Successful cartilage engineering requires the generation of biological grafts mimicking the structure, composition and mechanical behaviour of the native tissue. Here melt electrowriting (MEW) was used to produce arrays of polymeric structures whose function was to orient the growth of cellular aggregates spontaneously generated within these structures, and to provide tensile reinforcement to the resulting tissues. Inkjet printing was used to deposit defined numbers of cells into MEW structures, which self-assembled into an organized array of spheroids within hours, ultimately generating a hybrid tissue that was hyaline-like in composition. Structurally, the engineered cartilage mimicked the histotypical organization observed in skeletally immature synovial joints. This biofabrication framework was then used to generate scaled-up (50 mm × 50 mm) cartilage implants containing over 3,500 cellular aggregates in under 15 min. After 8 weeks in culture, a 50-fold increase in the compressive stiffness of these MEW reinforced tissues were observed, while the tensile properties were still dominated by the polymer network, resulting in a composite construct demonstrating tension-compression nonlinearity mimetic of the native tissue. Helium ion microscopy further demonstrated the development of an arcading collagen network within the engineered tissue. This hybrid bioprinting strategy provides a versatile and scalable approach to engineer cartilage biomimetic grafts for biological joint resurfacing.

Digging into nonclassical pathways to crystallization to unearth design principles for fabricating advanced functionalized materials shapes the future of materials science. Nature has long since been exploiting such nonclassical pathways to crystallization to build inorganic-organic hybrid materials that fulfill support, mastication, defense, attack, or optical functions. Especially, various biomineralizing taxa such as stony corals deposit metastable, magnesium-rich, amorphous calcium carbonate nanoparticles that further transform into higher-order mineral structures. Here we examine whether a similar process can be duplicate in abiogenic conditions using synthetic, amorphous calcium magnesium carbonate nanoparticles. Applying a combination of ultrahigh-resolution imaging, and, in situ, solidstate nuclear magnetic resonance (NMR) spectroscopy, we reveal the underlying mechanism of the phase transformation of these synthetic amorphous nanoparticles into crystals. When soaked in water, these synthetic amorphous nanoparticles are coated by a rigid hydration layer of bound water molecules. In addition, fast chemical exchanges occur between hydrogens from the nanoparticles and those from the free water molecules of the surrounding aqueous medium. At some stage, crystallization spontaneously occurs, and we provide spectroscopic evidence for a solid-state phase transformation of the starting amorphous nanoparticles into crystals. Depending on their initial chemical composition, and especially on the amount of magnesium, the starting amorphous nanoparticles can aggregate and form ordered mineral structures through crystal growth by particle attachment, or rather dissolve and reprecipitate into another crystalline phase. The former scenario offers promising prospects for exerting some control over such non-classical pathway to crystallization to design mineral structures that could not be achieved through a classical layer-by-layer growth.