Effect of Structure and Hydration Level on Water Diffusion in Chitosan Membranes

DOI: 10.1002/mats.202000064 traditional semiconductor devices rely exclusively on electronic currents rather than molecular or ionic movement, they are unsuitable for communicating with living organisms. To address this challenge, bioprotonic devices interface with living organisms by incorporating components that facilitate both molecular/ ionic as well as electronic transport. [8] The bioprotonic devices reported to date, have focused on delivering one type of ion, H+, to biological systems. [9,10] Expanding these devices range of molecular/ionic species is an anticipated future advancement. A standard bioprotonic device consists of a conductive polymer membrane deposited onto an inert substrate (typically SiO2) that lies between two palladium hydride (PdHx) protodes. The device is turned on when a voltage is applied to its gate electrode, and the flow of protonic current is initiated. [11] Protons are generated at the source protode through hydrogen oxidation (PdH → Pd + H+ + e−)[8,12] and flow across the conductive polymer membrane toward the drain protode. Because the flow of protonic current in these devices depends on the transport properties of the polymer membrane, designing membranes with suitable molecular/ionic transport properties and biocompatibility is essential for the advancement of bioprotonics. One candidate polymer for bioprotonic devices is chitosan, a de-acetylated derivative of chitin. [5,6] Its chemical structure is shown in Figure 1. Chitosan is an appealing candidate because its biologically derived parent material, chitin, is widely available and can be extracted from the shells of crustaceans. [13] The design refinement of chitosan-based membranes for bioprotonic applications necessitates an understanding of the factors that influence the polymer’s transport properties. While there have been several experimental and theoretical studies of ionic diffusion in chitosan-based membranes,[14–17] the diffusion of molecular species in these membranes is less well-characterized. Understanding molecular diffusion is essential for the design of not only devices that deliver small molecules but also those that deliver ions, for example, in cases where ionic transport depends on a network of solvent molecules. In this article, we present results from ab intio molecular dynamics (AIMD) simulations showing how membrane structure and hydration level influence water diffusion in chitosan membranes. We consider a set of six model chitosan membranes at varying levels of hydration. Based on our AIMD simulations, we calculate the structure of each membrane and the diffusion coefficient of water within each hydrated membrane. The factors that influence water diffusion are explored, and Bioprotonic devices show promise for medicinal/therapeutic treatments because of their ability to deliver a controlled flow of small molecules and ions to living organisms. These devices rely on biocompatible, conductive polymer membranes to facilitate molecular/ionic transport. Herein, ab initio molecular dynamics simulations are used to probe the effects structure and hydration level on water diffusion in chitosan-based polymer membranes. The diffusion coefficient of water is shown to be highest in membranes with low overall densities and weak non-covalent interactions. Insight from these results may aid in optimizing the molecular/ionic transport properties of polymer membranes for bioprotonic applications.