*2.2. Flexographic Printing*

Flexography is another high throughput, a roll-to-roll fabrication method for printed electronics the origins of which can be traced to late 19th-century image printing [29]. Flexographic printing consists of five subprocesses, as depicted in Figure 3a. First, ink is pulled from a reservoir by the fountain roller. Second, the ink is transferred to an intermediate anilox roller containing millions of miniature engraved cells. Third, a blade removes excess ink from the anilox roller. Fourth, ink is transferred from the anilox to a flexible photopolymer plate containing a mirror engraved pattern. Finally, the substrate is rolled between the flexographic plate and an impression cylinder, yielding an ink deposition on the substrate. Because the printing roller is made of a flexible polymer wrapped around a metal cylinder, the prototyping and startup costs are significantly lower in flexography than gravure printing. However, plate deformation is a significant limitation to be overcome in high-resolution flexographic printing [41]. Another key difference between gravure printing and flexography is the presence of an anilox roll, which allows for a wider range of ink rheology to be printed, but whose geometry, pressure, and speed must be carefully optimized [41]. These challenges exist in addition to those faced by gravure printing, which is one explanation for the greater adoption of gravure for printed electronics. However, recent works have significantly improved flexographic printing capabilities, making flexography an exciting and fast-developing approach with a significantly lower barrier to entry than gravure printing. For instance, the surface energy of the flexography roll relative to the anilox and substrate can be modified to improve ink transfer in each phase, the print speed and pressure can be optimized for the specific transfer chemistry and pattern geometries, and the geometries themselves can be improved [41,58,59]. However, these innovations still result in resolutions >50 µm because of fundamental material limitations in the photopolymer flexographic roll. As a result, Kim et al. developed a microstructured, nanoporous carbon nanotube (CNT) stamp to replace the traditional roll with carefully controlled porosity, mechanics, and surface chemistry [60]. As shown in Figure 3b, the CNT nanopillars leave a precise open area in which the ink can reside. During printing, the stamp is brought into conformal contact with the substrate due to the mechanical flexibility of the CNTs, and a highly controlled deposition is produced as the stamp is removed, as shown in Figure 3c. This mechanism overcomes many of the key challenges in flexographic transfer by storing the ink in the stamp pores, then transferring directly to the substrate, and yield high-quality prints with a variety of nanomaterial inks of <20 µm were demonstrated [60,61]. In light of these innovations, flexography is now considered an exciting new field in printed electronics with great opportunities for further improvement.

**Figure 2.** Gravure printing of hybrid bioelectronics. (**a**) Overview of the gravure printing process (reprinted with permission from *Flex. Print. Electron* (2016), 1 023003. Copyright 2016, IOP). (**b**) Illustration of gravure printing linear traces against an impression roll. (reprinted with permission from *Adv. Mater.* (2019), 31, 1806702. Copyright 2020, Wiley). (**c**) Optimizing graphene inks for gravure printing. (**i**) Characterization of viscosity for the three different ink formulations. (**ii**–**iv**) Images of printed dots for each ink using a gravure cell of 50 µm. (**v**–**vii**) Images showing line formation as the cell spacing is reduced, corresponding to 50, 25, and 5 µm spacing for a cell size of 50 µm. (reproduced with permission from *Adv. Mater.* (2014), 26: 4533–4538. Copyright 2014, Wiley). (**d**) Optimized cell patterns achieved with gradation engraving to achieve high-resolution gravure printing. (reprinted with permission from *Precis. Eng.* (2021), 69: 1–7. Copyright 2021, Elsevier). (**e**) Illustration of roll-to-roll printed sweat sensors. (reprinted with permission from *ACS Nano* (2018), 12(7): 6978–6987, Copyright 2018, ACS).
