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Abstract:
One promising strategy to reconstruct osteochondral defects relies on 3D bioprinted three-zonal structures comprised of hyaline cartilage, calcified cartilage, and subchondral bone. So far, several studies have pursued the regeneration of either hyaline cartilage or bone in vitro while—despite its key role in the osteochondral region—only few of them have targeted the calcified layer. In this work, we present a 3D biomimetic hydrogel scaffold containing β-tricalcium phosphate (TCP) for engineering calcified cartilage through a co-axial needle system implemented in extrusion-based bioprinting process. After a thorough bioink optimization, we showed that 0.5% w/v TCP is the optimal concentration forming stable scaffolds with high shape fidelity and endowed with biological properties relevant for the development of calcified cartilage. In particular, we investigate the effect induced by ceramic nano-particles over the differentiation capacity of bioprinted bone marrow-derived human mesenchymal stem cells in hydrogel scaffolds cultured up to 21 d in chondrogenic media. To confirm the potential of the presented approach to generate a functional in vitro model of calcified cartilage tissue, we evaluated quantitatively gene expression of relevant chondrogenic (COL1, COL2, COL10A1, ACAN) and osteogenic (ALPL, BGLAP) gene markers by means of RT-qPCR and qualitatively by means of fluorescence immunocytochemistry.

Keywords:
alginate, gelatin methacrylate, ß-tricalcium phosphate TCP, bioprinting, coaxial needle, calcified cartilage

Abstract:
In this study, we present an innovative strategy to reinforce 3D-printed hydrogel constructs for cartilage tissue engineering by formulating composite bioinks containing alginate and short sub-micron polylactide (PLA) fibers. We demonstrate that Young's modulus obtained for pristine alginate constructs (6.9 ± 1.7 kPa) can be increased threefold (up to 25.1 ± 3.8 kPa) with the addition of PLA short fibers. Furthermore, to assess the performance of such materials in cartilage tissue engineering, we loaded the bioinks with human chondrocytes and cultured in vitro the bioprinted constructs for up to 14 days. Live/dead assays at day 0, 3, 7 and 14 of in vitro culture showed that human chondrocytes were retained and highly viable (∼80%) within the 3D deposited hydrogel filaments, thus confirming that the fabricated composites materials represent a valid solution for tissue engineering applications. Finally, we show that the embedded chondrocytes during all the in vitro culture maintain a round morphology, a key parameter for a proper deposition of neocartilage extracellular matrix.

Keywords:
alginate, PLA, short fibers, hydrogel reinforcement, chondrocytes