Implantable pressure sensor

Abstract

An implantable pressure sensor. In some embodiments, a system includes a pressure sensor, the pressure sensor including a piezoresistive element including a plurality of carbon nanotubes in a polydimethylsiloxane matrix.

Description

269517WO (M24-202L-WO1-a)IMPLANTABLE PRESSURE SENSORCROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] The present application claims priority to and the benefit of U.S. Provisional Application No. 63 / 638,240, filed April 24, 2024, entitled "IMPLANTABLE PRESSURE SENSOR", the entire content of which is incorporated herein by reference.

[0002] The present application is related to U.S. Patent Application Ser. No. 17 / 426,497, filed July 28, 2021 , entitled “STABILIZING MATRICES FOR IMPLANTABLE ELECTROCHEMICAL BIOSENSORS”, and to U.S. Patent No. 11 ,554,958, issued January 17, 2023, entitled “ULTRA-SOFT COATINGS FOR INTERFACES WITH BRAIN AND OTHER SOFT TISSUES”, the entire contents of both of which are incorporated herein by reference.FIELD

[0003] One or more aspects of embodiments according to the present disclosure relate to implantable devices, and more particularly to implantable pressure sensors.BACKGROUND

[0004] In various circumstances, an internal pressure (e.g., intracranial pressure) in a subject may be of clinical importance, and it may be useful to monitor such a pressure. For example, in a hydrocephalic patient, it may be advantageous to monitor the pressure in a ventricle of the brain.

[0005] It is with respect to this general technical environment that aspects of the present disclosure are related.SUMMARY

[0006] According to an embodiment of the present disclosure, there is provided a system, including: a pressure sensor, the pressure sensor including a piezoresistive element including a plurality of carbon nanotubes in a polydimethylsiloxane matrix.

[0007] In some embodiments, the system further includes two conductive wires in contact with different locations on the piezoresistive element.-1 -4933-1832-4538 v.l269517WO (M24-202L-WO1-a)

[0008] In some embodiments, one of the conductive wires is composed of a material selected from the group consisting of carbon, platinum, indium, gold, indium tin oxide, graphene, titanium and stainless steel.

[0009] In some embodiments, one of the conductive wires is composed of carbon nanotubes.

[0010] In some embodiments, the system further includes a polydimethylsiloxane coating over the piezoresistive element.

[0011] In some embodiments, the piezoresistive element is at a proximal end of a catheter.

[0012] In some embodiments, the system further includes two conductive wires in contact with different locations on the piezoresistive element and extending longitudinally along the catheter.

[0013] In some embodiments, the system further includes a piezoelectric element connected to the piezoresistive element.

[0014] In some embodiments, the system further includes a first electrode and a second electrode.

[0015] In some embodiments: the piezoresistive element has a first end and a second end; the first electrode is in electrical contact with the first end; and the second electrode is in electrical contact with the second end.

[0016] In some embodiments, the piezoelectric element is connected in parallel with the piezoresistive element.

[0017] In some embodiments: the piezoresistive element is a hollow cylinder; and the piezoelectric element is inside the piezoresistive element.

[0018] According to an embodiment of the present disclosure, there is provided a system, including: a pressure sensor including a piezoresistive element; a first electrode connected to a first conductive wire; and a second electrode connected to a second conductive wire, the first electrode being in contact with a first location on the piezoresistive element, and the second electrode being in contact with a second location on the piezoresistive element.-2-4933-1832-4538 v.l269517WO (M24-202L-WO1-a)

[0019] In some embodiments, the first conductive wire is composed of a material selected from the group consisting of carbon, platinum, indium, gold, indium tin oxide, graphene, titanium and stainless steel.

[0020] In some embodiments, the first conductive wire is composed of carbon nanotubes.

[0021] In some embodiments, the system further includes a polydimethylsiloxane coating over the piezoresistive element.

[0022] In some embodiments, the system further includes a piezoelectric element connected to the piezoresistive element.

[0023] In some embodiments: the piezoresistive element has a first end and a second end; the first electrode is in electrical contact with the first end; and the second electrode is in electrical contact with the second end.

[0024] In some embodiments, the piezoelectric element is connected in parallel with the piezoresistive element.

[0025] In some embodiments: the piezoresistive element is a hollow cylinder; and the piezoelectric element is inside the piezoresistive element.BRIEF DESCRIPTION OF THE DRAWINGS

[0026] These and other features and advantages of the present disclosure will be appreciated and understood with reference to the specification, claims, and appended drawings wherein:

[0027] FIG. 1 A is a schematic diagram of a pressure sensor including a piezoresistive element connected in series with a piezoelectric signal source, according to an embodiment of the present disclosure;

[0028] FIG. 1 B is a schematic diagram of a pressure sensor including a piezoresistive element connected in parallel with a piezoelectric signal source, according to an embodiment of the present disclosure;

[0029] FIG. 2 includes a plurality of views of an embodiment including a pressure sensor on a catheter, according to an embodiment of the present disclosure;

[0030] FIG. 3 includes a plurality of views of another embodiment including a pressure sensor on a catheter, according to an embodiment of the present disclosure;-3-4933-1832-4538 v.l269517WO (M24-202L-WO1-a)

[0031] FIG. 4 shows a plurality of views of an implantable pressure sensor, according to an embodiment of the present disclosure;

[0032] FIG. 5A is a schematic diagram of a system including an implantable pressure sensor, according to an embodiment of the present disclosure;

[0033] FIG. 5B is a schematic diagram of a system including an implantable pressure sensor, according to an embodiment of the present disclosure; and

[0034] FIG. 5C is a schematic diagram of a system including an implantable pressure sensor, according to an embodiment of the present disclosure.DETAILED DESCRIPTION

[0035] The detailed description set forth below in connection with the appended drawings is intended as a description of exemplary embodiments of an implantable pressure sensor provided in accordance with the present disclosure and is not intended to represent the only forms in which the present disclosure may be constructed or utilized. The description sets forth the features of the present disclosure in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and structures may be accomplished by different embodiments that are also intended to be encompassed within the scope of the disclosure. As denoted elsewhere herein, like element numbers are intended to indicate like elements or features.

[0036] In various circumstances it may be advantageous to measure pressure within the body of a patient, or “subject”. For example, for a hydrocephalic patient, it may be useful to measure the pressure in a ventricle of the brain (in a “cerebral ventricle”). If the patient has a shunt that extends into the cerebral ventricle allowing cerebrospinal fluid to drain from the cerebral ventricle, such a pressure measurement may make it possible, for example, to detect clogging of the shunt, which may cause the fluid pressure, of the cerebrospinal fluid in the cerebral ventricle, to increase. As another example, it may be useful to measure the pressure applied to the dura by one of the cerebral hemispheres. Pressure measurements elsewhere in the body of a subject may similarly be useful.

[0037] In some embodiments, such pressure measurements may be performed using a pressure sensor that measures pressure using the piezoresistive effect. For example,-4-4933-1832-4538 v.l269517WO (M24-202L-WO1-a) the sensor may include a piezoresistive element with two contacts; in such a sensor, the resistance between the two contacts may depend on the strain in the piezoresistive element. In general, the resistance between the two contacts may depend on both bulk strain within the piezoresistive element and on shear strain within the piezoresistive element.

[0038] In some embodiments, the piezoresistive element includes (e.g., consists of) a matrix of polydimethylsiloxane (PDMS) containing a plurality of carbon nanotubes (CNTs). For example, a mixture of between 3% and 12% (e.g., 8%) by weight of CNTs and PDMS part A (base) (e.g., 92% by weight PDMS part A) may be used to form the piezoresistive element. The mixture of CNTs and PDMS part A may be placed in a beaker and mixed for 24 hours at 150 revolutions per minute (rpm). Alternatively, functionalized CNTs in solvent may be added to PDMS part A and mixed until the solvent evaporates. The mixture may then be placed in a 1 mm deep mold and cured. The resulting sheet may be cut into 2 mm x 5 mm x 1 mm parts (each having the shape of a rectangular parallelepiped), each of which may then be used as a piezoresistive element. The PDMS base may be, for example, Sylgard 184.

[0039] In some embodiments, the ratio of cross linker to PDMS part A is selected to be between 1 :30 and 1 :15 (e.g., 1 :20) so that the resulting cured piezoresistive element is relatively soft, which may improve biocompatibility. The Young’s modulus of such a piezoresistive element may be between 5 kPa and 20 MPa (e.g., 590 kPa).

[0040] Conductive wires may be attached to the piezoresistive element in two or more locations to form contacts to the piezoresistive element. For example, a first conductive wire may be put in contact with one face of a piezoresistive element having the shape of a rectangular parallelepiped, and a second conductive wire may be put in contact with the opposite face of the piezoresistive element. In some embodiments, more than two contacts may be used. For example, conductive wires may be put in contact with the piezoresistive element at four different locations on the piezoresistive element, and the wires may be used to perform a four-point measurement of the resistance of the piezoresistive element, with two of the wires being used to drive a known current through the piezoresistive element, and the other two wires being used to measure the resulting voltage drop in the piezoresistive element.-5-4933-1832-4538 v.l269517WO (M24-202L-WO1-a)

[0041] The wires may be carbon nanowires, for example, or carbon nanotube ribbons, or platinum, iridium, gold, indium tin oxide, graphene, titanium, or stainless steel wires. Once the conductive wires have been put in contact with the piezoresistive element, a coating of PDMS (e.g., a mixture of PDMS part A and cross-linker) may be applied over the entire surface of the piezoresistive element and over the conductive wires where they contact the piezoresistive element. The PDMS may then be allowed to cure, forming a protective insulating coating for the piezoresistive element and securing the conductive wires to the piezoresistive element. Other portions of the conductive wires may also be insulated, with a PDMS coating or by another insulator.

[0042] Referring to FIGs. 1A and 1 B, in some embodiments, the piezoresistive element 105 is connected (e.g., by the conductive wires 107) to a piezoelectric element 110 (e.g., a lead zirconate titanate (PZT) element), which receives acoustic energy, converts the acoustic energy to an electrical signal, and operates as the source of an electrical drive signal for the piezoresistive element 105. FIG. 1 A shows a circuit in which the piezoresistive element 105 is connected in series with the piezoelectric element 110, and the series combination is connected between two electrodes 115. Connections to the piezoelectric element 110 may be made using silver epoxy, for example.

[0043] In operation, the piezoelectric element 110 receives acoustic waves (e.g., ultrasonic waves, at a frequency (or “frequency of operation”) between 100 kHz and a few MHz (e.g., 10 MHz)), transmitted into the subject by a suitable ultrasonic transducer or ultrasonic transmitter. The acoustic waves produce time varying strain in the piezoelectric element 110 which causes a corresponding time-varying voltage to appear across the contacts of the piezoelectric element 110. The electrodes 115 form a connection to the tissue of the subject, and, in the embodiment of FIG. 1A, the timevarying (or “alternating current”, or AC) voltage causes a current to flow through the series combination of (i) the tissue of the subject and (ii) the piezoresistive element 105. As such, the series combination of (i) the tissue of the subject and (ii) the piezoresistive element 105 forms a voltage divider, with a greater proportion of the voltage drop (and, therefore, a greater AC voltage) appearing across the tissue of the subject when the resistance of the piezoresistive element 105 is lower (e.g., as a result of strain in the piezoresistive element 105). The combination of the piezoresistive element 105, the-6-4933-1832-4538 v.l269517WO (M24-202L-WO1-a) piezoelectric element 110, and the electrodes 115 may operate a pressure sensor, as discussed in further detail below. The piezoresistive element 105 may itself also be considered to be a pressure sensor (e.g., because it is capable of sensing pressure (e.g., when connected to other circuitry)). As such, as the term “pressure sensor” is used herein, a pressure sensor may include a pressure sensor and other elements.

[0044] FIG. 1 B shows an embodiment in which the piezoresistive element 105 is connected in parallel with the piezoelectric element 110, and the AC voltage appearing across the tissue of the subject is lower when the resistance of the piezoresistive element 105 is lower (e.g., as a result of strain in the piezoresistive element 105).

[0045] The AC voltage applied, by the electrodes 115, to the tissue of the subject may be detected using external electrodes placed on the skin of the subject (e.g., conductive patches adhered to the skin of the subject). The AC voltage applied to the tissue may appear at the surface of the skin of the subject through volume conduction through the tissue of the subject (if the distance from the electrodes to the skin is comparable to or larger than the separation between the electrodes 115) or by direct conduction (if the distance from the electrodes to the skin is less than the separation between the electrodes 115). Because the signal measured by the external electrodes depends on the resistance of the piezoresistive element 105, it may be possible to infer the strain in the piezoresistive element 105 (and the pressure to which the piezoresistive element 105 is subjected) from the amplitude of the signal measured by the external electrodes. Especially when the electrical signal reaches the external electrodes by volume conduction, the frequency of operation may be limited by attenuation of the electrical signal (within the tissue of the subject), which may be greater at higher frequencies.

[0046] FIG. 2 shows various views of a catheter 205 including a piezoresistive element 105. The piezoelectric element may be mounted sub-durally, cortically or sub- cortically on the catheter. The catheter 205, which may be used as a shunt, e.g., to drain cerebrospinal fluid from a cerebral ventricle, may include (e.g., consist of) a tube with a plurality of perforations 210 through which fluid may enter the catheter. The piezoresistive element 105, which may approximately have the shape of a hemisphere, may cover one end (which may be referred to as the proximal end) of the catheter 205.-7-4933-1832-4538 v.l269517WO (M24-202L-WO1-a)Conductive wires 107 may extend from two locations on the piezoresistive element 105 along the outside of the tube of the catheter 205. As mentioned above, the piezoresistive element 105 and the proximal ends of the conductive wires 107 may be covered in a protective, insulating (e.g., PDMS) covering. The distal ends of the conductive wires 107 may be bare and may operate as electrodes 115 (in such an embodiment each conductive wire may nonetheless be said to be “connected to” a respective electrode). If the catheter 205 exits the skull at the occipital protuberance, then at that point the conductive wires 107 may be sufficiently close to the skin that direct conduction to the external electrodes is feasible. In operation, the pressure in the ventricle may produce compressive strain (or “bulk” strain) in the piezoresistive element 105 (or shear strain if the perforations 210 are clogged and a pressure difference exists across the piezoresistive element 105), and, as such, the pressure in the ventricle may be measured by measuring the signal detected by the external electrodes.

[0047] FIG. 3 shows an embodiment in which the proximal end of the catheter 205 is sealed by a catheter tip 305 and the piezoresistive element 105 (instead of being at the proximal end of the catheter 205 as in the embodiment of FIG. 2) is a sleeve that fits over the catheter 205 at some distance from the proximal end of the catheter 205. The conductive wires 107 may make contact, at two different locations, with the inner surface of the piezoresistive element 105, which may slide over the catheter 205 and the proximal (bare) ends of the conductive wires 107. The sleeve may lack the protective insulating coating on its interior surface (or on a portion of its interior surface) so that it may be in electrical contact at each of the two locations with a respective one of the conductive wires 107. The sleeve may further be a sufficiently tight fit on the catheter 205 to remain in place once installed. As in the embodiment of FIG. 2, the distal ends of the conductive wires 107 may be bare and may operate as electrodes 115, and if the catheter 205 exits the skull at the occipital protuberance, then at that point the conductive wires 107 may be sufficiently close to the skin that direct conduction to the external electrodes is feasible. In operation, the pressure in the ventricle may produce compressive strain in the piezoresistive element 105, and, as such, the pressure in the ventricle may be measured by measuring the signal detected by the external electrodes.-8-4933-1832-4538 v.l269517WO (M24-202L-WO1-a)

[0048] FIG. 4 shows an embodiment that may be implanted, in the subject, between the dura and one of the cerebral hemispheres, and used to measure the pressure applied to the dura by the cerebral hemisphere. The implantable pressure sensor may include, as shown, two hemispherical conducting (e.g., stainless steel) electrodes 115 (e.g., stainless steel electrodes 115), and, sandwiched between the electrodes 115, the piezoresistive element 105, which has the shape of a tube or hollow cylinder. Each end of the piezoresistive element 105 is in physical and electrical contact with a respective one of the electrodes 115. A piezoelectric element 110 is within the interior (e.g., the lumen) of the tube that is the piezoresistive element 105. The piezoelectric element 110 may further be inside a polyimide tube 410 that is inside the piezoresistive element 105 and that insulates the piezoelectric element 110 from the interior surface of the piezoresistive element 105. A PDMS encapsulation layer 415 may cover a portion of the exterior of the implantable pressure sensor (e.g. the encapsulation layer 415 may cover the exterior of the piezoresistive element 105). The piezoelectric element 110 may be connected by conductive wires 405 to both of the electrodes 115. The piezoresistive element 105 and the piezoelectric element 110 are therefore wired in parallel, implementing (and operating in the manner of) the embodiment of FIG. 1 B. When implanted between the dura and a cerebral hemisphere, the pressure applied to the implantable sensor may be anisotropic, and, as such, the strain in the piezoresistive element 105 may include both shear strain and bulk strain.

[0049] FIG. 5A is a schematic drawing of the embodiment of FIG. 1 B, showing additional details. An acoustic source 505 (which may be a piezoelectric transducer) external to the subject may provide acoustic waves which are converted to a time-varying voltage by the piezoelectric element 110, as described above. External electrodes 510 may be used to sense the voltage across the piezoresistive element 105. In the embodiments of FIGs. 5B and 5C, a resistor 515 is connected in series between the piezoelectric element 110 and the piezoresistive element 105. In such an embodiment, the piezoresistive element 105 and the resistor 515 form a resistive divider, and the fraction of the voltage produced by the piezoelectric element 110 that appears across the resistor 515 is greater when the resistance of the piezoresistive element 105 is smaller. As such, the voltage across the resistor 515 may be sensed by external-9-4933-1832-4538 v.l269517WO (M24-202L-WO1-a) electrodes 510 and may be used as an indication of the resistance of the piezoresistive element 105 and of the pressure. In each of FIGs. 5A - 50, the piezoresistive element 105 may be in the parenchyma or in a cerebral ventricle.

[0050] In the embodiment of FIG. 5B, current paths between the internal electrodes 115 and the external electrodes 510 are relatively short, with, e.g., each of the internal electrodes 115 being just inside the skin and the corresponding external electrode 510 being just outside the skin. In the embodiment of FIG. 5C, the resistor is on the interior of the skull, and the current paths between the internal electrodes 115 and the external electrodes 510 may be relatively long, extending, for example, through brain tissue; in such an embodiment the system may be able to sense properties of the tissue through which the current propagates, in addition to sensing the pressure. In each of FIGs. 5A - 5C, each of the external electrodes 510 may be a conductive patch placed on the subject’s skin to provide (i) increased surface area for conduction through the skin, and (ii) a current path through the skin having correspondingly lower resistance.

[0051] In some embodiments the piezoresistive element 105 uses a piezoresistive material different from a matrix of polydimethylsiloxane (PDMS) containing a plurality of carbon nanotubes (CNTs). For example, the piezoresistive element 105 may include (e.g., be composed of) a doped semiconductor material (e.g., doped silicon), silicon carbide (or another piezoresistive ceramic), or a suitable (piezoresistive) metal alloy. In some embodiments, the pressure sensor includes a sealed cavity filled with gas (e.g., with air or with nitrogen) and the piezoresistive element 105 forms part of the container forming the cavity, so that the piezoresistive element 105 may deform in shear (e.g., it may flex inward when the external pressure increases and flex outward when the external pressure decreases). Such an embodiment may result in a different rate of change of resistance of the piezoresistive element 105 with pressure because (i) the shear modulus of the piezoresistive element 105 may be different from the bulk modulus and (ii) the rate of change of resistivity with shear strain may differ from the rate of change of resistivity with compressive strain.

[0052] In some embodiments, the internal electrodes 115 and the external electrodes 510 may be located at some distance from (e.g., on a different part of the subject’s head than) the piezoelectric element 110 (and from the acoustic source 505) so that-10-4933-1832-4538 v.l269517WO (M24-202L-WO1-a) interference (e.g., interference caused by capacitive or inductive coupling) between the drive signal used to drive the acoustic source 505 and the voltage sensed at the external electrodes 510 is reduced. In some embodiments, such interference is also, or instead, reduced by (i) using a pulsed drive signal (produced by a suitable drive circuit), and (ii) sensing the voltage, on the external electrodes 510, after the end of each pulse. For example, the acoustic drive signal may consist of a carrier signal (e.g., a carrier at a carrier frequency between 10 kHz and 100 MHz) that is on-off modulated with a rectangular wave (or “pulse wave”) having a frequency between 100 Hz and 10 kHz and a duty cycle between 1 % and 95%. In such an embodiment, the signal at the external electrodes 510 due to current flowing through the piezoresistive element 105 may persist after the end of each pulse of the pulsed drive signal because, for example, (i) the delay between the emission of the acoustic waves from the acoustic source 505 and their arrival at the piezoelectric element 110 may cause some such waves to arrive after the end of each pulse, or (ii) the capacitance of the piezoelectric element 110 may, at the end of each pulse, store some charge, which may cause a current to continue flowing after the end of the pulse. In some embodiments, the piezoelectric element 110 or the acoustic source 505 has a mechanical resonance with a frequency at or near the carrier frequency, and the piezoelectric element 110 or the acoustic source 505 may continue to ring after the end of the pulse, causing an electrical drive signal to continue to be applied to the piezoresistive element 105 (or to the piezoresistive element 105 and the resistor 515) after the end of the pulse. In an embodiment in which the signal at the external electrodes 510 due to current flowing through the piezoresistive element 105 persists after the end of each pulse of the pulsed drive signal, the signal at the external electrodes 510 may not be significantly affected, after the end of each pulse, by interference from the drive signal, and, as such, the signal from such intervals may be used for pressure sensing.

[0053] As used herein, “a portion of” something means “at least some of’ the thing, and as such may mean less than all of, or all of, the thing. As such, “a portion of” a thing includes the entire thing as a special case, i.e., the entire thing is an example of a portion of the thing. As used herein, when a second quantity is “within Y” of a first quantity X, it means that the second quantity is at least X-Y and the second quantity is at most X+Y.-11 -4933-1832-4538 v.l269517WO (M24-202L-WO1-a)As used herein, when a second number is “within Y%” of a first number, it means that the second number is at least (1 -Y / 100) times the first number and the second number is at most (1 +Y / 100) times the first number. As used herein, the word “or” is inclusive, so that, for example, “A or B” means any one of (i) A, (ii) B, and (iii) A and B.

[0054] As used herein, when a method (e.g., an adjustment) or a first quantity (e.g., a first variable) is referred to as being “based on” a second quantity (e.g., a second variable) it means that the second quantity is an input to the method or influences the first quantity, e.g., the second quantity may be an input (e.g., the only input, or one of several inputs) to a function that calculates the first quantity, or the first quantity may be equal to the second quantity, or the first quantity may be the same as (e.g., stored at the same location or locations in memory as) the second quantity.

[0055] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the inventive concept. As used herein, the terms “substantially,” “about,” and similar terms are used as terms of approximation and not as terms of degree, and are intended to account for the inherent deviations in measured or calculated values that would be recognized by those of ordinary skill in the art.

[0056] As used herein, the term “major component” refers to a component that is present in a composition, polymer, or product in an amount greater than an amount of any other single component in the composition or product. In contrast, the term “primary component” refers to a component that makes up at least 50% by weight or more of the composition, polymer, or product. As used herein, the term “major portion”, when applied to a plurality of items, means at least half of the items. As used herein, any structure or layer that is described as being “made of” or “composed of” a substance should be understood (i) in some embodiments, to contain that substance as the primary component or (ii) in some embodiments, to contain that substance as the major component.

[0057] Any numerical range recited herein is intended to include all sub-ranges of the same numerical precision subsumed within the recited range. For example, a range of "1.0 to 10.0" or “between 1.0 and 10.0” is intended to include all subranges between (and including) the recited minimum value of 1 .0 and the recited maximum value of 10.0,-12-4933-1832-4538 v.l269517WO (M24-202L-WO1-a) that is, having a minimum value equal to or greater than 1 .0 and a maximum value equal to or less than 10.0, such as, for example, 2.4 to 7.6. Similarly, a range described as “within 35% of 10” is intended to include all subranges between (and including) the recited minimum value of 6.5 (i.e. , (1 - 35 / 100) times 10) and the recited maximum value of 13.5 (i.e., (1 + 35 / 100) times 10), that is, having a minimum value equal to or greater than 6.5 and a maximum value equal to or less than 13.5, such as, for example, 7.4 to 10.6. Any maximum numerical limitation recited herein is intended to include all lower numerical limitations subsumed therein and any minimum numerical limitation recited in this specification is intended to include all higher numerical limitations subsumed therein.

[0058] It will be understood that when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. As used herein, “generally connected” means connected by an electrical path that may contain arbitrary intervening elements, including intervening elements the presence of which qualitatively changes the behavior of the circuit. As used herein, “connected” means (i) “directly connected” or (ii) connected with intervening elements, the intervening elements being ones (e.g., low-value resistors or inductors, or short sections of transmission line) that do not qualitatively affect the behavior of the circuit.

[0059] Some embodiments may include features of the following numbered clauses.1 . A system, comprising: a pressure sensor, the pressure sensor comprising: a piezoresistive element; and a piezoelectric element connected to the piezoresistive element.2. The system of clause 1 , wherein the piezoresistive element comprises a plurality of carbon nanotubes in a polydimethylsiloxane matrix.3. The system of clause 1 or clause 2, wherein the pressure sensor further comprises a resistor connected in series with the piezoresistive element.-13-4933-1832-4538 v.l269517WO (M24-202L-WO1-a)4. The system of any one of the preceding clauses, further comprising two conductive wires in contact with different locations on the piezoresistive element.5. The system of clause 4, wherein one of the conductive wires is composed of carbon nanotubes.6. The system of any one of the preceding clauses, further comprising a polydimethylsiloxane coating over the piezoresistive element.7. The system of any one of the preceding clauses, wherein the piezoresistive element is at a proximal end of a catheter.8. The system of clause 7, further comprising two conductive wires in contact with different locations on the piezoresistive element and extending longitudinally along the catheter.9. The system of any one of the preceding clauses, further comprising a first electrode and a second electrode.10. The system of clause 9, wherein: the piezoresistive element has a first end and a second end; the first electrode is in electrical contact with the first end; and the second electrode is in electrical contact with the second end.11 . The system of any one of the preceding clauses, wherein the piezoelectric element is connected in parallel with the piezoresistive element.12. The system of any one of the preceding clauses, wherein: the piezoresistive element is a hollow cylinder; and the piezoelectric element is inside the piezoresistive element.-14-4933-1832-4538 v.l269517WO (M24-202L-WO1-a)13. A system, comprising: a pressure sensor comprising a piezoresistive element; a first electrode connected to a first conductive wire; and a second electrode connected to a second conductive wire, the first electrode being in contact with a first location on the piezoresistive element, and the second electrode being in contact with a second location on the piezoresistive element.14. The system of clause 13, wherein the first conductive wire is composed of a material selected from the group consisting of carbon, platinum, indium, gold, indium tin oxide, graphene, titanium and stainless steel.15. The system of clause 13 or clause 14, wherein the first conductive wire is composed of carbon nanotubes.16. The system of any one of clauses 13 to 15, further comprising a polydimethylsiloxane coating over the piezoresistive element.17. The system of any one of clauses 13 to 16, further comprising a piezoelectric element connected to the piezoresistive element.18. The system of any one of clauses 13 to 17, wherein: the piezoresistive element has a first end and a second end; the first electrode is in electrical contact with the first end; and the second electrode is in electrical contact with the second end.19. The system of any one of clauses 13 to 18, comprising a piezoelectric element, wherein the piezoelectric element is connected in parallel with the piezoresistive element.-15-4933-1832-4538 v.l269517WO (M24-202L-WO1-a)20. The system of any one of clauses 13 to 19, comprising a piezoelectric element, wherein: the piezoresistive element is a hollow cylinder; and the piezoelectric element is inside the piezoresistive element.

[0060] Although exemplary embodiments of an implantable pressure sensor have been specifically described and illustrated herein, many modifications and variations will be apparent to those skilled in the art. Accordingly, it is to be understood that an implantable pressure sensor constructed according to principles of this disclosure may be embodied other than as specifically described herein. The invention is also defined in the following claims, and equivalents thereof.-16-4933-1832-4538 v.l

Claims

269517WO (M24-202L-WO1-a)WHAT IS CLAIMED IS:1 . A system, comprising: a pressure sensor, the pressure sensor comprising: a piezoresistive element; and a piezoelectric element connected to the piezoresistive element.

2. The system of claim 1 , wherein the piezoresistive element comprises a plurality of carbon nanotubes in a polydimethylsiloxane matrix.

3. The system of claim 1 , wherein the pressure sensor further comprises a resistor connected in series with the piezoresistive element.

4. The system of claim 1 , further comprising two conductive wires in contact with different locations on the piezoresistive element.

5. The system of claim 4, wherein one of the conductive wires is composed of carbon nanotubes.

6. The system of claim 4, further comprising a polydimethylsiloxane coating over the piezoresistive element.

7. The system of claim 1 , wherein the piezoresistive element is at a proximal end of a catheter.

8. The system of claim 7, further comprising two conductive wires in contact with different locations on the piezoresistive element and extending longitudinally along the catheter.

9. The system of claim 8, further comprising a first electrode and a second electrode.-17-4933-1832-4538 v.l269517WO (M24-202L-WO1-a)10. The system of claim 9, wherein: the piezoresistive element has a first end and a second end; the first electrode is in electrical contact with the first end; and the second electrode is in electrical contact with the second end.11 . The system of claim 10, wherein the piezoelectric element is connected in parallel with the piezoresistive element.

12. The system of claim 11 , wherein: the piezoresistive element is a hollow cylinder; and the piezoelectric element is inside the piezoresistive element.

13. A system, comprising: a pressure sensor comprising a piezoresistive element; a first electrode connected to a first conductive wire; and a second electrode connected to a second conductive wire, the first electrode being in contact with a first location on the piezoresistive element, and the second electrode being in contact with a second location on the piezoresistive element.

14. The system of claim 13, wherein the first conductive wire is composed of a material selected from the group consisting of carbon, platinum, iridium, gold, indium tin oxide, graphene, titanium and stainless steel.

15. The system of claim 14, wherein the first conductive wire is composed of carbon nanotubes.

16. The system of claim 13, further comprising a polydimethylsiloxane coating over the piezoresistive element.-18-4933-1832-4538 v.l269517WO (M24-202L-WO1-a)17. The system of claim 13, further comprising a piezoelectric element connected to the piezoresistive element.

18. The system of claim 17, wherein: the piezoresistive element has a first end and a second end; the first electrode is in electrical contact with the first end; and the second electrode is in electrical contact with the second end.

19. The system of claim 18, wherein the piezoelectric element is connected in parallel with the piezoresistive element.

20. The system of claim 19, wherein: the piezoresistive element is a hollow cylinder; and the piezoelectric element is inside the piezoresistive element.-19-4933-1832-4538 v.l

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