Human mesenchymal stem cells (hMSCs) sense and respond to the bulk elastic and viscoelastic properties of their microenvironment, as well as the spatial distribution of these mechanical signals. Hydrogel substrates with photo-controlled mechanical properties can allow one to probe the cellular response to localized variations in substrate viscoelasticity. Here, we report on a cytocompatible hydrogel culture system that allows photo-induced changes in viscoelasticity via an additionfragmentation

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... chain transfer reaction triggered by a network tethered photoinitiator. Tethering the photoinitiator to the network allowed for on-demand material property changes and spatiotemporal control of viscoelasticity. It was found that both the photoinitiation rate and chain transfer agent concentration contributed to the degree of photo-induced viscoelasticity. The loss factor (tan δ) of this system was tuned with the illumination intensity and chain transfer agent concentration, with a maximum value of 0.27 at 1 rad s -1 . In experiments with hMSCs cultured on the hydrogels, the cellular protrusions retracted in response to photo-induced viscoelasticity and this retraction could be confined to a single cellular protrusion through controlled photo-illumination. The retraction length and area of each protrusion was dependent on the initial proximity to the viscoelastic region.