AC frequency-based electrical stimulation of hydrogel microactuators employing Parylene-N coated electrodes

Abstract

This article demonstrates the first AC-frequency based electrical actuation of hydrogels employing coplanar microelectrodes coated with Parylene-N. For the first time hydrogel microactuators were operated at applied voltages up to 40Vpk-pk without any electrolyte electrolysis or external pumping equipment. An RC circuit model was developed to determine the characteristic frequency required to overcome electrostatic shielding, as the hydrogels were actuated in the polarizable potassium hydroxide (KOH). The characteristic actuation frequency was found to be between 10kHz and 1GHz, with the minimum frequency at an electrolyte concentration of 1mM and Parylene-N thickness of 100nm. This circuit model was also extended to calculate the total capacitance of the hydrogel-based system. To validate the circuit model, the total capacitance of the system was experimentally measured using capacitance and admittance spectroscopy, for Parylene-N thicknesses from 2145±55nm to 565±7nm and from 1mM to 10mM KOH. Good agreement was found at higher frequencies, while at lower frequencies the system demonstrated frequency and concentration dependent behavior. The spectroscopy results were used to calculate the apparent power of the system based off the maximum electric potential of 40Vpk-pk applied during actuation, with an absolute minimum and maximum of ~2×10−4VA and ~2×10−2VA, respectively. To determine actuation dynamics systems were fabricated with Parylene-N thicknesses from 907±28nm to 348±13nm and equilibrated with 1mM or 5mM KOH, providing a system above and a system below the predicted characteristic frequency, respectively. The 1mM KOH system displayed true strains from 18% to 30% with response times from 14.7s to 4.7s, with optimum response at applied electric fields of 16kV/m or 40Vpk-pk with an 80% duty cycle. The 5mM KOH system had a maximum true strain of ~7% and response time of 14.9s. The trend observed while increasing frequency was also observed when a 1mM KOH sample was subjected to increasing applied electric potential, which showed a 674% increase in true strain and a 703% decrease in response time, from 15Vpk-pk to 40Vpk-pk. All systems actuated displayed deformation at all frequencies tested; thus even a minimal frequency can disturb electrostatic shielding, but above the characteristic frequency deformation and response times were optimum. This work overcomes the previous operational challenges associated with electrical hydrogel actuation, and investigates the electrical and actuation characteristics of the hydrogel-based system. Moreover, as hydrogels operated at lower frequencies still displayed actuation, hydrogel actuation could occur at low power zones enabling its integration within ultra-low power portable systems.