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

Fabricating materials with tailored mechanical properties is a challenge and crucial for their successful application in a variety of fields such as tissue engineering. Here collagen and riboflavin were used to create hydrogels 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 increase 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 compressive elastic modulus of collagen-based hydrogels can be tuned between 0.9 and 3.6 kPa by changing collagen concentration, irradiation with UV in the presence of riboflavin and freeze-drying.