Vagus nerve stimulation (VNS) has the potential to treat various peripheral dysfunctions, but the traditional cuff electrodes for VNS are susceptible to off‐target effects. Microelectrodes may enable highly selective VNS that can mitigate off‐target effects, but they suffer from the increased impedance. Recent studies on microelectrodes with non‐Euclidean geometries have reported higher energy efficiency in neural stimulation applications. These previous studies use electrodes with mm/cm‐scale dimensions, mostly targeted for myelinated fibers. This study evaluates fractal microelectrodes for VNS in a rodent model (N = 3). A thin‐film device with fractal and circle microelectrodes is fabricated to compare their neural stimulation performance on the same radial coordinate of the nerve. The results show that fractal microelectrodes can activate C‐fibers with up to 52% less energy (p = 0.012) compared to circle microelectrodes. To the best of the knowledge, this work is the first to demonstrate a geometric advantage of fractal microelectrodes for VNS in vivo.

Vagus nerve stimulation (VNS) is a moderately effective treatment option for intractable epilepsy, depression, and pain, with promising applications in the treatment of gastrointestinal motility disorders like gastroparesis. The wide‐ranging potential indications of VNS suggest an opportunity for a better understanding of underlying neural circuits that can be targeted using microscale electrodes. However, microscale Platinum (Pt) electrodes often have limited charge injection capacity due to their high impedance. Recently, we experimentally demonstrated that microscale Pt electrodes with Vicsek fractal geometry can facilitate electrochemical charge transfer more efficiently with 57% higher cathodic charge storage capacity than conventional circular microelectrodes. Here we are investigating whether the fractal electrode may be more effective in eliciting small fibers in the vagus nerve with less energy consumption due to its enhanced charge injection capability. To evaluate the in vivo performance of these novel microelectrode designs and to determine whether these electrodes are capable of stimulating small, unmyelinated C fibers that supply the stomach, we designed a cuff electrode with an array with four pairs of fractal and circular microelectrode to explore the possibility of radial selective stimulation of vagus nerve. We determined the dimension of the cuff to be 760 μm in diameter to wrap around a rat's vagus nerve so that it can withstand the swelling of the nerve from chronic implantation. We patterned fractal microelectrode to have a geometrical surface area equivalent to that of a circle with 100 μm of diameter. Using a rat model, we have stimulated the right cervical vagus nerve with fractal/circular electrode and recorded the compound action potential (CAP) distal from the stimulation site with 5‐7 mm of conduction distance or the dorsal trunk of the abdominal vagus (conduction distance: 70 mm). We used an autonomous neural control (ANC) system, which can stimulate and record from the nerve with randomized stimulus parameter combinations (pulse amplitude: 0‐0.4 mA, pulse width: 0.02‐1 ms). We adapted a cathode‐first, alternating monophasic stimulation for the reliable suppression of stimulus artifact (pulse repetition frequency: 5 Hz). Results from right cervical VNS experiments in anesthetized rats showed that both the fractal and the circular electrodes can recruit small, myelinated Ad and B fibers to a similar degree (pulse amplitude: 0‐0.4 mA, pulse width: 0.1 ms) (Figure 1.), but fractal electrodes produce a more uniform and consistent fiber recruitment profile (Figure 2.). We observed volleys within the slow conduction velocity range (< 3 m/s), which we suspect to be signals from the C‐fibers. Our next plan is to obtain more experimental data to statistically identify the efficiency of various electrode designs in stimulating small fibers and to investigate the electrical potential profile of these electrodes in a biophysical model.