U.S. patent application number 10/643866 was filed with the patent office on 2004-03-18 for biomaterial.
This patent application is currently assigned to pSiMedica Limited. Invention is credited to Canham, Leigh T..
Application Number | 20040052867 10/643866 |
Document ID | / |
Family ID | 27267841 |
Filed Date | 2004-03-18 |
United States Patent
Application |
20040052867 |
Kind Code |
A1 |
Canham, Leigh T. |
March 18, 2004 |
Biomaterial
Abstract
Biomaterial, for example bioactive silicon, may be fabricated by
anodizing a silicon wafer to produce a wafer having a porous
silicon region. In vitro experiments have shown that certain types
of porous silicon cause the deposition of apatite deposits both on
the porous silicon and neighboring areas of bulk silicon when
immersed in a simulated body fluid solution. This deposition of
apatite provides an indication that porous silicon of appropriate
form is bioactive, and therefore also biocompatible. A form of
porous silicon is dissolved in the simulated body fluid solution
and this is an indication of a resorbable biomaterial
characteristic. In addition to porous silicon, certain types of
polycrystalline silicon exhibit bioactive characteristics.
Bioactive silicon may be used in the fabrication of biosensors for
in vitro or in vivo applications. The bioactivity of the bioactive
silicon may be controlled by the application of an electrical
potential thereto.
Inventors: |
Canham, Leigh T.; (Malvern,
GB) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
1100 N GLEBE ROAD
8TH FLOOR
ARLINGTON
VA
22201-4714
US
|
Assignee: |
pSiMedica Limited
Malvern
GB
|
Family ID: |
27267841 |
Appl. No.: |
10/643866 |
Filed: |
August 20, 2003 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10643866 |
Aug 20, 2003 |
|
|
|
09964361 |
Sep 28, 2001 |
|
|
|
6666214 |
|
|
|
|
09964361 |
Sep 28, 2001 |
|
|
|
09000258 |
Jan 30, 1998 |
|
|
|
6322895 |
|
|
|
|
09000258 |
Jan 30, 1998 |
|
|
|
PCT/GB96/01863 |
Aug 1, 1996 |
|
|
|
Current U.S.
Class: |
424/724 |
Current CPC
Class: |
A61L 27/32 20130101;
A61L 27/306 20130101; A61F 2310/0061 20130101; A61F 2310/00796
20130101; A61L 27/56 20130101; A61L 27/025 20130101 |
Class at
Publication: |
424/724 |
International
Class: |
A61K 033/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 3, 1995 |
GB |
9515956.3 |
Nov 28, 1995 |
GB |
9524242.6 |
May 31, 1996 |
GB |
9611437.6 |
Claims
1. Bioactive silicon (20, 520) characterized in that the silicon is
at least partly crystalline.
2. Bioactive silicon according to claim 1, characterized in that
when immersed in a simulated body fluid solution held at a
physiological temperature the silicon induces the deposition of a
mineral deposit (54, C) thereon.
3. Bioactive silicon according to claim 2, characterized in that
the mineral deposit is apatite.
4. Bioactive silicon according to claim 3, characterized in that
the apatite is continuous over at least an area of 100
.mu.m.sup.2.
5. Bioactive silicon according to claim 1, characterized in that
the silicon (20) is at least partially porous with a porosity
greater than 4% and less than 70%.
6. Bioactive silicon according to claim 5, characterized in that
the porous silicon is microporous.
7. Bioactive silicon according to claim 5, characterized in that
the porous silicon is mesoporous.
8. Bioactive silicon according to claim 5, characterized in that
the porous silicon is visibly luminescent.
9. Bioactive silicon according to claim 1 or claim 5, characterized
in that the silicon is impregnated with at least one species taken
from a list of calcium, sodium and phosphorus.
10. Bioactive silicon according to claim 1, characterized in that
the silicon is polycrystalline silicon (520).
11. A bioactiv silicon structure (10, 300, 500) characterized in
that th silicon is at least partly crystalline.
12. A bioactive silicon structure according to claim 11,
characterized in that the structure comprises a porous silicon
region (20) having a porosity greater than 4% and less than
70%.
13. A bioactive silicon structure according to claim 12,
characterized in that the porous silicon is microporous.
14. A bioactive silicon structure according to claim 12,
characterized in that the porous silicon is mesoporous.
15. A bioactive silicon structure according to claim 12,
characterized in that the structure also includes macropores.
16. A bioactive silicon structure according to claim 11 or claim
12, characterized in that the silicon is impregnated with at least
one species taken from a list of calcium, sodium and
phosphorus.
17. A bioactive silicon structure according to claim 16 wherein the
porous silicon is impregnated with calcium at a concentration
greater than 10.sup.21 cm.sup.-3.
18. A bioactive silicon structure according to claim 11,
characterized in that the structure includes resorbable silicon
material.
19. A bioactive silicon structure according to claim 11,
characterized in that the structure comprises a region of
polycrystalline silicon (520).
20. An electronic device (300, 500) for operation within a living
human or animal body, characterized in that the device includes
bioactive silicon (20, 520).
21. An electronic device according to claim 20, characterized in
that the bioactive silicon comprises at least partially porous
silicon having a porosity greater than 4% and less than 70%.
22. An electronic device according to claim 21, characterized in
that the porous silicon contains macropores for enhancing vascular
tissue ingrowth.
23. An electronic device according to claim 21, characterized in
that the porous silicon extends at least partially over an outer
surface of the device.
24. An electronic device according to any one of claims 20 to 23,
characterized in that the device is a sensor device.
25. An electronic device according to claim 20, characterized in
that the bioactive silicon is polycrystalline silicon.
26. A method of making silicon bioactive, the method comprising
making at least part of the silicon porous.
27. A method according to claim 26, characterized in that the
method includes the impregnation of the porous silicon with
calcium.
28. A method of fabricating bioactive silicon, characterized in
that the method comprises the step of depositing a layer of
polycrystalline silicon.
29. The use of bioactive silicon for the construction of a device
(300, 500) for use in a living human or animal body characterized
in that the silicon is at least partly crystalline.
30. Bioactive silicon (20, 520) for use in a method of treatment of
the human or animal body.
31. Bioactive silicon (20, 520) incorporated in a device (300, 500)
suitable for use in a living human or animal body characterized in
that the silicon is at least partly crystalline.
32. Biocompatible silicon (20, 520) characterized in that the
silicon is at least partly cystalline.
33. Biocompatible silicon according to claim 32, characterized in
that when immersed in a simulated body fluid solution held at a
physiological temperature the silicon induces the deposition of a
mineral deposit thereon.
34. Resorbable silicon.
35. Resorbable silicon according to claim 34, characterized in that
th resorbable silicon comprises a region of porous silicon such
that when immersed in a simulated body fluid solution the porous
silicon dissolves over a period of time.
36. A method of accelerating or retarding the rate of deposition of
a mineral deposit on silicon in a physiological electrolyte wherein
the method comprises the application of an electrical bias to the
silicon.
37. A method according to claim 36, characterized in that the
silicon is porous silicon.
38. Bioactive material (20) characterised in that the bioactivity
of the material is controllable by the application of an electrical
bias to the material.
39. Bioactive electrically conductive material (20, 520).
40. A composite structure (10, 300, 500) comprising bioactive
silicon region (20, 520) and a mineral deposit thereon
characterized in that the silicon region comprises silicon which is
at least partly crystallin .
41. A composit structure according to claim 40, characterized in
that the mineral deposit is apatite.
42. A composite structure according to claim 40 or claim 41,
characterized in that the bioactive silicon region is porous
silicon (20).
43. A composite structure according to claim 40 or claim 41,
characterized in that the bioactive silicon is polycrystalline
silicon (520).
44. A method of fabricating a biosensor, characterized in that the
method includes the step of forming a composite structure of
bioactive silicon and a mineral deposit thereon.
45. A biosensor for testing the pharmacological activity of
compounds including a silicon substrate, characterized in that at
least part of the silicon substrate is comprised of bioactive
silicon.
Description
[0001] The present invention relates to biomaterials.
[0002] A "biomaterial" is a non-living material used in a medical
device which is intended to interact with biological systems. Such
materials may be relatively "bioinert", "biocompatible",
"bioactive" or "resorbable", depending on their biological response
in vivo.
[0003] Bioactive materials are a class of materials each of which
when in vivo elicits a specific biological response that results in
the formation of a bond between living tissue and that material.
Bioactive materials are also referred to as surface reactive
biomaterials. Biomaterials may be defined as materials suitable for
implantation into a living organism. L. L. Hench has reviewed
biomaterials in a scientific paper published in Science, Volume
208, May 1980, pages 826-831. Biomaterials which are relatively
inert may cause interfacial problems when implanted and so
considerable research activity has been directed towards developing
materials which are bioactive in order to improve the
biomaterial-tissue interface.
[0004] Known bioactive materials include hydroxyapatite (HA), some
glasses and some glass ceramics. Both bioactive glasses and
bioactive glass ceramics form a biologically active layer of
hydroxycarbonateapatite (HCA) when implanted. This layer is
equivalent chemically and structurally to the mineral phase in bone
and is responsible for the interfacial bonding between bone and the
bioactive material. The properties of these bioactive materials are
described by L. L. Hench in the Journal of the American Ceramic
Society, Volume 74 Number 7, 1991, pages 1487-1510. The scientific
literature on bioactive materials often uses the terms HA and HCA
on an interchangeable basis. In this patent specification, the
materials HA and HCA are collectively referred to as apatite.
[0005] Li et al. have reported the deposition of apatite on silica
gel in the Journal of Biomedical Materials Research, Volume 28,
1994, pages 7-15. They suggest that a certain density of silanol
(SIOH) groups is necessary to trigger the heterogeneous nucleation
of hydroxyapatite. An apatite layer did not develop on the surface
of a silica glass sample and this is attributed to the lower
density of surface silanol groups compared with silica gel.
[0006] Thick films of apatite have previously been deposited on
silicon single crystal wafers by placing the wafers in close
proximity to a plate of apatite and wollastonite-containing glass
dipped into a physiological solution at 36.degree. C., as described
by Wang et al. in the Journal of Materials Science: Materials In
Medicine, Volume 6, 1995, pages 94-104. A physiological solution,
also known as a simulated body fluid (SBF), is a solution
containing ion concentrations similar to those found in the human
body and is widely used to mimic the behaviour of the body in in
vitro tests of bioactivity. Wang et al. reported the growth of
apatite on (111) Si wafers but reported that "hardly any" apatite
could be grown on (100) Si wafers. The silicon wafer itself is not
bioactive. Wang et al. state that "Si does not play any special
role in the growth of (the) apatite film except that Si atoms on
the substrate can bond strongly with oxygen atoms in apatite nuclei
to form interfaces with low energy". The presence of the apatite
and wollastonite containing glass is required to induce the
deposition of the apatite. Indeed, this so-called "biomimetic
process" whereby a bioactive material is used to treat another
material has been shown to induce apatite growth on a wide variety
of bioinert materials, as reported by Y. Abe et al. in the Journal
of Materials Science: Materials In Medicine, Volume 1, 1990, pages
233 to 238.
[0007] There is a long felt want for the ability to use silicon
based integrated circuits within the human body both for diagnostic
and therapeutic purposes. Silicon has been reported to exhibit a
poor biocompatibility in blood, Kanda et al. in Electronics
Letters, Volume 17, Number 16, 1981, pages 558 and 559, and in
order to protect integrated circuits from damage in biological
environments encapsulation by a suitable material is currently
required. Medical applications for silicon based sensors are
described in a paper by Engels et al. in the Journal of Physics E:
Sci. Instrum., Volume 16, 1983, pages 987 to 994.
[0008] The present invention provides bioactive silicon
characterized in that the silicon is at least partly
crystalline.
[0009] Bioactive silicon provides the advantage over other
bioactive materials that it is compatible with silicon based
integrated circuit technol gy. It has the advantag over
non-bioactive silicon that it exhibits a greater degree of
biocompatibility. In addition, bioactive silicon may be used for
forming a bond to bone or vascular tissue of a living animal.
Bioactive silicon may provide a material suitable for use as a
packaging material in miniaturised packaging applications.
[0010] The bioactive nature of the silicon may be demonstrated by
the immersion of the material in a simulated body fluid held at a
physiological temperature, such immersion producing a mineral
deposit on the bioactive silicon. The mineral deposit may be
apatite. The apatite deposit may be continuous over an area greater
than 100 .mu.m.sup.2. The bioactive silicon may be at least
partially porous silicon. The porous silicon may have a porosity
greater than 4% and less than 70%.
[0011] Bulk crystalline silicon can be rendered porous by partial
electrochemical dissolution in hydrofluoric acid based solutions,
as described in U.S. Pat. No. 5,348,618. This etching process
generates a silicon structure that retains the crystallinity and
the crystallographic orientation of the original bulk material. The
porous silicon thus formed is a form of crystalline silicon. At low
levels of porosity, for example less than 20%, the electronic
properties of the porous silicon resemble those of bulk crystalline
silicon.
[0012] Porous silicon may be subdivided according to the nature of
the porosity. Microporous silicon contains pores having a diameter
less than 20 .ANG.; mesoporous silicon contains pores having a
diameter in the range 20 .ANG. to 500 .ANG.; and macroporous
silicon contains pores having a diameter greater than 500 .ANG..
The bioactive silicon may comprise porous silicon which is either
microporous or mesoporous.
[0013] Silicon has never been judged a promising biomaterial, in
contrast with numerous metals, ceramics and polymers, and has never
been judged capable of exhibiting bioactive behaviour. Indeed, no
semiconductors have been reported to be bioactive. Silicon is at
best reported to be relatively bioinert but gen rally exhibits poor
biocompatibility. Despite the advances made in miniaturisation of
integrated circuitry, silicon VLSI technology is still under
development for invasive medical and biosensing applications, as
described by K. D. Wise et al. in "VLSI in Medicine" edited by N.
G. Einspruch et al., Academic Press, New York, 1989, Chapter 10 and
M. Madou et al. in Appl. Biochem. Biotechn., Volume 41, 1993, pages
109-128.
[0014] The use of silicon structures for biological applications is
known. International patent application PCT/US95/02752 having an
International Publication Number WO 95/24472 describes a capsule
having end faces formed from a perforated amorphous silicon
structure, whose pores are large enough to allow desired molecular
products through but which block the passage of larger
immunological molecules, to provide immunological isolation of
cells contained therein. No evidence as to the biocompatibility of
the silicon structure is provided, and workers skilled in the field
of biocompatible materials would expect that such a device would in
vivo stimulate the production of fibrous tissue which would block
the pores. It is known that when micromachined silicon structures
are used as sensors for neural elements a layer of fibrous tissue
forms between the silicon surfaces and the neural elements of
interest, as reported by D. J. Edell et al. in IEEE Transactions on
Biomedical Engineering, Volume 39, Number 6, 1992 page 635. Indeed
the thickness and nature of any fibrous issue layer formed is often
used as one measure of biocompatibility, with a thinner layer
containing little cell necrosis reflecting a higher degree of
biocompatibility.
[0015] U.S. Pat. No. 5,225,374 describes the use of porous silicon
as a substrate for a protein-lipid film which interacts with target
species to produce an electrical current when exposed to target
species in an in vitro solution. The porous silicon is oxidised to
produce a hydrophilic surface and is chosen since the pores act as
a conduit for an ion-current flow and the structure provides
structural support for the lipid layer. The porous silicon is
separated from the in vitro solution by the protein-lipid film and
so the question of the bioactivity or biocompatibility of the
porous silicon does not arise.
[0016] Porous silicon has been suggested as a substrate material
for in vitro biosensors by M. Thust et al. in Meas. Sci. Technol,
Volume 7 1996 pages 26-29. In the device structure described
therein, the porous silicon is subjected to a thermal oxidation
process to form a silicon dioxid lay r on the exposed silicon
surfaces of the pores.
[0017] Since the porous silicon is partially thermally oxidised,
the bioactivity or biocompatibility of the silicon is not of
relevance since it is only the silicon dioxide which is exposed to
test solutions. The porous silicon is effectively an inert host for
enzyme solutions.
[0018] Microperforated silicon membranes have been described as
being capable of supporting cell structures by E. Richter et al. in
Journal of Materials Scienc : Materials in Medicine, Volume 7,
1996, pages 85-97, and by G. Fuhr et al. in Journal of
Micromechanics and Microengineering, Volume 5, Number 2, 1995,
pages 77-85. The silicon membranes described therein comprises
silicon membranes of thickness 3 .mu.m perforated by square pores
of width 5 .mu.m to 20 .mu.m using a lithography process. Mouse
embryo fibroblasts were able to grow on cleaned membranes but
adherence of the cells was improved if the membranes were coated
with polylysine. This paper is silent as to the bioactivity of the
silicon membrane, and there is no mention of an apatite layer
having been formed when exposed to the cell culture medium. Indeed,
given the dimensions of the pores used, the structure is not likely
to exhibit a significant degree of bioactivity. Furthermore, it is
accepted by Fuhr et al. that there is still a need to find and
develop cell-compatible materials with long term stability.
[0019] A. Offenhusser et al. in Journal of Vacuum Science
Technology A, Volume 13, Number 5, 1995, pages 2606-2612 describe
techniques for achieving biocompatibility with silicon substrates
by coating the substrate with an ultrathin polymer film. Similarly,
R. S. Potember et al. in Proc. 16th Int. Conf. IEEE Engineering in
Medicine and Biology Society, Volume 2, 1994, pages 842-843
describe the use of a synthetic peptide attached to a silicon
surface to promote the development of rat neurons.
[0020] In a further aspect, the invention provides a bioactive
silicon structure characterized in that the silicon is at least
partly crystalline.
[0021] In a still further aspect, the invention provides an
electronic device for operation within a living human or animal
body, characterized in that the device includes bioactive
silicon.
[0022] Bioactive silicon of the invention may be arranged as a
protective covering for an electronic circuit as well as a means
for attaching a device to bone or other tissue.
[0023] The electronic device may be a sensor device or a device for
intelligent drug delivery or a prosthetic device.
[0024] In a still further aspect, the invention provides a method
of making silicon bioactive wherein the method comprises making at
least part of the silicon porous.
[0025] In another aspect, the invention provides a method of
fabricating bioactive silicon, characterized in that the method
comprises the step of depositing a layer of polycrystalline
silicon.
[0026] In a yet further aspect, the invention provides
biocompatible silicon characterized in that the silicon is at least
partly crystalline.
[0027] In a still further aspect, the invention provides resorbable
silicon.
[0028] In another aspect, the invention provides a method of
accelerating or retarding the rate of deposition of a mineral
deposit on silicon in a physiological electrolyte wherein the
method comprises the application of an electrical bias to the
silicon.
[0029] The silicon may be porous silicon.
[0030] In a further aspect, the invention provides bioactive
material characterised in that the bioactivity of the material is
controllable by the application of an electrical bias to the
material.
[0031] Conventional bioactive ceramics are electrically insulating
and therefore preclude their use in electrochemical applications.
Where the electrical simulation of tissue growth has been studied
previously, it has often been difficult to distinguish the direct
effects of electric fields from those associated with an altered
body chemistry near implanted "bioinert" electrodes.
[0032] In a still further aspect, the invention provides a
composite structure comprising bioactive silicon region and a
mineral deposit thereon characterized in that the silicon region
comprises silicon which is at least partly crystalline.
[0033] A possible application of the invention is as a substrate
for performing bioassays. It is desirable to be able to perform
certain tests on pharmaceutical compounds without resorting to
performing tests on living animals. There has therefore been a
considerable amount of research activity devoted to developing in
vitro tests in which cell lines are supported on a substrate and
the effects of pharmaceutical compounds on the cell lines
monitored. A composite structure of silicon and apatite might
provide a suitable substrate for such tests.
[0034] In a further aspect, the invention provides a method of
fabricating a biosensor, characterized in that the method includes
the step of forming a composite structure of bioactive silicon and
a mineral deposit thereon.
[0035] The invention further provides a biosensor for testing the
pharmacological activity of compounds including a silicon
substrate, characterized in that at least part of the silicon
substrate is comprised of bioactive silicon.
[0036] In order that the invention may be more fully understood,
embodiments thereof will now be described, by way of example only,
with reference to the accompanying drawings, in which:--
[0037] FIG. 1 is a schematic sectional diagram of a bioactive
silicon wafer;
[0038] FIG. 2 is a representation of a scanning electron microscope
(SEM) micrograph of an apatite deposit on a bulk silicon region
adjacent a porous region of the FIG. 1 wafer;
[0039] FIG. 3 is a representation of an SEM micrograph of a
cross-section of the FIG. 2 silicon region;
[0040] FIG. 4 is a representation of an SEM micrograph showing an
apatite spherulite deposited on a porous silicon region of porosity
31%
[0041] FIG. 5a is a representation of an SEM micrograph of an
unanodised region of a silicon wafer anodised to produce a porosity
of 48% after immersion in a simulated body fluid solution;
[0042] FIG. 5b is a representation of an SEM micrograph of an
anodised region of the FIG. 5a wafer;
[0043] FIG. 6 is a schematic diagram of a biosensor incorporating
bioactive silicon;
[0044] FIG. 7 is a schematic diagram of an electrochemical cell for
the electrical control of bioactivity;
[0045] FIG. 8 is a plot of a calcium concentration profile in
porous silicon wafers after treatment in the FIG. 7 cell; and
[0046] FIG. 9 is a schematic diagram of a biosensor device
incorporating bioactive polycrystalline silicon of the
invention.
[0047] Referring to FIG. 1 there is shown a section of a bioactive
silicon wafer, indicated generally by 10. The silicon wafer 10
comprises a porous silicon region 20 and a non-porous bulk silicon
region 22. The porous region 20 has a thickness d of 13.7 .mu.m and
an average porosity of 18%. The silicon wafer 10 has a diameter l
of three inches or 75 mm. The porous region 20 has a surface area
per unit mass of material of 67 m.sup.2 g.sup.-1. This was measured
using a BET gas analysis technique, as described in "Adsorption,
Surface Area and Porosity" by S. J. Gregg and K. S. W. Sing, 2nd
edition, Academic Press, 1982.
[0048] The wafer 10 was fabricated by the anodisation of a heavily
arsenic doped Czochralski-grown (CZ) n-type (100) silicon wafer
having an initial resistivity of 0.012 .OMEGA.cm. The anodisation
was carried out in an electrochemical cell, as described in U.S.
Pat. No. 5,348,618, containing an electrolyte of 50 wt % aqueous
HF. The waf r was anodised using an anodisation current density of
100 mAcm.sup.-2 for one minute. The wafer was held in place in the
electrochemical cell by a synthetic rubber washer around the
outside of the wafer. Consequently, an outer ring of the wafer
remained unanodised after the anodisation process. This outer
unanodised ring is shown in FIG. 1 as a non-porous bulk silicon
region 22. The unanodised ring has a width s of 4 mm.
[0049] In order to determine the bioactivity of anodised wafers,
cleaved wafer segments were placed in a simulated body fluid (SBF)
solution for a period of time ranging from 2 hours to 6 weeks. The
SBF solution was prepared by dissolving reagent grade salts in
deionised water. The solution contained ion concentrations similar
to those found in human blood plasma. The SBF solution ion
concentrations and those of human blood plasma are shown at Table
1. The SBF solution was organically buffered at a pH of
7.30.+-.0.05, equivalent to the physiological pH, with
trihydroxymethylaminomethane and hydrochloric acid. The porous
wafers were stored in ambient air for at least several months prior
to immersion in the SBF solution and were therefore hydrated porous
silicon wafers. The porous silicon thus comprised a silicon
skeleton coated in a thin native oxide, similar to that formed on
bulk silicon as a result of storage in air.
1TABLE 1 Concentration (mM) Ion Simulated Body Fluid Human Plasma
Na.sup.+ 142.0 142.0 K.sup.+ 5.0 5.0 Mg.sup.2+ 1.5 1.5 Ca.sup.2+
2.5 2.5 HCO.sub.3.sup.- 4.2 27.0 HPO.sub.4.sup.2- 1.0 1.0 Cl.sup.-
147.8 103.0 SO.sub.4.sup.2- 0.5 0.5
[0050] Cleaved wafer segments having typical dimensions of
0.4.times.50.times.20 mm.sup.3 were placed in 30 cm.sup.3 capacity
polyethylene bottles filled with the SBF solution and held at
37.degree..+-.1.degree. C. by a calibrated water bath.
[0051] After a known period of tim , the segments were removed from
the SBF solution, rinsed in deionised water and allowed to dry in
ambient air prior to characterisation. The SBF treated segments
were examined using scanning electron microscopy (SEM) and x-ray
microanalysis (EDX) on a JEOL 6400F microscope. Secondary ion mass
spectrometry was carried out using a Cameca 4F instrument and
infrared spectroscopy was performed using a Biorad FTS-40
spectrometer.
[0052] After periods of immersion in the SBF solution of 2, 4, and
17 hours, there were negligible apatite deposits on both the porous
silicon region 20 and the non-porous bulk silicon region 22.
[0053] Referring to FIG. 2 there is shown a reproduction of an SEM
micrograph indicated generally by 50. The micrograph 50 is an image
of part of the region 22 after the wafer 10 had been placed in the
SBF solution for a period of 6 days. A scale bar 52 indicates a
dimension of 2 .mu.m. The micrograph 50 shows a continuous layer of
apatite spherulites 54 covering the surface of the region 22. The
apatite spherulites had nucleated at a sufficiently high density to
create a relatively smooth film in which boundaries between
spherulites such as boundary 56 are indistinct The film was
continuous over an area of at least 100 .mu.m.sup.2.
[0054] Referring to FIG. 3 there is shown a reproduction of an SEM
micrograph, indicated generally by 100, of a cross-section of the
wafer 10 in the region 22 after the wafer had been immersed in the
SBF solution for 6 days. A scale bar 102 indicates a dimension of
1.0 .mu.m. The micrograph 100 indicates three distinct regions,
indicated by the letters A, B, and C. EDX analysis confirmed that
region A is silicon, corresponding to the original material of the
non-porous bulk silicon region 22. Region B exhibited both silicon
and oxygen peaks under EDX analysis, indicating that region B
comprises silicon oxide. Region C exhibited calcium, phosphorus and
oxygen peaks under EDX analysis, consistent with this region
comprising spherulites of apatite. The combined SEM and EDX
analysis demonstrates that a porous silicon oxide layer (region B)
has formed on the bulk silicon (region A), thereby enabling
nucleation and coverage with apatite (region C).
[0055] SEM analysis of the wafer 10 in the area of the porous
silicon region 20 after 6 days immersion in th SEF solution
indicated a much lower l vel of apatite coverage compared with the
region 22. The porous silicon region 20 contains a high level of
mesoporosity. After 10 days immersion in the SBF solution in which
significant layer erosion of the porous silicon had occurred,
macropores were visible under SEM analysis in the region 20. The
combined SEM and EDX analysis demonstrates that, in contrast to the
bulk silicon region 22, apatite nucleation can occur directly on
the porous silicon region 20 and does not require the formation of
an intermediate porous silicon oxide layer. The intentional
introduction of very large (greater than 100 .mu.m diameter)
macropores may be advantageous in that it may enable vascular
tissue to grow within the structure of the porous silicon.
[0056] The formation of apatite deposits has also been observed on
wafers having porous silicon porosities other than 18%. A
microporous wafer having a porous silicon region with a porosity of
31% was fabricated from a 0.03 .OMEGA.cm heavily boron doped p-type
CZ silicon wafer by anodisation at an anodisation current density
of 100 mAcm.sup.-2 for one minute in 50 wt % HF. The resulting
porous silicon region had a thickness of 9.4 .mu.m and a surface
area per unit mass of 250 m.sup.2 g.sup.-1. The porous silicon
wafer was heavily aged prior to immersion in the SBF solution.
[0057] FIG. 4 shows a representation of an SEM micrograph,
indicated generally by 150, of the surface of the 31% porosity
porous silicon layer after a segment of the wafer had been immersed
in 30 cm.sup.3 of the SBF solution for 7 days. The micrograph 150
shows spherulites such as a spherulite 152 of apatite on the
surface 154 of the porous silicon.
[0058] Microporous wafers having a porous silicon region of a
porosity of 48% were fabricated by anodising a lightly boron doped
p-type silicon wafer having a resistivity of 30 .OMEGA.cm in 50 wt
% HF at an anodisation current density of 20 mAcm.sup.-2 for five
minutes. The resulting porous silicon region had a thickness of
6.65 .mu.m and a surface area per unit mass of approximately 800
m.sup.2 g.sup.-1. The porous silicon wafer segment was heavily aged
prior to immersion in a 150 cm.sup.3 polyethylene bottle filled
with the SBF solution.
[0059] FIG. 5a shows a representation of a SEM micrograph,
indicated g nerally by 200, of an apatit deposit 202 on an
unanodised region of the 48% porosity wafer after a four week
immersion period. FIG. 5b shows a representation of a SEM
micrograph, indicated generally by 250 of an apatite spherulite 252
deposited on the 48% porosity porous region. The spherulite 252
exhibits a morphology having a columnar structure characteristic of
apatite growth on bioactive ceramics as described by P. Li et al.
in Journal of Biomedical Materials Research, Volume 28, pages 7-15,
1994. Apatite spherulites having a similar morphology were observed
on the unanodised region of the wafer. Cross-sectional EDX spectra
of the 48% porosity wafer after immersion in the SBF solution taken
across the unanodised region indicated that spherulites contained
calcium, phosphorus and oxygen, consistent with apatite. Away from
the spherulites, an interfacial layer having a thickness of only
150 nm comprising predominantly silicon and oxygen was observed.
Fourier transform infrared spectroscopy confirms the presence of
apatite in both the porous and non-porous regions. Both the P--O
bending vibrational modes of P0.sub.4 tetrahedra at wavenumbers of
around 600 cm.sup.-1 and a broad band around 1400 cm.sup.-1,
attributed to vibrational modes of carbonate groups, were
observed.
[0060] Some forms of porous silicon are known to be
photoluminescent. The observation of red or orange
photoluminescence from porous silicon generally indicates the
presence of quantum wires or quantum dots of silicon material.
Prior to immersion in the SBF solution, the heavily aged 48%
porosity wafer exhibited photoluminescence, indicating that despite
being hydrated by exposure to ambient air, the porous silicon
region maintains a high concentration of quantum wires or dots. The
luminescent property was preserved both during and after immersion
in the SBF solution. This shows that apatite may be deposited on
porous silicon such that the luminescent properties are preserved.
Preservation of the luminescent properties after growth of an
apatite layer may be a useful property for the development of an
electro-optical biosensor.
[0061] A wholly mesoporous luminescent porous silicon wafer having
a 1 .mu.m thick porous region with a porosity of 70% and a surface
area per unit mass of 640 m.sup.2 g.sup.-1 was placed in the SBF
solution. After approximately one day the porous region had been
completely removed by dissolution in the SBF solution and the wafer
was no longer lumin sc nt. No apatite deposits were observed on
either the porous silicon region or the non-porous region. It is
thought that the mesoporous silicon is wetted more efficiently by
the SBF solution and hence the rate of dissolution is higher for
mesoporous silicon than microporous silicon. The mesoporous silicon
thus shows resorbable biomaterial characteristics. It might be
possible to construct a bioactive silicon structure having a
limited area of mesoporous silicon to act as a source of soluble
silicon. This could produce a locally saturated silicon solution
and hence the promotion of apatite deposition.
[0062] A macroporous silicon wafer having a porous region of 4%
porosity and a thickness of 38 .mu.m behaved like a bulk,
unanodised silicon wafer in as much as it did not exhibit growth of
an apatite deposit when immersed in the SBF solution for four
weeks. In addition, no apatite growth has been observed on a porous
silicon region having a porosity of 80% and a thickness of 50 .mu.m
which retains its luminescent properties after two weeks immersion
in the SBF solution.
[0063] As a further control, a cleaved non-porous silicon wafer
segment of similar dimensions to the porous silicon wafer segments
was placed in 30 cm.sup.3 of the SBF solution. An extremely low
density of micron size deposits, less than 5000/cm.sup.2 was
observed after immersion in the SBF solution for five weeks. These
deposits were possibly located at surface defects of the silicon
wafer. Bulk, non-porous silicon is therefore not bioactive since
the rate of growth of apatite deposits is too low for a bond to be
formed with living tissue.
[0064] These experiments thus indicate that by appropriate control
of pore size and porosity, silicon structures can cover virtually
the entire bioactivity spectrum. Bulk and purely macroporous
silicon are relatively bioinert; high porosity mesoporous silicon
is resorbable and microporous silicon of moderate porosity is
bioactive.
[0065] It is known that changes in chemical composition of
biomaterials can also affect whether they are bioinert, resorbable
or bioactive. The above experiments were carried out on porous
silicon wafers which had not been int ntionally doped with any
specific elements other than the impurity doping for controlling
the semiconductor properties of the silicon.
[0066] Th elution of calcium from bioactive glass containing
SiO.sub.2, Na.sub.2O, CaO and P.sub.2O.sub.5 is believed to
significantly assist apatite growth by promoting local
supersaturation. Calcium has been impregnated into a freshly etched
layer of microporous silicon of 55% porosity and having a thickness
of 1.2 .mu.m formed in a lightly doped p-type (30 .OMEGA.m) CZ
silicon wafer by anodisation at 20 mAcm.sup.-2 for one minute in
40% aqueous HF. The calcium impregnation was achieved through mild
oxidation by storage in a solution containing 5 g of CaCl.2H.sub.2O
in 125 cm.sup.3 pure ethanol for 16 hours. The impregnation of the
porous silicon with calcium, sodium or phosphorus or a combination
of these species may promote apatite formation on silicon.
[0067] The presence of the silicon oxide layer underneath the
apatite deposit at the non-porous region adjacent the porous
silicon region of the anodised wafers after immersion in the SBF
solution indicates that the dissolution of silicon from the porous
silicon region may be an important factor for the bioactivity of
the porous silicon. The dissolution of the silicon may form a local
supersaturated solution which results in the deposition of a porous
silicon oxide layer. Apatite is then deposited on the porous
silicon oxide. This suggests that a variety of non-porous
crystalline, polycrystalline or amorphous silicon based structures
containing impregnated calcium and having a higher solubility than
normal bulk crystalline silicon in the SBF solution may be
bioactive. To significantly assist apatite growth, the level of
calcium impregnation needs to be much higher than previously
reported calcium doped silicon, though the crystallinity of the
silicon need not necessarily be preserved.
[0068] Calcium is generally regarded as an unattractive dopant for
silicon and consequently there have been few studies of calcium
doped silicon. Sigmund in the Journal of the Electrochemical
Society, Volume 129, 1982, pages 2809 to 2812, reports that the
maximum equilibrium solubility of calcium in monocrystalline
silicon is 6.0.times.10.sup.18 cm.sup.-3. At this concentration,
calcium is unlikely to have any significant effect upon apatite
growth. Supersaturated levels of calcium are needed with
concentrations in excess of 10.sup.21 cm.sup.-3 (2 at %). Such very
high concentrations may be achieved by:
[0069] (a) solution doping of porous silicon as previously
described;
[0070] (b) ion implantation of porous silicon or bulk silicon with
calcium ions; or
[0071] (c) epitaxial deposition of calcium or calcium compounds
followed by thermal treatments.
[0072] Referring to FIG. 6 there is shown a schematic diagram of a
generalised sensor, indicated generally by 300, for medical
applications incorporating bioactive silicon. The sensor 300
comprises two silicon wafer segments 302 and 304. The segment 302
incorporates CMOS circuitry 306 and a sensing element 308 linked to
the circuitry 306. The sensing element 308 may be an oxygen sensor,
for instance a Clark cell. The CMOS circuitry is powered by a
miniaturised battery (not shown) and signals are produced for
external monitoring using standard telemetry techniques.
[0073] The wafer segment 304 is a micromachined top cover for the
segment 302. The segment 304 has two major cavities 310 and 312
machined into it The cavity 310 has a dome shape. When the segments
302 and 304 are joined together, the cavity 310 is above the CMOS
circuitry 306. The cavity 312 is circular in cross-section and
extends through the segment 304 to allow the sensing element 308 to
monitor the environment surrounding the sensor. The cavity 312 is
covered by a permeable membrane 314. In addition to the major
cavities 310 and 312, minor cavities, such as cavities 316, are
distributed over a top surface 322 of the segment 304. The minor
cavities are frusto-conical in shape, with the diameter of its
cross-section increasing into the segment. The minor cavities are
present to enable the growth of vascular tissue or bone for
biological fixation. The cavities 310, 312, and 316 are formed by
standard etching techniques, for example ion-beam milling and
reactive ion etching through a photoresist mask. At least part of
the outer surfaces of the segments 302 and 304 are anodised to form
a porous silicon region in order to promote the deposition of
apatite and the bonding of the sensor with the tissue. In FIG. 6,
the porous silicon is indicated by rings 330 on the top surface of
the segment 304 and grooves 332 in the other surfaces. Although
FIG. 6 indicates that the outer surfaces of the segments 302 and
304 are covered entirely by porous silicon, it may be sufficient
for only the surface 322 and a bottom surface 334 of the segment
302 to incorporate porous silicon. Such an arrangement would be
simpler to fabricate. The segments 302 and 304 are bonded together
using techniqu s d veloped for silicon on insulator technologies.
Whilst an anodisation technique has been described for the
production of the porous silicon, stain etching techniques are also
known for the production of porous silicon. Such techniques may be
advantageous for producing porous silicon surfaces on complex
shaped structures.
[0074] In addition to sensors, bioactive silicon might find
applications in electronic prosthetic devices, for example
replacement eyes. Other electronic devices which may incorporate
bioactive silicon might include intelligent drug delivery
systems.
[0075] As well as sensors for incorporation into the bodies of
humans and other animals, bioactive porous silicon may be used in
the fabrication of biosensors for in vitro applications. A
composite structure of porous silicon with a layer of apatite
thereon may have improved cell compatibility compared with prior
art biosensor arrangements. Biosensors are of potentially great
importance in the field of in vitro pharmaceutical testing. For
automated pharmaceutical testing, a bioasay device might comprise a
silicon wafer having a matrix array of porous silicon regions.
Cells could then be preferentially located at the porous silicon
regions and this would facilitate automated cell analysis after
exposure to a pharmaceutical product The luminescent properties of
porous silicon might be utilised to enable an optical cell analysis
technique. Workers skilled in the field of biosensors would use
their experience to identify which cell cultures were suitable and
how the cells' behaviour could be monitored.
[0076] Whilst the results of in vitro experiments have been
described, no in vivo experiments have been described. However, the
in vitro experiments are designed to mimic the environment within a
human body. From the results of the in vitro experiments it may be
concluded that those silicon wafers which produced significant
deposits of apatite in the SBF solution would also exhibit
bioactive behaviour in vivo.
[0077] The formation of a film of apatite over a silicon or porous
silicon surface in vitro indicates that the bioactive silicon may
be to a certain extent a biocompatible form of silicon. The term
"biocompatible" does not necessarily indicate that the material is
biologically acceptable for all applications but that the material
is biologically acceptable for specific applications. Some workers
skilled in the field of biocompatibility might regard "tissue
compatible" as a more appropriate term to describe this definition
of biocompatibility. The lay r of apatite may act as a protective
barrier reducing the physiological effects of the silicon.
[0078] As stated above, mesoporous silicon shows resorbable
biomaterial characteristics. From the previously referenced paper
by Hench in the Journal of the American Ceramic Society, resorbable
biomaterials are materials which are designed to degrade gradually
over a period of time and be replaced by the natural host tissue.
The characteristics of the mesoporous silicon in the simulated body
fluid indicate that mesoporous silicon of an appropriate porosity
may be a resorbable biomaterial. As previously discussed the porous
region 20 of the bioactive silicon wafer 10 of FIG. 1 contains a
high level of mesoporosity. This indicates that controlling the
porosity of mesoporous silicon can control whether a porous silicon
region is bioactive or resorbable. It may be possible to control
the rate at which a porous silicon region is absorbed by tuning the
porosity.
[0079] Although the dissolution of porous silicon in the SBF
solution provides an indication of resorbable biomaterial
characteristics, the behaviour of a porous silicon region in a
living body may be affected by factors which are not reproducible
in the SBF solution. If living cells grow on the surface of the
porous silicon, these cells may interact with the porous silicon.
Thus experiments carried out in the SBF solution do not provide a
clear indication of the suitability of a particular form of porous
silicon for resorbable material applications. Experiments may have
to be carried out in vivo to determine whether a particular desired
physiological response is achieved.
[0080] Further experiments have been performed which show that it
is possible to either enhance or retard the formation of an apatite
layer on the porous silicon by the application of a bias current in
the SBF solution.
[0081] Referring to FIG. 7 there is shown a schematic diagram of an
electrochemical cell 400 for applying a galvanostatic loading to a
whole silicon wafer 402. The wafer 402 is a heavily doped n-type
(100) oriented silicon wafer of resistivity 0.012 .OMEGA.cm which
prior to loading in the cell 400 was anodized in 40 wt % aqueous HF
at 100 mA cm.sup.-2 for one minute to form a bioactive porous
silicon layer of approximately 20% porosity having a thickness of
11 .mu.m with a BET measured surface area of approximately 70
m.sup.2 g.sup.-1. After anodisation, the wafers are spun dry in air
until their weight has stabilised and then immediately loaded into
the cell 400.
[0082] The wafer 402 is inserted into a PTFE cassette 404 and
mounted using a threaded PTFE ring 406 which is screwed into the
cassette 404 and which compress PTFE coated O-rings 408 and 410. In
the cassette 404, the silicon wafer is pushed against a metal back
plate 412. The plate 412 provides an electrical contact to a rear
face of the silicon wafer, and in the cassette an area of 36
cm.sup.2 of the front porous face of the silicon wafer is exposed.
The cassette 404 is placed in a polycarbonate tank 414, within a
waterbath, containing two litres of SBF solution maintained at
37.+-.1.degree. C. with organic buffering at pH=7.3.+-.0.05. A
spiral platinum counterelectrode 416 is also inserted into the SBF
solution. A d.c. galvanostatic power supply 418 is used to maintain
a constant electrical current between the wafer 402 and the
counterelectrode 416. The wafer 402 may either be under cathodic or
anodic bias control. The power supply 418 provides a constant
current of 36 mA, which corresponds to a current density at the
silicon wafer of approximately 1 mA cm.sup.2 if current flow is
primarily through the silicon skeleton or approximately 1 .mu.A
cm.sup.2 if current flow is uniformly distributed across the entire
silicon-SBF interface via the pore network of the porous silicon.
The current flow is maintained for three hours. After removal from
the cell 400, the wafers 402 are rinsed in deionised water and spun
dried.
[0083] After the three hour SBF exposure, the porous silicon wafer
surface was examined in a JEOL 6400F scanning electron microscope
(SEM) at an accelerating potential of 6 kV. Porosified wafers which
were anodically biased, together with control porosified wafers
which received no bias showed no evidence of surface deposits on
the porous silicon. The wafer which was cathodically biased however
was completely covered with spherulites which had merged to form a
continuous layer. Plan view EDX analysis showed that this overlayer
is a predominantly calcium and phosphorous containing mineral, with
other SBF constituents such as carbon, magnesium, sodium and
chlorine being close to EDX detection limits (i.e. <1 atomic %).
Plan view EDX analysis of the unbiased and anodically biased wafers
showed only the presence of silicon and oxygen.
[0084] Cross-sectional SEM and EDX analysis showed that th calcium
and phosphorous rich mineral developed under cathodic bias is
restricted to the top of the porous silicon layer and is relatively
thin, having a thickness of approximately 0.2 .mu.m. Within the
porous silicon the calcium and phosphorous levels are below EDX
detection limits for all samples. The porous silicon layer given
the anodic loading showed a significant build up of oxygen within
the top 0.5 .mu.m of the layer.
[0085] Secondary ion mass spectrometry (SIMS) was utilised to
compare the extent and depth to which layers were calcified after
the three differing treatments, together with the depth
distribution of other specific elements. Freshly etched microporous
silicon has been shown to contain very low levels of for example
calcium and sodium (present in SBF) but appreciable levels of
fluorine (not present in SBF).
[0086] FIG. 8 is a SIMS plot shows the varying levels of
calcification resulting from the electrical biasing treatments. In
FIG. 8, the SIMS plot from a cathodically biased wafer is shown by
a line 450, the SIMS plot from an unbiased wafer is shown by line
452, and a SIMS plot from an anodically biased wafer is shown by a
line 454. Although deposition has primarily occurred near the
surface of the porous silicon, in all cases calcium levels were
above the background level throughout the 11 .mu.m thick layer. The
line 450 shows that cathodic biasing has raised the degree of
calcification and anodic biasing has lowered it compared with the
unbiased wafer. The SIMS measurements also indicated the presence
of the SBF constituents throughout the porous silicon layer and
that there had been significant movement and loss of fluorine as a
result of the cathodic biasing, together with some degree of
retention within the overlayer.
[0087] It is well established that in vitro and in vivo tissues
only respond favourably over quite restricted ranges of input
power, current and voltage in electrostimulation experiments. These
ranges are sensitive to many factors including the nature of the
stimulating electrodes. The biasing experiments described above
indicate that the kinetics of the calcification process of porous
silicon can be accelerated in vitro and th refore possibly in vivo
by the application of a cathodic bias. They also suggest that when
dissimilar silicon structures such as porous and bulk silicon are
immersed together in physiological electrolyt s, galvanic corrosion
processes may favour calcification at any cathodic sites that
develop.
[0088] The potential applications for the bias control of mineral
deposition are varied. It is known that the insertion of electrodes
into a living organism may result in the formation of a fibrous
layer around the electrode, with the thickness of the layer being
an indication of the biocompatibility of the electrode. The rapid
formation of a stable mineral deposit around microelectrodes in
vivo offers potential benefits for the electrostimulation of tissue
growth or the stimulation of muscles of paraplegics. The localised
control of mineral deposition, where localised regions may be
arranged so that a mineral deposit is not formed thereon might have
applications in the field of biosensing devices, both in vivo and
in vitro. The process of enhanced mineral deposition may be
beneficial in the coating of silicon based integrated circuits
prior to their implantation in the body.
[0089] Whilst the above description of the electrical control of
the deposition of a mineral is concerned with the deposition on
porous silicon, mineral deposits have also been observed when a
cathodic bias is applied to an unanodised wafer in the SBF
solution.
[0090] In a further embodiment, it has been found that certain
types of polycrystalline silicon (polysilicon) are also capable of
inducing calcium phosphate deposition from an SBF solution and are
hence bioactive.
[0091] In order to produce bioactive polycrystalline silicon, 100
mm diameter <100>p-type CZ silicon wafers having a
resistivity in the range 5 to 10 .OMEGA.cm are coated front and
back with a 0.5 .mu.m thick wet thermal oxide and subsequently a 1
.mu.m thick polysilicon layer of varying microstructure. The oxide
layer is grown in a Thermco TMX9000 diffusion furnace and the
polysilicon layer is grown in a Thermco TMX9000 low pressure
chemical vapour deposition hot walled furnace. For thermal oxide
growth, the furnace tube is held at a uniform temperature of
1000.degree. C., and the wet thermal oxid is grown using steam
oxidation for 110 minutes. Th subsequent deposition of the
polysilicon layer involves the pyrolysis of SiH.sub.4 at a pressure
in the range 250 to 300 mtorr with the furnace tube held at a
temperature in the range 570 to 620 .degree. C.
[0092] It is well established that th microstructure of the
polysilicon layer is sensitive to many deposition parameters such
as temperature, pressure, gas flow rate, and substrate type, as
described in Chapter 2 of "Polycrystalline Silicon for Integrated
Circuit Applications" by T. Kamins, published by Kluwer Acad. Publ.
1988. Polysilicon layers of widely varying microstructure and
morphology were obtained by using different deposition temperatures
of 570.degree. C., 580.degree. C., 590.degree. C., 600.degree. C.,
610.degree. C., and 620.degree. C. Cross-sectional transmission
electron microscopy analysis revealed that the layer deposited at
570.degree. C. was virtually amorphous near its surface whereas the
layers deposited at 600.degree. C. and 620.degree. C. were
polycrystalline throughout their depths. The grain size varies
appreciably with deposition temperature and significantly with
depth for a given layer.
[0093] Cleaved wafer segments having typical dimensions of
0.5.times.50.times.20 mm.sup.3 were then placed in separate 30
cm.sup.3 polyethylene bottles filled with SBF solution as
previously described, with the temperature of the SBF maintained at
37.degree. C..+-.1.degree. C. The different polysilicon layers were
observed to have varying levels of stability in the SBF solution as
determined by cross-sectional SEM imaging. After 64 hours in the
SBF solution, the polysilicon layer deposited at 620.degree. C. was
thinned to approximately 60% of its original thickness, whereas the
thickness of the layer deposited at 570.degree. C. was
substantially unchanged after 160 hours in the SBF solution.
[0094] Mineral deposits were observed to nucleate and proliferate
over certain of the polysilicon layers. These deposits were
observed using plan-view SEM. After two weeks immersion in the SBF
solution, mineral deposits were observed on the polysilicon layers
deposited at 600.degree. C. and 620.degree. C. but not on the layer
deposited at 570.degree. C. These observations indicate that as for
the porous silicon there is a reactivity window, dependent on the
microstructure, for optimum bioactivity. The greatest density of
mineral deposits were observed with the polysilicon layer deposited
at 600.degree. C. Significant levels of mineral deposits were
observed on both the front and back of the silicon wafers,
consistent with there having been polysilicon deposition on both
sides.
[0095] EDAX analysis of the deposits indicated the presence of
calcium, phosphorous and oxygen, consistent with som form of
apatite having nucleated. The morphology of the deposits however
differs from that of the spherulites previously described in
connection with the porous silicon, with the deposits appearing to
be more angular. The reasons for this are not understood but could
reflect a slightly different local pH at the nucleation sites on
the polysilicon. P. Li et al. in Journal of Applied Biomaterials,
Volume 4, 1993, page 221, reported that the apatite morphology
observed at a pH of 7.3 is significantly different from that
observed at a pH of 7.2 for growth on silica gel.
[0096] The potential applications for bioactive polysilicon are
potentially broader than those for bioactive porous silicon. It is
possible to coat a variety of substrates with polysilicon which
could not be coated with monocrystalline silicon. Surgical implants
could be coated with a layer of polysilicon in order to improve
adhesion with bone. Polysilicon is also highly compatible with VLSI
technology offering the prospect of complex electronic circuitry
being made biocompatible. Polysilicon can be surface micromachined
in order to produce a variety of devices and packaging
arrangements.
[0097] One possible bioactive silicon packaging concept has already
been described with reference to FIG. 6. With bioactive
polysilicon, it might be possible to construct smaller biochips.
Referring to FIG. 9 there is shown a schematic diagram of a
biosensor device 500 incorporating bioactive polysilicon. The
device 500 comprises a bulk silicon wafer 510 onto which a CMOS
circuit 512 and a sensor element 514 are fabricated. The sensor
element 514 is electrically connected to the circuit 512. The
circuit 512 is protected by a barrier layer 516 of for example
silicon oxide and silicon nitride. The whole of the device 500
except for a window 518 to the sensor element 514 is covered with a
layer 520 of bioactive polysilicon. The barrier layer 516 is
required because polysilicon itself is not a good protective layer
for silicon based circuitry due to diffusion through grain
boundaries. The barrier layer 516 is therefore interposed between
the circuit 512 and the polysilicon layer 520.
[0098] By analogy with the results using porous silicon, the
bioactivity of polycrystalline silicon might be improved by doping
it with calcium, sodium or phosphorus or a combination of these
species.
[0099] Bioactive polysilicon might be a suitable substrate for
bioassay device applications. L. Bousse et al. in IEEE Engineering
in Medicine and Biology, 1994 pages 396 to 401 describe a biosensor
for performing in vitro measurements in which cells are trapped in
micromachined cavities on a silicon chip. Such an arrangement might
beneficially incorporate a composite structure of polysilicon with
a layer of apatite thereon, the cells locating themselves
preferentially on regions of apatite.
* * * * *