Hydrogel biomaterials are effective tools to probe and direct the complex interplay between cells and their microenvironment, an array of chemical and physical cues that dictate cellular functions and fate. For this, materials are needed that afford property control at specific locations and times in response to different stimuli (e.g., light, enzymes, reducing conditions). Enabling technologies to achieve this level of property control are combinations specific ‘click’ and cleavable chemistries, owing to their often rapid, cytocompatible, and orthogonal reactivity for additional or removal of components from synthetic matrices. Here, we have sought to create hydrogel-based materials for controlling cellular microenvironments in vitro or in vivo using select thiol–ene click chemistries and degradable moieties.
For in vitro microenvironments, building blocks that are accessible and allow the formation and modification of the base hydrogel are needed toward the widespread use of synthetic extracellular matrix mimics. To address this, we designed a hydrogel formed by the reaction of allyloxycarbonyl-functionalized peptides and thiol-functionalized poly(ethylene glycol), monomers that are modular and created using straightforward syntheses.1 These monomers were polymerized with cytocompatible doses of light (~ 1 min, 10 mW/cm2 at 365 nm) to form hydrogels with moduli mimicking a range of soft tissues (E ~ 1-10 kPa). Additionally, by polymerizing hydrogels with excess thiol functional groups, a biochemical cue (e.g., the adhesion peptide RGDS) modified with an allyloxycarbonyl group was added after hydrogel formation using photolithographic techniques, where good pattern fidelity was observed using Ellman’s reagent, a simple and cost effective assay. Human mesenchymal stem cells (hMSCs) remained viable and spread after encapsulation and photopatterning within enzymatically degradable matrices, demonstrating the utility of this approach for three-dimensional cell culture. Similar materials currently are in use for the controlled differentiation of hMSCs and culture of breast cancer cells [In preparation].
For in vivo applications, chemistries that respond to stimuli found within cellular microenvironments (e.g., enzymes, pH, reducing conditions) are desirable for controlling material properties. Harnessing the power of reversible click reactions, we created injectable hydrogels that form under cytocompatible conditions and degrade in response to thiol-rich reducing conditions found in tissues containing cells with high metabolic activity (e.g., tumors).2 Specifically, the reaction between multifunctional monomers decorated with maleimides and thiols was utilized for hydrogel formation, and the selection of thiol dictated the degradability of the resulting linkage under thiol-rich reducing conditions. In a reducing environment, arylthiol-based thioether succinimide linkages underwent degradation by click linker cleavage and thiol exchange (short time scales ~ days) and ester hydrolysis (long time scales ~ weeks), whereas alkylthiol-based thioether succinimide linkages only underwent ester hydrolysis. Building upon this, we recently incorporated a photolabile moiety within these responsive materials to enable material degradation and therapeutic release in response to both endogenous and exogenous stimuli toward patient-specific treatment regimens [In preparation]. These unique chemistries are promising for controlling the delivery of proteins and cells within in vivo microenvironments.
1. L. A. Sawicki and A. M. Kloxin, Biomaterials Science, 2014, 2, 1612-1626.
2. P. M. Kharkar, A. M. Kloxin, K. L. Kiick, Journal of Materials Chemistry B, 2014, 2, 5511-5521.