*2.3. Screen Printing*

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Screen printing is an ancient printing method that has been employed in garment processing for centuries [19]. Today, it is a mature industrial process used in textiles, graphics, printed circuit silkscreens, in-mold electronics, capacitive touch sensors, printed heaters, and chemical sensors [47]. Significantly, screen printing is highly suitable for roll-to-roll manufacturing and high throughput processing. Unlike gravure printing and flexography, Screen printing involves the active transfer of ink from a mesh to a target substrate mediated by pressure and shear applied by a blade termed the squeegee [62,63]. Printing occurs in six distinct phases, as shown in Figure 4a [43,62]. First (I), ink enters the mesh after the application of gentle pressure such that it occupies the enter open mesh area,

but does not run out from the bottom of the mesh [63]. Second (II), the mesh is brought into contact with the substrate as a result of applied pressure and the highly pseudoplastic ink becomes highly thin with applied pressure [64]. Third (III), the ink adheres to both the mesh and the substrate [43]. Fourth (IV), the mesh is pulled upwards as the squeegee progresses down the print, causing the ink to rise [43]. Fifth (V), the ink begins to form filaments underneath the mesh wires as the mesh is continually raised [43]. Finally (VI), the filaments break and the print levels, resulting in a deposition thickness that depends on the mesh open area and the ink adhesion to both the mesh and the substrate [43]. In traditional screen-printing applications, the substrate is placed on a flat plate below the mesh, as shown in Figure 4b [34]. In roll-to-roll screen-printing, the mesh is folded into a cylinder with the squeegee blade inside the cylinder [35]. The substrate is then rolled against the mesh and the impression cylinder, which causes an applied pressure against the squeegee and shear proportional to the print velocity. This process is depicted in Figure 4c, and an example roll-to-roll machine is shown in Figure 4d [35]. The same six steps described previously also apply to roll-to-roll screen printing, but some non-idealities in the mesh liftoff are caused by the curved substrate, especially when the radius of curvature is small [35,65].

**Figure 3.** Advanced flexographic printing techniques. (**a**) Diagram of a standard flexographic printer to illustrate the key operating principles. (reproduced under creative commons license CC BY-SA 4.0). (**b**) Scanning electron microscope images of a CNT array (100 µm pillar diameter, 150 µm height) used for high-resolution flexography, and close-up top and side surfaces of a micropillar (reproduced with permission from *Langmuir* (2019), 35, 24, 7659–7671. Copyright 2019, ACS). (**c**) Simplified schematic of ink transfer from carbon nanotubes (CNTs) micropillar stamp loaded with ink. (reproduced with permission from *Langmuir* (2019), 35, 24, 7659–7671. Copyright 2019, ACS).

In screen printing, the mesh height from the substrate and mesh geometry are crucial parameters, but the squeegee speed and pressure are not highly correlated with print quality [66]. This is because the sheer and compressive forces applied are typically large enough to elicit a strong sheer thinking response in ink and prevent ink hydroplaning before the squeegee [24]. Instead, optimizing ink rheology for this complicated fluid dynamics is crucial in screen printing [64]. Ink viscosities are typically high (10–30 PaS) and highly pseudoplastic so that they can avoid running through the mesh preprinting, flow easily during applied shear, and rapidly coalesce post print into a steep deposition without slumping on the substrate [11,67]. Furthermore, the mesh area that is not to be printed is blocked by an ultraviolet (UV) cured emulsion mask, and limiting this emulsion's

roughness is important in creating a high resolution and even print [66]. Like gravure and flexographic printing, the minimal resolution achievable in screen printing is limited fundamentally by the mesh quality, even if many inks with suboptimal rheology cannot approach this limit [63]. Specifically, screen printing meshes are limited by lithography resolution in emulsion etching, emulsion smoothness, and mesh geometries [11]. Creating a finer mesh with more weaves per unit area improves print resolution, but the reduction in mesh open area leads to a thinner print deposition [11]. Mesh counts generally reach their minimization limits beyond 140 threads per centimeter, and screens with around these mesh counts and optimized emulsions are capable of printing resolutions of around 70 µm [62]. However, Hyun et al. recently demonstrated a screen-printing stencil derived from a thin silicon wafer (90 µm thickness) with photolithographically defined openings to produce high quality depositions of graphene and AgNP inks with widths of 40 µm, and the silicon stencil fabrication and graphene printing is illustrated in Figure 4e [11]. In summary, screen printing is attractive for high throughput printed bioelectronics because it is a mature industrial process with significantly lower startup and prototyping costs than gravure and flexographic printing, and new innovations in mesh or stencil design open new opportunities for increased print resolutions.
