Engineering a microfluidic-assisted 3D bioprinting approach for the hierarchical control deposition and compartmentalisation of graded bioinks.

The advent of 3D bioprinting has revolutionised tissue engineering and regenerative medicine (TERM). Today, tissues of single cell type can be fabricated with extreme resolution and printing fidelity. However, the ultimate functionality of the desired tissue is limited, due to the absence of a multicellular population and diversity in micro-environment distribution. Currently, 3D bioprinting technologies are facing challenges in delivering multiple cells and biomaterials in a controlled fashion. The use of interchangeable syringe-based systems has often favoured the delamination between interfaces, greatly limiting the fabrication of interconnected tissue constructs. Microfluidic-assisted 3D bioprinting platforms have been found capable of rescuing the fabrication of tissue interfaces, but often fails to guarantee printing fidelity, cell density control and compartmentalisation. Herein, we present the convergence of microfluidic and 3D bioprinting platforms into a deposition system capable of harnessing a microfluidic printhead for the continuous rapid fabrication of interconnected functional tissues. The use of flow-focusing and passive mixer printhead modules allowed for the rapid and dynamic modulation of fibre diameter and material composition, respectively. Cells were compartmentalised into discrete three-dimensional layers with defined density patterns, confirming the punctual control of the presented microfluidic platform in arranging cells and materials in 3D. In ovo and in vivo studies demonstrated the seminal functionality of 3D bioprinted constructs with patterned vascular endothelial growth factor (VEGF) and transforming growth factor-β1 (TGF-β1), respectively. This, in turn, facilitated the simulation of diverse cellular environments and proliferation pathways within a single construct, which is currently unachievable with conventional 3D bioprinting techniques, offering new opportunities for the fabrication of functionally graded systems and physiologically-relevant skeletal tissue substitutes.