*2.4. Roll to Roll Inkjet Printing*

Inkjet printing is an extensively developed technology that is widely employed in conventional printing applications, and it is exceptionally well suited to rapid, low-cost prototyping [25,31,68]. In inkjet printing, pressurized ink is forced through a nozzle, forming droplets that fall onto the substrate and collapse due to their momentum and substrate wettability [46]. Inkjet printing is achieved through two approaches, although drop on demand (DOD) printing is greatly preferred over continuous inkjet printing (CIJ) for bioelectronics because it allows for higher placement accuracy and higher resolutions [17]. In DOD printing, the ink is forced through the nozzle through either a contractile force applied from a piezoelectric actuator or a thermal disturbance that produces a shockwave capable of ejecting the ink [25]. In contrast, a CIJ printer charges ink droplets and continually passes them through an electric field formed between two deflection plates, allowing one to control the ink depositions [25]. Both inkjet printing processes are illustrated in Figure 5a. DOD printing is highly attractive because it allows for excellent control over deposition thickness, high resolution down to 40 µm, very inexpensive prototyping, and minimal startup costs, but it is also limited by clogging in the minuscule nozzle head, the uneven flow of material to the edge of the print in a process termed the coffee ring effect, and lower throughputs than the previously described methods [11,22,68].

Each of these limitations has been thoroughly studied using traditional graphics inks, and the challenge in bioelectronics fabrication is to apply these lessons to nanomaterialbased inks [25,69,70]. The fluid mechanics during printing is characterized primarily by three dimensionless quantities, the Weber number (We), Reynolds number (Re), and Ohnesorge number (Oh):

$$\begin{array}{c} \text{We} = \frac{\zeta \rho v^2}{\gamma} \\ \text{Re} = \frac{\zeta \rho v}{\eta} \\ \text{Oh} = \frac{\sqrt{\text{We}}}{\text{Re}} = \frac{\eta}{\sqrt{\zeta \rho \gamma}} \end{array}$$

where *η*, *ρ*, and *γ* are the ink viscosity, density, and surface tension, respectively, *v* is the print velocity, and *ζ* is characteristic printing length, which is in most cases simply the diameter of the print head nozzle [25,26,70]. In almost all inkjet applications, Oh must be between 1 and 1/10 to achieve a quality print, as illustrated in Figure 5b [28]. At high Oh values, the ink viscosity will prevent stable drop formation [28]. When Oh is too low, the ink forms many uncontrolled drops instead of a single, well-defined drop, which results in an unusable print [28,69]. In addition, the particle size cannot be >*ζ*/50 in order to avoid immediate nozzle clogging [71]. As we will discuss in Section 3, these requirements greatly complicate the printing of Ag nanowires (AgNWs) and CNTs, which

are usually much longer than *ζ*/50, and carbon-based nanomaterials, which are difficult to disperse with both low viscosity and high material loadings [47,72,73]. Another crucial challenge in inkjet printing is the accumulation of the deposited material along the edge of the print, commonly termed the coffee ring effect [48]. This occurs when the edge of a droplet on a substrate is fixed in place and capillary flow induced by evaporation of the drop causes material to flow from the interior towards this fixed edge [48]. This process is combatted by Marangoni flow within the drop, but many surfactants and even added water tend to have very weak Marangoni flows [74]. There are numerous methods employed to combat coffee ring formation, including careful control of the surfactant mediated interactions between particles and the liquid–gas interface [72,74], mixing high and low boiling point solvents [75], heating the substrate [76], depinning the contact line (which reduces print definition) [77], alternating voltage electrowetting [78], and dual drop inkjet printing [33]. In an example of the first method, Anyfantakis et al. mixed surfactants and colloids with opposite charges and observed that particles that absorbed the surfactants become hydrophobic, giving them a greater affinity to the liquid–gas interface [72]. These particles on the drop surface prevented capillary flow from collapsing the structure, leading to a uniform deposition, as shown in Figure 5c [72]. In the later method, two main approaches are employed. First, the Langmuir–Blodgett concept is applied to the picolitre depositions by first depositing a supporting layer, then adding a functional ink on top containing colloidal nanoparticles that assemble as the solvent dissolves to produce a highly uniform layer, as illustrated in Figure 5d,e [33]. Second, antisolvent crystallization can be used to form highly uniform semiconducting films at the liquid–air interface in a mixed droplet [79]. This occurs after printing an antisolvent layer, then a semiconductor solution. The undissolved nuclei form a cohesive film on the drop surface, preventing the drop from collapsing as the solvent evaporates [79].

Finally, significant commercial interest in inkjet printing has led to many efforts to improve manufacturing throughput, and numerous inkjet printers can achieve speeds far beyond those achieved in home-use graphics printers [73]. However, there are still key tradeoffs between print resolution, deposition uniformity, and throughput [26,73]. The greatest improvements in throughput generally come through roll-to-roll processing, stringent quality control on component manufacturing, and precise temperature control, all of which have been thoroughly investigated by private companies [73]. Even in the most advanced systems, nozzle clogging is still a crucial issue with nanomaterial inks that limits manufacturing throughput, and continuous cleaning of the systems is therefore necessary [73]. Inkjet printing is highly attractive for printing bioelectronics because complex systems can be very rapidly prototyped during development, then easily scaled to mass production, but there are also very strict requirements on nanomaterial ink properties, lower demonstrated throughputs than alternative methods, and key challenges relating to nozzle clogging that complicate high throughput fabrication.

#### *2.5. Slot Die and Blade Coating*

Slot die and blade coating, which are sometimes referred to as bar coating or knife coating, are high throughput methods to deposit homogenous films for applications that do not require complex patterns to be formed [34,49]. In blade coating, ink is placed before the blade, and deposition is left as the blade swipes across the substrate [34]. The thickness of the resultant deposition depends largely on the blade height relative to the substrate, the print velocity, ink viscosity, and ink-substrate wetting contact angle [34]. In slot coating, ink is continually pumped from a slot inside a print head, which can be masked to print unidirectional lines [49]. In addition, the print head can be displaced perpendicular to the print direction to yield curved lines [80]. The print quality and film thickness in slot die coating is determined by the meniscus forming between the print head and the substrate, and this meniscus can be controlled by the same parameters mentioned for the doctor blade in addition to the pumping rate and temperature control of the ink [30]. Slot die and blade coating are mature manufacturing processes for depositing homogeneous films, which

are desired in pressure, chemical, and electrophysiological sensors for soft bioelectronics; however, these methods are not well suited to more complicated printing applications that require sophisticated patterning.

**Figure 4.** High-throughput screen-printing approaches. (**a**) Illustration of the six stages of screen printing, as proposed by Messerschmitt et al. and investigated Abbott et al. (**i**) Ink flooded into the mesh. (**ii**) Squeegee pressure brings the mesh in contact with the substrate. (**iii**) Ink adheres to both the substrate and mesh. (**iv**–**vi**) As the mesh is raised off the substrate, the ink first (**iv**) forms a continuous structure, then (**v**) forms filaments, which then (**vi**) collapse and level to form a deposition. (reprinted with permission from *ACS Omega* (2021), 6, 14, 9344–9351. Copyright 2021, ACS). (**b**) Illustration of a sheet-to-sheet screen-printer. (reprinted with permission from *Adv. Mater.* (2019), 31, 1806702. Copyright 2020, Wiley). (**c**) Illustration of a roll-to-roll screen printer, demonstrating the key operating principles. (reprinted with permission from Ind. *Eng. Chem. Res.* (2019), 58, 43, 19909–19916, Copyright 2020, ACS). (**d**) Image of a roll-to-roll screen-printer used in nanomaterial printing. (reprinted with permission from *Ind. Eng. Chem. Res.* (2019), 58, 43, 19909–19916, Copyright 2020, ACS). (**e**) Fabrication of a thin silicon screen printing stencil for high-resolution printing and printing process implanting this stencil. (reprinted with permission from *Adv. Mater.* (2014), 27: 109–115. Copyright 2014, Wiley).

ℎ = √ **Figure 5.** Roll-to-roll inkjet printing for hybrid bioelectronics. (**a**) Illustration of CIJ (**left**) and DOD (**right**) inkjet printing techniques. (reprinted with permission from *Micromachines* (2017), 8(6), 194. Copyright 2017, MDPI). (**b**) Reynolds number and Ohnesorge numbers that yield a high-quality inkjet deposition. Weber numbers can be calculated based on the ratio Oh = √ We/Re. (**c**) Example images and illustrations of coffee ring formation due to capillary flow in evaporating droplets (**i**,**vi**–**viii**). This is compared to uniform depositions produced with added DTAB to promote particle trapping at the liquid–gas interface, which created particle skins that lead to homogenous disk like patterns upon drying (**ii**–**v**). (reprinted with permission from *Langmuir* (2015), 31, 14, 4113–4120, Copyright 2015, ACS). (**d**) Illustration of the dual drop inkjet printing process, where the blue ink is the supporting droplet, the red ink is the wetting droplet, and the gold represents the nanoparticles to be deposited. (reprinted with permission from *Adv. Mater. Interfaces* (2018), 5, 1701561. Copyright 2018, Wiley). (**e**) Illustration of the dual drop process used to deposit a uniform nanoparticle monolayer. (reprinted with permission from *Adv. Mater. Interfaces* (2018), 5, 1701561. Copyright 2018, Wiley).
