An ECM scaffold combined with a compliant 3D printed spring-shaped reinforcement for cartilage engineering applications

Articular cartilage is a soft tissue lining the ends of the long bones in our joints. Even minor lesions in articular cartilage (AC) can cause underlying bone damage creating an osteochondral (OC) defect. OC defects can cause pain, impaired mobility and can develop osteoarthritis (OA). OA is a disease that affects nearly 10% of the population worldwide, and represents a significant economic burden to patients and society. While significant progress has been made in this field, realising an efficacious therapeutic option for unresolved OA remains elusive and is considered one of the greatest challenges in the field of orthopaedic regenerative medicine. Therefore, there is a societal need to develop new strategies for AC regeneration. In recent years there has been increased interest in the use of tissue-specific aligned porous freeze-dried extracellular matrix (ECM) scaffolds as an off-the-shelf approach for AC repair, as they allow for cell infiltration, provide biological cues to direct target-tissue repair and permit aligned tissue deposition, desired in AC repair. However, most ECM-scaffolds lack the appropriate mechanical properties to withstand the loads passing through the joint. One solution to this problem is to reinforce the ECM with a stiffer framework made of synthetic materials, such as polylactic acid (PLA). Such framework can be 3D printed to produce anatomically accurate implants, attractive in personalized medicine. However, typical 3D prints are static, their design is not optimized for soft-hard interfaces (OC interface), and they may not adapt to the cyclic loading passing through our joints, thus risking implant failure. To tackle this limitation, more compliant or dynamic designs can be printed, such as coil-shaped structures. Thus, in this study we use finite element modelling to create different designs including single triple, single quadruple, double triple and double quadruple helix and prototype them in PLA. The optimal design is combined with an ECM slurry. Briefly, the ECM slurry is combined with the PLA coil and freeze-casted under directional freezing prior to freeze-drying the samples to obtain an off-the-shelf scaffold with a dynamic reinforcement. The scaffold will be combined with mesenchymal stem cells (MSCs) to investigate the chondrogenic potential of such metamaterial. The double helix has a higher stiffness modulus than the single helix and the quadruple helix a higher stiffness modulus than the triple helix. The single helixes have a better recovery after compression, while the doble helixes have a higher plastic deformation under compression. The directional freeze-casting results in ECM scaffolds (either alone or PLA reinforced) containing a tailored microarchitecture mimicking aspects of native AC. To conclude, it was possible to design and simulate stiffness of different coil-shaped reinforcements and 3D print the PLA prototypes without support material. We were able to isolate and incorporate ECM into the coil structure and produce dynamic scaffolds that have the potential to be used in cartilage tissue engineering. Graphical abstract