Tissue-engineered mitral valve chordae using directed collagen gel shrinkage

The increase in mitral valve repair has stimulated an interest in developing alternative materials for artificial chordae. We have turned to collagen-based tissue engineering technologies. The principle of directed collagen gel shrinkage involves combining cells and reconstituted type I collagen. However, inappropriate mechanical properties have been one of the main limitations of most collagen-based tissue equivalents.;To improve the strength of chordae equivalents, the parameters that regulate gel contraction and collagen synthesis need to be better understood. I have demonstrated that mechanical properties of the constructs were improved by optimizing cell seeding density, initial collagen concentration, cell passage, serum concentration and culture time. Since the microstructure of tissue components determines their mechanical behavior, I also thoroughly characterized the collagen constructs. The collagen constructs contained a dense collagen core and an outer elastin sheath similar to native chordae, even though only collagen was in the original mixture. I have demonstrated that cells migrated to the surface, where there is less tension, proliferated to multiple layers, and remained in the synthetic phenotype. These cells therefore continued to synthesize elastin, until a continuous sheath eventually developed.;I designed a motorized dynamic loading system to stretch the constructs during culture to improve the mechanical properties further. Data analysis showed that dynamically cultured constructs have better mechanical properties than statically cultured constructs after 8 weeks of culture (P < 0.05). Cyclic strain increased their failure strength (from 1.2 to 3.2 MPa), modulus (from 6.5 to 18 MPa), and cell number (by 155%). However, the modulus and failure stress are still at least an order of magnitude less than those of normal human chordae (from adult). Transmission electron microscopy revealed an elastin sheath around the collagen core and the presence of proteoglycan filaments associated with collagen fibrils. In dynamically loaded constructs, collagen fibrils were longer and more aligned and compacted. Dynamic strain produced replacement tissue-engineered materials with good mechanical properties and ultrastructure.;This project has demonstrated that directed collagen gel shrinkage can be used to fabricate mitral valve chordae in vitro.