Somps, C.J., Madou, M.J., and Hines, J.W.
SENSORS 2000! Program
NASA-Ames Research Center, Mail Stop 213-2
Moffett Field, CA 94035-1000

INTRODUCTION The space life sciences research community needs physical, chemical, and biological sensors and associated instrumentation to monitor physiologic adaptation to the space flight environment. Since crew time is limited, automated data acquisition, processing, and storage are required, and wireless systems are clearly preferred. In response to these needs, NASA’s Sensors 2000! (S2K!) program is developing wireless telemetry systems for physiologic monitoring in animal models. In many cases, implanted sensors, in direct contact with biologic tissue, are the only way to acquire the needed information. However the performance of chronically implanted sensors, especially chemical and biological sensors, is not well understood.

METHODS To determine if sensors based on ion selective, polymeric membranes retain their sensing properties following chronic exposure to a physiologic environment, miniaturized pH sensors were implanted chronically in the subcutaneous tissue of alert rats. Sensors were constructed from a medical-grade PVC tubing (0.07" O.D.) and a sensing membrane cast over the end of the tubing which incorporated a plasticized, PVC support matrix doped with an H⁺ ionophore (tri-n-dodecylamine). Sensor performance, including stability, sensitivity, and response time were measured on the bench following as much as 28 days of implantation and compared to pre-implant values. Sensor stability and sensitivity were also measured in vivo for implant times up to 8 days. In vivo measurements of sensor sensitivity were made in rats subjected to elevated CO₂ levels.

RESULTS We found that our polymeric membrane pH sensors showed little change in sensitivity, selectivity, and response time following up to 28 days of implantation. The only significant change observed was a drift in the response baseline, which diminished to less than 0.03 pH units/day for implant times beyond 4 days. Similar drift was observed with direct measurements from implanted sensors. We also found that sensor sensitivity determined in vivo showed a close correlation with values obtained on the bench, at least over most of an 8 day implant period.