Thinking Small: Progress on Microscale Neurostimulation Technology

Abstract

Objectives Neural stimulation is well-accepted as an effective therapy for a wide range of neurological disorders. While the scale of clinical devices is relatively large, translational and pilot clinical applications are underway for microelectrode-based systems. Microelectrodes have the advantage of stimulating a relatively small tissue volume which may improve selectivity of therapeutic stimuli. Current microelectrode technology is associated with chronic tissue response which limits utility of these devices for neural recording and stimulation. One approach for addressing the tissue response problem may be to reduce physical dimensions of the device. “Thinking small” is a trend for the electronics industry, and for implantable neural interfaces, the result may be a device that can evade the foreign body response. Materials and Methods This review paper surveys our current understanding pertaining to the relationship between implant size and tissue response and the state-of-the-art in ultra-small microelectrodes. A comprehensive literature search was performed using PubMed, Web of Science (Clarivate Analytics), and Google Scholar. Results The literature review shows recent efforts to create microelectrodes that are extremely thin appear to reduce or even eliminate the chronic tissue response. With high charge capacity coatings, ultra-microelectrodes fabricated from emerging polymers and amorphous silicon carbide appear promising for neurostimulation applications. Conclusion We envision the emergence of robust and manufacturable ultra-microelectrodes that leverage advanced materials where the small cross-sectional geometry enables compliance within tissue. Nevertheless, future testing under in vivo conditions is particularly important for assessing the stability of thin film devices under chronic stimulation.

Full activation pattern mapping by simultaneous deep brain stimulation and fMRI with graphene fiber electrodes

“…11). However, we found that CF electrodes had a very low charge-injection-capacity (CIL of 0.05 mC cm −2 vs. 10.1 mC cm −2 for GF electrodes), which is consistent with previous reports 20 . Due to this very low charge-injection-capacity, a much larger electrode size is required to inject sufficient charge for DBS to elicit the desired physiological response without damages to tissues or electrodes ( Supplementary Fig.…”

“…The as-prepared GFs had a loose structure, with the graphene sheets randomly oriented. After being air-dried, the GO sheets became densely stacked and aligned parallel to the fiber's main axis, which causes a shrinkage of the fiber diameter and provides GFs with high electrical conductivity, as well as excellent mechanical strength and robustness, which is superior to carbon fibers (CFs), known to have poor fracture resistance and is inconvenient to use [18][19][20] . The fiber diameter was determined by the pipeline dimension and the GO concentration 18 .…”

“…We attribute the high charge-injection-capacity of the GF electrodes to the porous structure and large surface area of their active stimulation sites that are accessible to ions. The high charge-injection-capacity of the GF electrodes allows for the use of small electrodes without the loss of stimulation efficacy, which is not only important to maintain a small MRI artifact size but could also activate a comparatively smaller population of neurons, thus improving the spatial resolution and selectivity of the neural stimulation 20 . STN-DBS in hemi-Parkinsonian rats using GF microelectrodes.…”

“…In addition, the large size of the CF bipolar electrodes (~586 μm, Supplementary Figs. 12, 13 and Supplementary Table 2) would also decrease stimulation selectivity, as well as lead to severe acute tissue damages and sustained chronic inflammatory responses 20,34 , all of which are undesirable for neural electrical modulation.…”

Development of visual Neuroprostheses: trends and challenges

“…Thereby, if the electrodes are too large, can interact with hundreds and thousands of neurons, but if they are too small, the impedances are too high and may miss nearby neurons entirely. On the other hand, the electrodes cannot be arbitrary small, since the amount of safe charge is reduced with electrode area (Pancrazio et al, 2017;Cogan, n.d.). In addition, the substrates that host the connecting pathways to individual electrodes have to remain perfectly functional within the biological microenvironment and must be completely insulated to prevent cross-talking between electrodes (Heiduschka & Thanos, 1998;Marin & Fernandez, 2010).…”