**4. Advanced inkjet printing techniques**

IJP is a mature technology and recently has been used to print functional inks for PE devices. This novel use demanded developments in terms of new printing equipment and inks. As a result, advanced IJP technologies have emerged, responding to the requirements of novel applications. These advanced technologies are focused on increasing printing resolution and speed, printing of high viscosity inks (with higher electrical conductivity), printing over non-planar substrates, and enlarging the range of materials that can be printed.

A novel double-shot IJP technique has been developed, which allows for the deposition of two types of inks at the same position [163]. In this way, conductive and dielectric inks can be printed at the same position, allowing the construction of devices. Reactive inkjet printing, which also uses two nozzles, combines the processes of material deposition and chemical reaction to print over a substrate material, enlarging the type of ink materials.

EHD-IJP allowed high-resolution printing, paving its use in micro/nano manufacturing of electronic devices [45]. EHD-IJP is a direct patterning technique that can also be used as a thin film deposition technique (e.g., electrospraying, electrospinning). Furthermore, multi-nozzle implementation has been proposed, but nozzle density is still low [50]. EHD-IJP also allows printing of high viscosity inks and consequently has huge potential for fabricating 3D patterns [163].

Needle-based printing is a recent technology to dispense relatively high viscosity ink through a fine nozzle [50]. Droplets are ejected from the nozzle exit by the motion of the needle, which can be operated by air pressure or piezoelectric actuator. This technique can handle high viscosity inks.

Micro-plotter is a technology that also allows dispensing of relatively high viscosity ink through a fine nozzle [50]. The dispensing mechanism is based on the ultrasonic pumping action at the core of the micro-plotter head, a micropipette. The mechanism is capable of depositing ink droplets with dot size of less than 2 μm, which is smaller than an inkjet system, even when using relatively high viscosity inks (up to 450 cP).

Other droplet-based techniques have been developed, mainly for printing on nonplanar substrates, such as aerosol jet printing, surpassing the limitations of both inkjet and EHD-IJP techniques. Aerosol jet printing uses high-speed ejection of aerosols instead of liquid droplets [164, 165]. The aerosol is produced by atomization of ink, which produces very small droplets with diameters in the range of 1 to 5 μm. Due to the aerodynamic effect, it also allows higher resolution for fabricating micro- and nanoscale devices. Furthermore, aerosol jetting allows different inks to be conformably printed onto the substrate.

High-resolution 3D patterning combines IJP with 3D printing technologies [166]. High-resolution insulating and conductive layers can be printed using multiple printheads in the same printing system. However, this requires a high degree of deposition precision that can be achieved by the use of phase-change inks (activated by chemical or thermal triggers) with no solvents. 3D structures can also be produced by IJP, by deposition of layer-by-layer of two reactive components, followed by polymerization. This is a technology of current intense research and fast growth.
