A Hydrogel Ionic Circuit-Based Electrical Stimulation System for Restoration of Denervated Muscles after Long-Gap Peripheral Nerve Injury

Introduction:: Skeletal muscle is one of the most abundant tissues and plays a role as one of the important effectors of the peripheral nervous system (PNS) in locomotion and respiration. The PNS also maintains the muscular structure and function through constant neural signal. Following peripheral nerve injury (PNI), there is loss of the neural signals to the muscles as they become denervated leading to muscle atrophy. Electrical stimulation (EStim) has been used to preserve muscle physiology during nerve regeneration to mimic the electrical neural signal from the nerves. An intensity-dependent improved muscle contraction force has been earlier reported with the maximum intensity of 16 mA [1]. Attempt to increase the intensity of EStim using carbon and stainless steel electrodes lead to tissue damage due to high temperature and pH change resulting from conversion of electronic current to ionic current at electrode/tissue interface (ETI) [2]. A hydrogel ionic circuit (HIC) was previously developed with the ability to absorb and neutralize the temperature and pH changes associated with the electrochemical reaction at ETI [2]. These properties made the HIC suitable for delivering high intensity EStim on tissues without tissue damage. We hypothesize that the HIC can be used to safely deliver a high intensity EStim to preserve the structure and function of denervated muscle after long-gap PNI.


Materials and Methods:: The HIC is composed of a hydrogel and saturated phosphate salt solution. The hydrogel is made from an ultraviolet-crosslinked poly (ethylene glycol) diacrylate (PEGDA)/polyethylene glycol dimethacrylate (PEGDMA 400). The HIC was set-up for EStim as depicted in Fig 1A. A pilot study was carried out for the safety of the HIC when used to deliver a high intensity of 32 mA at a frequency of 20 Hz and pulse width of 25 ms for 1 hr on the gastrocnemius muscle (GM) over the skin in a rat model. The temperature and pH changes were recorded. A 10 mm-long sciatic nerve transection and orthotopically sutured as autograft was used to induce denervation in the right hindlimb. A weekly EStim of the GM was carried out beginning from the second week of the surgery for 16 weeks. The sensory functions recovery was evaluated every two weeks using von Frey filament and Hargreaves tests for mechanical and heat responses respectively. Following 16 weeks of EStim, muscle contraction test was done after which the rats were euthanized. GMs were isolated to measure their wet weights. They were then processed for hematoxylin and eosin staining (H&E) to measure the average cross-sectional area of the myofibers.


Results, Conclusions, and Discussions:: The HIC is safe to deliver to a higher EStim intensity of 32 mA without any tissue damage. The temperature and pH of the skin over which the EStim was applied did not differ significantly from the baseline (Fig 1B, C). A weekly EStim of 32 mA also showed an early sensory recovery following sciatic nerve injury in the group that received EStim compared to the untreated group (Fig 1D, E). Following 16 weeks of weekly EStim, the group that received EStim has a significantly higher muscle contraction force when compared to the untreated group (Fig 1F). The rats were sacrificed, and GMs were isolated, weighed and processed for H&E. The muscle weight ratio (right/left) showed that the muscles of the EStim group were better preserved following denervation with respect to the untreated group (Fig 1G, I). The myofiber cross-sectional area, a hallmark of muscle atrophy, was also preserved significantly in the EStim group when compared to the untreated group (Fig 1H, J).

This study showed that the HIC can safely be used to deliver a higher current of EStim that could reach deeper muscle tissues without tissue damage, which is the problem associated with the commercially available electrodes when used to deliver a higher current. The higher EStim current delivered with the HIC can preserve both structure and function of denervated muscle. Therefore, the HIC is a potential substitute for the common electrodes used in muscle stimulation.


Acknowledgements (Optional): : This work was funded by NIH/NIAMS R21R21AR078439, University of Nebraska Collaboration Initiative Grant, Nebraska Stem Cell Research Project Grant.

References (Optional): : [1] Tamaki, H., et al., Electrical stimulation of denervated rat skeletal muscle retards trabecular bone loss in early stages of disuse musculoskeletal atrophy. J Musculoskelet Neuronal Interact, 2014. 14(2): p. 220-8. [2] Zhao, S., et al., Programmable Hydrogel Ionic Circuits for Biologically Matched Electronic Interfaces. Advanced Materials, 2018. 30(25): p. 1800598.