We have developed a microcontroller-based implantable passive telemetry system. The passive telemetry system, which employs an 8-bit microcontroller and a minimum parts count, was developed to fill the need for continuous recording of ECG and sympathetic nerve activity under unrestrained and freely moving conditions. A number of passive telemetry systems have been recently developed to reduce power limitations, size and weight. These telemeters are powered from an external source of energy, and the data from the implantable telemeter is transmitted by using the grid-dip or reflected impedance principle. The developed device also uses magnetic coupling between coils to transmit power and data to the implanted telemeter, however, the telemeter digital output data is transmitted by an electromagnetic signal generated with a RLC passive series resonant circuit.

This is the developed passive telemetry system. The backpack contains a power controller, an oscillator, a tuned amplifier, a FM transmitter and a 9V battery. A power transmitting primary spiral coil is attached to the other side of this backpack. This is the implantable telemeter. The size is 35 mm x 45 mm x 10 mm, and weight is 16 grams.

The complete telemetry system block diagram. The computer sends the system power switch control command and the gain control codes of the ECG and NS amplifiers in the implantable telemeter to the backpack by an infrared beam. When the power controller receives the system power switch ON command from the computer, the controller then turns on the all circuits in the backpack. The oscillator generates a sinusoidal wave to drive a transmitting primary spiral coil. The telemeter is powered by the power receiving circuit. The variable gain amplifiers for ECG and NS are controlled through a microcontroller by the external on-off-keying transmitted power. The amplified ECG and NS are converted to digital data, and then the data is transmitted to the tuned amplifier by the RLC passive series resonant circuit. The tuned amplifier and FM transmitter transmit the data to the computer through the FM receiver.

The detailed circuit diagram of the power controller and oscillator. The power controller consists of a photo diode, four ultra low power operational amplifiers, and an 8-bit microcontroller. The oscillator and audio power amplifier generate a 8 Volt peak to peak sinusoidal wave at 200 kHz to drive a transmitting primary spiral coil. When the microcontroller receives the gain control codes of the ECG and NS amplifiers, the codes are sent by 144 bps on-off-keying of the 200 kHz signal. The total circuit current drain is 40 mA at 9 V. The power is furnished by a lithium 1.2 Ah battery, which provides 30 hours of continuous operation.

The detailed circuit diagram of the implantable telemeter. The power receiving circuit supplies plus and minus 1.5 volt power to the telemeter. The gain control codes are demodulated by the circuit, and then the codes are inputted to a microcontroller. The controller is the low power 8-bit CMOS RISC-like CPU with two 8-bit A/D converters. The variable gain instrumentation amplifiers are designed with low-power instrumentation amplifiers and an addressable dual digital potentiometer. The potentiometer controlled by the microcontroller sets the gain of the amplifiers. The maximum gains for ECG and NS are set at 40 dB and 80dB. The amplified ECG and NS signals are supplied to the A/D converters in the microcontroller. The data is transmitted to the tuned amplifier by the RLC passive series resonant circuit.

The microcontroller output waveform. The individual channel A/D converters simultaneously sample the amplified signals at the rate of 2kHz and convert each channel to an 8-bit digital binary code with a 1 bit duration of 23.8 ƒĘs. A frame synchronizing pulse 35.7us is incorporated into the pulse train. The rising and falling edges are transmitted by electromagnetic wave with a frequency of 8 MHz.

The detailed circuit diagram of the tuned amplifier and FM transmitter. The transmitted edges are received by the high gain 8 MHz tuned amplifier. The amplified edges are shaped and transmitted to the computer by the FM transmitter.

The experiments were carried out on rats. The left renal nerve@signal (RNA ) was recorded simultaneously by both the telemetry system and a direct lead wire connected to an amplifier with band pass filtering of 50 to 1000Hz. An example of the data sampled after surgery was shown. Discharge patterns of RNA were regular and showed a grouped activity synchronous with cardiac cycle. The results indicate that two recording methods are the same.

The ECG and renal nerve activity record from a cat.
We developed a new microcontroller-based implantable passive telemetry system.