Microcantilever sensing arrays from biodegradable,pH-responsive hydrogels

Biodegradable, pH-responsive hydrogels composed of poly(methacrylic acid) crosslinked with varying molar percentages of polycaprolactone diacrylate were synthesized. The equilibrium swelling properties of these pH-responsive materials were studied. Methods were developed to incorporate these novel hydrogels as sensing components in silicon-based microsensors. Extremely thin layers of hydrogels were prepared by photopolymerization atop silicon microcantilever arrays that served to transduce the pH-responsive volume change of the hydrogel into an optical signal. Organosilane chemistry allowed covalent adhesion of the hydrogel to the silicon beam. As the hydrogel swelled, the stress generated at the surface between the hydrogel and the silicon caused a beam deflection downward. The resulting sensor demonstrated a maximum sensitivity of 1 nm/5.7×10⁻⁵ pH unit. Sensors were tested in protein-rich solutions to mimic biological conditions and found to retain their high sensitivity. The existing theory was evaluated and developed to predict deflection of these composite cantilever beams.