gels Article Physical Interactions Strengthen Chemical Gelatin Methacryloyl Gels Lisa Rebers 1 , Tobias Granse 1,2 , Günter E.M. Tovar 1,2, *, Alexander Southan 1 and Kirsten Borchers 1,2, * 1 Institute of Interfacial Process Engineering and Plasma Technology IGVP, University of Stuttgart, Nobelstr. 12, 70569 Stuttgart, Germany; lisa.rebers@igvp.uni-stuttgart.de (L.R.); tobias_granse@gmx.de (T.G.); alexander.southan@igvp.uni-stuttgart.de (A.S.) 2 Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB, Nobelstr. 12, 70569 Stuttgart, Germany * Correspondence: guenter.tovar@igvp.uni-stuttgart.de (G.E.M.T.); kirsten.borchers@igb.fraunhofer.de (K.B.); Tel.: +49-(0)711-970-4109 (G.E.M.T.); +49-(0)711-970-4121 (K.B.) Received: 20 October 2018; Accepted: 12 January 2019; Published: 17 January 2019   Abstract: Chemically cross-linkable gelatin methacryloyl (GM) derivatives are getting increasing attention regarding biomedical applications. Thus, thorough investigations are needed to achieve full understanding and control of the physico-chemical behavior of these promising biomaterials. We previously introduced gelatin methacryloyl acetyl (GMA) derivatives, which can be used to control physical network formation (solution viscosity, sol-gel transition) independently from chemical cross-linking by variation of the methacryloyl-to-acetyl ratio. It is known that temperature dependent physical network formation significantly influences the mechanical properties of chemically cross-linked GM hydrogels. We investigated the temperature sensitivity of GM derivatives with different degrees of modification (GM2, GM10), or similar degrees of modification but different methacryloyl contents (GM10, GM2A8). Rheological analysis showed that the low modified GM2 forms strong physical gels upon cooling while GM10 and GM2A8 form soft or no gels. Yet, compression testing revealed that all photo cross-linked GM(A) hydrogels were stronger if cooling was applied during hydrogel preparation. We suggest that the hydrophobic methacryloyl and acetyl residues disturb triple helix formation with increasing degree of modification, but additionally form hydrophobic structures, which facilitate chemical cross-linking. Keywords: compression testing; physical and chemical network; hybrid network 1. Introduction Gelatin is a collagen-derived biopolymer forming physical hydrogels due to the occurrence of secondary and tertiary structures and triple helix formation [1]. Physical gelling of the gelatin solutions is temperature dependent: Upon cooling, solutions gel at the gelation temperature and liquefy at the melting temperature [2]. Due to their inherent biocompatibility and bioactivity, gelatin-based hydrogels are frequently investigated for medical applications. For this purpose, hydrogels usually must be mechanically stable at body temperature, e.g., for drug release or tissue engineering [3,4], which is not fulfilled by physical gelatin hydrogels. Hence, thermally stable covalent cross-links are needed in gelatin hydrogels. This can be achieved for example with the use of carbodiimides, such as 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) [5] for cross-linking of unmodified gelatin, or by chemical modification of gelatin with cross-linkable groups such as methacryloyl groups, resulting in gelatin methacryloyl (GM, also known as GelMA) as originally introduced by van den Bulcke et al. [6]. Gels 2019, 5, 4; doi:10.3390/gels5010004 www.mdpi.com/journal/gels