Gelatin and Tannic Acid Based Iongels for Muscle Activity Recording and Stimulation Electrodes

Abstract

Iongels are soft ionic conducting materials, usually composed of polymer networks swollen with ionic liquids (ILs), which are being investigated for applications ranging from energy to bioelectronics. The employment of iongels in bioelectronic devices such as bioelectrodes or body sensors has been limited by the lack of biocompatibility of the ILs and/or polymer matrices. In this work, we present iongels prepared from solely biocompatible materials: (i) a biobased polymer network containing tannic acid as a cross-linker in a gelatin matrix and (ii) three different biocompatible cholinium carboxylate ionic liquids. The resulting iongels are flexible and elastic with Young‘s modulus between 11.3 and 28.9 kPa. The morphology of the iongels is based on a dual polymer network system formed by both chemical bonding due to the reaction of the gelatin’s amines with the polyphenol units and physical interactions between the tannic acid and the gelatin. These biocompatible iongels presented high ionic conductivity values, from 0.003 and up to 0.015 S·cm–1 at room temperature. Furthermore, they showed excellent performance as a conducting gel in electrodes for electromyography and electrocardiogram recording as well as muscle stimulation.

Injectable PEDOT:PSS/cholinium ionic liquid mixed conducting materials for electrocardiogram recordings

“…34 In the same direction, cholinium carboxylate ILs are an attractive family of biocompatible and biodegradable electrolytes that have been proven to have low biological activity and non-potential skin irritation. [35][36][37] Cholinium carboxylate ILs have already demonstrated an arresting performance in iongels for long-term cutaneous electrophysiology, [38][39][40] but the worth of these ILs as conductivity enhancers for PEDOT:PSS is still a matter of research.…”

Recent Advances and Developments in Injectable Conductive Polymer Gels for Bioelectronics

“…66,67 The resulting iongels exhibited flexibility and high ionic conductivity (∼1.8 × 10 −2 S cm −1 performance in electrodes for electrophysiology. 69 Nevertheless, although the iongels exhibited a thermo-reversible transition at 60 °C, this high-temperature injectability limited direct use as injectable iongels in the human body. Intriguingly, an alternative approach involved incorporating ionic liquids (ILs) into conductive PEDOT:PSS aqueous dispersions.…”

“…In addition, ILs containing the cholinium cation and different phenolic acid anions (i.e., cholinium salicylate ([Ch][Sal]), cholinium gallate ([Ch][Ga]), cholinium vanillate ([Ch][Van]), and cholinium caffeate ([Ch][Caff])) demonstrated noncytotoxic properties and commendable anti-inflammatory, antioxidant, and antibacterial activities . Harnessing the interaction between phenolic compounds and proteins, Minari et al developed physically cross-linked iongels formed by a gelatin–tannic acid polymer network and cholinium carboxylate ILs, which possessed high ionic conductivity values at room temperature (∼1.5 × 10 –2 S cm –1 ), cell compatibility, and excellent performance in electrodes for electrophysiology . Nevertheless, although the iongels exhibited a thermo-reversible transition at 60 °C, this high-temperature injectability limited direct use as injectable iongels in the human body.…”

Recent Advances in Stretchable and Wearable Capacitive Electrophysiological Sensors for Long-Term Health Monitoring

“…The achieved ion gels were soft, stable, good flexible (ionic conductivity of 1.8 × 10 −2 S cm −1 , Young’s modulus of 14 to 70 kPa). Aguzin et al [ 251 ] also prepared ion gels for body sensors and bio-electrodes, considering the lack of biocompatibility of conventional ILs and polymer matrices where tannic acid is used as the cross-linker in the gelatin matrix and three different biocompatible cholinium carboxylate ILs. Their prepared ion gels provided good ionic conductivity (0.003 to 0.015 S cm −1 ) and were flexible and elastic with Young’s modulus of 11.3 to 28.9 kPa at room temperature, which is more adaptable to human skin.…”