Constraint stress, microstructural characteristics, and enhanced mechanical properties of a special fibroblast-embedded collagen construct

Cell-contracted collagen gels could provide rejection-free biomaterials for tissue engineering, but their application is limited by relatively low mechanical strength. We developed a special type I collagen construct (based on embedded fibroblasts) that was formed into a gel thread by using two anchors to constrain gel contraction in one direction. Each gel thread contained 2 mg of type I collagen and 1.0 x 10(6) fibroblasts, and had an initial volume of 3 mL. After 9 days in culture, this preparation was transformed into a thread-like construct measuring 26 x 2.3 x 0.21 mm. Investigation of the microstructure showed that the collagen fibrils longitudinally between two cells had most aligned with the direction of the constraint stress and had assumed higher density than those in the freely contracted controls. During culturing, the constraint stress first increased then decreased, with implications for the nature of the interaction between the embedded cells and collagen matrix. Under uniaxial tensile testing, the ultimate stress and material modulus increased by factors of 6 and 16, respectively, compared with controls, while the maximal strain decreased by 590%. Compared with the similar constructs in the literature, the thread gel was fabricated by means of a novel mold configuration so that it contracted to thread shape much faster, and more importantly, the constraint force was firstly reported in this article. The improved mechanical properties show that the gel thread could be an effective biomaterial for such tissue engineering applications as the fabrication of blood vessels, ligaments, and tendon grafts.