Tunable Cross-Linking and Adhesion of Gelatin Hydrogels via Bioorthogonal Click Chemistry

Engineering cytocompatible hydrogels with tunable physico-mechanical properties as a biomimetic three-dimensional extracellular matrix (ECM) is fundamental to guide cell response and target tissue regeneration or development of in vitro models. Gelatin represents an optimal choice given its ECM biomimetic properties; however, gelatin cross-linking is required to ensure structural stability at physiological temperature (i.e., T > T_{sol-gel gelatin}). Here, we use a previously developed cross-linking reaction between tetrazine (Tz)- and norbornene (Nb) modified gelatin derivatives to prepare gelatin hydrogels and we demonstrate the possible tuning of their properties by varying their degree of modification (DOM) and the Tz/Nb ratio (R). The percentage DOM of the gelatin derivatives was tuned between 5 and 15%. Hydrogels prepared with higher DOM cross-linked faster (i.e., 10-20 min) compared to hydrogels prepared with lower DOM (i.e., 60-70 min). A higher DOM and equimolar Tz/Nb ratio R resulted in hydrogels with lower weight variation after immersion in PBS at 37 °C. The mechanical properties of the hydrogels were tuned by varying DOM and R by 1 order of magnitude, achieving elastic modulus E values ranging from 0.5 (low DOM and nonequimolar Tz/Nb ratio) to 5 kPa (high DOM and equimolar Tz/Nb ratio). Human dental pulp stem cells were embedded in the hydrogels and successfully 3D cultured in the hydrogels (percentage viable cells >85%). An increase in metabolic activity and a more elongated cell morphology was detected for cells cultured in hydrogels with lower mechanical properties (E < 1 kPa). Hydrogels prepared with an excess of Tz or Nb were successfully adhered and remained in contact during in vitro cultures, highlighting the potential use of these hydrogels as compartmentalized coculture systems. The successful tuning of the gelatin hydrogel properties here developed by controlling their bioorthogonal cross-linking is promising for tissue engineering and in vitro modeling applications.