(2b) Novel Microsensors From Biodegradable Smart Hydrogels

This research utilizes environmentally-responsive hydrogels as sensing components in novel microscale devices. Hydrogels are excellent materials for in vivo applications because they mimic important qualities of natural tissue and are biocompatible. Environmentally-responsive hydrogels can be designed to respond to a wide range of conditions such as temperature, ionic strength, and pH. Environmental changes result in a change to the equilibrium swollen volume of the hydrogel. This response is both predictable and repeatable. When coupled with the right transducers, environmentally-responsive hydrogels gain functionality as microscale biosensing components.

Biodegradable, pH-responsive hydrogels composed of poly(methacrylic acid) crosslinked with poly(caprolactone) diacrylate were synthesized. These novel hydrogels are very sensitive in the region just below the biological pH of 7.4, making them ideal for sensing localized acidosis in the human body. Hydrogels were photopolymerized atop silicon microcantilever beams which had been treated with an organosilane agent to allow covalent adhesion to the substrate. As the hydrogel swelled, force generated at the surface between the hydrogel and the silicon caused the beam to bend. This bending could be output as a change in resistivity by utilizing piezoresistive cantilevers or by deflection of a laser beam as in atomic force microscopy. When these microsensors were alternated between solutions of pH 3 and pH 7, they demonstrated durability and robustness. The microsensors also exhibited extreme sensitivity between pH 4.5 and pH 6.5. This work represents a significant step forward in the development of biodegradable microsensors for in vivo applications.