According to the specific enduse, during the formulation of graphenebased inks, several criteria should be taken into consideration. First, the stability of graphene and/or its derivatives against the precipitation and aggregation in ink should be considered to ensure reproducibility. Depending on its end-use, the formulated ink should possess the expected fluidic properties, such as surface tension and viscosity, according to its application methods on the substrate. For instance, an ink formulated for imparting through printing methods should have higher surface tension and lower viscosity, facilitating the uninterrupted ejection through a printing applicator, such as a printing mesh, or printing nozzle, without bleeding, drying out, coagulation, or breaking away. Second, the printed pattern or track fabricated by the graphene-based inks must exhibit good electrical properties with maximum adhesion to the substrates at predefined environmental stimuli. Finally, the formulation methods should be free from complicacy and ensure maximum yield. [10] The preparation of graphene ink with good stability takes two routes: ultrasonic exfoliation of graphene sheets with stabilizers and stabilizers during the chemical reduction of graphene oxides that can wrap around the graphene and may electrostatically or sterically create a barrier for restacking. [11] Water, ethanol, N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), terpineol, and dimethyl sulfoxide (DMSO) are currently employed as solvents in producing appropriate graphene ink. Table 1 presents the results of some of the previous studies on electroconductive graphene inks using various solvents. The table shows that some solvents (NMP and DMF) are toxic and unsuitable for future printed flexible/wearable electronic applications. [12] In this situation, Polycyclic aromatic hydrocarbons, polymers (polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA)), surfactants (e.g., sodium dodecyl benzenesulfonate, sodium cholate, and sodium deoxycholate), ethylcellulose are often used to stabilize the exfoliated flakes against re-aggregation by Coulomb repulsion (e.g., pluronic). [13][14][15][16][17][18][19] However, wrapping the dispersant molecules around the graphene/graphitic flakes is not the greatest solution for realizing conductive channels. The presence of these molecules might reduce inter-flake connections, resulting in the electrical conductivity of the printed patterns. [14,[20][21][22][23][24] Moreover, such stabilizers in ink affect the optoelectronic characteristics of graphene once printed on a target substrate. [25] Formulating highly stable graphene-based conductive inks with consistency in electrical properties over the storage period has remained a significant challenge in the development of wearable electronics. Two highly stable graphene-based inks (Cyclohexanone:Ethylene glycol (CEG) ink and Cyclohexanone:Terpineol (CT) ink) are prepared by using two different organic binary solvents, for the first time, without using solvent exchange methods. Both the...

An isophorone‐based quaternary compound with two positively charged polymer chains has been synthesized to modify charge‐neutral graphene without affecting its unique intrinsic properties. A highly scalable dip and dry coating method is employed to coat the textile‐based substrates without subsequent post‐treatment. The resultant electronic textiles exhibit remarkably low sheet resistance ≈140 Ω sq−1) after 10 washing cycles, which is considerably better compared to any reported graphene‐based e‐textiles without post‐treatment. Graphene‐based nonwoven piezoresistive sensors developed for wearable e‐textiles use this approach to exhibit an impressive piezoresistive sensitivity in the low‐pressure range of 0 to 40 Pa and display good repeatability after washing cycles. Hence this novel modification strategy is envisaged to have strong potential for multifunctional applications in next‐generation graphene‐based wearable e‐textiles.

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