Generation and recovery of β-cell spheroids from step-growth PEG-peptide hydrogels

Orthogonal enzymatic reactions for rapid crosslinking and dynamic tuning of PEG–peptide hydrogels

A biocompatible PEG–peptide hydrogel with dynamically tunable stiffness was developed through sortase A-mediated crosslinking and mushroom tyrosinase-triggered stiffening.

Thiol-norbornene photo-click hydrogels for tissue engineering applications

Thiol-norbornene (thiol-ene) photo-click hydrogels have emerged as a diverse material system for tissue engineering applications. These hydrogels are cross-linked through light mediated orthogonal reactions between multi-functional norbornene-modified macromers (e.g., poly(ethylene glycol), hyaluronic acid, gelatin) and sulfhydryl-containing linkers (e.g., dithiothreitol, PEG-dithiol, bis-cysteine peptides) using low concentration of photoinitiator. The gelation of thiol-norbornene hydrogels can be initiated by long-wave UV light or visible light without additional co-initiator or co-monomer. The cross-linking and degradation behaviors of thiol-norbornene hydrogels are controlled through material selections, whereas the biophysical and biochemical properties of the gels are easily and independently tuned owing to the orthogonal reactivity between norbornene and thiol moieties. Uniquely, the cross-linking of step-growth thiol-norbornene hydrogels is not oxygen-inhibited, therefore the gelation is much faster and highly cytocompatible compared with chain-growth polymerized hydrogels using similar gelation conditions. These hydrogels have been prepared as tunable substrates for 2D cell culture, as microgels or bulk gels for affinity-based or protease-sensitive drug delivery, and as scaffolds for 3D cell culture. Reports from different laboratories have demonstrated the broad utility of thiol-norbornene hydrogels in tissue engineering and regenerative medicine applications, including valvular and vascular tissue engineering, liver and pancreas-related tissue engineering, neural regeneration, musculoskeletal (bone and cartilage) tissue regeneration, stem cell culture and differentiation, as well as cancer cell biology. This article provides an up-to-date overview on thiol-norbornene hydrogel cross-linking and degradation mechanisms, tunable material properties, as well as the use of thiol-norbornene hydrogels in drug delivery and tissue engineering applications.

The influence of matrix degradation and functionality on cell survival and morphogenesis in PEG-based hydrogels

Two norbornene-functionalized PEG macromers are synthesized to render hydrogels with different hydrolytic degradability. Dithiol-containing linkers such as dithiothreitol or biscysteine-containing peptides are used to control proteolytic degradability. The influence of thiol-ene gel degradability on cell survival and morphogenesis in 3D is assessed using hMSCs and pancreatic MIN6 β cells. The initial cell viability can be negatively affected in highly crosslinked thiol-ene hydrogels. When cells are encapsulated in thiol-ene gels lacking cell-adhesive motifs, their survival and proliferation are promoted in more hydrolytically labile hydrogels. The degree of 3D cell spreading in encapsulated hMSCs is enhanced when the matrices are immobilized with cell-adhesive motifs.