We report here a synthetically simple yet highly tunable and diverse visible light mediated thiol-vinyl gelation system for fabricating cell-instructive hydrogels. Gelation was achieved via a mixed-mode step-and-chain-growth photopolymerization using functionalized 4-arm poly(ethylene glycol) as backbone macromer, eosin-Y as photosensitizer, and di-thiol containing molecule as dual purpose co-initiator/cross-linker. N-vinylpyrrolidone (NVP) was used to accelerate gelation kinetics and to adjust the stiffness of the hydrogels. Visible light (wavelength: 400–700nm) was used to initiate rapid gelation (gel points: ~20 seconds) that reached completion within a few minutes. The major differences between current thiol-vinyl gelation and prior visible light mediated photopolymerization are that: (1) the co-initiator triethanolamine (TEOA) used in the previous systems was replaced with multifunctional thiols and (2) mixed-mode polymerized gels contain less network heterogeneity. The gelation kinetics and gel properties at the same PEG macromer concentration could be tuned by changing the identity of vinyl groups and di-thiol cross-linkers, as well as concentration of cross-linker and NVP. Specifically, acrylate-modified PEG afforded the fastest gelation rate, followed by acrylamide and methacrylate-functionalized PEG. Increasing NVP concentration also accelerated gelation and led to a higher network cross-linking density. Further, increasing di-thiol peptide concentration in the gel formulation increased hydrogel swelling and decreased gel stiffness. Due to the formation of thiol-ether-ester bonds following thiol-acrylate reaction, the gels degraded hydrolytically following a pseudo first order degradation kinetics. Degradation rate was controlled by adjusting thiol or NVP content in the polymer precursor solution. The cytocompatibility and utility of this hydrogel system were evaluated using in situ encapsulation of human mesenchymal stem cells (hMSC). Encapsulated hMSCs remained alive (>90%) throughout the duration of the study and the cells were differentiated down osteogenic lineage with varying degrees by controlling the rate and mode of gel degradation.

A degradable poly(ethylene glycol)-diacrylate (PEGDA) hydrogel system was developed using simple macromer formulations and visible light initiated thiol-acrylate photopolymerization. In addition to PEGDA, other components in this gelation system include eosin-Y as a photo-sensitizer, bi-functional thiol (dithiothreitol, DTT) as a dual-purpose co-initiator and cross-linker, and N-vinylpyrrolidone (NVP) as a co-monomer. Gelation was achieved through a mixed-mode step-chain growth polymerization mechanism under bright visible light exposure. Increasing photo-sensitizer or NVP concentrations accelerated photo-crosslinking and increased final gel stiffness. Increasing bi-functional thiol content in the prepolymer solution only increased gel stiffness to some degree. As the concentration of thiol surpassed certain range, thiol-mediated chain-transfer events caused thiol-acrylate gels to form with lower degree of cross-linking. Pendant peptide, such as integrin ligand RGDS, was more effectively immobilized in the network via a thiol-acrylate reaction (using thiol-bearing peptide Ac-CRGDS. Underline indicates cross-linkable motif) than through homo-polymerization of acrylated peptide (e.g., acryl-RGDS). The incorporation of pendant peptide comes with the expense of a lower degree of gel cross-linking, which was rectified by increasing co-monomer NVP content. Without the use of any readily degradable macromer, these visible light initiated mixed-mode cross-linked hydrogels degraded hydrolytically due to the formation of thiol-ether-ester bonds following thiol-acrylate reactions. An exponential growth relationship was identified between the hydrolytic degradation rate and bifunctional thiol content in the prepolymer solution. Finally, we evaluated the cytocompatibility of these mixed-mode cross-linked degradable hydrogels using in situ encapsulation of hepatocellular carcinoma Huh7 cells. Encapsulated Huh7 cells remained alive and proliferated as time to form cell clusters. The addition of NVP at a higher concentration (0.3%) did not affect Huh7 cell viability but resulted in reduction of cell metabolic activity, which was accompanied by an elevated urea secretion from the encapsulated cells.

This paper explores the application potential of a biodegradable PLLA/chitosan electrospun composite membrane for guided periodontal tissue regeneration which in addition serves as a fibroblast barrier. Electrospinning was applied to fabricate the PLLA membrane and aminolysis method was applied to graft chitosan on its surface. The morphology of the PLLA/chitosan membrane was observed by SEM. The surface chemical composition was analyzed by XPS. The appearance of N 1s peak in XPS demonstrated the successful grafting of chitosan on the PLLA electrospin membrane. After the modification, the water contact angle decreased from 136.9 ± 2.18°to 117.0 ± 2.10°, representing an improved hydrophilicity of the membrane. The bioactivity of the membrane was analyzed by XPS after soaking in SBF. The deposits had a Ca/P ratio of 1.6, indicating the hydroxyapatite formation on PLLA/chitosan membrane. The degradation rate was determined by measuring mass loss after immersion in PBS at different time periods. Compared to pure PLLA electrospun membrane which was almost nondegradable, the degradation rate of PLLA/chitosan composite membrane was up to 20 % in 6 weeks while maintaining its basic architecture to keep supporting the regenerated tissue. Live-dead cell staining of MC3T3 E1 cells cultured on the surface of the membrane showed a good biocompatibility of the PLLA/chitosan membrane. Furthermore, fibroblast cell line NIH 3T3 was cultured on surface of the membrane for the evaluation of cell penetration. The result demonstrated that the membrane worked as a fibroblast barrier to minimize the unfavorable effect of fibroblasts on periodontal tissue regeneration. Therefore, this electrospun PLLA/chitosan composite membrane has more potential for clinical application compared to old generation regeneration membrane with both suitable degradation rate and non-fibroblast penetration property.