Characterization of the Network Structures of PEGDA and Dextran Hydrogels by Complementary Methods

The physical properties of hydrogels are directly affected by their network structure; thus, characterization of that structure is a key part of their development for specific applications. There are generally several different characteristic length scales from the molecular to macroscopic dimensions. Additive manufacturing of hydrogel components introduces additional levels of network structures, depending on the manufacturing methods. Thus, many different methods of characterizing network structures have been developed and used to characterize gels. In this presentation, two fundamentally different but complementary methods will be examined for their potential to evaluate the network structure. The first is the most widely used method – mechanical testing –, but the data is examined in terms of multi-parameter models. The second is the use of time domain low-field NMR (LF NMR), a method that has not been widely used but has the potential for rapid screening of hydrogel formulations. Two hydrogels with quite different network structures widely used in a variety of applications, including 3D printing, are examined: poly(ethylene glycol diacrylate) (PEGDA) and thiol-ene crosslinked dextran.<br/><br/>PEGDA hydrogels were synthesized from monomers of molecular weight from 575 to 8000 Da by photopolymerization at polymer concentrations ranging from 7% to 50 wt.% in water. Dextran gels were made by functionalization of dextran with pentanoate groups which makes it possible to crosslink them by a photopolymerized thiol-ene reaction with dithiothreitol.<br/><br/>Generally, hydrogels are assumed to follow the classic neo-Hookean elastic model under compression or tension, from which network mesh size is estimated. However, not all gels fit this model well, often with deviation increasing with strain. Multiple parameter models can accurately fit data over a broader range of strains and provide further structural information than the standard model. For PEGDA, molecular weights, and concentrations were varied. As the polymer concentration at hydrogel formation increases, the covalent and the entanglement contributions of the modulus both increase. For dextran, a range of molecular weights and polymer concentrations were tested, as well as pentanoate functionalization and thiol:ene ratio. As the thiol:ene ratio was varied, the covalent contributions increased, but the entanglement contributions were less strongly affected. When the polymer concentration at formation was increased, however, both contributions increased.<br/><br/>LF-NMR is an experimentally simple, quick, and non-destructive technique that has the potential to determine a distribution of characteristic length scales in hydrogels. Its speed and experimental simplicity suggest a potential use for the rapid characterization of hydrogel formulations in application development. Proton relaxation times in the time domain (T₂) are related to surface-solvent interactions, and theory can be used to convert the T₂ distributions into network mesh sizes. PEGDA gels were characterized in solution before polymerization, after polymerization, and after equilibration with water. A noticeable reduction in T₂ was observed for PEGDA gels upon cross-linking, indicative of the mobility constraint caused by the formation of the hydrogel network.