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

Abstract

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.

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

“…The model of Figure further expands on the work of Soltanzadeh et al It shows the developed 3D multilayer model of the skin, that consists of two nerves (target and adjacent) and a blood‐filled capillary that is located between the nerves. The electrical and thermal properties of each layer were adopted from former studies …”

“…Although the electrical impedance of the skin is not uniform in different parts of the body, it was shown that increasing the stimulation intensity can compensate the high impedance of the skin . However, in a former study, we showed that using flat surface electrodes for TENS may lead to skin burns in long term applications . Therefore, it can be concluded that developing a new type of TENS electrodes that can bypass the skin's impedance and target a specific tissue with high selectivity is beneficial.…”

“…In almost all of the former studies involving conventional surface electrodes, it was assumed that the tissues around a nerve fiber are homogenous conductive mediums, and in none of them the effect of blood flow in capillaries was studied. In our prior study, a multilayer three‐dimension (3D) model of the skin was developed and studied in COMSOL Multiphysics FEA software. Using both experimental work and modeling, it was proved that the performance of replicated painless microneedle arrays (MNAs) is more effective than the conventional surface electrodes for TENS, in the parameters of wasted power on the skin and selectivity of a tissue.…”

“…9 However, in a former study, we showed that using flat surface electrodes for TENS may lead to skin burns in long term applications. 10 Therefore, it can be concluded that developing a new type of TENS electrodes that can bypass the skin's impedance and target a specific tissue with high selectivity is beneficial. In the other words, electrodes with high efficiency are required to enable effective TENS for neuromodulation either in pain relief or chronic nervous disorder syndromes (like bladder overactive disorder).…”

Microfluidic dielectrophoretic cell manipulation towards stable cell contact assemblies

“…The advancement of MEMS technology, in which DEP forces has been widely employed, has been adapted to biomedical studies and technology development. So-called BioMEMS, these microdevices include the application for biosensors and diagnostics (Longo et al 2016;Shi et al 2014;Tsai et al 2010;Wasilewski et al 2018;Xiong et al 2015), drug delivery systems (Coffel and Nuxoll 2018;Nuxoll 2013), microsurgical (Abe et al 2016;Soltanzadeh et al 2017) as well as cell biology, proteomics and genomics studies (Ferrari 2007;Menon et al 2013). In this work, the application of BioMEMS for cell biology studies is demonstrated, particularly in the formation of stable cell contact, towards applications for cellular bioelectricity and intercellular interactions.…”