Strategies for Stratified Cartilage Bioprinting

Multiple materials, cells and growth factors can be combined into one construct by the use of a state–of-the-art bioprinter. This technique may in the future make the fabrication of complete tissues or organs possible. In this thesis the feasibility of the bioprinting of cartilage and the differences between layers of cartilage are investigated. First, it was demonstrated that enriched populations of mature cartilage cells (chondrocytes) from the superficial, middle, and deep zones of articular cartilage can reacquire their zonal characteristics when cultured under adequate circumstances. This is importance for regenerative therapies, for which cells need to be expanded. When pellets of cells from the different subtypes are combined in one construct, this does not lead to enhancement of cartilaginous properties or zonal chondrocyte characteristics. Constructs containing gels, with or without cells, were fabricated using a bioprinter. It was shown that a construct consisting of bone and cartilage could be printed, and after three weeks, cartilage- and bone-like tissue could be observed. Since the printability and cellular behaviour in the used gel could be optimized, three other hydrogels were investigated. The mechanical properties of these biomaterials can be influenced by a change in temperature and by the application of UV light. The investigated gels allowed for the fabrication of stiffer and higher constructs, and in one of the used biomaterials (modified gelatine) cartilaginous tissue formation was shown. Finally, the novel technique of hybrid bioprinting was introduced. For this method, a gel with incorporated cell is combined with a stiff degradable plastic to fabricate cell-laden constructs which have similar mechanical properties as native tissues, such as cartilage or intervertebral discs.