(192j) Construction of a Hydrogel System for Bioadsorption and Bioseparations By Molecular Modeling and Simulation

Hydrogels are formed by three-dimensionally cross-linked hydrophilic polymers. They have highly porous structures, good affinity for water, and consequently extraordinary capability to swell in water and retain a large amount of water. Hydrogels can also possess desirable biocompatibility either by starting from biocompatible polymers or through functionalization with biocompatible ligands. More importantly, they can exhibit dramatic volume phase transition (VPT), i.e., quasi-reversible swelling and collapse of hydrogel structures, in response to external physical or chemical stimuli. A great deal of effort has thus been devoted to developing hydrogels into material systems that fulfill important needs ranging from super absorbers to innovative biomedical applications including drug storage and delivery. In order to fully realize their potential, a thorough understanding of their various structural and dynamic properties is of essential importance and necessitates molecular-level characterization of their porous structures and volume phase transitions. However, our understanding in these aspects are still quite limited to date because (i) current hydrogel studies are mostly experimental and experiential with limited resolution and (ii) the few relevant theoretical studies are either based on statistical mechanics of idealized network (lattice) models or focused on specific fine-scale factors (e.g., hydrogen bond) using atomistic modeling and simulations of individual polymer chains or segments, which are not sufficient for representing the properties and structures of hydrogels. Additionally, dynamic properties, for the most part, have been overlooked. In this work, dextran hydrogel is selected to be studied because it has significant and desirable hydrophilicity and biocompatibility, and consequently greater potential for industrial and biotechnological applications. In order to examine practically important aspects of hydrogel properties and applications that are mostly unexplored and poorly understood, model systems are constructed with multiple dextran chains cross-linked together by H₂N(-CH₂)_n-NH₂. This cross-linking agent has two terminal -NH₂ function groups that can react with two -OH groups in different dextran chains to form -C-O-NH- cross-linkages. The number of methylene groups, n, take on a few different values in order to study the effects of the cross-linker length. We have successfully modeled and simulated such dextran hydrogel using a coarse-grain force field derived from the so-called M3B force field. The model hydrogel system is immersed in water which is also coarse grained into proper spherical beads. The current focus is on the volume phase transition (VPT) phenomenon, i.e., quasi-reversible swelling and collapse of hydrogel structures, which is triggered by temperature change and accompanied by significant swings of water content in the hydrogel. Water molecules are found to exist in diverse energetic states and hence different mobility in the hydrogel, which underlies the VPT phenomenon. The results and mechanisms will be presented and discussed in detail in this presentation.