Printed electronics (PE) is an emerging technology that uses functional inks to print electrical components and circuits on variety of substrates. This technology has opened up new possibilities to fabricate flexible, bendable, and form-fitting devices at low-cost and fast speed. There are different printing technologies in use, among which droplet-based techniques are of great interest as they provide the possibility of printing computer-controlled design patterns with high resolution, and greater production flexibility. Nanomaterial inks form the heart of this technology, enabling different functionalities. To this end, intensive research has been carried out on formulating inks with conductive, semiconductive, magnetic, piezoresistive, and piezoelectric properties. Here, a detailed landscape view on different droplet-based printing technologies (inkjet, aerosol jet, and electrohydrodynamic jet) is provided, with comprehensive discussion on their working principals. This is followed by a detailed research overview of different functional inks (metal, carbon, polymer, and ceramic). Different sintering methods and common substrates being used in printed electronics are also discussed, followed by an in-depth review of different physical sensors fabricated by droplet-based techniques. Finally, the challenges facing the field are considered and a perspective on possible ways to overcome them is provided.

Droplet-Based Techniques for Printing of Functional Inks for Flexible Physical Sensors.

Sheet resistance is one of the essential properties of thin films, and the change of it under different conditions is the basis of various sensor designs. However, the contact resistances between the thin-film materials and the metal electrodes can seriously affect the sensing accuracy. Due to various material properties, the contact resistances are usually difficult to be managed or reduced, and even nonlinear Schottky barriers can form in some cases. A modified four-terminal method is proposed to eliminate the impact of contact resistances through simple resistance measurements and algebraic calculations, which can be easily integrated into sensors and accurately measure the sheet resistances/resistivities of thin films of different materials under different conditions. The carbon black/polydimethylsiloxane (CB/PDMS) thin film is taken as an example for demonstration of the method. The applications for pressure-sensitive and temperature-sensitive resistance measurement are also demonstrated.

A General Method to Accurately Measure the Thin-Film Sheet Resistance for Sensors

Ultra-broadband wide-angle terahertz absorber realized by a doped silicon metamaterial

Fractal structures in flexible electronic devices

A mechanically flexible and optically transparent millimeter-wave broadband absorber was proposed and fabricated. The absorber showed >90% absorption in (73.5–110) GHz, covering the whole W-band. The key frequency selective surface used a concise metal mesh structure fabricated by the electrohydrodynamics (EHD) printing technology, which formed a classic sandwich structure together with a transparent dielectric layer and a transparent and conductive backplane. The transmittance of the sample in the visible light region was 58.1%. To testify the flexibility, the absorber was conformed to cylinders with a radius of 34 and 16 mm, respectively. The measured normalized S 11 increased with bending. Simulation results also showed that the absorption over W-band was >80% within (0°–45°) incident angle and was insensitive to the polarization angle.

Flexible and Transparent W-Band Absorber Fabricated by EHD Printing Technology

Flexible pressure sensors (FPSs) are widely demanded for accurate pressure sensing on complex surfaces. FPSs using metal nanoparticles (NPs) as the sensing material have high conductivity and good mechanical properties, but poor gauge factor as a trade-off. In this work, a novel design of the AgNP-based FPS is proposed and testified with the help of the electrohydrodynamics-printing technology, so that the gauge factor can be improved from 8 to 286.8, resulting from the localized micro-zone piezoresistance effect. The working principle of the piezoresistive characteristic is discussed, and two methods to further improve and adjust the gauge factor are proposed and experimentally demonstrated.

Electrohydrodynamics-Printed Silver Nanoparticle Flexible Pressure Sensors With Improved Gauge Factor

Terahertz (THz) devices, including the THz broadband absorbers, have attracted much attention in the recent decade. However, due to the micron-level precision requirement, the mainstream fabrication technology for THz devices still rely on complex and expensive photolithography or nanoimprinting techniques. Electrohydrodynamics (EHD) printing technology offers a novel, convenient, and cost-effective solution to this issue. A broadband THz absorber with over 90% absorption in the range of 98–353 GHz was designed and fabricated by EHD printing technology, and the inductive fishnet grid structure was used for the metasurface. The equivalent circuit model of the absorber was established based on the transmission line theory and impedance matching theory. Simulation results matched well with the theoretical and experimental results, and further demonstrated that the absorbers were polarization insensitive. Furthermore, both simulation and experiments proved that the absorption spectrum was insensitive to the fishnet grid linewidth and surface resistance, i.e., the design has high tolerance to possible fabrication errors. Therefore, the EHD printing technology can be a promising fabrication method of THz absorbers for future mass production.

THz Broadband Absorber Fabricated by EHD Printing Technology With High Error Tolerance

How to obtain a jet with smaller diameter is one of the key issues of the electrohydrodynamics (EHD) based printing technology. One important approach is to use smaller nozzles. The jet diameter dependence on the nozzle size was studied in theory and experiment in this work for the continuous and stable cone-jet mode. The theoretical model showed that the jet diameter should be proportional to the square of the nozzle diameter, which was verified by experiments.

https://iopscience.iop.org/article/10.1088/2631-8695/ab54a3/pdf

Jet diameter dependence on nozzle size in DC-driven electrohydrodynamics printing

This letter proposes an electrohydrodynamics-based printing system developed with high-precision on-off control, which has the merits of anticlog printing, maskless patterning, and precise control. Silver-colloid-based precursor can be conveniently focused into a single and continuous liquid jet and directly printed on glass or aluminum foil substrates just like a dip-pen. The liquid jet itself can be as thin as a couple of microns, and the minimum silver linewidth could reach 30 μm. With high-precision on-off control of the spraying, complex pictures, letters, as well as a specially designed radio frequency identification (RFID) tag antenna were printed using this system. The printed RFID tag antenna was characterized and analyzed, which meets the basic requirements of practical applications.

Maskless Direct Printing of Radio Frequency Identification Antenna Based on Electrohydrodynamics Technology

A broadband THz metamaterial absorber was designed based on the transmission line theory, and the inductive mesh structure was selected as the meta-surface. The absorber samples were fabricated by electrohydrodynamics(EHD)-based printing technology, which is a cost-effective and high-precision technology for flexible electronic device fabrication. The absorption in (98-353) GHz exceeded 90%, and the experimental data matched well with the theoretical and simulation results. It was also proven that the absorption spectra were insensitive to the linewidth and surface resistance of the inductive mesh structure, so that the design has high tolerance to possible fabrication error.

EHD-printing technology: a novel approach to THz broadband absorber fabrication

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