Kai Guo

Scaffolds play a crucial role in tissue engineering as regenerative templates. Fabricating scaffolds with good biocompatibility and appropriate mechanical properties remains a major challenge in this field. This study proposes a method for preparing multi-material scaffolds, enabling the 3D printing of collagen and thermoplastic elastomers at room temperature. Addressing the previous challenges such as the poor printability of pure collagen and the difficulty of maintaining structural integrity during multilayer printing, this research improved the printability of collagen by optimizing its concentration and pH value and completed the large-span printing of thermoplastic elastomer using a precise temperature-control system. The developed hybrid scaffold has an interconnected porous structure, which can support the adhesion and proliferation of fibroblasts. The scaffolds were further treated with different post-treatment methods, and it was proven that the neutralized and cross-linked collagen scaffold, which has both nano-fibers and a certain rigidity, can better support the osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs). The research results show that the collagen thermoplastic elastomer hybrid scaffold has significant clinical application potential in soft tissue and hard tissue regeneration, providing a versatile solution to meet the diverse needs of tissue engineering.

Fabricating connective tissue with printing fidelity, structural stability, biocompatibility, and cellular orientation remains a challenge for bioink. Collagen, as inherent fibers to provide strength in vivo, should be the ideal material for tissue printing. However, current collagen-bioink exhibits poor printability and mechanical properties. Here, the light-crosslinkable norbornene-collagen (NorCol) is developed to solve this dilemma. NorCol with complete collagen structure exhibits outstanding shear-thinning properties and light-crosslinking strength, realizing the construction of complicated constructs with excellent printing fidelity and structural stability. Neutral-dissolved NorCol with different concentrations is directly integrated to print pure collagen structure with composite modulus, providing top-class biocompatibility that mimics the heterogeneous microarchitecture of the extracellular matrix (ECM). This composite structure also permits extensive infiltration of host tissue and capillaries during subcutaneous transplants. By the designed tight interface of printed filaments, a geometric-guided and modulus-independent anisotropic mechanical environment is constructed, realizing cellular orientation. Furthermore, the cellular orientation is in the same direction as the printed high-concentration NorCol, which is the same case of cells and collagen fibers in vivo. This capability of NorCol breaks new ground for printing pure ECM protein-based, implantable and functional constructs, applicable in connective tissue engineering for human applications.

As the essential foundation of bioprinting technology, cell-laden bio-ink is confronted with the inevitable contradiction between printability and bioactivity. For example, type I collagen has been widely applied for its excellent biocompatibility; however, its relatively low self-assembly speed restricts the performance in high-precision bioprinting of cell-laden structures. In this study, we synthesize norbornene-functionalized neutral soluble collagen (NorCol) by the reaction of acid-soluble collagen (Col) and carbic anhydride in the aqueous phase. NorCol retains collagen triple-helical conformation and can be quickly orthogonally cross-linked to build a cell-laden hydrogel via a cell-friendly thiol-ene photoclick reaction. Moreover, the additional carboxyl groups produced in the reaction of carbic anhydride and collagen obviously improve the solubility of NorCol in neutral buffer and miscibility of NorCol with other polymers such as alginate and gelatin. It enables hybrid bio-ink to respond to multiple stimuli, resulting in continuous cross-linked NorCol networks in hybrid hydrogels. For the first time, the collagen with a triple helix structure and gelatin can be mixed and printed, keeping the integrity of the printed construct after gelatin’s dissolution. The molecular interaction among giant collagen molecules allows NorCol hydrogel formation at a low concentration, which leads to excellent cell spreading, migration, and proliferation. These properties give NorCol flexible formability and excellent biocompatibility in temperature-, ion-, and photo-based bioprinting. We speculate that NorCol is a promising bio-ink for emerging demands in tissue engineering, regenerative medicine, and personalized therapeutics.