The relationship between polymeric hydrogel microstructure and macroscopic properties is of specific interest to the materials science and polymer science communities for the rational design of materials for targeted applications. Specifically, research has focused on elucidating the role of network formation and connectivity on mechanical integrity and degradation behavior. Here, we compared the mechanical properties of chain- and step-polymerized, photodegradable hydrogels. Increased ductility, tensile toughness, and shear strain to yield were observed in step-polymerized hydrogels, as compared to the chain-polymerized gels, indicating that increased homogeneity and network cooperativity in the gel backbone improves mechanical integrity. Furthermore, the ability to degrade the hydrogels in a controlled fashion with light was exploited to explore how hydrogel microstructure influences photodegradation and erosion. Here, the decreased network connectivity at the junction points in the step-polymerized gels resulted in more rapid erosion. Finally, a relationship between the reverse gelation threshold and erosion rate was developed for the general class of photodegradable hydrogels. In all, these studies further elucidate the relationship between hydrogel formation and microarchitecture with macroscale behavior to facilitate the future design of polymer networks and degradable hydrogels, as well as photoresponsive materials such as cell culture templates, drug delivery vehicles, responsive coatings, and anisotropic materials.

Wound healing results from complex signaling between cells and their environment in response to injury. Fibroblasts residing within the extracellular matrix (ECM) of various connective tissues are critical for matrix synthesis and repair. Upon injury or chronic insult, these cells activate into wound-healing cells, called myofibroblasts, and repair the damaged tissue through enzyme and protein secretion. However, misregulation and persistence of myofibroblasts can lead to uncontrolled accumulation of matrix proteins, tissue stiffening, and ultimately disease. Extracellular cues are important regulators of fibroblast activation and have been implicated in their persistence. Hydrogel-based culture models have emerged as useful tools to examine fibroblast response to ECM cues presented during these complex processes. In this Mini-Review, we will provide an overview of these model systems, which are built upon naturally-derived or synthetic materials, and mimic relevant biophysical and biochemical properties of the native ECM with different levels of control. Additionally, we will discuss the application of these hydrogel-based systems for the examination of fibroblast function and fate, including adhesion, migration, and activation, as well as approaches for mimicking both static and temporal aspects of extracellular environments. Specifically, we will highlight hydrogels that have been used to investigate the effects of matrix rigidity, protein binding, and cytokine signaling on fibroblast activation. Last, we will describe future directions for the design of hydrogels to develop improved synthetic models that mimic the complex extracellular environment.

Thiol–ene photoclick hydrogels with tunable biomechanical and biochemical properties for biological applications, including controlled cell culture, regenerative medicine, and drug delivery.

A method of cysteine alkylation using cyclopropenyl ketones is described. Due to the significant release of cyclopropene strain energy, reactions of thiols with cyclopropenyl ketones are both fast and irreversible and give rise to stable conjugate addition adducts. The resulting cyclopropenyl ketones have a low molecular weight and allow for simple attachment of amides via N-hydroxysuccinimide (NHS)-esters. While cyclopropenyl ketones do display slow background reactivity toward water, labeling by thiols is much more rapid. The reaction of a cyclopropenyl ketone with glutathione (GSH) proceeds with a rate of 595 M s in PBS at pH 7.4, which is considerably faster than α-halocarbonyl labeling reagents, and competitive with maleimide/thiol couplings. The method has been demonstrated in protein conjugation, and an arylthiolate conjugate was shown to be stable upon prolonged incubation in either GSH or human plasma. Finally, cyclopropenyl ketones were used to create PEG-based hydrogels that are stable to prolonged incubation in a reducing environment.

Late recurrences of breast cancer are hypothesized to originate from disseminated tumor cells that re‐activate after a long period of dormancy, ≥5 years for estrogen‐receptor positive (ER+) tumors. An outstanding question remains as to what the key microenvironment interactions are that regulate this complex process, and well‐defined human model systems are needed for probing this. Here, a robust, bioinspired 3D ER+ dormancy culture model is established and utilized to probe the effects of matrix properties for common sites of late recurrence on breast cancer cell dormancy. Formation of dormant micrometastases over several weeks is examined for ER+ cells (T47D, BT474), where the timing of entry into dormancy versus persistent growth depends on matrix composition and cell type. In contrast, triple negative cells (MDA‐MB‐231), associated with early recurrence, are not observed to undergo long‐term dormancy. Bioinformatic analyses quantitatively support an increased “dormancy score” gene signature for ER+ cells (T47D) and reveal differential expression of genes associated with different biological processes based on matrix composition. Further, these analyses support a link between dormancy and autophagy, a potential survival mechanism. This robust model system will allow systematic investigations of other cell‐microenvironment interactions in dormancy and evaluation of therapeutics for preventing late recurrence.