Continuous pore size gradient enhances zonal-specific differentiation of stem cells in an osteochondral scaffold

Cartilage and bone in articular joints are intimately linked within the osteochondral (OC) unit. Scaffold-based regenerative approaches in the joint often target both cartilage and the subchondral bone, taking advantage of the endogenous bone marrow stem cells made available by breaching the OC junction. However, the production of scaffolds for OC regeneration is challenging, as scaffolds must provide mechanical strength while also mimicking the local cartilage and bone microenvironments. To create an osteochondral scaffold, we used Thermally Induced Phase Separation (TIPS) that allows us to create a wide range of morphologies in terms of pore size and distribution by tuning thermal history. We created a poly-l-lactic acid (PLLA) scaffold with a continuous pore size gradient from 70 μm diameter on the cartilage repair side to over 200 μm diameter on the bone repair side. We hypothesized that the smaller pore size will support chondrogenesis while the larger pore size will induce an osteogenic phenotype. This hypothesis was confirmed using an innovative biphasic bioreactor capable of providing distinct and separate signaling cues for cartilage and bone differentiation, while allowing communication across the osteochondral junction, similar to the in vivo environment. Our findings suggested that the PLLA continuous pore-gradient structure may offer a clinically translatable solution to osteochondral defect repair by supporting zone-specific differentiation.