(Invited) Emerging Nanomaterial Inks for Additive Manufacturing of Wearable Sensors
Authors: Fujimoto Kiyo , Pandhi Twinkle , Mansoor Naqsh E , Muramutsa Florent , Subbaraman Harish , Estrada David 
Published: February 2020

The versatility of direct write techniques such as aerosol jet printing (AJP) and ink jet printing (IJP) has recently intersected with two-dimensional (2D) materials research to enable novel wearable sensors. Graphene and 2D transition metal nitrides, carbides and carbonitrides (MXenes) are of particular interest for wearable sensor applications due to their biocompatibility and multifunctional nature. However, these 2D materials must be contacted by biocompatible metals, e.g. platinum, in order to enable functional devices such as wearable and/or implantable electrochemical sensors and neural interfaces.[1 - 3]
In this study, we report the printing of 2D materials, graphene, titanium carbide Mxenes (Ti3C2), and platinum nanoparticles via AJP and IJP. Ink characterization was performed with atomic force microscopy, scanning and transmission electron microscopy (SEM and TEM), DLS and Raman spectroscopy. Graphene sheets were obtained via solvent assisted exfoliation and then dispersed in 85:15 cyclohexanone/terpineol to formulate graphene inks at a concentration of 3.5 mg/ml.[4,5] Graphene sheets exhibited an average thickness of 16 nm +/- 15 nm and crystal domains with lateral dimensions ranging from ~50 nm to 200 nm. Platinum ink (20 wt.%) was formulated with the dispersion of platinum nanopowders and 70:30 water/ethylene glycol mix. Dispersed platinum nanoparticles exhibited an average particle size of 7.8 nm +/- 2.3 nm. Finally, an AJP compatible multi-layered MXene ink was obtained by in situ HF etching of the pre-cursor Ti3AlC2 MAX phase.[6] The obtained colloidal suspension containing 2D MXene flakes was sonicated with deionized water for 3 hours at 55 W. The resultant ink had a concentration of 1 mg/mL and an average flake size of 100 nm +/- 10 nm, as confirmed by dynamic light scattering (DLS) measurements. Platinum, graphene and Ti3C2 inks have demonstrated good compatibility with an Optomec Aerosol Jet 200 and a Dimatix 2850 materials inkjet printer, while platinum and graphene have shown additional compatibility with IJP. For good electrical conductivity graphene was sintered at 390 C for one hour and platinum was sintered at 450 C for 130 hours, while MXenes showed good conductivity without sintering.
Optical microscopy, SEM, stylus profilometry, and two/four-point resistance measurements were used to examine the quality of the printed MXene and platinum structures. Initial printing with MXenes resulted in feature heights ranging from ~ 620 nm to 4.80 µm, depending on the number of print passes denoted as N. Our most uniform prints were N = 15 and N = 20 having a height of 2.50 µm +/- 0.10 µm and 3.50 µm +/- 0.10 µm, respectively. Electrical resistance varied between R = 568 kΩ and R = 60 kΩ for N = 5 and N = 25 respectively. Platinum exhibited good electrical conductivity as R = 70 Ω for N = 10, having a height of 3 µm and a feature width of ~50 µm. Furthermore, the electrochemical properties of fully inkjet printed graphene electrodes on a flexible polyimide substrate for pH and electrolyte sensing were investigated. Printed electrodes consisted of a printed 1x1 cm graphene square with a total of 17 layers with printed silver contact pads and a SU-8 as a passivation layer. Electrochemical activity of inkjet printed graphene electrodes was demonstrated with both cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measurements in a room temperature 10 mM ferricyanide aqueous solution (1 M KCl supporting electrolyte). Ag/AgCl and platinum electrodes were used as the reference and counter electrode, respectively. Additionally, pH sensing capabilities were investigated by measuring the potentiometric response of polyaniline coated graphene electrodes over three different pH values (4, 7 and 10). The response of printed and flexible graphene electrodes was comparable to that of a conventional glass electrode.
In conclusion, AJP compatible Ti3C2 MXene inks along with AJP and IJP compatible platinum and graphene inks have been developed. Printing of these biocompatible materials is intended to support the fabrication of nanomaterial based flexible hybrid electronics such as biosensors for electrochemical sensing of a broad spectrum of biomarkers, and also neural interface systems. Furthermore, our results provide new fundamental insights on role of structure-property-performance correlations in printed graphene electrodes for electrochemical sensing. The combination of our works highlight the significant potential of novel material inks for additively manufactured wearable sensors.
[1] Kenry et al., Microsystems Nanoeng., Sep. 2016. [2] Sinha et al.,
Trends Analyt Chem, 2018. [3] A. Sridhar et al., Mater. Matters, 2009. [4] J. N. Coleman et al., Acc. Chem. Res., 2013. [5] F. Torrisi et al., ACS Nano, 2012. [6] M. Alhabeb et al., Chemistry of Materials, 2017.