3.2.2. Post Print Processing

After printing, NP depositions must be cured to remove the solvent, and many, but not all, inks are also sintered to form conductive sheets [94,100]. Sintering is not typically employed for printing on TPU, PET and paper because of low substrate melting points, in printing stretchable interconnects, where the unconnected particles form effective conductive networks with strain, and for biosensor applications where increased surface area is preferred (e.g., glucose sensors) [104]. Sintering, however, is highly advantageous for forming conductive sheets with low resistances and high yield stress [47,105,106]. The SEM images in Figure 6c clearly show the formation of a more uniform metal sheet with increased temperature in thermal sintering, and this is reflected in the decreased resistivity [107]. Although thermal and chemical sintering are easily employed in sheetto-sheet processes, alternative methods are needed for roll-to-roll integration [105]. One approach with significant promise is photonic sintering, where energy is provided by an ultrafast pulsed laser source with a wavelength tuned to match the ink's absorption spectrum [105,106]. For instance, Hösel et al. demonstrated a single exposure system integrated into roll-to-roll flexography printing with speeds of 2.5 m/min [106]. In addition, electric, plasma, and microwave sintering are well suited for roll-to-roll processes [108,109]. Allen et al. demonstrated effective electric sintering with a directly applied voltage, but the method has not been explored for roll-to-roll processes, likely because of the need to create direct and secure contact between the pattern and electrode [108]. In contrast, indirect methods, such as microwave sintering, can be well integrated into roll-to-roll processes, but their throughput is greatly limited compared to photonic and electrical methods [105]. For instance, Fujii et al. demonstrated effective sintering in 1.5 min, compared to milliseconds in other methods [110]. Finally, plasma sintering is a promising sintering method, but it is limited for thick or multilayer depositions by a slow depth penetration, which is an issue for high throughput applications [105].
