In vitro electrochemical assessment of electrodes for neurostimulation in roach biobots

Abstract

Biobotics investigates the use of live insects as biological robots whose locomotion can be controlled by neurostimulation through implanted electrodes. Inactivity in the biobots (biological robots) can sometimes be noticed following extended neurostimulation, partly owing to incompatibility of implanted electrodes with the biobotic application or gradual degradation of the tissue-electrode interface. Implanted electrodes need to sufficiently exhibit consistent, reliable, and stable performance during stimulation experiments, have low tissue-electrode impedance, facilitate good charge injection capacity, and be compact in size or shape. Towards the goal of finding such electrodes suitable for biobotic applications, we compare electrochemical performances of five different types of electrodes in vitro with a saline based electrolytic medium. These include stainless steel wire electrodes, microfabricated flexible gold electrodes coated with PEDOT:PSS conductive polymer, eutectic gallium indium (EGaIn) in a tube, and “hybrid” stainless steel electrodes coated with EGaIn. We also performed accelerated aging of the electrodes to help estimate their longitudinal performance. Based on our experimentation, microfabricated electrodes with PEDOT:PSS and stainless steel electrodes coated with EGaIn performed remarkably well. This is the first time conductive polymer and liquid metal electrodes were studied comparatively for neurostimulation applications. These in vitro comparison results will be used in the future to provide a benchmark for subsequent in vivo tests with implanted electrodes in cockroach biobots.

State‐of‐the‐Art Electronic Materials for Thin Films in Bioelectronics

“…[ 78 ] In the work of Latif and colleagues EGaIn in a flexible conduit was tested as a neural stimulation electrode, and was reported to have charge injection capacity and impedance values about an order of magnitude superior to gold thin film reference samples. [ 79 ] However, the long‐term electrochemical stability in physiological media of such electrodes remains to be established, as well as fundamental properties like charge injection capacity should be definitively studied. Another remaining issue is the in vivo toxicity of gallium, indium, and other metals used in such eutectics.…”

Implantable Flexible Sensors for Health Monitoring

“…[ 40,41 ] For example, intramuscular electrodes made of metal have a significantly higher Young's modulus (approximately tens of GMPa) than the muscle tissue (between 10 and 100 kPa), which has the potential to cause chronic inflammation around the implant, especially in studies where there is motion‐applied stress between the electrode and the tissue. [ 42–45 ] In recent researches, mechanically pliable devices have already been demonstrated to cause fewer foreign body reactions in vivo. [ 46,47 ] Electronics devices with stretchable capabilities could provide even better mimicry of the sensing behavior of biological systems and measure vital signs reflecting the general physiological states.…”

Novel Electrode Designs for Neurostimulation in Regenerative Medicine: Activation of Stem Cells

“…The surface and geometry of implanted electrodes can play important roles in modulating galvanotaxis for neurostimulation since electrode surface features and overall geometry impact on electrode parameters such as electrical impedance, charge injection capacity, and stimulation efficiency. 94 Surface and geometry should be optimized to maximize the clinical effectiveness while minimizing the risks associated with electrical stimulation of the brain or tissue trauma during electrode implantation. In this section, the effect of surface morphology and geometrical features on neurostimulation is outlined.…”