A miniaturized Nafion-based glucose sensor: in vitro and in vivo evaluation in dogs.

We have developed an implantable glucose sensor based on a new tri-layer membrane configuration. The needle-type sensor integrates a Pt working electrode and a Ag/AgCl reference electrode. Its size is equivalent to a 25 gauge needle (0.5 mm in diamater). Poly (o-phenylenediamine) was used as an inner coating to reduce interference by small compounds present in the body fluids, and the perfluorinated ionomer, Nafion as a biocompatible, protective, outer coating. Glucose oxidase trapped in an albumin/glutaraldehyde matrix was sandwiched between these coatings. In vitro tests in buffer showed the sensors had a good selectively, a sensitivity of about 25 nA/mM, and a 90% response time of 33 s. Stabilization of the current following polarization required 10 to 30 min in vitro and 30 to 40 in vivo. Although these sensors remained stable for many weeks in saline solution, their implantation in animals resulted in the degradation of the protective Nafion outer coating, which in turn, led to the failure of the incorporated reference electrode. We demonstrated that if unprotected, the AgCl layer of the reference electrode rapidly dissolves in the biological environment. However, we later showed that in vivo degradation of Nafion can be prevented by heat curing. When heat cured sensors were subcutaneously implanted in dogs, the sensors' signal closely followed the plasma glucose level during glucose tolerance tests. The response of the sensors implanted in dogs was retained for 10 days.