U.S. patent application number 12/325502 was filed with the patent office on 2009-06-04 for sensor unit and procedure for monitoring intracranial physiological properties.
This patent application is currently assigned to Integrated Sensing Systems, Inc.. Invention is credited to David Joseph Goetzinger, Catherine Hook-Morgan, Sonbol Massoud-Ansari, Nader Najafi.
Application Number | 20090143696 12/325502 |
Document ID | / |
Family ID | 40676475 |
Filed Date | 2009-06-04 |
United States Patent
Application |
20090143696 |
Kind Code |
A1 |
Najafi; Nader ; et
al. |
June 4, 2009 |
SENSOR UNIT AND PROCEDURE FOR MONITORING INTRACRANIAL PHYSIOLOGICAL
PROPERTIES
Abstract
An anchor for an implantable sensing device, a sensor unit
formed by the anchor and sensing device, and a surgical procedure
for implanting the sensor unit for monitoring a physiological
parameter within a cavity of a living body, such as an intracranial
physiological property. The anchor includes a shank portion and a
head portion. The shank portion defines a distal end of the anchor
and has a bore defining an opening at the distal end. The head
portion defines a proximal end of the anchor and has a larger
cross-sectional dimension than the shank portion. The sensor unit
comprises the anchor and the sensing device placed and secured
within the bore of the anchor so that a sensing element of the
sensing device is exposed for sensing the physiological parameter
within the cavity.
Inventors: |
Najafi; Nader; (Ann Arbor,
MI) ; Hook-Morgan; Catherine; (Ann Arbor, MI)
; Goetzinger; David Joseph; (Livonia, MI) ;
Massoud-Ansari; Sonbol; (El Dorado Hills, CA) |
Correspondence
Address: |
HARTMAN & HARTMAN, P.C.
552 EAST 700 NORTH
VALPARAISO
IN
46383
US
|
Assignee: |
Integrated Sensing Systems,
Inc.
Ypsilanti
MI
|
Family ID: |
40676475 |
Appl. No.: |
12/325502 |
Filed: |
December 1, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61004508 |
Nov 29, 2007 |
|
|
|
61008202 |
Dec 19, 2007 |
|
|
|
Current U.S.
Class: |
600/561 |
Current CPC
Class: |
A61B 2560/0219 20130101;
A61B 5/076 20130101; A61B 5/031 20130101; A61B 5/0031 20130101;
A61B 5/6849 20130101; A61B 5/6864 20130101 |
Class at
Publication: |
600/561 |
International
Class: |
A61B 5/00 20060101
A61B005/00 |
Claims
1. An sensor unit configured to position a sensing element for
monitoring a physiological parameter within a cavity of a living
body, the sensor unit having an anchor comprising: a shank portion
defining a distal end of the anchor and having a bore defining an
opening at the distal end; and a head portion defining a proximal
end of the anchor and having a larger cross-sectional dimension
than the shank portion.
2. The sensor unit according to claim 1, wherein the shank portion
comprises means for securing the anchor within a hole.
3. The sensor unit according to claim 2, wherein the securing means
is at least one biocompatible attachment device chosen from the
group consisting of inserts, threads, nails, screws, springs, and
adhesives.
4. The sensor unit according to claim 2, wherein the anchor
consists of the shank portion, the head portion, the bore, and the
securing means.
5. The sensor unit according to claim 1, further comprising a
sensing device within the bore of the anchor, the sensing device
comprising a sensing element exposed and adapted to sense the
physiological parameter within the cavity.
6. The sensor unit according to claim 5, wherein the sensing device
is operable to telemetrically communicate a reading of the
physiological parameter to a readout device that is not adapted to
be implanted in the living body.
7. The sensor unit according to claim 5, wherein the sensing device
has a distal end and the sensing element is disposed at the distal
end of the sensing device.
8. The sensor unit according to claim 7, wherein the distal end of
the sensing device protrudes from the bore of the anchor such that
the sensing device defines a distal end of the sensor unit.
9. The sensor unit according to claim 7, wherein the sensing device
has an oppositely-disposed proximal end concealed within the
anchor.
10. The sensor unit according to claim 5, wherein the physiological
parameter is pressure.
11. The sensor unit according to claim 10, wherein the sensing
element comprises a diaphragm responsive to pressure.
12. The sensor unit according to claim 11, wherein the diaphragm is
at a distal surface of the sensing device.
13. The sensor unit according to claim 5, wherein the sensing
element comprises a micromachined structure.
14. The sensor unit according to claim 5, further comprising a
telemetry antenna adapted for telemetrically communicating a
reading of the physiological parameter sensed by the sensing
element and optionally electromagnetically receiving power for the
sensing device.
15. The sensor unit according to claim 14, wherein the telemetry
antenna is within the sensing device.
16. The sensor unit according to claim 14, wherein the sensor unit
is wirelessly coupled with the telemetry antenna to a readout
device that is not adapted to be implanted in the living body.
17. The sensor unit according to claim 16, wherein the sensor unit
is wirelessly coupled to the readout device for telemetric
communication therewith using a resonant scheme in which the
sensing device telemetrically receives power from the readout
device.
18. The sensor unit according to claim 16, wherein the sensor unit
is wirelessly coupled to the readout device for telemetric
communication therewith using a passive scheme in which the sensing
device telemetrically receives electromagnetic power from the
readout device.
19. The sensor unit according to claim 16, wherein the sensing
device further comprises processing circuitry for processing
electrical communications between the sensing element and the
telemetry antenna.
20. The sensor unit according to claim 19, wherein the processing
circuitry causes the telemetry antenna to transmit an amplitude
modulation transmission.
21. The sensor unit according to claim 5, wherein the sensor unit
consists of the sensing device, the anchor, and means for
telemetrically communicating a reading of the physiological
parameter to a readout device.
22. A surgical procedure comprising: assembling a sensor unit by
placing a sensing device within a bore of an anchor so that a
sensing element of the sensing device is exposed at a distal end of
the anchor, the sensing element being adapted to sense a
physiological parameter; making an incision in the scalp of a
patient to expose a portion of the skull; making a hole through the
skull; placing the sensor unit in the hole such that the distal end
of the sensor unit is flush with or protrudes into the cranial
cavity within the skull and an oppositely-disposed proximal end of
the sensor unit is outside the skull; securing the anchor to the
skull such that the sensing device is secured to the skull by the
anchor and the hole is occluded by the sensor unit; and then
telemetrically communicating with the sensing device to obtain a
reading of the physiological parameter using a readout device
located outside the patient.
23. The surgical procedure according to claim 22, wherein the
sensing device has a distal end, the sensing element is disposed at
the distal end of the sensing device, and the distal end of the
sensing device protrudes from the anchor such that the sensing
device defines the distal end of the sensor unit.
24. The surgical procedure according to claim 22, wherein the
anchor comprises a shank portion at the distal end of the sensor
unit and a head portion that defines the proximal end of the sensor
unit, the shank portion is inserted into the hole during the
placing step and occludes the hole as a result of the placing step,
and the head portion is not inserted into the hole during the
placing step but instead is external to the skull following the
placing step.
25. The surgical procedure according to claim 24, wherein the
securing step comprises securing the shank portion of the anchor to
the skull.
26. The surgical procedure according to claim 25, wherein the shank
portion of the anchor is secured within the hole in the skull by an
interference fit therebetween.
27. The surgical procedure according to claim 25, wherein the shank
portion of the anchor is secured within the hole in the skull by an
element chosen from the group consisting of inserts, threads,
nails, screws, springs, and adhesives.
28. The surgical procedure according to claim 24, wherein an
interference fit does not exist between the shank portion of the
anchor and the hole in the skull.
29. The surgical procedure according to claim 24, wherein the
securing step comprises securing the head portion of the anchor to
the skull.
30. The surgical procedure according to claim 29, wherein the head
portion of the anchor is secured to the skull by an element chosen
from the group consisting of nails, screws, springs, and
adhesives.
31. The surgical procedure according to claim 24, wherein the bore
of the anchor is located within the shank portion of the
anchor.
32. The surgical procedure according to claim 22, wherein the
physiological parameter is pressure.
33. The surgical procedure according to claim 22, wherein the
telemetric communicating step between the sensing device and the
readout device is established using a resonant scheme in which the
sensing device telemetrically receives power from the readout
device.
34. The surgical procedure according to claim 22, wherein the
telemetric communicating step between the sensing device and the
readout device is established using a passive scheme in which the
sensing device telemetrically receives electromagnetic power from
the readout device.
35. The surgical procedure according to claim 22, further
comprising processing electrical communications between the sensing
element and a telemetry antenna of the sensor unit.
36. The surgical procedure according to claim 35, wherein the
telemetry antenna of the sensor unit transmits an amplitude
modulation transmission to the readout device.
37. The surgical procedure according to claim 22, wherein the
surgical procedure is part of at least one of the following medical
procedures: diagnosis, treatment intervention, tailoring of
medications, disease management, identification of complications,
and chronic disease management.
38. The surgical procedure according to claim 22, wherein the
readout device is used to perform at least one of the following:
remote monitoring of the patient, closed-loop drug delivery of
medications to treat the patient, warning of changes in the
physiological parameter, portable or ambulatory monitoring or
diagnosis, monitoring of battery operation, data storage, reporting
global positioning coordinates for emergency applications, and
communication with other medical devices.
39. The surgical procedure according to claim 22, wherein the
sensor unit consists of the sensing device, the anchor, and means
for telemetrically communicating the reading of the physiological
parameter to the readout device.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application Nos. 61/004,508 filed Nov. 29, 2007, and 61/008,202
filed Dec. 19, 2007. The contents of these prior patent
applications are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] The present invention generally relates to implantable
medical devices, monitoring systems and associated procedures. More
particularly, this invention relates to a sensor unit comprising an
anchor and an implantable medical sensing device, and to a
procedure for implanting the sensing device for monitoring
intracranial physiological properties.
[0003] Wireless devices such as pressure sensors have been
implanted and used to monitor heart, brain, bladder and ocular
function. With this technology, capacitive pressure sensors are
often used, by which changes in pressure cause a corresponding
change in the capacitance of an implanted capacitor (tuning
capacitor). The change in capacitance can be sensed, for example,
by sensing a change in the resonant frequency of a tank or other
circuit coupled to the implanted capacitor.
[0004] Telemetric implantable sensors that have been proposed
include batteryless pressure sensors developed by CardioMEMS, Inc.,
Remon Medical, and the assignee of the present invention,
Integrated Sensing Systems, Inc. (ISSYS). For example, see
commonly-assigned U.S. Pat. Nos. 6,926,670 and 6,968,734 to Rich et
al., and N. Najafi and A. Ludomirsky, "Initial Animal Studies of a
Wireless, Batteryless, MEMS Implant for Cardiovascular
Applications," Biomedical Microdevices, 6:1, p. 61-65 (2004). With
such technologies, pressure changes are typically sensed with an
implant equipped with a mechanical (tuning) capacitor having a
fixed electrode and a moving electrode, for example, on a diaphragm
that deflects in response to pressure changes. The implant is
further equipped with an inductor in the form of a fixed coil that
serves as an antenna for the implant, such that the implant is able
to receive a radio frequency (RF) signal transmitted from outside
the patient to power the circuit, and also transmit the resonant
frequency as an output of the circuit that can be sensed by a
reader outside the patient. The implant can be placed with a
catheter, for example, directly within the heart chamber whose
pressure is to be monitored, or in an intermediary structure, for
example, the atrial or ventricular septum of the heart.
[0005] Presently in the United States, roughly one million people
are treated for head injuries each year, with over a quarter
million of these being moderate or severe injuries. Traumatic brain
injuries currently account for approximately 70,000 deaths each
year in the United States, with an additional 80,000 patients
having severe long-term disabilities. Monitoring intracranial
pressure (ICP) to identify intracranial hypertension (ICH) is one
of the most important steps in treatment of severe head injuries.
The ability to accurately monitor and identify high ICP levels
enables physicians to diagnose and treat the underlying causes and
significantly reduce the morbidity and mortality rates of these
patients.
[0006] ICP is currently measured and recorded through a variety of
systems, such as intraventricular catheters, subarachnoid bolts,
and catheter tip strain gauges. However, each of these systems has
significant drawbacks, including the need for repositioning and
balancing, the occurrence of occlusions and blockages, and the risk
of infection.
BRIEF SUMMARY OF THE INVENTION
[0007] The present invention provides an anchor for an implantable
sensing device, a sensor unit formed by the anchor and sensing
device, and a surgical procedure for implanting the sensor unit for
monitoring a physiological parameter within a cavity of a living
body, such as an intracranial physiological property.
[0008] The anchor includes a shank portion and a head portion. The
shank portion defines a distal end of the anchor and has a bore
defining an opening at the distal end. The head portion defines a
proximal end of the anchor and has a larger cross-sectional
dimension than the shank portion. The sensor unit is configured to
position a sensing element for monitoring a physiological parameter
within a cavity of a living body, and includes the anchor and a
sensing device that comprises the sensing element and is configured
to be placed and secured within the bore of the anchor.
[0009] The surgical procedure generally entails assembling the
sensor unit by placing the sensing device within the bore of the
anchor so that the sensing element of the sensing device is exposed
at the distal end of the anchor for sensing a physiological
parameter. An incision is made in the scalp of a patient to expose
a portion of the skull, a hole is made through the skull, and the
sensor unit is placed in the hole such that the distal end of the
sensor unit (as defined by the sensing device or the distal end of
the anchor) is flush with or protrudes into the cranial cavity
within the skull, while an oppositely-disposed proximal end of the
sensor unit (as defined by the proximal end of the anchor) remains
outside the skull. The anchor is secured to the skull so that the
hole in the skull is occluded by the sensor unit. A readout device
located outside the patient can be used to telemetrically
communicate with the sensing device to obtain a reading of the
physiological parameter sensed by the sensing element.
[0010] The sensor unit and implantation procedure are intended to
be particularly well suited for providing safe, fast, detailed,
real-time, and continuous intracranial pressure measurements.
Compared to existing systems used for ICP monitoring, particular
advantages of the invention include a miniature wireless unit with
an uncomplicated anchoring system and implantation/placement
procedure that enables accurate placement of a sensing element at
various depths in the cranial cavity. The invention also offers
reduced infection risk and patient discomfort, increased patient
mobility, and improved post-surgical patient care. Preferred
embodiments of the sensor unit are very small, allowing the unit to
be easily placed under the scalp with minimal discomfort to the
patient.
[0011] Other objects and advantages of this invention will be
better appreciated from the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIGS. 1a and 1b are block diagrams of wireless pressure
monitoring systems that utilize resonant and passive sensing
schemes, respectively, which can be utilized by the present
invention.
[0013] FIGS. 2a and 2b are schematic representations of a wireless
sensing device and a readout device suitable for use in wireless
monitoring systems of this invention.
[0014] FIG. 3 schematically represents internal components of
processing circuitry suitable for use in the sensing device of FIG.
2a.
[0015] FIG. 4 represents a perspective view of a cylindrical
self-contained sensing device of the type represented in FIG.
2a.
[0016] FIG. 5 represents the sensing device of FIG. 4 assembled
with an anchor in accordance with a preferred embodiment of the
invention.
[0017] FIGS. 6 through 8 schematically represent sensor units
equipped with alternative anchors implanted through a hole in the
skull of a subject.
DETAILED DESCRIPTION OF THE INVENTION
[0018] FIGS. 1a through 4 schematically illustrate monitoring
systems and components thereof that implement one or more
implantable sensing devices (10,30,60) adapted to be placed through
a hole in the skull of a patent for monitoring one or more
intracranial physiological parameters, a notable but nonlimiting
example of which is intracranial pressure (ICP). Each monitoring
system preferably makes use of a readout unit (20,50,80) adapted to
wirelessly communicate with the sensing device. The sensing device
is placed at a desired location within the skull with an anchor
120, of which several embodiments are shown in FIGS. 5 through 8.
Together, the sensing device and its anchor 120 define a sensor
unit 150. Because the sensing device communicates wirelessly with a
readout unit, the sensor unit 150 lacks a wire, cable, tether, or
other physical component that conducts the output of the sensing
device to the readout unit or another processing or transmission
device outside the body of a patent. As such, the sensor unit 150
defines the only implanted portion of the monitoring system.
[0019] FIGS. 1a and 1b represent two types of wireless pressure
sensing schemes disclosed in U.S. Pat. Nos. 6,926,670 and 6,968,734
to Rich et al., and capable of use with the present invention. In
FIG. 1a, an implant 10 is shown as operating in combination with a
non-implanted external reader unit 20, between which a wireless
telemetry link is established using a resonant scheme. The implant
10 contains a packaged inductor coil 12 and a pressure sensor in
the form of a mechanical capacitor 14. Together, the inductor coil
12 and capacitor 14 form an LC (inductor-capacitor) tank resonator
circuit that has a specific resonant frequency, expressed as
1/(LC).sup.1/2, which can be detected from the impedance of the
circuit. At the resonant frequency, the circuit presents a
measurable change in magnetically-coupled impedance load to an
external coil 22 associated with the reader unit 20. Because the
resonant frequency is a function of the capacitance of the
capacitor 14, the resonant frequency of the LC circuit changes in
response to pressure changes that alter the capacitance of the
capacitor 14. Based on the coil 12 being fixed and therefore having
a fixed inductance value, the reader unit 20 is able to determine
the pressure sensed by the implant 10 by monitoring the resonant
frequency of the circuit.
[0020] FIG. 1b shows another wireless pressure sensor implant 30
operating in combination with a non-implanted external reader unit
50. A wireless telemetry link is established between the implant 30
and reader unit 50 using a passive, magnetically-coupled scheme, in
which onboard circuitry of the implant 30 receives power from the
reader unit 50. In the absence of the reader unit 50, the implant
30 lays passive and without any internal means to power itself.
When a pressure reading is desired, the reader unit 50 must be
brought within range of the implant 30. The implant 30 contains a
packaged inductor coil 32 and a pressure sensor in the form of a
mechanical capacitor 34. The reader unit 50 has a coil 52 by which
an alternating electromagnetic field is transmitted to the coil 32
of the implant 30 to induce a voltage in the implant 30. When
sufficient voltage has been induced in the implant 30, a
rectification circuit 38 converts the alternating voltage on the
coil 32 into a direct voltage that can be used by electronics 40 as
a power supply for signal conversion and communication. At this
point the implant 30 can be considered alert and ready for commands
from the reader unit 50. The implant 30 may employ the coil 32 as
an antenna for both reception and transmission, or it may utilize
the coil 32 solely for receiving power from the reader unit 50 and
employ a second coil 42 for transmitting signals to the reader unit
50. Signal transmission circuitry 44 receives an encoded signal
generated by signal conditioning circuitry 46 based on the output
of the capacitor 34, and then generates an alternating
electromagnetic field that is propagated to the reader unit 50 with
the coil 42. The implant 30 is shown in FIG. 1b without a battery,
and therefore its operation does not require occasional replacement
or charging of a battery. Instead, the energy required to perform
the sensing operation is entirely derived from the reader unit 50.
However, the implant 30 of FIG. 1b could be modified to use a
battery or other power storage device to power the implant 30 when
the reader unit 50 is not sufficiently close to induce a voltage in
the implant 30.
[0021] While the resonant and passive schemes described in
reference to FIGS. 1a and 1b are within the scope of the invention,
FIG. 2a represents a more preferred sensing device 60 that
translates a physiologic parameter into a frequency tone and
modulates the impedance of an antenna with the frequency tone to
communicate the physiologic parameter to an external readout unit
80 (FIG. 2b). FIG. 2a represents the wireless implantable sensing
device 60 as comprising a transducer 62, electronic circuitry 64
(e.g., an application-specific integrated circuit, or ASIC), and an
antenna 66. The antenna 66 is shown as comprising windings 68
(e.g., copper wire) wrapped around a core 70 (e.g., ferrite),
though other antenna configurations and materials are foreseeable.
The transducer 62 is preferably a MEMS device, more particularly a
micromachine fabricated by additive and subtractive processes
performed on a substrate. The substrate can be rigid, flexible, or
a combination of rigid and flexible materials. Notable examples of
rigid substrate materials include glass, semiconductors, silicon,
ceramics, carbides, metals, hard polymers, and TEFLON. Notable
flexible substrate materials include various polymers such as
parylene and silicone, or other biocompatible flexible materials. A
particular but nonlimiting example of the transducer 62 is a MEMS
capacitive pressure sensor for sensing pressure, such as
intracranial pressure (ICP) of the cerebrospinal fluid, though
other materials and any variety of sensing elements, e.g.,
capacitive, inductive, resistive, piezoelectric, etc., could be
used. For example, the transducer 62 could be configured to sense
temperature, flow, acceleration, vibration, pH, conductivity,
dielectric constant, and chemical composition, including the
composition and/or contents of cerebrospinal fluid. The sensing
device 60 may be powered with a battery or other power storage
device, but in preferred embodiments is powered entirely by the
readout unit 80 schematically represented in FIG. 2b.
[0022] In addition to powering the sensing device 60, the readout
unit 80 is represented as being configured to receive an output
signal from the sensing device 60, process the signal, and relay
the processed signal as data in a useful form to a user. The
readout unit 80 is shown equipped with circuitry 82 that generates
a high-frequency (e.g., 13.56 MHz), high-power signal for an
antenna 84 to create the magnetic field needed in communicate with
the sensing device 60. The readout unit 80 contains additional
circuitry 86 to receive and demodulate a backscattered signal from
the sensing device 60, which is then processed with a processing
unit 88 using calibration coefficients to quantify the
physiological parameter of interest. The readout unit 80 is further
shown as equipped with a user interface 90, by which the operation
of the readout unit 80 can be controlled to allow data logging or
other user control and data examination. The readout unit 80 can be
further configured for wireless or wired communication with a
computer, telephone, or web-based system.
[0023] FIG. 3 represents a block diagram showing particularly
suitable components for the electronic circuitry 64 of FIG. 2a. The
circuitry 64 includes an oscillator 92, for example a relaxation
oscillator, connected to a resistor 93 and a MEMs mechanical
capacitor 94 as an example of the transducer 62 of FIG. 2a. A
preferred MEMS capacitor 94 comprises a fixed electrode and a
moving electrode on a diaphragm that deflects relative to the fixed
electrode in response to pressure, such that the capacitor 94 is
able to serve as a pressure sensing element for the transducer 62.
A nonlimiting example of a preferred MEMS capacitor 94 has a
pressure range of about -100 to about +300 mmHg, with an accuracy
of about 1 mmHg. Alternatively, a variable resistor transducer
could be used with a fixed capacitance, or an inductor could be
substituted for the transducer or fixed circuit element. Based on
the RC or other time constant (1/(LC).sup.1/2), the oscillator 92
produces a frequency tone that directly relates to the capacitive
value of the capacitor 94 and, therefore, the physiologic parameter
of interest.
[0024] The circuitry 64 is further shown as including a modulator
96, with which the frequency tone of the oscillator 92 is encoded
on a carrier frequency, placed on the antenna 66, and then
transmitted to the readout unit 80. This is accomplished simply by
opening and closing a switch 98 and adding a capacitance 100 to the
antenna matching circuit, resulting in an AM (amplitude modulation)
LSK (load shift keying) type modulation. This transmission approach
is similar to that used in RFID (radio frequency identification)
communications, except RFID does not typically encode analog
information but instead encodes a few digital bits either on an AM
LSK or FSK (frequency shift keying) modulation.
[0025] Because the preferred embodiment of the sensing device 60
does not utilize wires to transmit data or power to the readout
unit 80 (or another remote device), nor contains an internal power
source, the circuitry 64 further includes a regulator/rectifier 102
to extract its operating power from electromagnetic (EM) energy
generated by the readout unit 80 or another EM power source. The
regulator/rectifier 102 rectifies incoming power from the inductive
antenna 66 and conditions it for the other circuit components
within the circuitry 64. Finally, a matching circuit 104 is shown
as comprising a trimmable capacitor bank 106 to resonate the
inductor antenna 66, which is energized by the magnetic field and
backscatters data as previously described.
[0026] As an alternative to the embodiment of FIG. 3, the modulator
96 could use a 13.56 MHz (or other frequency) magnetic field as a
clock reference to create a second carrier frequency, such as one
that is one-quarter or another sub-multiple or multiple of the
original frequency. The second carrier frequency can then be
amplitude modulated (AM) using the oscillator frequency tone and
transmitted to the readout unit 80 via the same antenna 66. In this
embodiment, the readout unit 80 may or may not have a second
antenna to receive the second carrier frequency-based AM
signal.
[0027] The communication scheme described above differs from
resonate tank communication systems that use capacitive pressure
transducer elements in conjunction with an inductor/antenna. In
particular, the circuitry 64 allows the use of any frequency for
the high power readout unit 80, which in preferred embodiments
utilizes an industrial, scientific, medical (ISM) band frequency.
In contrast, the frequencies and potentially large bandwidths
required of resonate tank communication systems are subject to FCC
emission limitations, likely requiring the use of extra shielding
or potentially other measures taken in the facilities where the
sensing device 60 and readout unit 80 are to be used. Another
feature of the circuitry 64 is the allowance of more combinations
of oscillator elements to be used. Because resonator tank systems
require an inductive element and a capacitive element in which at
least one of the elements serves as a transducer, resonator tank
systems do not lend themselves well to resistive-based or other
based sensors. Finally, the circuitry 64 also allows for signal
conditioning, such as transducer compensation, which allows for
such items as removing temperature dependence or other
non-idealities that may be inherent to the transducer 62. In the
embodiment of FIG. 3, a negative temperature coefficient of the
MEMS capacitor 94 can be compensated with simple circuitry relying
on the positive temperature coefficient of resistor elements
arranged in a trimmable bank of two resistor units with largely
different temperature coefficients that can be selectively added in
a trimming procedure in production to select the precise level to
compensate the transducer variation.
[0028] Restrictive levels of energy available to small implantable
medical sensing devices and the desire to maximize data rates to
capture more detailed physiological parameter response have
typically been met with a robust type of analog communication that
places information on the frequency rather than amplitude of the
carrier. In U.S. Pat. No. 6,929,970 to Rich et al., a secondary
carrier frequency is used for communication with an interrogator
unit, resulting in a technique that consumes substantially more
power in the implant and requires a second external antenna to
receive the signal. The greater power consumption of the implant
necessitates a tradeoff between smaller size and longer
communication range. In contrast, the communication scheme
described above in reference to FIGS. 2a, 2b and 3 draws upon the
RFID-type communications, such as those described in U.S. Pat. Nos.
7,015,826 and 6,622,567, whose contents are incorporated herein by
reference. However instead of communicating digital data using a
fixed rate clock, the present invention transmits analog
information as the frequency of the clock to lower power
consumption and enhance powering and communication range. In this
way, much of the readout unit 80 can utilize hardware that is
commercially available for RFID, except that a different
demodulator is required. An early example of RFID can be found in
U.S. Pat. No. 4,333,072.
[0029] The transducer 62 (e.g., mechanical capacitor 94), the
electronic circuitry 64 (including chips, diodes, capacitors, etc.,
thereof), the antenna 66 and any additional or optional components
(e.g., additional transducers 62) of the sensing device 60 (or any
alternative sensing device, such as the devices 10 and 30 of FIGS.
1a and 1b) are preferably contained in a single hermetically-sealed
housing. FIG. 4 depicts a preferred example as being a cylindrical
housing 110, which is convenient for placing the sensing device 60
within the anchor 120 discussed in reference to FIGS. 5 through 8
below. Other exterior shapes for the housing 110 are also possible
to the extent that the exterior shape permits assembly of the
sensing device 60 with the anchor 120 as discussed below. The
cylindrical-shaped housing 110 of FIG. 4 includes a flat distal
face 112, though other shapes are also possible, for example, a
torpedo-shape in which the peripheral face 114 of the housing 110
immediately adjacent the distal face 112 is tapered or conical (not
shown). The housing 110 can be formed of glass, for example, a
borosilicate glass such as Pyrex Glass Brand No 7740 or another
suitable material capable of forming a hermetically-sealed
enclosure for the electrical components of the sensing device 60. A
biocompatible coating, such as a layer of a hydrogel, titanium,
nitride, oxide, carbide, silicide, silicone, parylene and/or other
polymers, can be deposited on the housing 110 to provide a
non-thrombogenic exterior for the biologic environment in which the
sensing device 60 will be placed. As can be seen in FIG. 4, the
inductive antenna 66 (for example, comprising the coil 68
surrounding the core 70 as represented in FIG. 2a) occupies most of
the internal volume of the housing 110. The size of the antenna 66
is governed by the need to couple to a magnetic field to enable
telepowering with the readout unit 80 from outside the body, for
example, a transmission distance of about ten centimeters or more.
The circuitry 64 is disposed between the antenna 66 and the distal
face 112 of the housing 110 that preferably carries the transducer
62. A nonlimiting example of an overall size for the housing 110 is
about 3.7 mm in diameter and about 16.5 mm in length.
[0030] A preferred aspect of the invention is to locate the
transducer 62 at or near the distal end of the sensing device 60,
for example, the flat distal face 112 of the cylindrical housing
110 or on the peripheral face 114 of the housing 110 immediately
adjacent the distal face 112. The distal face 112 can be defined by
a biocompatible semiconductor material, such as a heavily
boron-doped single-crystalline silicon, in whose outer surface the
transducer 62 (for example, a pressure-sensitive diaphragm of the
capacitor 94) is formed. In this manner, only the distal face 112
of the housing 110 need be in contact with cerebrospinal fluid,
whose pressure (or other physiological parameter) is to be
monitored. In the case of monitoring intracranial pressures, this
aspect of the invention can be used to minimize the protrusion of
the sensing device 60 into the cranial cavity. For example, the
sensing device 60 can be placed so that the transducer 62 presses
against the dura mater (extradural), though it is also within the
scope of the invention that the transducer 62 is placed beneath the
dura (subdural) in the subarachnoid space or beneath the pia mater
and extend into brain tissue.
[0031] FIGS. 5 through 8 represent different embodiments of the
anchor 120 assembled with the sensing device 60 to form the sensor
unit 150. In FIG. 5, the sensor unit 150 is represented as a
coaxial assembly of the sensing device 60 and anchor 120, with the
distal face 112 of the sensing device 60 exposed and the
oppositely-disposed proximal end of the sensing device 60 concealed
within the anchor 120. As represented in FIG. 6, the sensor unit
150 can be anchored to the skull 134, for example, by making an
incision in the scalp 142, drilling a hole 136 in the skull 134,
and then inserting the sensor unit 150 in the hole 136 so that the
anchor 120 secures the sensing device 60 to the skull 134. The
protrusion of the sensor unit 150 and its sensing device 60
relative to the skull 134 can be determined by the anchor 120. For
example, the distal end of the unit 150 (for example, as defined by
the distal face 112 of the housing 110 or the distal end 128 of the
anchor 120) may be slightly recessed or flush with the interior
surface of the skull 134 so that the transducer 62 presses against
the dura mater 138, or may be placed beneath the dura mater 138
into the subarachnoid space or into brain tissue. As such, the
length of the shank portion 122 can be varied depending on the
desired location of the transducer 92. Furthermore, the shank
portion 122 could be configured as a catheter through which
pressure is conducted to the sensing device 60, which can then be
located within the shank portion 122 nearer the head portion 124
than the distal end 128 of the anchor 120.
[0032] The anchor 120 can be fabricated as a unitary component or
as an assembly, and can be formed of various biocompatible
materials, nonlimiting examples of which include NITINOL, TEFLON,
polymers such as parylene, silicone and PEEK, metals, glass, and
ceramics. The anchor 120 is represented in FIGS. 5 through 8 as
having a shank portion 122 and a head portion 124 that define,
respectively, the distal end 128 and an oppositely-disposed
proximal end of the anchor 120. The head portion 124 is represented
as having a larger cross-sectional dimension than the shank portion
122 to prevent the entire anchor 120 from being placed within the
skull hole 136. The shank and head portions 122 and 124 are
represented as having coaxial tubular and disk shapes,
respectively, though a round outer periphery is nota requirement
for either portion 122 and 124. The shank portion 122 is further
represented as having an internal bore 126 that defines an opening
at the distal end 128 of the anchor 120. The sensing device 60 is
axially disposed within the anchor bore 126 such that the distal
face 112 carrying the transducer 62 is exposed outside the anchor
120. The distal face 112 of the sensing device 60 is shown as
protruding from the shank portion 122, though it is also within the
scope of the invention that the distal face 112 could be recessed
within the anchor bore 126. The anchor bore 126 and sensing device
housing 110 are represented as having complementary shapes,
providing a close fit that prevents biological material (for
example, cerebrospinal fluid) from infiltrating the bore 126. The
sensing device 60 can be temporarily or permanently secured within
the bore 126, for example, with an interference fit or another
mechanical securement device, or a biocompatible adhesive such as a
cement, glue, epoxy, etc. While the antenna 66 of the sensing
device 60 is shown enclosed with the housing 110 in FIG. 5, the
antenna 66 could be placed within the head portion 124 of the
anchor 120, or within a separate subassembly (not shown) placed
remotely on the patient and electrically coupled to the remaining
components of the sensing device 60 via the anchor 120.
[0033] In FIG. 6, the sensor unit 150 is represented as anchored to
the skull 134, with the shank portion 122 of the anchor 120
received in the skull hole 136, and the distal end of the unit 150
(as defined by the distal face 112 of the housing 110) placed by
the anchor 120 beneath the dura mater 138 in the subarachnoid space
140. The head portion 124 of the anchor 120 abuts the exterior
surface of the skull 134, and may be exposed through the scalp 142
(as shown) or covered by the scalp 142. The anchor 120 can be
secured to the skull 134 with an interference fit between the shank
portion 122 and the skull hole 136, and/or with threads formed on
the exterior of the shank portion 122, or with a biocompatible
cement, glue or epoxy, spring, etc., placed between the skull 134
and the shank portion 122.
[0034] In FIG. 7, the shank portion 122 is shown to have a smaller
cross-section than the skull hole 136, for example, as a result of
the hole 136 being formed for another medical procedure. The anchor
120 is secured to the skull 134 with the head portion 124 assisted
by an attachment element 144, for example, a biocompatible cement,
glue or epoxy, screws, nails, etc.
[0035] In FIG. 8, the sensor unit 150 is shown as further including
an insert 146 between the shank portion 122 and the skull 134. The
insert 146 can have a tubular shape, can be secured to the anchor
120 by an interference fit, and can provide for an interference fit
with the skull hole 136. Alternatively or in addition, the insert
146 can be or comprise a spring or threads capable of securing the
shank portion 122 to the skull 134, optionally assisted by a
biocompatible cement, glue or epoxy, nails, etc. A preferred aspect
of the embodiment of FIG. 8 is that the anchor 120 is not
permanently joined to the insert 146, which permits the insert 146
to remain secured to the skull 134 while allowing the sensor unit
150 and/or its sensing device 60 and/or anchor 120 to be
replaced.
[0036] In addition to the above-noted features, the anchor 120 can
be modified to provide other functional features useful to the
sensing device 60 or sensor unit 150, for example, a device similar
to an RFID tag can be added to the anchor 120 to wirelessly
transmit ID information concerning the sensing device 60. The ID
information may include an ID number, ID name, patient name/ID,
calibration coefficients/information, range of operation, date of
implantation, valid life of the device (operation life), etc. The
anchor 120 may further include additional capabilities such as
features for connection to a catheter, shunt, or other device (not
shown) that may be useful when monitoring ICP or treating
intracranial hypertension (ICH) and severe head injuries.
[0037] In addition to the sensing device 60, sensor unit 150 and
reader unit 80 described above, the monitoring systems of this
invention can be combined with other technologies to achieve
additional functionalities. For example, the reader unit 80 can be
implemented to have a remote transmission capability, such as home
monitoring that may employ telephone, wireless communication, or
web-based delivery of information received from the sensor units
150 by the reader unit 80 to a physician or caregiver. In this
manner, the reader unit 80 can be adapted for remote monitoring of
the patient, closed-loop drug delivery of medications to treat the
patient, warning of changes in the physiological parameter
(pressure), portable or ambulatory monitoring or diagnosis,
monitoring of battery operation, data storage, reporting global
positioning coordinates for emergency applications, and
communication with other medical devices such as deep brain
stimulation (DBS) devices, drug delivery systems, non-drug delivery
systems, and wireless medical management systems. Furthermore, the
placement of the sensor unit 150 can be utilized as part of a
variety of different medical procedures, including diagnosis,
treatment intervention, tailoring of medications, disease
management, identification of complications, and chronic disease
management.
[0038] While the invention has been described in terms of specific
embodiments, it is apparent that other forms could be adopted by
one skilled in the art. Therefore, the scope of the invention is to
be limited only by the following claims.
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