Computational and experimental study on silk fibroin and silk fibroin polymer electrolyte for application in transient energy storage devices

Transient implantable medical bionics (TIMBs), such as, degradable and biocompatible batteries that disappear after their operation, are gaining attention because they potentially facilitate the deployment of novel instructive biomaterials for regenerative medicine. In the wider context, the generation of degradable electronics potentially addresses problems associated with electronic waste (E-waste) and these materials can influence biological processes in a controllable manner, (e.g. tissue regeneration and drug delivery via electrical stimulation). Implantable degradable and biocompatible batteries may be capable of satisfying the power requirements of some biomedical devices and then harmlessly degrading.1 Therefore, these batteries are of great interest and a number of different battery designs have been reported in the literature. In this work, Mg and Zn primary air batteries utilising a degradable and biocompatible polymer electrolyte (PE) (silk fibroin [SF] and choline nitrate [Ch]NO3] ionic liquid [IL]) is reported. The batteries detailed in this work offer up to 7.18 Wh L-1 and 3.89 Wh L-1, respectively, which is sufficient to power ultralow power devices (e.g. 10 to 1000 µW pacemakers).5 However, the chemistry that underpins the interactions and performance of the materials utilised in the batteries reported in the literature has yet to be fully explored. Therefore, classical molecular dynamics (MD) simulations have been employed to investigate the interactions between SF and water molecules which are essential to the functionality of the batteries detailed in this work.1 An alanine-glycine (Ala-Gly) crystal model is implemented to represent the SF11, 68 with 7.5 % water by weight, which is analogous to regenerated SF films.1, 116 The silk crystal structure, reported in this work, is in the silk I form (i.e. repeated β-turn type II conformation), because β-sheets are not the predominant secondary structure (ca. 26 %), instead, the 310-helix is the predominant secondary structure (ca. 37 %). Furthermore, the trajectory of water diffusion is reported to be anisotropic (diffusion is prominent along the X-axis of the crystal model) with a diffusivity calculated at 1.60x10-6 cm2 s-1 at 298 K. Similar results were observed for experimentally determined water diffusivity in SF films at 5.79x10-6 cm2 s-1 at 298 K, possessing 36 % β-sheet content. By drawing inspiration from experimental studies on degradable and biocompatible batteries in the literature and producing an appropriate computational model to represent the pertinent materials theoretically (and experimental validation of them), should result in an improved understanding of the science underpinning the interactions and performance of these devices. Thus, a greater control over these devices should be achieved and potentially enable new applications of transient energy storage devices