Introduction Bioprinting has emerged as a promising technology for cartilage tissue engineering. In cartilage, the mechanical properties are closely associated with the extracellular matrix (ECM) and of paramount importance for tissue functionality. However, to date mechanical analysis of biofabricated cartilaginous constructs is often limited to single parameters, and a detailed analysis of the concomitant development of ECM and mechanical properties is lacking. Therefore, in this study, we investigate the post-fabrication development of the ECM and comprehensively analyze the large-strain viscoelastic properties of cartilaginous constructs made from mesenchymal stromal cells (MSC) in a hyaluronic acid (HA)-based bioink. Methods MSC are embedded in a HA-based bioink and chondrogenically differentiated post-production for 6 weeks. A time-matched analysis is performed monitoring the ECM development represented by glycosaminoglycans and collagen as well as the changes in mechanical properties, such as hysteresis, classical shear modulus, nonlinearity and stress relaxation, using a multimodal testing approach at six different time points. Results Distinctly increasing amounts of ECM components are detected in the cartilaginous constructs over the whole cultivation period. Glycosaminoglycans and collagen content increase by 57- and 52-fold, respectively, from day 1 to day 42. In turn, the ECM development markedly influences the overall multimodal mechanical response over time, leading to distinct changes in nonlinear and stress relaxation behavior and a 265-fold increase in hysteresis and a 174-fold increase in classical shear modulus reaching 50 kPa in total. We find a strong correlation between the amount of glycosaminoglycans and collagen, and the classical shear modulus as well as the hysteresis during cyclic loading. When partially inhibiting collagen production in the constructs, the robustness of the correlation between collagen and classical shear modulus is confirmed. Discussion Our study underlines the substantial impact of matrix components on the mechanical behavior in biofabricated chondrogenic constructs. The results emphasize the importance of a comprehensive analysis of the mechanical properties. We propose a matrix-based prediction function that can estimate the classical shear modulus of constructs made from HA-based bioinks. Overall, our findings contribute to the understanding of tissue maturation in engineered constructs for cartilage regeneration.
Büttner et al. (Wed,) studied this question.