Nanotechnology-based flexible electronics

Flexible electronics have received considerable attention in academies and industries for their promising applications in enormous fields, such as flexible displays, wearable sensors, artificial skins, and flexible energy devices. Challenges remain in developing a flexible and scalable manufacturing method to facilitate the fabrication of multi-functional structures in a flexible electronic system. Nanoimprint lithography is a high resolution and low-cost approach to fabricate nanostructures over a large area. This paper reviews recent progress of nanoimprint lithography and its applications in flexible electronics. The basic principles, classification, research focus, and critical issues of nanoimprint lithography are elaborated. Then, the advantages of nanoimprint lithography are demonstrated in several typical applications related to flexible electronics, including conductive films, optoelectronic devices, flexible sensors, energy harvesting and storage devices, and bioinspired electronic devices. Finally, the challenges and perspectives of nanoimprint lithography in flexible electronic systems are discussed.

Flexible electronics and sensors are a key enabling element for the realization of wearables and geometry adaptive devices needed to follow current trends such as the Internet of things or Industry 4.0. Within this paper, we present a new and flexible packaging platform by the fabrication of an ultra-thin and highly flexible printed circuit board (PCB).The thermoplastic polymer Parylene, which combines a variety of unique properties such as optical transparency, biostability and biocompatibility according to ISO 10993, thermal stability as well as low permeability for gases and water, was used as the base for the new PCB.Particularly, its mechanical properties, especially the absence of intrinsic stresses, as well as its low Young’s modulus and good bendability are advantageous when using Parylene films as a free-standing substrate for flexible applications. The chemical inertness of Parylene against all common acids, bases and solvents ensures its compatibility with established standard microsystem technologies.For the realization of ultra-thin flexible PCBs, Parylene was used as a substrate as well as for the encapsulation and protection layer, respectively. Furthermore, Parylene was the dielectric between the metallic conductive layers. These redistribution layers (RDL) were deposited and patterned by standard microsystem technologies, such as sputtering, lithography and wet chemical etching. Hence, structure sizes of down to 10 µm were successfully realized. Different metals, such as gold, copper, and aluminum were tested for the RDL. Alternatively, printing technologies such as screen printing and aerosol jet printing were successfully demonstrated for the if conductive silver layerse. The electrical connection between the various RDL was of special interest throughout the study and was realized by etching vertical vias through the Parylene using oxygen plasma. Due to the low total thickness of the flexible Parylene PCB of 20 µm or even less, the obtained vias through the Parylene feature an advantageous low aspect ratio. For the contact formation through the vias, different methods were investigated, e. g. sputtering, printing and electrochemical deposition.Finally, the fabricated flexible Parylene PCB were characterized with respect to their bendability and electrical properties. Doing so, the resistivity and capacitance were measured as well as the frequency response between 20 kHz and 500 MHz. Particularly, for frequencies below 1 MHz the realized flexible Parylene PCBs show a good electrical performance.A third function of Parylene with respect to the realization of an ultra-thin and flexible PCB is its usage as a barrier or capping layer. Doing so, new approaches such as embedding electronic or sensor components in the flexible PCB can be enabled. Other integration technologies such as wire bonding, printing or gluing were investigated with respect to their usability for the new flexible PCB as well. Besides, also the direct fabrication of sensors on the Parylene flexible PCB was successfully demonstrated by the realization of a flexible potentiometric pH sensor.The presented new type of ultra-thin PCB is a versatile platform for the realization of flexible electronics, sensors and devices. Due to its biocompatible properties, the flexible Parylene PCB can be used for medical applications but also for the integration of MEMS, chemical sensors and optical components, respectively. Doing so, it enables the realization of smart systems for wearable applications and their integration into light weight constructions. Within this paper, the concept of this new approach, the respected fabrication and integration technologies as well as possible applications are presented in detail. Figure 1

Flexible tactile sensors play important roles in many areas, like human-machine interface, robotic manipulation, and biomedicine. However, their flexible form factor poses challenges in their integration with wafer-based devices, commercial chips, or circuit boards. Here, we introduce manufacturing approaches, device designs, integration strategies, and biomedical applications of a set of flexible, modular tactile sensors, which overcome the above challenges and achieve cooperation with commercial electronics. The sensors exploit lithographically defined thin wires of metal or alloy as the sensing elements. Arranging these elements across three-dimensional space enables accurate, hysteresis-free, and decoupled measurements of temperature, normal force, and shear force. Assembly of such sensors on flexible printed circuit boards together with commercial electronics forms various flexible electronic systems with capabilities in wireless measurements at the skin interface, continuous monitoring of biomechanical signals, and spatial mapping of tactile information. The flexible, modular tactile sensors expand the portfolio of functional components in both microelectronics and macroelectronics.

Variation in human brains creates difficulty in implementing electroencephalography into universal brain–machine interfaces. Conventional electroencephalography systems typically suffer from motion artefacts, extensive preparation time and bulky equipment, while existing electroencephalography classification methods require training on a per-subject or per-session basis. Here, we introduce a fully portable, wireless, flexible scalp electronic system, incorporating a set of dry electrodes and a flexible membrane circuit. Time-domain analysis using convolutional neural networks allows for accurate, real-time classification of steady-state visually evoked potentials in the occipital lobe. Compared to commercial systems, the flexible electronics show the improved performance in detection of evoked potentials due to significant reduction of noise and electromagnetic interference. The two-channel scalp electronic system achieves a high information transfer rate (122.1 ± 3.53 bits per minute) with six human subjects, allowing for wireless, real-time, universal electroencephalography classification for an electric wheelchair, a motorized vehicle and a keyboard-less presentation. Brain–machine interfaces using steady-state visually evoked potentials (SSVEPs) show promise in therapeutic applications. With a combination of innovations in flexible and soft electronics and in deep learning approaches to classify potentials from two channels and from any subject, a compact, wireless and universal SSVEP interface is designed. Subjects can operate a wheelchair in real time with eye movements while wearing the new brain–machine interface.

How is flexible electronics advancing neuroscience research?

Flexible electronics have inspired many novel and very important applications in recent years and various flexible electronic devices such as diodes, transistors, circuits, sensors, and radiofrequency (RF) passive devices including antennas and inductors have been reported. However, the lack of a high-performance RF resonator is one of the key bottlenecks to implement flexible wireless electronics. In this study, for the first time, a novel ultra-flexible structured film bulk acoustic resonator (FBAR) is proposed. The flexible FBAR is fabricated on a flexible polyimide substrate using piezoelectric thin film aluminum nitride (AlN) for acoustic wave excitation. Both the shear wave and longitudinal wave can be excited under the surface interdigital electrodes configuration we proposed. In the case of the thickness extension mode, a flexible resonator with a working frequency as high as of 5.2325 GHz has been realized. The resonators stay fully functional under bending status and after repeated bending and re-flattening operations. This flexible high-frequency resonator will serve as a key building block for the future flexible wireless electronics, greatly expanding the application scope of flexible electronics.

Printed electronics have gained increased interest in recent years, and printed sensors, which promise mechanically flexible multifunctional electronics over large areas, are raising remarkable attention. Numerous low cost printing technologies have been developed on different flexible/stretchable substrates including textiles. This paper presents a brief summary of various passive sensors (humidity, temperature, and pH) printed on diverse flexible substrates from an integrated circuits and systems' design standpoint and for use in flexible hybrid-printed electronics systems. A description of possible application domain for flexible sensors is provided.

Flexible Hybrid Electronics combine the best characteristics of printed electronics and silicon ICs to create high performance, ultra-thin, physically flexible systems. New static and dynamic tests are being developed to evaluate the performance of these systems. Dynamic radius of curvature and torsional test results are presented for a flexible hybrid electronics system with a FleX Silicon-on-Polymer operational amplifier manufactured in an 180nm CMOS process with 4-levels of metal interconnect mounted on a PET substrate.

The flexible wearable electronic systems have great potential in medical treatment, electronic skin, energy storage and other applications. A successful flexible wearable electronic system not only has to fulfill its functionality and adapt to the complex environments of human skin, but also needs to consider the comfortability during wearing. However, the normal structured flexible wearable systems would always make discomforts to human skin, owing to relatively large local stress caused by these rigid devices. The island-bridge structures with raised islands have proven advantages in the improvement of stretchability and alleviation of skin’s local stress. To further exploit this advantage, the influence of the geometric parameters (e. g. shape, size and thickness) on stretchability and comfortability have been analyzed, thereby providing guidance to the further optimization. Both the simulation and experiment results indicate that smaller size and larger thickness of the raised islands can decrease the surface deformation of the raised island and reduce the mismatch between rigid devices and largely-deformed human skin. This finding is exemplified by optimizing the geometric parameters on an island-bridge structured temperature detection patch. The testing results show that the optimized island-bridge structure is available to reduce the skin stress and improve the comfort of the human skin compared with the normal structure. This work provides design guidelines for the island-bridge structured flexible wearable electronic systems, which can substantially improve the comfortability and stretchability.

Stretchable electronic materials and devices have important applications in flexible electronic systems including wearable electronics and bioelectronics. Convenient electricity generation such as thermoelectric conversion is required for the flexible electronic systems. Hence, it is development of high‐performance thermoelectric materials with high mechanical stretchability would be highly desirable. Here, stretchable and transparent ionogels with high thermoelectric properties are demonstrated. The ionogels made of elastomeric waterborne polyurethane and 1‐ethyl‐3‐methylimidazolium dicyanamide (EMIM:DCA, an ionic liquid) are prepared by solution processing. Their mechanical and electrical properties depend on the loading of EMIM:DCA. The ionogels with 40 wt% EMIM:DCA can have a high mechanical stretchability of up to 156%, low tensile strength of 0.6 MPa, and low Young's modulus of 0.6 MPa. They also exhibit a high ionic thermovoltage of 34.5 mV K−1, high ionic conductivity of 8.4 mS cm−1 and low thermal conductivity of 0.23 W m−1 K−1 at a relative humidity of 90%. As a result, it can have a high ionic figure of merit (ZTi) of 1.3 ± 0.2. Both the thermovoltage and the ZTi value are the highest for stretchable thermoelectric materials. They can be used in ionic thermoelectric capacitors to convert heat into electricity.

Conductive hydrogels (CHs) have emerged as a promising class of multifunctional materials that integrate the softness, stretchability, and high water content of traditional hydrogels with the electrical conductivity essential for modern electronic applications. Their unique ability to conform to dynamic biological interfaces while maintaining stable electrochemical performance makes them ideal candidates for next‐generation flexible and wearable devices. This review provides a comprehensive and critical analysis of the recent progress in CH development, focusing on the design strategies, synthesis routes, and structural innovations that enhance their electrical, mechanical, and environmental performance. Various types of conductive elements, including metallic nanostructures, carbon‐based nanomaterials, ionic species, and intrinsically conducting polymers are discussed in the context of hybrid network formation and charge transport mechanisms. Special attention is given to the multifunctional properties of CHs, such as antifreezing behavior, self‐healing capability, tissue adhesion, and long‐term stability, which are pivotal for real‐world deployment. Application‐driven sections explore the role of CHs in wearable strain sensors, energy storage systems, and triboelectric nanogenerators, highlighting state‐of‐the‐art device performance and integration strategies. Finally, current challenges and future perspectives are outlined, emphasizing the need for scalable fabrication methods, sustainable material systems, and intelligent functionalities. This review aims to guide interdisciplinary efforts in advancing CHs as transformative materials for soft electronics, bio‐integrated technologies, and adaptive human–machine interfaces.

Predicting the reliability is an important task of product life cycle analysis, especially during the product development stages. The uncertainty in the operating conditions and the presence of multistress factors makes this reliability prediction more difficult. In this work, a probabilistic reliability prediction framework is proposed using the linear damage accumulation and degradation modeling for multistress conditions. The multistress-life (S-L) curve for corresponding multistress has been developed using the equivalent stress. The multi S-L model allows extrapolating the expected life product under given operating or test conditions as well as provides input to estimate the reliability as a function of time for both a single multistress factor level and a sequence of multistress-factor levels. The proposed methodology has been demonstrated with newly developed flexible hybrid electronics products.

Depletion of energy sources and need for efficient, cost-effective materials for flexible electronics is on demand and has led to the search for alternative materials that could be used in energy storage systems and devices for flexible electronics. The discovery of two-dimensional nanomaterials graphene and MXene with their flexible properties has led to the development of composite materials which could be used in energy storage devices and flexible electronic products like wearable devices, displays, and touch panels. This article reviews the process of synthesis of MXene, properties of graphene and MXene. Also, a review of the applications of graphene and MXene in flexible and transparent electronics and how these emerging popular nanomaterials could be used in energy storage systems has been presented.

Influence of interlayers on the interfacial behavior of Ag films on polymer substrates

Since the first design of tactile sensors was proposed by Harmon in 1982, tactile sensors have evolved through four key phases: industrial applications (1980s, basic pressure detection), miniaturization via MEMS (1990s), flexible electronics (2010s, stretchable materials), and intelligent systems (2020s-present, AI-driven multimodal sensing). With the innovation of material, processing techniques, and multimodal fusion of stimuli, the application of tactile sensors has been continuously expanding to a diversity of areas, including but not limited to medical care, aerospace, sports and intelligent robots. Currently, researchers are dedicated to develop tactile sensors with emerging mechanisms and structures, pursuing high-sensitivity, high-resolution, and multimodal characteristics and further constructing tactile systems which imitate and approach the performance of human organs. However, challenges in the combination between the theoretical research and the practical applications are still significant. There is a lack of comprehensive understanding in the state of the art of such knowledge transferring from academic work to technical products. Scaled-up production of laboratory materials faces fatal challenges like high costs, small scale, and inconsistent quality. Ambient factors, such as temperature, humidity, and electromagnetic interference, also impair signal reliability. Moreover, tactile sensors must operate across a wide pressure range (0.1 kPa to several or even dozens of MPa) to meet diverse application needs. Meanwhile, the existing algorithms, data models and sensing systems commonly reveal insufficient precision as well as undesired robustness in data processing, and there is a realistic gap between the designed and the demanded system response speed. In this review, oriented by the design requirements of intelligent tactile sensing systems, we summarize the common sensing mechanisms, inspired structures, key performance, and optimizing strategies, followed by a brief overview of the recent advances in the perspectives of system integration and algorithm implementation, and the possible roadmap of future development of tactile sensors, providing a forward-looking as well as critical discussions in the future industrial applications of flexible tactile sensors.