Frequency dependent sensitivity of hydrogel iontronic sensor

The hydrogel iontronic sensor (HIS) has attracted much attention in recent years due to its high sensitivity. The physical model to analyze the effects of various parameters on the sensitivity of the HIS is still lacking. In this work, we conduct experiments to study the effects of voltage frequency, sensor size, and ion concentration on the sensitivity of the HIS. The experimental results show that the sensitivity is highly dependent on frequency in the range of 20-1 MHz. We establish a theoretical model consisting of Possion-Nerust-Planck equations to describe the ion migration and incompressible Neo-Hookean constitutive equation to describe the hydrogel deformation. The theoretical results divide the sensitivity into three regions of frequency: the region dominated by fully formed electric double layers at the electrode-hydrogel interfaces, the region dominated by ionic relaxation of the hydrogel, and the region dominated by the dielectric property of hydrogel. The model agrees well with the experiments on the frequency dependence of sensitivity as well as the effect of size and ion concentration. This work may provide a guidance for the design of highly sensitive HIS.