Articular cartilage and the underlying subchondral bone are crucial in human movement and when damaged through disease or trauma impacts severely on quality of life. Cartilage has a limited regenerative capacity due to its avascular composition and current therapeutic interventions have limited efficacy. With a rapidly ageing population globally, the numbers of patients requiring therapy for osteochondral disorders is rising, leading to increasing pressures on healthcare systems. Research into novel therapies using tissue engineering has become a priority. However, rational design of biomimetic and clinically effective tissue constructs requires basic understanding of osteochondral biological composition, structure, and mechanical properties. Furthermore, consideration of material design, scaffold architecture, and biofabrication strategies, is needed to assist in the development of tissue engineering therapies enabling successful translation into the clinical arena. This review provides a starting point for any researcher investigating tissue engineering for osteochondral applications. An overview of biological properties of osteochondral tissue, current clinical practices, the role of tissue engineering and biofabrication, and key challenges associated with new treatments is provided. Developing precisely engineered tissue constructs with mechanical and phenotypic stability is the goal. Future work should focus on multi-stimulatory environments, long-term studies to determine phenotypic alterations and tissue formation, and the development of novel bioreactor systems that can more accurately resemble the in vivo environment.

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A challenge for tissue engineering is to produce synthetic scaffolds of adequate chemical, physical, and biological cues effectively. Due to the hydrophobicity of the commonly used synthetic polymers, the printed scaffolds are limited in cell-seeding and proliferation efficiency. Furthermore, non-uniform cell distribution along the scaffolds with rare cell attachment in the core region is a common problem. There are no available commercial systems able to produce multi-type material and gradient scaffolds which could mimic the nature tissues. This paper describes a plasma-assisted bio-extrusion system (PABS) to overcome the above limitations and capable of producing functional-gradient scaffolds; it comprises pressure-assisted and screw-assisted extruders and plasma jets. A hybrid scaffold consisting of synthetic biopolymer and natural hybrid hydrogel alginate-gelatin (Alg-Gel) methacrylate anhydride, and full-layer N2  plasma modification scaffolds were produced using PABS. Water contact angle and in vitro biological tests confirm that the plasma modification alters the hydrophilicity properties of synthetic polymers and promotes proliferation of cells, leading to homogeneous cell colonization. The results confirm the printing capability for soft hard material integration of PABS and suggest that it is promising for producing functional gradient scaffolds of biomaterials.

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