3.5.1. Material Properties, Synthesis, and Ink Formation

CNTs offer very attractive elasticity, biocompatibility, surface area, aspect ratios, strength, and conductivity, making them of great interest for electronics applications, but very strong van der Waals interactions greatly complicate particle dispersion [83]. CNTs consist of rolled graphene sheets that consist of either one tube (single-walled CNT, or SWCNT) or multiple tubes (multi-walled CNT, or MWCNT) held together with Van der Waals attractions [90]. The direction in which CNTs are rolled greatly affects their observed properties, and illustrations of several common orientations are provided in Figure 9a [134]. "Armchair" CNTs are highly preferred for interconnects or conductive planes because their identical chiral indices create highly uniform conductivity [92], but zigzag or chiral CNT orientations are widely employed for their semiconducting effects, and they are also of great interest for printed transistor fabrication [91]. CNTs are typically synthesized through three processes: chemical vapor deposition (CVD), arc discharge, and laser ablation, although CVD is the most widely employed. In CVD, metal NPs of the CNT diameter are introduced in the presence of a carbon-based gas, such as CO2, to form CNTs, and this process is illustrated in Figure 9b [82,91]. In order to remove the NPs and other impurities, the CNT powder is typically sonicated or treated with acid, and CNT purity is crucial in achieving optimal material properties [83,135]. The final product is CNTs like those shown in the AFM images in Figure 9c [135].

Once the CNTs have been synthesized, dispersing them in printable ink is a key challenge [92]. In designing a screen printable CNT ink, Menon et al. dispersed CNTs in an ethanol SDS solution optimized to 7.5 wt.%, then added various PVP loadings and assessed printability [135]. It was determined that PVP weights equal to half that of the CNTs were most suited for screen printing [135]. In addition, Shi et al. demonstrated that sonication is crucial in SDS facilitated CNT dispersion because the sonication forcibly breaks apart CNT clusters, exposing the CNT surface to SDS [87]. Figure 9d depicts TEM

image results of one such experiment, where dispersion clearly improves after sonicating for 6 instead of 4 h [87]. Gravure printed semiconducting SWCNTs have been thoroughly studied for thin-film transistor applications, and Sun et al. recently demonstrated a thin film transistor active-matrix (TFT-AM) electrophoretic sheet on PET that could be used as a wearable display [136]. Metallic CNTs were removed from a mixed semiconducting-metal powder with poly(9,9-didodecylfluorene) (PFDD) in a PFDD/CNT ratio of 1.25/1.00 to yield a semiconducting purity of 99.9%. The PFDD was then exchanged with a polythiophene derivative (P3ME4MT) in toluene and dispersed in 1-octanol to produce a printable viscosity and suitable capillary number. After gravure printing at 6 m/min at 30 µm depth and 150 µm cell opening for 10 PPI resolution and 10 µm depth and 35 µm opening for 40 PPI, TFT-AMs with average mobility of 0.23 <sup>±</sup> 0.12 cm<sup>2</sup> <sup>V</sup> −1 s −1 , the average on-off ratio of 104.1, and threshold voltage variation of ±13% was demonstrated [136]. Images of the printed TFT-AM are provided in Figure 9d [136]. Finally, inkjet printing of CNTs has been demonstrated for numerous biosensor, conductor, and semiconductor applications [137]. For instance, Okimoto et al. improved on previous CNT semiconductor performances by optimizing the CNT density in a novel SWCNT inkjet printing ink [138]. The SWCNTs were prepared by laser vaporizing carbon rods doped with Co/Ni in an argon environment and purified with H2O2, HCl, and NaOH. The SWCNTs were dispersed in DMF in a mixture of 0.04 µg/mL, sonicated, centrifuged, and filtered through poly(tetrafluoroethylene) membrane filters. The ink was printable with a 30 µm nozzle at 500 Hz, and the fabricated CNT TFT yielded mobility of 1.6 to 4.2 cm<sup>2</sup> V −1 s <sup>−</sup><sup>1</sup> and an on/off ratio of 4–5 digits [138]. Because CNT inks are both highly desirable for commercial sensing and TFT applications and the large challenges in designing printable inks, numerous commercial inks are now available, and many reported works in the literature are using these inks for inkjet and gravure printing (Figure 9e) [91,92]. In summary, CNT ink printing is highly attractive for many essential bioelectronics' applications, and new continued investigations into high throughput fabrication methods are essential in translating these novel discoveries to industrial and clinical use.
