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Solid state sensors offer high performance, small size, lower cost, and compatibility with computing circuits. The current status of these sensors for biomedical applications is reviewed. Physical transducers may be used to measure pressure, flow, acceleration, velocity, etc. Chemical transducers may measure H+, K+, Na+, etc. Research required for their implantation is suggested.

A mathematical model of ventricular volume regulation based on fluid mechanical principles has been constructed using a systems engineering approach. The parameters used in the model are based on clinical observation, laboratory investigation, and presumptions that will be tested later. The model was constructed to be the basis of a computer simulation. Using the computer simulation, information obtained from the literature and laboratory hypotheses regarding pathophysiology, several enigmatic conditions were tested. The model predicted that over-production of cerebrospinal fluid, as in the case of choroid plexus papilloma, could by itself lead to distention of the ventricular system. In simulating pseudotumor cerebri, if cerebrospinal fluid absorption at the arachnoid villi is impaired and the brain itself is rendered incompressible by swelling, intracranial pressure rises and ventricular volume diminishes. Conversely, in normal-pressure hydrocephalus, if cerebrospinal fluid flow is restricted between the spinal and cortical subarachnoid spaces and the brain is made more compressible, the ventricular volume increases with minimal increases in intracranial pressure. This mathematical model and its associated computer simulation is useful in predicting the behavior of the volume of the cerebral ventricles to a variety of pathological phenomena.

Results in the development of intracranial pressure(ICP) telemetry systems are reported. Included are a single-channel ICP system and a two-channel ICP and temperature system. The unit used hybrid-integrated circuits housed in a Kovar flatpack and packaged with polyurethane or Hysol epoxy. Battery and radio frequency (RF) induction power supplies were tested. The two-channel systems used RF power at 3.5 MHz and transmitted signals around 120 MHz. The package measures 3 x 2 x 0.8 cm and weighs 9 g. The pressure range is -20 to +100 mm Hg with accuracy to 1.0 mm Hg. The implant measures absolute pressure and has a baseline stability of better than +/- 2 mm Hg/month. 17 dogs and 4 goats were used for in vivo evaluation. A summary of results is presented. Detailed evaluation is given in section II of this paper.

An implantable RF-powered dual channel stimulator was designed for hip stabilization. The approach included: small size, low power consumption, external powering, adjustable output current stimulus, dual channel operation an hermetic sealed electronic packaging. Thick film hybrid circuit techniques were used. The circuit and RF powering scheme were designed for low power operation with compromise to allow for linear and lateral displacements between the transmitting and receiving antenna. The output stimuli are biphasic negative going current pulses of 175 microseconds duration and 0-5 mA amplitude. Hermeticity is provided by enclosure of the electronics in a flat pack and subsequent epoxy coating of the implant assembly.

A small, implantable, telemetric device for the long-term monitoring of intracranial pressure has been described in part I of this article. This portion of the study is designed to demonstrate the in vivo operational characteristics of that implant device in experimental animals. Results indicate that this system can provide long-term in vivo operation with rapid and accurate responses to acute changes in pressure. Data also indicate some drift in the baseline measurement of pressure. No signs of abnormal body reactions to the units were observed.

Command receivers that control the functioning of additional implanted circuitry can be useful in many medical telemetry applications. The authors have designed and constructed a thick-film hybrid command receiver system that directly interfaces with the second-generation ICP telemetry system. This command receiver controls telemetry on/off state through enabling/disabling the telemetry pulse generator stage. Lithium battery powering of the implant is made practical through use of this command receiver. An automatic power shutdown feature is incorporated to guard against accidental battery drain. The unit uses two commercial CMOS integrated circuits, a transistor, a resistor and an RF coil.