3.4.1. Material Properties, Synthesis, and Ink Formation

Graphene is a zero-gap semiconductor with exceptional conductivity, biocompatibility, and high mechanical strength that is easily functionalized with numerous materials for sensing applications [81]. As a result, graphene is highly attractive for printed bioelectronics [53,67,86]. However, graphene is very difficult to print because of its low dispersibility in printable inks [86]. Typically, graphene is formed through exfoliation from graphite, either through ultrasonic or mechanical methods, but numerous additional mechanisms have been explored, including electrochemical synthesis, chemical vapor deposition, laser processing and sodium ethoxide pyrolysis [81]. An example SEM image of graphene sheets for screen printing is provided in Figure 8a [121]. Once synthesized, dispersing graphene in printable inks is a key challenge. Although graphene oxide (GO) is easily dispersed, it must be reduced after printing, limiting throughput, and creating numerous defects that detract from the material's electrical and sensing capabilities.

To create a screen printable ink, He et al. dispersed 5 g of graphene nanoplatelets (GNPs) in 50 mL EG and 0.5 g PVP and printed traces on the PI with conductivities of 8.81 <sup>×</sup> <sup>10</sup><sup>4</sup> S/m [67]. Graphene's natural tendency to agglomerate was used to increase the ink viscosity to a range reasonable for screen printing (1 Pa·s) [67]. For gravure printing, Secor et al. noted that stabilizing graphene with ethyl cellulose (EC) greatly aids in dispersion [53]. Graphene was exfoliated from graphite in ethanol with EC, excess graphite was removed by centrifuging. The resultant graphene–EC precipitate was then redispersed in ethanol and terpineol, and the specific quantities of each substance were altered to yield inks with various viscosities [53]. As described in Section 2.1, the optimized ink was able to be printed with trace widths of <30 µm with conductivities >10,000 S/m [53]. Although initial feasibility studies have investigated flexographic graphene printing, no successful use in soft bioelectronics has been reported to date [122]. Likewise, many inkjet applications

select to use GO instead of graphene, but graphene printing has been successfully reported. For instance, Li et al. exfoliated graphene from graphite in dimethylformamide (DMF), and the toxic DMF is distilled out in a terpineol solution [86]. Graphene in this state is normally stable for only hours, but in this work, EC was added to protect the graphene from agglomeration. After the solvent exchange, the graphene/toluene dispersion was mixed in ethanol in a volume ratio of 3:1 to yield a printable viscosity and rheology. Figure 8b,c depicts this optimized ink (b) directly after printing and (c) after curing, demonstrating a mostly uniform deposition with a minimal coffee ring effect [86]. Finally, the resultant ink was printed in 80 µm traces on both plastic, and silicon substrates and printed supercapacitors were able to achieve a specific capacitance of 0.59 mF cm−<sup>2</sup> [86].
