**3. Conductive Nanomaterial Printing**

## *3.1. Fundamentals*

Printed nanomaterial applications typically follow the same four-step process: first, nanomaterials are produced either through top-down methods, where the nanomaterial is broken off from bulk material, or bottom-down approaches, where the particles are synthesized from atomic precursors [81–85]. Second, these nanomaterials are dispersed in printable inks with viscosities and rheology that are optimized for the printing method of choice [11,26,86]. Third, the inks are printed on a substrate and create a deposition based on the fluid mechanics during printing and free energy effects at the liquid–gas and liquid–solid interfaces [11,44,63,69]. Finally, the solvent is evaporated, and, in some cases, the nanomaterials are sintered to yield a conductive structure [50,87,88]. When choosing a nanomaterial for a specific bioelectronics' application, the material's electrical and mechanical properties, the tendency to agglomerate, required loading to produce a conductive network, and particle aspect ratio are crucial considerations. For instance, graphene nanoplatelets and carbon nanotubes (CNTs) have excellent conductivities, are easily functionalized, and have high durability, but they are difficult to disperse in printable inks because of strong intermolecular forces [67,83,89]. In most nanomaterial inks, the solvent is highly polar, and the nanomaterial is nonpolar [90,91]. An amphiphilic dispersion agent, such as polyvinyl pyrrolidone (PVP) and sodium dodecyl sulfate (SDS), is introduced [64]. The nonpolar region binds to the nanomaterial surface, leaving a polar tail that allows the material complex to be dissolved in the solvent and creates interparticle repulsive forces that prevent agglomeration. In silver nanomaterials, PVP is highly attractive because the nitrogen and oxygen atoms enable effective absorption into the surfaces of Ag seeds or particles, whereas SDS is effectively absorbed into CNT surfaces in the presence of ultrasonication energy [64,91,92]. However, SDS is not biocompatible, and it must be effectively removed or reduced in concentration either before or after printing if the CNTs will be skin-contacting [91]. On the other hand, PVP is biocompatible, making it more attractive for many bioelectronics applications [93]. In the following sections, we will summarize recent developments in the synthesis, dispersion, high throughput printing, and sintering for each nanomaterial and demonstrated the soft electronics devices created with these methods.
