Tuning collagen and collagen-alginate mechanics through extrusion bioprinting process parameters

Bioprinting allows the fabrication of biopolymers into complex and hierarchical structures reminiscent of their organization in vivo. As the main structural protein found in connective tissues, type I collagen is of particular interest as a biomaterial due to its biochemical activity and ease of physical or chemical crosslinking. However, several limitations of collagen-based constructs include poor mechanical strength and inability to bear loads in dynamic conditions. Towards overcoming these challenges, this study explores the impact of higher concentration collagen bioinks (35 and 70 mg mL^-1) and the incorporation of an alginate hydrogel during synthesis to create designs with shape fidelity and tunable mechanical properties. Using bioprinting processes, we quantify the relationship between bioink composition, printing parameters, and post-processing on printability and mechanical behavior. Results show that both pure collagen bioinks and low-concentration collagen to alginate volume ratios of 1 : 1, 1 : 5, and 1 : 10 exhibited good printability, but increasing the alginate concentration led to greater shrinkage of scaffolds after thermo-ionic crosslinking. Uniaxial compression results indicated a directly increased modulus and compressive strength after 24 hours of crosslinking, which was also seen in tensile modulus after 12 hours of crosslinking. Notably, blend composition demonstrated the greatest influence on material stiffness, with crosslinking duration serving as a secondary factor. Scanning electron microscopy used to visualize the cross-section of these collagen constructs reveals a dense fibrous microstructure that may help reinforce mechanical properties and promote cell adhesion. Ultimately, designing collagen-based biomaterials that can be mechanically tailored through printing process parameters will inform customizable extrusion of soft tissues for regenerative medicine.