Fluorescently Labeled Gradient Hydrogels Reveal Matrix-Dependent Cell Responses to Substrate Stiffness

Abstract

Microfabricated stiffness gradient hydrogels hold significant value for advancing mechanobiology, tissue engineering, and in vitro tissue models. However, it remains challenging to design these materials given their broad processing parameter space. The continuum of stiffness values also makes it difficult to precisely correlate the local substrate properties and observed biological responses, often relying on cumbersome characterization methods such as atomic force microscopy. To address these bottlenecks, we present a straightforward thermophoresis-based fabrication strategy to pattern stiffness gradients in fluorescein isothiocyanate-labeled hydrogel network, which displays a polymer concentration-dependent fluorescence readout. This approach enables quantitative assessment of the gradient formation process and contactless stiffness mapping via standard microscopy imaging. Using gelatin methacryloyl and Gellan gum as model systems, it is shown that substrate stiffness and extracellular matrix protein composition work together to affect 3T3-L1 fibroblast cell morphology and migration, with the underlying hydrogel type also affecting the outcome. By offering a simple and reliable approach for characterizing stiffness gradient hydrogels, this work advances the thermophoretic fabrication platform, opening avenues for new biomaterial systems for understanding and controlling the cell-material interplay.