Sensing contact between microneedle array and epidermis using frequency response measurement

This paper presents a preliminary proof of concept where contact between a microneedle array (MNA) and epidermis was sensed using frequency response measurement. The microneedles used in this paper had a height of ∼350 μm, a pitch of 450 μm and were filled with saline. With the Randles electrodeto- electrolyte interface model, the cutoff frequency of MNA in contact with the epidermis was calculated to be significantly lower than that of the dry electrode on stratum corneum. This is in agreement with the experimental measurement of two different MNA configurations. In the first configuration, a pair of MNAs was used and in the second configuration, a MNA was paired with a wet (ECG) electrode. Using a frequency sweep from 10 Hz to 200 kHz on the first configuration, the cutoff frequency of the MNA pair was ∼6 kHz lower than that of the dry electrode pair. Similarly in the second configuration, the MNA-ECG electrode pair and dry electrode-ECG electrode pair were differentiated by a significant peak-to-peak voltage difference of ∼300 mV across series resistances of 10 and 100 kσ at frequencies ∼90-100 Hz. Through fitting the analytical and experimental frequency response plots of the MNA (R² = 0.994), the component values in the Randles model were estimated to be R₁ = 10 kσ, C₁ = 15 nF, and R _1s = 300 σ.