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Articular cartilage (AC) transmits large mechanical loads in synovial joints. This tissue’s properties derive from its unique composition and structure, which consists of glycosaminoglycans and type II collagen arranged into arcade-like structures [1]. AC has a limited capacity for regeneration, hence damage here typically leads to the development of osteoarthritis, a disease impacting the quality of life of millions [2]. Current clinical treatments for AC repair such as mosaicplasty and autologous chondrocyte implantation fail to regenerate AC that mimics the structure and mechanical properties of the native tissue [3]. This has motivated the development of new tissue engineering strategies to recapitulate the zonal architecture of the tissue. We previously explored the integration of melt-electro written (MEW) scaffolds with inkjet bioprinting to generate AC grafts that mimicked some, but not all, features of the native tissue [5]. Challenges remain to scale up such approaches to engineer clinically relevant cartilage and osteochondral grafts. The current study aims to address these challenges by exploring how different cell types and densities, as well as alternative MEW scaffold pore sizes, influence the organization, composition and mechanical properties of the resultant graft, with a view to engineering a tissue with similar properties to native AC.
Bone marrow-derived MSCs were isolated from mature female goats [6]. MEW scaffolds (50 layers) were fabricated from PCL with 0.2×0.2, 0.4×0.4 and 0.8×0.8mm pore sizes using a custom MEW printer [7]. Cells were manually seeded into the scaffolds at a density of 30 million cells per milliliter. Constructs were cultured in chemically defined medium supplemented with 10ng/ml TGF-β3 for 6 weeks and assessed histologically, biochemically and mechanically in tension and compression.
Additionally, articular cartilage progenitors (ACPs) were isolated from mature female goats [1]. MEW scaffolds were fabricated from PCL at a more clinically relevant height of 130 layers with a 0.8x0.8 pore size. Cells were manually seeded into the scaffolds at 15, 30 and 45 million cells per milliliter. Constructs were cultured and assessed in the same manner as described above.
Our results show that MEW scaffold pore size influences collagen organization, mechanical strength and tissue composition, with less arcade-like organization but higher ramp and dynamic moduli for the smallest pore size (Fig. 1A-D). The tensile modulus of the 0.2×0.2 construct approached 6 MPa (average in X & Y directions) after 6 weeks in culture, while its compressive modulus approached 290 kPa. We also observed differences in tissue formation between ACPs and MSCs across different cell densities. Larger, more dense cellular aggregates appear in constructs casted with a higher cell density as compared to lower densities (Fig 1E).
This study indicates that we can engineer scaled-up cartilage grafts with more biomimetic organization and mechanical properties by tailoring cell type, density and scaffold architecture. Results from this study will inform the fabrication of cartilage grafts via electromagnetic droplet printing which will be evaluated using in vitro and in vivo models to assess their potential for osteochondral defect repair. In conclusion, this approach may potentially address current challenges in engineering articular cartilage grafts.
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