Focal articular cartilage (AC) defects, if left untreated, can lead to debilitating diseases such as osteoarthritis. While several tissue engineering strategies have been developed to promote cartilage regeneration, it is still challenging to generate functional AC capable of sustaining high load‐bearing environments. Here, a new class of cartilage extracellular matrix (cECM)‐functionalized alginate bioink is developed for the bioprinting of cartilaginous tissues. The bioinks are 3D‐printable, support mesenchymal stem cell (MSC) viability postprinting and robust chondrogenesis in vitro, with the highest levels of COLLII and ACAN expression observed in bioinks containing the highest concentration of cECM. Enhanced chondrogenesis in cECM‐functionalized bioinks is also associated with progression along an endochondral‐like pathway, as evident by increases in RUNX2 expression and calcium deposition in vitro. The bioinks loaded with MSCs and TGF‐β3 are also found capable of supporting robust chondrogenesis, opening the possibility of using such bioinks for direct “print‐and‐implant” cartilage repair strategies. Finally, it is demonstrated that networks of 3D‐printed polycaprolactone fibers with compressive modulus comparable to native AC can be used to mechanically reinforce these bioinks, with no loss in cell viability. It is envisioned that combinations of such biomaterials can be used in multiple‐tool biofabrication strategies for the bioprinting of biomimetic cartilaginous implants.

Engineering constructs that mimic the complex structure, composition and biomechanics of the articular cartilage represents a promising route to joint regeneration. Such tissue engineering strategies require the development of biomaterials that mimic the mechanical properties of articular cartilage whilst simultaneously providing an environment supportive of chondrogenesis. Here three-dimensional (3D) bioprinting is used to develop polycaprolactone (PCL) fibre networks to mechanically reinforce interpenetrating network (IPN) hydrogels consisting of alginate and gelatin methacryloyl (GelMA). Inspired by the significant tension-compression nonlinearity of the collagen network in articular cartilage, we printed reinforcing PCL networks with different ratios of tensile to compressive modulus. Synergistic increases in compressive modulus were observed when IPN hydrogels were reinforced with PCL networks that were relatively soft in compression and stiff in tension. The resulting composites possessed equilibrium and dynamic mechanical properties that matched or approached that of native articular cartilage. Finite Element (FE) modelling revealed that the reinforcement of IPN hydrogels with specific PCL networks limited radial expansion and increased the hydrostatic pressure generated within the IPN upon the application of compressive loading. Next, multiple-tool biofabrication techniques were used to 3D bioprint PCL reinforced IPN hydrogels laden with a co-culture of bone marrow-derived stromal cells (BMSCs) and chondrocytes (CCs). The bioprinted biomimetic composites were found to support robust chondrogenesis, with encapsulated cells producing hyaline-like cartilage that stained strongly for sGAG and type II collagen deposition, and negatively for type X collagen and calcium deposition. Taken together, these results demonstrate how 3D bioprinting can be used to engineer constructs that are both pro-chondrogenic and biomimetic of the mechanical properties of articular cartilage.

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Electrospun fibers offer tremendous potential for tendon and ligament tissue engineering, yet developing porous scaffolds mimicking the size, stiffness and strength of human tissues remains a challenge. Previous studies have rolled, braided, or stacked electrospun sheets to produce threedimensional (3D) scaffolds with tailored sizes and mechanical properties. A common limitation with such approaches is the development of low porosity scaffolds that impede cellular infiltration into the body of the implant, thereby limiting their regenerative potential. Here, we demonstrate how varying the rotational speed of the collecting mandrel during the electrospinning of poly(ε-caprolactone) (PCL) can be used to limit inter-fiber fusion (or fiber welding). Increasing the fraction of unfused fibers reduced the flexural rigidity of the electrospun sheets, which in turn allowed us to bundle the fibers into 3D scaffolds with similar dimensions to the human anterior cruciate ligament (ACL). These unfused fibers allowed for higher levels of porosity (up to 95%) that facilitated the rapid migration of mesenchymal stem cells (MSCs) into the body of the scaffolds. Mechanical testing demonstrated that the fiber-bundles possessed a Young's modulus approaching that of the native human ACL. The scaffolds were also capable of supporting the differentiation of MSCs towards either the fibrocartilage or ligament/tendon lineage. This novel electrospinning strategy could be used to produce mechanically functional, yet porous, scaffolds for a wide range of biomedical applications.