Mechanical characterization of sequentially layered photo-clickable thiol-ene hydrogels

Multi-layer hydrogels are promising for tissue engineering due to the ability to control the local properties within each layer. However, the interface that forms between each layer has the potential to affect the performance of the hydrogel. The goals of this study were to characterize how the interface forms via its thickness and mechanical properties, identify its impact on the overall hydrogel properties, and provide new insights into how to control the interface. A photo-clickable poly(ethylene glycol) hydrogel was used to form bilayer hydrogels that were sequentially polymerized in a step-and-repeat process. Different processing conditions were studied: the time (0-20min) before initiating polymerization of the second layer (soak time, t_s) and the hydrogel crosslink density (the same, less crosslinked, or more crosslinked) of the first layer as compared to the second layer. Interface thickness was characterized by confocal microscopy, monomer transport by Fickian diffusion, single and bilayer hydrogel mechanics by bulk moduli measurements, and interface moduli measurements using AFM, nanoindentation, and strain mapping. The interface thickness ranged from ~70 to 600μm (1-10% of total height) depending on processing conditions, but did not affect the bulk hydrogel modulus. Analysis of monomer transport revealed that convection, due to changes in hydrogel swelling, and diffusion contribute to interface thickness. Nanomechanical analysis of bilayer hydrogels formed from soft (75kPa) and stiff (250kPa) layers showed a gradient in elastic modulus across the interface, which corresponded to strain maps. In summary, this work identifies that diffusive and convective transport of monomers across the interface controls its thickness and that a mechanically robust interface forms, which does not affect the hydrogel modulus. By controlling the processing conditions, the thickness of the interface can be tuned without affecting the mechanical properties of the bulk hydrogel.