Rational Design of Flexible and Stretchable Electronics based on 3D Printing

Flexible and stretchable electronics have been considered as the key component for the next generation of flexible devices. There are many approaches to prepare the devices, such as dip coating, spin coating, Mayer bar coating, filtration and transfer, and printing, etc. The effectiveness of these methods has been proven, but some drawbacks cannot be ignored, such as lacking pattern control, labor consuming, requiring complex pretreatment, wasting conductive materials, etc. In this investigation, we propose to adopt 3D printing technology to design flexible and stretchable electronics. The objective is to rationally design flexible and stretchable sensors, simplify the preparation process, form the sample with the complex desirable patterns, and promote the performance of the samples. The dissertation comprises of three major parts: water-induced polymer swelling and its application in soft electronics, utilizing 3D printing to transfer conductive layer into elastomer for building soft electronics, and 3D printing of functional devices. In the first part, we developed the soft electronics with wrinkled structure via 3D printing and water-induced polymer swelling, which can avoid some disadvantages in conventional method, e.g., pre-stretching and organic solvent-induced polymer swelling, including mechanical loss, negative effect to human health, and unidirectionally response to external deformation. Water-induced polymer swelling was achieved by introducing soluble particles into silicone matrixes and soaking the polymer composites in aqueous solution. We have investigated the characteristics and mechanisms of water-induced polymer swelling. Then, the conductive materials were deposited on the swollen sample to form the desired wrinkled structures for stretchable sensors. Furthermore, a dopamine layer was adopted to enhance the adhesion of matrix and conductive layer. The improvement was a key enabler to achieve superior electrical properties of 3D printed stretchable sensors for long-term cyclic stretching. We have demonstrated a series of human motion detection by using these stretchable strain sensors. Another part is designing flexible electrodes with desirable complex pattern by transferring a conductive layer into soft substrates during a 3D printing process. Taking advantage of extrusion pressure and polymer adhesion, the thin conductive layers were embedded into the printed polymer patterns, which can achieve conductive flexible electronics with desirable complex patterns. High-quality transfer has been achieved through adjusting conductive layer thickness, nozzle-to-substrate distance, and printing parameters, etc. Moreover, various printing patterns were created, and their properties were exhibited. The stretchable sensors showed an outstanding stress-strain relationship and electrical response to external deformations. The third part is about 3D printing of functional devices. In the collaborated study, the drug particles were introduced into silicone matrix to prepare the drug-eluting devices. When water molecules transported into the silicone matrix, the loaded drug particles decomposed and released nitric oxide (NO) enabling antibacterial properties. It is noted that 3D printing is creatively employed to form the desirable patterns. We also observed a self-wiring effect in the printing process, i.e., the printed device is covered by a drug-free layer due to the diffusion of a low viscosity silicone component during printing, which can be utilized to prevent drug release bursts and to form a gradient drug-loaded device. The printed samples showed a sustainable NO release and good antibacterial property. Furthermore, the water-induced polymer swelling was possible to be used as actuator in humidity environment. There are some highlights deserving emphasis in the dissertation. Firstly, the water-induced polymer swelling is proposed to develop the flexible and stretchable electronics. The findings have a wide potential application. Additionally, a drug-eluting polymer device with a drug-loaded bulk and a drug-free coating is prepared via leveraging self-wiring effect in 3D printing. The structure can regulate the drug release rate. On the other hand, the additive manufacturing platform offers unique opportunities to produce drug-eluting silicone devices in a customized manner. Finally, 3D printing is employed to encapsulate the conductive layers to achieve the flexible electronics with patterned structure and high performances. The facile and effective approach provides a distinctive view in advancing the development of stretchable electronics

Flexible and Stretchable Electronics

Flexible and stretchable electronics are receiving tremendous attention as future electronics due to their flexibility and light weight, especially as applications in wearable electronics. Flexible electronics are usually fabricated on heat sensitive flexible substrates such as plastic, fabric or even paper, while stretchable electronics are usually fabricated from an elastomeric substrate to survive large deformation in their practical application. Therefore, successful fabrication of flexible electronics needs low temperature processable novel materials and a particular processing development because traditional materials and processes are not compatible with flexible/stretchable electronics. Huge technical challenges and opportunities surround these dramatic changes from the perspective of new material design and processing, new fabrication techniques, large deformation mechanics, new application development and so on. Here, we invited talented researchers to join us in this new vital field that holds the potential to reshape our future life, by contributing their words of wisdom from their particular perspective

Scalable Microfabrication Procedures for Adhesive-Integrated Flexible and Stretchable Electronic Sensors.

New classes of ultrathin flexible and stretchable devices have changed the way modern electronics are designed to interact with their target systems. Though more and more novel technologies surface and steer the way we think about future electronics, there exists an unmet need in regards to optimizing the fabrication procedures for these devices so that large-scale industrial translation is realistic. This article presents an unconventional approach for facile microfabrication and processing of adhesive-peeled (AP) flexible sensors. By assembling AP sensors on a weakly-adhering substrate in an inverted fashion, we demonstrate a procedure with 50% reduced end-to-end processing time that achieves greater levels of fabrication yield. The methodology is used to demonstrate the fabrication of electrical and mechanical flexible and stretchable AP sensors that are peeled-off their carrier substrates by consumer adhesives. In using this approach, we outline the manner by which adhesion is maintained and buckling is reduced for gold film processing on polydimethylsiloxane substrates. In addition, we demonstrate the compatibility of our methodology with large-scale post-processing using a roll-to-roll approach

Design of Electrode Materials for Stretchable Triboelectric Nanogenerators

Triboelectric nanogenerator (TENG), a recently emerging technology that is based on the combination of triboelectric effect and electrostatic induction, has been found to be a promising strategy to harvest large amount of underutilized and low-frequency mechanical energy. One major challenge for TENGs is that the practical application requires flexible, deformable, multifunctional materials to ensure its favorable accommodation to arbitrary surfaces or moving object or harsh environment. Recent research interests mainly focus on the design and fabrication of electrode materials for TENG, making it a perfect candidate for wearable power source. In this chapter, we will introduce a couple of recent achievements regarding highly flexible/deformable TENGs based on stretchable electrodes, including geometrically designed electrode, mixture of conductive materials with elastomeric materials and intrinsically stretchable electrode, etc. In addition, we will address stretchable and self-healing electrodes of flexible TENGs for potential wearable and implantable electronics

Selective crack formation on stretchable silver nano-particle based thin films for customized and integrated strain-sensing system

Wrinkle structure effectively suppresses the crack formation in a material under tensile strain, thus enabling intrinsically brittle material to be strain-tolerant. If the brittle nature of the material can be selectively engineered on the wrinkle structure, combination of mechanically different structures can be obtained within the same material. In this work, we introduce a simple and facile approach to effectively turn strain-tolerant conductive metal thin film with wrinkle structure into crack-rich thin film. The intended phenomenon was controlled by additionally inkjet-printing Ag thin film onto the wrinkle structure, and the structure and performance were also systematically optimized to enhance sensitivity and stability of the sensor. Our strategy to induce intended cracks on corrugated metal thin film enables not only to stably combine stretchable interconnectors and strain sensors but also to fabricate an integrated strain sensor system that can be custom-tailored to the subject's hand size. We believe the facile strategy will be a good step to realize an integrated strain sensor system.N

Development of Multifunctional E-skin Sensors

Electronic skin (e-skin) is a hot topic due to its enormous potential for health monitoring, functional prosthesis, robotics, and human-machine-interfaces (HMI). For these applications, pressure and temperature sensors and energy harvesters are essential. Their performance may be tuned by their films micro-structuring, either through expensive and time-consuming photolithography techniques or low-cost yet low-tunability approaches. This PhD thesis aimed to introduce and explore a new micro-structuring technique to the field of e-skin – laser engraving – to produce multifunctional e-skin devices able to sense pressure and temperature while being self-powered. This technique was employed to produce moulds for soft lithography, in a low-cost, fast, and highly customizable way. Several parameters of the technique were studied to evaluate their impact in the performance of the devices, such as moulds materials, laser power and speed, and design variables. Amongst the piezoresistive sensors produced, sensors suitable for blood pressure wave detection at the wrist [sensitivity of – 3.2 kPa-1 below 119 Pa, limit of detection (LOD) of 15 Pa], general health monitoring (sensitivity of 4.5 kPa-1 below 10 kPa, relaxation time of 1.4 ms, micro-structured film thickness of only 133 µm), and robotics and functional prosthesis (sensitivity of – 6.4 × 10-3 kPa-1 between 1.2 kPa and 100 kPa, stable output over 27 500 cycles) were obtained. Temperature sensors with micro-cones were achieved with a temperature coefficient of resistance (TCR) of 2.3 %/°C. Energy harvesters based on micro-structured composites of polydimethylsiloxane (PDMS) and zinc tin oxide (ZnSnO3) nanowires (NWs; 120 V and 13 µA at > 100 N) or zinc oxide (ZnO) nanorods (NRs; 6 V at 2.3 N) were produced as well. The work described herein unveils the tremendous potential of the laser engraving technique to produce different e-skin devices with adjustable performance to suit distinct applications, with a high benefit/cost ratio

A sub-150-nanometre-thick and ultraconformable solution-processed all-organic transistor

Recent advancements in the field of electronics have paved the way to the development of new applications, such as tattoo electronics, where the employment of ultraconformable devices is required, typically achievable with a significant reduction in their total thickness. Organic materials can be considered enablers, owing to the possibility of depositing films with thicknesses at the nanometric scale, even from solution. However, available processes do not allow obtaining devices with thicknesses below hundreds of nanometres, thus setting a limit. Here, we show an all-organic field effect transistor that is less than 150 nm thick and that is fabricated through a fully solution-based approach. Such unprecedented thickness permits the device to conformally adhere onto nonplanar surfaces, such as human skin, and to be bent to a radius lower than 1 μm, thereby overcoming another limitation for field-effect transistors and representing a fundamental advancement in the field of ultrathin and tattoo electronics

Wearable Intrinsically Soft, Stretchable, Flexible Devices for Memories and Computing

A recent trend in the development of high mass consumption electron devices is towards electronic textiles (e-textiles), smart wearable devices, smart clothes, and flexible or printable electronics. Intrinsically soft, stretchable, flexible, Wearable Memories and Computing devices (WMCs) bring us closer to sci-fi scenarios, where future electronic systems are totally integrated in our everyday outfits and help us in achieving a higher comfort level, interacting for us with other digital devices such as smartphones and domotics, or with analog devices, such as our brain/peripheral nervous system. WMC will enable each of us to contribute to open and big data systems as individual nodes, providing real-time information about physical and environmental parameters (including air pollution monitoring, sound and light pollution, chemical or radioactive fallout alert, network availability, and so on). Furthermore, WMC could be directly connected to human brain and enable extremely fast operation and unprecedented interface complexity, directly mapping the continuous states available to biological systems. This review focuses on recent advances in nanotechnology and materials science and pays particular attention to any result and promising technology to enable intrinsically soft, stretchable, flexible WMC

Graphene-based flexible sensors towards electronic wearables

Flexible electronics and wearable devices have attracted considerable attention because they produce mechanical liberty, in terms of flexibility and stretchability that can enable the possibility of a wide range of new applications. The term “wearable electronics” can be used to define devices that can be worn or mated with the sensed surface to continuously monitor signals without limitations on mechanical deformability of the devices and electronic performance of the functional materials. The use of polymeric substrates or other nonconventional substrates as base materials brings novel functionalities to sensors and other electronic devices in terms of being flexible and light weight. Conductive nanomaterials, such as carbon nanotubes and graphene have been utilized as functional materials for flexible electronics and wearable devices. Graphene has specifically been considered for producing next-generation sensors due to its impressive electrical and mechanical properties and a result, incorporation of flexible substrates and graphene-based nanomaterials has been widely utilized to form versatile flexible sensors and other wearable devices through use of different fabrication processes. Creation of a large-scale, simple, high-resolution and cost-effective technique that overcomes fabrication limitations and supports production of flexible graphene-based sensors with high flexibility and stretch ability is highly demanding. Soft lithography can be merged with a mechanical exfoliation process using adhesive tape followed by transfer printing to form a graphene sensor on a desired final substrate. In situ microfluidic casting of graphene into channels is another promising platform driving the rapid development of flexible graphene sensors and wearable devices with a wide dynamic detection range. Selective coating of graphene-based nanomaterials (e.g. graphene oxide (GO)) on flexible electrode tapes can, because of its flexibility and adhesive features, be used to track relative humidity (RH) variations at the surface of target surfaces. This thesis describes the design and development of flexible and wearable strain, pressure and humidity sensors based on a novel tape-based cost-effective patterning and transferring technique, an in situ microfluidic casting method, and a novel selective coating technique for graphene-based nanomaterials. First of all, we present a tape-based graphene patterning and transferring approach to production of graphene sensors on elastomeric substrates and adhesive tapes. The method utilizes the work of adhesion at the interface between two contacting materials as determined by their surface energies to pattern graphene on PDMS substrate and transfer it onto a target tape. We have achieved patterning and transferring method with the features of high pattern spatial resolution, thickness control, and process simplicity with respect to functional materials and pattern geometries. We have demonstrated the usage of flexible graphene sensors on tape to realize interaction with structures, humans, and plants for real-time monitoring of important signals. Secondly, we present a helical spring-like piezo resistive graphene sensor formed within a microfluidic channel using a unique and easy in situ microfluidic casting method. Because of its helical shape, the sensor exhibits a wide dynamic detection range as well as mechanical flexibility and stretch ability. Finally, we present a flexible GO-based RH sensor on an adhesive polyimide thin film realized by selectively coating and patterning GO at the surface of Au Interdigitated electrodes (IDEs) and subsequently peeling the device from a temporary PDMS film. Real-time monitoring of the water movement inside the plant has been demonstrated by installing GO-based RH sensor at the surfaces of different plant leaves