TECHNIQUES FOR THE LABORATORY CONSTRUCTION OF MINIATURE HYBRID-CIRCUITS: A 350-MG ECG-TRANSMITTER

YORK WINTER
Erlangen University, Institute of Zoology II
Staudtstrasse 5, 91058 Erlangen, Germany,
ywinter@biologie.uni-erlangen.de

INTRODUCTION: Telemetric methods are necessary for the investigation of small (<15g) flying animals in a wind-tunnel to minimize any disturbances of flight aerodynamics or kinematics by the measuring devices. The small transmitter size required here cannot be achieved with standard SMD (surface mount device) -components. I therefore developed a method for the laboratory construction of hybrid-circuits, i.e. for the connection of passive electronic components (resistors and capaci-tors) with unencapsulated ('naked') semiconductor chips, which are only 2% in volume of the SMD-package type.

METHODS: Transmission of the ECG-signal is done here by conventional pulse-frequency modulation using a biopotential amplifier which modulates an asymmetric subcarrier oscillator which in turn pulses a UHF-oscillator transmitting from its coil. Passive components (resistors, capacitors) were used in sizes from 1.0 x 0.75 down to 0.25 x 0.25 mm. To obtain 'naked' semiconduc-tor chips, standard SMD-components were decapsulated by immersing them within a teflon-gaze net into boiling, fumic nictric acid (99.9% HNO3). This treatment leaves the gold bonding wires intact. The printed circuit board was etched from a laminate of 25 m m Polimid film with double sided 18 m m copper sheets (Pyralux LF-8515R, Du Pont) by using a conventional photo-chemical method. The components were mounted onto the circuit board under a stereomicroscope by soldering (resistors and capacitors) or glueing with silver-filled conduc-tive epoxy glue (semiconductor bond wires). The epoxy glue was applied with the help of a micromanipulator after it was either filled into a glass electrode of the type used for intra-cellular recordings or placed as a small droplet onto the tip of a 100 m m stainless steel needle (minuten pin).

CIRCUIT SIZE AND PERFORMANCE: The final circuitboard with 22 components including a dual Op-Amp (TLC 25L2 C) and three transistors was 5.8 mm in diameter and 750 m m high. The complete transmitter weighed 350 mg including electrodes, transmitting coil, outer case and a 190 mg 1.55 volt silver oxide battery (no. 317; 9 mAh). The biocompatible coating for the implant specimen of this transmitter weighed an additional 100 mg. Average power consumption was 15 m A at a subcarrier oscillation frequency of 300 Hz. A battery lasted for about 3 weeks. The transmission range was between 2 and 5 m.

DISCUSSION: With the techniques described here it is possible to assemble very small electronic hybrid-circuits with the tools available in a standard biological laboratory. This was used here for the construction of an ECG-transmitter for a 10 g mammal. With a potential sampling rate of up to several kHz this same transmitter is usable for a variety of purposes from ECG to neurophysiology in small animals.