Abstract:
An improved antenna for use with an implantable microdevice, such as a microstimulator or microsensor, comprises a loop antenna on the case of the microdevice. The antenna receives data transmitted from an external device, and transmits data to an external device. Such a loop antenna may be formed from two cylindrical sections separated by an insulating material on the case of the microdevice, or by separating a metal cylinder into two parallel semi-cylinders separated by an insulating material. A tuning circuit comprising capacitors and/or varactors is used to obtain resonance in the loop antenna, thus creating a sufficiently large effective antenna aperture. In a preferred embodiment, the electrodes of the microdevice are modified to both perform their primary task of tissue stimulation and to perform a secondary task as the radiating elements of a loop antenna.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/239,289, filed Oct. 11, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to implantable medical devices, and more particularly to implantable micro stimulators or sensors, hereafter referred to as microstimulators or microsensors. Such devices have electrodes attached to muscle or nerve fibers, through which the devices electrically stimulate the muscle or nerve fibers, or sense one or more physiological states present in the muscle or nerve fibers. More particularly, the invention relates to an improved antenna for such implantable microdevices, for both receiving signals from an external device, and transmitting signals to an external device. 
     Neurological disorders are often caused by neural impulses failing to reach their natural destination in otherwise functional body systems. Local nerves and muscles may function, but, for various reasons, injury, stroke, or other cause, the stimulating signals do not reach their natural destination. 
     For example, paraplegics and quadriplegics have intact muscles and only lack the complete brain-to-muscle nerve link which conducts the signal to the muscles. 
     Prosthetic devices have been used for some time to provide electrical stimulation to excite muscle, nerve or other cells to provide relief from paralysis, and various other physical disorders have been identified which may be treated by electrical stimulation devices. 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 have been developed that are fully implantable and are controlled through high-frequency, modulated RF, telemetry signals. Such systems designed to stimulate nerves or muscles to provide motion are know as Functional Electrical Stimulation (FES) systems. 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 in accordance with desired control information, to selectively power and control numerous, small stimulators, disposed at various locations within the body. Thus, for example, a desired progressive muscular motion 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 implanted stimulators which are small in size, and in passing sufficient energy and control information to the stimulators to satisfactorily operate them without direct connection. A design of a small functionally suitable stimulator, a microstimulator, is taught is U.S. Pat. No. 5,324,316 issued Jun. 28, 1994 for “Implantable Microstimulator.” 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 defining pulse duration, current amplitude and shape. The microstimulator of the &#39;316 patent can also be easily implanted, such as by expulsion through a hypodermic needle. The &#39;316 patent is incorporated herein by reference. 
     Known microstimulators utilize a telemetry receiver based on modulating an inductive power signal provided to the microstimulator. Similarly, signals are back transmitted from the microstimulator using the same circuits. By using components already present in the microstimulator, these telemetry systems do not require substantial additional circuitry. However, such inductive telemetry methods are limited by the resonant frequencies of the existing coil, which are typically below 2 MHz. While this approach has proven adequate for many applications, there are potential problems with interfering signals. Further, much higher frequencies, 402 MHz to 405 MHz, have been designated by the Federal Communications Commission (FCC) for use with medical devices. 
     Telemetry methods utilizing monopole and dipole antennas are known for use in the FCC designated frequency range, however, such antennas are, primarily, electrical field devices. Electrical field devices suffer from high tissue detuning (i.e., the surrounding tissue interacts with the electrical nature of circuit components to the extent that some effectiveness of tuning is lost) and may not provide the best performance for implantable devices. Other telemetry systems utilizing a loop antenna inside the microdevice are also known in the art. Loop antennas have the advantage of being magnetic field devices, and are therefore less susceptible to tissue detuning. However, placing the loop antenna inside the case of a microdevice exhausts scarce space within the microdevices. 
     What is needed is a telemetry system, suitable for operation in the 402 MHz to 405 MHz frequency range, that does not suffer from high tissue detuning loss, and that does not take up substantial space within the implantable microdevice. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the above and other needs by providing a loop antenna formed on the case of an implantable microdevice. The improved antenna receives data transmitted from an external device, and transmits data to an external device. Such a loop antenna may be formed from two cylindrical sections separated by an insulating material on the case of the microdevice, or by separating a metal cylinder into two parallel semi-cylinders separated by an insulating material. A tuning circuit comprising capacitors and/or varactors is used to obtain resonance in the loop antenna, thus creating a sufficiently large effective antenna aperture. Advantageously, such a loop antenna is suitable for operation in the 402 MHz to 405 MHz frequency range, is a magnetic field device and therefore not susceptible to high absorption losses, and does not require space in the interior of the microdevice. 
     In accordance with one aspect of the invention, a loop antenna is formed on the case of an implantable microdevice. By forming the antenna on the case, space inside the microdevice is available for circuit components. In one embodiment of the invention, the existing electrodes, on the case of a microstimulator, are combined with a reactive circuit to create a loop antenna. 
     It is a feature of the invention to provide an implantable medical device having a loop antenna, which loop antenna is advantageously a magnetic field device. Magnetic field devices are less prone to degradation from tissue absorption than are electrical field devices, such as dipole and monopole antennas. Accordingly, once implanted, a magnetic field device is more stable and predictable than an electrical field device. 
     In accordance with another aspect of the invention, a loop antenna provided in an implantable medical device may be tuned with an array of capacitors and/or varactors. Because of the small physical size of the antenna, the antenna is not an effective radiator at the targeted operating frequencies without tuning. Accordingly, the capacitance provided by an array of capacitors and/or varactors is adjusted to be equal to the inductive reactance of the loop, resulting in a high Q circuit and a larger effective antenna size. 
     In accordance with yet another aspect of the invention, a telemetry system using a loop antenna provides non-inductive telemetry capability. Inductive telemetry requires that the transmitter and receiver be in very close proximity for effective operation. The telemetry system provided by the loop antenna does not include such limitations. 
    
    
     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 patient with an implanted microdevice and an external device adapted to be in telecommunicative contact with the implanted microdevice; 
     FIG. 2A depicts a first embodiment of a loop antenna formed on the case of an implantable microdevice; 
     FIG. 2B depicts a second embodiment of a loop antenna located on the case of an implantable microdevice; 
     FIG. 3A shows a preferred embodiment of a loop antenna created from the electrodes of a microdevice; 
     FIG. 3B shows a second preferred embodiment of a loop antenna created from one electrode of a microdevice; 
     FIG. 4A shows a telemetry system with a parallel connection; and 
     FIG. 4B shows a telemetry system with a series connection. 
     Corresponding reference characters indicate corresponding components throughout the several views of the drawings. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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. 
     As seen in FIG. 1, the present invention applies to a microdevice  12  implanted in a patient  10 . An external device  14  transmits signals, represented in FIG. 1 by the arced lines  18 , to the microdevice  12  and the microdevice  12  transmits signals, represented by the arced lines  16 , to the external device  14 . The signals  18  transmitted to the microdevice  12  are principally control signals. The signals  16  transmitted from the microdevice  12  may be status signals, including diagnostic signals and/or performance signals (e.g., battery voltage), or signals that represent sensed physiological values. Those skilled in the art will appreciate that signals used for other purposes may also be transmitted from an implanted device, and the transmission of those signals using a loop antenna formed on the case of an implantable device falls within the scope of the present invention. 
     The present invention pertains to a loop antenna  13  formed on the case of the microdevice  12 . Such a loop antenna  13  is shown in FIG. 2A in the form of two cylindrical sleeves  20 A and  20 B. The cylindrical sleeves  20 A and  20 B form the radiating element of the loop antenna. The cylindrical sleeves  20 A and  20 B are separated either by a gap or by an insulating material. 
     A tuning element is typically required to increase the effective aperture of a loop antenna. The tuning element is reactively matched to the radiating element to create a resonant circuit. A tuning element comprising a tuning circuit  26 A and a short  24  is shown in FIG.  2 A. The tuning circuit  26 A is electrically connected between the cylindrical sleeves  20 A and  20 B at adjacent points. The short  24  is electrically connected between the cylindrical sleeves  20 A and  20 B at adjacent points on the side of the microdevice  12  opposite the tuning circuit  26 A. 
     An alternative embodiment of a loop antenna  13 ′ is shown in FIG.  2 B. In this embodiment, a pair of parallel semi-cylinders  28 A and  28 B, with concave sides facing each other, on the case of the microdevice  12 , form the radiating element of the loop antenna. The edges of the semi-cylinders  28 A and  28 B are separated by an insulating material or by gaps. The tuning element for the antenna  13 ′ comprises a tuning circuit  26 B and a short  24 . The tuning circuit  26 B is electrically connected between the semi-cylinders  28 A and  28 B at one end of the semi-cylinders  28 A and  28 B, and the short  24  is electrically connected between the semi-cylinders  28 A and  28 B at the opposite end. 
     The embodiments described in FIGS. 2A and 2B are intended for use with a microdevice having a cylindrical case. Such a cylindrical microdevice is well suited for implanting using a large gauge needle or a cannula. However, those skilled in the art will recognize that many other shapes are viable for implantable microdevices. While the cylindrical and semi-cylindrical radiating elements of FIGS. 2A and 2B may not be appropriate for a non-cylindrical microdevice, the concepts taught for a cylindrical microdevice are readily adaptable to other shapes, and fall within the scope of the present invention. 
     Known microstimulators include electrodes at each end of the microstimulator body. A preferred embodiment of the present invention is shown in FIG. 3A, which uses the existing microstimulator electrodes  32  and  34  as the radiating element of the loop antenna. This embodiment is similar to the embodiment described in FIG. 2A, except that in FIG. 3A, the two cylinders that form the radiating element are not connected by a short. Such a connection would prevent the electrodes from performing their primary task of tissue stimulation. Here, the tuning circuit  26 A and short  24  of FIG. 2A are replaced by a first tuning circuit  36 A and a second tuning circuit  36 B. The first and second tuning circuits  36 A and  36 B and the electrodes  32  and  34  are designed to provide a resonant circuit at the transmit frequency, but the first and second tuning circuits  36 A and  36 B also are designed to have high impedance at stimulation frequencies. Thus, the electrodes  32  and  34  may serve both for stimulation and for data transmission. 
     A second embodiment of a loop antenna using a microstimulator electrode as the radiating element is shown in FIG.  3 B. In this embodiment, the electrode on one end of the microstimulator is divided by a gap, or an insulator, into two semi-cylindrical halves  38 A and  38 B. These semi-cylinders  38 A and  38 B are then electrically connected by a tuning circuit  40  at one end, and a short  24  at the opposite end. 
     Other electrode arrangements will be apparent to those skilled in the art. Many of these arrangements may be modified to provide a radiating element for a loop antenna, and such arrangements are intended to fall within the scope of the present invention. 
     The design of a tuning circuit to combine with the radiating elements described by FIGS. 2A,  2 B,  3 A, and  3 B, or other suitable radiating elements, is often difficult because of the difficulty in modeling the electrical behavior of such radiating elements. In the case of a receive circuit, this difficulty may be dealt with by using a tuning circuit comprising an array of capacitors and varactors. The varactors may be adjusted to arrive at the desired resonant circuit needed for efficient operation of the receive circuit. 
     The loop antenna of the present invention may be matched electrically to communication circuits in several ways to create an effective telemetry system. These ways include the use of series and parallel matching circuits. An example of a parallel matching circuit is shown in FIG. 4A. A transmit/receive switch  42  functionally has a first switched contact  43 A and a second switched contact  43 B, and one fixed contact  43 C. A transmit driver  44  is connected to the first switched contact  43 A, and a receiver amplifier  46  is connected to the second switched contact  43 B. The receiver amplifier  46  amplifies received signals and provides the amplified signal to the receiver  48 . The fixed contact  43 C of the transmit/receive switch  42  connects to a matching network  50 , and the matching network  50  connects to a tuning element  52  and a radiating element  54 , which tuning element  52  and radiating element  54  are configured in a parallel relationship. 
     In operation, the telemetry system of FIG. 4A functions as a transmit circuit by controlling the switch  42  so that the fixed contact  43 C is connected to the first switched contact  43 A. With the switch  42  in this position, the output of the transmit driver  44  is applied through the matching network  50  to the parallel-configured tuning element  52  and radiating element  54 , and is transmitted from the radiating element  54 . 
     When the fixed contact  43 C is connected to the receiver amplifier  46 , the telemetry system of FIG. 4A functions as a receiving circuit. That is, signals received through the parallel combination of the tuning element  52  and radiating element  54  are applied through the matching network  50  to the receiver amplifier  46 . The output of the receiver amplifier  46  is then sent to the receiver  48 . 
     A telemetry system including a series matching circuit is shown in FIG. 4B. A transmit/receive switch  56  functionally has a first switched contact  57 A and a second switched contact  57 B, and one fixed contact  57 C. A transmit driver  58  is connected to the first switched contact  57 A, and a matching network  60  is connected to the second switched contact  57 B. The matching network  60  provides received signals to a receiver amplifier  62 , and the receiver amplifier  62  provides an amplified signal to a receiver  64 . The fixed contact  57 C of the transmit/receive switch  56  is connected to a tuning element  66 , and the tuning element  66  is connected in series to a radiating element  68 . 
     The telemetry system of FIG. 4B functions operationally as a transmit circuit by controlling the switch  56  so that the fixed contact  57 C is connected to the first switched contact  57 A. With the switch  56  in this position, the output of the transmit driver  58  is applied to the serial-configured tuning element  66  and radiating element  68 , and is transmitted from the radiating element  68 . 
     When the fixed contact  57 C is connected to the second switched contact  57 B, the telemetry system of FIG. 4B functions as a receiving circuit. That is, signals received through the series combination of the tuning element  66  and radiating element  68  are sent through the matching network  60  to the receiver amplifier  62 . The output of the receiver amplifier  62  is then sent to the receiver  64 . 
     Other telemetry systems configurations will be apparent to those skilled in the art. The present invention relates to the use of a radiating element formed on the case of a microdevice, and the examples of telemetry systems shown in FIGS. 4A and 4B are merely provided as particular embodiments of systems within which the invention may be practiced. Any application of a radiating element as described herein, formed on the case of a microdevice, is intended to fall 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.