Abstract Graphene-based inks have emerged as promising functional materials for printed and flexible electronics because of their electrical, mechanical, and chemical properties, together with their compatibility with solution-based processing. Among the various graphene-based materials, graphene oxide (GO), reduced graphene oxide (rGO), graphene flakes, and graphene-based composites are particularly attractive for ink formulations due to their tunable dispersibility, electrical conductivity, and processability. This review discusses graphene-based inks from the perspective of material selection, ink formulation, and printability, with a focus on flexible sensor applications. First, the key graphene derivatives used in printable inks are outlined, followed by a discussion of their electrical, optical, thermal, and mechanical properties in the context of printed films. The major synthesis routes, including top-down and bottom-up approaches, are then compared with an emphasis on scalability and compatibility with ink-based processing. Particular attention is given to ink formulation strategies, including solvent selection, binder and additive incorporation, rheological control, dispersion stability, and post-print processing such as thermal annealing, chemical reduction, and photonic treatment. The role of these factors in determining print quality, interflake charge transport, and device performance is highlighted. Representative applications in flexible sensing, including biosensors, gas, humidity, strain, and pressure sensors, are also reviewed. Finally, current challenges related to dispersion stability, conductivity–printability trade-offs, reproducibility, and long-term device reliability are discussed, together with future directions involving hybrid inks, predictive rheological modelling, and sustainable processing routes. Overall, graphene-based inks offer a versatile platform for the development of printed and wearable sensors, with further advances in formulation, processing, and scale-up needed for broader technological adoption.
Abstract Amorphous indium gallium zinc oxide (IGZO) based flexible and robust chemiresistive NO 2 gas sensors prepared on a polyimide substrate are presented in this work. A different target composition of IGZO (In:Ga:Zn ratio of 1:1:5) than the usually reported In:Ga:Zn:1:1:1 composition for gas sensing is explored in the present study. IGZO (115) composition has been reported for transparent thin film devices and circuits previously. In fact, this is the first study to explore this particular composition for the fabrication of flexible gas sensors. The devices are semi-transparent, adding another functionality that can be useful for see-through devices. Therefore, this study can open new paths for transparent, flexible, and fully integrated all IGZO based devices and circuits. The sensor gives an excellent response to sub-ppm exposures of NO 2 gas. A 262% increase in the resistance of the device is observed for 1 ppm NO 2 exposure at 125 °C which is amongst the best responses reported for metal oxide based flexible NO 2 gas sensors. The sensor device shows a highly linear response ( R 2 = 0.9928) to sub-ppm NO 2 concentrations with an experimentally observable limit of detection reaching 100 ppb. Furthermore, the devices are robust in the sense that they can be subjected to a large number of bending cycles and multiple squeegee cycles and can still retain good responsivity. Squeegeeing is one of the most commonly used methods for sticking tapes and stickers on flat surfaces. This is the first report where a gas sensor has been subjected to squeegee cycles to demonstrate its capabilities to withstand extreme mechanical forces. The proposed devices have potential to be used as sticker type gas sensing devices as their capability to withstand bending can allow rolling of sensor arrays for storage and transportation while their capability to withstand squeegeeing can be useful during sticking the devices.
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 S 11 < −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 −1 at 0.1 W input power, well below the EU limit of 2.0 W kg −1 . 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.
