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

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 40 Vpk-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 10 kHz and 1 GHz, with the minimum frequency at an electrolyte concentration of 1 mM and Parylene-N thickness of 100 nm. 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 ± 55 nm to 565 ± 7 nm and from 1 mM to 10 mM 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 40 Vpk-pk applied during actuation, with an absolute minimum and maximum of ~2 × 10−4 V A and ~2 × 10−2 V A, respectively. To determine actuation dynamics systems were fabricated with Parylene-N thicknesses from 907 ± 28 nm to 348 ± 13 nm and equilibrated with 1 mM or 5 mM KOH, providing a system above and a system below the predicted characteristic frequency, respectively. The 1 mM KOH system displayed true strains from 18% to 30% with response times from 14.7 s to 4.7 s, with optimum response at applied electric fields of 16 kV/m or 40 Vpk-pk with an 80% duty cycle. The 5 mM KOH system had a maximum true strain of ~7% and response time of 14.9 s. The trend observed while increasing frequency was also observed when a 1 mM 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 15 Vpk-pk to 40 Vpk-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.