desired.
[0110] The solid portion preferably includes one or more materials
such as silicone, polytetrafluoroethylene, expanded
polytetrafluoroethylene, polyethylene-co-tetrafluoroethylene,
polyolefin, polyester, polycarbonate, biostable
polytetrafluoroethylene, homopolymers, copolymers, terpolymers of
polyurethanes, polypropylene (PP), polyvinylchloride (PVC),
polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA),
polybutylene terephthalate (PBT), polymethylmethacrylate (PMMA),
polyether ether ketone (PEEK), polyamides, polyurethanes,
cellulosic polymers, polysulfones and block copolymers thereof
including, for example, di-block, tri-block, alternating, random
and graft copolymers. In some embodiments, the material selected
for the first domain is an elastomeric material, for example,
silicone, which is able to absorb stresses that can occur in vivo,
such that sheer and other environmental forces are significantly
minimized at the second domain. The solid portion can comprises a
silicone composition with a hydrophile such as Polyethylene Glycol
(PEG) covalently incorporated or grafted therein, such as described
in co-pending U.S. patent application Ser. No. 10/695,636, filed
Oct. 28, 2003, and entitled, "SILICONE COMPOSITION FOR
BIOCOMPATIBLE MEMBRANE," which is incorporated herein by reference
in its entirety. Additionally, elastomeric materials with a memory
of the original configuration can withstand greater stresses
without affecting the configuration, and thus the function, of the
device.
[0111] The first domain can include a macro-architecture and a
micro-architecture located within at least a portion of the
macro-architecture, such as is described in co-pending U.S.
Provisional Patent Application 60/544,722, entitled, "BIOINTERFACE
WITH MACRO- AND MICRO-ARCHITECTURE," which is incorporated herein
by reference in its entirety. For example, the macro-architecture
includes a porous structure with interconnected cavities such as
described with reference to the solid portion of the first domain,
wherein at least some portion of the cavities of the first domain
are filled with the micro-architecture that includes a fibrous or
other fine structured material that aids in preventing formation of
a barrier cell layer, for example in pockets in the bottom of the
cavities of the macro-architecture adjacent to the implantable
device.
[0112] In certain embodiments, other non-resorbable implant
materials can be used in forming the first domain, including but
not limited to, metals, ceramics, cellulose, hydrogel polymers,
poly (2-hydroxyethyl methacrylate, pHEMA), hydroxyethyl
methacrylate, (HEMA), polyacrylonitrile-polyvinyl chloride
(PAN-PVC), high density polyethylene, acrylic copolymers, nylon,
polyvinyl difluoride, polyanhydrides, poly(l-lysine), poly
(L-lactic acid), hydroxyethylmetharcrylate, hydroxyapeptite,
alumina, zirconia, carbon fiber, aluminum, calcium phosphate,
titanium, titanium alloy, nintinol, stainless steel, and CoCr
alloy.
Architecture of the Second Domain
[0113] FIGS. 2A and 2B, illustrate the second domain of the
membrane. The second domain is impermeable to cells or cell
processes, and is composed of a biostable material. In one
embodiment, the second domain is comprised of polyurethane and a
hydrophilic polymer, such as is described in co-pending U.S.
application Ser. No. 09/916,858 filed Jul. 27, 2001, which is
incorporated herein by reference in its entirety. Alternatively,
the hydrophilic polymer can include polyvinylpyrrolidone.
Alternatively, the second domain is polyurethane comprising about 5
weight percent or more polyvinylpyrrolidone and about 45 weight
percent or more polyvinylpyrrolidone. Alternatively, the second
domain comprises about 20 weight percent or more
polyvinylpyrrolidone and about 35 weight percent or more
polyvinylpyrrolidone. Alternatively, the second domain is
polyurethane comprising about 27 weight percent
polyvinylpyrrolidone. In certain embodiments, however, the second
domain can comprise about 5 weight percent or more than about 45
weight percent polyvinylpyrrolidone.
[0114] Alternatively, the second domain can be formed from
materials such as copolymers or blends of copolymers with
hydrophilic polymers such as polyvinylpyrrolidone (PVP),
polyhydroxyethyl methacrylate, polyvinylalcohol, polyacrylic acid,
polyethers such as polyethylene glycol, and block copolymers
thereof, including, for example, di-block, tri-block, alternating,
random and graft copolymers (block copolymers are disclosed in U.S.
Pat. Nos. 4,803,243 and 4,686,044). In some embodiments, the second
domain can comprise a silicone composition with a hydrophile such
as Polyethylene Glycol (PEG) covalently incorporated or grafted
therein, such as described in co-pending U.S. patent application
Ser. No. 10/695,636, entitled, "SILICONE COMPOSITION FOR
BIOCOMPATIBLE MEMBRANE," which is incorporated herein by reference
in its entirety. In one embodiment, the second domain is comprised
of a silicone copolymer including a hydrophilic component, which
can be formed as a unitary structure with the first domain or a
separate structure adhered thereto.
[0115] In general, the materials preferred for the second domain
prevent or hinder cell entry or contact with device elements
underlying the membrane and prevent or hinder the adherence of
cells, thereby further discouraging formation of a barrier cell
layer. Additionally, because of the resistance of the materials to
barrier cell layer formation, membranes prepared therefrom are
robust long-term in vivo.
[0116] The thickness of the cell impermeable biomaterial of the
second domain (also referred to as a cell impermeable domain) is
typically about 1 .mu.m or more, preferably from about 1, 5, 10,
15, 20, 25, 30, 35, 40, 45, or 50 .mu.m to about 55, 60, 65, 70,
75, 80, 85, 90, 95, 100, 110, 120, 130, 140, 150, 160, 170, 180,
190, or 200 .mu.m. In some embodiments, thicker or thinner cell
impermeable domains can be desired. Alternatively, the function of
the cell impermeable domain is accomplished by the implantable
device, or a portion of the implantable device, which may or may
not include a distinct domain or layer.
[0117] The characteristics of the cell impermeable membrane prevent
or hinder cells from entering the membrane, but permit or
facilitate transport of the analyte of interest or a substance
indicative of the concentration or presence of the analyte.
Additionally the second domain, similar to the first domain, is
preferably constructed of a biodurable material (for example, a
material durable for a period of several years in vivo) that is
impermeable to host cells, for example, macrophages, such as
described above.
[0118] In embodiments wherein the biointerface membrane is employed
in an implantable glucose-measuring device, the biointerface
membrane is permeable to oxygen and glucose or a substance
indicative of the concentration of glucose. In embodiments wherein
the membrane is employed in a drug delivery device or other device
for delivering a substance to the body, the cell impermeable
membrane is permeable to the drug or other substance dispensed from
the device. In embodiments wherein the membrane is employed for
cell transplantation, the membrane is semi-permeable, for example,
impermeable to immune cells and soluble factors responsible for
rejecting transplanted tissue, but permeable to the ingress of
glucose and oxygen for the purpose of sustaining the transplanted
tissue; additionally, the second domain is permeable to the egress
of the gene product of interest (for example, insulin).
[0119] The cell disruptive (first) domain and the cell impermeable
(second) domain can be secured to each other by any suitable method
as is known in the art. For example, the cell impermeable domain
can simply be layered or cast upon the porous cell disruptive
domain so as to form a mechanical attachment. Alternatively,
chemical and/or mechanical attachment methods can be suitable for
use. Chemical attachment methods can include adhesives, glues,
lamination, and/or wherein a thermal bond is formed through the
application of heat and pressure, and the like. Suitable adhesives
are those capable of forming a bond between the materials that make
up both the barrier cell disruptive domain and the cell impermeable
domain, and include liquid and/or film applied adhesives. An
appropriate material can be designed that can be used for preparing
both domains such that the composite is prepared in one step,
thereby forming a unitary structure. For example, when the cell
disruptive domain and the cell impermeable domain comprise
silicone, the materials can be designed so that they can be
covalently cured to one another. However in some embodiments
wherein the second domain comprises a part of the implantable
device, it can be attached to or simply lie adjacent to the first
domain.
[0120] In some embodiments wherein an adhesive is employed, the
adhesive can comprise a biocompatible material. However, in some
embodiments adhesives not generally considered to have a high
degree of biocompatibility can also be employed. Adhesives with
varying degrees of biocompatibility suitable for use include
acrylates, for example, cyanoacrylates, epoxies, methacrylates,
polyurethanes, and other polymers, resins, and crosslinking agents
as are known in the art. In some embodiments, a layer of non-woven
material (such as ePTFE) is cured to the first domain after which
the material is bonded to the second domain, which allows a good
adhesive interface between the first and second domains using a
biomaterial known to respond well at the tissue-device interface,
for example.
Bioactive Agents
[0121] The biointerface membranes of the preferred embodiments
preferably include a bioactive agent, which is incorporated into at
least one of the first and second domains of the biointerface
membrane, or which is incorporated into the device and adapted to
diffuse through the first and/or second domains, in order to modify
the tissue response of the host to the membrane. The architectures
of the first and second domains support vascularized tissue growth
in or around the biointerface membrane, interfere with and resist
barrier cell layer formation, and allow the transport of analytes
across the membrane. However, certain outside influences, for
example, faulty surgical techniques, acute or chronic movement of
the implant, or other surgery-, patient-, and/or implantation
site-related conditions, can create acute and/or chronic
inflammation at the implant site. When this occurs, the
biointerface membrane architecture alone may not be sufficient to
overcome the acute and/or chronic inflammation. Alternatively, the
membrane architecture can benefit from additional mechanisms that
aid in reducing this acute and/or chronic inflammation that can
produce a barrier cell layer and/or a fibrotic capsule surrounding
the implant, resulting in compromised solute transport through the
membrane.
[0122] In general, the inflammatory response to biomaterial
implants can be divided into two phases. The first phase consists
of mobilization of mast cells and then infiltration of
predominantly polymorphonuclear (PMN) cells. This phase is termed
the acute inflammatory phase. Over the course of days to weeks,
chronic cell types that comprise the second phase of inflammation
replace the PMNs. Macrophage and lymphocyte cells predominate
during this phase. While not wishing to be bound by any particular
theory, it is believed that short-term stimulation of
vascularization, or short-term inhibition of scar formation or
barrier cell layer formation, provides protection from scar tissue
formation, thereby providing a stable platform for sustained
maintenance of the altered foreign body response.
[0123] Accordingly, bioactive intervention can modify the foreign
body response in the early weeks of foreign body capsule formation,
thereby fundamentally altering the long-term behavior of the
foreign body capsule. Additionally, it is believed that the
biointerface membranes of the preferred embodiments can
advantageously benefit from bioactive intervention to overcome
sensitivity of the membrane to implant procedure, motion of the
implant, or other factors, which are known to otherwise cause
inflammation, scar formation, and hinder device function in
vivo.
[0124] In general, bioactive agents that are believed to modify
tissue response include anti-inflammatory agents, anti-infective
agents, anesthetics, inflammatory agents, growth factors,
angiogenic (growth) factors, adjuvants, wound factors, resorbable
device components, immunosuppressive agents, antiplatelet agents,
anticoagulants, ACE inhibitors, cytotoxic agents, anti-barrier cell
compounds, vascularization compounds, anti-sense molecules, and the
like. In some embodiments, preferred bioactive agents include S1P
(Sphingosine-1-phosphate), Monobutyrin, Cyclosporin A,
Anti-thrombospondin-2, Rapamycin (and its derivatives), and
Dexamethasone. However, other bioactive agents, biological
materials (for example, proteins), or even non-bioactive substances
can be preferred for incorporation into the membranes of preferred
embodiments.
[0125] Bioactive agents suitable for use in the preferred
embodiments are loosely organized into two groups: anti-barrier
cell agents and vascularization agents. These designations reflect
functions that are believed to provide short-term solute transport
through the biointerface membrane, and additionally extend the life
of a healthy vascular bed and hence solute transport through the
biointerface membrane long term in vivo. However, not all bioactive
agents can be clearly categorized into one or other of the above
groups; rather, bioactive agents generally comprise one or more
varying mechanisms for modifying tissue response and can be
generally categorized into one or both of the above-cited
categories.
Anti-Barrier Cell Agents
[0126] Generally, anti-barrier cell agents include compounds
exhibiting affects on macrophages and foreign body giant cells
(FBGCs). It is believed that anti-barrier cell agents prevent
closure of the barrier to solute transport presented by macrophages
and FBGCs at the device-tissue interface during FBC maturation.
[0127] Anti-barrier cell agents generally include mechanisms that
inhibit foreign body giant cells and/or occlusive cell layers. For
example, Super Oxide Dismutase (SOD) Mimetic, which utilizes a
manganese catalytic center within a porphyrin like molecule to
mimic native SOD and effectively remove superoxide for long
periods, thereby inhibiting FBGC formation at the surfaces of
biomaterials in vivo, is incorporated into a biointerface membrane
of a preferred embodiment.
[0128] Anti-barrier cell agents can include anti-inflammatory
and/or immunosuppressive mechanisms that affect the wound healing
process, for example, healing of the wound created by the incision
into which an implantable device is inserted. Cyclosporine, which
stimulates very high levels of neovascularization around
biomaterials, can be incorporated into a biointerface membrane of a
preferred embodiment [see U.S. Pat. No. 5,569,462 to Martinson et
al., which is incorporated herein by reference in its entirety.]
Alternatively, Dexamethasone, which abates the intensity of the FBC
response at the tissue-device interface, can be incorporated into a
biointerface membrane of a preferred embodiment. Alternatively,
Rapamycin, which is a potent specific inhibitor of some macrophage
inflammatory functions, can be incorporated into a biointerface
membrane of a preferred embodiment.
[0129] Other suitable medicaments, pharmaceutical compositions,
therapeutic agents, or other desirable substances can be
incorporated into the membranes of preferred embodiments,
including, but not limited to, anti-inflammatory agents,
anti-infective agents, and anesthetics.
[0130] Generally, anti-inflammatory agents reduce acute and/or
chronic inflammation adjacent to the implant, in order to decrease
the formation of a FBC capsule to reduce or prevent barrier cell
layer formation. Suitable anti-inflammatory agents include but are
not limited to, for example, nonsteroidal anti-inflammatory drugs
(NSAIDs) such as acetometaphen, aminosalicylic acid, aspirin,
celecoxib, choline magnesium trisalicylate, diclofenac potassium,
diclofenac sodium, diflunisal, etodolac, fenoprofen, flurbiprofen,
ibuprofen, indomethacin, interleukin (IL)-10, IL-6 mutein,
anti-IL-6 iNOS inhibitors (for example, L-NAME or L-NMDA),
Interferon, ketoprofen, ketorolac, leflunomide, melenamic acid,
mycophenolic acid, mizoribine, nabumetone, naproxen, naproxen
sodium, oxaprozin, piroxicam, rofecoxib, salsalate, sulindac, and
tolmetin; and corticosteroids such as cortisone, hydrocortisone,
methylprednisolone, prednisone, prednisolone, betamethesone,
beclomethasone dipropionate, budesonide, dexamethasone sodium
phosphate, flunisolide, fluticasone propionate, paclitaxel,
tacrolimus, tranilast, triamcinolone acetonide, betamethasone,
fluocinolone, fluocinonide, betamethasone dipropionate,
betamethasone valerate, desonide, desoximetasone, fluocinolone,
triamcinolone, triamcinolone acetonide, clobetasol propionate, and
dexamethasone.
[0131] Generally, immunosuppressive and/or immunomodulatory agents
interfere directly with several key mechanisms necessary for
involvement of different cellular elements in the inflammatory
response. Suitable immunosuppressive and/or immunomodulatory agents
include anti-proliferative, cell-cycle inhibitors, (for example,
paclitaxel, cytochalasin D, infiximab), taxol, actinomycin,
mitomycin, thospromote VEGF, estradiols, NO donors, QP-2,
tacrolimus, tranilast, actinomycin, everolimus, methothrexate,
mycophenolic acid, angiopeptin, vincristing, mitomycine, statins, C
MYC antisense, sirolimus (and analogs), RestenASE,
2-chloro-deoxyadenosine, PCNA Ribozyme, batimstat, prolyl
hydroxylase inhibitors, PPAR.gamma. ligands (for example
troglitazone, rosiglitazone, pioglitazone), halofuginone,
C-proteinase inhibitors, probucol, BCP671, EPC antibodies,
catchins, glycating agents, endothelin inhibitors (for example,
Ambrisentan, Tesosentan, Bosentan), Statins (for example,
Cerivasttin), E. coli heat-labile enterotoxin, and advanced
coatings.
[0132] Generally, anti-infective agents are substances capable of
acting against infection by inhibiting the spread of an infectious
agent or by killing the infectious agent outright, which can serve
to reduce immuno-response without inflammatory response at the
implant site. Anti-infective agents include, but are not limited
to, anthelmintics (mebendazole), antibiotics including
aminoclycosides (gentamicin, neomycin, tobramycin), antifungal
antibiotics (amphotericin b, fluconazole, griseofulvin,
itraconazole, ketoconazole, nystatin, micatin, tolnaftate),
cephalosporins (cefaclor, cefazolin, cefotaxime, ceftazidime,
ceftriaxone, cefuroxime, cephalexin), beta-lactam antibiotics
(cefotetan, meropenem), chloramphenicol, macrolides (azithromycin,
clarithromycin, erythromycin), penicillins (penicillin G sodium
salt, amoxicillin, ampicillin, dicloxacillin, nafcillin,
piperacillin, ticarcillin), tetracyclines (doxycycline,
minocycline, tetracycline), bacitracin; clindamycin; colistimethate
sodium; polymyxin b sulfate; vancomycin; antivirals including
acyclovir, amantadine, didanosine, efavirenz, foscarnet,
ganciclovir, indinavir, lamivudine, nelfinavir, ritonavir,
saquinavir, silver, stavudine, valacyclovir, valganciclovir,
zidovudine; quinolones (ciprofloxacin, levofloxacin); sulfonamides
(sulfadiazine, sulfisoxazole); sulfones (dapsone); furazolidone;
metronidazole; pentamidine; sulfanilamidum crystallinum;
gatifloxacin; and sulfamethoxazole/trimethoprim.
Vascularization Agents
[0133] Generally, vascularization agents include substances with
direct or indirect angiogenic properties. In some cases,
vascularization agents may additionally affect formation of barrier
cells in vivo. By indirect angiogenesis, it is meant that the
angiogenesis can be mediated through inflammatory or immune
stimulatory pathways. It is not fully known how agents that induce
local vascularization indirectly inhibit barrier-cell formation,
however it is believed that some barrier-cell effects can result
indirectly from the effects of vascularization agents.
[0134] Vascularization agents include mechanisms that promote
neovascularization and accelerate wound healing around the membrane
and/or minimize periods of ischemia by increasing vascularization
close to the tissue-device interface. Sphingosine-1-Phosphate
(S1P), which is a phospholipid possessing potent angiogenic
activity, is incorporated into a biointerface membrane of a
preferred embodiment. Monobutyrin, which is a potent vasodilator
and angiogenic lipid product of adipocytes, is incorporated into a
biointerface membrane of a preferred embodiment. In another
embodiment, an anti-sense molecule (for example, thrombospondin-2
anti-sense), which increases vascularization, is incorporated into
a biointerface membrane.
[0135] Vascularization agents can include mechanisms that promote
inflammation, which is believed to cause accelerated
neovascularization and wound healing in vivo. In one embodiment, a
xenogenic carrier, for example, bovine collagen, which by its
foreign nature invokes an immune response, stimulates
neovascularization, and is incorporated into a biointerface
membrane of the preferred embodiments. In another embodiment,
Lipopolysaccharide, which is a potent immunostimulant, is
incorporated into a biointerface membrane. In another embodiment, a
protein, for example, a bone morphogenetic protein (BMP), which is
known to modulate bone healing in tissue, is incorporated into a
biointerface membrane of a preferred embodiment.
[0136] Generally, angiogenic agents are substances capable of
stimulating neovascularization, which can accelerate and sustain
the development of a vascularized tissue bed at the tissue-device
interface. Angiogenic agents include, but are not limited to, Basic
Fibroblast Growth Factor (bFGF), (also known as Heparin Binding
Growth Factor-II and Fibroblast Growth Factor II), Acidic
Fibroblast Growth Factor (aFGF), (also known as Heparin Binding
Growth Factor-I and Fibroblast Growth Factor-I), Vascular
Endothelial Growth Factor (VEGF), Platelet Derived Endothelial Cell
Growth Factor BB (PDEGF-BB), Angiopoietin-1, Transforming Growth
Factor Beta (TGF-Beta), Transforming Growth Factor Alpha
(TGF-Alpha), Hepatocyte Growth Factor, Tumor Necrosis Factor-Alpha
(TNF-Alpha), Placental Growth Factor (PLGF), Angiogenin,
Interleukin-8 (IL-8), Hypoxia Inducible Factor-I (HIF-1),
Angiotensin-Converting Enzyme (ACE) Inhibitor Quinaprilat,
Angiotropin, Thrombospondin, Peptide KGHK, Low Oxygen Tension,
Lactic Acid, Insulin, Copper Sulphate, Estradiol, prostaglandins,
cox inhibitors, endothelial cell binding agents (for example,
decorin or vimentin), glenipin, hydrogen peroxide, nicotine, and
Growth Hormone.
[0137] Generally, pro-inflammatory agents are substances capable of
stimulating an immune response in host tissue, which can accelerate
or sustain formation of a mature vascularized tissue bed. For
example, pro-inflammatory agents are generally irritants or other
substances that induce chronic inflammation and chronic granular
response at the wound-site. While not wishing to be bound by
theory, it is believed that formation of high tissue granulation
induces blood vessels, which supply an adequate, or rich supply of
analytes to the device-tissue interface. Pro-inflammatory agents
include, but are not limited to, xenogenic carriers,
Lipopolysaccharides, S. aureus peptidoglycan, and proteins.
[0138] Other substances that can be incorporated into membranes of
preferred embodiments include various pharmacological agents,
excipients, and other substances well known in the art of
pharmaceutical formulations.
Bioactive Agent Delivery Systems and Methods
[0139] There are a variety of systems and methods by which the
bioactive agent is incorporated into the biointerface membranes of
the preferred embodiments. In some embodiments, the bioactive agent
is incorporated at the time of manufacture of the biointerface
membrane. For example, the bioactive agent can be blended prior to
curing the biointerface membrane, or subsequent to biointerface
membrane manufacture, for example, by coating, imbibing,
solvent-casting, or sorption of the bioactive agent into the
biointerface membrane. Although the bioactive agent is preferably
incorporated into the biointerface membrane, in some embodiments
the bioactive agent can be administered concurrently with, prior
to, or after implantation of the device systemically, for example,
by oral administration, or locally, for example, by subcutaneous
injection near the implantation site. A combination of bioactive
agent incorporated in the biointerface membrane and bioactive agent
administration locally and/or systemically can be preferred in
certain embodiments.
[0140] The biointerface membranes of the preferred embodiments
preferably include a bioactive agent, which is incorporated into at
least one of the first and second domains of the biointerface
membrane, and/or which is incorporated into the device and adapted
to diffuse through the first and/or second domains, in order to
modify the tissue response of the host to the membrane. In some
embodiments wherein the biointerface membrane is used with an
analyte-measuring device, the bioactive agent is incorporated only
into a portion of the biointerface membrane adjacent to the sensing
region of the device, over the entire surface of the device except
over the sensing region, or any combination thereof, which can be
helpful in controlling different mechanisms and/or stages of the
maturation of the FBC. In some alternative embodiments however, the
bioactive agent is incorporated into the implantable device
proximal to the biointerface membrane, such that the bioactive
agent diffuses through the biointerface membrane to the host
tissue.
[0141] The bioactive agent can include a carrier matrix, wherein
the matrix includes one or more of collagen, a particulate matrix,
a resorbable or non-resorbable matrix, a controlled-release matrix,
and/or a gel. In some embodiments, the carrier matrix includes a
reservoir, wherein a bioactive agent is encapsulated within a
microcapsule. The carrier matrix can include a system in which a
bioactive agent is physically entrapped within a polymer network.
In some embodiments, the bioactive agent is cross-linked with the
biointerface membrane, while in others the bioactive agent is
sorbed into the biointerface membrane, for example, by adsorption,
absorption, or imbibing. The bioactive agent can be deposited in or
on the biointerface membrane, for example, by coating, filling, or
solvent casting. In certain embodiments, ionic and nonionic
surfactants, detergents, micelles, emulsifiers, demulsifiers,
stabilizers, aqueous and oleaginous carriers, solvents,
preservatives, antioxidants, or buffering agents are used to
incorporate the bioactive agent into the biointerface membrane. The
bioactive agent can be incorporated into a polymer using techniques
such as described above, and the polymer can be used to form the
biointerface membrane, coatings on the biointerface membrane,
portions of the biointerface membrane, and/or a portion of an
implantable device.
[0142] The biointerface membrane can be manufactured using
techniques known in the art. The bioactive agent can be sorbed into
the biointerface membrane, for example, by soaking the biointerface
membrane for a length of time (for example, from about an hour or
less to about a week or more, preferably from about 4, 8, 12, 16,
or 20 hours to about 1, 2, 3, 4, 5, or 7 days). Absorption of
Dexamethasone into a porous silicone membrane is described in the
experimental section.
[0143] The bioactive agent can be blended into uncured polymer
prior to forming the biointerface membrane. The biointerface
membrane is then cured and the bioactive agent thereby cross-linked
and/or encapsulated within the polymer that forms the biointerface
membrane. For example, Monobutyrin was covalently bonded to a
silicone matrix in such a manner that is slowly cleavable under in
vivo conditions. The alcohol groups of Monobutyrin react with a
silanol group, resulting in a C--O--Si bond. This bond is known to
be susceptible to hydrolysis, and is therefore cleaved to yield the
original alcohol and silanol. Thus, the Monobutyrin is released
from the silicone matrix according to the rate of hydrolysis. Other
bioactive agents, such as Dexamethasone, comprise alcohol groups
and can be bound to a silicone matrix in a similar manner.
[0144] In yet another embodiment, microspheres are used to
encapsulate the bioactive agent. The microspheres can be formed of
biodegradable polymers, most preferably synthetic polymers or
natural polymers such as proteins and polysaccharides. As used
herein, the term polymer is used to refer to both to synthetic
polymers and proteins. U.S. Pat. No. 6,281,015, which is
incorporated herein by reference in its entirety, discloses some
systems and methods that can be used in conjunction with the
preferred embodiments. In general, bioactive agents can be
incorporated in (1) the polymer matrix forming the microspheres,
(2) microparticle(s) surrounded by the polymer which forms the
microspheres, (3) a polymer core within a protein microsphere, (4)
a polymer coating around a polymer microsphere, (5) mixed in with
microspheres aggregated into a larger form, or (6) a combination
thereof. Bioactive agents can be incorporated as particulates or by
co-dissolving the factors with the polymer. Stabilizers can be
incorporated by addition of the stabilizers to the factor solution
prior to formation of the microspheres.
[0145] The bioactive agent can be incorporated into a hydrogel and
coated or otherwise deposited in or on the biointerface membrane.
Some hydrogels suitable for use in the preferred embodiments
include cross-linked, hydrophilic, three-dimensional polymer
networks that are highly permeable to the bioactive agent and are
triggered to release the bioactive agent based on a stimulus.
[0146] The bioactive agent can be incorporated into the
biointerface membrane by solvent casting, wherein a solution
including dissolved bioactive agent is disposed on the surface of
the biointerface membrane, after which the solvent is removed to
form a coating on the membrane surface.
[0147] In yet another embodiment, the interconnected cavities of
the biointerface membrane are filled with the bioactive agent.
Preferably, a bioactive agent, with or without a carrier matrix,
fills the cavities of the membrane, depending on the loading and
release properties desired, which are discussed in more detail
below.
[0148] The bioactive agent can be compounded into a plug of
material, which is placed within the implantable device, such as is
described in U.S. Pat. Nos. 4,506,680 and 5,282,844, which are
incorporated herein by reference in their entirety. In contrast to
the method disclosed in U.S. Pat. Nos. 4,506,680 and 5,282,844, in
the preferred embodiments it is preferred to dispose the plug
beneath a membrane system, for example, beneath the sensing
membrane or biointerface membrane. In this way, the bioactive agent
is controlled by diffusion through the membrane, which provides a
mechanism for sustained-release of the bioactive agent long-term in
the host.
Release of Bioactive Agents
[0149] Numerous variables can affect the pharmacokinetics of
bioactive agent release. The bioactive agents of the preferred
embodiments can be optimized for short- and/or long-term release.
In some embodiments, the bioactive agents of the preferred
embodiments are designed to aid or overcome factors associated with
short-term effects (for example, acute inflammation) of the foreign
body response, which can begin as early as the time of implantation
and extend up to about one month after implantation. In some
embodiments, the bioactive agents of the preferred embodiments are
designed to aid or overcome factors associated with long-term
effects, for example, chronic inflammation, barrier cell layer
formation, or build-up of fibrotic tissue of the foreign body
response, which can begin as early as about one week after
implantation and extend for the life of the implant, for example,
months to years. In some embodiments, the bioactive agents of the
preferred embodiments combine short- and long-term release to
exploit the benefits of both.
[0150] As used herein, "controlled," "sustained," or "extended"
release of the factors can be continuous or discontinuous, linear
or non-linear. This can be accomplished using one or more types of
polymer compositions, drug loadings, selections of excipients or
degradation enhancers, or other modifications, administered alone,
in combination or sequentially to produce the desired effect.
[0151] Short-term release of the bioactive agent in the preferred
embodiments generally refers to release over a period of from about
1 day or less to about 2, 3, 4, 5, 6, or 7 days, 2 or 3 weeks, 1
month, or more. More preferably, the short-term release of the
bioactive agent occurs over from about 14, 15, 16, 17, or 18 days
up to about 19, 20, or 21 days.
[0152] Conventional devices, such as implantable analyte
measuring-devices, drug delivery devices, and cell transplantation
devices that require transport of solutes across the device-tissue
interface for proper function, tend to lose their function after
the first few days following implantation. At least one reason for
this loss of function is the lack of direct contact with
circulating fluid for appropriate analyte transport to the device.
Therefore, in some embodiments, short-term release of certain
bioactive agents, for example vascularization agents, can increase
the circulating fluid to the device for an extended period of
time.
[0153] Additionally, it is believed that short-term release of the
bioactive agent can have a positive effect of the functionality of
porous biointerface membranes during the initial tissue ingrowth
period prior to formation of a capillary bed. For example, when a
device requiring analyte transport across its device-tissue
interface is implanted, a "sleep period" can occur which begins as
early as the first day after implantation and extends as far as one
month after implantation. However shorter sleep periods are more
common. During this sleep period, extensive ingrowth of tissue into
the porous structure causes the inflammatory cells responsible for
facilitating wound healing to proliferate within the local
environment of the wound region. Because these cells are respiring,
they consume some or all of the glucose and oxygen that is within
the wound environment, which has shown to block adequate flow of
analytes to the implantable device. Accordingly in some
embodiments, it is believed that short-term release of certain
bioactive agents, for example vascularization agents, can aid in
providing adequate vascularization to substantially overcome the
effects of the sleep period, and thereby allow sufficient analytes
to pass through to the implantable device.
[0154] Additionally, it is believed that short-term release of the
bioactive agent can have an enhanced effect on neovascularization
at the tissue-device interface. Although neovascularization alone
is generally not sufficient to provide sufficient analyte transport
at the device-tissue interface, in combination with other
mechanisms, enhanced neovascularization can result in enhanced
transport of analytes from the host to the implanted device.
Therefore in some embodiments, short-term release of certain
bioactive agents, for example angiogenic agents, can have a
positive effect on neovascularization and thereby enhance transport
of analytes at the device-tissue interface.
[0155] Additionally, it is believed that short-term release of the
bioactive agent can be sufficient to reduce or prevent barrier cell
layer formation. Formation of a cohesive monolayer of closely
opposed cells, e.g., macrophages and foreign body giant cells,
interfere with the transport of analytes across the tissue-device
interface, also known as a barrier cell layer, and are large
contributors to poor device performance. See U.S. Pat. No.
6,702,857, which is incorporated herein by reference in its
entirety. Therefore in some embodiments, it is believed that
short-term release of certain bioactive agents, for example,
anti-barrier cell agents, can aid in preventing barrier cell layer
formation.
[0156] Additionally, it is believed that short-term release of the
bioactive agent can be sufficient to prevent negative effects of
acute inflammation caused, for example, by surgical trauma,
micro-motion, or macro-motion of the device in the soft tissue.
Short-term release of anti-inflammatory agents can be sufficient to
rescue a biointerface membrane from the negative effects associated
with such acute inflammation, rendering adequate analyte
transport.
[0157] Long-term release of the bioactive agent in the preferred
embodiments generally occurs over a period of from about 1 month to
about 2 years or more, preferably from at least about 2 months to
at least about 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, or 23
months, and more preferably from at least about 3 months to at
least about 4, 5, 6, 7, 8, 9, 10, 11, or 12 months.
[0158] Long-term glucose-measuring device experiments demonstrate
that many biointerface materials experience a distinct and
continual decline in sensitivity, for example, reduced analyte
transport, beginning at three months after implantation in some
cases. It is believed that this decline in analyte transport can be
a result of barrier cell layer formation, cellular growth at the
membrane, and/or thickening of the fibrous elements of the foreign
body capsule. Other contributing factors can include chronic
inflammation, which is believed to be due to micro-motion or
macro-motion of the device; delamination of the biointerface
membrane, which is believed to be due to cellular ingrowth within
and under the biointerface membrane; compression of the
biointerface membrane due to increasing compression of the foreign
body capsule around the device; and distortion of the biointerface
membrane, which is believed to be a result of a combination of
compression and cellular ingrowth, for example.
[0159] Accordingly, long-term release of certain bioactive agents
can modulate the foreign body response sufficiently to prevent
long-term thickening of the foreign body capsule, reduce or prevent
barrier cell layer formation, reduce or prevent chronic
inflammation, reduce or prevent extensive cellular ingrowth, and/or
reduce or prevent compression of the foreign body capsule on the
biointerface membrane.
Loading of Bioactive Agents
[0160] The amount of loading of the bioactive agent into the
biointerface membrane can depend upon several factors. For example,
the bioactive agent dosage and duration can vary with the intended
use of the biointerface membrane, for example, cell
transplantation, analyte measuring-device, and the like;
differences among patients in the effective dose of bioactive
agent; location and methods of loading the bioactive agent; and
release rates associated with bioactive agents and optionally their
carrier matrix. Therefore, one skilled in the art will appreciate
the variability in the levels of loading the bioactive agent, for
the reasons described above.
[0161] In some embodiments, wherein the bioactive agent is
incorporated into the biointerface membrane without a carrier
matrix, the preferred level of loading of the bioactive agent into
the biointerface membrane can vary depending upon the nature of the
bioactive agent. The level of loading of the bioactive agent is
preferably sufficiently high such that a biological effect is
observed. Above this threshold, bioactive agent can be loaded into
the biointerface membrane so as to imbibe up to 100% of the solid
portions, cover all accessible surfaces of the membrane, and/or
fill up to 100% of the accessible cavity space. Typically, the
level of loading (based on the weight of bioactive agent(s),
biointerface membrane, and other substances present) is from about
1 ppm or less to about 1000 ppm or more, preferably from about 2,
3, 4, or 5 ppm up to about 10, 25, 50, 75, 100, 200, 300, 400, 500,
600, 700, 800, or 900 ppm. In certain embodiments, the level of
loading can be 1 wt. % or less up to about 50 wt. % or more,
preferably from about 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, or 20 wt. %
up to about 25, 30, 35, 40, or 45 wt. %.
[0162] When the bioactive agent is incorporated into the
biointerface membrane with a carrier matrix, such as a gel, the gel
concentration can be optimized, for example, loaded with one or
more test loadings of the bioactive agent. It is generally
preferred that the gel contain from about 0.1 or less to about 50
wt. % or more of the bioactive agent(s), preferably from about 0.2,
0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or 0.9 wt. % to about 6, 7, 8, 9, 10,
15, 20, 25, 30, 35, 40, or 45 wt. % or more bioactive agent(s),
more preferably from about 1, 2, or 3 wt. % to about 4 or 5 wt. %
of the bioactive agent(s). Substances that are not bioactive can
also be incorporated into the matrix.
[0163] Referring now to microencapsulated bioactive agents, the
release of the agents from these polymeric systems generally occur
by two different mechanisms. The bioactive agent can be released by
diffusion through aqueous filled channels generated in the dosage
form by the dissolution of the agent or by voids created by the
removal of the polymer solvent or a pore forming agent during the
original micro-encapsulation. Alternatively, release can be
enhanced due to the degradation of the polymer. With time, the
polymer erodes and generates increased porosity and microstructure
within the device. This creates additional pathways for release of
the bioactive agent.
Implantable Devices
[0164] Biointerface membranes of the preferred embodiments are
suitable for use with implantable devices in contact with a
biological fluid. For example, the biointerface membranes can be
utilized with implantable devices and methods for monitoring and
determining analyte levels in a biological fluid, such as
measurement of glucose levels for individuals having diabetes. In
some embodiments, the analyte-measuring device is a continuous
device. Alternatively, the device can analyze a plurality of
intermittent biological samples. The analyte-measuring device can
use any method of analyte-measurement, including enzymatic,
chemical, physical, electrochemical, spectrophotometric,
polarimetric, calorimetric, radiometric, or the like.
[0165] Although some of the description that follows is directed at
glucose-measuring devices, including the described biointerface
membranes and methods for their use, these biointerface membranes
are not limited to use in devices that measure or monitor glucose.
These biointerface membranes are suitable for use in a variety of
devices, including, for example, those that detect and quantify
other analytes present in biological fluids (including, but not
limited to, cholesterol, amino acids, and lactate), cell
transplantation devices (see, e.g., U.S. Pat. Nos. 6,015,572,
5,964,745, and 6,083,523), drug delivery devices (see, e.g., U.S.
Pat. Nos. 5,458,631, 5,820,589, and 5,972,369) and electrical
delivery and/or measuring devices such as implantable pulse
generation cardiac pacing devices (see, e.g., U.S. Pat. Nos.
6,157,860, 5,782,880, and 5,207,218), electrocardiogram devices
(see, e.g., U.S. Pat. Nos. 4,625,730 and 5,987,352) electrical
nerve stimulating devices (see, e.g., U.S. Pat. Nos. 6,175,767,
6,055,456, and 4,940,065), and in combination with angiogenic
factor gene transfer technology to enhance implantable device
function (see, e.g., Klueh U, Dorsky D I, Kreutzer D L. Use of
vascular endothelial cell growth factor gene transfer to enhance
implantable device function in vivo. J Biomed Mater Res. 2003 Dec.
15; 67A(4):1072-86), to name but a few The biointerface membranes
can be utilized in conjunction with transplanted cells, for
example, transplanted genetic engineered cells of Langerhans,
either allo, auto or xeno geneic in origin, as pancreatic beta
cells to increase the diffusion of nutrients to the islets, but
additionally utilizing a biointerface membrane of the preferred
embodiment on a measuring-device proximal to the transplanted cells
to sense glucose in the tissues of the patient to monitor the
viability of the implanted cells. Preferably, implantable devices
that include the biointerface membranes of the preferred
embodiments are implanted in soft tissue, for example, abdominal,
subcutaneous, and peritoneal tissues, the brain, the intramedullary
space, and other suitable organs or body tissues.
[0166] In addition to the glucose-measuring device described below,
the biointerface membranes of the preferred embodiments can be
employed with a variety of known continuous glucose
measuring-devices. For example, the biointerface membrane can be
employed in conjunction with a continuous glucose measuring-device
that comprises a subcutaneous measuring-device such as is described
in U.S. Pat. No. 6,579,690 to Bonnecaze et al. and U.S. Pat. No.
6,484,046 to Say et al. In another alternative embodiment, the
continuous glucose measuring-device comprises a refillable
subcutaneous measuring-device such as is described in U.S. Pat. No.
6,512,939 to Colvin et al. All of the above patents are
incorporated in their entirety herein by reference. In general, it
is understood that the disclosed embodiments are applicable to a
variety of continuous glucose measuring-device configurations.
[0167] Implantable devices for detecting the presence of an analyte
or analyte concentrations in a biological system can utilize the
biointerface membranes of the preferred embodiments to increase
local vascularization and interfere with the formation of a barrier
cell layer, thereby assuring that the measuring-device receives
analyte concentrations representative of that in the vasculature.
Drug delivery devices can utilize the biointerface membranes of the
preferred embodiments to protect the drug housed within the device
from host inflammatory or immune cells that might potentially
damage or destroy the drug. In addition, the biointerface membrane
can prevent or hinder the formation of a barrier cell layer that
can interfere with proper dispensing of drug from the device for
treatment of the host. Correspondingly, cell transplantation
devices can utilize the biointerface membranes of the preferred
embodiments to protect the transplanted cells from attack by the
host inflammatory or immune response cells while simultaneously
preventing the formation of a barrier cell layer, thereby
permitting nutrients as well as other biologically active molecules
needed by the cells for survival to diffuse through the
membrane.
[0168] FIG. 3 is a graph of signal output from a glucose-measuring
device implanted in a human, wherein the device included a
biointerface membrane without a bioactive agent incorporated
therein. The graph shows the data signal produced by the device
from time of implant up to about 21 days after implant. The x-axis
represents time in days; the y-axis presents the data signal from
the device output in counts. The term "counts," as used herein, is
a broad term and is used in its ordinary sense, including, without
limitation, a unit of measurement of a digital signal. In one
example, a raw data signal measured in counts is directly related
to a voltage (converted by an A/D converter), which is directly
related to current. The glucose-measuring device of this experiment
is described in more detail with reference to FIGS. 4A and 4B.
[0169] Referring to FIG. 3, the device associated with the signal
output was implanted during day 1. The associated signal output is
shown beginning at day 1 and substantially tracks the rise and fall
of the patient's glucose levels during the first few days after
implant. It is noted that approximately 5 days after device
implant, the signal output experienced a temporary decrease in
sensitivity, sometimes referred to as a "sleep period." It is
believed that this loss in sensitivity is due to migration of
cells, which consume glucose and oxygen during formation of a
vascularized foreign body capsule (tissue bed) into and around the
biointerface membrane. In this example, the sleep period continues
for approximately 7 days during which time the glucose-measuring
device does not accurately track the patient's glucose levels.
Approximately 12 days after implant, the signal output resumes
function, as indicated by the rise and fall of the signal output,
which correlates with the rise and fall the patient's glucose
levels. It is believed that this resuming of signal output
correlates with a reduction in the numbers of inflammatory cells
and a mature vascularized tissue bed within and around the
biointerface membrane that allows glucose and oxygen to transport
through the biointerface membrane to the glucose-measuring device.
The difference in sensitivity of the device before and after the
sleep period is attributed to the effect of the vascularized tissue
bed on the transport of glucose and oxygen therethrough. In
summary, it has been shown that the an implantable device with a
biointerface membrane but without a bioactive agent incorporated
therein sometimes undergoes a sleep period in the device during the
formation of the vascularized tissue bed and/or a foreign body
capsule surrounding and within the implant.
[0170] In order to overcome the sleep period described above, it is
believed that by incorporating bioactive agents that enhance local
vascularization and inhibit inflammatory cells within or around the
biointerface membranes of the preferred embodiments on implanted
devices, accelerated maturation of a vascularized tissue bed and
decreased inflammatory response will occur, which increases the
rate at which devices become functional, reducing or eliminating
the loss insensitivity seen in the experiment above. The bioactive
agents that are incorporated into the biointerface membrane 30 used
on implantable devices of certain preferred embodiments are chosen
to optimize the rate of biointerface formation.
[0171] In some embodiments, the bioactive agents that are
incorporated into the biointerface membrane 30 used on implantable
devices are chosen to optimize reliable biointerface formation. In
some situations, stable device function does not occur due to
faulty surgical techniques, acute or chronic movement of the
implant, or other surgery-, patient-, or implantation site-related
complications, which can create acute and/or chronic inflammation
at the implant site and subsequent formation of barrier cell layer
and/or thick fibrotic tissue build-up. While not wishing to be
bound by theory, it is believed that bioactive agents described in
the preferred embodiments, for example anti-inflammatory agents
and/or anti-barrier cell agents, can provide sufficient biological
activity to reduce the effects of site-related complications, and
thereby increase reliability of device functionality.
[0172] In some embodiments, the bioactive agents that are
incorporated into the biointerface membrane 30 used on implantable
devices are chosen to optimize the stability of the biointerface.
Even after devices have been implanted for some length of time and
begin to function, it is observed that device stability can be lost
gradually or suddenly. It is believed that this loss of stability
or function can be attributed the biointerface, based on
post-explantation histological examinations. This conclusion is
further supported by the observation that devices typically
function in vitro after removal from animals or humans. It is
therefore believed that delivery of bioactive agents described in
the preferred embodiments can increase the stability of the
biointerface so that device calibration values remain sufficiently
stable so as to provide accurate measurements.
[0173] FIGS. 4A and 4B are perspective views of an implantable
glucose measuring-device of a preferred embodiment. FIG. 4A is a
view of the assembled glucose measuring-device, including sensing
and biointerface membranes incorporated thereon. FIG. 4B is an
exploded view of the glucose measuring-device 60, showing the body
62, the sensing membrane 64, and the biointerface membrane 30 of a
preferred embodiment, such as is described in more detail
above.
[0174] The body 62 is preferably formed from epoxy molded around
the measuring-device electronics (not shown), however the body can
be formed from a variety of materials, including metals, ceramics,
plastics, or composites thereof. Co-pending U.S. patent application
Ser. No. 10/646,333, entitled, "Optimized Device Geometry for an
Implantable Glucose Device" discloses suitable configurations
suitable for the body 62, and is incorporated by reference in its
entirety.
[0175] In one preferred embodiment, the measuring-device 60 is an
enzyme-based measuring-device, which includes an electrode system
66 (for example, a platinum working electrode, a platinum counter
electrode, and a silver/silver chloride reference electrode), which
is described in more detail with reference to U.S. patent
application Ser. No. 09/916,711, entitled "Sensor head for use with
implantable devices," which is incorporated herein by reference in
its entirety. However, a variety of electrode materials and
configurations can be used with the implantable glucose
measuring-devices of the preferred embodiments. The top ends of the
electrodes are in contact with an electrolyte phase (not shown),
which is a free-flowing fluid phase disposed between a sensing
membrane 64 and the electrode system 66. In this embodiment, the
counter electrode is provided to balance the current generated by
the species being measured at the working electrode. In the case of
a glucose oxidase based glucose measuring-device, the species
measured at the working electrode is H.sub.2O.sub.2. Glucose
oxidase catalyzes the conversion of oxygen and glucose to hydrogen
peroxide and gluconate according to the following reaction:
Glucose+O.sub.2.fwdarw.Gluconate+H.sub.2O.sub.2
[0176] The change in H.sub.2O.sub.2 can be monitored to determine
glucose concentration because for each glucose molecule
metabolized, there is a proportional change in the product
H.sub.2O.sub.2. Oxidation of H.sub.2O.sub.2 by the working
electrode is balanced by reduction of ambient oxygen, enzyme
generated H.sub.2O.sub.2, or other reducible species at the counter
electrode. The H.sub.2O.sub.2 produced from the glucose oxidase
reaction further reacts at the surface of working electrode and
produces two protons (2H.sup.+), two electrons (2e.sup.-), and one
oxygen molecule (O.sub.2).
[0177] In this embodiment, a potentiostat is employed to monitor
the electrochemical reaction at the electroactive surface(s). The
potentiostat applies a constant potential to the working and
reference electrodes to determine a current value. The current that
is produced at the working electrode (and flows through the
circuitry to the counter electrode) is substantially proportional
to the amount of H.sub.2O.sub.2 that diffuses to the working
electrode. Accordingly, a raw signal can be produced that is
representative of the concentration of glucose in the user's body,
and therefore can be utilized to estimate a meaningful glucose
concentration.
[0178] In some embodiments, the sensing membrane 64 includes an
enzyme, for example, glucose oxidase, and covers the electrolyte
phase. The sensing membrane 64 preferably includes a resistance
domain most distal from the electrochemically reactive surfaces, an
enzyme domain less distal from the electrochemically reactive
surfaces than the resistance domain, and an electrolyte domain
adjacent to the electrochemically reactive surfaces. However, it is
understood that a sensing membrane 64 modified for other devices,
for example, by including fewer or additional domains, is within
the scope of the preferred embodiments. Co-pending U.S. patent
application Ser. No. 10/838,912, filed May 3, 2004 entitled,
"IMPLANTABLE ANALYTE SENSOR," and U.S. patent application Ser. No.
09/916,711, entitled, "SENSOR HEAD FOR USE WITH IMPLANTABLE
DEVICES," each of which are incorporated herein by reference in
their entirety, describes membranes that can be used in some
embodiments of the sensing membrane 64. In some embodiments, the
sensing membrane 64 can additionally include an interference domain
that blocks some interfering species; such as described in the
above-cited co-pending patent application. Co-pending U.S. patent
application Ser. No. 10/695,636, entitled, "SILICONE COMPOSITION
FOR BIOCOMPATIBLE MEMBRANE" also describes membranes that can be
used for the sensing membrane 64 of the preferred embodiments, and
is incorporated herein by reference in its entirety.
[0179] The biointerface membrane 30 includes a biointerface
membrane of a preferred embodiment, which covers the sensing
membrane and supports tissue ingrowth, interferes with the
formation of a barrier cell layer, and protects the sensitive
regions of the measuring-device 60 from host inflammatory response.
Preferably, the biointerface membrane 30 is a formed from a
non-resorbable membrane and includes a porous architecture with a
bioactive agent incorporated therein.
[0180] The biointerface membranes of the preferred embodiments can
incorporate a variety of mechanisms, including materials,
architecture, cavity size, and incorporation of one or bioactive
agents, which can be function alone or in combination to enhance
wound healing, which when incorporated into an analyte
measuring-device, result in enhanced device performance.
[0181] In one embodiment, an anchoring material (not shown) is
formed substantially around the device body in order to stabilize
the device in vivo. Controlled release of a bioactive agent from
the biointerface membrane 30, such as an anti-inflammatory agent,
is provided for a period of time up to about one month, which is
believed to be sufficient to reduce the effects of tissue trauma at
the device interface prior to stabilization of the device in vivo.
Consequently, when the device is stable (for example, when
sufficient tissue ingrowth into the anchoring material occurs to
ensure minimal motion and less broken fat cells, seepage and other
inflammatory factors), it is safe to permit the biointerface to
heal with good vascularization.
Experiments
[0182] The following examples serve to illustrate certain preferred
embodiments and aspects and are not to be construed as limiting the
scope thereof.
[0183] In the preceding description and the experimental disclosure
which follows, the following abbreviations apply: Eq and Eqs
(equivalents); mEq (milliequivalents); M (molar); mM (millimolar)
.mu.M (micromolar); N (Normal); mol (moles); mmol (millimoles);
.mu.mol (micromoles); nmol (nanomoles); g (grams); mg (milligrams);
.mu.g (micrograms); Kg (kilograms); L (liters); mL (milliliters);
dL (deciliters); .mu.L (microliters); cm (centimeters); mm
(millimeters); .mu.m (micrometers); nm (nanometers); h and hr
(hours); min. (minutes); s and sec. (seconds); .degree. C. (degrees
Centigrade).
Example 1
Preparation of Biointerface Membrane with Porous Silicone
[0184] A porous silicone cell disruptive (first) domain was
prepared by mixing approximately 1 kg of sugar crystals with
approximately 36 grams of water for 3-6 minutes. The mixture was
then pressed into a mold and baked at 80.degree. C. for 2 hours.
The silicone was vacuumed into the mold for 6 minutes and cured at
80.degree. C. for at least 2 hours. The sugar was dissolved using
heat and deionized water, resulting in a flat sheet, porous
membrane. Different architectures were obtained by varying the
crystal size (crystals having an average diameter of about 90, 106,
150, 180, and 220 .mu.m) and distribution within the mold that the
silicone was cast from. After removal of silicone from the mold,
the resulting membranes were measured for material thickness.
[0185] The cell-impermeable (second) domain was prepared by placing
approximately 706 gm of dimethylacetamide (DMAC) into a 3 L
stainless steel bowl to which a polycarbonate urethane solution
(1325 g, CHRONOFLEX.TM. AR 25% solids in DMAC and a viscosity of
5100 cp) and polyvinylpyrrolidone (125 g, PLASDONE.TM. K-90D) were
added. The bowl was then fitted to a planetary mixer with a paddle
type blade and the contents were stirred for one hour at room
temperature. The cell-impermeable domain coating solution was then
coated onto a PET release liner (Douglas Hansen Co., Inc.
(Minneapolis, Minn.)) using a knife over roll set at a 0.012'' (305
.mu.m) gap. This film was then dried at 305.degree. F. (152.degree.
C.). The final film was approximately 0.0015'' (38 .mu.m) thick.
The biointerface membrane was prepared by pressing the porous
silicone onto the cast cell-impermeable domain.
[0186] The advantages of using porous silicone included the
mechanical robustness of the material, the ability to mold it into
various structural architectures, the ability to load lipid-soluble
bioactive agents into the membrane without a carrier, the ability
to fill the large pores of the material with collagen-coupled
bioactive agents, and the high oxygen solubility of silicone that
allowed the membrane to act as an oxygen antenna domain.
[0187] Various bioactive agents can be incorporated into the
biomaterials of preferred embodiments. In some embodiments, such
bioactive agent containing biomaterials can be employed in an
implantable glucose device for various purposes, such as extending
the life of the device or to facilitate short-term function. The
following experiments were performed with a porous silicone
biointerface membrane prepared as described above, in combination
with bioactive agents, for the purpose of accelerated device
initiation and long-term sustentation.
Example 2
Neovascularizing Agents in Biointerface Membranes
[0188] In a first experiment, disks were employed, which were
prepared for three-week implantation into the subcutaneous space of
rats to test a neovascularizing agent. Monobutyrin was chosen based
on its hydrophobic characteristics and ability to promote
neovascularization. This experiment consisted of soaking the porous
silicone prepared as described above in the concentrated solution
of the bioactive compound at elevated temperature. This facilitated
a partitioning of the agent into the porous silicone dependent upon
its solubility in silicone rubber. Porous silicone disks were
exposed to phosphate buffer mixed with Monobutyrin (500 mg/ml) for
four days at 47.degree. C. These disks were then autoclaved in the
same solution, then rinsed in sterile saline immediately prior to
implant. Disks were implanted into the subcutaneous dorsal space.
Rats were euthanized and disks explanted at 3 weeks. Disks were
fixed in 10% NBF and histologically processed and analyzed. The
numbers of vessels per high power field were evaluated from porous
silicone disks embedded with and without Monobutyrin after 3 weeks
of implantation.
[0189] FIG. 5 is a bar graph that shows average number of vessels
(per high-powered field of vision) of porous silicone (PS)
materials embedded with and without Monobutyrin (MBN) after three
weeks of implantation. MBN was chosen because of its reported
neovascularizing properties. See Halvorsen et al., J. Clin. Invest.
92(6):2872-6 (1993); Dobson et al., Cel 61(2) l (1990); and English
et al., Cardiovasc. Res 49(3):588-99. (2001). An overall increase
in the numbers of vessels per high power field was seen with MBN as
compared to porous silicone alone (p<0.05). These preliminary
data suggested that bioactive agents absorbed into porous silicone
can alter healing in the first month. It is believed that this
increase in vessels results in improved device performance.
Example 3
Anti-Inflammatory Agents in Biointerface Membranes
[0190] Dexamethasone was loaded into a porous silicone biointerface
membrane by sorption. In this experiment, 100 mg of Dexamethasone
was mixed with 10 mL of Butanone (solvent) and the mixture heated
to about 70.degree. C.-80.degree. C. to dissolve the Dexamethasone
in the solvent. The solution was then centrifuged to ensure
solubility. The supernatant was pipetted from the solution and
placed in a clean glass vial. Disks of porous silicone were placed
in the Dexamethasone solution at 40.degree. C. for 5 days, after
which the disks were air-dried. The disks were sprayed with 70%
isopropanol to remove trapped air from the porous silicone,
attached to glucose sensors, and sterilized in 0.5% glutaraldehyde
for 24 hours. After rinsing, the glucose sensors were placed in a
40 mL phosphate buffer solution conical. These conicals were placed
on a shaker table with a setting of about 7 or 8. Dexamethasone
release in PBS solution was measured daily for the first five days
and then every three days until the end of the experiment using a
UV spectrometer. After each measurement when the absorbance was
above 0.1, the PBS solution was changed to ensure that it did no
reach its maximum solubility). The release kinetics are graphed on
FIG. 6.
[0191] FIG. 6 is a graph that shows the cumulative amount of
Dexamethasone released over time as described above. Namely, during
the first 19 days, about 0.4 mg of Dexamethasone was released in
PBS solution. The amount of Dexamethasone released is at least
partially dependent upon the surface area of the biointerface
membrane, including throughout the cavities of the cell disruptive
domain. While not wishing to be bound by theory, it is believed
that Dexamethasone released over time can modify a tissue response
to the biointerface membrane in vivo, thereby substantially
overcoming the effects of a "sleep period", 2) aid in preventing
barrier cell layer formation, and/or 3) rescuing a biointerface
membrane from the negative effects associated with such acute
inflammation, rendering adequate analyte transport to an
implantable device.
[0192] Methods and devices that are suitable for use in conjunction
with aspects of the preferred embodiments are disclosed in
copending U.S. patent application Ser. No. 10/838,912 filed May 3,
2004 and entitled, "IMPLANTABLE ANALYTE SENSOR"; U.S. patent
application Ser. No. 10/789,359 filed Feb. 26, 2004 and entitled,
"INTEGRATED DELIVERY DEVICE FOR A CONTINUOUS GLUCOSE SENSOR"; U.S.
application Ser. No. 10/685,636 filed Oct. 28, 2003 and entitled,
"SILICONE COMPOSITION FOR BIOCOMPATIBLE MEMBRANE"; U.S. application
Ser. No. 10/648,849 filed Aug. 22, 2003 and entitled, "SYSTEMS AND
METHODS FOR REPLACING SIGNAL ARTIFACTS IN A GLUCOSE DEVICE DATA
STREAM"; U.S. application Ser. No. 10/646,333 filed Aug. 22, 2003
entitled, "OPTIMIZED DEVICE GEOMETRY FOR AN IMPLANTABLE GLUCOSE
DEVICE"; U.S. application Ser. No. 10/647,065 filed Aug. 22, 2003
entitled, "POROUS MEMBRANES FOR USE WITH IMPLANTABLE DEVICES"; U.S.
application Ser. No. 10/633,367 filed Aug. 1, 2003 entitled,
"SYSTEM AND METHODS FOR PROCESSING ANALYTE MEASURING-DEVICE DATA";
U.S. application Ser. No. 09/916,386 filed Jul. 27, 2001 and
entitled "MEMBRANE FOR USE WITH IMPLANTABLE DEVICES"; U.S.
application Ser. No. 09/916,711 filed Jul. 27, 2001 and entitled
"SENSOR HEAD FOR USE WITH IMPLANTABLE DEVICE"; U.S. application
Ser. No. 09/447,227 filed Nov. 22, 1999 and entitled "DEVICE AND
METHOD FOR DETERMINING ANALYTE LEVELS"; U.S. application Ser. No.
10/153,356 filed May 22, 2002 and entitled "TECHNIQUES TO IMPROVE
POLYURETHANE MEMBRANES FOR IMPLANTABLE GLUCOSE DEVICES"; U.S.
application Ser. No. 09/489,588 filed Jan. 21, 2000 and entitled
"DEVICE AND METHOD FOR DETERMINING ANALYTE LEVELS"; U.S.
application Ser. No. 09/636,369 filed Aug. 11, 2000 and entitled
"SYSTEMS AND METHODS FOR REMOTE MONITORING AND MODULATION OF
MEDICAL DEVICES"; and U.S. application Ser. No. 09/916,858 filed
Jul. 27, 2001 and entitled "DEVICE AND METHOD FOR DETERMINING
ANALYTE LEVELS," as well as issued patents including U.S. Pat. No.
6,001,067 issued Dec. 14, 1999 and entitled "DEVICE AND METHOD FOR
DETERMINING ANALYTE LEVELS"; U.S. Pat. No. 4,994,167 issued Feb.
19, 1991 and entitled "BIOLOGICAL FLUID MEASURING DEVICE"; and U.S.
Pat. No. 4,757,022 filed Jul. 12, 1988 and entitled "BIOLOGICAL
FLUID MEASURING DEVICE."
[0193] The above description discloses several methods and
materials of the present invention. This invention is susceptible
to modifications in the methods and materials, as well as
alterations in the fabrication methods and equipment. Such
modifications will become apparent to those skilled in the art from
a consideration of this disclosure or practice of the invention
disclosed herein. Consequently, it is not intended that this
invention be limited to the specific embodiments disclosed herein,
but that it cover all modifications and alternatives coming within
the true scope and spirit of the invention as embodied in the
attached claims. All patents, applications, and other references
cited herein are hereby incorporated by reference in their
entirety.
* * * * *