A Microsensor Based on a Microcantilever Patterned with an Environmentally Sensitive Hydrogel
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A Microsensor Based on a Microcantilever Patterned with an Environmentally Sensitive Hydrogel J. Zachary Hilt1, 2, Amit K. Gupta3, Rashid Bashir1,3,4 * and Nicholas A. Peppas1, 2, 4 1. NSF Program on Therapeutic and Diagnostic Devices 2. Biomaterials and Drug Delivery Laboratories, School of Chemical Engineering 3. Department of Electrical and Computer Engineering 4. Department of Biomedical Engineering Purdue University West Lafayette, IN 47907-1283 U.S.A. * To whom correspondence should be addressed
ABSTRACT A process was developed for patterning thin films of environmentally sensitive hydrogels onto silicon microcantilevers. Microcantilevers have been shown to be ultra-sensitive transducers for chemical, physical, and biological microsensors. By patterning environmentally sensitive hydrogels onto silicon microcantilevers, novel microsensors were prepared for MEMS and BioMEMS applications. Specifically, a cross-linked poly(methacrylic acid) (PMAA) network containing significant amounts of poly(ethylene glycol) dimethacrylate (PEGDMA) was studied. This hydrogel exhibits a swelling dependence on pH. By increasing the environmental pH above the pKa of PMAA to cause ionization of the carboxylic acid groups, electrostatic repulsion is produced along the main polymer chain causing the polymer network to expand and swell. Therefore, a pH change induces swelling or shrinking of the polymer network and creates stress on the microcantilever surface causing it to bend. In this study, silicon microcantilevers were fabricated on p-type (100) SOI wafers. Covalent adhesion was gained between the polymer and the silicon surface through the modification of the silicon surface with γ-methacryloxypropyl trimethoxysilane. Hydrogels were patterned onto the silicon microcantilevers utilizing a mask aligner to allow for precise positioning. The micropatterned hydrogels were analyzed using optical microscopy and profilometry. The bending response of patterned cantilevers with a change in environmental pH was observed, providing proof-of-concept for a MEMS/BioMEMS sensor based on microcantilevers patterned with environmentally sensitive hydrogels. INTRODUCTION Microfabrication techniques originally developed for manufacturing integrated circuits have been refined over the last few decades and are now being applied to create microstructures for additional applications, such as microelectromechanical systems (MEMS), bioMEMS, microfluidic devices, lab-on-a-chip, and other microdevices. Because of the versatility of polymers as structural, sensing, and/or actuating elements, researchers have recently begun to integrate them into these devices. In particular, hydrogels have received a great deal of attention due to their favorable properties, such as high biocompatibility and ability to be made sensitive
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