Patent Publication Number: US-2021187311-A1

Title: Relay module for implant

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 15/983,355, filed May 18, 2018, now allowed, which is a continuation of U.S. application Ser. No. 15/002,610, filed Jan. 21, 2016, now U.S. Pat. No. 9,974,965, issued May 22, 2018, which is a continuation of U.S. application Ser. No. 13/621,530, filed Sep. 17, 2012, now U.S. Pat. No. 9,242,103, issued Jan. 26, 2016, which claims the benefit of U.S. Provisional Application No. 61/535,295, filed Sep. 15, 2011, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     Active implanted stimulation devices have been utilized for applications such as pacing, defibrillation, spinal and gastric stimulation. Such devices typically include wired electrodes on a lead module hardwired to an implanted pulse generator (IPG) that contains an internal battery that can be recharged periodically with an inductive coil recharging system. 
     SUMMARY 
     In one aspect, a system includes a control module including a first antenna, the control module being configured to generate a first radio frequency (RF) signal and transmit the first RF signal using the first antenna; an implantable lead module including a second antenna and at least one electrode configured to stimulate excitable tissue of a subject; and a relay module configured to: receive the first RF signal; generate a second RF signal based on the first RF signal with the second RF signal encoding a stimulus waveform to be applied to the electrodes of the implantable lead module to stimulate excitable tissue of a subject; and transmit the second RF signal, wherein the implantable lead module is configured to receive the second RF signal using the second antenna, generate the stimulus waveform from the received second RF signal, and apply the stimulus waveform to the excitable tissue of the subject. 
     Implementations of this and other aspects may include the following features: a control module which may include a programming interface to allow a user to adjust parameters of the stimulation waveform; a first antenna of the control module which may include a dipole antenna, a folded dipole antenna, a microstrip antenna, or a phased array of antennas. 
     The relay module may include: a receive antenna layer configured to receive the first RF signal transmitted by the first antenna of the control module; at least one dielectric insulating layer; and a transmit antenna layer separated from the receive antenna layer by the dielectric insulating layer, the transmit antenna layer being configured to transmit the second RF signal to the second antenna of the implantable lead module, the second RF signal being generated based on the first RF signal, and the second RF signal encoding a stimulus waveform to be applied by the at least one electrode of the implantable lead module to stimulate the excitable tissue of the subject. 
     The receive antenna layer of the relay module may include one of: a patch antenna, or a dipole antenna. The receive antenna layer may further include at least one quarter wavelength antenna. The transmit antenna layer of the relay module may include one of: a patch antenna, or a dipole antenna. The transmit antenna layer may further include at least one quarter wavelength antenna. 
     The relay module may further include a flexible circuit, wherein the flexible circuit may include a rectifier and a capacitor, and wherein the capacitor is coupled to the rectifier and configured to store a charge during an initial portion of the first RF signal. The flexible circuit may further include a counter configured to cause the flexible circuit to generate a trigger upon an end of the initial portion. The flexible circuit may further include an oscillator, coupled to the counter and configured to generate, upon the trigger, a carrier signal, and wherein the flexible circuit may modulate the carrier signal with a stimulus waveform encoded in the first RF signal to generate the second RF signal. The flexible circuit may be configured to generate the second RF signal based on the stimulus waveform during a stimulation portion of the first RF signal, wherein the second RF signal has a corresponding carrier frequency that is substantially identical to that of the first RF signal. The flexible circuit may further include a power amplifier configured to amplify the second RF signal, and wherein the transmit antenna layer may be configured to transmit the amplified second RF signal to the second antenna of the implantable lead module. The power amplifier may be powered by the charge stored in the capacitor during the initial portion of the first RF signal. The oscillator may be triggered by an amplitude shift keying in the first RF signal. 
     The first RF signal and the second RF signal may have respective carrier frequencies that may be within a range of about 800 MHz to about 6 GHz. The respective carrier frequencies of the first and second RF signals may be different. 
     The relay module may be placed exterior to the subject and the relay module may further include a battery. The relay module may be subcutaneously placed underneath the subject&#39;s skin. The relay module may be placed on the subject&#39;s skin. The relay module is placed on a wearable item. 
     The relay module may further include a position sensor configured to read positional information of the relay module. The position sensor comprises one of: a touch sensor, a gyroscope, or an accelerometer. The control module may be further configured to: receive the positional information from multiple relay modules; and choose a particular relay module to transmit the second RF signal to the implantable lead module, based on the positional information received, wherein the particular relay module chosen is better coupled to the implantable lead module than at least one other relay module. 
     In another aspect, a method of stimulating excitable tissue in a subject by using a relay module includes: transmitting a first RF signal from a first antenna on a control module; receiving, by the relay module, the first RF signal from the first antenna on the control module; generating, by the relay module, a second RF signal based on the first RF signal, the second RF signal containing power and encoding a stimulus waveform to be applied by the at least one electrodes of the implantable lead module to stimulate excitable tissue of the subject; transmitting, by the relay module, the second RF signal to an implantable lead module; receiving, by the implantable lead module the second RF signal; generating, by the implantable lead module the stimulation waveform; and applying, through at least one electrode on the implantable lead module, the stimulation waveform to the excitable tissue. 
     Implementations of this and other aspects may further include rectifying an initial portion of the first RF signal to provide energy to store a charge on the relay module; generating the second RF signal at an end of the initial portion; and amplifying the second RF signal by using the stored charge before transmitting the second RF signal. 
     The method may further include: generating the second RF signal based on a trigger caused by an amplitude shift keying in the first RF signal, the amplitude shift keying corresponding to the end of the initial portion of the first RF signal. The method may further include: generating the second RF signal based on a trigger caused by counting a number of cycles during the initial portion of the first RF signal. 
     The second RF pulse may include a portion to provide energy to power the implantable lead module. The method may further include: configuring polarity of at least one electrode of the implantable lead module based on a subsequent portion of the second RF signal that encodes polarity setting information of the at least one electrode. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  shows an example of a wireless stimulation system including a relay module. 
         FIGS. 2A and 2B  show example of a portable Microwave Field Stimulator (MFS) device. 
         FIG. 3  is a block diagram showing an example of implantable lead module. 
         FIGS. 4A-4C  show examples of configurations of a relay module. 
         FIGS. 5A-5C  show examples of configurations of a relay module with a flexible circuit. 
         FIG. 6  is a block diagram showing an example of a circuit, such as a flexible circuit, used on a relay module. 
         FIG. 7  is a block diagram showing another example of a circuit, such as a flexible circuit, used on the relay module. 
         FIG. 8  is a timing diagram showing examples of the first RF signal received at the relay module  130  and subsequent waveforms generated by the flexible circuit. 
         FIG. 9  is a flow chart showing an example process in which the wireless stimulation system selects a particular relay module. 
         FIG. 10  shows example of a configuration of the relay module with a position sensor. 
         FIG. 11  illustrates an example workflow of a wireless stimulation system with the relay module of  FIG. 10 . 
         FIG. 12A-E  show example placements of the relay module. 
         FIG. 13A-L  show example placements of the relay module as a wearable item. 
         FIG. 14A-14D  show example configurations of a portable MFS device. 
         FIG. 15  depicts relay module in the configuration of a watch. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an example of a wireless stimulation system including a relay module  130 . The wireless stimulation system includes a control module, such as a portable microwave field simulator (MFS) device  100 , the relay module  130 , and an implantable lead module  140 , which may be an implantable neural stimulator. In the example shown in  FIG. 1 , the lead module  140  is implanted in a subject, such as a human patient, or an animal. 
     The portable MFS device  100  includes an antenna  110 . Antenna  110  may be configured to transmit a first radio frequency (RF) signal that propagates to relay module  130 . The first RF signal may have a characteristic carrier frequency within a range from about 800 MHz to about 6 GHz. 
     As shown by  FIG. 1 , the relay module  130  may be placed subcutaneously under the skin of a subject. The first RF signal from antenna  110  may propagate through body boundary  120  to reach relay module  130 . Relay module  130  may also be placed outside body boundary, for example, on the patent&#39;s skin topically. Relay module  130  may also be placed as a wearable item, as will be discussed in further detail later. 
     Relay module  130  may include a receive antenna  131  and a transmit antenna  132 . Receive (Rx) antenna  131  is configured to receive the RF signal from antenna  110 . The coupling between antenna  110  and Rx antenna  131  may be inductive, radiative, or any combinations thereof. The Rx antenna  131  may be coupled to transmit (Tx) antenna  132  by a dielectric insulating layer(s) and flexible circuits, as will be discussed in further detail below. The Tx antenna  132  transmits a second RF signal to an implantable lead module  140 . The second RF signal may be derived from, or otherwise based on, the first RF signal and may or may not have the same characteristic carrier frequency of the first RF signal, as will be discussed in further detail below. A RF module  130  may use, for example, a conditioning circuit in combination with a power amplifier to shape and enhance the second RF signal before transmitting the second RF signal to implantable lead module  140 , as will be discussed below in further detail. 
     An implantable lead module  140  has been implanted inside the body of a subject. The subject can be a live human or animal. The implantable lead module  140  is a passive device without an onboard power source, such as a battery. An implantable lead module  140  includes an antenna  141  configured to receive the second RF signal from antenna  132 . The coupling between antenna  141  and Tx antenna  132  may be inductive, radiative, or any combinations thereof. The implantable lead module  140  includes one or more electrodes placed in close proximity to an excitable tissue, such as, for example, neural tissue. The second RF signal may contain energy to power the lead module  140 , and may encode a stimulus waveform. The lead module  140  may generate the stimulus waveform from the second RF signal, and apply the stimulus waveform to the excitable tissue using the electrodes. Examples of the lead module  140  are described in, for example, U.S. patent application Ser. No. 13/584,618, filed on Aug. 13, 2012, the entire contents of which are incorporated herein by reference. 
       FIGS. 2A and 2B  show examples of a portable Microwave Field Stimulator (MFS) device. A portable MFS device  100  may include a power system  201 , a controller  202 , a user interface (UI)  203 , a feedback subsystem  204 , and antenna  110 . Examples of the MFS are described in, for example, U.S. patent application Ser. No. 13/584,618, filed on Aug. 13, 2012. 
     As illustrated by  FIG. 2A , a power system  201  may include a battery, for example, a rechargeable power source such as, for example, a lithium-ion battery, a lithium polymer battery, etc. The power system  201  provides power to a portable MFS device  100 . 
     The controller  202  can create the first RF signal to be transmitted from the antenna  110  to the relay module  130 , which in turn may generate and transmit the second RF signal to the antenna  141  on the implantable lead module  140 . As shown in  FIG. 2A , the controller  202  may include memory  211 , pulse generator  212 , modulator  213 , and amplifier  214 . 
     Memory  211  may be local memory on board of the portable MFS device  100 . Memory  211  may include any type of non-volatile memories, such as, for example, EEPROM, flash memory, etc. Memory  211  may store stimulation parameter settings, such as for example, pulse amplitude, waveform shape, repetition frequency, pulse duration, etc. Based on the stored stimulation parameter settings, pulse generator  212  may generate stimulation waveforms. Modulator  213  may generate a carrier frequency, for example, within a range from about 600 MHz to about 6 GHz. The stimulation waveforms generated by pulse generator  212  may modulate the carrier frequency. The resulting modulated carrier frequency signal may be amplified by amplifier  214  to generate the first RF signal to be transmitted by antenna  110 . 
     The controller  202  may receive input from the UI  203  and the feedback subsystem  204 . UI  203  may include a Bluetooth circuit board, or a USB interface connector. UI  203  may include a programmer interface for a user, such as a manufacturer&#39;s representative, to adjust stimulation parameters, such as, for example, stimulation frequency, pulse width, power amplitude, duration of treatment, waveform shape, pre-programmed options and patient reminders. The programming interface can cause the selected settings to be stored on memory  211  of controller  202 . The selected settings are used to create, for example, the appropriate stimulation waveforms for driving the electrodes on implantable lead module  140 . 
     Feedback subsystem  204  also may provide input to the controller  202  in creating the first RF signal. The feedback may be based on measurements of reflected power on antenna  110 . The reflected power may indicate the coupling between antenna  110  and surrounding medium, as will be discussed in further detail in association with  FIG. 10 . 
     Antenna  110  may include a dipole antenna, a folded dipole antenna, a patch antenna, a microstrip antenna, or a phased array of antennas. Antenna  110  may be impedance matched to air to improve coupling efficiency with relay module  130 . Antenna  110  can be located on the top of a flexible fixation housing that encloses the MFS circuitry connected with a low loss cable, or within the MFS enclosure, or remote from the MFS connected through a low loss cable. 
       FIG. 2A  illustrates an implementation in which the antenna  110  is housed within the enclosure of the portable MFS device  100 . The housing enclosure of portable MFS device  100  can be made of materials such as neoprene, or polyurethane, or other similar material with similar dielectric properties. 
     In another example, shown in  FIG. 2B , antenna  110  may be located on the outside of the portable MFS device  100  within a separate encasement by which the MFS power is hardwired to the antenna by a low loss cable. The antenna  110  can be located as far as three feet from the relay module  130 , or alternatively may be coupled directly to the skin in the proximity of the implanted lead module  140 . 
       FIG. 3  is a block diagram showing an example of implantable lead module  140 . Implantable lead module  140  is a passive device without an active power supply, such as a battery. Implantable lead module  140  may be an implantable neural stimulator. Implantable lead module  140  may include antenna  141 , power management circuitry  310 , passive charge balance circuitry  318 , and electrodes  322 . 
     Antenna  141  is configured to receive the second RF signal from antenna  132  on relay module  130 . The Antenna  141  may be embedded as a dipole, a patch, a microstrip, folded dipole, other antenna configuration. The second RF signal may have a carrier frequency in the GHz range and contain electrical energy for powering the wireless implantable lead module  140  and for providing stimulation pulses to electrodes of implantable lead module. Once received by the antenna  141 , the second RF signal is routed to power management circuitry  310  as the input signal. 
     Power management circuitry  310  is configured to rectify the input signal and convert it to a DC power source. For example, the power management circuitry  310  may include a diode rectification bridge and a capacitor. The rectification may utilize one or more full wave diode bridge rectifiers within the power management circuitry  310 . 
     The DC power source provides power to the stimulation circuitry  311  and lead logic circuitry  313 . Stimulation circuitry  311  may extract the stimulation waveforms from the received input signal. The stimulation waveforms may be shaped by pulse shaping RC timer circuitry  312  and then applied to the electrodes  322 . Passive charge balancing circuitry  318  may balance charges applied at the electrodes. Lead logic circuitry  313  may detect a portion of the input signal containing polarity setting information for each electrode of the electrode array  322 . This information may be used to set the polarity of electrode interface  314  controlling the polarity assignment of each electrode on electrodes  322 . A particular electrode on the electrode array  322  may be implanted near target excitable tissue. The excitable tissue can be, for example, a cardiac tissue, a neural tissue, etc. 
       FIGS. 4A-4C  show examples of configurations of a relay module  130 . A relay module  130  may include encapsulation materials  400  and antenna layers  401 , as shown by  FIG. 4A . Encapsulation materials  400  may be any material that encapsulates relay module  130 , such as most plastics. The antenna layers  401  may be encapsulated underneath encapsulation material  400 . 
       FIG. 4B  shows a profile view of one example of a layered configuration for the Rx antenna  131  and the Tx antenna  132 . Rx  131  in  FIG. 4B  is a patch antenna formed by a layered structure of two conductor layers  404  and one insulator layer  405  in between. The conductor layers  404  may include any appropriate conducting metal, for example, copper, silver, etc. The insulator layer  405  may include insulating dielectric materials, such as, for example, porcelain, glass, and most plastics. 
     As discussed above, relay module  130  may be placed either in proximity of the tissue medium within a few millimeters or subcutaneously under the skin of a subject, such as a human or an animal. If placed outside the subject&#39;s body, the Rx antenna  131  may be coupled to the air and may be impedance-matched to the air. If placed subcutaneously, the Rx antenna  131  may still be coupled to the air since the skin layer covering the antenna is sufficiently thin, having minimal effect on the coupling efficiency between the antenna  110  and Rx antenna  131  of the relay module  130 . The separation of the two conductor layers  404  and the electromagnetic properties of the insulator layer  405  may determine the resonant frequency of Rx antenna  131 . Rx antenna  131  may generally be a quarter wavelength antenna at this resonant frequency. 
     The Tx antenna  132  in  FIG. 4B  is also a patch antenna formed by a layered structure of two conductor layers  404  and one insulator layer  405  in between. Likewise, the separation of the two conductor layers  404  and the electromagnetic properties of the insulator layer  405  may determine the resonant frequency of Tx antenna  132 . Similarly, Tx antenna  131  may also be a quarter wavelength antenna at this resonant frequency. In contrast to the Rx antenna  131 , which may be coupled to the air, Tx antenna  132  may be coupled to the tissue, especially when relay module  130  is placed subcutaneously. Tx antenna  132  may then be impedance matched to tissue to improve coupling efficiency when transmitting the second RF signal to implantable lead module  140  inside the subject&#39;s body. The transmitting metal layer may have a smaller surface area than the ground plane and may have a specific shape for improved coupling with surrounding tissue (e.g., if placed topically on the subject&#39;s skin). As illustrated, Tx antenna  132  in  FIG. 4B  is separated by another insulator layer  405  from Rx antenna  131 . 
     Generally, a patch antenna may include a conducting material layer that serves as a conducting plane; a dielectric insulating plane the size of the conducting plane placed over the conducting layer; and another conducting layer, smaller than the ground plane, shaped in a desired pattern. If two patch antennas are separated by another insulating plane, as illustrated by  FIG. 4B , the E-field of the transmit patch antenna does not interact with the E-field of the receive patch antenna on the other side of the relay module, when no edge-effects are present. 
       FIG. 4C  shows a profile view of another configuration of Rx antenna  131  and Tx antenna  132  configured as dipole antennas. In this configuration, the Rx antenna  131  is formed by the shape and contour of the surface of one conductor layer  404  while the Tx antenna  132  is formed by the shape and contour of another conductor layer  404 . The two conductor layers are separated by an insulator layer  405 . The shape and contour of each conductor layer may generally determine the corresponding resonant frequency. In this configuration, the Rx antenna  131  and the Tx antenna may also be quarter-wavelength antennas at their respective resonant frequencies. 
     In  FIGS. 4B and 4C , the ground plane of the Tx antenna  132  may face away from the active radiator of the antenna  110  and the transmitting surface of Tx antenna  132  may face towards tissue in order to improve the efficiency of the Tx antenna  132  in relaying energy to the antenna  141  on implantable module  140 . Additionally, Rx antenna  132  may have a surface area much larger than antenna  141  on the implantable module  140 . For example, in certain embodiments, the Rx antenna  132  may have surface area of four square centimeters or above, while the antenna  141  within the implanted lead module may have a surface area less than one tenth of a square centimeter. The Rx antenna  131  may thus capture a much larger portion of the flux of EM energy (for example, hundreds of times larger) and relay that energy to the antenna  141  through the relay module Tx antenna  132 . Although  FIGS. 4B and 4C  respectively show a patch-on-patch configuration and a dipole-on-dipole configuration, other arrangements may be implemented, such as, for example, a patch-on-dipole or a dipole-on-patch configuration. 
       FIGS. 5A-5C  show examples of configurations of a relay module  130  with a flexible circuit. The RF signal may be received by a Rx antenna  131  from the antenna  110 . This received RF signal may be modulated and amplified via circuitry on a flexible circuit within the relay module  130 . The flexible circuit may be implemented in a flexible circuit board substrate that is easily bendable within the body or on the surface of the skin. These electronics may be isolated from the antenna ground planes by a layer of insulation. A layer of conductive material may provide the interconnections to route the input signal from the Rx antenna  131  and send the conditioned and amplified signal out through the Tx antenna  132 . This circuitry may include amplification and conditioning functions, as will be discussed in detail in association with  FIGS. 6-8 . 
     The flexible circuit may be placed relative to the Rx antenna  131  and the Tx antenna  132 . For example,  FIGS. 5A and 5B  respectively show the front view and the profile view of a configuration in which the flex circuit  506 , along with the components, are placed on the side of the antenna layers. In another example,  FIG. 5C  shows the profile view of another configuration in which the flexible circuit  506  and the surface mount (SMT) flexible circuit components  507  are placed in between the antenna layers. Additionally, although not shown, the flexible circuit may also be placed on the top or bottom of the antenna layers. 
     The relay module  130  may operate in two modes, a relay mode and a repeater mode. In relay mode, the relay module  130  may not alter the stimulation portion of the received first RF signal when transmitting the second RF signal to the implantable lead module  140 . In the repeater mode, however, the relay module  130  may enhance the stimulation portion of the received first RF signal when transmitting the second RF signal to the implantable lead module  140 . 
       FIG. 6  is a block diagram showing an example of a circuit, such as a flexible circuit, used on the relay module  130 . In this mode, relay module  130  operates as an RF signal replicator to transmit the second RF signal at the same carrier frequency as the stimulus portion of the received first RF signal from the portable MFS device  100 . 
     The first RF signal transmitted from the portable MFS device  100  contains two separate portions of encoded carrier waveforms. The first RF signal is received by Rx antenna  131  on relay module  130 . A charging portion of the received first RF signal may contain a long (e.g., about 1 ms or above) burst of pulses at a carrier frequency. This charging portion may be the initial portion of a particular signal pattern to be repeated in the first RF signal. This charging portion is used to charge a power storage reservoir circuit including a capacitor  605  within the relay module  130 . For example, the flexible circuit may contain a rectifier  601  to generate a DC power supply by rectifying and smoothing the initial portion of the received first RF signal. The DC power supply may store charges in, for example, capacitor  605 . The stored charge may then be used to power subsequent operations of relay module  130 . These subsequent operations may include, for example, subsequent transmission of the second RF signal that powers the electrodes on implantable lead module  140 . Specifically, implantable lead module  140  is a passive device without a power supply. In contrast, some implementations of the relay module  130 , however, may include a power source, such as a rechargeable battery. Once the second RF signal is received at the passive implantable lead module  140 , it may be demodulated to provide the stimulation waveforms to be applied at the electrodes  322 . As discussed above in association with  FIG. 3 , in some implementations, the second RF signal may also contain polarity setting information to be applied in assigning the polarity of each electrode of the electrode array  322 . Details are discussed in U.S. patent application Ser. No. 13/584,618, filed on Aug. 13, 2012. Thus, by transmitting the second RF signal, derived from or otherwise based on the first RF signal transmitted from portable MFS device  100 , relay module  130  of  FIG. 6  can power a passive lead module  140 . 
     A stimulation portion of the received first RF signal encodes stimulus waveforms. This stimulation portion may be the later portion of the signal pattern being repeated in the first RF signal. The stimulation portion of the first RF signal will be conditioned by stimulus conditioning circuitry  602  before transmission to implantable lead module  140 . The stimulus waveforms may contain short (e.g., about 0.5 ms or shorter) bursts of pulses. A low-noise amplifier  603  detects the stimulation portion of the first RF signal from Rx antenna  131  and feeds the stimulation portion to a high power amplifier  604 . In one implementation, the first RF signal contains amplitude shift keying to indicate the end of the initial portion (for charging, e.g., capacitor  605 ) and the start of the stimulation portion. The amplitude shift keying may cause the stimulus conditioning circuitry  602  to generate a trigger to allow DC power to be received from the stored charge in capacitor  605 . In another implementation, the stimulus conditioning circuit may include a counter that is set to expire upon a pre-determined number of pulse wave cycles. When the counter counts the number of pulse cycles in the received first RF signal has reached the pre-determined threshold, the counter will expire and generate a trigger. Upon the trigger, stored charge in capacitor  605  may be harvested to power, for example, stimulus conditioning circuit  602 , low-noise amplifier  603  and power amplifier  604 . In either example implementation, the output from the power amplifier  604  drives the Tx antenna  132  to transmit the amplified stimulus waveform at the original carrier frequency to the implantable lead module  140 . The stored charge can be recharged by the next repetition of the initial portion in the first RF signal received from portable MFS device  100 . 
       FIG. 7  is a block diagram showing another example of a circuit, such as a flexible circuit, used on the relay module  130 . In this mode, relay module  130  acts as an active modulated pulse transmitter. The modulator  600  can provide a carrier signal at a different frequency than the frequency of the first RF signal received from the portable MFS device  100 . The first RF signal is received by the Rx antenna  131  coupled to air. 
     The first RF signal received from portable MFS device  100  by Rx antenna  131  contains two separate portions of encoded carrier waveforms. As discussed above, an initial portion of the first RF signal may contain a long (e.g., about 1 ms or above) burst of pulses at a carrier frequency. This initial portion is used to charge a power storage reservoir circuit including a capacitor  605  within the relay module  130 . For example, the flexible circuit may contain a rectifier  601  to generate a DC power supply by rectifying and smoothing the initial portion of the first RF signal. The DC power supply may store charges in, for example, capacitor  605 . The stored charge may then be used to power subsequent power subsequent operations of relay module  130 . These subsequent operations may include, for example, subsequent transmission of the second RF signal that powers the electrodes on implantable lead module  140 . As discussed above, implantable lead module  140  is a passive device without a power supply. In contrast, some implementations of the relay module  130 , however, may include a power source, such as a rechargeable battery. Once the second RF signal is received at the passive implantable lead module  140 , it may be demodulated to provide the stimulation waveforms to be applied at the electrodes  322 . As discussed above in association with  FIG. 3 , in some implementations, the second RF signal may also contain polarity setting information to be applied in assigning the polarity of each electrode of electrodes  322 . Details of discussed in U.S. patent application Ser. No. 13/584,618, filed on Aug. 13, 2012. Thus, by transmitting the second RF signal, derived from or otherwise based on the first RF signal transmitted from portable MFS device  100 , relay module  130  of  FIG. 7  can also power a passive lead module  140 . 
     A stimulation portion of the first RF signal encodes stimulus waveforms. This stimulation portion may be a later portion in a pattern being repeated in the first RF signal. The simulations portion of the first RF signal will be conditioned by stimulus conditioning circuitry  602  and further modulated by TX modulator  700  before transmission to implantable lead module  140 . The stimulus waveforms contain short (e.g., about 0.5 ms or shorter) bursts of pulses. In one implementation, the first RF signal contains amplitude shift keying to indicate the end of the initial portion (for charging, e.g., capacitor  605 ) and the start of the stimulation portion. The amplitude shift keying may cause the stimulus conditioning circuitry  602  to generate a trigger to allow DC power to be received from the stored charge in capacitor  605 . In another implementation, the stimulus conditioning circuit may include a counter that is set to expire upon a pre-determined number of pulse wave cycles. When the counted number of pulse cycles in the received first RF signal has reached the pre-determined threshold, the counter will expire and generate a trigger. Upon the trigger, stored charge in capacitor  605  may be harvested to power, for example, Tx modulator  700  and power amplifier  604 . In either example implementation, the stimulus waveform is mixed with a carrier frequency of Tx modulator, the result is fed to power amplifier  604 , and the output from the power amplifier  604  drives the Tx antenna  132  to transmit the amplified stimulus waveform modulated at the carrier frequency of Tx modulator  132  to the implantable lead module  140 . As discussed above, the stored charge can be recharged by the next instance of the initial portion of the first RF signal received from portable MFS device  100 . 
     In this mode, the carrier frequency of the first RF signal transmitted by the portable MFS device  100  can be decoupled from the carrier frequency of the stimulus waveform transmitted by the relay module  130 . As long as the two carrier frequencies are sufficiently apart and the pass band of antenna  141  on implantable lead module  140  is sufficiently selective, the electrodes on the implantable lead module may only be driven by the stimulus waveform transmitted from relay module  130 . 
       FIG. 8  is a timing diagram showing examples of the first RF signal received at the relay module  130  and subsequent waveforms generated by the flexible circuit. For example, in microwave relay mode (illustrated in  FIG. 6 ), the charging portion  801  utilized for charge storage may include a burst of pulses 1 millisecond or longer in pulse duration. Between each repetition of the charging portion of long bursts, a short burst, with pulse durations of 500 microseconds or less, encodes the stimulus waveforms. This portion is the stimulation portion  802 . In one implementation, after every 1000 cycles of the short bursts, the stored power is recharged/replenished by the long bursts for pulse durations of 1 millisecond or longer. The cyclic pattern is repeated as needed to power the amplification circuitry on board the relay antenna module so that stimulus waveforms are sent to passive, implantable lead module  140 . 
     Multiple implantable lead modules  140  may be implanted inside a subject&#39;s body. Multiple relay modules  130  may be configured to relay energy from a portable MFS device  100  to the implantable lead modules  140 . 
       FIG. 9  is a flow chart  900  showing an example process in which the wireless stimulation system chooses a particular relay module for relaying energy to a particular implantable lead module  140 . 
     Initially, a user may input stimulation parameters into the portable MFS device  100  ( 902 ). The stimulation parameters may include, for example, frequency, amplitude, pulse width, treatment duration, etc. These parameters may be entered into portable MFS device  100  through a programmer module, e.g., UI  203  ( 904 ). Afterwards, the portable MFS device  100  may send power to each relay module  130  ( 906 ). As discussed below in  FIGS. 10 and 11 , each relay module  130  may include position sensors to provide positional information of the respective relay module  130 . Example position sensors may include radio-frequency identification (RFID) devices, touch sensors, gyroscopes, etc. 
     Subsequently, the portable MFS device  100  may read the positional information generated by the position sensors at the respective relay module  130  ( 908 ). Based on the positional information collected, portable MFS device  100  may determine the relay module  130  best positioned to relay energy to power a particular implantable lead module  140 . The relay module best positioned to relay energy may be the relay module with one of the following characteristics: the lowest amount of transmission loss, best coupling to tissue, closest proximity to the portable MFS device  100 , or closest proximity to a particular implantable lead module  140 . For example, a software algorithm may be implemented on the portable MFS device  100  to determine the position of a particular relay module  130  relative to a given implanted implantable lead module  130 . The portable MFS device  100  may then determine which relay module should be selected to transmit energy most efficiently to the given implanted implantable lead module  130 . In this example, the relay module that will transmit energy most efficiently to the given implantable lead module may be the relay module closest to the given implantable lead module. The portable MFS device  100  can digitally control a multiplexor to selectively transmit energy to a chosen relay module  130 . 
     Thereafter, the portable MFS device  100  may generate the first RF signal by modulating a carrier signal with a particular stimulation waveform, for example, according to stimulation parameters stored in memory  211  ( 910 ). The portable MFS device  100  may then send the first RF signal to the optimal relay module as determined above ( 911 ). The selected optimal relay module may be the only relay module activated to receive the first RF signal. The activation may be achieved remotely by portable MFS device  100  before transmission of the first RF signal. 
     When the selected optimal relay module receives the first RF signal at its Rx antenna  131 , the relay module may utilize a charging portion of the received first RF signal to charge a reservoir, such as, for example, capacitor  605 , and then utilize the stored charge to power the relay circuitry ( 912 ). For example, the stored charge may be used to modulate a carrier wave with a stimulation waveform, amplifier the modulated carrier wave to provide the second RF signal, and then transmit the second RF signal to the given implantable lead module ( 914 ). 
     Subsequently, the given implantable lead module receives the second RF signal. As a passive device, the given implantable lead module is powered by the energy contained in the second RF signal and extracts the stimulation waveform from the received second RF signal ( 916 ). In capturing the energy contained in the second RF signal, the implantable lead module  140  may store a charge in a capacitor. The stored charge will be utilized to apply the extracted stimulation waveform to the electrodes  322  ( 918 ). 
       FIG. 10  shows an example of a configuration of a relay module  130  with a position sensor  1000 . As illustrated, position sensor  1000  may be integrated on flexible circuit  506 . As shown in the left panel of  FIG. 10 , the flexible circuit  506  may be placed on top of antenna layers  401  and occupying part of the surface area of antenna layers  401 . Encapsulation material  400  may enclose flexible circuit  506  (with components) and antenna layers  401 , as discussed above. 
     The right panel shows a profile view of the example configuration of relay module  130  with positional sensor  1000 . Position sensor  1000  may be a component of the surface mount (SMT) components  507  mounted on flexible circuit  506 . As discussed above, the Rx antenna  131  and the Tx antenna  132  may be implemented as patch-on-patch antennas. The Tx antenna  132  of each relay module  130  can be circularly polarized to substantially obviate directional dependence, thereby permitting a wider acceptance angle at the antenna  141  on implantable lead module  140 . 
     In one implementation, a semiconductor gyroscope can be used as a position sensor to determine the orientation of Rx antenna  131  and Tx antenna  132 . In other implementations, touch sensors can be used as a position sensor to detect, for example, if the Tx antenna  132  of the relay module  130  is coming in contact with an object. The touch sensor may also detect any force gradients to determine whether the side of Tx antenna  132  is touching something pliable, such as clothing, or something hard. In particular, when Tx antenna  132  is touching a lossy surface, like the thigh, it could be considered a worst case scenario. A lossy surface may have different impedance than the impedance of the antenna. When the Rx antenna  131  or the Tx antenna  132  is touching a side pocket material, or other clothing, antenna coupling could be closer to that of air coupling, which may be considered the best-case scenario. 
     In yet other implementations, an additional coupler can be used to detect the forward power and reflection outputted by a given Tx antenna  132 . A lossy surface may be detected when the measured reflection measurement is high, such as, for example, over 25% of the transmission energy. The presence of a lossy surface on a particular relay module may provide feedback to portable MFS device  100  that the particular relay module should be avoided. As a result, an alert may be provided to UI  203  on portable MFS device  100  to notify a user of the situation. Unless the situation has been remedied, the portable MFS device  100  may refrain from using the given relay module to relay energy to an implantable lead module. 
       FIG. 11  illustrates an example workflow of a wireless stimulation system with the relay module  130  of  FIG. 10 . In step  1 , the portable MFS device  100  transmits omnidirectional charging signal to all relay modules in range. In step  2 , position sensors on the relay module  130  provide positional readings for the host relay module and utilize a telemetry antenna within the relay module to transmit the positional information to the portable MFS device as a feedback signal from the position sensors. In some implementations, Rx antenna  131  may serve as a transceiver to transmit the telemetry signal to the portable MFS device  100 . In these implementations, relay module  130  may include a power source, such as, for example, a rechargeable battery. In step  3 , the portable MFS device  100  receives the information from the position sensors on the respective relay modules. Based on the positional information received, the portable MFS device  100  software algorithms determine which relay module  130  is in the most optimal position to relay the maximum amount of energy to power a given implantable lead module  140  that has already been implanted in the subject, as discussed above. In step  4 , portable MFS device  100  sends energy directed to the chosen relay module  130 . Thereafter, the relay module  130  harvests the energy to power the given implantable lead module  140 , as discussed above. 
       FIG. 12A-E  show example placements of the relay module. The relay module  130  can be placed nearby a variety of anatomical targets that contain the implanted lead module. Example targeted sites for relay module  130  include, but are not limited to, behind the neck or at the small of the back as shown in  FIG. 12A ; the waistline or abdomen, as shown in  FIG. 12B ; the side of the buttock as shown in  FIG. 12C . The relay module  130  may also be placed under the skin in the skullcap, as illustrated in  FIG. 12D , and just under the skin over the vagus nerve around the neck area, as illustrated in  FIG. 12E . 
       FIGS. 13A-L  show example placements of the relay module as a wearable item. Relay module  130  may be placed, for example, a bandage, a strap, an adhesive surface, a sleeve cover, or a piece of cloth worn on the body, for instance behind the neck or at the small of the back.  FIG. 13A  shows an example placement of relay module  130  in an eyeglass frame  1301 .  FIG. 13B  depicts a dress shirt  1310  with relay modules  130  attached to the inside and outside.  FIG. 13C  depicts relay module  130  placed on the inside and outside of a general use shirt  1320 .  FIG. 13D  depicts an example placement of relay module  130  in a neck brace or other stabilization brace  1330 .  FIG. 13E  shows example placement of relay module  130  in a ball cap  1340 .  FIG. 13F  shows example placement of relay module  130  PR on a flexible ace bandage  1350  housing which can be utilized at a multitude of locations on the body.  FIG. 13G  shows example placement of relay module  130  on an ankle brace  1360 .  FIG. 13H  d shows an example of placing relay module  130  within a girdle or haulter  1370 .  FIG. 13I  shows example placement of relay module  130  on the body of a bra structure  1380 .  FIG. 13J  shows example placement of relay module  130  on trunks  1390 .  FIG. 13K  depicts example placement of relay module  130  in multiple locations on a leg brace  1391 .  FIG. 13L  depicts example placement of relay module  130  within a scarf material  1392 . 
     The design of the relay module  130  is intended to be convenient for patient use in daily activities such as exercise, working, and other leisure activities. A strap holding the relay module  130  over an implanted antenna  141  on implantable lead module  140  can become inconvenient in situations such as swimming, such as where the relay module  130  can shift, for example, during the sleeping time of the subject; or where the relay module  130  could press against the skin potentially uncomfortably. Additionally, bulky medical devices tend to be unaesthetic and are undesirable in many situations where skin is exposed. 
     The implementations discussed above address these issues by placing the pulse generator on the portable MFS device  100  wirelessly away from the body up to three feet. The implementations utilize a compact relay module  130  that may seamlessly integrate into a wearable item or be subcutaneously placed. The relay module  130  may relay energy received from portable MFS device  100  to power implantable lead module  140 . Some implementations may further detect which relay module is in contact with lossy materials and guides the pulsed microwave energy from portable MFS device  100  to be directed to the relay module with the best coupling to a particular implantable lead module. 
       FIG. 14A-14D  show example configurations of a portable MFS device. As discussed above, the portable MFS device  100  may be typically located outside the body and is not physically connected to the skin; however can be located subcutaneously (not shown). In certain embodiments, a programmer is embedded into the portable MFS device  100  that interfaces with a user to provide options to change the frequency, amplitude, pulse width, treatment duration, and other system specifications. In certain circumstances, a manufacturer&#39;s representative will set specific parameters for the MFS device and the patient will be given the option to adjust certain subsets of those parameters, within a specified range, based on a user&#39;s experience. 
       FIG. 14A  shows an example portable MFS device  100  with a strap  1401 , surface  1402 , and control buttons  1403 - 1406  for a user to make adjustments to the stimulation parameters. Antenna  110  may be mounted under surface  1402 .  FIG. 14B  shows another example portable MFS device  100  with a display  1410  on surface  1402 , and control buttons  1405  and  1406 . Antenna  110  may be mounted under surface  1402 . The display  1410  may provide visual information to a user about the progress of the therapy and associated stimulation parameters. Control buttons  1405  and  1406  may allow a user to make adjustments to the stimulation parameters.  FIG. 14C  shows yet another example portable MFS device  100  with a surface  1402 , and control buttons  1403  to  1406 . Antenna  110  may be mounted under surface  1402 . Control buttons  1403 - 1406  may allow a user to make adjustments to the stimulation parameters.  FIG. 14D  shows still another example portable MFS device  100  with antenna  110  and control buttons  1403 - 1406  for a user to make adjustments to the stimulation parameters. 
       FIG. 15  depicts the MFS and Tx antenna in the configuration of a watch or other strap on arm unit. In certain embodiments, the Tx antenna is located on the perimeter of the watch face, or optionally on the strap of the watch or arm unit. 
     A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made. Accordingly, other implementations are within the scope of the following claims.