Hydrogel-Piezoelectric Bilayer Thin Film For Wireless Biochemical Sensing

This paper reports on a novel transducer for wireless biochemical sensing. The bilayer transducer consists of a fractal piezoelectric membrane, laminated with a pH-sensitive chemo-mechanical hydrogel. The proposed scheme utilizes the piezoelectric element as a wireless transducer and the phase transition behavior of hydrogels as a sensing element. Notably, the fractal design on the piezoelectric membrane enhances frequency response and linearity by employing a periodically repeated pore architecture. As a basis of the pore, a modified Hilbert space-filling curve is used. By laminating a pH-sensitive hydrogel onto a fractal-porous piezoelectric thin film, the overall transducer curls inward or outward, depending on the environmental pH (e.g., pH = 4, 8, and 12). The curvature exhibits a sensitivity of 10.5°/pH, yielding accordant ultrasound responses under excitation. The measured voltage outputs from an ultrasonic receiver were 0.393, 0.341, and 0.250 mV/cm<sup>2</sup> for curvature angles of 30°, 60°, and 120°, respectively. Overall pH sensitivity was 0.017 mV/cm<sup>2</sup>/pH. Ultimately, the biochemical sensing principle using a novel bilayer ultrasound transducer suggests a simple, low-cost, battery-less, and long-range wireless readout system as compared to traditional biochemical sensing.