Investigation of transcutaneous electrical nerve stimulation improvements with microneedle array electrodes based on multiphysics simulation.

This paper investigates microneedle array electrodes for transcutaneous electrical nerve stimulation, and compares their performance with conventional surface electrodes. A three-dimensional model of tissue was developed for finite element multiphysics simulations. Investigations included current density in different depths of a tissue, space constant under electrodes, specific absorption ratio of tissue, selectivity of stimulation, temperature rise, and blood flow. Results showed that microneedle electrodes have up to 10% higher selectivity than the surface electrodes. Furthermore, it was found that stimulation using microneedle electrodes provides more robust current density at different tissue depths compared to the surface electrode stimulation. Microneedle electrodes showed enhanced stimulation parameters, particularly for targeting a specific nerve in a specific depth of a tissue.

Molybdenum coated SU-8 microneedle electrodes for transcutaneous electrical nerve stimulation.

Electrophysiological devices are connected to the body through electrodes. In some applications, such as nerve stimulation, it is needed to minimally pierce the skin and reach the underneath layers to bypass the impedance of the first layer called stratum corneum. In this study, we have designed and fabricated surface microneedle electrodes for applications such as electrical peripheral nerve stimulation. We used molybdenum for microneedle fabrication, which is a biocompatible metal; it was used for the conductive layer of the needle array. To evaluate the performance of the fabricated electrodes, they were compared with the conventional surface electrodes in nerve conduction velocity experiment. The recorded signals showed a much lower contact resistance and higher bandwidth in low frequencies for the fabricated microneedle electrodes compared to those of the conventional electrodes. These results indicate the electrode-tissue interface capacitance and charge transfer resistance have been increased in our designed electrodes, while the contact resistance decreased. These changes will lead to less harmful Faradaic current passing through the tissue during stimulation in different frequencies. We also compared the designed microneedle electrodes with conventional ones by a 3-dimensional finite element simulation. The results demonstrated that the current density in the deep layers of the skin and the directivity toward a target nerve for microneedle electrodes were much more than those for the conventional ones. Therefore, the designed electrodes are much more efficient than the conventional electrodes for superficial transcutaneous nerve stimulation purposes.