Abstract:
An RF-energy modulation system dynamically adjusts tuned receiving circuits within a plurality of slave devices, thereby regulating the level of power reception in each slave device. The slave devices receive power from a single master device, through coupling of a primary antenna in the master device with a secondary antenna in each slave device. The amount of the power received by each slave device is a function of the antenna separation distance, and is thus different at each slave device location. The RF-energy modulation system monitors the power requirements of the slave device within which the modulation system is included, and modulates the tuning of the secondary antenna to maintain the proper power reception level. Advantageously, such modulation controls the power reception by the slave device, versus dissipating energy already received by the slave device. In a preferred embodiment, the RF-energy modulation system controls the power received by a multiplicity of microstimulators implanted in a single patient.

Description:
The present application claims the benefit of U.S. Provisional Application Ser. No. 60/293,302, filed May 24, 2001, which application is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to implantable medical devices and more particularly to an improvement to known methods for providing power from a single external device to a multiplicity of implantable microstimulators implanted within the same patient. Known implantable microstimulators receive power inductively from an external device. The improvement provided by the present invention dynamically adjusts the power reception of individual microstimulators to compensate for differences in the separation of each microstimulator from the external device. 
     Implantable microdevices have the potential to address a number of useful purposes. These purposes range from pain mitigation to Functional Electrical Stimulation (FES). Some of these applications require that a multiplicity of microdevices be implanted in a single patient. For example, paraplegics and quadriplegics often have muscles capable of functioning, but are paralyzed due to damage to nerves that carry impulses to the muscles. Additionally, individuals afflicted with neuro degenerative diseases such as polio and Amyotrophic Lateral Sclerosis (also known as ALS, or Lou Gehig&#39;s disease) may be similarly disabled. Functional Electrical Stimulation provides such individuals with use of their muscles by providing artificial stimulation pulses to the patient&#39;s muscles, which stimulation pulses result in a desired movement. However, in order to provide effective use of muscles, a number of microstimulators must stimulate various muscles in the arms or legs of the patient in a coordinated fashion. 
     Prosthetic devices have been used for some time to provide electrical stimulation to excite muscle, nerve or other tissue. Some of these devices have been large bulky systems providing electrical pulses through conductors extending through the skin. Disadvantageously, complications, including the possibility of infection, arise in the use of stimulators which have conductors extending through the skin. 
     Other smaller stimulators are implants which are controlled through high-frequency, modulated, RF telemetry signals. An FES system using telemetry signals is set forth in U.S. Pat. No. 4,524,774, issued Jun. 25, 1985 for “Apparatus and Method for the Stimulation of a Human Muscle.” The &#39;774 patent teaches a source of electrical energy, modulated by desired control information, to selectively control and drive numerous, small stimulators, disposed at various locations within the body. Thus, for example, a desired progressive muscular stimulation may be achieved through the successive or simultaneous stimulation of numerous stimulators, directed by a single source of information and energy outside the body. 
     Many difficulties arise in designing RF powered implantable stimulators which are small in size, and are also capable of receiving sufficient energy and control information to satisfactorily operate without direct connection. A design for a small functionally suitable stimulator, a microstimulator, is taught is U.S. Pat. No. 5,324,316 issued Jun. 28, 1994 for “Implantable Microstimulator,” incorporated herein by reference. The &#39;316 patent teaches all the elements required for successful construction and operation of a microstimulator. The microstimulator is capable of receiving and storing sufficient energy to provide the desired stimulating pulses, and also is able to respond to received control information specifying pulse duration, amplitude and shape. The microstimulator of the &#39;316 patent can also be easily implanted, such as by expulsion through a large gauge hypodermic needle. 
     A large number of microstimulators may be required to stimulate different nerves or muscles in a coordinated manner to achieve a desired motion, or to treat some other medical condition. Known microdevices may either be continuously powered through an RF signal, or may contain a battery that is periodically recharged through an RF signal. In either case, power is provided inductively from an external device to the implantable microdevices. Further, it is desirable to utilize a single external device to provide power to all of the microdevices in a single patient. When using such a single external device, the distance between the external device and the implantable microdevices may vary greatly. 
     For distances much shorter than the wavelength of the RF signal (e.g., a 3 MHz signal has a wavelength of 100 meter), the magnetic field decays as 1/r 3 . The energy received by the microdevice is proportional to the square of the magnetic field strength. Therefore, the energy received by a microdevice, with separation r from the external device, attenuates as 1/r 6 . As a result, small variations in the separation of individual microdevices from the external device, result in very large variations in the energy received by the microdevices. For example, a first microdevice located half the distance from the external device as a second microdevice receives sixty four times more energy than the second microdevice. The microdevices proximal to the external device may have difficulty dissipating the energy they receive, and overheat as a result. 
     Known microdevices have a surface area of about 2 cm 2 . Power dissipation from such a device is about 40 mW/cm 2  for a 2 degree centigrade temperature rise, resulting in a capability to safely dissipate 80 mW of power with a 2 degree centigrade temperature increase. A typical microdevice requires about 6 mW of power to charge. Thus, if the ratio of separation of individual microdevices from the external device is two, and the microdevice farther from the external device receives 6 mW, the microdevice nearer the external device receives 384 mW of total power, and therefore 378 mW of excess power. The requirement to dissipate the excess power results in a dangerous increase in the temperature of the microdevice and unacceptable heating of adjacent tissue. 
     What is therefore needed is a method for limiting the power received by microdevices that are near the external device. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the above and other needs by providing power reception circuits including control circuits that dynamically adjust the power reception circuits of a plurality of individual slave devices (e.g., implantable microdevices implanted in a single patient), and thereby maintain an acceptable level of power reception in each slave device. A master device provides the RF-energy to the slave devices through inductive coupling. The control circuit in each slave device monitors the power reception of the slave device they are included in, and detunes the power reception circuit within the slave device, when necessary, to limit the power reception level of the slave device. 
     In accordance with one aspect of the invention, there is provided a detuning circuit corresponding to each power reception circuit of each slave device. The slave devices receive power from a single master device through inductive coupling of a primary coil in the master device with secondary coils in the slave devices. The power transferred through the inductive coupling is proportional to the coil separation distance to the sixth power. Thus, the slave devices near the master device receive significantly more power than slave devices farther from the master device, and may over heat. The energy reception is also related to how well the power reception circuit is tuned to the resonant frequency of the primary coil. The detuning circuit monitors the power received by the slave devices, and detunes the power reception circuit from the primary coil when the power level exceeds a threshold, thereby reducing the energy received by the slave devices. 
     It is a feature of the invention that the detuning circuit reduces the coupling between the primary coil in the master device and the power reception circuit in the slave device. Known master devices are battery powered, and the life of the battery is an important feature of such master device. The master device provides power to implantable slave devices over an inductive link, and the power drawn from the master device by such inductive link is related to the magnetic coupling between the primary and secondary coils and the matching factor of the transmitting and receiving circuits. Advantageously, limiting power received by the slave devices, by reducing the matching factor between the master device and the slave devices, reduces the power drawn from the master device, and extends battery life. 
     It is an additional feature of the present invention to provide a controlled range of voltages to battery charging circuits within the implantable slave devices. The battery charging circuit is adapted to provide the correct power to the battery for effective charging. However, when the voltage provided to the charging circuit is higher than the battery charging voltage, power is dissipated within the charging circuit to achieve the necessary voltage reduction. Such power dissipation results in the generation of heat within the charging circuit, and a reduction of the life of the charging circuit. By controlling the range of voltage provided to the charging circuit, the present invention reduces the power dissipated within the charging circuit, and extends the life of the charging circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein: 
         FIG. 1  shows a master device and various muscle groups of a patient, which muscle groups may be stimulation sites; 
         FIG. 2  depicts a group of microstimulators implanted in a single muscle and a master device residing outside the skin of the patient; 
         FIG. 3  functionally depicts a power reception circuit and load, which circuit is retunably detunable according to the present invention; 
         FIG. 4A  plots a typical control signal for modulating the tuning and detuning of the power reception circuit; 
         FIG. 4B  plots a typical voltage level resulting from the modulation depicted in  FIG. 4A ; 
         FIG. 5  schematically depicts a first embodiment of a power reception circuit according to the present invention; 
         FIG. 6  schematically depicts a second embodiment of a power reception circuit according to the present invention; 
         FIG. 7  schematically depicts a third embodiment of a power reception circuit according to the present invention; and 
         FIG. 8  schematically depicts a fourth embodiment of a power reception circuit according to the present invention. 
     
    
    
     Corresponding reference characters indicate corresponding components throughout the several views of the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims. 
     A Functional Electrical Stimulation (FES) system generates electrical signals to stimulate nerves and muscles to provide movement for paraplegics, quadriplegics, and other individuals with a neural lesion, a neuro degenerative disease, or other physical condition depriving the patient of the use of the muscles. Similarly, heart or lung muscles may be stimulated to provide the appropriate blood flow and oxygen for various activities, and various sensors may be implanted to monitor body functions. A master device  14  of an FES system centrally carried on a patient  10  is shown in  FIG. 1 . The master device  14  includes a primary antenna to transmit power and control signals to secondary antennas in a multiplicity of slave devices. In an FES system, the slave devices typically comprise slave devices implanted in various muscle groups including lungs  12   a ,  12   b , heart  12 C, arm muscles  12   d – 12   g , and leg muscles  12   h – 2   k . As can be seen from  FIG. 1 , the distances, separating the master device  14  from slave devices implanted in various muscles, may vary greatly in a single patient. 
     A view of slave devices  16   a – 16   i  implanted in the leg muscle  12   j  is shown in  FIG. 2 . The master device  14  is separated from slave devices  16   a – 16   i  by the patient&#39;s skin  18 . U.S. Pat. No. 5,324,316 issued Jun. 28, 1994, for “Implantable Microstimulator,” incorporated by reference above, teaches the elements required for successful construction and operation of a microstimulator, which may serve as a slave device. A slave device according to the &#39;316 patent advantageously may be implanted through a large gauge needle, thus providing a minimally invasive implant procedure. 
     As is evident in both  FIGS. 1 and 2 , the separation of individual slave devices  16   a – 16   i  from master device  14  may vary greatly. Known implantable slave devices receive power inductively from an external device, such as the master device  14 , through a magnetic field. The primary antenna in the master device  14  thus comprises a primary coil, and the secondary antenna in each slave device thus comprises a secondary coil. The magnetic field received by a slave device, with separation distance r from the master device  14 , decays as 1/r 3 . The power received by the slave device is proportional to the square of the magnetic field strength, and as a result the power decays as 1/r 6 . Therefore, a first slave device half the distance from the master device  14  as a second slave device, receives about 64 times the power the second slave device receives. 
     A generalized power reception circuit  19  is shown connected to a load  28  in  FIG. 3 . The power reception circuit  19  may be included in each slave device  16   a – 16   i . An antenna  20  may be any device that receives energy. The antenna  20  is electrically connected by at least one conductor  30  to a matching network  22  (in the case of a single conductor  30 , the antenna  20  and matching network  22  may also be connected to a common ground). The matching network  22  electrically cooperates with the antenna  20  to match (i.e., to tune, or to increase the matching factor), the power reception circuit  19  to the power signals received by the antenna  20 . The load  28  is electrically connected by at least one conductor  30  to the power reception circuit  19  (in the case of a single conductor  30 , the load  28  and the power reception circuit  19  may also be connected to a common ground). Monitoring signal paths  32  connect the antenna  20 , the matching network  22 , and/or the load  28  to a control circuit  24 . The control circuit  24  may receive monitoring signals from the antenna  20 , the matching network  22 , or the load  28 , or any combination of the antenna  20 , the matching network  22 , or the load  28 , over the signal paths  32 . The control circuit  24  preferably monitors the voltage at a circuit node, but could monitor a current, energy level, or temperature of a circuit component, and these and other measures of power reception are intended to come within the scope of the present invention. Control signals  26  are sent from the control circuit  24  to the antenna  20  or the matching network  22 , or both the antenna  20  and the matching network  22 , to modify the electrical characteristics (e.g., resonate frequency) of the power reception circuit  19 , thus controlling the amount of power received by the power reception circuit  19  from the master device  14 . In a first example, when the power reception level measured by the control circuit  24  exceeds a first limit, the control circuit  24  uses the control signals  26  to reduce the power reception level (i.e., detune the power reception circuit  19 ), and when the power reception level drops below a second limit, the control circuit  24  uses the control signals  26  to increase the power reception level (i.e., retune the power reception circuit  19 ). The power reception circuit  19  may thus be retunably detuned. In a second example, a control circuit continuously adjusts the power reception circuit  19  to substantially maintain (i.e., maintain within a range) a target power reception level. Other methods of controlling the power reception level will be apparent to those skilled in the art and are intended to come within the scope of the present invention. 
     A plot of one example of the control signal  26  of the power reception circuit  19  is shown in  FIG. 4A , and a plot of the resulting power reception is shown in  FIG. 4B . When power is first received by the slave device, the power reception level begins to rise. The rate of rise depends on the strength of the magnetic field in the vicinity of the slave device. The control circuit  24  monitors the power being received, and when the power level reaches or exceeds a predetermined threshold, for example, a high voltage threshold Vh (preferable 6.0 volts), the control circuit  24  detunes the power reception circuit. When the power reception circuit is detuned, the secondary antenna is no longer tuned to the primary antenna, the magnetic coupling between the primary antenna and the secondary antenna is reduced, and the current induced in the secondary antenna is reduced. The reduction in current results in the power reception dropping. The control circuit  24  continues to monitor the power reception, and when the power reception drops to or below a predetermined minimum threshold, for example a low voltage threshold Vm (preferable 5.0 volts), the control circuit  24  retunes the power reception circuit, and power reception level begins to rise again. 
     A first power reception circuit  19   a  is shown schematically in  FIG. 5 . Such circuit may be included in each slave device. The antenna (i.e., secondary coil) of the power reception circuit  19   a  comprises an inductor L. The inductor L is electrically connected between ground and a first voltage node V 1 . A tuning capacitor Ct and first switch M 1  are electrically connected in series between the node V 1  and ground. A diode D is electrically connected between the node V 1  and a rectified voltage node Vr. A storage capacitor Cs is electrically connected between the node Vr and ground. A load in the form of a charging circuit  32  and a battery B are electrically connected in series between the node Vr and ground. A first control circuit  34   a  monitors the voltage at the node Vr, and controls the switch M 1 . The power signals are represented by solid lines, the monitoring signal is represented by a dotted line, and the control signal is represented by a dashed line. When the switch M 1  is closed, the capacitor Ct is electrically connected to the inductor L, thereby tuning the power reception circuit  19   a  to the primary antenna for efficient reception of the power transmitted by the master device  14 . When the switch M 1  is opened, the capacitor Ct is electrically disconnected to the inductor L, thereby detuning the power reception circuit  19   a  from the primary antenna and attenuating reception of power transmitted by the master device  14 . The control circuit  34   a  includes a first reference voltage and a second reference voltage. When the voltage at the node Vr exceeds the first reference voltage, the control circuit  34   a  opens the switch M 1 . When the voltage at the node Vr is below the second reference voltage, the control circuit  34   a  closes the switch M 1 . 
     A second power reception circuit  19   b  is shown schematically in  FIG. 6 . Such circuit may be included in each slave device. The antenna (i.e., secondary coil) of the power reception circuit  19   b  comprises an inductor L. The inductor L and a second switch M 2  are electrically connected in series between ground and a first voltage node V 1 . A tuning capacitor Ct is electrically connected between the node V 1  and ground. A diode D is electrically connected between the node V 1  and a rectified voltage node Vr. A storage capacitor Cs is electrically connected between the node Vr and ground. A load in the form of a charging circuit  32  and a battery B are electrically connected in series between the node Vr and ground. A second control circuit  34   b  monitors the voltage at the node Vr, and controls the switch M 2 . The power signals are represented by solid lines, the monitoring signal is represented by a dotted line, and the control signal is represented by a dashed line. When the switch M 2  is closed, the inductor L is electrically connected to ground to complete a tuned antenna circuit, thereby tuning the power reception circuit  19   b  to the primary antenna for efficient reception of the power transmitted by the master device  14 . When the switch M 1  is opened, the inductor L is in an open circuit, thereby detuning the power reception circuit  19   b  from the primary antenna and attenuating reception of the power transmitted by the master device  14 . The control circuit  34 B controls the switch M 2  in the same manner that the control circuit  34   a  controls the switch M 1 . 
     A third power reception circuit  19   c  is shown schematically in  FIG. 7 . Such circuit may be included in each slave device. The antenna (i.e., secondary coil) of the power reception circuit  19   c  comprises an inductor L. The inductor L is electrically connected between ground and a first voltage node V 1 . A tuning capacitor Ct is electrically connected in series with a varactor Dt, between the node V 1  and ground. A diode D is electrically connected between the node V 1  and a rectified voltage node Vr. A storage capacitor Cs is electrically connected between the node Vr and ground. A load in the form of a charging circuit  32  and a battery B are electrically connected in series between the node Vr and ground. A third control circuit  34   c  monitors the voltage at the node Vr, and controls the variable capacitance of the varactor Dt, and thereby controls the total capacitance between the node V 1  and ground. The power signals are represented by solid lines, the monitoring signal is represented by a dotted line, and the control signal is represented by a dashed line. When the voltage at the node Vr drifts away from a third reference voltage, the control circuit  34   c  adjusts the variable capacitance of the varactor Dt to drive the voltage at the node Vr toward the third reference voltage. 
     A fourth power reception circuit  19   d  is shown schematically in  FIG. 8 . Such circuit may be included in each slave device. The antenna (i.e., secondary coil) of the power reception circuit  19   d  comprises a second inductor L 2 . The inductor L 2  is electrically connected between ground and a first voltage node V 1 . The inductor L 2  includes a tap point substantially centered on the windings of the inductor L 2 . A third switch M 3  is electrically connected between the tap point and ground. A tuning capacitor Ct is electrically connected between the node V 1  and ground. A diode D is electrically connected between the node V 1  and a rectified voltage node Vr. A storage capacitor Cs is electrically connected between the node Vr and ground. A load in the form of a charging circuit  32  and a battery B are electrically connected in series between the node Vr and ground. A fourth control circuit  34   d  monitors the voltage at the node Vr, and controls the switch M 3 . The power signals are represented by solid lines, the monitoring signal is represented by a dotted line, and the control signal is represented by a dashed line. When the voltage at the node Vr exceeds the first reference voltage, the switch M 3  is closed, and the tap point on the inductor L 2  is electrically connected to ground, thereby detuning the power reception circuit  19   d  from the primary antenna and attenuating reception of the power transmitted by the master device  14 . When the voltage at the node Vr drops below a second reference voltage, the switch M 3  is opened, and the tap point of the inductor L 2  is electrically disconnected from ground, thereby tuning the power reception circuit  19   d  to the primary antenna for efficient reception of the power transmitted by the master device  14 . Those skilled in the art will recognize that a third inductor L 3  could be substituted for the inductor L 2 , wherein the circuit is tunned when the switch M 3  is closed, and detuned when the switch M 3  is open. Similarly, the tap point could be moved away from the center of the inductor L 2  or L 3 . 
     The power reception circuits described above in  FIGS. 5–8  represent exemplary embodiments, and any one of these circuits could be used to practice the invention. Various equivalent circuits exist also. In many applications, the power received by the slave device is used to charge a battery. In other applications, the power may be used to charge a capacitor or other energy storage device, or used directly to perform a useful task. These variations are intended to come within the scope of the present invention. Further, in the embodiments described herein, a single diode was used as a rectifying element. Those skilled in the art will recognize that other rectifying elements, such as a diode bridge, synchronous switch, etc. may be used, and are intended to come within the scope of the present invention. While the examples in  FIGS. 5 ,  6 ,  7 , and  8 , show circuit elements connected to a common ground, the circuit elements may also be connected to any common reference potential. Similarly, power may be transferred between a master device and a slave device using methods other than inductive coupling. Thus, any method for controlling power reception by detuning (i.e., reducing the matching factor) a secondary antenna circuit from a primary antenna circuit, regardless of variations in the power reception circuit or method of power transmission, is intended to come within the scope of the present invention. 
     While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.