Patent Publication Number: US-2012041310-A1

Title: Apparatus, System and Method for Ultrasound Powered Neurotelemetry

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made in part with government support under Grant No. 5R21NS063213-01 awarded by the National Institute of Health. The United States Government has certain rights in the invention. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present embodiments relate generally to biomedical engineering and, more particularly, to an apparatus, system, and method for ultrasound powered neurotelemetry. 
     2. Description of Related Art 
     Recording of bioelectrical event from the brain, spine, and nervous system in a wireless and minimally invasive manner is an important capability that has received much attention by the National Institute of Health (NIH) in recent years. Investigations of the neural system of the body have been made possible by modern electrophysiological tools. However, such tools have been fundamentally limited with respect to therapeutic uses that go beyond mere research because such devices typically require wires to communicate information. Wires are not desirable and can be sites of infection, mechanical failure, and present dangers of being scraped by abrasion or caught and torn by clothing or environmental objects. Effective biotelemetry obviates the need to pass neural carrier signals through wired connectors on the skin or skull. 
     There have been some advances in miniature telemetry applications for bioelectrical recording, mostly including batteries or inductive power coupling. There is wide recognition that batteries are undesirable in wireless implant applications and that powering techniques must be by other techniques such as Radio Frequency (RF) induction. Heetderks (1988) performed some early work that examined the limitations on inductive power coupling between two separated loop antennas, one external and one internal to the body at various frequencies up to 20 MHz. There are some fundamental limitations on this process relative to the needed and relatively large size of the implanted antenna size for at-depth applications. 
     Sophisticated analog and more recently digital circuitry mated to wireless telemetry have been reported for neuroprostheses. A review of this activity has been conducted by Wise et al. (2004). Present methods of achieving multichannel wireless interfaces involve silicon VSLI circuitry and are relatively complex devices involving arrays of high performance bioamplifiers, multiplexers, and wideband RF communication. These devices tend to have thermal dissipation problems, and supplying power to neuroprosthetics becomes a major issue. 
     The use of passive RF circuitry for biotelemetry has a long history. These devices typically use changes in mutual inductance or reflected impedance between two resonant circuits. Passive techniques have the advantage of low power needs and the potential for reduced dependency on RF power induction for active circuitry. In 1986 Towe (1986) demonstrated a low power quasi-passive technique of resonant frequency shifting to telemeter analog bioelectrical waveforms on a subcarrier. The NIH has supported the development of passive biotelemetry devices at the WIMS center at the University of Michigan (http://www.wimserc.org/). There have been reports by Najafi, Wise, and others at Michigan (Harpster et al., 2002; Takahata et al., 2003) of passive telemetry applications for humidity, for stents, as well as for parameters such as pressure (DeHennis et al., 2002). 
     Recently Towe (2007) presented a method to considerably reduce the complexity of passive telemetry by exploiting the unique properties of semiconductor RF varactor diodes. This wireless biotelemetry system is similar to the technology of RF-ID tags and presents an RF backscatter method to telemeter low level bioelectric events over short distances, without the use of integrated bioamplifiers or conventional transmitters. The approach employs the voltage-variable capacitance function of varactor diodes to allow biopotentials to directly alter the tuning of an Inductive/Capacitive (LC) resonant circuit. The tradeoff is the need for relatively more complex synchronous carrier demodulation schemes external to the body. 
     The human body can be electrically modeled as a volume conducting medium. Natural or artificial current sources in the interior of the body will thereby produce skin surface potentials. This principle has been used for biotelemetry by Mingui et al. (2003) and Linsey et al. (1998) by using implanted amplifiers connected to sensors or biopotential electrodes and then driving relatively higher local currents in tissues to cause large signals at the skin surface. Difficulties include the achievement of multichannel operation, the relatively large bulk of devices reported so far, and the need for induced power to run the amplifier. 
     This application incorporates by reference provisional patent application No. 60/916,152 filed on May 4, 2007 in its entirety. 
     SUMMARY 
     Multichannel, totally integrated neuro-recording by ultra-miniature wireless systems is a long-sought goal in neuroengineering. It would allow us to achieve multiple simultaneous recordings of bioelectrical events such as to constitute a map of the activity at multiple sites. Mapping would allow a more complete understanding of ensembles of activity that are further apart than a few millimeters of each other and so useful to record from multiple sites in the brain such as motor and sensory centers spine, or nervous system. 
     Ultraminiature wireless bioelectric monitoring tools could be useful and important in design of neuroprosthetics medical rehabilitation, diagnostics, therapeutics, and to the relatively new field of man-machine interfaces. It is widely recognized that microminiature wireless interfaces to the body interior would enhance the development of advanced neural interfaces leading to prostheses in many forms. 
     The present embodiments provide an apparatus, system, and method for ultrasound powered neurotelemetry. In one embodiment, the apparatus includes a piezoelectric element configured to receive an ultrasonic pulse and convert the electronic pulse into an electric potential. A diode may be coupled to the piezoelectric element, the diode configured to cause an electric current to flow in response to the electric potential. The apparatus may additionally include a reference electrode and a stimulating electrode coupled to the diode. The reference electrode may sense bioelectric activity in a region of body tissue located in proximity to the reference diode. The stimulating electrode may emit a carrier signal, wherein the carrier signal is modulated in response to the bioelectric activity sensed by the reference electrode. 
     In a further embodiment, the apparatus may include a housing configured to house the piezoelectric element and the diode. The housing may also house at least a portion of both the reference electrode and the stimulating electrode. The housing may reduce potential infection due to immune system response to the apparatus. 
     In one embodiment, the diode may be a semiconductive mixer diode. The reference electrode may be coupled to a cathode portion of the diode and the stimulating electrode may be coupled to an anode portion of the diode. Additionally, the piezoelectric element may apply an electric potential to the diode that is slightly below the threshold voltage of the diode. In another embodiment, the diode may be zero-potential biased. In a further embodiment, the diode may be further configured to mix a bioelectric signal generated by bioelectric activity sensed by the reference electrode with the carrier signal. 
     An alternative embodiment of an apparatus is also presented. In this embodiment, the apparatus may include a biopotential electrode configured to detect a carrier signal on a skin surface. The apparatus may also include an amplifier coupled to the biopotential electrode, the amplifier configured to amplify the carrier signal across a predetermined frequency range. The apparatus may further include a range gate circuit coupled to the amplifier, the range gate circuit configured to capture the carrier signal within a specified time range. In a further embodiment, the apparatus may include a sample and hold circuit coupled to the range gate circuit, the sample and hold circuit configured to construct a waveform associated with the carrier signal. Additionally, the apparatus may include a bandpass filter coupled to the sample and hold circuit, the bandpass filter configured to smooth the waveform. The apparatus may also include a waveform output device coupled to the bandpass filter, the waveform output device configured to produce a waveform display. 
     A system in accordance with the present embodiments is also presented, the system including an ultrasound source configured to generate an ultrasound pulse, an implant configured to be implanted in body tissue, and a receiver configured to detect the carrier signal. The implant may include a piezoelectric element configured to receive an ultrasonic pulse and convert the electronic pulse into an electric potential, a diode coupled to the piezoelectric element, the diode configured to cause an electric current to flow in response to the electric potential, a reference electrode coupled to the diode, the reference electrode configured to sense bioelectric activity in a region of the body tissue located in proximity to the reference diode, and a stimulating electrode coupled to the diode, the stimulating diode configured to emit an carrier signal, wherein the carrier signal is modulated in response to the bioelectric activity sensed by the reference electrode. 
     A method is also presented in accordance with the present embodiments. In one embodiment, the method includes receiving an ultrasound pulse, converting the ultrasound pulse into an electric potential, causing an electric current to flow through a diode from a reference electrode to a stimulating electrode in response to the electric potential, and emitting an carrier signal from the stimulating electrode, wherein the carrier signal is modulated in response to bioelectric activity in a region of body tissue located in proximity to the reference electrode. 
     A further embodiment of the method may include detecting the carrier signal on a skin surface, amplifying the carrier signal across a predetermined frequency range, capturing the carrier signal within a specified time range, constructing a waveform associated with the carrier signal, smoothing the waveform, and producing a waveform display. 
     The term “coupled” is defined as connected, although not necessarily directly, and not necessarily mechanically. The terms “a” and “an” are defined as one or more unless this disclosure explicitly requires otherwise. The terms “substantially,” “approximately,” “about,” and variations thereof are defined as being largely but not necessarily wholly what is specified, as understood by a person of ordinary skill in the art. In one non-limiting embodiment, the term substantially refers to ranges within 10%, preferably within 5%, more preferably within 1%, and most preferably within 0.5% of what is specified. 
     The terms “comprise” (and any form of comprise, such as “comprises” and “comprising”), “have” (and any form of have, such as “has” and “having”), “include” (and any form of include, such as “includes” and “including”) and “contain” (and any form of contain, such as “contains” and “containing”) are open-ended linking verbs. As a result, a method or device that “comprises,” “has,” “includes” or “contains” one or more steps or elements possesses those one or more steps or elements, but is not limited to possessing only those one or more elements. Likewise, a step of a method or an element of a device that “comprises,” “has,” “includes” or “contains” one or more features possesses those one or more features, but is not limited to possessing only those one or more features. Furthermore, a device or structure that is configured in a certain way is configured in at least that way, but it may also be configured in ways other than those specifically described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       For a more complete understanding of the present embodiments, reference is now made to the following drawings, in which: 
         FIG. 1  is a schematic block diagram illustrating one embodiment of a system for ultrasound powered neurotelemetry; 
         FIG. 2  is a schematic block diagram illustrating another embodiment of a system for ultrasound powered neurotelemetry; 
         FIGS. 3A-3C  are schematic diagrams illustrating various embodiments of an implant for ultrasound powered neurotelemetry; 
         FIG. 4  is a schematic block diagram illustrating one embodiment of a receiver; 
         FIG. 5  is a schematic flowchart diagram illustrating one embodiment of a method for ultrasound powered neurotelemetry; 
         FIG. 6A  is a graph illustrating a voltage response of a diode in accordance with the present embodiments; 
         FIG. 6B  illustrates a response of a piezoelectric element in response to an ultrasound pulse; 
         FIG. 7A  is a frequency measurement of an unmodulated carrier in accordance with the present embodiments; 
         FIG. 7B  is a frequency measurement of a modulated carrier in accordance with the present embodiments; 
         FIG. 8  is a graph of a voltage level as a function of depth of placement of the implant; and 
         FIG. 9  is an illustration of one embodiment of an implant with size comparison reference objects. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     In the following detailed description, reference is made to the accompanying drawings that illustrate embodiments of the present invention. These embodiments are described in sufficient detail to enable a person of ordinary skill in the art to practice the invention without undue experimentation. It should be understood, however, that the embodiments and examples described herein are given by way of illustration only, and not by way of limitation. Various substitutions, modifications, additions, and rearrangements may be made without departing from the spirit of the present invention. Therefore, the description that follows is not to be taken in a limited sense, and the scope of the present invention is defined only by the appended claims. 
     Bioelectrical currents flowing in excitable tissue in the body may be modeled as current sources in the range of tens to hundreds of microamperes and with associated electric fields in the range of microvolts to tens of millivolts in the case of transmembrane potentials. These devices can be understood from volume conductor propagation of a small dipolar current source in tissue that follows well understood rules. The potential V appears on the skin surface as: 
         V=id  cos θ/4 πσr   2  
 
     where i is the current flow over a dipole length d, σ is the medium conductivity, and r is the distance from the center of the dipole to the skin surface. Thus there is a square law loss of the signal strength generated by the current source at depth from the body surface and there is a vector relationship to orientation of the electrode pairs. 
     In the system  100  illustrated in  FIG. 1 , the bioelectrical event waveforms are relayed to the skin for detection by a small implant  104  device that senses local events and then modulates them on an electrical carrier for remote detection at the body surface. 
     The characteristics of p-n junction diodes, such as those that may be suitable for diode  114 , can be substantially varied in their characteristics by biopotentials when reverse biased or when biased near their turn-on threshold. Parameters such as junction capacitance, effective resistance, and nonlinear second harmonic production can all be substantially affected by submillivolt level electrical signals applied to them. This process can be conceived as the diode acting as a (nonlinear) multiplying element. The Shockley equation shows the relationship of the diode forward current to an applied bias voltage. 
         I=I   S ( e   V     D     /(nV     T     ) −1)
 
     where I is the diode current, I s  is a scale factor called the saturation current, V D  is the voltage across the diode, V T  is the thermal voltage, and n is emission coefficient.  FIG. 6A  shows the sharp knee in the i-v curve near threshold. By operating V D  slightly below this point (which moves towards the origin in zero-bias type Shottky diodes) millivolt biopotential signals may amplitude modulate an externally applied and relatively high frequency carrier current also passing through the diode. This process is known as mixing or sometimes as intermodulation when applied to the design of radio devices. This process may be accomplished using high performance low-noise mixer diodes  114 , such as those used in RF communications, at microvolt signal levels. Accordingly, in such an embodiment, the mixing process may not be a significant source of noise or limitation on the biopotential intermodulation process. 
     In one embodiment, a high frequency (megahertz) carrier current signal may be applied to the diode  114  from a small attached piezoelectric element  110 . The piezoelectric element  110  may include a polymer material (PVDF). Alternatively, the piezoelectric element  110  may include a crystalline and ceramic materials such as quartz, barium titanate, lead zirconium titanate (PZT), or the like. The piezoelectric element  110  may be driven to generate an oscillating current through the diode  114  by an ultrasound wave or pulse.  FIG. 1  illustrates one embodiment of an electrical circuit configuration where the impedance of the tissue volume conductivity  120  is in parallel with the mixer diode  114  and piezoelectric element  110 . 
     The carrier current through the diode  114  may be amplitude modulated by a lower frequency (0-10 kHz) signal from local microelectrodes  116 - 118 . When placed in tissue, volume conductivity carries the biopotential modulated carrier current to the surface where it is detected by a second set of surface bioelectrodes (illustrated as elements  214  in  FIG. 2 ). Demodulation of the detected signal reproduces the original biopotential waveform. 
     In such an embodiment, the implant  104  assembly may intermodulate a bioelectrical event on a superimposed high frequency carrier whose energy is obtained piezoelectrically from an ultrasound frequency pressure wave. Over a small change in biopotential, characterized by the impedance value  120 , the changes in the carrier current through the diode  114  may be reasonably linear. At low drive levels, the diode  114  may present a relatively high source impedance to the electrodes  214  which, according to system tests, appears to work satisfactorily. 
       FIG. 7A  shows a spectrum analyzer output when connected to skin surface electrodes  214 . The implant  104  rectifies the 1 MHz ultrasound to the spike seen.  FIG. 7B  shows the effect of driving simulated bioelectric activity in a test setup at microamperes and a frequency of 30 kHz using two small silver electrodes immersed in a fluid tank in a region in proximity to the reference electrode  116  and the stimulating electrode  118 . The spread spectrum with multiple sideband spikes in addition to the main carrier is a clear indication of amplitude intermodulation of the volume conducted current with the carrier current generated by the piezoelectric element  110  in response to the ultrasound beam  108 . 
     Preliminary test data shows a strong intermodulation effect that is exhibited in currents  112  in volume conductors. Effectively, the volume current captures the bioelectrical event of interest. Remote detection and demodulation of the surface-detected carrier  112  may reproduce the bioelectric event waveform. 
     In one embodiment, the intermodulation effect may be relatively frequency independent with modern RF diodes  114 , and at least extends over the range from dc to tens of MHz with typical diodes  114 . This easily encompasses the ultrasound and bioelectrical frequency ranges. 
     For example, the ultrasound source  102  may include an ultrasound transducer. The transducer may generate a variable power ultrasound pulse  106  at a frequency range of about 400 kHz to 5 MHz. The ultrasound pulse power may be varied in order to provide consistent power levels to the implant at varying depths in tissue. The ultrasound pulse  106  may generate sound pressure waves that pass through the skin and other tissue in a body. In one embodiment, the ultrasound source  102  may generate the ultrasound pulse  106  in response to a control signal  108 . The power level and/or frequency of the ultrasound pulse  106  may be determined by a combination of the control signal  108  properties and the characteristics of the ultrasound source  102 . 
     In one embodiment, the piezoelectric element  110  may receive an incident ultrasound pulse  106 . The piezoelectric element  110  may convert the mechanical pressure of the incident ultrasound pulse  106  into electrical power. The electrical power generated by the piezoelectric element  110  may be conducted by one or more conductive plates  112  coupled to the piezoelectric element  110 . The diode  114  may be coupled in electric parallel to the piezoelectric element  110  through the conductive plates  112 . 
     In one embodiment, a reference electrode  116  may be coupled to a cathode portion of the diode  114 . Additionally, a stimulating electrode  118  may be coupled to an anode portion of the diode  114 . In a further embodiment, the implant  104  may be placed in a body, and the reference electrode  116  and the stimulating electrode  118  may be placed in contact with a portion of body tissue. The impedance of the body tissue is represented by an equivalent impedance value  120 . 
     An electric potential may be generated by the piezoelectric element  110  in response to the incident ultrasound pulse  106 . The level of electric potential applied to the diode  114  may put the diode  114  in a state that is near its threshold value. Additionally, the electric potential may cause a current generated by the piezoelectric element  110  to flow from the reference electrode  116 , through the diode  114 , to the stimulating electrode  118 . In this embodiment, the current may have a frequency of around 400 kHz to 5 MHz. For example, the diode  114  may conduct a 2 MHz carrier current in response to the electric potential applied by the piezoelectric element  110 .  FIG. 6B  illustrates one embodiment of a carrier current generated by a piezoelectric element  110  in response to illumination by an ultrasound beam  108  with a frequency of 1 MHz and a power of 10 W/cm 2 .  FIG. 6  B illustrates that the carrier current may also have a frequency of 1 MHz. Indeed, the carrier current may have the same frequency as the frequency of the incident ultrasound pulse  108 . 
     In such an embodiment, the diode  114  may mix or intermodulate bioelectric activity occurring in the proximity of the electrodes  116 ,  118  with the carrier current. In one embodiment, the carrier current may be amplitude modulated by the bioelectric signal detected by the local electrodes  116 - 118  at a frequency between 0-10 kHz. The modulated carrier current may then be transmitted as a modulated carrier signal  122  through volume conduction to the skin. 
       FIG. 2  illustrates another embodiment of a system  200  for ultrasound powered neurotelemetry. As depicted, the ultrasound source  102  is replaced by a ultrasound driver  206  coupled to a transducer  208 . The transducer  208  may be placed in contact with the skin  202 . An implant  104  may be placed under the skin  202  within body tissue  204 . For example, the implant  104  may be placed in brain tissue, heart tissue, or other body tissues. The ultrasound driver  206  may generate a driving signal causing the transducer  208  to emit an ultrasound pulse  210  through the skin  202  into the tissue  204 . The implant  104  may receive the ultrasound pulse  210  and emit a modulated carrier signal  212  through a volume conduction of electrical field lines back to the skin. 
     One or more electrodes  214  may be in electrical contact with the skin  202 . The electrodes  214  may detect the modulated signal  212  and transmit the modulated signal  212  over a wired connection  216  to a receiver  218 . Alternatively, the electrodes  214  may communicate the signal  212  to the receiver  218  over a wireless RF link (not shown). The receiver  218  may demodulate the signal  212  to obtain information about the bioelectric activity sensed by the implant  104 . The receiver  218  may use amplitude and/or phase demodulation to decode the bioelectrical event signal. Advantageously, such a system  200  may be implanted directly in the tissue  204  without the need for internally coupled lead wires or bulky open-loop inductive components. 
     In one embodiment, the receiver  218  may provide single-channel demodulation for a single implant  104 . This implant  104  design approach can operate in at least two different modes but in each case it drives a high frequency carrier wave  212  in tissue  204  containing a volume current driven by an additional set of electrodes that mimic a bioelectrical current. 
     The highest system sensitivity to low level bioelectric events can be achieved by driving the implant  104  with a continuous-duty ultrasound beam  210  such as comparatively might be used in medical Doppler flow or similar applications. For single implant  104 , or in situations where implants  104  are spaced such that individual ultrasound beams  210  may be directed at individual implants  104  without overlap, the demodulation process that recovers the biopotential waveform from the surface detected carrier wave performed by the receiver  218  may be relatively straightforward. 
     For example, commercially available high frequency lock-in amplifiers may be used for demodulation directly from surface electrodes  214 . In such an embodiment, digital outputs of the lock-in amplifier may be recorded by a computer configured to make plots and tables of the data. 
       FIGS. 3A-3C  illustrate various embodiments of an implant  104 . In  FIG. 3A , the implant  104  includes an elongated piezoelectric element  110  coupled to two conductive plates  112 . This example also includes a semiconductor diode  114  coupled in electric parallel to the piezoelectric element  110  by electrical coupling lines  304 ,  306 . These lines may be soldered to the conductive plates  112  and the semiconductor diode  114 . Alternatively, the coupling lines  304 ,  306  may be deposited through physical deposition or chemical deposition processes. In another embodiment, the coupling lines  304 ,  306  may be coupled to the conductive plates  112  using silver epoxy or by hot pressing a silver coating. The diode  114  may also be coupled to a reference electrode  116  and a stimulating electrode  118 . In one embodiment, these electrodes  116 ,  118  may protrude through a protective coating or housing  302  which houses the other elements of the implant. The housing may protect the components from corrosion and may reduce infection resulting from immune system reactions to the components. 
     In one specific example, the implant  104  may be constructed using a commercial quality PVDF plastic and a packaged Shottky diode. In a further embodiment, the piezoelectric current response of the PVDF may be increased by stacking thin sheets of approximately 25 micrometer thickness in electric parallel. The overall thickness of the piezoelectric element, including bonding thicknesses, may be around 250-350 micrometers, and form a solid structure. The piezoelectric element may be cut into various sizes depending on power requirements. For example, the piezoelectric element may have a width-height measurement of 0.8 mm×2 mm, 1.5 mm×3 mm, 2.5 mm×5 mm, or the like. 
     The diode  114  may comprise an ultraminiature surface mount diode, such as an SOT-363 package, having an epoxy overcoat. Indeed the size of these packages may be reduced, by sanding the package with light grit sand paper, to a thickness of between 0.6 mm and 0.9 mm. In such an embodiment, the piezoelectric element  110  and the diode  114  may be sized to fit through the lumen of a #16 gauge syringe needle. 
     In the embodiment depicted in  FIG. 3A , the reference electrode  116  and the stimulating electrode  118  both protrude from the housing  302  at the same end. The electrodes  116 ,  118  may protrude from the housing  302  by about millimeter. In various embodiments, the electrodes  116 ,  118  may protrude more or less depending on the particular bioelectrical characteristics of the tissue in which the implant  104  is placed. 
       FIG. 3B  illustrates an alternative embodiment of the implant  104  in which the reference electrode  116  is positioned on a first end of the implant  104  and the stimulating electrode  118  is positioned on a second end of the implant  104 . 
       FIG. 3C  illustrates yet another embodiment of the implant  104  in which the electrodes  116 ,  118  are substantially spherical or ball shaped. The spherical electrodes  116 ,  118  may have a diameter of approximately 0.9 mm. In one embodiment, the spherical electrodes  116 ,  118  may be formed of silver chloride. The silver chloride balls may be formed in a flame by melting silver wire and then attached to the body of the implant  104  using silver bearing epoxy and held in place using UltraViolet (UV) curing epoxy. The silver-chloride balls may be chlorided through exposure to saline. 
       FIG. 9  illustrates a size comparison of one embodiment of an implant  104 . In the depicted embodiment, the implant  104  may be sized to pass through the lumen of a syringe needle, such as a #16 gauge needle. 
       FIG. 4  illustrates a further embodiment of a receiver  208 . In the depicted embodiment, the receiver  208  may include a wideband amplifier  402 , a range-gate circuit  404 , a samplehold circuit  406 , a bandpass filter  408 , and a waveform output  410 . 
     One advantage of a telemetry approach using ultrasound is that it permits time-serial excitation and readout of multiple implants  104 . Advantageously, there is no added complexity on the implants  104  to achieve this. In one embodiment, multichannel operation may be accomplished by simply placing additional implants  104  within the path of the incident ultrasound beam  210 . 
     The wideband amplifier  402  may be required to amplified low level carrier signals  112  detected by the surface electrodes  214 . Amplification may be particularly useful as the depth of the implant  104  placement increases. An additional feature for the wideband amplifier  402  may be a low noise contribution level. As shown in  FIG. 8 , the body tissue  2014  may significantly attenuate the carrier signal  112  as the volume conduction current carries it to the skin surface from various depths.  FIG. 8  shows that at depths of 10 mm and more, the surface electrodes  214  may only detect 4 millivolts or less of the carrier signal  112 . Thus, the wideband amplifier  402  may include a high degree of noise isolation in order to provide a sufficient signal to noise ratio (SNR). 
     Pulsed ultrasound drivers  206  employed for multichannel applications may require a more complex process of demodulation by the receiver. For example, transit time range gating by a range-gate circuit  404  may separate the signals from multiple implants  104  along a beam of ultrasound  210 . 
     In one embodiment, an ultrasound pulse  210  may travel at a quantifiable rate through the various body tissues  204 . For example, 15 microsecond delay may occur between a transmitted ultrasonic pulse  210  and detected electrical response  112  from a 2.2 cm spacing between the implant  104  and the ultrasound transducer  208 . Thus the delay time indicates implant  104  distance from the transducer  208 . Such data may be used by the range-gate circuit  404  to separate received carrier signals  112  by time delay and identify corresponding implants  104  based on a correlation between known depth of placement and response timing. For example, the electrical responses  112  of multiple implants  104  placed at various depths along a line can potentially be discriminated from each other by noting delay times and by using an electronic gate to admit signals from only specific depths for further processing. 
     The electrical design connects the surface biopotential electrodes  214  to a high gain broad bandwidth amplifier  402  to accommodate low level surface potentials whose amplitudes may fall into the tens of microvolt range. The amplified signal may be passed to the range-gate circuit  404 . The range gate circuit  404  may comprise a series of electronic gates that pass only electrode signals that are delayed by a selectable windowed interval. The width and timing of the gate opening may define the depth of the implant  104 . Additionally, the width and timing of the gate opening may contribute to the system range resolution. This allows directing of the bioelectrically modulated carrier currents to be directed into separate data channel streams. 
     In one embodiment ultrasound imaging system pulse repetition rates be in the range of 4 kHz to 10 kHz for near surface imaging. This suggests that bioelectrical bandwidths in a pulsed sampling mode could be as much as 2 kHz to 5 kHz under conditions of Nyquist limitation. Ultrasound imaging systems can have millimeter-order spatial resolution which thus suggests an ability to separate signals from closely spaced implants  104 . 
     Each data stream may be reconstructed into continuous waveform data using a sample-hold circuit  406 . The sample-hold circuit may be synced to the range gate circuit  404 . High speed sampled waveform segments may be connected and smoothed by a low pass filter or a bandpass filter  408 . Multiple bioelectric waveforms may be recovered by the receiver  218  with a range resolution that is determined by a multitude of factors including the gate timing width, the transducer frequency and pulse ring-down, the size of the implants  104 , and practical constraints on the density of placement of the implants  104 . The resulting waveforms may be stored in a database on a computer using an interface to the waveform output  410 . Alternatively, the waveforms may be displayed on a graphical display or screen, plotted, or printed using a data driven printing device. 
       FIG. 5  illustrates one embodiment of a method  500  for ultrasound powered neurotelemetry. In one embodiment, the method  500  starts when the piezoelectric element  110  of the implant  104  receives  502  an ultrasound pulse  108 . The ultrasound pulse  108  may be generated by an ultrasound source  102 . In one embodiment, the ultrasound source  102  includes a transducer  208  and an ultrasound driver  206 . The ultrasound pulse may have a frequency of between 400 kHz and 5 MHz. 
     The method  500  may continue when the piezoelectric element  110  converts  504  the ultrasound pulse  108  into electrical potential. In a further embodiment, the piezoelectric element may convert the physical pressure power of the ultrasound pulse into sufficient electrical power to supply both the voltage and current needs of the diode  114  circuit. 
     In a further embodiment, the method  500  may include causing  506  an electric current to flow through the diode  114  from a reference electrode  116  to a stimulating electrode  118  in response to the electric potential generated by the piezoelectric element  110 . The piezoelectric element  110  may also supply sufficient current to establish a carrier current through the diode. The carrier current may be modulated through the diode  114  by mixing the carrier current with any bioelectric activity that may occur in proximity of the reference electrode  116 . In a further embodiment, the implant  104  may emit  508  the modulated carrier signal  112  into body tissue  204 . The modulated carrier signal  112  may be conducted through volume conduction to the skin  202  where it may be detected by one or more surface electrodes  214 , and the method  500  ends. In a further embodiment, the surface electrodes  214  may communicate the carrier signal  112  to the receiver  218  for demodulation. 
     The present method  500  may be performed by multiple implants  104  placed in different regions of the body tissue  204 . In a certain embodiment, multiple implants  104  may respond according to the present method  500  in response to a single ultrasound pulse  108 . In such an embodiment, the receiver  218  may demodulate and assemble the various carrier signals  112  for analysis using techniques described above with reference to  FIG. 4 . 
     Although certain embodiments of the present invention and their advantages have been described herein in detail, it should be understood that various changes, substitutions and alterations can be made without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present invention is not intended to be limited to the particular embodiments of the processes, machines, manufactures, means, methods, and steps described herein. As a person of ordinary skill in the art will readily appreciate from this disclosure, other processes, machines, manufactures, means, methods, or steps, presently existing or later to be developed that perforin substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufactures, means, methods, or steps. 
     REFERENCES 
     
         
         DeHennis et al., In:  A Double - Sided Single - Chip Wireless Pressure Sensor , Micro Electro Mech. Sys., 15 th  IEEE Intl. Conf., 252-255, 2002. 
         Harpster et al.,  Sensors Actuators A: Physical,  95(2-3):100-07, 2002. 
         Heetderks,  IEEE Trans. Biomed. Engineering,  35(5):323, 1988. 
         Lindsey et al.,  IEEE Trans. Biomed. Engineering,  45(5):614-619, 1998. 
         Minqui et al.,  IEEE Trans. Neural Sys. Rehab. Engin.,  11(2):189-192, 2003. 
         Takahata et al., In:  Stentenna: A Micromachined Antenna Stent For Wireless Monitoring Of Implantable Microsensors , Eng. Med. Biol. Soc., Proc. 25 th  Ann. Intl. Conf. IEEE, 4:3360-3363, 2003. 
         Towe,  IEEE Trans. Biomed. Engineering , BME-33:10, 1986. 
         Towe, In:  Passive Backscatter Biotelemetry for Neural Interfacing,  3 rd  Intl. IEEE/EMBS Conf., 144-147, 2007. 
         Wise et al.,  IEEE Trans. Biomed. Engineering,  92(1):76-97, 2004.