Abstract:
A microstimulator or neural prosthesis is powered and operative in response to an externally applied RF signal which includes a power component and control component. A slow rise time storage circuit stores RF power during a charging period and a fast rise time triggering circuit responsive to a fast rise time input triggers the device for producing output pulses following a selected delay time. The duration of the delay controls the current level of the output pulses.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to an implantable microstimulator or neural prosthesis powered by an operative in response to an externally applied signal. In particular, the invention relates to a device responsive to an input signal having a power component and a control component wherein the device output is controllable in accordance with encoding associated with the input signal. 
   Autonomic problems occur in areas of the bowel, bladder and respiratory systems following spinal cord injuries. Methods for managing these problems are active areas of medical research. An important approach is the use of electrical stimulation or neural prosthetics. A particular direction in the application of electrical stimulation is the use of a small, fully implanted microstimulator that is activated with radio frequency (RF) energy. 
   Conventional approaches employ various techniques for controlling the amplitude and duration of the applied electrical stimulation. Such techniques suffer from problems associated with the inefficient coupling of the input power to the device. As a result, systems have been developed which are designed to provide a selective level of power output. Such systems are often inefficient and expensive to implement. In addition, such systems do not provide for versatile controls. For example, in some systems the signal level may only be varied in accordance with the duration of the applied input signal. Other systems may allow for variable signal strength and duration but require adjustments be effected by electromechanical coupling, i.e., by the application of external magnetic fields. 
   Also, conventional devices have limited capability to deliver relatively high currents over prolonged application cycles. Such systems may not be satisfactory for use in connection with bowel stimulation, which may require application of relatively high currents during a treatment period for as long as 30 minutes. 
   SUMMARY OF THE INVENTION 
   The present invention is based upon the discovery that an implantable device having a selectable relatively high current output is powered and controlled by an encoded input signal having an energy component and a control component. 
   In a particular embodiment, the invention comprises an implantable electrical stimulator including a power storage circuit responsive to the power component of an RF input signal for storing energy to a predetermined level; and a pulse generating circuit coupled to the power storage circuit for receiving stored energy therefrom and being operative to produce stimulation pulses at a selectable output level and rate. A control circuit is coupled between the power storage circuit and the pulse generating circuit and is responsive to the energy stored in the power storage circuit for controlling the output level and pulse rate of the stimulator pulses. 
   In the exemplary embodiment, the power component has a relatively slow temporal characteristic, and the energy storage circuit is responsive to said relatively slow temporal characteristic whereas the pulse generating circuit is non-responsive thereto. 
   In the exemplary embodiment, the control component has a relatively fast temporal response and the pulse generating circuit is triggerably responsive to the relatively fast temporal characteristic for producing output pulses. 
   In accordance with another feature of the invention, the power component and control component of the RF signal are selectively temporally separated so that in one phase, the strength of the input pulse is determined and thereafter the pulse is triggered. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic illustration of a microstimulator in accordance with the present invention sutured or attached to the external wall of an organ. 
       FIG. 2  is a schematic block diagram of the implantable portion of the microstimulator according to the present invention. 
       FIG. 3  is a schematic block diagram illustrating the transmitter for delivering an encoded power and control signal to the microstimulator circuit illustrated in  FIG. 2 . 
       FIG. 3A  is an illustration of the input signal. 
       FIG. 4  is a schematic illustration of the circuit of  FIG. 2 . 
       FIG. 5  is a time chart illustrating the encoding technique according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates a microstimulator  10  in accordance with the present invention, which comprises an encapsulated device  12  having external electrodes  14  which may be sutured or attached to an excitable organ or muscle, such as the bowel  16  of a patient, as shown in the drawing. The microstimulator has an internal antenna ( FIG. 2 ) for receiving an input radio frequency (RF) input signal  30 . A transmitter  26 , shown in  FIG. 3 , having a transmitter antenna  28 , produces the RF signal  30  which powers and controls the microstimulator  10 . According to the invention, when treatment is necessary, the transmitter antenna  28  is located proximate to the patient and the implanted microstimulator  10  which is located near the internal organ  16  is powered and controlled wirelessly by the transmitter  26 . 
     FIG. 2  illustrates the microstimulator  10  in schematic form. The antenna receiver  20  receives the RF signal  30  and couples the signal to a power supply  40  which produces a high voltage (HV) and a low voltage (LV) outputs. A demodulator circuit  42  coupled to the power supply  40  produces a demodulator control signal (DM) which is employed for controlling the operation of the device  10 . Power supply  40  delivers LV power to a charging circuit  44  which stores energy necessary for controlling the energization of the electrodes  14 . The charging circuit  44  is coupled to a pulse generating circuit  46  which produces output pulses  48  on output electrodes  14  for energizing the organ represented by load resistor L. The pulses have a selectable current amplitude, pulse rate and a duration. The charging circuit  44  is responsibly coupled to the demodulator signal DM and receives the low voltage input LV as well. The pulse generating circuit  46  is likewise responsively coupled to the demodulator output and has a high voltage input as shown. 
     FIG. 3  illustrates the exemplary transmitter  26  in the form of RF generator  50  and wave form modulator  52 . The transmitter  26  may include a variable delay control  54  and a variable time control  56  discussed hereinafter. 
   The transmitter  26  produces the output  30  which is a modularized RF signal  57  produced by the signal generator  50 , modulated by wave form  58  produced by the wave form generator  52  as shown. 
     FIG. 4  schematically illustrates the circuit of the device  12 . It should be understood that one or more of the various functions illustrated by the blocks in  FIG. 2  may be performed by the elements described below. 
   Power supply  40  and the demodulator  42  are illustrated in the upper portion of  FIG. 4 . The power supply comprises voltage doubler circuits  60 ,  62  and  64 . Voltage doubler  60  includes capacitors CA 1  and CB 1  paired with respective diodes DA 1  and DB 1 . Voltage doubler  62  includes capacitors CA 2  and CB 2  paired with diodes DA 2  and DB 2 . Voltage doubler  64  includes capacitors CA 3  and CB 3  paired with diodes DA 3  and DB 3 . The diodes in each of the voltage doubler circuits are coupled in antiparallel configuration as shown. Voltage doubler  60  is coupled to a tank circuit  68  which includes an inductor LT and a parallel capacitor CT. The inductor LT represents the antenna  20 . The tank circuit  68  is responsive to the modulated signal  30  produced by the transmitter  26  for producing a voltage input V 1  to the input of voltage doubler  60 . The input voltage V 1  is doubled at the demodulator output  70  and has a wave shape  58  as illustrated in  FIG. 3A , and as further detailed in  FIG. 5 . The wave form  58  operates to control the timing of various functions as hereinafter discussed. 
   Voltage doubler  62  receives input V 1  which is likewise doubled at output  72 . This voltage is coupled to a regulator  74  which in combination with a capacitor CR produces regulated low voltage LV. 
   Voltage doubler  64  receives the output of voltage doubler  62  and produces doubled voltage at high voltage output node  76  which is coupled to ground through capacitor CH. 
   The voltage doubler circuits  60 ,  62  and  64  and the tank circuit  68  have component values which establish certain voltage levels as illustrated. The selected component values noted on the drawing are exemplary of an embodiment of the invention which has operated satisfactorily. 
   The lower portion of  FIG. 4  illustrates the charging circuit  44  and the pulse generating circuit  46 . The regulator output LV is coupled to a node  80  on one side of resistor R 5 . The other side of the resistor R 5  is coupled to the cathode of a diode D 2  at node  82 . The diode D 2  is series connected to resistor R 6  which in turn is series connected to charging capacitor at node  84 . The other side of the capacitor C 1  is connected to ground G as illustrated. 
   The demodulator output DM, noted above, is coupled to one side of a charging capacitor C 2  at node  86 . The other side of the charging capacitor C 2  is series connected to the relatively high resistance charging resistor R 4  at node  88 . 
   The circuit of  FIG. 4  includes a number of control switches including a switch Q 1  which is coupled between node  84  and ground G through a relatively low resistance discharge resistor R 1 . 
   Switch Q 2  is coupled between node  80  and ground G through a relatively low resistance resistor R 2 . The output  92  of Q 2  is coupled to gate  94  of switch Q 1 , as shown. The demodulator input node  86  is coupled to ground G through the relatively low resistance discharge resistor R 3 , which is coupled to the gate  96  of switch Q 2  at node  86 . Diode D 1  is coupled between node  88  and is forward biased relative to the ground G. 
   A switch Q 3  is coupled between node  82  and ground as illustrated. The switch Q 3  has a gate  98  coupled to the anode of diode D 1  and an output  100  coupled to node  82  as illustrated. 
   Switch Q 4  is coupled between the node  84  and ground G through a relatively high resistance resistor R 7 . The gate  102  of switch Q 4  is coupled to node  82 . 
   The high voltage output HV is coupled to the high voltage node  104  which represents one of the output electrodes  14 , and which is coupled to the organ represented by the load resistor L. The other electrode  14  is coupled to one side  106  of a switch Q 5  and to ground G through output  108  and relatively low resistance series resistor R 8 . Gate  110  of switch Q 5  is coupled to output  112  of Q 4 . When the switch Q 5  is on, the high voltage signal HV is applied to the load L or organ to thereby stimulate the organ function. 
     FIG. 5  illustrates the wave shape  58  of the output signal  30  which controls the various phases and operation and the invention. The area under the curve represents the power delivered to the device  10  during the various phases of operation. According to the invention the circuit of  FIG. 4  has a Charging Phase I, Set Current Phase II, an Output Phase III and an End Output Phase IV. The various phases of operation occur during the corresponding portions of the control wave shape  58  illustrated in  FIG. 5 . In addition, certain transitions T 1 –T 4  in the wave shape  58  of  FIG. 5  results in corresponding responses in the circuit  12 . For example, down going transition T 2  on the leading edge of Phase I ends an initial Charging Phase I at time t 2 . The rising edge T 3  of the Output Phase III initiates a discharge of energy through the load L; and transition T 4  at the leading edge of the End Output Phase IV at t 4  terminates the discharge. 
   Referring once again to the lower portion of  FIG. 4  and the wave shape  58  in  FIG. 5 , the charging Phase I is described below. The wave shape  58  is received in the demodulator as noted above. The control portion of the wave shape  58  ramps up at T 1  during time t 0 –t 1  as indicated, and charges capacitor C 1  from the low voltage supply through R 5 , D 2  and R 6 . The charging rate of capacitor C 1  is relatively fast and depends upon the combined resistance and capacitance of the R 5 , R 6  and D 1  of the charging circuit. At the same time capacitor C 2  charges from the demodulated input through R 4  as illustrated. The charging rate is relatively slow, again as a result of a combined resistance and capacitance of the elements R 4  and C 2  of the circuit. 
   During the Charging Phase I, the switchs Q 1 –Q 5  are all off or open circuit. The charging time of C 1 , set by charge time control  56  ( FIG. 3 ) is sufficient to permit the capacitor C 1  to become charged to a level sufficient to operate the system. This charging level during Phase I is illustrated by the level LI on wave shape  58  when capacitors C 1  and C 2  are fully charged. During Phase I the switches Q 1 –Q 5  are off. At time t 2  the wave shape  58  transitions down at transition T 2  initiating Set Current Phase II. The gate  96  on switch Q 2  goes low through resistor R 2  to the node  86  causing switch Q 2  to turn on, which in turn causes switch Q 1  to turn on as well. At this time capacitor C 1  discharges through resistor R 1  and switch Q 1 ; and capacitor C 2  discharges through diode D 1  and resistor R 3 . At this time the switches Q 3 , Q 4  and Q 5  remain off. Capacitor C 1  continues to receive a slow charge from the regulated supply through R 5 , D 2  and R 6 . 
   During Phase II the capacitor C 1  discharges to some voltage Vx, as determined by the time constant of C 1  and R 1  less the recharge supplied by the low voltage source. The length of Set Current Phase II is determined by Vx. This in turn may be established by the set current delay control  54  in the transmitter  26  ( FIG. 3 ). 
   At time t 3  the Set Current Phase II ends by wave form transition T 3 . Switchs Q 3 –Q 5  remain on during Phase III. 
   At transition T 3  occurring at time t 3 , the wave shape  58  steps up to initiate Output Phase III. At this time capacitor C 2  recharges through the resistor R 4 . The rising signal at node  86  drives the gate  96  of switch Q 2  high causing it to shut off which in turn shuts off switch Q 1 . At the same time, node  88  is driven high by the discharge of C 2  through R 4 , causing the gate  98  on switch Q 3  to go high, thereby turning switch Q 3  on. As switch Q 3  conducts its output at node  82  goes low thereby driving the gate  102  of switch Q 4  low turning it on and thereby establishing a discharge path for capacitor C 1  through Q 4  and resistor R 7 . The discharge of C 1  is relatively slow due to the high resistors of R 7 . The voltage drop across R 7  varies as Vy. As C 1  discharges, node  83  feeding the gate  10  of switch Q 5  goes high thereby closing the output circuit allowing the high voltage HV supply to stimulate the muscle, represented by the load resistor L, through switch Q 5  and resistor R 8 . The discharge for output Output Phase III terminates at time t 4  upon the occurrence of transition T 4 , the End Output Phase IV begins. 
   The duration of the Output Phase III depends on the voltage level at gate  110  of switch Q 5 , which in turn, is a function of the charge level on capacitor C 1 . As the voltage on R 7  drops below Vy the switch Q 5  turns off. When the wave shape  58  goes low at time t 4 , Q 3 , Q 4  and Q 5  turn off, and in a fashion similar to the occurrence at time t 2 , switch Q 1  and Q 2  turn on discharging capacitor C 1  through resistor R 1 , and discharging capacitor C 2  through diode D 1  and R 3 . After some fixed dead time D, during End Output Phase IV, the cycle may be restarted at time t 0 . In this connection it is possible to eliminate the charging period t 0 –t 1  as the capacitor C 1  retains some residual charge and may be brought up to its selected operating level LI relatively quickly thereafter. 
   The system according to the invention has the capability to provide electrical stimulation for muscles and organs requiring pulses with relatively large currents, for example, around 10–100 ma; a relatively long duration, for example, 1,000 microseconds per pulse. The fully implantable microstimulator of the invention uses RF signals for both power and control. Since the energy and control is externally provided, batteries and complex control circuitry are not needed for the implant. The use of RF energy eliminates the need for wires or cables that would be prone to breakage and requite an exit site through the skin of the patient that would be a route for infection. 
   The system uses a signaling method, which allows varying the parameters of stimulation so that the current amplitude, pulse width and pulse repetition rate can be controlled externally as needed. A prototype device has been produced which has an output exceeding 50 miliamps, 1,000 microseconds and 40 pulses per second. However, depending on the operation, the parameters may be charged by varying the value of the various components. The design for the implant allows varying these parameters on a pulse-by-pulse basis so that with a sophisticated external transmitter, complex results such as ramping and current or pulse with up and down are possible. 
   The exemplary device was devised for using aiding bowel movements. However, other possible uses are available such as for aiding in bladder control and respiratory cough assistance.