Rational Design and Fabrication of Mineral-Graded Scaffolds for Use in Tendon-to-Bone Insertion Repair

The enthesis is a specialized fibrocartilaginous tissue responsible for transmitting mechanical loads between the connective tissue and bone during joint motion. Due to the complex nature of the soft-to-hard tissue interface, it has been a challenge to regenerate the transitional tissue and thereby restore its function. Critical features, including a spatially graded composition, hierarchical structure of the extracellular matrix, and a unique population of cells with a phenotypic gradient, are not recreated during the healing process. To address this problem, we are developing biomimetic scaffolds with a controllable mineral gradient to facilitate surgical repairs of the enthesis through the regeneration of tendon-to-bone insertion. The gradient in mineral content can be created by sequentially spin-coating with hydroxyapatite/poly(e-caprolactone) suspensions containing hydroxyapatite nanoparticles in decreasing concentrations. To produce an interdigitation geometry mimicking that of the natural enthesis for stress alleviation and facilitate cell infiltration, the mineral-graded film is then patterned with an array of funnel-shaped microchannels by laser machining. The unique design provided both mechanical (<i>i.e.</i>, substrate stiffness) and biochemical (<i>e.g.</i>, hydroxyapatite content) cues to spatially control the graded differentiation of mesenchymal stem cells (MSCs). Immunocytochemical analysis of human-derived MSC-seeded scaffolds demonstrates the creation of a gradient in cell phenotype from osteoblasts to mineralized chondrocytes depending on the level of mineralization in the scaffold. We can also incorporate a gradient of developmentally-inspired biofactors in the scaffold to further promote the phenotypic transition from mineralized chondrocytes to tendon fibroblasts. By recreating compositional and cellular features of the native tendon enthesis, the biomimetic scaffolds offer a promising avenue for improved tendon-to-bone insertion repair. This strategy potentially has broad applications in many clinically relevant scenarios, including rotator cuff repair, anterior cruciate ligament reconstruction, and meniscal repair.