Paulo José Abatti, Sérgio Francisco Pichorim & Thorsten Enders
CPGEI / CEFET-PR - Paraná Federal Center of Technological Education
Av. Sete de Setembro, 3165 CEP : 80230-901 - Curitiba - Paraná - Brazil

INTRODUCTION This works describes the design and preliminary results in vitro of an injectable passive resonant circuit useful to monitor body internal pressure (in the muscle, brain or blood vessels).

METHODS The injectable resonant circuit, composed of a 68µH inductor, a 4.7nF capacitor (frequency @ 280kHz) and a moveable ferrite core, is encapsuleted in a glass tube (diameter of 1.5 mm and length of 10 mm). The inductor is constructed using a ferrite core (diameter of 0.75mm, length of 5.5 mm, µ=2000), and a 43 AWG wire (135 turns). The moveable core (0.9 x 0.9 mm) has its relative position (inside the glass tube) proportional to the external pressure, changing total inductance and, consequently, the circuit resonant frequency. The circuit resonant frequency is measured using three external coils (one primary excitation coil, located between two other receiving coil). The receiving coils are carefully constructed so that the direct influence of the primary on them is minimized. Thus, applying a trapezoidal current signal in primary coil, the internal circuit is excited during current changing period, and then set free to oscillate when current is constant. These oscillations are then received externaly, and after amplifying and filtering, the signal is processed to determinate its frequency, and therefore, the pressure value.

The receiving frequency measurement has been realized using an A/D-converter based on Schmitt-Trigger, a high frequency oscillator and counter circuits. Because of the noise due to practical problems of compensation, windows of time to get the right position of the natural oscillation has been used. A squarewave oscillator with a frequency of 44 MHz and six 4-bit-counters measure the width of the impulses, which is related to the resonance frequency of the implanted circuit. Multiplexers transfer these informations to a computer to calculate and display the transmitted values.

RESULTS The injectable circuit has shown a sensitivity of 16.6 kHz per millimeter of ferrite core movement, presenting for a specific membrane (air volume of 6.6 mm³), a final sensitivity of 880Hz/10mmHg. Preliminary receiver tests demonstrated that average measurement of frequency (ten measurements) yielded in a stable value (S.D. of 0.5 %).

DISCUSSION & CONCLUSIONS A injectable passive resonant circuit for pressure biotelemetry has been presented and its characteristics discussed. Preliminary results has shown that the system is able to monitor body internal localized pressure.