Abstract:
A method and apparatus are disclosed for transcutaneous communication using infrared light signals to communicate bidirectionally between electronic apparatus implanted within a living organism and electronic apparatus external to the body by which measurements of physiological data can be obtained and command signals can be imposed on implanted apparatus.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation-in-part of application Ser. No. 336,337, filed Dec. 31, 1981, and now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The invention pertains to a method and apparatus which minimize the power requirements for transcutaneous communication. Infrared light signals are used to control the bidirectional transfer of information between electronic apparatus implanted beneath the skin of a living organism and other apparatus which is located externally. Physiological and biochemical data can be measured and transmitted through the skin to the external apparatus. 
     BACKGROUND OF THE INVENTION 
     A primary concern with any electronic device intended for implantation within the body is its power consumption as the power requirements dictate the ultimate size and operational lifetime of an implant. Most devices are powered by batteries that must be replaced to restore power, and surgical removal always involves risk and inconvenience to the patient. This undesirable feature greatly detracts from the benefits which may be gained from an implant device. 
     Transcutaneous transmission of information requires that the transmitter power be minimized. Over a period of time, fibrotic tissue may increase the density of the dermal cover and affect transmitted signal power requirements in light-based communication systems. In addition, the proximity and alignment of transmitter and external receiver may affect signal power requirements. 
     The type of information that is to be transmitted must also be considered. Information may be transmitted in a weak signal by making it highly redundant, but this will decrease the rate at which information may be acquired. 
     With certain variables of physiological interest, it may be necessary to monitor baseline of slow variations to observe long-term trends and also to monitor more rapid dynamic changes of interest for diagnostic purposes. 
     Past designs which use a transmitter power level based on a worst-case analysis of the above factors have a shorter lifetime than necessary. 
     The present invention provides a method whereby implant transmitter power is controlled externally, based upon the time resolution desired. Thus, the methodology of the invention maximizes the working lifetime of the implants and/or allows a reduction of size for the implanted device. These benefits of the present invention will extend the range of practicality to include implants which heretofore have not been justifiable. 
     SUMMARY OF THE INVENTION 
     The method and apparatus of the invention consist of an external apparatus controlled by an operator and an internal implanted apparatus which responds to the external unit by transmitting a signal derived from an internal condition such that the external apparatus receives the signal with the desired clarity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The FIGURE is a block diagram of the preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the embodiment illustrated in the FIGURE, the external part of the system transmits a modulated infrared light signal to the implant by means of a voltage-to-current converter (21) driving a light emitting diode (22). The frequency of this externally transmitted signal is variable within a predetermined range. 
     The implant detects this light signal via a photodiode (1) which converts the externally transmitted light signal to a current signal which is transformed into a voltage signal by a current-to-voltage converter (2). The resulting voltage signal (a) is applied to a band-pass filter (3). The range of frequencies that the filter will pass is matched to the range of the external transmitter. The output of the filter (b) which is a replica of external signal (1) is applied to a frequency-to-voltage converter (4). One such type converter is known in the art as a so-called phase-locked-loop (PLL) demodulator which is used in this embodiment. The frequency range to which the PLL will respond is matched to that of the external transmitter. The PLL produces two output signals, one (c) indicating the presence of a received signal in the correct range which controls a transistor (5) which in turn switches power from battery (6) to the remaining internal circuitry (7-11). Thus the bulk of the internal circuitry consumes power only when the external apparatus requests implant activation. The other signal (d) is a voltage proportional to the specific frequency of the received signal (b). This level signal (d) is used to determine the voltage which will be applied to voltage-to-current converter (11). 
     The implant sensor (8) and signal conditioning circuity (9) produce a voltage signal (e) proportional to the physiological variable the implant is designed to measure. This voltage signal (e) is applied to a voltage-to-frequency converter (10) which converts the voltage signal (e) to a frequency-modulated on-off signal (f) whose frequency is in a range that is separated from that of the external transmitter by at least a factor of two. This signal (f) controls the switching of electronic switch (7) to produce a voltage signal (g) at the same frequency as that of the switching signal (f) but whose amplitude is governed by the received external light signal as described above (derived signal d). This sensor-dependent frequency and externally derived amplitude signal (g) is applied to a voltage-to-current converter (11) to drive the implant phototransmitter light-emitting diode (12). 
     The external photoreceiver (13-14) converts the received internal transmitter light signal to a voltage signal which is then filtered by band-pass filter (15) to remove frequency components outside that of the implant transmitter. This signal(h) is then directed along two different paths. 
     On one path, signal (h) is applied to rectifier (16) and low pass filter (17) to produce signal (i), the amplitude of which is proportional to the received light-signal average peak-to-peak amplitude. This signal (i) is applied to one input of a differencing or error amplifier (19), the other input of which (j) is supplied by an operator resolution control (18). The output of differencing amplifier (19) is error signal (k) which is proportional to the difference between the desired implant transmitter signal amplitude as set by control (18) and the externally received amplitude as measured by signal (i). This error signal (k) is applied to the external apparatus&#39; voltage-to-frequency converter (20) and phototransmitter (21-22) in such a manner as to cause the external transmitter frequency signal to vary such that the implant transmitter power will be varied by means described above to reduce the error. 
     The other path of signal (h) is to a phase-locked-loop demodulator (23), the output of which (signal m) is a voltage proportional to the frequency of the externally received implant light signal. The quality of signal-to-noise ratio of this demodulated signal (m) depends upon the quality of the received signal. Thus, at low implant transmitter power levels, the demodulated received signal (m) will be noisier than at higher implant power levels. The demodulated signal (m) is applied to a variable low pass filter (24) to remove noise which may be present in signal (m). At low implant power levels filter (24) may be adjusted by operator control (25) to pass to the output (signal n) only very low frequencies so that random noise in signal (m) would not appear in the output (n). However, this also requires that higher speed dynamic components of the signal (m) which are real and not noise would be lost. 
     If the operator, by means of control (18), sets desired implant power level to a greater value, filter (24) can be set to pass higher frequencies which may be detected with the subsequent increase in received signal-to-noise ratio. 
     Not shown in the embodiment represented in the FIGURE is a method of monitoring implant transmitter power. The output of external voltage controlled oscillator (20), signal (1), indirectly controls implant transmitter power. The frequency of this signal (1) can be monitored by devices known in the art to indicate implant transmitter power at any moment in time. This can be used in adjusting the location of the external receiver over the implant transmitter for greater efficiency. Also recording this frequency as a function of time can be used to accumulate knowledge of total implant power expended and to predict remaining implant lifetime. 
     It is understood that various other modifications will be apparent to, and readily be made by, those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be construed as encompassing all the features of patentable novelty which reside in the present invention, including all features which would be treated as equivalents thereof, by those skilled in the art to which this invention pertains.