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Injectable delivery systems that respond to biologically relevant stimuli present an attractive strategy for tailorable drug release. Here, the design and synthesis of unique polymers are reported for the creation of hydrogels that are formed in situ and degrade in response to clinically relevant endogenous and exogenous stimuli, specifically reducing microenvironments and externally applied light. Hydrogels are formed with polyethylene glycol and heparin-based polymers using a Michael-type addition reaction. The resulting hydrogels are investigated for the local controlled release of low molecular weight proteins (e.g., growth factors and cytokines), which are of interest for regulating various cellular functions and fates in vivo yet remain difficult to deliver. Incorporation of reduction-sensitive linkages and light-degradable linkages affords significant changes in the release profiles of fibroblast growth factor-2 (FGF-2) in the presence of the reducing agent glutathione or light, respectively. The bioactivity of the released FGF-2 is comparable to pristine FGF-2, indicating the ability of these hydrogels to retain the bioactivity of cargo molecules during encapsulation and release. Further, in vivo studies demonstrate degradation-mediated release of FGF-2. Overall, our studies demonstrate the potential of these unique stimuli-responsive chemistries for controlling the local release of low molecular weight proteins in response to clinically relevant stimuli., (© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Fibroblasts residing in connective tissues throughout the body are responsible for extracellular matrix (ECM) homeostasis and repair. In response to tissue damage, they activate to become myofibroblasts, which have organized contractile cytoskeletons and produce a myriad of proteins for ECM remodeling. However, persistence of myofibroblasts can lead to fibrosis with excessive collagen deposition and tissue stiffening. Thus, understanding which signals regulate de-activation of myofibroblasts during normal tissue repair is critical. Substrate modulus has recently been shown to regulate fibrogenic properties, proliferation and apoptosis of fibroblasts isolated from different organs. However, few studies track the cellular responses of fibroblasts to dynamic changes in the microenvironmental modulus. Here, we utilized a light-responsive hydrogel system to probe the fate of valvular myofibroblasts when the Young's modulus of the substrate was reduced from ∼32 kPa, mimicking pre-calcified diseased tissue, to ∼7 kPa, mimicking healthy cardiac valve fibrosa. After softening the substrata, valvular myofibroblasts de-activated with decreases in α-smooth muscle actin (α-SMA) stress fibers and proliferation, indicating a dormant fibroblast state. Gene signatures of myofibroblasts (including α-SMA and connective tissue growth factor (CTGF)) were significantly down-regulated to fibroblast levels within 6 hours of in situ substrate elasticity reduction while a general fibroblast gene vimentin was not changed. Additionally, the de-activated fibroblasts were in a reversible state and could be re-activated to enter cell cycle by growth stimulation and to express fibrogenic genes, such as CTGF, collagen 1A1 and fibronectin 1, in response to TGF-β1. Our data suggest that lowering substrate modulus can serve as a cue to down-regulate the valvular myofibroblast phenotype resulting in a predominantly quiescent fibroblast population. These results provide insight in designing hydrogel substrates with physiologically relevant stiffness to dynamically redirect cell fate in vitro. [ABSTRACT FROM AUTHOR]