Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/02—Detecting, measuring or recording for evaluating the cardiovascular system, e.g. pulse, heart rate, blood pressure or blood flow
  • A61B5/021—Measuring pressure in heart or blood vessels
  • A61B5/0215—Measuring pressure in heart or blood vessels by means inserted into the body
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/68—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient
  • A61B5/6846—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient specially adapted to be brought in contact with an internal body part, i.e. invasive
  • A61B5/6879—Means for maintaining contact with the body
  • A61B5/6882—Anchoring means

Definitions

• This invention relates to chronically implanted sensors for wirelessly sensing pressure, temperature, and/or other physical properties within the human body. More particularly, the invention concerns a wireless, un-powered micromachined blood pressure sensor that can be delivered using endovascular or simple surgical techniques to the interior of a human artery.
• Systemic arterial blood pressure measurement provides important diagnostic and health monitoring information, especially for people at risk for hypertension. Blood pressure is also an important measurement in most animal research studies. Intravascular measures of blood pressure, typically via a pressure sensor mounted on a catheter inserted directly into a blood vessel, are considered the "gold standard" for measurement accuracy; however, these intravascular measures require invasive surgery and patient immobilization and cannot be used for simple diagnostic or chronic measurements.
• MEMS Micro- Electro-Mechanical Systems
• a number of patents detail pressure sensors (some capacitive in nature, some manufactured using MEMS-based technology) that are specifically designed for implantation into the human body. These sensors suffer from many of the limitations already mentioned with the additional concern that they require either the addition of a power source to operate the device or a physical connection to a device capable of translating the sensor output into a meaningful display of a physiologic parameter. [0007] To overcome these two problems (power and physical connection), the concept of an externally modulated LC circuit has been applied to development of implantable pressure sensors. Of a number of patents that describe a sensor design of this nature, Chubbuck, U.S. Patent No. 6,113,553 is a representative example.
• the device embodied by the Chubbuck patent is manufactured using conventional techniques, thus requiring surgical implantation and thus limiting its applicability to areas that are easily accessible to surgery (e.g., the skull).
• An ideal method of accomplishing all of the above objectives would be to place a device capable of measuring pressure within or adjacent to an artery.
• a healthcare provider or patient will obtain an immediate readout of blood pressure, which could averaged over time or tracked for diurnal variation.
• FIG. 6 An example of an implantable pressure sensor designed to monitor blood is shown in Kensey et al, U.S. Patent No. 6,015,386. While this sensor accomplishes some of the above objectives, it has multiple problems that would make its use impractical.
• the sensor disclosed in the Kensey patent relies on a mechanical sensing element. Elements of this kind cannot be practically manufactured in dimensions that would allow for endovascular introduction.
• this type of pressure sensor would be subject to many problems in use that would limit its accuracy and reliability.
• One example would be exposure of the mechanical sensing element to body fluids or tissue ingrowth that could disrupt its function.
• the device fails to account for vascular remolding which would result in baseline drift and could render the device inoperable, as the device requires that the artery be permanently deformed by the clamping action of the sensing element.
• a biocompatible, wireless, un-powered pressure sensor that for the purposes of introduction and delivery within the human artery can be manipulated into a smaller shape and size by rolling or folding it into a reduced diameter form and loaded into a small diameter catheter. Then, upon positioning the catheter in the desired location, the sensor can be deployed and secured to the interior of the artery.
• the present invention describes a sensor that can be fabricated using micro- machining techniques and can be implanted into the human body using non-surgical methods for the measurement of physical parameters.
• Specific target locations could include the interior or exterior of a blood vessel, such as the aorta (preferably just below the renal arteries), or the femoral or the brachial artery.
• the device is implanted in the arm (radial or brachial artery), as the relative proximity of these arteries to the surface allows for further reduction in sensor size and ease of taking a blood pressure reading.
• blood pressure measurements in the brachial artery correlate well with aortic blood pressures.
• the sensor according to the invention is fabricated using MicroElectroMechanical Systems (MEMS) technology, which allows the creation of a flexible device that is small, accurate, precise, durable, robust, biocompatible, radiopaque and insensitive to changes in body chemistry, biology or external pressure. This device will not require the use of wires to relay pressure information externally nor need an internal power supply to perform its function.
• MEMS MicroElectroMechanical Systems
• the pressure sensor can be manufactured using Micro-machining techniques that were developed for the integrated circuit industry.
• An example of this type of sensor features an inductive-capacitive (LC) resonant circuit with a variable capacitor and is described in Allen et al., U.S. Patent No. 6,111,520, incorporated herein by reference.
• LC inductive-capacitive
• the pressure sensor is made of completely passive components having no active circuitry or power sources such as batteries.
• the pressure sensor is completely self-contained, having no leads to connect to an external circuit or power source.
• these same manufacturing techniques can be used to add additional sensing capabilities, such as the ability to measure temperature by the addition of a resistor to the basic LC circuit.
• the pressure sensor When introduced into artery, can provide pressure related data by use of an external measuring device.
• an external measuring device As disclosed in the Allen et al. patent, several different excitation systems can be used.
• the sensor can be electromagnetically coupled to a transmitting antenna. Consequently, a current is induced in the sensors, which oscillates at the resonant frequency of the sensor. This oscillation causes a change in the frequency spectrum of the transmitted signal. From this change, the bandwidth and resonant frequency of the particular sensor may be determined, from which the corresponding change in pressure can be calculated.
• the present invention provides for a transmit and receive system and method for determining the resonant frequency and bandwidth of a resonant circuit within a particular sensor.
• an excitation signal of white noise or predetermined multiple frequencies is transmitted from a transmitting antenna, the sensor being electromagnetically coupled to the transmitting antenna.
• a current is induced in the resonant circuit of the sensor as it absorbs energy from the transmitted excitation signal, the current oscillating at the resonant frequency of the resonant circuit.
• a receiving antenna also electromagnetically coupled to the transmitting antemia, receives the excitation signal minus the energy which was absorbed by the sensor.
• the power of the received signal experiences a dip or notch at the resonant frequency of the sensor.
• the resonant frequency and bandwidth are determined from this notch in the power.
• Yet another system and method for determining the resonant frequency and bandwidth of a resonant circuit within a particular sensor includes a chirp interrogation system.
• This system provides for a transmitting antenna which is electromagnetically coupled to the resonant circuit of the sensor.
• An excitation signal of white noise or predetermined multiple frequencies is applied to the transmitting antenna for a predetermined period of time, thereby inducing a current in the resonant circuit of the sensor at the resonant frequency.
• the system listens for a return signal which radiates from the sensor.
• the resonant frequency and bandwidth of the resonant circuit are determined from the return signal.
• the chirp interrogation method for determining the resonant frequency and bandwidth of a resonant circuit within a particular sensor includes the steps of transmitting a multi-frequency signal pulse from a transmitting antenna, electromagnetically coupling a resonant circuit on a sensor to the transmitting antenna, thereby inducing a current in the sensor circuit, listening for and receiving a return signal radiated from the sensor circuit, and determining the resonant frequency and bandwidth from the return signal.
• the analog method for determining the resonant frequency and bandwidth of a resonant circuit within a particular sensor includes the steps of generating a transmission signal using a tank circuit which includes a transmitting antenna, modifying the frequency of the transmission signal by electromagnetically coupling the resonant circuit of a sensor to the transmitting antenna, and converting the modified transmission signal into a standard signal for further application.
• the slit By cutting the longitudinal slit at angle that is offset from the main axis of the outer tube, the sensor will be biased into a planar configuration as it is forced through the slit during the deployment process.
• the sensor ring shaped or flat
• the sensor could be crimped or otherwise mounted on an intravascular balloon catheter, common in the art, and delivered to the target location. This balloon catheter is then inflated, forcing the sensor in contact with the vessel wall where it attaches as previously described.
• Fig. 2 is a lateral view of the embodiment of the invention shown in Fig. 1;
• Fig. 3 is a lateral view of an embodiment of the invention of Fig. 1 folded for delivery;
• FIG. 9 is a schematic representation of an embodiment of the invention with distributed capacitance
• FIG. 15 is a schematic representation of another, preferred embodiment of the invention.
• Fig. 16 is a partly cross-sectional view of a preferred delivery system according to the invention.
• Fig. 22 is a block diagram of an electrical circuit useful according to the invention.
• FIG. 1 One embodiment of a sensor according to the invention is shown in Figures 1, 2, and 3, where a disc-shaped sensor 10 comprises a capacitor disk 12 and a wire spiral 14.
• Figure 2 is a lateral view of sensor 10
• Figure 3 is a lateral view of sensor 10 in a folded configuration for insertion.
• sensor 10 is sufficiently flexible to be folded as shown in Figure 4 is an important aspect of the invention.
• a ring 20 comprised of a shape memory alloy such as nitinol has been attached to, for example, with adhesive, or incorporated into, for example, layered within, a sensor 22.
• Figure 5 is a lateral cross-sectional view of a circular sensor 30 having a ring 32 comprised of a shape memory alloy such as nitinol encompassing the outer edge 34 of sensor 30.
• Ring 32 preferably is attached to outer edge 34 by a suitable physiologically acceptable adhesive 36, such as an appropriate epoxy or cyanoacrylate material.
• a suitable physiologically acceptable adhesive 36 such as an appropriate epoxy or cyanoacrylate material.
• the ring will be radiopaque.
• the size of the circular sensors of the invention will vary according to factors such as the intended application, the delivery system, etc.
• the circular sensors are intended to be from about 0.5 to about 3 cm in diameter, with a thickness of from about 0.05 to about 0.30 in.
• the thickness of the ring i.e., the width of the outside surface 38, will preferably be from about 1.5 to about 3.5 times the thickness of the sensor.
• LC inductive-capacitive
• the sensor contains two types of passive electrical components, namely, an inductor and a capacitor.
• the sensor is constructed so that the fluid pressure at the sensor's surface changes the distance between the capacitor's parallel plates and causes a variation of the sensor's capacitance.
• the senor of the invention is constructed by laminating several layers of material together, as shown, for example, in Figure 8.
• a first layer 142 is fabricated from a sheet of polyimide film (e.g., KAPTON, available from Du Pont) upon which a micro-machined copper pattern 144 is deposited.
• Pattern 144 preferably consists of a circular conductive segment in the center of the sheet surrounded by a spiral coil.
• a second layer 148 comprises a sheet of flexible adhesive through which hole 150 has been cut in the center. (Optionally there may be more than one such layer 148.)
• a final layer 152 is another sheet of polyimide film with a copper pattern 154 that is a mirror image of pattern 144.
• the first, second, and third layers are aligned such that the holes in the middle adhesive layers are centered between the circular conductive segments in the middle of the two outer polyimide layers 142 and 152.
• a capacitor defined as an electric circuit element used to store charge temporarily, consisting in general of two metallic plates separated and insulated from each other by a dielectric
• the two metal spirals on the polyimide sheets 142 and 152 form an inductor component of a miniature electrical circuit.
• This frequency is a function of the capacitance of the device. Therefore, if the sensor's capacitance changes, so will the frequency at which it minimally absorbs energy from the readout device. Since the sensor's capacitance is mechanically linked to the fluid pressure at the sensor's surface, a measurement of this frequency by the readout device gives a relative measurement of the fluid pressure. If calibration of the device is performed, then an absolute measurement of pressure can be made. See, for example, the extensive discussion in the Allen et al. patent, again incorporated herein by reference, as well as Gershenfeld et al., U.S. Patent No. 6,025,725, incorporated herein by reference.
• the capacitor element consists of two plates that are separated by a suitable dielectric material, such as air, inert gas, fluid or a vacuum.
• a suitable dielectric material such as air, inert gas, fluid or a vacuum.
• various coatings could be applied to the surface or between the polymeric layers used to form the sensor. These coating can he used to provide a hermetic seal that will prevent leakage of body fluids into the cavity or permeation of the cavity material (gas, vacuum or fluid) out of the sensor.
• a sensor 170 has a multitude of capacitors 175 formed either as separate elements or as an array.
• the device is constructed using multiple layers upon lie the necessary circuit elements. Disposed on the top and bottom layer are metal patterns constructed using micro-machining techniques which define a top and bottom conductor and a spiral inductor coil. To provide for an electrical contact between the top and bottom layers small vias or holes are cut through the middle layers. When the layers are assembled, a metal paste is forced into the small vias to create direct electrical connections or conduits.
• a vialess operational LC circuit can be created. This absence of via holes represents a significant improvement to the sensor in that it simplifies the manufacturing process and, more importantly, significantly increases the durability of the sensor making it more appropriate for use inside the human body.
• Figure 10 is a partial cross-sectional review of the sensor shown in Figure 8, where first layer 142, second layer 148, and third layer 152 are sandwiched together.
• a cylindrical space 156 comprises a pressure sensitive capacitor. No via holes are present.
• the sensor 178 shown in Figure 11 comprises a first polyimide layer 180, a second, adhesive layer 182, and a third, polyimide layer 184.
• First layer 180 has a copper pattern comprising a coil 186 and a disk 188, and third layer 184 comprises a coil 190 and a disk 192.
• a cylindrical space 196 comprises a pressure sensitive capacitor.
• a diode 194 connected between coils 186 and 190 creates a non-linear sensor, i.e., a sensor where the frequency change is non-linear as compared to a change in pressure.
• a foldable sensor is delivered to a patient's artery in the distal end of a delivery catheter.
• the sensor can be regularly- or irregularly- shaped so that outer portions of the sensor can fold to about a 90° angle as compared to a relatively flat, middle portion of the sensor.
• FIG. 12 Another embodiment of a sensor is shown in Figure 12, where circular sensor 230 comprises flexible cut-outs 232.
• the first outer layer 234 comprises a polymide substrate with a copper pattern comprising a coil 240 and several, from 2 to 6, disks 242 to form pressure sensitive capacitors.
• Sensor 230 also comprises at least one adhesive layer (not shown) and a third outer layer corresponding to the first outer layer (not shown).
• Preferably sensor 230 has at least one diode connecting the copper coils of the first and third layers.
• the flexible cut-outs 232 facilitate, among other things, folding of sections of sensor 230 for placement in, or arrangement upon, a delivery catheter, such as in Figure 13.
• the sections can also be folded to create either a "Z" shape or, for example, a "U” shape, for other applications. It is within the scope of the invention that variously numbered and shaped cut-outs could be used for particular applications.
• a preferred delivery system is described above, it is within the scope of the invention that other delivery systems could be employed. Other such delivery systems are described in, for example, co-pending, commonly assigned U.S. patent application Serial No. 10/054,671, filed January 22, 2002, incorporated herein by reference.
• a preferred embodiment of the invention and a preferred delivery system are described in Figs. 15 to 20.
• a pressure sensor 250 has a slightly curved cross-section in a lateral direction 252.
• dilatation balloon 262 of dilatation balloon catheter 258 has been inflated to press the outer surface of sensor 250 against the inner wall 272 of artery 268.
• sensor 250 remains, attached to inner wall 272.
• a lateral cross-sectional view across line 20-20 is shown in Fig. 20.
• the transmitted energy will decay exponentially as it travels away from the sensor, the lower the energy available to be transmitted, the faster it will decay below a signal strength that can be detected by the receiving antenna and the closer the sensor needs to be situated relative to the receiving electronics. In general then, the lower the Q, the greater the energy loss and the shorter the distance between sensor and recieving antenna.
• the Q of the sensor will be dependent on multiple factors such as the shape, size, diameter, number of turns, spacing between turns and cross-sectional area of the inductor component. In addition, Q will be greatly affected by the materials used to construct the sensors. Specifically, materials with low loss tangents will provide the sensor with higher Q factors.
• the implantable sensor accending to the invention is preferably constructed of various polymers that provide the required flexibility, biocompatibility and processing capabilities.
• the materials used are flexible, biocompatible, and result in a high Q factor.
• KAPTON a polyimide
• suitable materials include polyimides, polyesters (e.g., polyethylene terephthalate), liquid crystal polymers (LCP), and polytetrafluoroethylene (PTFE) and copolymers thereof.
• a thin (i.e., 200 micron) coating of silicone was applied to the LCP sensor detailed above.
• This coating provided sufficient insulation to maintain the Q at 40 in a conductive medium. Equally important, despite the presence of the silicone, adequate sensitivity to pressure changes was maintained and the sensor retained sufficient flexibility to be folded for endovascular delivery.
• One additional benefit of the silicone encapsulation material is that it can be loaded with a low percentage (i.e., 10 - 20%) of radio-opaque material (e.g., barium sulfate) to provide visibility when examined using fluoroscopic x-ray equipment. This added barium sulphate will not affect the mechanical and electrical properties of the silicone.
• the display may be created by integrating a commercially available hand-held computing device such as a Palm® or micro-PC into the electronic circuitry and using this device's display unit as the visual interface between the equipment and its operator.
• a further advantage of this approach is that the hand-held computer could be detached from the read-out unit and linked to a standard desktop computer. The information from the device could thus be downloaded into any of several commercially available data acquisition software programs for more detailed analysis or for electronic transfer via hard media or the internet to a remote location.
• the electronics could be reduced is size such that they are capable of being formed into a band that could be placed around the wrist or leg directly above the location of the implanted sensor. In this manner, continuous readings of pressure could be made and displayed.
• the present invention provides for an impedance system and method of determining the resonant frequency and bandwidth of a resonant circuit within a particular sensor.
• the system includes a transmitting antenna, which is coupled to an impedance analyzer.
• the impedance analyzer applies a constant voltage signal to the transmitting antenna scanning the frequency across a predetermined spectrum.
• the current passing through the transmitting antenna experiences a peak at the resonant frequency of the sensor.
• the resonant frequency and bandwidth are thus determined from this peak in the current.
• the method of determining the resonant frequency and bandwidth using an impedance approach may include the steps of transmitting an excitation signal using a transmitting antenna and electromagnetically coupling a sensor having a resonant circuit to the transmitting antenna thereby modifying the impedance of the transmitting antenna. Next, the step of measuring the change in impedance of the transmitting antenna is performed, and finally, the resonant frequency and bandwidth of the sensor circuit are determined. [0098] In addition, the present invention provides for a transmit and receive system and method for determining the resonant frequency and bandwidth of a resonant circuit within a particular sensor.
• an excitation signal of white noise or predetermined multiple frequencies is transmitted from a transmitting antenna, the sensor being electromagnetically coupled to the transmitting antenna.
• a current is induced in the resonant circuit of the sensor as it absorbs energy from the transmitted excitation signal, the current oscillating at the resonant frequency of the resonant circuit.
• a receiving antenna also electromagnetically coupled to the transmitting antenna, receives the excitation signal minus the energy which was absorbed by the sensor.
• the power of the received signal experiences a dip or notch at the resonant frequency of the sensor. The resonant frequency and bandwidth are determined from this notch in the power.
• the transmit and receive method of determining the resonant frequency and bandwidth of a sensor circuit includes the steps of transmitting a multiple frequency signal from transmitting antenna, and, electromagnetically coupling a resonant circuit on a sensor to the transmitting antenna thereby inducing a current in the sensor circuit. Next, the step of receiving a modified transmitted signal due to the induction of current in the sensor circuit is performed. Finally, the step of determining the resonant frequency and bandwidth from the received signal is executed.
• a representative block diagram of an electrical circuit that can be used to interrogate the sensor and determine the resonant frequency is shown in Fig. 22.
• a transmitter and receiver i.e., a transceiver 322
• Transceiver 322 is an electronic or digital connection with a phase detector 330, a microprocessor 332, and a frequency synthesizer 334.
• Microprocessor 332 is in turn connected to an interface 336 such as a terminal.
• Power supply 338 regulates and provides electrical power to the system.
• the invention further includes an alternative method of measuring pressure in which a non-linear element such as a diode or polyvinylidenedifloride piezo-electric polymer, is added to the LC circuit.
• a non-linear element such as a diode or polyvinylidenedifloride piezo-electric polymer
• a diode with a low turn-on voltage such as a Schottky diode can be fabricated using micro-machining techniques.
• the presence of this non-linear element in various configurations within the LC circuit can be used to modulate the incoming signal from the receiving device and produce different harmonics of the original signal.
• the read-out circuitry can be tuned to receive the particular harmonic frequency that is produced and use this signal to reconstruct the fundamental frequency of the sensor.
• the advantage of this approach is two-fold; the incoming signal can be transmitted continuously and since the return signal will be at different signals, the return signal can also be received continuously.
• One additional concern regarding devices designated for long term implantation in the human body is maintenance of electrical stability over time as the environment the sensor has been placed in changes. Under this scenario the sensor's accuracy may drift from its original baseline. It would thus be desirable to have available to the user of the device, a method for determining if the sensor is functioning properly and also to be able to recalibrate the device anytime after it has been implanted.
• This invention therefore also includes a method of using acoustic energy to challenge the sensor and determining to what degree (if any) sensor performance has been degraded. In this method, energy in the ultrasound range is directed towards the sensor and a measurement is made of the mechanical resonance of the sensor membrane. This same measurement can be made at point after the sensor has been implanted.
• a determination of the degree of change in mechanical resonance frequency can be established. This value can then be used to create a calibration factor that can be applied to the pressure reading taken post-implantation in order to adjust the measured value to reflect the actual pressure within the artery.

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• Health & Medical Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• Surgery (AREA)
• Biophysics (AREA)
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• Engineering & Computer Science (AREA)
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• Physiology (AREA)
• Measuring Pulse, Heart Rate, Blood Pressure Or Blood Flow (AREA)
• Measuring And Recording Apparatus For Diagnosis (AREA)
• Measuring Fluid Pressure (AREA)

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• 2003
  • 2003-08-07 AU AU2003265380A patent/AU2003265380A1/en not_active Abandoned
  • 2003-08-07 CA CA002494989A patent/CA2494989A1/en not_active Abandoned
  • 2003-08-07 WO PCT/US2003/024751 patent/WO2004014456A2/en not_active Ceased
  • 2003-08-07 EP EP03785007A patent/EP1545303A4/de not_active Withdrawn
• 2009
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