Abstract The growing demand for wearable wireless systems has revealed significant limitations in conventional rigid antennas, which lack the flexibility, stretchability, and on-body stability required under mechanical deformation. Although various flexible and stretchable antenna designs have been proposed, many suffer from narrow impedance bandwidth, gain degradation under strain, complex and costly fabrication processes, and inadequate consideration of human-body electromagnetic effects such as specific absorption rate (SAR). This paper presents a stretchable coplanar waveguide (CPW)-fed antenna fabricated on a 0.3 mm thick polydimethylsiloxane (PDMS) elastomer substrate using low-cost screen printing with a commercial stretchable silver-based conductive paste. The antenna incorporates a periodic wavy geometry applied not only to the radiating patch but also to the CPW ground planes. This wavy topology serves a dual function: mechanically, it distributes tensile strain uniformly to prevent conductive failure; electromagnetically, it elongates the effective current path, lowers the resonant frequency, and provides additional degrees of freedom for broadband impedance matching. The fabricated antenna operates from 1.75 to 4.4 GHz, covering the 2.4 GHz ISM band and portions of sub 6 GHz applications. Measured results demonstrate a peak gain of 5.2 dBi and a maximum radiation efficiency of 92%. Under uniaxial tensile strains up to 15%, the antenna maintains S11< −10 dB across the operating band. SAR simulations using a three-layer human tissue phantom yield a maximum 10 g average SAR of 1.189 W/kg at 0.1 W input power, well below the EU limit of 2.0 W/kg. Comparative evaluation against state of the art flexible and stretchable antennas confirms that the proposed design uniquely combines broadband operation, moderate stretchability (15%), low cost screen printing, and rigorous on body safety validation, positioning it as a practical candidate for next generation wearable electronics.
Patients with bladder underactivity experience difficulty in sensing bladder fullness and emptying the bladder efficiently, leading to chronic urinary retention. An implantable bidirectional device that combines capturing bladder smooth muscle activities with stimulation could enable closed-loop assistance and offer benefits compared to the common timed intermittent catheterization. Most existing bidirectional systems lean on passive metal electrodes as the interface, which provide no intrinsic signal amplification for low-amplitude bladder electromyography sensing and offer limited charge-injection capacity for effective and safe stimulation. Organic electrochemical transistors (OECTs) offer an attractive alternative with high sensitivity for biopotential measurements and effective stimulation capability due to their high-capacitance polymer channels. In this study, we used a custom-made portable system for a flexible ultrathin OECT array to shape a bidirectional recording and stimulation interface for bladder smooth muscle. We validate the system's recording and stimulation performance under ex vivo conditions by using an organ bath setup equipped with a commercial force transducer as a mechanical reference. The array recordings showed stable baselines during inactive periods and clear and repeatable low-frequency output changes that matched force changes during spontaneous muscle contractions. The system also delivered programmed pulse trains that consistently triggered measurable contractions in the muscle, proving its reliable stimulation capability. These findings establish a practical foundation for OECT-enabled closed-loop bladder monitoring and stimulation and support the broader use of active organic bioelectronics as bidirectional interfaces for soft and dynamic organs.
Abstract The advancement of soft robotics, wearable electronics, and deformable wireless systems necessitates radio-frequency (RF) filters that maintain stable performance under mechanical deformation. Although various stretchable conductive materials and filter designs have been explored, existing solutions often suffer from complex fabrication, high cost, significant RF loss, or limited evaluation under diverse deformation modes. To address these challenges, this work presents a low-cost, scalable stretchable combline bandpass filter operating at approximately 2.4 GHz, fabricated by screen-printing a stretchable silver-copper (Ag–Cu) conductive paste on a flexible polydimethylsiloxane (PDMS) substrate. The thermal behavior, sintering-induced microstructural evolution, and electrical properties of the printed films are systematically investigated, enabling optimization of the sintering process and filter geometry. Experimental results demonstrate that the filter retains a clear passband with insertion loss of 5–6 dB under planar, bending, uniaxial stretching (up to 15% strain), and biaxial balloon-inflation deformations, exhibiting only minor variations in impedance matching. This work highlights the feasibility of using screen-printed Ag–Cu films to achieve mechanically robust and electromagnetically stable RF filters, offering a cost-effective and manufacturable solution for next-generation deformable wireless systems.
The increasing adoption of wearable technologies and flexible displays has intensified exposure to short-wavelength blue light and ultraviolet (UV) radiation, raising concerns regarding visual comfort and long-term ocular safety. However, despite growing awareness of the phototoxic effects associated with high-energy blue light (400-450 nm) and UV emissions, most optical filters used in flexible systems are still adapted from rigid multilayer structures originally developed for conventional displays. Their limited mechanical compliance, complex fabrication requirements, and difficulty conforming to curved or deformable surfaces make them poorly suited for emerging soft electronic platforms. In this study, a graphene quantum dot (GQD)-based optical filtering layer designed specifically for flexible and printed systems is presented. The GQD films deposited on Polyethylene Terephthalate (PET) exhibit strong suppression of UV and harmful blue wavelengths, attenuating 94.3% at 415 nm and absorbing more than 74% across the 445-515 nm region. In addition to passive filtering, the films down-convert absorbed UV energy into visible emission, enabling enhanced optical functionality. To demonstrate this dual behavior, the proposed filter was integrated into a simple UV-sensing configuration using a CuSO 4 photoluminescent solution and was compared with a commercial 515 nm long-pass filter. The GQD layer demonstrated both efficient UV blocking and enhanced emission behavior, validating its capability as a combined spectral filter and photoluminescent conversion layer. Overall, the mechanical flexibility, solution-process compatibility, and optical conversion properties of the GQD films highlight their potential for applications including flexible UV dosimeters, wearable optical indicators, and protective photonic layers for next-generation soft electronics.
Abstract Internet of things (IoT) systems rely on the broad deployment of electronic devices and sensors in diverse environments. In some applications, including agriculture, packaging, and medical, there is a need for devices with relatively short lifetimes. Thus, there is growing interest in bio-based substrates for printed electronics and sensors that improve sustainability and minimize the environmental impact of IoT devices. Paper is a natural choice as a substrate for disposable printed devices; however, the high surface roughness and porosity of typical papers lead to printed structures with high variability. Here, we report a nanocellulose-infiltrated paper consisting of a cellulose nanofibrils (CNFs) film supported on a cardstock substrate. The CNF solution was coated onto the cardstock and then press-dried to form a smooth and dense surface. The CNF partially infiltrates the substrate, but forms a distinct film on the surface. The CNF coating process reduced the root mean square surface roughness of the cardstock from 4.3 µ m to 189 nm. This improvement in surface properties enables the screen-printing of silver patterns with geometric uniformity comparable to that of patterns printed on conventional polyimide substrates. The moisture sensitivity of these cellulose-based substrates can be exploited for moisture sensing, and the moisture absorption/desorption and resulting change in relative permittivity of these substrates are characterized.
Abstract Gravure printing is a two-dimensional patterning, high-precision, high-speed, and roll-to-roll large-area fabrication method that is expected to directly and rapidly fabricate solar cell modules with high geometric fill factor without the need for laser scribing, thus significantly reducing the fabrication cost of solar cells. Furthermore, gravure printing can produce solar cells of different shapes and colors, further expanding their application range. Here, we review the research progress of gravure printing solar cells. First, we introduce the basic process of gravure printing, key issues in gravure printing solar cells, and the rheological properties of the inks. Then, we summarize the research progress of gravure printing organic solar cells (OSCs). The efficiency of gravure printing OSCs has reached a level comparable to spin-coating. Patterned OSC modules have been fabricated using roll-to-roll gravure printing. Finally, we review the research progress of gravure printing perovskite solar cells (PSCs). High-efficiency PSC cells have been fabricated using roll-to-roll gravure printing.
Abstract This roadmap presents key advances and future directions in the field of the conducting polymer polyethylene dioxythiophene (PEDOT), with a focus on its role as multifunctional material in emerging electronic applications. It covers a broad spectrum of topics, including self-healing materials, plant bioelectronics, energy harvesting and storage, and neural interfaces. The roadmap also addresses recent progress in the synthesis and functionalization of PEDOT derivatives, as well as insights from molecular dynamics simulations and transport physics. Emphasis is placed on a range of processing techniques, such as electropolymerization, vapor phase polymerization, and 3D printing, and their implications for device performance. Specific applications explored include thermoelectrics, biosensors, organic electrochemical transistors, and advanced bioelectronic platforms for neuromodulation, stretchable electronics, and neuromorphic computing. This collective effort aims to serve as a comprehensive resource outlining the current state of PEDOT research and identifying challenges and opportunities for its integration across diverse electronic technologies.
Abstract Van der Waals (vdW) materials on paper are promising candidates for low-cost, flexible optoelectronics, combining the materials’ optical and electronic properties with paper’s mechanical compliance and biodegradability. Established dry abrasion methods enable the deposition of continuous vdW films by rubbing their crystal traces across the rough surface of paper. However, this method is limited to resolutions> 100 µ m, reducing their potential for high-density integration (devices/cm 2 ) and improving device performance with shorter channels. Therefore, new mass-scalable production routes that mimic conventional lithographic techniques are needed for reproducible vdW on paper with enhanced feature resolution. In this work, we demonstrate a scalable, latex-based approach to realize controlled abrasion and patterning of vdW materials on a paper substrate. Our method combines the benefits of improved feature definition, maintaining low cost and excellent area scalability. To highlight the simplicity of our method, we fabricate a flexible broadband photodetector using graphite only. This approach is not only very general and applicable to various substrates and vdW materials, but it also paves the way for advancing lower-cost, higher-performance paper-based technologies.
The development of flexible and neuromorphic electronic devices relies on materials and systems that can sustain high performance despite mechanical deformation, fatigue, and environmental stress. Inspired by the self-repair mechanisms of human skin, self-healing materials present a promising approach to improving the durability, reliability, and lifespan of next-generation electronics. This review offers a comprehensive overview of self-healing processes in artificial materials, comparing them to biological healing. It discusses the fundamentals of biological synapses and their significance in neuromorphic computing, and examines the intrinsic and extrinsic self-healing mechanisms used in electronic devices. Special emphasis is placed on designing and integrating self-healing functions into flexible transistors and memristors for neuromorphic applications, along with their role in creating resilient, adaptable, and stretchable flexible electronics. The key device requirements, current challenges, and future prospects for scalable manufacturing and material development are analyzed. By connecting biology-inspired strategies with flexible and neuromorphic electronics, self-healing technologies are set to significantly influence sustainable, reliable, and intelligent electronic systems across various fields.
Ultra-soft conductive elastomers are promising materials for wearable electronics, but achieving high conductivity without compromising mechanical compliance remains challenging. We present a hybrid sandwich architecture here, in which a spray-deposited silver nanowire (AgNW) network is embedded between two layers of carbon nanotube-incorporated bottlebrush elastomer (CNT/BBE). The CNT/BBE matrix provides mechanical softness and bulk electrical conductivity, while the AgNW interlayer introduces a tunable interfacial conduction pathway. By varying the number of AgNW spray-deposition cycles, we systematically control network density and examine its effect on electrical transport, tensile strain sensing, pressure response, dynamic cyclic behavior, and wearable motion detection. The AgNW interlayer produces deformation-mode-dependent performance: lower-density networks enhance bending sensitivity, intermediate AgNW coverage yields the highest apparent tensile sensitivity, and higher-density networks improve signal reproducibility during repeated motion. In contrast, pressure measurements reveal nonlinear responses for the sandwich architectures, indicating that the interlayer suppresses pressure-proportional through-thickness transduction relative to the CNT/BBE control. These results establish interfacial network engineering as a versatile strategy for tuning ultra-soft conductive elastomers and highlight the importance of matching conductive architecture to the target sensing mode.
Ag paste is a key electrode material for carbon nanotube (CNT) composite devices designed for next-generation human-conformable electronics. However, because Ag paste is typically used as supplied, its influence on contact resistance, which generally dominates the electrical characteristics of devices, remains poorly understood. Here, we demonstrate that contact resistance at the interface between Ag paste and CNT-polydimethylsiloxane (CNT-PDMS) composites can be significantly reduced by controlling their interfacial structure through the regulation of Ag particle diameter (DAg). The CNT-PDMS surface exhibits concavities formed between CNT bundles protruding from the surface. Using Ag paste with small DAg results in an interfacial structure with Ag particles penetrating the surface concavities, thereby increasing the number of Ag-CNT contact points within the concave regions and reducing contact resistance. The contact resistance obtained with DAg = 0.2 mu m is only 2.4% of that obtained with DAg = 5.0 mu m. Additionally, the power generation performance of flexible thermoelectric devices using Ag-paste/CNT-PDMS contacts more than doubles when DAg is reduced from 5.0 to 0.2 mu m. Overall, the introduction of low-resistance contacts by interfacial-structure engineering via Ag particle size can enhance the performance of flexible CNT-based composite devices using Ag paste.
This study evaluates the thermal performance of a hot-air drying chamber for roll-to-roll (R2R) flexible electronics using experiments and three-dimensional heat transfer simulations. A computational fluid dynamic model incorporating a moving reference frame for the polyethylene terephthalate film was developed in ANSYS Fluent (ANSYS Inc., USA) and validated against measured surface temperature data over film speeds ranging from 10 to 90 mm s-1, yielding errors below 10.4%. At 90 mm s-1, the baseline chamber with 6 mm distributor holes showed a time-averaged spatial coefficient of variation ( CV & horbar;s) of 3.87% and a temporal coefficient of variation ( CVt) of 1.84% in the transverse direction. Increasing the distributor hole diameter from 6 to 8 or 9 mm resulted in little to no improvement in transverse temperature uniformity, due to the reduced jet momentum and enhanced thermal stratification. In contrast, a multi-diameter configuration (6 mm bottom half, 7 mm middle quarter, and 8 mm top quarter) effectively compensated for hydrostatic pressure drop, ensuring balanced airflow, enhanced mixing, and fewer recirculation zones. This optimized design reduced CV & horbar;s to 2.18% and CVt to 1.71%, representing improvements of 43.67% and 7.07% relative to the baseline case. As a result, the chamber delivered more uniform and stable thermal conditions, particularly in the downstream region which critically governs the drying process. The proposed multi-diameter distributor design offers a simple and cost-effective retrofit that significantly enhances thermal uniformity, which is expected to reduce drying-related defects such as cracking, delamination, and poor adhesion in printed electronics. These results offer practical guidelines for optimizing thermal design in high-performance R2R drying systems.
In response to the application requirements of aerosol jet-printed (AJP) silver electrodes in flexible electronic devices, this study employs a combined experimental and simulation approach to investigate the influence of pre-sintering drying time on their electrical resistance performance. In the experiments, a single batch of silver electrode samples was subjected to a precisely controlled drying-time gradient ranging from 0 to 45 min. Subsequently, all samples were co-sintered, and their electrical resistance was characterized using the four-point probe method. For the simulation, a finite-element model based on porous-media two-phase flow theory was constructed to quantify the solvent-evaporation kinetics and the degree of drying during the process. The results show that under 50 degrees C drying conditions, the critical drying time for AJP silver electrodes is 35 min, at which point the electrodes reach a completely dry state. The resistance of the sintered electrodes decreases with prolonged drying time and enters a stable plateau after 35 min; insufficiently dried samples exhibit resistance several times higher than that of sufficiently dried samples. SEM characterization reveals that insufficient drying leads to a porous, cracked structure that disrupts the continuity of the conductive network, whereas sufficient drying yields a dense, uniform electrode with well-fused particles. The solvent-content variation predicted by the simulation aligns closely with the experimental resistance-stabilization point and the structural transition observed via SEM, validating the reliability of the model. This work identifies a key process parameter for AJP silver electrodes, elucidates the drying-structure-performance relationship, and provides a theoretical basis and data support for process optimization in flexible electronics manufacturing.
To address process instability in aerosol jet printing (AJP), this study proposes AeroOptima-DRL, a deep reinforcement learning-based closed-loop process optimization system. A data-driven virtual environment, termed NeuroPrintMapper, is developed to model the relationship between process parameters and printed line quality, enabling efficient agent training without extensive physical experiments. Based on this environment, a reinforcement learning agent is trained to autonomously generate optimal process parameters in real time. Experiments on silver nanoparticle ink deposition on polyimide substrates demonstrate that the proposed system achieves accurate line width control within a target range of 10-30 mu m, with a mean absolute deviation of 0.19 mu m and an average error rate of 1.09%. During a 5 h continuous printing test, the closed-loop system effectively suppresses process drift, limiting resistance fluctuations to 2.5 times the initial value, compared to approximately 30-fold fluctuations under open-loop control. These results verify the effectiveness of DRL-based closed-loop optimization for improving the stability and consistency of high-resolution AJP processes.
Long-term continuous electrophysiological monitoring is crucial for accurate health assessment. However, the practical implementation of flexible dry electrodes is frequently constrained by difficulty of simultaneously achieving high stretchability, stable skin adhesion, and sufficient breathability. This work presents a flexible electrode encapsulation strategy utilizing an in-situ electrospinning process integrated with direct-write transfer to fabricate embedded flexible composite electrodes. A composite of poly (3,4-ethylenedioxythiophene)/poly (styrene sulfonate) and silver nanowires was embedded within a poly (vinyl alcohol)-glycerol substrate, yielding epidermal electrodes exhibiting high conductivity, low Young's modulus, and superior stretchability. The electrode exhibits low impedance (18 Omega) and biomimetic mechanical properties (Young's modulus of 1.23 MPa), with minimal resistance variation prior to exceeding 80% strain. Significantly, an in-situ nanofiber encapsulation technique was introduced to address the poor skin adhesion characteristic of dry electrodes. This approach achieves a breathability rate 3.5 times higher than that of medical polyurethane tape encapsulation while maintaining effective waterproofing. A 12 h patch test on human skin revealed no irritation, confirming excellent biocompatibility. In practical use, the electrode enables stable acquisition of long-term electrocardiogram signals during continuous wear, offering a comfortable and user-friendly solution for wearable dry electrodes in prolonged electrophysiological monitoring.
Flexible platinum thin-film temperature sensors were designed and fabricated on polyimide substrates using a sputtered Ti/Pt bilayer to achieve high mechanical flexibility and thermal stability. The effects of film thickness (100-400 nm) and post-deposition annealing temperature (200 C-degrees-350 C-degrees) on the microstructure and electromechanical response were systematically investigated. Field-emission scanning electron microscopy revealed uniform grain coarsening at 300 C-degrees without microcrack formation, corresponding to enhanced bending resilience. Electrical characterization on flat and curved surfaces (12.7-25.4 mm radii) demonstrated that 100 nm films annealed at 300 C-degrees exhibited the lowest bending-induced resistance variation (< 0.7% under tensile and < 0.5% under compressive curvature). In contrast, thicker or improperly annealed films showed higher strain sensitivity due to residual stress and microstructural defects. The results establish an optimized process window that minimizes deformation-induced resistance drift while preserving temperature accuracy, offering practical design and fabrication guidelines for mechanically robust platinum-based flexible temperature sensors suitable for wearable, robotic, and aerospace applications.
The advancement of next-generation flexible electronics relies on the development of high-performance components that are lightweight, conformable, and compatible with scalable additive manufacturing processes. Here, we report a fully 3D-printed capacitive pressure sensor featuring a novel dielectric composite ink composed of nickel nanowires (NiNWs), hexagonal boron nitride (h-BN), and polyethylene oxide (PEO). This eco-friendly, water-based ink was engineered for direct ink writing, enabling the fabrication of printed planar capacitive sensors, with dimensions of 1.5 cm & times; 0.6 cm, comprised of interdigitated silver electrodes topped with composite dielectric layer on flexible, polyimide substrates. The composite dielectric ink included: h-BN, a 2D nanomaterial which served as a charge storage medium with low dielectric loss, and a PEO binder, which stabilized the nanosheets and provided to the necessary viscoelastic properties for print fidelity and structural stability. Adding a small concentration of NiNWs to the hBN/PEO composite enhanced the dielectric properties due to an increase in interfacial polarization. The printed capacitive sensors incorporating an hBN/PEO/NiNWs dielectric layer exhibited reliable pressure-responsive behavior across a wide sensing range (0-466 kPa), with sensitivities of 0.024 and 0.008 kPa-1 in low-medium, and high-pressure regimes, respectively; these sensitivities were 7-8 times higher than for similar devices with hBN/PEO dielectric layers without NiNWs. Sensors demonstrated fast response times, low hysteresis, mechanical durability over cyclic loading, and high signal fidelity under dynamic pressure. Furthermore, the sensors enabled real-time monitoring of physiological signals including pulse, voice, and motion, underscoring their applicability in wearable health diagnostics and soft human-machine interfaces.
The presence of both metallic (M-) and semiconducting (S-) single-walled carbon nanotubes (SWCNTs) in as-synthesized samples severely limits their application in flexible electronics, where the efficient separation of large-diameter (>1.2 nm) semiconducting tubes and the preparation of low-temperature-compatible inks pose key challenges. This study proposes a fully integrated 'material -> ink -> device' solution. First, by optimizing the NaOH concentration (0.3 mmol l(-1)), the coating state of sodium dodecyl sulfate (SDS) was modulated, enhancing the separation selectivity of large-diameter S-SWCNTs via gel chromatography (GC). Second, a 'filtration -> washing -> exchange' process was designed to successfully transfer the S-SWCNTs into an organic ink based on a low-decomposition-temperature surfactant (poly(propylene carbonate), PPC), making it compatible with low-temperature aerosol-jet printing. Finally, a fully printed Ag-SWCNT-Pt Schottky diode was realized using this ink. The device exhibits excellent electrical performance: a rectification ratio of up to 10(4), a subthreshold swing of 65-68 mV dec(-1), and a Schottky barrier height of approximately 0.38 eV. This work provides a feasible material foundation and process route for high-purity separation, low-temperature printing, and high-performance flexible electronic integration of large-diameter S-SWCNTs.
Screen-printed electrode (SPE)-based electroanalysis is widely employed in the development of wearable sensing devices due to its multiple advantages in probing interfacial phenomena in electrochemical systems. SPEs, as core elements of electrochemical biosensors as a result of their cost-effectiveness, versatility, and ease of mass production, are gaining popularity due to their facile integration and embedment with wearable devices. In spite of the multiple technological solutions for enhancing their electrochemical performance, the SPE geometry remains insufficiently explored, although the shape, dimension, and spacing of their functional components are known to play critical roles in influencing the accuracy, sensitivity, and reproducibility of electroanalytical results. Thus, our paper investigates the impact of six configurations related to various geometrical parameters of SPEs on the electrochemical performance, evaluated through three electrochemical techniques: cyclic voltammetry, differential pulse voltammetry, and electrochemical impedance spectroscopy. The experimental study focuses on emphasizing the effects of a tailored design on the charge transfer resistance, double-layer capacitance, and diffusion pathways. Significant electrochemical performance improvements were noted when optimizing the counter-electrode-working electrode distance and the width of the counter-electrode coupled with Na2CO3 base activation.
The large-scale production of flexible electronics requires cost-effective, high-precision printing processes and optimized conductive inks. Screen printing with carbon-based formulations offers uniformity, scalability, and low production costs; however, achieving the required balance between rheology, conductivity, and mechanical stability remains challenging. This work presents a formulation strategy for graphite-graphene hybrid ink designed for screen printing on PET substrates. We systematically investigate the effects of graphite particle size, graphene content, and a binary solvent system on rheological behavior, print fidelity, and electrical performance. The optimized ink-comprising 200-mesh graphite, 2 wt% graphene, and a 47:13 wt% mixture of cyclohexanone and ethyl acetate-exhibits pronounced shear-thinning (viscosity drop from 318.72 Pa.s to 8.16 Pa.s between 0.1 and 500 s-1), high thixotropic recovery (99.11%), strong adhesion (ASTM D3359, 5B), and a low resistivity of (1.19 +/- 0.10) & times; 10-3 Omega.m at 14.20 +/- 2 & micro;m thickness. These features enable sharp pattern definition and mechanical robustness, with only 1.6% resistivity change after 300 bending cycles. The results highlight the synergistic role of particle size control, nanosheet incorporation, and solvent engineering in advancing carbon-based inks for scalable, high-performance flexible electronics.
