Membranes formed of thermodynamically stable cubic phase lyotropic liquid crystals (LLCs) could replace the presently used polymeric membranes, applied to reduce the flux of
glucose in semicontinuous, subcutaneously implanted, user-replaced, miniature, amperometric
glucose sensors, assisting in the management of diabetes. LLC-forming amphiphilic compounds set and toughen spontaneously after mixing with water, without undergoing chemical change. When applied by doctor-blading, they form membranes having three-dimensionally interconnected
water channels of uniform diameter, with reproducible
glucose transport-characteristics. We find that the best studied cubic phase LLCs, which are formed of
monoolein and water, are not useful in their intended application because they are hydrolyzed by serum lipases. Those formed of
phytantriol, a liquid at ambient temperature, and water, are not hydrolyzed but change their shape and size in a
dehydration and
rehydration cycle. Because
glucose sensors are sterilized and stored in a sealed package in a dry atmosphere, drying and
rehydration must not change the transport characteristics. A third, novel, LLC-forming, amphiphile 1-O-beta-(3,7,11,15-tetramethylhexadecyl)-d-ribopyranoside, I, was synthesized, and its phase diagram was tailored by adding
Vitamin E acetate, to form a cubic phase. The phase was stable through the 20 degrees C-90 degrees C temperature range in excess of water and had the desired
glucose-transport characteristics. A preferred LLC, II, was formed of water and I containing 7 wt % of
Vitamin E acetate. When II was applied to a wired
glucose oxidase bioelectrocatalyst, sensors of reproducible
glucose-sensitivity were formed. At a 0.1 mm thickness of II, the membrane reduced the
glucose flux 5-fold and increased the 90% response-time by less than 2 min. The membrane was mechanically rugged, withstanding the approximately 1 N m(-2) maximal shear stress at 5 mm diameter
electrodes rotating at 4000 rpm. The activation energy for
glucose permeation through II was reduced to 15.6 kJ/mol, making the sensors's current less temperature-dependent than that of the polymeric-membrane overcoated implantable
glucose sensors.