Abstract
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 addition-fragmentation 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.
Highlights
Many cells are anchorage-dependent and reside within the interstitial space of tissues, embedded in a proteinrich extracellular matrix (ECM)
Design of hydrogels with photochemically derived viscoelasticity A poly(ethylene glycol) (PEG) based hydrogel was designed with an allyl sulfide cross-linker and a covalently tethered photoinitiator to enable photo-controlled viscoelasticity
The precise location, duration, and magnitude of the hydrogel viscoelasticity was controlled through an addition-fragmentation chain transfer (AFCT) reaction using allyl sulfide cross-linkers and tethered thiyl radicals
Summary
Many cells are anchorage-dependent and reside within the interstitial space of tissues, embedded in a proteinrich extracellular matrix (ECM). The resulting deformation of the ECM informs the cells about the mechanical properties of their microenvironment and can lead to a change in the production of cytoskeletal proteins and cytoskeletal stress [1, 2] These biophysical signals provide cells with critical information necessary for the development, maintenance, function, and wound healing of tissues [3,4,5]. Under an imposed load or stress, the viscous component of a viscoelastic substrate is energy dissipative and gives rise to various time-dependent phenomena including creep—a time-dependent increase in deformation (i.e. strain) under an imposed constant stress—and stress relaxation—a time-dependent decrease in stress at constant strain These viscoelastic processes are thought to be an important component of the biophysical signals that direct tissue specific cell function through mechanotransduction [10]. The spatiotemporal variations of viscoelasticity within the body may act as a cue to guide specific cellular processes throughout development
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