Traditional invasive and synchrotron-based non-invasive assessments of 3D-printed hybrid cartilage constructs

Three-dimensional (3D)-printed constructs made of polycaprolactone (PCL) and chondrocyte-impregnated alginate hydrogel (hybrid cartilage constructs) mimic the biphasic nature of articular cartilage, offering promise for cartilage tissue engineering (CTE) applications. However, the regulatory pathway for medical device development requires validation of such constructs through in vitro bench tests and in vivo preclinical examinations premarket approval. Furthermore, non-invasive imaging techniques are required for effective evaluation of the progress of these cartilage constructs, especially when implanted in animal models or human subjects. However, characterization of the individual components of the hybrid cartilage constructs and their associated time-dependent structural changes by currently available non-invasive techniques is challenging as these constructs contain a combination of hydrophobic and hydrophilic biomaterials with different refractive indices. Here, we report the use of a novel synchrotron-radiation inline phase contrast imaging computed tomography (SR-inline-PCI-CT) approach for non-invasive (in situ) characterization of 3D-printed hybrid cartilage constructs that has been implanted subcutaneously in mice over a 21-day period. In parallel, traditional invasive assays was used to evaluate the in vivo performance of the implanted hybrid cartilage constructs with respect to their cell viability and secretion of cartilage-specific extracellular matrix (ECM) over the 21-day period post-implantation in mice. SR-inline-PCI-CT allowed striking visualization of the individual components within the 3D-printed hybrid cartilage constructs as well as characterization of the time-dependent structural changes after implantation. In addition, the relationship between the implanted constructs and the surrounding tissues was delineated. Furthermore, traditional assays showed that cell viability within the cartilage constructs was at least 70% at all three time points, and secretion of alcian blue- and collagen type 2-positive matrices increased progressively over the 21-days period post-implantation. Overall, these results demonstrate the 3D-printed hybrid cartilage constructs have good in vivo performance and validate their potential for regeneration of articular cartilage in vivo. In addition, SR-inline-PCI-CT has demonstrated potential for longitudinal and non-invasive monitoring of the functionality of 3D-printed hybrid cartilage constructs in a way that is translatable to other soft tissue engineering applications.