A Wireless Chemical Sensing Scheme using Ultrasonic Imaging of Silica-Particle-Embedded Hydrogels (Silicagel)

In this paper, we introduce a wireless chemical sensing technique featuring ultrasonic imaging of a silica-bead-embedded hydrogel, named “silicagel”. By incorporating silica beads within a hydrogel network, its volume change in response to environmental stimuli can be remotely interrogated by ultrasonic imaging, either by directly measuring its dimensions (cross-section of the silicagel) or indirectly evaluating the back-scattered wave intensity. Various sizes (4 and 2 mm cube) and concentrations (0.05 and 0.1 w/v%) of silicagel fabricated with pH-sensitive poly (methacrylic acid-co-acrylamide) hydrogel is studied in-vitro, using a 40 MHz ultrasonic imaging system. For the silicagel with 4 mm cube and 0.05 w/v% (as a representative), its cross-section sensing method results in a sensitivity of 0.83 (mm2/mm2)/pH with a theoretical resolution of 0.001 pH while back-scattered intensity technique exhibit a sensitivity of 17.5 gray-scale intensity/pH with a resolution of 0.06 pH unit in the maximal response region (between pH 4 and 6). It is anticipated that the same technique can be applied to hydrogels sensitive to other stimuli (e.g., glucose, specific ions, biomarkers, etc.).