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The development of techniques for synthesizing graphene and its derivatives, as well as currently available nanocomposite fabrication techniques, coupled with the diverse applications of strain and pressure sensors, has made this field of growing interest in the last decade. This article provides an overview of conventional strain sensor manufacturing techniques, such as in situ polymerization, solution blending, and electrospinning. It also covers various additive manufacturing techniques such as vat-photopolymerization, material extrusion, material jetting, sheet lamination, and the most common graphene synthesis techniques like chemical-based, vapor deposition, exfoliation, and mechanical-based methods. The review is also completed with a discussion about the sensing mechanisms of strain sensors, considering the various process parameters to characterize and compare the performance of a strain sensor. Finally, we examine several key aspects of the sensor’s component materials, the type of sensing mechanism, and the appropriate manufacturing process. © 2022, The Minerals, Metals & Materials Society.

The safety issue of robot becomes a key challenge in modern manufacturing industry, especially in the scenario of frequent human-robot collaboration. This paper presents a flexible ultrasonic proximity sensing skin which can be integrated on the surface of robot arm to ensure the safe human-robot collaboration. The sensing skin is composed of three pieces of aluminum nitride based piezoelectric micromachined ultrasonic transducers (PMUTs) arrays at the resonance frequency of 115 kHz and chip size of 4 mm × 4 mm × 0.5 mm. The three pieces of PMUTs arrays are evenly integrated on the flexible printed circuit board (FPCB) with a device size of 8.8 mm × 60 mm × 1.7 mm. The proximity sensing skin can detect obstacles within 300 mm and 60 degrees in front, and shows a large sensing coverage in comparison with capacitive/infrared devices. Besides, it is capable of sensing transparent materials like glass. Finally, since the small size and good scalability of micro-electro-mechanical system (MEMS), the PMUTs are easy to install onto robots and to integrate with other ultrasonic transducers in the future. This PMUTs based sensing skin will greatly enhance the sensing capability of robots and human safety during collaborative work.

Highly durable and stretchable ionic conductors are indispensable components for flexible electronics. However, fabricating such ionic conductors that are also non-toxic and biodegradable remains a...

This work was developed within the scope of project i3N (LA/P/0037/2020. I.P. N. F. Santos thanks i3N for the BPD Grant BPD/UI96/5177/2020. S. O. Pereira thanks i3N for the BPD Grant BPD/UI96/5808/2017. The authors also thank Jonas Deuermeier for the XPS measurements. Publisher Copyright: © 2022 Elsevier Ltd Laser-induced graphene from paper (paper-LIG) was applied in non-enzymatic electrochemical sensing of uric acid (UA) in human urine. Paper-LIG was formed by CO2 laser modification of paper into a 3D graphene arrangement. Kinetic analysis of paper-LIG electrodes returned effective heterogeneous electron transfer standard rate constants of 1.4 × 10−3 cm s−1 and 7.8 × 10−4 cm s−1 for [Ru(NH3)6]2+/3+ and [Fe(CN)6]4−/3− redox probes, respectively. These electrodes were able to detect and quantify uric acid in PBS within the 10–300 μM range at pH between 5.6 and 7.4. At pH 7.4, a linear response (R2 = 0.999) from 10 to 250 μM was achieved, with a limit of detection of 3.97 μM and a sensitivity of 0.363 μA cm−2 μM−1. Paper-LIG electrodes denoted adequate selectivity in synthetic urine as well as in ascorbic acid (AA) and dopamine (DA)-containing electrolytes. Determination of urinary UA content in human samples returned a concentration of c.a. 1.8–1.9 mM, within the range for healthy individuals. Recoveries of samples spiked with 50 and 100 μM UA were 100.6% and 95.4%, respectively, with satisfactory reproducibility and stability. These cheap, lightweight, flexible, and eco-friendly paper-LIG biosensors for non-enzymatic quantification of UA in human urine pave the way to widespread application in the detection of other important biomarkers. publishersversion published

With the increase in the popularity of wearable and integrated electronics, a proper way to manufacture electronics on textiles is needed. This study aims to analyze the effect of different parameters of the heat transfer process on the electrical and mechanical properties of flexible electronics made on textiles, presenting it as a viable method of producing such electronics. Wires made from different composites based on silver microparticles and an insulating layer were screen-printed on a release film. Then, they were transferred onto a polyester cloth using heat transfer with different parameters. Research showed that different heat transfer parameters could influence the electrical properties of screen-printed wires, changing their resistance between −15% and +150%, making it imperative to adjust those properties depending on the materials used. Changes in the settings of heat transfer also influence mechanical properties, increasing adhesion between layers at higher temperatures. This study shows the importance of tailoring heat transfer properties and the differences that these properties make.

The hydrothermal synthesis of a nanosized NiCo2O4 oxide with several levels of hierarchical self-organization was studied. Using X-ray diffraction analysis (XRD) and Fourier-transform infrared (FTIR) spectroscopy, it was determined that under the selected synthesis conditions, a nickel-cobalt carbonate hydroxide hydrate of the composition M(CO3)0.5(OH)·0.11H2O (where M–Ni2+ and Co2+) is formed as a semi-product. The conditions of semi-product transformation into the target oxide were determined by simultaneous thermal analysis. It was found by means of scanning electron microscopy (SEM) that the main powder fraction consists of hierarchically organized microspheres of 3–10 μm in diameter, and individual nanorods are observed as the second fraction of the powder. Nanorod microstructure was further studied by transmission electron microscopy (TEM). A hierarchically organized NiCo2O4 film was printed on the surface of a flexible carbon paper (CP) using an optimized microplotter printing technique and functional inks based on the obtained oxide powder. It was shown by XRD, TEM, and atomic force microscopy (AFM) that the crystalline structure and microstructural features of the oxide particles are preserved when deposited on the surface of the flexible substrate. It was found that the obtained electrode sample is characterized by a specific capacitance value of 420 F/g at a current density of 1 A/g, and the capacitance loss during 2000 charge–discharge cycles at 10 A/g is 10%, which indicates a high material stability. It was established that the proposed synthesis and printing technology enables the efficient automated formation of corresponding miniature electrode nanostructures as promising components for flexible planar supercapacitors.

Quasi-two-dimensional (q2D) conducting polymer thin film synergizes the advantageous features of long-range molecular ordering and high intrinsic conductivity, which are promising for flexible thin film-based micro-supercapacitors (MSCs). Herein, we present the high-performance flexible MSCs based on highly ordered quasi-two-dimensional polyaniline (q2D-PANI) thin film using surfactant monolayer assisted interfacial synthesis (SMAIS). Owing to high electrical conductivity, rich redox chemistry, and thin-film morphology, the q2D-PANI MSCs show high volumetric specific capacitance (ca. 360 F/cm3) and energy density (17.9 mWh/cm3), which outperform the state-of-art PANI thin-film based MSCs and promise for future flexible electronics.

Actively reconfigurable flexible electronics enabled by robotic technologies would provide opportunities to extensively broaden the application areas of flexible electronics compared with their passively flexible counterparts. Diverse reconfigurable shapes can be enabled by localized control of the film-type electronics while keeping a thin form factor for flexible electronics. The flexible electronics are usually designed to be thin to lower mechanical stress or strain when bending. In this article, we present a locally actuatable film designed to control actuating regions, bending directions, and curvatures by the electrical inputs. The film is in a thin form factor with multichannels realized by the proposed method of crimping electrical wires over shape memory alloy wires. The industry standard process should be convenient in terms of scaling up, increasing channels, or adding reconfigurable shapes for new applications. We demonstrate shape-changeable displays, self-rollable photovoltaics, and light art to prove the feasibility of the concept.

When developing flexible electronic devices, trade-offs between desired functional properties and sufficient mechanical flexibility must often be considered. The integration of functional ceramics on flexible materials is a major challenge. However, aerosol deposition (AD), a room-temperature deposition method, has gained a reputation for its ability to combine ceramics with polymers previously considered incompatible with the conventional high-temperature sintering process. In this work, 0.9Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$−0.1PbTiO$_3$ (PMN−10PT) thick films were deposited directly on a polyimide substrate using the AD method. As a result, dense and flexible relaxor-ferroelectric thick films were produced by a one-step direct-integration, suitable for large-scale production. After annealing of as-deposited PMN−10PT films at 400 °C, stress-relaxation occurs, which is responsible for the development of a relaxor-ferroelectric character. Achieved high polarization (38 μC·cm$^{−2}$), high dielectric breakdown strength (∼1000 kV·cm$^{−1}$), and low hysteresis losses lead to improved recoverable energy density and energy-storage efficiency of the annealed thick films, reaching 10 J·cm$^{−3}$ and 73% (at 1000 kV·cm$^{−1}$), respectively. The thick films were subjected to flexural bending tests, which showed high flexibility (1.1% bending strain) and high durability (10$^5$ bending cycles). This stable energy-storage operation makes ceramic-polymer layered structures promising for integration into a wide range of flexible electronic devices.

Food poisoning, infection of open wounds, and sepsis are the serious clinical consequences that pathogens can cause. Rapid identification of these pathogens allows prompt monitoring of infections, which improves clinical outcomes. An integrated platform is proposed to manipulate and identify the microorganisms (e.g., foodborne pathogens) in this work. An electrokinetic-based Actuator and capacitive-based sensor are the two primary components of the proposed platform. Dielectrophoresis-based microelectrode is proposed to manipulate the microorganisms. A novel vertical array of capacitive sensors is suggested To identify the levitated microorganisms. The mass of microorganisms can be extracted according to the applied dielectrophoretic force respecting the gravitational force. The whole platform is simulated using 2D numerical simulation, COMSOL Multiphysics, to evaluate design efficiency. Furthermore, the proposed platform is developed and experimentally evaluated using the most common food bacterium type (Escherichia coli (E. coli)). The practical prototype is implemented using cheap technology (rigid and flexible printed circuit board technology). Experiments demonstrated that the proposed biochip could identify E. coli without the need of complex and expensive tools and with a simple manufacturing method.

Advances in flexible electronics are driving dielectric capacitors with high energy storage density toward flexibility and miniaturization. In the present work, an all-inorganic thin film dielectric capacitor with the coexistence of ferroelectric (FE) and antiferroelectric (AFE) phases based on Pb0.96La0.04(Zr0.95Ti0.05)O3 (PLZT) was prepared on a 2D fluorophlogopite mica substrate via a simple one-step process. The flexible capacitor exhibits a high recoverable energy density (Urec) of ≈ 44.2 J/cm3, a large electric breakdown strength (EBDS) of 3011 kV/cm, excellent frequency stability (500 Hz-20 kHz) and high thermal stability over 30–190 °C. It also demonstrates an outstanding bending endurance, which can maintain a high energy storage performance under various bending radii (R = 2–10 mm) or 103 repeated bends at 4 mm. The FE phase is stable near the film surface and the interface with the bottom electrode. The AFE phase with multi-domains has incommensurate modulation structures with super-periodicity of 6.5, 6.9 and 5.2. It indicates that the PLZT/LNO/F-Mica capacitor has high potential for energy storage application and may provide great opportunities for exploring new energy storage materials.

Conventional ultrasound probes have inherent limitations in some application scenarios. Because of their fixed shapes and sizes, they can hardly pass through small orifices to examine the target closely. With the recent development of micro-electro-mechanical systems (MEMS) and flexible electronic technologies, collapsible and self-assembling devices are being considered as a potential approach to address this problem. This paper presents an innovative transformable ultrasonic transducer (TUT), which can transform between a 1D linear array and a 2D planar array with the ability of switching between different functions including 2D/3D imaging and high-intensity focused ultrasound beamforming. The transforming process is accomplished with a specially designed flexible printed circuit board inspired by paper-folding mechanism. The electrical and acoustic properties of the developed transducer along with the acoustic field distribution in its different forms have been characterized. The 2D/3D imaging performances of the TUT have been also evaluated under 1D and 2D configurations, respectively. In addition, the performance of a focused concave transforming mode has been also discussed. The evaluation results suggest that TUT can maintain its performance under different structures with precise transforming control. The TUT device has the potential to considerably expand the applications of industrial and clinical ultrasound.

By engineering strain gradients in dielectrics, the flexoelectric effect can be created, which yields interesting physical properties via electromechanical coupling. Here, we report flexoelectric-induced photovoltaic effects in centrosymmetric LaFeO3 thin-film heterostructures grown on flexible mica substrates, in which partial relaxation of lattice-mismatch strain against LaAlO3 stretching layers results in giant strain gradients and pronounced electrical polarizations. The flexoelectric polarization modulates band alignment and leads to significant photovoltaic effects with a short-circuit current density of ∼0.4 mA/cm2 and an open circuit voltage of ∼ -0.45 V in Pt/LaFeO3/LaNiO3 devices. In addition, by concavely/convexly bending the mica substrate, mechanical strain gradients give rise to bi-directionally tunable photocurrents, in which continuously change of short-circuit current density with a magnitude of ∼100% and good reproducibility in repetitive bending operations are observed in the Pt/LaFeO3/LaNiO3 devices. The present work demonstrates an approach to design self-powered photoelectric devices with an electromechanical degree of freedom based on the flexoelectric effect in flexible thin-film heterostructures.

One of the key challenges in the field of flexible electronics relies on finding conductive materials that can withstand bending and stretching stresses while maintaining their performance. In this context, this work presents a comparative study of laser-induced conductive materials from the direct laser-scribing of two commercial flexible films: the benchmark Kapton® HN polyimide (PI) precursor and the UltemTM 1000 polyetherimide (PEI) alternative contender. The synthesis process on both materials is optimized in terms of electrical conductivity using a high-performance galvanometric laser with a wavelength of 532 nm for the fabrication of multiple samples at different laser powers and speeds. The samples are structurally characterized using Scanning Electron Microscopy (SEM), Raman spectroscopy, X-ray Photoelectron Spectroscopy (XPS), and Fourier-Transform Infrared Spectroscopy (FTIR) aiming at understanding the chemical and physical changes of the ablated material. The results demonstrate that the proposed setup is feasible for the synthesis of uniform and reliable conductive patterns on the surface of both substrates with high reproducibility. In particular, it is proved that PEI is more suitable precursor for flexible electronics applications which demand high electrical conductivity, leading to a sheet resistance of 3.62 ± 0.35 Ω/sq at 0.8 W and 5 mm/s once the laser-synthesis process is optimized (against the 6.04 ± 0.63 Ω/sq at 0.6 W and 5 mm/s offered by the LIG on PI). The performance of both laser-induced patterns as electrodes for the fabrication of electrochemical capacitors is also studied and compared in terms of areal specific capacitance., FEDER/Junta de Andalucía-Consejería de Transformaci´on Econ´omica, Industria, Conocimiento y Universidades Project P20_00265 and Project BRNM-680-UGR20; Project TED2021-129949A-I00, MCIN/AEI/10.13039/ 501100011033, European Union NextGenerationEU/PRTR, Grant PID2020-117344RB-I00, MCIN/AEI 10.13039/ 501100011033, Junta de Andalucía – Consejería de Universidad, Investigaci´on e Innovaci´on through the project ProyExcel_00268, Spanish Ministry of Sciences and Innovation through the Ram´on y Cajal fellow RYC2019- 027457-I, María Zambrano fellow C21.I4.P1, grant PRE2021-096886.

Currently, an urgent need in the field of wearable electronics is the development of flexible sensors that can be attached to the human body to monitor various physiological indicators and movements. In this work, we propose a method for forming an electrically conductive network of multi-walled carbon nanotubes (MWCNT) in a matrix of silicone elastomer to make stretchable sensors sensitive to mechanical strain. The electrical conductivity and sensitivity characteristics of the sensor were improved by using laser exposure, through the effect of forming strong carbon nanotube (CNT) networks. The initial electrical resistance of the sensors obtained using laser technology was ~3 kOhm (in the absence of deformation) at a low concentration of nanotubes of 3 wt% in composition. For comparison, in a similar manufacturing process, but without laser exposure, the active material had significantly higher values of electrical resistance, which was ~19 kOhm in this case. The laser-fabricated sensors have a high tensile sensitivity (gauge factor ~10), linearity of >0.97, a low hysteresis of 2.4%, tensile strength of 963 kPa, and a fast strain response of 1 ms. The low Young’s modulus values of ~47 kPa and the high electrical and sensitivity characteristics of the sensors made it possible to fabricate a smart gesture recognition sensor system based on them, with a recognition accuracy of ~94%. Data reading and visualization were performed using the developed electronic unit based on the ATXMEGA8E5-AU microcontroller and software. The obtained results open great prospects for the application of flexible CNT sensors in intelligent wearable devices (IWDs) for medical and industrial applications.

Nickel plating electrolytes prepared by using a simple salt solution can achieve nickel plating on laser-induced graphene (LIG) electrodes, which greatly enhances the electrical conductivity, electrochemical properties, wear resistance, and corrosion resistance of LIG. This makes the LIG–Ni electrodes well suited for electrophysiological, strain, and electrochemical sensing applications. The investigation of the mechanical properties of the LIG–Ni sensor and the monitoring of pulse, respiration, and swallowing confirmed that the sensor can sense insignificant deformations to relatively large conformal strains of skin. Modulation of the nickel-plating process of LIG–Ni, followed by chemical modification, may allow for the introduction of glucose redox catalyst Ni2Fe(CN)6 with interestingly strong catalytic effects, which gives LIG–Ni impressive glucose-sensing properties. Additionally, the chemical modification of LIG–Ni for pH and Na+ monitoring also confirmed its strong electrochemical monitoring potential, which demonstrates application prospects in the development of multiple electrochemical sensors for sweat parameters. A more uniform LIG–Ni multi-physiological sensor preparation process provides a prerequisite for the construction of an integrated multi-physiological sensor system. The sensor was validated to have continuous monitoring performance, and its preparation process is expected to form a system for non-invasive physiological parameter signal monitoring, thus contributing to motion monitoring, disease prevention, and disease diagnosis.

Flexible electronics are of great interest and importance due to their applications in a range of fields, from wearable electronics to solar cells. While resolutions of printed flexible electronics have been improving in recent years, there remain problems with mechanical fatigue and substrate cost, curtailing the use of such devices due to performance-related issues, and resulting in increased cost and waste products. Here we present a novel method for improving the fatigue resistance of printed flexible electronics by a factor of ~ 40 by sintering the electronic circuits prior to transferring them into low-cost polymer substrates with low melting temperatures, such that the circuits remain embedded in the substrates. This method is demonstrated using circuits printed using silver nanoparticulate ink with an aerosol jet printer, and could be applicable to multiple different metallic inks and polymer substrates. Importantly, this method can be used to transfer-print circuits into polymer substrates with low melting temperatures, without subjecting the polymers to the otherwise detrimentally high sintering temperatures required for ink curing.

The growing interest in materials derived from biomass has generated a multitude of solutions for the development of new sustainable materials with low environmental impact. We report here, for the first time, a strategy to obtain bio-based nanocomposites from epoxidized linseed oil (ELO), itaconic acid (IA), and surface-treated nanofibrillated cellulose (NC). The effect of nanofibrillated cellulose functionalized with silane (NC/S) and then grafted with methacrylic acid (NC/SM) on the properties of the resulted bio-based epoxy systems was thoroughly investigated. The differential scanning calorimetry (DSC) results showed that the addition of NCs did not influence the curing process and had a slight impact on the maximum peak temperature. Moreover, the NCs improved the onset degradation temperature of the epoxy-based nanocomposites by more than 30 °C, regardless of their treatment. The most important effect on the mechanical properties of bio-based epoxy nanocomposites, i.e., an increase in the storage modulus by more than 60% at room temperature was observed in the case of NC/SM addition. Therefore, NC’s treatment with silane and methacrylic acid improved the epoxy–nanofiber interface and led to a very good dispersion of the NC/SM in the epoxy network, as observed by the SEM investigation. The dielectric results proved the suitability of the obtained bio-based epoxy/NCs materials as substitutes for petroleum-based thermosets in the fabrication of flexible electronic devices.

Compressibility of zinc-manganese oxide (Zn-MnO2) batteries is an essential element of modern flexible electronics. Hydrogel electrolytes with superior elasticity and compressibility are highly demand to guarantee a stable energy output of the flexible Zn-MnO2 battery. Herein, a highly compressible hydrogel electrolyte was developed by introducing soybean protein isolate nanoparticles (SPI) into covalently cross-linked polyacrylamide (PAAM) polymer networks. The SPI/PAAM hydrogel electrolyte for Zn-MnO2 battery possessed outstanding reversible compressibility due to the aggregation of SPI nanoparticles on the PAAM chains through the weak electrostatic interaction, which could dissipate energy effectively. Consequently, the Zn-MnO2 battery based on the compressible hydrogel electrolyte displayed a decent specific capacity (299.3 mA h g−1) and desirable capacity retention rate (78.2%) after 500 charge/discharge cycles. Notably, the device could maintain stable power output under 96% compress strain and light the bulb even under severe mechanical stimulation like being-bent and hammered. It’s believed that the compressible Zn-MnO2 batteries hold enormous potential as the energy storage devices in the field of flexible wearable electronics.

Flexible narrowband near infrared (NIR) photodetectors (PDs) are urgently in demand in the fast-developing era of flexible electronics, due to their crucial roles in various innovative applications. Hence, we designed and synthesized the core–shell structured NaYF4:Yb3+,Er3+@NaYF4:Nd3+ upconversion nanoparticles (UCNPs), which can be pumped by the 808 and 980 nm lights. The upconversion luminescence (UCL) are significantly enhanced after being assembled with the opal photonic crystals (OPCs) due to the photonic crystal effect, with 55 and 48 folds of enhancement factors under illuminations of 808 and 980 nm lights, respectively. Based on this hybrid, the flexible narrowband PDs were successfully fabricated on the PET substrate with the structure of OPCs/NaYF4:Yb3+,Er3+@Nd3+/MAPbI3, which displays excellent detection performance to double narrowband NIR light (808 and 980 nm) benefiting from the amplified UCL, with responsivity of 8.79 and 7.39 A/W, detectivity of 3.01 × 1011 and 2.68 × 1011 cm⋅Hz1/2/ W for 808 and 980 nm lights detection respectively, along with short response time in the range of 120–160 ms. Furthermore, the OPCs/NaYF4:Yb3+,Er3+@NaYF4:Nd3+/MAPbI3 double narrowband PDs display low photodetection power threshold (0.05 W/cm2), outstanding flexibility, prominent moisture resistance, and good long-time stability. This work displays a new concept of narrowband NIR PDs, which open a new field for specific NIR light detections.

"Learning from nature" provides endless inspiration for scientists to invent new materials and devices. Here, we review state-of-the-art technologies in flexible electronics, with a focus on bio-inspired smart skins. This review focuses on the development of E-skin for sensing a variety of parameters such as mechanical loads, temperature, light, and biochemical cues, with a trend of increased integration of multiple functions. It highlights the most recent advances in flexible electronics inspired by animals such as chameleons, squids, and octopi whose bodies have remarkable camouflage, mimicry, or self-healing attributes. Implantable devices, being overlapped with smart E-skin in a broad sense, are included in this review. This review outlines the remaining challenges in flexible electronics and the prospects for future development for biomedical applications. STATEMENT OF SIGNIFICANCE: This article reviews the state-of-the-art technologies of bio-inspired smart electronic skin (E-skin) developed in a "learning-mimicking-creating" (LMC) cycle. We emphasize the most recent innovations in the development of E-skin for sensing physical changes and biochemical cues, and for integrating multiple sensing modalities. We discuss the achievements in implantable materials, wireless communication, and device design pertaining to implantable flexible electronics. This review will provide prospective insights integrating material, electronics, and mechanical engineering viewpoints to foster new ideas for next-generation smart E-skin.

Graphene is a two-dimensional material that can be folded into diverse and yet interesting nanostructures like macro-scale paper origami. Folding of graphene not only makes different morphological configurations but also modifies their mechanical and thermal properties. Inspired by paper origami, herein we studied systemically the effects of creases, where sp2 to sp3 bond transformation occurs, on the thermal properties of graphene origami using molecular dynamics (MD) simulations. Our MD simulation results show that tensile strain reduces (not increases) the interfacial thermal resistance owing to the presence of the crease. This unusual phenomenon is explained by the micro-heat flux migration and stress distribution. Our findings on the graphene origami enable the design of the next-generation thermal management devices and flexible electronics with tuneable properties.

For the first time, the necessity to thermally pre-treat ubiquitously used PET substrates for printed electronics, to improve dimensional stability during manufacturing, is clearly defined. The experimental results have proven this phenomenon for both roll-to-roll (R2R) and sheet-to-sheet (S2S) processing of printed electronics. The next generation of electronics manufacturing has pushed the boundaries for low-cost, flexible, printed, and mass produced electronic components and systems. A driving force, and enabling production method, are the R2R printing presses. However, to produce electronics with increasing complexity and high yield in volume production, one must have a highly accurate process. In this article, R2R processing accuracy of printed electronics is evaluated from the point of dimensional accuracy of the flexible polyester substrate (DuPont Teijin Films’ PET Melinex ST504 with and without indium tin oxide, Melinex ST506, and Melinex PCS), precision of printing, and accuracy of layer-to-layer registration with stages that involve tension and elevated temperatures. This study has confirmed that dimensional changes during R2R processing will occur only in the first processing stage and that if a thermal pre-treatment run for the substrate is made—at identical temperature and tension of the processing stage—there is improved stability originating from a new-level strain in the crystalline PET film structure and freezing it in at the tensions and temperatures it is exposed to (i.e. 1400 μm machine direction stretching reduced to 8 μm). Furthermore, it is explained how the dimensional accuracy can be improved and reproducibly maintained in multilayer printing of electronics devices such as organic photovoltaics (OPV). These devices provide a valuable baseline of how the layer-to-layer alignment accuracy plays a crucial role in fully printed electronics devices, which lessons can be applied in all aspects of this field including hybrid systems and system fabrication involving multiple processing methods.

Due to the scarcity of indium (In) in the earth and its potential harm to individuals, the development of In-free transparent conductive film is considered crucial. In this work, In-free SnO2:Sb/Au/SnO2:Sb (ATO/Au/ATO, SAS) tri-layer films with high transparency and conductivity were successfully prepared on polycarbonate (PC) substrates by RF and DC magnetron sputtering at room temperature. The influence of the Au layer thickness on microstructure, electrical and optical performances was systematically studied after fixing the ATO thickness to 50 nm. It was indicated by X-ray diffraction patterns that ATO is amorphous and Au is oriented along (111). The trend of increasing and then decreasing light transmission with Au layer thickness was observed in both experimental and simulation results. The improved figure of merit (FoM, 1.89 × 10−2 Ω−1) was achieved in SAS tri-layer film, the resistivity and average transmittance of which was lowered to 7.50 × 10−5 Ω cm and 81.4%, respectively, when Au layer thickness is 11 nm. Moreover, the mechanism of the variation of optical and electrical properties at different Au layer thickness was proposed. Particularly, the SAS tri-layer films also exhibit superior flexibility, durability and adhesion. These results demonstrate SAS tri-layer films are promising alternative to ITO in flexible electronics applications.

Pressure vessels are always under different hazards such as leaking, fracturing, and unknown deformation. In this work, the novel micro-nano sensors were introduced based on flexible printed circuit (FPC). The temperature cycling and hydraulic pressure cycling experiments were carried out. This kind of combined sensor can be embedded and arranged between inner tank and fiber layer without any defect. The experimental results show the new sensor can detect not only thermal strain, but also elastic deformation and plastic deformation caused by pressure. Also can reflect the yield limit of COPV, which can ensure the safety of manufacture.

To obtain a flexible supercapacitor with excellent capacitance, good mechanical properties while maintaining self-healing ability, β-NiS/Ti3C2Tx with 2D/2D heterostructure was synthesized by hydrothermal reaction and introduced as filler to the polymer hydrogel PAA/CS(PC) network. Through mechanical and self-healing performance tests, it was found that the hydrogel electrode had high tensile properties with tensile strain up to 566% and excellent self-healing properties with efficiency as high as 84%. The electrode not only exhibits a specific capacitance of 10.92 F/g at a current density of 6 mA/g, but also still shows an excellent stability of 87% at a high current density of 20 mA/g for 2000 cycles. This novel hydrogel facilitates the further development of multifunctional, smart and flexible electronics.

Range of materials from polymeric to inorganic materials encompasses a wide array of performance benchmarks as reported in the International Roadmap for Devices and Systems (IRDS) in the field of Large Area and Flexible Electronics for market greater than $100 Billion. It is these benchmarks of device characteristics (electrical and mechanical), and processes involved to make the device that will ultimately determine the suitable application. This paper focuses on simulation and development of a flexible thin film sensor for sensing of biomedical micro forces or displacement. These include, identification and characterization of biological and electrophysiological changes induced by controlled, focal application of mechanical force for biomedical application. Measurement of the nonlinear behavior of the thin films fabricated using various spectroscopic instruments. The study on surface morphology using SEM for surface roughness, phase crystalline nature using X- ray Diffraction, optical behavior using UV–Visible spectroscopy for analytic study of band gap and stress strain using Universal Test Machine. The study of electromechanical response to understand the piezo electric behavior or pressure sensing using fabricated sensor was done for wearable electronics used for sensing physiological signals Electromyography (EMG) and Electro-oculogram (EoG). The sensors were used as dry electrode for testing EMG and EoG using amplifiers and filters as compared to the performance of standard available wet electrodes.

With the development of lightweight and flexible electronics, flexible transparent electrodes (TEs) have attracted huge attention in both academia and industry, and play a central role in high-performance flexible electronics. As a kind of emerging conducting material for TEs, metallic micro–nano architectures (MMNAs) possess both low sheet resistance and high optical transmittance. In addition, the high flexibility and low-cost solution processing make MMNAs promising candidates to replace the traditional conductive metal oxides, which suffer from the high-cost fabrication process and low conductivity on flexible substrates. In this review, we summarize the recent progress in flexible TEs based on MMNAs, as well as the comparison to other conducting materials, such as conducting polymers, graphene, carbon nanotubes, etc. Specially, we highlight the applications of flexible TEs based on MMNAs in perovskite solar cells (PSCs), including inverted PSCs, conventional PSCs, and semi-transparent PSCs. Finally, the challenges and prospects in this field are proposed.

Silver nanowires (AgNWs) based electrode are the potential candidates to replace expensive ITO electrodes in the next generation of flexible electronics. Herein, AgNWs of ∼1000 aspect ratio were synthesised by using simple and cost-effective salt mediated polyol synthesis method. The AgNWs based transparent conducting electrodes (TCEs) have demonstrated the transparency and sheet resistance of 81% and 20 Ω/sq. The effect of AgNO3 injection rate, PVP/AgNO3 molar ratio, and NaCl/KBr ratio on the AgNWs morphology was extensively studied and optimized to 0.2 mL/min, 4.5:1 and 3.8:1, respectively. The effect of post annealing on TCE has also been studied and optimized to 150 °C.

Compact model of flexible radio frequency (RF) electronic devices is crucial for flexible circuit designs. In this article, a compact model, including external strain effects for flexible RF gallium nitride (GaN) high-electron-mobility transistor, is presented. First, the carrier density and threshold voltage with external mechanical uniaxial strain is analytically characterized by introducing the strained piezoelectric charge Δ σ , Schottky barrier height $Δ φ_{B}$ , and surface state density $D_{{it}}$ into the model. Then, the variations of the conduction band offset at the AlGaN/AlN and AlN/GaN interfaces are considered. Finally, the complete large-signal model is established after embedding strain effect into intrinsic drain current and nonlinear capacitance models. The proposed model is verified by measurements, including direct current I-V characteristics, small-signal S-parameters up to 40 GHz, and large-signal power performance--output power, efficiency, and gain. The proposed model will be useful for the RF application of flexible electronics.

Flexible devices play an important role in various fields such as electronics, industry, healthcare, military, space exploration, and so on. Traditional materials used for flexible devices include silicon, inorganic oxides, and polymers. However, these materials show some drawbacks such as high rigidity, low electrical conductivity, or being costly for large-scale manufacturing. Transition metal dichalcogenides (TMDs) have attracted great interest in the last decade for their layered structures, and relevant publications have gained rapid growth. TMDs exhibit tunable properties as a function of layers. TMDs show the semiconducting/metallic transition, direct/indirect band transition, strengthened flexibility, and increased transparency with the decreasing layer number, Such tunable properties of TMDs allow them to be promising materials for flexible devices. As part of this review, the initial discussion comes to the structural information of TMDs. Subsequently, recent improved synthetic routes and phase engineering for TMDs will be introduced. Then, we introduce the studies and challenges of flexible and stretchable devices. And then we review some recent work upon TMD-based flexible devices, including transistors, energy storage devices, and sensors. In the end, some comprehensive discussion and perspectives for the future trend of flexible devices will be given.

Cellulose paper derived from plants is used for daily writing and recording, and has historically played an important role in promoting cultural dissemination and inheritance. However, it attracted more and more attentions in recent years serving as a flexible platform or substrate for application in environment and energy, attributing from its many inherent advantages such as biocompatibility, biodegradability, sustainability, low cost, and great capability to be modified. With the development of various industries, especially the flexible electronics industry, the demand for two-dimensional (2D) paper-like materials has increased. Several kinds of inorganic paper are manufactured and have been widely used in many fields. Among them, there are hydroxyapatite (HAP)-based and carbon-based inorganic paper that give play to their own advantages and have been well applied in some fields. In addition, there are many composite papers, which are assembled from several raw materials (cellulose fibers, HAP, carbon), exhibiting excellent characteristics. Cellulose paper is no longer limited to traditional writing applications, but has great potential to be used in many aspects, achieving a gorgeous change in its role. The emergence of inorganic paper and hybrid paper has further demonstrated the application prospects of paper-like two-dimensional materials. This paper will start from the characteristics of the three types of paper and composite paper, and summarize their typical applications in the environment and energy fields.

Ferroelectric tunnel junctions (FTJs) as the artificial synaptic devices have been considered promising for constructing brain-inspired neuromorphic computing systems. However, the memristive synapses based on the flexible FTJs have been rarely studied. Here, we report a flexible FTJ memristor grown on a mica substrate, which consists of an ultrathin ferroelectric barrier of BiFeO3, a semiconducting layer of ZnO, and an electrode of SrRuO3. The obtained flexible FTJ memristor exhibits stable voltage-tuned multi-states, and the resistive switchings are robust after 103 bending cycles. The capability of the FTJ as a flexible synaptic device is demonstrated by the functionality of the spike-timing-dependent plasticity with bending, and the accurate conductance manipulation with small nonlinearity (−0.24) and low cycle-to-cycle variation (1.77%) is also realized. Especially, artificial neural network simulations based on experimental device behaviors reveal that the high recognition accuracies up to 92.8% and 86.2% are obtained for handwritten digits and images, respectively, which are close to the performances for ideal memristors. This work highlights the potential applications of FTJ as flexible electronics for data storage and processing.

With the rapid development of the flexible electronics field, flexible pressure sensors are widely used in wearable devices, medical monitoring and other fields. However, it is still a huge challenge to design a sensor with high sensitivity, low cost and a simple manufacturing process. In this paper, a resistive pressure sensor based on a composite conductive layer is developed. The polydimethylsiloxane (PDMS) is used as the matrix of the composite. The electrolyte fully absorbed by the super absorbent polymer (SAP+) and used as the conductive filler of the composite. The elastic polymer is used to replace the common rigid conductive fillers (such as carbon nanotubes, graphene, etc.), can be better combined with the matrix, greatly improving the stability and reducing the Young’s modulus of the sensor. After describing the working mechanism of the sensor, the influence of the composite conductive layer of different thicknesses and the content of SAP+ on the electrical output of the sensor is discussed. In addition, the sensor exhibits high sensitivity (0.062 kPa-1), a fast response time (0.136 s) and excellent cyclic loading/unloading stability (> 500 cycles).

Organic single crystals (OSCs) have received increasing interest in the last decades for their potential applications in flexible electronics. Although there are various subtractive manufacturing methods of organic electronics, the subtractive manufacturing of OSCs is still a challenge, since OSCs are assembled via weak van-der-Waals interactions which are vulnerable and cannot afford damages and suffer the degradation of performances after the process. Here, we develop an epitaxial etching strategy which clips the OSCs and keeps high-quality crystalline nature of the resulting materials. As a result, high-quality organic micro-ribbon arrays are fabricated which maintains 89% charge mobility in average compared with original OSCs, showing great potential of this subtractive manufacturing method in future organic electronics.

Printed electronics using inks based on graphene and other two-dimensional materials can be used to create large-scale, flexible, and wearable devices. However, the complexity of the ink formulations, and the polycrystalline nature of the resulting thin films, have made it difficult to examine charge transport in such devices. Here we report the charge transport mechanisms of surfactant- and solvent-free inkjet-printed thin-film devices based on few-layer graphene (semi-metal), molybdenum disulfide (MoS2, semiconductor) and titanium carbide MXene (Ti3C2, metal) by investigating the temperature, gate and magnetic field dependencies of their electrical conductivity. We find that charge transport in printed few-layer MXene and MoS2 devices is dominated by the intrinsic transport mechanism of the constituent flakes: MXene exhibits a weakly-localized 2D metallic behaviour at any temperature, whereas MoS2 behaves as an insulator with a crossover from 3D-Mott variable-range hopping to nearest-neighbour hopping around 200 K. Charge transport in printed few-layer graphene devices is dominated by the transport mechanism between different flakes, which exhibit 3D-Mott variable range hopping conduction at any temperature.

This paper presents a relative pressure sensor fabricated by a standard industrial flexible printed circuit board (FPCB) technology. The passive wireless sensor consists of a variable sensing capacitor and a planar coil inductor. When the external pressure is applied to the sensing capacitor, it changes the air gap and shifts the LC resonance frequency which is detected by an antenna wirelessly. The sensing cavity of the variable capacitor is extended to the opening at the end of an embedded pressure channel in the FPCB strip to enable relative pressure measurement. Measurement results showed that the proposed sensor with such an open sensing cavity had much higher frequency sensitivity (up to 0.4 %/mmHg) compared with the sensor with a sealed cavity (< 0.01 %/mmHg in this work and ≤ 0.08 in literature). With the enhanced sensitivity, the measured pressure resolution could reach 0.2 mmHg. The potential application of the proposed sensor includes gauge pressure measurement with respect to local atmospheric pressure, such as intracranial pressure (ICP) monitoring, without complicated re-calibration procedures. The flat FPCB structure facilitates applications with limited space for sensor installation, such as in the subdural implantation procedures. Furthermore, the proposed technology enables future development of flexible microfluidic devices.

Radio frequency identification (RFID) is a well-established technology utilized in inventory management, security, logistics, product marketing as well as next generation smart flexible sensor platforms. This work reports the development of printed, wearable graphene temperature sensor on flexible textile substrates with structural configuration of RFID coil. Patterning of graphene in the form of sensing RFID coils on textile surfaces was achieved with cost-effective and scalable spray printing of graphene oxide and sequential reduction to reduced graphene oxide. Graphene RFID coil operating at 13.56 MHz with temperature sensing capability, are printed on flexible, ordinary fabrics to enable wearable and flexible electronics applications. The demonstration of reduced graphene oxide on textile as temperature sensitive layer is the distinct feature of this work. Good sensitivity of 0.0125 degrees C-1 is achieved with linear response in the range of 25 degrees C to 45 degrees C and similar to 12 % decrease in resistance was observed over the measured temperature span. Induced voltage of 100 mV peak-to-peak at an area of 30.25 cm(-2) is measured. RFID performance of the sensor coil under different bending radii starting from fully flat (0 degrees) to complete bending (90 degrees) was also studied, showing that sensor coil performs well during flat operation (100 mV) and stays above noise floor even under fully bended condition (12 mV). Finally, flexible temperature sensing RFID antenna attached to human body to investigate performance for wearable applications, indicating that the sensor can effectively monitor temperatures in the range of 25 degrees C to 45 degrees C.

Ultrathin, soft, flexible and stretchable sensors offer good opportunity to design personal health diagnosis and treatment systems by fabricating wearable biomedical devices for continuous health monitoring and preventive medicine. This paper summarizes the recent advances of flexible sensors for next generation biomedical applications. Materials used for flexible sensors are first briefly introduced. Followed to this part, the latest developments of flexible sensors for biomedical applications are discussed in detail, including flexible sensors for skin-attached devices and implantable devices. Finally, the challenges and future research directions of flexible sensors are also discussed in the development of biomedical devices.

Flexible photodetectors have attracted much attention due to their great potentials in wearable electronics, bendable image sensors, and environmental monitoring, which requires functional components with good machinal properties. Herein, we introduced a novel metallic two-dimensional (2D) material, MXene, to form the electrodes by a spray-coating method and zero-dimensional (0D) PbS quantum dots (QDs) to work as photoactive layer. All solution-processable fabrication technique contributes to a simplified, low-cost and large-scale route to develop flexible photodetectors. Benefiting from the excellent conductivity of MXene electrodes and broadband photoresponse of PbS QDs, our devices realize a high detectivity up to 2.4×1011 Jones and exhibit good stability even after 1000 bending cycles. This work provides a new device configuration with 2D electrodes and 0D photoactive layers for designing high-performance flexible electronics.

A reconfigurable wearable textile antenna with wide frequency tuning range is demonstrated in this work. The frequency agility is realized by adding a coplanar reconfiguration module to a planar inverted-F antenna (PIFA) constructed in textile technology. The reconfiguration module consists of a small flexible printed circuit board (PCB) which integrates a tuning circuitry with commercial snap-on buttons as electronic-to-textile connectors. Using this module equipped with a varactor diode, the frequency reconfigurability of the PIFA is investigated and optimized parametrically for maximum tuning range, exploiting a smooth transition from a quarter-wave to a half-wave patch resonant mode. A fabricated prototype demonstrates a very wide tuning range of approximately 70% (or one octave) with stable directive radiation characteristics, as predicted by simulations. The robust performance and mechanical stability of the reconfigurable antenna in various on-body and bending configurations validates the viability of the antenna tuning concept and the practicality of the proposed coplanar reconfiguration module.

Conventional rigid batteries find limited applications in the wearable and flexible electronics field, as difficulties in device integration due to lack of shape conformability to unconventional substrates remain an obstacle. Accordingly, new fabrication techniques are under investigations and printing techniques have attracted great interest for their selectivity, low material waste and scalability. In this frame, inkjet printing (IJP) has emerged as a potential fabrication technique to obtain flexible patterned thin-film electrodes with high resolution. However, a more in-depth systematic study of how IJP can be applied to lithium-ion batteries fabrication is still missing. Herein, we propose a study that focuses on how different carbon-based additives (Nitta et al., 2015; Choi and Ahn, 2018 [1,2]), i.e. carbon black and multi-walled carbon nanotubes, affect the electrochemical performances of inkjet printed thin-films Li4Ti5O12 electrodes. A simple ink formulation is proposed, which is aqueous-based, non-toxic and safe to handle. The fabricated thin-film electrodes showed different specific capacity, the highest associated with the carbon nanotubes-based (CNTs) electrode, i.e. 150.3 mAh g−1 at 0.2 C, showing that CNTs improving electrochemical performances can be applied also to printed electrodes. The carbon nanotubes-based electrodes showed excellent cycle stability, with negligible capacity loss for over 100 cycles. The work highlights the importance of a valid material choice to reach the desired electrode performances and may inspire alternatives paths to deepen IJP of lithium-ion batteries fabrication.

Emerging trends like the Internet of Things require an increasing number of different sensors, actuators and electronic devices. To enable new applications, such as wearables and electronic skins, flexible sensor technologies are required. However, established technologies for the fabrication of sensors and actuators, as well as the related packaging, are based on rigid substrates, i.e., silicon wafer substrates and printed circuit boards (PCB). Moreover, most of the flexible substrates investigated until now are not compatible with the aforementioned fabrication technologies on wafers due to their lack of chemical inertness and handling issues. In this presented paper, we demonstrate a conceptually new approach to transfer structures, dies, and electronic components to a flexible substrate by lift-off. The structures to be transferred, including the related electrical contacts and packaging, are fabricated on a rigid carrier substrate, coated with the flexible substrate and finally lifted off from the carrier. The benefits of this approach are the combined advantages of using established semiconductor and microsystem fabrication technologies as well as packaging technologies, such as high precision and miniaturization, as well as a variety of available materials and processes together with those of flexible substrates, such as a geometry adaptivity, lightweight structures and low costs.

The electrical properties of pristine fluoropolymers are inferior due to their low polar crystalline phase content and rigid dipoles that tend to retain their fixed moment and orientation. Several strategies, such as electrospinning, electrohydrodynamic pulling, and template-assisted growing, have been proven to enhance the electrical properties of fluoropolymers; however, these techniques are mostly very hard to scale-up and expensive. Here, a facile interfacial engineering approach based on amine-functionalized graphene oxide (AGO) is proposed to manipulate the intermolecular interactions in poly(vinylidenefluoride-trifluoroethylene) (PVDF-TrFE) to induce β-phase formation, enlarge the lamellae dimensions, and align the micro-dipoles. The coexistence of primary amine and hydroxyl groups on AGO nanosheets offers strong hydrogen bonding with fluorine atoms, which facilitates domain alignment, resulting in an exceptional remnant polarization of 11.3 µC cm −2. PVDF-TrFE films with 0.1 wt.% AGO demonstrate voltage coefficient, energy density, and energy-harvesting figure of merit values of 0.30 Vm N −1, 4.75 J cm −3, and 14 pm 3 J −1, respectively, making it outstanding compared with state-of-the-art ceramic-free ferroelectric films. It is believed that this work can open-up new insights toward structural and morphological tailoring of fluoropolymers to enhance their electrical and electromechanical performance and pave the way for their industrial deployment in next-generation wearables and human-machine interfaces.

Optical transparency is highly desirable in bioelectronic sensors because it enables multimodal optical assessment during electronic sensing. Ultrathin (90%) and high transconductance (≈1 mS) in low-voltage operations (

A lot of every day devices are electrically powered and dependent on an internal energy source. One of the most used energy sources for portable devices are rechargeable batteries. Throughout the last decades, battery research and development gave rise to significant performance improvements. New electrode materials, design strategies and advanced fabrication techniques have equipped modern-day batteries with ever-rising capacity. However, little changed in their appearance, shape, and functionalities. On the other hand, development of flexible devices stays on the horizon. Without adequate evolution of the energy storage unit, development of those devices is severely hampered. Powering flexible devices requires flexible, mechanically strong and lightweight energy storage units. In contrary, commercial batteries are mechanically susceptible to external stresses from bending, highlighting the current hurdle of powering flexible devices with commercial batteries. High-capacity materials, namely metal and alloying anodes, as well as conversion cathodes would make cells and devices lighter and smaller. However, these materials currently possess unsolved challenges in material integrity and cycling stability. Adding bending stresses in flexible devices make the battery even more vulnerable to capacity decay. All in all, this leads to a distinguished trade-off between batteries with high capacity and ones with good bending stability. This thesis aims to tackle the challenge by investigating and optimizing the compatibility of high-capacity electrode materials with a flexible substrate. The work shows that carbon-based materials provide a versatile, conductive platform to stabilize and utilize high-capacity materials in flexible devices, demonstrated with two battery chemistries – namely zinc-ion and lithium-sulfur batteries. Carbon materials are introduced as a lightweight conductive platform to process stiff, brittle and/or powdery electrode materials into structures that are flexible. A conductive network is formed via self-assembly of single-walled carbon nanotubes (CNT) upon drying. Their bending stability could be verified even upon integration of 40 wt% active material. In particular, high aspect ratio CNTs formed flexible substrates with incorporated materials for silicon anode and sulfur cathode in lithium-sulfur cells. Accordingly, silicon-gallium composite with Page | III chitosan-alginate binder and sulfur with nitrogen-doped titanium oxide integrated into CNT network exhibited stable electrochemical performance in half-cells. Furthermore, the preparation method was adapted to the respective active materials for zinc-ion batteries in a Zn-MnO2 cell and has the potential to incorporate various other active materials. One approach to flexibility relies on thinning down the material layers. However, among the challenges in thin batteries, dendrite formation and their control of metal anodes is one of the crucial factors to maintain the cell performance. Dendrite growth mechanism is unique to the respective metal surface and interacting battery components. Zinc and its cycling behaviour were studied as one of the battery systems with decent capacity, but prone to dendrite formation. Without mitigation measures, zinc surfaces form dendrites under stripping and plating in near neutral electrolyte, which leads to short circuit. This thesis demonstrates a functionalization process on zinc anodes by simple surface assembled monolayer of thiols. The additives improve the cycle of short circuit at 1 mAh/cm2, one order of magnitude compared to pristine zinc surfaces. Intrinsic electrochemical and mechanical stability, together with interfacial adhesion, define the robustness of a flexible battery. With the CNT network proven to show compatibility with two, rather different battery chemistries, it can be assumed that further investigations will show universal application of this carbon-based material as substrate for flexible electrode fabrication. Furthermore, the investigations on different aspects of stability in high-capacity active materials show progress towards better cycling stability of materials in zinc and lithium batteries. Light, thin and flexible electrochemical energy storage devices meet the criteria of the fast-evolving fields of wearable sensing devices and rollable personal electronics. New functionalities of electrochemical cells potentially create new integration opportunities or even facilitate completely new applications. This research paves the way toward developing flexible devices and digitalization in everyday life.

Hydrogels have emerged as promising materials for flexible electronics due to their unique properties, such as high-water content, softness, and biocompatibility. In this perspective, we provide an overview of the development of hydrogels for flexible electronics, with a focus on three key aspects: mechanical properties, interfacial adhesion, and conductivity. We discuss the principles of designing high-performance hydrogels and present representative examples of their potential applications in the field of flexible electronics for healthcare. Despite significant progress, several challenges remain, including improving the anti-fatigue capability, enhancing interfacial adhesion, and balancing water content in wet environments. Additionally, we highlight the importance of considering the hydrogel-cell interactions and the dynamic properties of hydrogels in future research. Looking ahead, the future of hydrogels in flexible electronics is promising, with exciting opportunities on the horizon, but continued investment in research and development is necessary to overcome the remaining challenges. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Nanyang Technological University National Research Foundation (NRF) Submitted/Accepted version The authors acknowledge the AME programmatic funding scheme of Cyber Physiochemical Interfaces (CPI) project #A18A1b0045 and the Singapore Ministry of Education (MOE) Tier 2 Grant (MOE-T2EP30220-0006). Y. Tan is supported by the National Research Foundation, Singapore (NRF) under NRF’s Medium Sized Centre: Singapore Hybrid-Integrated Next-Generation μ-Electronics (SHINE) Centre funding programme. J. Lao acknowledges the research scholarship awarded by the Institute of Flexible Electronics Technology of Tsinghua, Zhejiang (IFET-THU), Nanyang Technological University (NTU), and Qiantang Science and Technology Innovation Center, China (QSTIC). H. Gao acknowledges a start-up grant (002479-00001) from Nanyang Technological University and Agency for Science, Technology and Research (A*STAR).

El uso de electrónica flexible para instrumentación biomédica, supone un gran avance para conseguir una monitorización continua mediante dispositivos wearables. En esta Tesis, se estudia el uso de nuevos materiales y técnicas de impresión para la consecución de un dispositivo de monitorización biomédica. En primer lugar, se han desarrollado electrodos sobre diferentes materiales flexibles basados en grafeno, polímeros y tintas conductoras. Estos electrodos flexibles se han realizado empleando técnicas de impresión como screen printing o grabado láser. Además, se han estudiado sobre distintos substratos flexibles como pueden ser láminas de plásticos flexibles o textiles, siendo este último el más interesante para los dispositivos wearables. Las técnicas de impresión también se han utilizado para crear sensores flexibles a través de materiales orgánicos innovadores como son los Metal-Organic Frameworks (MOFs). Por otra parte, también se ha estudiado la electrónica flexible aplicada al campo del energy harvesting, un campo muy interesante para aumentar la autonomía de los dispositivos wearable. En este aspecto se han desarrollado electrodos flexibles para el almacenamiento de energía. Se han empleado materiales como los MXenes y junto con polímeros conductores para crear dispositivos con capacidad de almacenar energía en un reducido espacio y manteniendo su flexibilidad. Finalmente, toda la tecnología desarrollada, se ha combinado en un dispositivo wearable basado en electrónica reconfigurable que permite la monitorización de distintas señales biomédicas en un solo dispositivo., The use of flexible electronics for biomedical instrumentation represents a great advance to achieve continuous monitoring through wearable devices. In this thesis, the use of new materials and printing techniques to achieve a biomedical monitoring device is studied. First, electrodes have been developed on different flexible materials based on graphene, polymers and conductive inks. These flexible electrodes have been created using printing techniques such as screen printing or laser engraving. In addition, they have been studied on different flexible substrates such as flexible plastic sheets or textiles, the latter being the most interesting for wearable devices. Printing techniques have also been used to create flexible sensors through innovative organic materials such as Metal-Organic Frameworks (MOFs). On the other hand, flexible electronics applied to the field of energy harvesting have also been studied, since it is a very interesting field to increase the autonomy of wearable devices. In this regard, flexible electrodes have been developed for energy storage. Materials such as MXenes and conductive polymers have been used to create devices capable of storing large amounts of energy in a small space while maintaining flexibility. Finally, all the technology developed has been combined in a wearable device based on reconfigurable electronics that allows the monitoring of different biomedical signals in a single device., Tesis Univ. Granada.