Figure 4: Response of hydrogel microphone to underwater acoustic waves.

Hydrogel microphone with (a) a low (10⁻² mM) or (b) high (100 mM) ion concentration responds to sound waves with different voltage output. (c) Capacitance change in microphone under sound pressures from 4 to 70 Pa, where gel membrane of 100 mM ions (unfilled squares) gives a higher sensitivity (217 nF kPa⁻¹; device area of 9 mm²) than that of 10⁻² mM (unfilled circles). (d) Frequency response of the hydrogel microphone from 20 Hz to 3 kHz, where ion concentration and bias direction affect the performance of the hydrogel microphone. Error bars (95% confidence) represent the variation of measurements due to sound interference inside the water tank. (e) Fourier transform on the frequency response further reveals a periodicity that is supposed to be caused by some form of wave propagating and interference through the thickness of the hydrogel. The wave velocity can be calculated from the periodicity (55 Hz) extracted in e and the thickness of the hydrogel (1 mm) to be 0.055 m s⁻¹. This is further verified by the phase delay measurement shown in f, where a linearly increasing negative phase shift can be observed in the hydrogel devices (red—10⁻² mM; dark blue—100 mM) but not the commercial hydrophone (light blue). This phase shift indicates a time delay of 15–20 ms, which suggest a wave velocity of 0.05–0.067 m s⁻¹ (using 1 mm hydrogel thickness). The nature of this slowly propagating wave in the hydrogel is suspected to be ion concentration wave.