As part of our overall long-term objective of designing a glucose sensor for long-term subcutaneous implantation, a coil-type implantable glucose sensor loaded with excess glucose oxidase (GOD) inside the coils of a 0.125mm diameter coiled platinum-iridium wire has been developed. The excess GOD was immobilized in a glutaraldehyde/bovine serum albumin (BSA) gel reinforced with cotton and located inside the coils chamber of the sensor. The excess GOD increased the lifetime of the sensor. Based on this coil-type design, various coil-type glucose sensors with cellulose acetate (CA), poly(vinyl chloride)(PVC), polyurethane (PU), poly(bisphenol A carbonate) (PC) and Nafion outer membranes were investigated and compared. Comparatively, Nafion based biosensors provided the best long-term response stability. However, Nafion can still not meet the lifetime requirement of the coil-type sensor with high enzyme loading because the observed function failure of these sensors was indeed caused by outer membrane damage rather than loss of enzyme activity. Additional experiments also revealed that hydrogen peroxide accumulation occurred in the GOD impregnated cotton when the sensors were not polarized which could cause a small false positive measurement. However, this artifact can be easily avoided by using an appropriate measurement technique.