MEMS Hydrogel Sensors for pH and Glucose Measurements

The new pH-responsive hydrogel precursor was developed from the standard pH-responsive  hydrogel precursor by preparing with50 % v/v of glycerol which increased the viscosity of the hydrogel and made possible thicker films in one spinning step. The glycerol did not chemically modify the hydrogel and was rinsed away after photo-polymerization. With its higher viscosity, the new pH-responsive hydrogel precursor was able to be spin-cast at more various spinning speeds and its film was thicker than the standard one. New dehydrated film thicknesses were approximately 45-98% more than standard dehydrated film thicknesses compared at the same spinning speeds varying from 300 to 600 rpm.The new pH-responsive hydrogel and the standard one could maximally swell about 4.7 and 2.2 times of their minimum deswelling thickness, respectively.

The glucose-responsive hydrogel precursor used in this study contained acrylamide as a monomer, 2-(dimethylamino)ethyl methacrylate as a pH-responsive monomer, glucose oxidase (GOx, EC 1.1.3.4, Type X-S from Aspergillus niger, 100,000-250,000 units/g of solid),N, N’-methylenebisacrylamide as a crosslinking agent, and 2,2-diethyoxyacetophenone as a photoinitiator dissolved in deionized water.This glucose-responsive hydrogel precursor was photopolymerized on gold electrodes of a sensor chip which was fabricated on a PMMA/ABS plastic substrate which is the copolymer of poly(methyl methacrylate) and acrylonitrile butadiene styrene.

This glucose sensor chip was immersed in 2-10 mM glucose in 10 mM NaCl. The resistance of the glucose sensor chip increased following the increasing in glucose concentration. Its time constant of each resistance response to glucose is 10 minutes.In 200-600 mM glucose, the capacitance of the glucose sensor chip decreased while the glucose concentration was increased. Its time constant of each capacitance response to glucose in 200-500 mM glucose is 4 minutes while the one in 600 mM glucose is 5 minutes