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Flexible electronic devices require metal interconnects to facilitate the flow of electrical signals among the device components, ensuring its proper functionality. There are multiple factors to consider when designing metal interconnects for flexible electronics, including their conductivity, flexibility, reliability, and cost. This article provides an overview of recent endeavors to create flexible electronic devices through different metal interconnect approaches, with a focus on materials and structural aspects. Additionally, the article discusses emerging flexible applications, such as e-textiles and flexible batteries, as essential considerations. [ABSTRACT FROM AUTHOR]

Thin-film transistors have been extensively developed due to their process merit: high compatibility with various substrates, large-area processes, and low-cost processes. Despite these advantages, most efforts for thin-film transistors still remain at the level of unit devices, so the circuit level for practical use needs to be further developed. In this regard, this review revisits digital and analog thin-film circuits using carbon nanotubes (CNTs), organic electrochemical transistors (OECTs), organic semiconductors, metal oxides, and two-dimensional materials. This review also discusses how to integrate thin-film circuits at the unit device level and some key issues such as metal routing and interconnection. Challenges and opportunities are also discussed to pave the way for developing thin-film circuits and their practical applications. [ABSTRACT FROM AUTHOR]

Herein, flexible circuits with enhanced adhesion and antibending resistance are fabricated by heat‐assisted direct ink writing. The experimental parameters are explored under different nozzle sizes, angles, heights, direct writing speeds, and heating temperatures by selecting an appropriate extrusion speed. The solvent evaporation caused by heating temperature enables the metal particles to densify quickly to form a conductive path. This method is suitable for fabricating devices on polyimide (PI) with appropriate direct writing parameters and is simple to operate. A flexible Ag circuit with a resistivity of 4.31 × 10−8 Ω m and good resistance after 20 000 consecutive bending cycles is fabricated with a nozzle size of 32G, a nozzle height of 10 μm, a nozzle angle of 90°, an extrusion speed of 5 μm min−1, a substrate heating temperature of 70 °C, and a direct writing speed of 1 mm s−1. Large‐area complex flexible display LED arrays are fabricated using thermally assisted direct ink writing. [ABSTRACT FROM AUTHOR]

The assembly of passive components on flexible electronics is essential for the functionalization of circuits. For this purpose, adhesive bonding technology by isotropic conductive adhesive (ICA) is increasingly used in addition to soldering processes. Nevertheless, a comparative study, especially for bending characterization, is not available. In this paper, soldering and conductive adhesive bonding of 0603 and 0402 components on flexible polyimide substrates is compared using the design of experiments methods (DoE), considering failure for shear strength and bending behavior. Various solder pastes and conductive adhesives are used. Process variation also includes curing and soldering profiles, respectively, amount of adhesive, and final surface metallization. Samples created with conductive adhesive H20E, a large amount of adhesive, and a faster curing profile could achieve the highest shear strength. In the bending characterization using adhesive bonding, samples on immersion silver surface finish withstood more cycles to failure than samples on bare copper surface. In comparison, the samples soldered to bare copper surface finish withstood more cycles to failure than the soldered samples on immersion silver surface finish. [ABSTRACT FROM AUTHOR]

Flexible electronic devices require metal interconnects to facilitate the flow of electrical signals among the device components, ensuring its proper functionality. There are multiple factors to consider when designing metal interconnects for flexible electronics, including their conductivity, flexibility, reliability, and cost. This article provides an overview of recent endeavors to create flexible electronic devices through different metal interconnect approaches, with a focus on materials and structural aspects. Additionally, the article discusses emerging flexible applications, such as e-textiles and flexible batteries, as essential considerations.

Three commercially-available conductive filaments were evaluated for 3D printing flexible circuits on paper. While all three filaments were printed successfully, the resulting conductive traces were found to have significantly different impedances when characterized by electrochemical impedance spectroscopy. Using a graphite-doped polylactic acid filament, the flexibility of paper-based conductive traces was evaluated, methods of integrating common electrical and electronic components with the conductive traces were demonstrated, and the resistive heating of the traces was characterized. The ability to 3D print conductive traces on paper using commercially available materials opens many opportunities for rapid prototyping of flexible electronics and for integrating electronic circuits with paper-based microfluidic devices. [ABSTRACT FROM AUTHOR]

Thin-film transistors have been extensively developed due to their process merit: high compatibility with various substrates, large-area processes, and low-cost processes. Despite these advantages, most efforts for thin-film transistors still remain at the level of unit devices, so the circuit level for practical use needs to be further developed. In this regard, this review revisits digital and analog thin-film circuits using carbon nanotubes (CNTs), organic electrochemical transistors (OECTs), organic semiconductors, metal oxides, and two-dimensional materials. This review also discusses how to integrate thin-film circuits at the unit device level and some key issues such as metal routing and interconnection. Challenges and opportunities are also discussed to pave the way for developing thin-film circuits and their practical applications.

Subsurface imaging of Au circuit structures embedded in poly(methyl methacrylate) (PMMA) thin films with a cover thickness ranging from 52 to 653 nm was carried out by using contact resonance atomic force microscopy (CR-AFM). The mechanical difference of the embedded metal layer leads to an obvious CR-AFM frequency shift and therefore its unambiguous differentiation from the polymer matrix. The contact stiffness contrast, determined from the tracked frequency images, was employed for quantitative evaluation. The influence of various parameter settings and sample properties was systematically investigated by combining experimental results with theoretical analysis from finite element simulations. The results show that imaging with a softer cantilever and a lower eigenmode will improve the subsurface contrast. The experimental results and theoretical calculations provide a guide to optimizing parameter settings for the nondestructive diagnosis of flexible circuits. Defect detection of the embedded circuit pattern was also carried out, which indicates the capability of imaging tiny subsurface structures smaller than 100 nm by using CR-AFM.

The assembly of passive components on flexible electronics is essential for the functionalization of circuits. For this purpose, adhesive bonding technology by isotropic conductive adhesive (ICA) is increasingly used in addition to soldering processes. Nevertheless, a comparative study, especially for bending characterization, is not available. In this paper, soldering and conductive adhesive bonding of 0603 and 0402 components on flexible polyimide substrates is compared using the design of experiments methods (DoE), considering failure for shear strength and bending behavior. Various solder pastes and conductive adhesives are used. Process variation also includes curing and soldering profiles, respectively, amount of adhesive, and final surface metallization. Samples created with conductive adhesive H20E, a large amount of adhesive, and a faster curing profile could achieve the highest shear strength. In the bending characterization using adhesive bonding, samples on immersion silver surface finish withstood more cycles to failure than samples on bare copper surface. In comparison, the samples soldered to bare copper surface finish withstood more cycles to failure than the soldered samples on immersion silver surface finish.

Stretchable transparent conductive films (STCFs) show great potential for application in flexible electronics. Herein, a facile and scalable method to prepare STCFs based on Ag nanowires (AgNWs) for flexible circuits and tension sensors is introduced for the first time. AgNWs were embedded in a substrate, and its bonding property with the substrate was tested using 3M Scotch tape. The transmittance of STCFs with a sheet resistance of 20 Ω/sq was found to be more than 85% at a wavelength of 550 nm. The STCFs with less than 20 Ω/sq almost stayed constant even after twisting and bending for 10,000 cycles and stretching for 1000 cycles, respectively. Then, the current signal variation was studied using an electrochemical workstation with STCFs stretched periodically. Finally, we fabricated transparent flexible circuits for light-emitting diodes (LEDs) and tension sensors for knuckles to detect their real-time motion. The LEDs worked well in the circuits, and the sensor quickly responded to knuckle bending, which demonstrated that STCFs would have good prospects in flexible electronics. [ABSTRACT FROM AUTHOR]

This paper reviews the material properties, fabrication and functionalities of liquid metal-based devices. In modern wireless communication technology, adaptability and versatility have become attractive features of any communication device. Compared with traditional conductors such as copper, the flow characteristics and lack of elastic limit of conductive fluids make them ideal alternatives for applications such as flexible circuits, soft electronic devices, wearable stretch sensors, and reconfigurable antennas. These fluid properties also allow for innovative manufacturing techniques such as 3-D printing, injecting or spraying conductive fluids on rigid/flexible substrates. Compared with traditional high-frequency switching methods, liquid metal (LM) can easily use micropumps or an electrochemically controlled capillary method to achieve reconfigurability of the device. The movement of LM over a large physical dimension enhances the reconfigurable state of the antenna, without depending on nonlinear materials or mechanisms. When LM is applied to wearable devices and sensors such as electronic skins (e-skins) and strain sensors, it consistently exhibits mechanical fatigue resistance and can maintain good electrical stability under a certain degree of stretching. When LM is used in microwave devices and paired with elastic linings such as polydimethylsiloxane (PDMS), the shape and size of the devices can be changed according to actual needs to meet the requirements of flexibility and a multistate frequency band. In this work, we discuss the material properties, fabrication and functionalities of LM.

As a critical part of flexible electronics, flexible circuits inevitably work in a dynamic state, which causes electrical deterioration of brittle conductive materials (i.e., Cu, Ag, ITO). Recently, gallium‐based liquid metal particles (LMPs) with electrical stability and self‐repairing have been studied to replace brittle materials owing to their low modulus and excellent conductivity. However, LMP‐coated Ga2O3 needs to activate by external sintering, which makes it more complicated to fabricate and gives it a larger short‐circuit risk. Core–shell structural particles (Ag@LMPs) that exhibit excellent initial conductivity(8.0 Ω sq−1) without extra sintering are successfully prepared by coating nanosilver on the surface of LMPs through in situ chemical reduction. The critical stress at which rigid Ag shells rupture can be controlled by adjusting the Ag shell thickness so that LM cores with low moduli can release, achieving real‐time self‐repairing (within 200 ms) under external destruction. Furthermore, a flexible circuit utilizing Ag@LMPs is fabricated by screen printing, and exhibits outstanding stability and durability (R/R0 < 1.65 after 10 000 bending cycles in a radius of 0.5 mm) because of the functional core–shell structure. The self‐repairable Ag@LMPs prepared in this study are a candidate filler for flexible circuit design through multiple processing methods. [ABSTRACT FROM AUTHOR]

This paper describes the design, development, and testing of flexible hybrid silicone-textile sensors and a flexible switching circuit that were integrated into a wearable system for monitoring neurodegenerative diseases. A total of 6 planar monopole antenna sensors were fabricated that propagates at two separate resonant frequencies: 800 MHz and 2.1 GHz respectively. In addition, 2 switching circuits, each having 3 switches and 4 SMA breakout boards, were assembled and placed on the wearable neurodegeneration monitoring system. Each switching circuit connects 3 sensors to a single port on a vector network analyzer (VNA) that is used to generate and receive microwave signals. Experiments were performed using the wearable device with the developed sensors and switching circuit on phantoms mimicking two common physiological changes in the brain caused by neurodegenerative diseases: 1) brain atrophy and 2) lateral ventricle enlargement. The dual nature of the sensors’ resonance allows it to detect both brain atrophy and lateral ventricle enlargement separately at different operating frequency. This provides the advantage of minimizing the number of sensor elements needed to monitor neurodegenerative disease. The use of a switching circuit also allows for quick and convenient measurements by choosing which sensors are active for ports 1 and 2 on the VNA respectively. In addition to being low-cost, the flexibility of the materials used in fabrication allows the sensors and switching circuit to be conformal to the patient's head. Results from the experiments indicates that the sensors and switching circuit were working successfully when integrated into the wearable device. [ABSTRACT FROM AUTHOR]

Silver circuits prepared by new hybrid pastes exhibit high electrical conductivity and mechanical properties, which are ideal for modern flexible electronics. Herein, the hybrid silver pastes composed of different silver nanoparticle content (0, 10, 20, 30 wt%), micron-sized silver flakes and epoxy-based binders were prepared. The corresponding electrical conductivity and bending resistance of the screen-printed silver circuits were studied. The experimental results demonstrated that the synergistic effect of micro-size silver flakes and nanoparticles can greatly improve the electrical conductivity and bending resistance of flexible circuits. Specifically, the silver circuits with 46 wt% micro-size silver flakes and 20 wt% silver nanoparticles incorporation exhibit a lower electrical resistivity of 8.1 × 10 ^−5 Ω·cm. Moreover, 10 wt% silver nanoparticles can be applied to significantly reduce the resistance change of flexible circuit, indicting a superior bending property. Our designed hybrid silver pastes with excellent performance might enable valuable applications in advanced electronic devices.

Abstract: The transfer of synthesized large‐area 2D materials to arbitrary substrates is expected to be a vital step for the development of flexible device fabrication processes. The currently used hazardous acid‐based wet chemical etching process for transferring large‐area MoS2 films is deemed to be unsuitable because it significantly degrades the material and damages growth substrates. Surface energy‐assisted water‐based transfer processes do not require corrosive chemicals during the transfer process; however, the concept is not investigated at the wafer scale due to a lack of both optimization and in‐depth understanding. In this study, a wafer‐scale water‐assisted transfer process for metal–organic chemical vapor‐deposited MoS2 films based on the hydrofluoric acid treatment of a SiO2 surface before the growth is demonstrated. Such surface treatment enhances the strongly adhering silanol groups, which allows the direct transfer of large‐area, continuous, and defect‐free MoS2 films; it also facilitates the reuse of growth substrate. The developed transfer method allows direct fabrication of flexible devices without the need for a polymeric supporting layer. It is believed that the proposed method can be an alternative defect‐ and residue‐free transfer method for the development of MoS2‐based next‐generation flexible devices. [ABSTRACT FROM AUTHOR]

In analog electronic communication and signal processing applications, programmable analog filters are desirable to reduce the hardware requirement. Memristor is two states configurable nonvolatile resistor, which is small size, low power, and fast switching device. To achieve tunable cut-off frequency and bandwidth, flexible memristor-capacitor (MC) circuits are proposed, which can be applied as low pass, high pass, and band pass filters. They can be fabricated on a flexible polyethylene terephthalate (PET) substrate through electrohydrodynamic (EHD) technique. In the proposed filters, two cut-off frequencies are selected through switching two resistive states of the memristor either in high resistance state (HRS) or low resistance state (LRS), whereas the capacitor has a fixed value. The memristor is composed of graphene quantum dots (GQDs)/poly 4-vinlyphenol (PVP) composite to achieve highly stable and linear behavior and the dielectric layer of capacitor is consisted of graphene/PVP composite. The fabricated MC filters are electrically and mechanically characterized, which are matched quite well with theoretical results. From these results, it can be a good basis for tunable analog filters in wearable electronics. [ABSTRACT FROM AUTHOR]

Flexible conductive circuits on PET fabrics were fabricated by a simple approach. Firstly, well dispersed nano-silver colloids with average size of 87 nm were synthesized with poly (vinyl pyrrolidone). Then, by adding polyurethane and thickening agent into these colloids, Ag NP-based ink was produced and printed on PET fabrics by screen printing. Conductive patterns were achieved through the swelling process of polyurethane and the decrease of contact resistance between nano-silver particles when immersed in dichloromethane (DCM) and diallyldimethylammonium chloride (DMDAAC) mixed solution. After it was dried at 40 °C,the surface resistivity was about 0.197 Ω cm with width 1.9 mm, and thickness 20 μm. Moreover, the effects of different DCM contents on the conductivity and the film forming ability have been investigated. We believe these foundings will provide some important analysis for printing flexible conductive circuits on textiles. [ABSTRACT FROM AUTHOR]

The quest for high-performance flexible circuits call for scaling of the minimum feature size in thin-film transistors (TFTs). Although reduced channel lengths can guarantee an improvement in the electrical properties of the devices, proper design rules also play a crucial role to minimize parasitics when designing fast circuits. In this letter, systematic computer-aided design simulations have guided the fabrication of high-performance flexible operational amplifiers (opamps) and logic circuits based on indium–gallium–zinc-oxide TFTs. In particular, the performance improvements due to the use of an additional third metal layer for the interconnections have been estimated for the first time. Encouraged by the simulated enhancements resulting by the decreased parasitic resistances and capacitances, both TFTs and circuits have been realized on a free-standing 50- \mu \textm -thick polymide foil using three metal layers. Despite the thicker layer stack, the TFTs have shown mechanical stability down to 5-mm bending radii. Moreover, the opamps and the logic circuits have yielded improved electrical performance with respect to the architecture with two metal layers: gain-bandwidth-product increased by 16.9%, for the first one, and propagation delay ( tpd ) decreased by 43%, for the latter one. [ABSTRACT FROM PUBLISHER]

Fused depositing modeling (FDM) is a fast, efficient process among 3D printing techniques. In this paper, we report the fabrication of the 3D printed flexible circuits based on graphene. Modified two-step in-situ reduced method is used to synthesize reduced graphene oxide (r-GO), whose conductivity can reach to 600 S/cm. Polylactic acid (PLA) and r-GO are mixed by melt blending. The SEM images show that the r-GO can be homogenous dispersed in the PLA. The 3D print-used composites filaments with the diameter of 1.75 mm are fabricated through melt extrusion. The conductivity of the composite filaments from 3D printer can reach to 4.76 S/cm (6 wt% r-GO). The orientation of r-GO occurs during the extrusion process, which contributing to increase the conductivity of the filaments. The composite also exhibit superior mechanical property. The printed 2D and 3D flexible circuits have strong interface bonding force between the layers. The filaments from 3D printer can replace the copper wire because of the high conductivity. This arbitrary 3D graphene-based structure printing technic may open a new prospect in electronic and energy storage fields. [ABSTRACT FROM AUTHOR]

In this paper, we present the fabrication and characterization of highly flexible indium-gallium–zinc-oxide (IGZO)-based thin-film transistors (TFTs) and integrated circuits on a transparent and thin polymer substrate. Mechanical reliability tests are performed under bending conditions down to a bending radius of 2 mm. All the TFT parameters show only a weak dependence on mechanical strain. TFTs can withstand bending strain up to 0.75% without any significant change in the device operation. Mechanical reliability is further demonstrated to a higher TFT integration level by ring oscillators and 8-b transponder chips operating at a bending radius of 2 mm. [ABSTRACT FROM PUBLISHER]

Flexible EMMO IGZO TFTs fabricated on 20 µm thick PI film are reported here. The utilized technology results in high electrical stability, of devices, combined to high robustness against mechanical deformation down to 3%. [ABSTRACT FROM AUTHOR]

Sorting of semiconducting single-walled carbon nanotubes (SWNTs) by conjugated polymers has attracted considerable attention recently because of its simplicity, high selectivity, and high yield. However, up to now, all the conjugated polymers used for SWNT sorting are electron-donating (p-type). Here, a high-mobility electron-accepting (n-type) polymer poly([N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)) (P(NDI2OD-T2)) is utilized for the sorting of high-purity semiconducting SWNTs, as characterized by Raman spectroscopy, dielectric force spectroscopy and transistor measurements. In addition, the SWNTs sorted by P(NDI2OD-T2) have larger diameters than poly(3-dodecylthiophene) (P3DDT)-sorted SWNTs. Molecular dynamics simulations in explicit toluene demonstrate distinct linear or helical wrapping geometry between P(NDI2OD-T2) and different types of SWNTs, likely as a result of the strong interactions between the large aromatic core of the P(NDI2OD-T2) backbone and the hexagon path of SWNTs. By using high-mobility n-type P(NDI2OD-T2) as the sorting polymer, ambipolar SWNT transistors with better electron transport than that attained by P3DDT-sorted SWNTs are achieved. As a result, flexible negated AND and negated OR logic circuits from the same set of ambipolar transistors are fabricated, without the need for doping. The use of n-type polymers for sorting semiconducting SWNTs and achieving ambipolar SWNT transistor characteristics greatly simplifies the fabrication of flexible complementary metal-oxide-semiconductor-like SWNT logic circuits. [ABSTRACT FROM AUTHOR]

The paper presents characterization of an Inkjet conductor printing technology utilizing silver conductive ink on flexible Kapton substrate for microwave applications. Microstrip ring resonator method has been used for measurement in the frequency band up to 20 GHz. Specific details related to application of the chosen method along with limitations of a technology on hand are provided. Results of the measurements suggest that the technology can not be used at microwave frequencies due to very low value of Q factor of the resonances and prohibitively high value of insertion loss. Measures to reduce losses have been proposed including the increase of conductor thickness and the increase of substrate thickness. [ABSTRACT FROM AUTHOR]