In situ cross-linkable hybrid hydrogels composed of gelatin and 4-arm-polypropylene oxide−polyethylene oxide (Tetronic) was developed as an injectable scaffold for tissue regeneration. The gelatin was modified by hydroxyphenyl propionic acid (HPA) and the Tetronic was conjugated with tyramines (Tet−TA). The hydrogels were rapidly formed by mixing the polymer solutions containing horseradish peroxidase (HRP) and hydrogen peroxide (H 2 O 2 ). The gelation time and mechanical properties of the hydrogels could be controlled by varying the HRP and H 2 O 2 concentrations. In vitro degradation study of the hybrid hydrogels was carried out using collagenase and the prolonged proteolytic degradation was obtained due to the presence of the Tetronic. Human dermal fibroblast (hDFB) was cultured in the hydrogel matrices to evaluate the cyto-compatibility. The encapsulated cells were shown to be highly viable and spread over the gel matrices, suggesting that the hybrid hydrogels have an excellent cyto-compatibility. The hydrogels were also subcutaneously injected in the back of mice and the results demonstrated that the hydrogels were rapidly formed at the injected site. From these results, we demonstrate that the in situ cross-linkable hydrogels formed by hybridization of gelatin and Tetronic via enzyme-mediated reactions hold great promise for use as injectable matrices for tissue regenerative medicine due to their tunable physico-chemical properties and excellent bioactivity.

Tissue engineering therapies require biocompatible and bioactive biomaterials that are capable of encouraging an angiogenic response for effective tissue regeneration. In this study, a SVVYGLR peptide, which functions as a potent angiogenic factor, was conjugated into injectable gelatin-poly(ethylene glycol)-tyramine (GPT) hydrogels in situ to enhance endothelial cell activities and neo-vascularization. SVVYGLRGGY (SV-Y) conjugated GPT (SV-GPT) hydrogels were formed in situ via enzyme-mediated reaction using horseradish peroxidase (HRP) and hydrogen peroxide (H(2)O(2)). The physico-chemical properties were characterized and could be controlled depending on the feed peptide and H(2)O(2) concentration. The concentration of conjugated peptide ranged from 0.37 to 0.81 μmol/mL, and the elastic moduli (G') of the hydrogels were 600-4900 Pa. In vitro cell studies using human umbilical vein endothelial cells (HUVECs) and in vivo subcutaneous injection studies were performed to confirm the effect of the SVVYGLR peptide on HUVEC activity and neo-vascularization. Obtained results demonstrated that the in situ conjugation of SVVYGLR sequences into phenol residues of GPT hydrogels enhanced the activity of HUVECs in vitro and stimulated the formation of new blood vessels in the hydrogel matrices in vivo. From the results, we suggest that in situ conjugation of SV-Y to GPT hydrogels via the enzymatic reaction may be an efficient tool to prepare injectable bioactive hydrogels that can enhance endothelial cell activities and promoting angiogenesis for tissue regeneration.

Reactive oxygen species (ROS) have been implicated as a critical modulator for various therapeutic applications such as treatment of vascular disorders, wound healing, and cancer treatment. Specifically, growing evidence has recently demonstrated that transient or low levels of hydrogen peroxide (HO) facilitates tissue regeneration and wound repair through acute oxidative stress that can evaluate intracellular ROS levels in cells or tissues. Herein, we report a gelatin-based HO-releasing hydrogel formed by dual enzyme-mediated reaction using horseradish peroxidase and glucose oxidase (GO ). The release behavior of HO from the hydrogel matrices can be precisely controlled by varying the GO concentrations. We demonstrate that HO-releasing hydrogels with the optimal condition increase transient upregulation of intracellular ROS levels in the endothelial cells (ECs), enhance proliferative activities of ECs in vitro, and facilitate neovascularization in ovo. We suggest that our HO-releasing hydrogels hold great potential as an injectable and dynamic matrix for the treatment of vascular disorders as well as in tissue regenerative medicine.

Horseradish peroxidase (HRP) and hydrogen peroxide (H O )-mediated crosslinking reaction has become an attractive method to create in situ forming hydrogels. While the crosslinking system has been widely utilized, there are certain issues require improvement to extend their biomedical applications, including creation of stiff hydrogels without compromising cytocompatibility due to initially high concentrations of H O . A gelatin-based hydrogels formed through a dual enzyme-mediated crosslinking reaction using HRP and glucose oxidase (GOx) as an H O -generating enzyme to gradually supply a radical source in HRP-mediated crosslinking reaction is reported. The physicochemical properties can be controlled by varying enzyme concentrations. Furthermore the hydrogel matrices provide 3D microenvironments for supporting the growth and spreading of human dermal fibroblasts with minimized cytotoxicity, despite the cells being encapsulated within stiff hydrogels. These hydrogels formed with HRP/GOx have great potential as artificial microenvironments for a wide range of biomedical applications.

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