Wireless chemical sensing schemes using environmentally sensitive hydrogels

In situ sensing of chemical parameters such as pH, glucose, enzymes, and other analytes is of immense importance in health care and environmental monitoring. There has been considerable research in this area using hydrogels as a chemo-mechanical transducer. Hydrogels are water-swollen polymers containing chemical groups that are sensitive to environmental stimuli. When there is a chemical change, e.g. alkali to acidic pH, hydrogel swells or shrinks. This reversible volume change can be coupled to capacitive, inductive or other sensing mechanisms. Despite many years of research, hydrogel-based sensors have not been of practical utility. This has been due to several reasons such as: 1) manipulation difficulties of hydrated gel and their integration with hard inorganic materials, 2) slow response of transducer due to diffusion limited sensing mechanism, 3) baseline and sensitivity drift, and 4) biofouling. In 2006, Ziaie\u27s group presented a wireless MEMS-based pH/glucose sensitive sensor using hydrogel. Despite successful demonstration for a proof of concept, the device had several shortcomings such as a complicated fabrication processes resulting in low yield, difficulty in snugfilling of a small cavity with hydrogel, and slow response time. The doctoral work presented here reports on several investigations undertaken to overcome the aforementioned shortcomings by developing novel, simple to fabricate, and inexpensive methods using 1) immobilized ferrogel (magnetic nanoparticles embedded in hydrogel) on top of an integrated flat coil, 2) hydrogel/polymeric magnet bilayer, and 3) microbubbles embedded hydrogel. The reversible swelling/shrinking of hydrogel results in 1) a change of inductance, which can be wirelessly monitored by standard phase 6 detection methods, 2) vertical movement of a polymeric magnet, which can be remotely detected by a giant magneto resistance (GMR) sensor, and 3) a variation of microbubble-density inside of the hydrogel, which can be evaluated using an ultrasound imaging equipment. The primary engineering contributions of this research includes design, fabrication, and characterization of the systems using pH and glucose sensitive hydrogels