In recent years, traditional printing methods have been integrated to print flexible electronic devices and circuits. Since process requirements for electronics differ from those for graphic printing, the fundamentals require rediscovery mainly to optimize manufacturing techniques and to find cost reduction methods without compromising functional performance. In addition, alternative inks need to be formulated to increase the variety of functional inks and to pioneer new product developments. In this report, we investigate a thermoplastic-based nickel ink prototype to print electrodes using a screen-printing process. Process fundamentals are explored, and cost reduction methods are addressed by studying the effect of substrate roughness, print direction, and number of ink layers on the electrical performance of printed nickel. Multilayered electrodes are printed on paper and heat stabilized engineered film. A novel fundamental mechanism is found that explains the effect of substrate roughness on ink film roughness in screen printing, including the roughness measurement of the screen mesh wire that is reported for the first time. Results demonstrated that (i) surface roughness of substrates does not have significant effect on printed ink film roughness in screen printing; (ii) ink film thickness is higher on nonabsorbent materials, while line gain is higher on absorbent materials; (iii) the effect of electrode orientation on electrical performance is insignificant; and (iv) the effect of substrate roughness on the electrical performance for the first print layer can be eliminated by printing multiple layers. The results significantly affect substrate choice and number of ink layers, which are considered the major cost factors in the manufacturing of printed electronics.

The interaction between inks and substrates is critical during printing. Adhesion of the ink film is determined by the reciprocal interactions of polar and nonpolar (dispersive) components between polymer films and inks. The greater the similarity between the polar and dispersive components of inks, coating and substrates, the better the wetting and adhesion on the surface of printing substrate. Various liquid materials in printing such as inks, varnishes, lacquers, and adhesives contain high ratios of water. The highly polar nature of water makes the interaction of these materials unsuitable with predominantly disperse polymer surfaces. Some films with polyolefin structure, especially polypropylene, and polyethylene, are nonpolar and cannot form strong bonds with ink, varnish, or lacquer coatings due to their chemical structure. Increasing surface energy components overcomes the poor wetting and adhesion on polymer surfaces. In this review, the topics of water contact angle measurement and determination of surface energy, surface tension, and using sessile drop method for the wettability and ink adhesion of polymer films are surveyed. Information on structural and chemical processes was given that assists in obtaining wettable film surfaces. Recommendations were made for good adhesion and printability based on surface treatment methods and ink modification.

An interpretation of solid surfaces is generated based on physical considerations and the laws of thermodynamics. Like the widely used Owens–Wendt (OW) method, the proposed method uses liquids for characterization. Each liquid provides an absolute lower bound on the surface energy with some uncertainty from measurement variations. If multiple liquids are employed, the largest lower bound is taken as the most accurate, with uncertainty due to measurement errors. The more liquids used, the more accurate is the greatest lower bound. This method links generalizations of the Good–Girifalco equation with a general thermodynamic inequality relating the three‐interfacial tensions in a three‐phase equilibrium system. The method always satisfies this inequality with better than a 65% certainty. However, the OW seldom, if ever, conforms to this inequality and even then, the degree of satisfaction is insignificant. A reconciliation of the two methods is proposed based on rescaling the OW surface energies to conform to the inequality. This enables interpretations of dispersion and polar components of the surface energy, which are thermodynamically self‐consistent. The proposed method is also capable of dealing with material exchange between liquid and solid phases, when the surface tension and contact angle of the saturated liquids can be measured.

One of the keys to improving print quality is to experimentally characterize the paper surface, structure and printability to obtain quality control mechanisms. In multi-color prints, determining the differences in the acceptance of the next color ink by the surface of the paper or the ink film that was previously printed is important for print quality. The criteria, such as ink setting, adhesion, color, gloss and density, in the printing process, depend on the wettability and absorbency of the paper. The surface structure of the paper is the most important factor in determining the hydrophobic properties. In this study, wetting and absorption dynamics of the printed partially hydrophobized paper surface were investigated. The aim was to measure the effect of the printed ink film on the wetting (surface free energy) and liquid absorption behavior of the paper. Liquid absorption changes on printed/unprinted paper surfaces were measured by the sessile drop method, using a contact angle-measuring device. The surface energies of the papers were calculated and evaluated according to the surface contact angle of the droplet.

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A novel nickel (Ni)-based resistance temperature detector (RTD) was successfully developed for temperature monitoring applications. The RTD was fabricated by depositing Ni ink on a flexible polyimide substrate using the screen printing process. Thermogravimetric analysis was performed to study the thermal behavior of the Ni ink, and it was observed that the Ni ink can withstand up to 200 • C before the decomposition of the binder in the ink system. Scanning electron microscopy and white light interferometry were used to analyze the surface morphology of the printed Ni. X-ray diffractometry was used to obtain structural information, phase, and crystallite size of the deposited Ni nanoparticles. Energy dispersive X-ray spectroscopy was used to obtain semi-quantitative information of the elements present in the fabricated RTD. The capability of the RTD to detect temperatures varying from −60 • C to 180 • C, in steps of 20 • C, was investigated at a constant relative humidity of 20%RH. The results of the RTD demonstrated a linear response with resistive changes as high as 113% at 180 • C when compared with its base resistance at −60 • C. An average TCR of 0.44%/ • C was calculated for the printed RTD with a response time of <10 s. The obtained results demonstrated the feasibility of employing Ni on flexible substrates for the development of flexible temperature sensors. INDEX TERMS Flexible temperature sensor, nickel, RTD, screen printing, TCR, TGA, XRD. PAUL D. FLEMING received the B.Sc. degree in physics from The Ohio State University, Columbus, OH, USA, in 1964, and the master's degree in physics and the Ph.D. degree in chemical physics

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Next-generation printed electronics is required to be of high performance, cost-effective, multifunctional, sustainable, and environmentally benign. Herein, we report the manufacturing of a flexible carbon-coated paper for printed electronics ensuring the abovementioned requirements. The multifunctional carboncoated substrate is achieved by employing copy paper, cellulose nanofibers (CNFs), and lignin-derived graphitic carbon as a base substrate, binder, and conductive pigment, respectively, using the scalable and simple Mayer rod coating and calendering methods. The pressure applied by the calendering led to a 96% decrease in electrical resistance for the carbon-coated paper. Wettability analysis revealed that the carbon layer is hydrophilic and capable of hydrogen bonding. The dielectric constant is obtained to be an order of magnitude less than that of the copy paper, indicating that the CNF binder between the carbon particles impedes the polarization mechanism. The impedance is measured as frequency-dependent and capacitive. The resistance, impedance, and dielectric values suggest that the carbon-coated paper is favorable for capacitive applications. Fabricating the flexible substrate with the biobased binder and conductive pigment introduces a significant step in sustainable and environmentally friendly printed electronics for future flexible capacitors, sensors, and wearable devices, especially for the applications where end-of-life disposal needs to be sustainable.