NASA is developing miniaturized electrolyte and blood gas sensors to aid investigations into the influence of space flight on physiologic systems. These sensors are being applied in ex vivo blood flow loops as well as in in vivo wireless telemetric configurations. Our development approach is to first implement sensors in simple hand-made miniaturized catheter shaped configurations, and then migrate to micro planar configurations compatible with low- cost mass production. Catheter-based sensors are used for materials performance and biocompatibility testing, and for systems level integration, demonstration, and evaluation. For example, we have shown that pH sensitive polymer membranes cast on miniaturized catheters survive chronic implantation in rat subcutaneous tissue for periods up to 12 weeks with little loss in performance characteristics such as drift, sensitivity, selectivity, and response time. Microfabrication options for electrochemical sensors are based on a combination of thin and thick film technology with inexpensive non-silicon substrates. For the inorganic layers we are working with thin film technology with inexpensive non-silicon substrates. For the inorganic layers we are working with thin film evaporation and silk- screening, and for the organic layers we are comparing drop delivery and silk-screen approaches. The electrochemical cells are contacted from the back-side and each type of sensor is optimized on a separately fabricated substrate. Sensor combinations are then put into any desired array configuration with pick-and-place technology. This modular approach has many advantages over the integrated sensor approach which has been promoted as the ideal sensor solution for many years.