Figure 1 3D printing techniques Figure 1 | 3D printing techniques. After creation of a virtual 3D model, four different 3D printing techniques can be used to create the construct. A | Inkjet printers are classified as thermal or acoustic on the basis of their extrusion force. In thermal printers, the print head is electrically heated to produce pressure pulses that force droplets from the nozzle. In acoustic printers, the force for droplet release is generated by a pressure wave, for example, by sending voltage through a piezoelectric actuator. Different substrates can be printed using the same print head. Ba | In extrusion printers, the dispensing force is created through air-pressure-driven pneumatic extrusion or mechanical, piston-based extrusion, forcing the substrate through a syringe-like extrusion head. Bb | Extruded tubes of a cell-containing hydrogel mature over time into cellularized tubes, in which the hydrogel has been replaced by extracellular matrix (ECM). C | In laser sintering of organic components, the energy source (for example, a laser) is directed at a transparent carrier ribbon that has the desired substrate on its underside. The biomaterial is then propelled onto a collecting dish, enabling stepwise production of a 3D construct. D | In stereolithography, a focused energy beam selectively heats an area in a bath of a liquid polymer. As the heated area of the polymer hardens, the completed layer gradually descends, and the next layer is created from the overlying substrate. Figure adapted with permission from Ref. 9, Springer. Figure adapted with permission from Skardal, A. & Atala, A. Biomaterials for integration with 3D bioprinting. Ann. Biomed. Eng.43, 730–746 (2015). Colaco, M. et al. (2018) The potential of 3D printing in urological research and patient care Nat. Rev. Urol. doi:10.1038/nrurol.2018.6