<p>Natural killer (NK) cells are innate cytotoxic immune cells at the forefront of cancer immunotherapies. However, a key limitation is their poor infiltration into solid tumors. Our lab has established a model system to study NK cell migration mechanisms in hydrogels mimicking the extracellular matrix of tumors. This model comprises 3D poly(ethylene glycol) (PEG) hydrogels engineered with a point source of chemoattractant proteins to stimulate NK cell migration. The development of chemoattractant gradients is dependent on the physical properties of the matrix and its effect on protein diffusion. The objective of this study was to establish a diffusion model for the hydrogels using a model protein, bovine serum albumin (BSA).</p><p>Hydrogels were formed by photo-crosslinking 10% PEG-diacrylate in the presence of 0.05% photoinitiator for 4 minutes by long UVA light. The hydrogel mesh size, swelling ratio, and free volume were measured 24 hours after crosslinking. For release studies, BSA was encapsulated in the hydrogel and quantified using CBQCA assay. The normalized diffusion coefficient (D/Do) was determined using the obstruction, combined obstruction and hydrodynamic, Brinkman, and Fickian diffusion models in MATLAB.</p><p>The D/Do for BSA ranged from 0.002-0.65, with the Brinkman and combined obstruction and hydrodynamic models describing this system the most accurately. Effective diffusion coefficients calculated from the Fickian equations supported D/Do data and was comparable with literature, with our findings in the range of 1.6  to 2.7  cm²/sec. These models provide a valid way to understand protein diffusion in our PEG hydrogels and can be used to study protein diffusion and establishment of chemoattractant gradients.</p>