DEVELOPMENT AND CHARACTERIZATION OF MEMBRANES FOR BIOMEDICAL AND ELCTROCHEMICAL APPLICATIONS

Abstract

This dissertation focuses on developing polymeric membranes for two distinct applications. 1) Exosome isolation: Exosomes are nano-sized membrane-bound vesicles secreted by cells and present in body fluids. Exosomes play a critical role in intercellular communication by transforming information to maintain physiological homeostasis. Recent studies indicated that exosomes derived from mesenchymal stem cells (MSCs) are considered potential therapeutics for many diseases. One of the key challenges is to obtain efficient exosome isolation with high yield and purity. To address this challenge, a stimulus-responsive polymer-based membrane was developed to isolate MSC-derived exosomes directly from culture media in a single step, without any pretreatment. The membrane was obtained by modifying the surface of a sulfonated polysulfone support with subsequent polyelectrolyte layers and a temperature- and pH-responsive cationic pentablock copolymer (PBC) via electrostatic interactions. A total exosome yield of ~3x1011 particles/mL and total exosomal protein concentration of ~2000 µg/mL were obtained with a purity of ~8x109 particles/µg protein at 37 ˚C and pH 7.4, which was higher than the conventionally used isolation methods. The isolated exosomes demonstrated enhanced cell migration on 3T3 cells and neurite outgrowth on PC12-TrkB cells, confirming biological activity. Therefore, the PBC-modified membrane could be a simple and viable alternative to isolate exosomes with high yield and purity. 2) Vanadium redox flow battery (VRFB): VRFB utilizes vanadium ions in different oxidation states to store and release energy. Currently, perfluorosulfonic acid membranes, such as Nafion117, are widely used in VRFBs. However, Nafion-based membranes are costly, suffer from vanadium-ion crossover, and raise environmental concerns. Therefore, in this dissertation, poly(styrene-isobutylene-styrene) triblock copolymer-based proton exchange membranes with varying sulfonation degrees were developed as an alternative. The 34%-S-SIBS membrane exhibited physicochemical and proton-transfer properties that were better than or at least comparable to those of Nafion117. Moreover, the coulombic, voltage, and energy efficiencies of 34%-S-SIBS, along with its stable cycling performance, were on par with Nafion117. To further enhance performance, S-SIBS was blended with polyvinylidene fluoride or graphene. The modified membranes demonstrated performance that was at least as good as, and in some cases better than, Nafion117, providing a cost-effective alternative.