(176at) Hydrogels for Biomedical Applications: Characterization of Structure and Performance Via NMR Relaxometry

Crosslinked gelatin methacryloyl (GelMA) and polyethylene glycol diacrylate (PEGDA) hydrogels are widely used in biomedical engineering applications including drug delivery, stem cell delivery and tissue engineering. These network structures hold high water content and have tunable mechanical and diffusion properties based on changes in crosslink density and porosity.

Characterization of hydrogel pore volume and pore size distribution in these systems is often relegated to scanning electron microscopy of dried or freeze-dried hydrogel samples, although it is well-known that drying conditions influence the final structure. Porosity of these dried samples is, likely, not representative of the porosity in the native state and thus there is a need for methods characterizing porosity in the wetted state. Nuclear magnetic resonance (NMR) relaxometry applied to imbibed fluids provides insight into porosity of pore structures based on correlations between relaxation rate and pore size (i.e. surface area to volume ratio of a pore). The interpretation of NMR relaxometry data is dependent upon pore surface / fluid interactions, which enhance relaxation rates, as well as, diffusivity of the imbibed fluid. Characterization of wetted hydroxycellulose hydrogels via NMR relaxometry has been reported and revealed unimodal pore size distribution.

In the present study, transverse NMR relaxation rates have been measured for crosslinked GelMA and PEGDA gels at varying water concentration using a benchtop low-field NMR instrument. Predominantly, unimodal distributions of the relaxation rate of pore water were observed and the rates were found to be linearly proportional to weight percent of the polymer. While initial inspection of the data suggests a uniform pore size throughout each sample, rapid diffusion of water between pores could be an alternate hypothesis describing the uniformity. The results of this investigation and correlations between NMR relaxation rates and drug diffusion rates from the hydrogels will be described.