**2.1 Inkjet printing**

 Inkjet printing is a non-contact method of fabrication, which limits the risk of contamination during printing. Scaffolds are constructed using the deposition of droplets and can achieve excellent resolution ranging between 20 and 100 μm [9–11]. Droplets are ejected from a printhead via thermal, piezoelectric or microvalve processes. Thermal inkjet printing uses the nucleation of an air bubble to induce droplet formation, but a localized temperature between 100 and 300°C could inflict damage on cells and limits its use for printing natural polymers [10, 12]. Piezoelectric-based inkjet printing uses acoustic waves to eject a droplet, however this limits the use of viscous and therefore more concentrated inks, as these will dampen out the acoustic waves before they can expel a droplet [13]. Microvalve printing uses pneumatically controlled pistons to eject droplets. Inkjet printing can print multiple inks at once that can be used to interact with each other to control scaffold properties and create complex structures with multiple types of cells [14–16].
