Riboflavin and collagen: New crosslinkin methods to tailor the stiffness of hydrogels

Fabricating materials with tailored mechanical properties is a challenge and crucial for their successful appli- cation in a variety of fields such as tissue engineering. Here collagen and riboflavin were used to create hydro- gels with controlled mechanical properties mimicking those of soft tissues (e.g. liver). Collagen-based hydrogels were obtained using a two-step gelation method. Firstly a physical gelation step (i.e. modulation of temperature and pH) was used to fix a specific shape; then photo-initiated cross-links were formed to in- crease the stiffness. Specifically the chemical cross-linking step was initiated with UV (ultra-violet) radiation to obtain riboflavin derivatised radical polymerization of collagen chains. Cylindrical shaped samples with controlled dimensions were fabricated, and then tested using compressive loading. We show that the com- pressive elastic modulus of collagen-based hydrogels can be tuned between 0.9 and 3.6 kPa by changing col- lagen concentration, irradiation with UV in the presence of riboflavin and freeze-drying.