High-density stretchable electronics: toward an integrated multilayer composite

E-Tattoos: Toward Functional but Imperceptible Interfacing with Human Skin

The human body continuously emits physiological and psychological information from head to toe. Wearable electronics capable of noninvasively and accurately digitizing this information without compromising user comfort or mobility have the potential to revolutionize telemedicine, mobile health, and both human-machine or human-metaverse interactions. However, state-of-the-art wearable electronics face limitations regarding wearability and functionality due to the mechanical incompatibility between conventional rigid, planar electronics and soft, curvy human skin surfaces. E-Tattoos, a unique type of wearable electronics, are defined by their ultrathin and skin-soft characteristics, which enable noninvasive and comfortable lamination on human skin surfaces without causing obstruction or even mechanical perception. This review article offers an exhaustive exploration of e-tattoos, accounting for their materials, structures, manufacturing processes, properties, functionalities, applications, and remaining challenges. We begin by summarizing the properties of human skin and their effects on signal transmission across the e-tattoo-skin interface. Following this is a discussion of the materials, structural designs, manufacturing, and skin attachment processes of e-tattoos. We classify e-tattoo functionalities into electrical, mechanical, optical, thermal, and chemical sensing, as well as wound healing and other treatments. After discussing energy harvesting and storage capabilities, we outline strategies for the system integration of wireless e-tattoos. In the end, we offer personal perspectives on the remaining challenges and future opportunities in the field.

The development of stretchable and self-repairing materials applied to electronic skin

Flexible electronic devices play a key role in the fields of flexible batteries, electronic skins, and flexible displays, which have attracted more and more attention in the past few years. Among them, the application areas of electronic skin in new energy, artificial intelligence, and other high-tech applications are increasing. Semiconductors are an indispensable part of electronic skin components. The design of semiconductor structure not only needs to maintain good carrier mobility, but also considers extensibility and self-healing capability, which is always a challenging work. Though flexible electronic devices are important for our daily life, the research on this topic is quite rare in the past few years. In this work, the recently published work regarding to stretchable semiconductors as well as self-healing conductors are reviewed. In addition, the current shortcomings, future challenges as well as an outlook of this technology are discussed. The final goal is to outline a theoretical framework for the design of high-performance flexible electronic devices that can at the same time address their commercialization challenges.

Solvent-Assisted Filling of Liquid Metal and Its Selective Dewetting for the Multilayered 3D Interconnect in Stretchable Electronics

As stretchable electronics are rapidly developing and becoming complex, the requirement for stretchable, multilayered, and large-area printed circuit boards (PCBs) is emerging. This demands a stretchable electrode and its vertical interconnect access (via) for 3-dimensional (3D) connectivity between layers. Here, we demonstrate solvent-assisted liquid metal (LM) filling into the submicrometer channel (∼400 nm), including via-hole filling and selective dewetting of LM. We provide the theoretical background of solvent-assisted LM filling and selective dewetting and reveal the osmotic pressure arising from anomalous mass transport phenomena, case II diffusion, which drives negative pressure, the spontaneous pulling of LM into the open channel. Also, we suggest design criteria for the geometry and dimension of LM interconnects to obtain structural stability without dewetting, based on the theoretical and computational background. We demonstrate a simple stretchable near-field communication (NFC) device including transferred micrometer-size light-emitting diodes (LEDs) with only 230 μm to the stretchable liquid metal PCB, without any soldering process. The device operates stably under repetitive stretching and releasing (∼50% uniaxial strain) due to the stable connection through the LM via between the upper and lower layers. Finally, we propose a concept for modular-type stretchable electronics, based on the cohesive liquid nature of LM. As a building block, the functional module can be easily removed from a mainframe, and replaced by another functional module, to suit user demand.

Permeable superelastic liquid-metal fibre mat enables biocompatible and monolithic stretchable electronics

Stretchable electronics find widespread uses in a variety of applications such as wearable electronics, on-skin electronics, soft robotics and bioelectronics. Stretchable electronic devices conventionally built with elastomeric thin films show a lack of permeability, which not only impedes wearing comfort and creates skin inflammation over long-term wearing but also limits the design form factors of device integration in the vertical direction. Here, we report a stretchable conductor that is fabricated by simply coating or printing liquid metal onto an electrospun elastomeric fibre mat. We call this stretchable conductor a liquid-metal fibre mat. Liquid metal hanging among the elastomeric fibres self-organizes into a laterally mesh-like and vertically buckled structure, which offers simultaneously high permeability, stretchability, conductivity and electrical stability. Furthermore, the liquid-metal fibre mat shows good biocompatibility and smart adaptiveness to omnidirectional stretching over 1,800% strain. We demonstrate the use of a liquid-metal fibre mat as a building block to realize highly permeable, multifunctional monolithic stretchable electronics.

A soft and stretchable bilayer electrode array with independent functional layers for the next generation of brain machine interfaces

OBJECTIVE

Brain-Machine Interfaces (BMIs) hold great promises for advancing neuroprosthetics, robotics, and for providing treatment options for severe neurological diseases. The objective of this work is the development and in vivo evaluation of electrodes for BMIs that meet the needs to record brain activity at sub-millimeter resolution over a large area of the cortex while being soft and electromechanically robust (i.e. stretchable).

APPROACH

Current electrodes require a trade-off between high spatiotemporal resolution and cortical coverage area. To address the needs for simultaneous high resolution and large cortical coverage, the prototype electrode array developed in this study employs a novel bilayer routing of soft and stretchable lead wires from the recording sites on the surface of the brain (electrocorticography, ECoG) to the data acquisition system.

MAIN RESULTS

To validate the recording characteristics, the array was implanted in healthy felines for up to 5 months. Neural signals recorded from both layers of the device showed elevated mid-frequency structures typical of local field potential (LFP) signals that were stable in amplitude over implant duration, and also exhibited consistent frequency-dependent modulation after anesthesia induction by Telazol.

SIGNIFICANCE

The successful development of a soft and stretchable large-area, high resolution micro ECoG electrode array (lahrμECoG) is an important step to meet the neurotechnological needs of advanced BMI applications.

Integrated multilayer stretchable printed circuit boards paving the way for deformable active matrix

Conventional rigid electronic systems use a number of metallization layers to route all necessary connections to and from isolated surface mount devices using well-established printed circuit board technology. In contrast, present solutions to prepare stretchable electronic systems are typically confined to a single stretchable metallization layer. Crossovers and vertical interconnect accesses remain challenging; consequently, no reliable stretchable printed circuit board (SPCB) method has established. This article reports an industry compatible SPCB manufacturing method that enables multilayer crossovers and vertical interconnect accesses to interconnect isolated devices within an elastomeric matrix. As a demonstration, a stretchable (260%) active matrix with integrated electronic and optoelectronic surface mount devices is shown that can deform reversibly into various 3D shapes including hemispherical, conical or pyramid.

Flexible piezoelectric ultrasonic energy harvester array for bio-implantable wireless generator

Ultrasonic driven wireless charging technology has recently attracted much attention in the next generation bio-implantable systems; however, most developed ultrasonic energy harvesters are bulky and rigid and cannot be applied to general complex surfaces. Here, a flexible piezoelectric ultrasonic energy harvester (PUEH) array was designed and fabricated by integrating a large number of piezoelectric active elements with multilayered flexible electrodes in an elastomer membrane. The developed flexible PUEH device can be driven by the ultrasonic wave to produce continuous voltage and current outputs on both planar and curved surfaces, reaching output signals of more than 2 Vpp and 4 μA, respectively. Potential applications of using the flexible PUEH to charge energy-storage devices and power commercial electronics were demonstrated. Its low attenuation performance was also evaluated using the in vitro test of transmitting power through pork tissue, demonstrating its potential use in the next generation of wirelessly powered bio-implantable micro-devices.

Materials and Structures toward Soft Electronics

Soft electronics are intensively studied as the integration of electronics with dynamic nonplanar surfaces has become necessary. Here, a discussion of the strategies in materials innovation and structural design to build soft electronic devices and systems is provided. For each strategy, the presentation focuses on the fundamental materials science and mechanics, and example device applications are highlighted where possible. Finally, perspectives on the key challenges and future directions of this field are presented.

Stretchable ultrasonic transducer arrays for three-dimensional imaging on complex surfaces

Ultrasonic imaging has been implemented as a powerful tool for noninvasive subsurface inspections of both structural and biological media. Current ultrasound probes are rigid and bulky and cannot readily image through nonplanar three-dimensional (3D) surfaces. However, imaging through these complicated surfaces is vital because stress concentrations at geometrical discontinuities render these surfaces highly prone to defects. This study reports a stretchable ultrasound probe that can conform to and detect nonplanar complex surfaces. The probe consists of a 10 × 10 array of piezoelectric transducers that exploit an "island-bridge" layout with multilayer electrodes, encapsulated by thin and compliant silicone elastomers. The stretchable probe shows excellent electromechanical coupling, minimal cross-talk, and more than 50% stretchability. Its performance is demonstrated by reconstructing defects in 3D space with high spatial resolution through flat, concave, and convex surfaces. The results hold great implications for applications of ultrasound that require imaging through complex surfaces.

Review of Flexible Temperature Sensing Networks for Wearable Physiological Monitoring

Physiological temperature varies temporally and spatially. Accurate and real-time detection of localized temperature changes in biological tissues regardless of large deformation is crucial to understand thermal principle of homeostasis, to assess sophisticated health conditions, and further to offer possibilities of building a smart healthcare and medical system. Additionally, continuous temperature mapping in flexible and stretchable formats opens up many other potential areas, such as artificially electronic skins and reflection of emotional changes. This review exploits a comprehensive investigation onto recent advances in flexible temperature sensors, stretchable sensor networks, and platforms constructed in soft and compliant formats for wearable physiological monitoring. The most recent examples of flexible temperature sensors are first discussed regarding to their materials, structures, electrical and mechanical properties; temperature sensing network technologies in new materials and structural designs are then presented based on platforms comprised of multiple physical sensors and stretchable electronics. Finally, wearable applications of the sensing network are described, such as detection of human activities, monitoring of health conditions, and emotion-related bodily sensations. Conclusions are made with emphasis on critical issues and new trends in the field of wearable temperature sensor network technologies.

Flexible and Stretchable Gold Microstructures on Extra Soft Poly(dimethylsiloxane) Substrates

Stretchable gold microstructures are reliably transferred onto an extra-soft elastomeric substrate. Several major challenges, including failure-free transfer and reliable bonding with the substrate, are addressed. The simple and reproducible fabrication allows extensive study and optimization of the stretchability of meanders in terms of thickness, geometry, and substrate. The results provide new insights for designing stretchable electronics and novel routes for stretchrelated, mechanobiological cell-interface applications.

Resistivity optimization and properties of silver nanoparticles-filled alcohol-soluble conductive coating based on acrylic resin

An alcohol-soluble acrylic resin used to improve the dispersity of modified silver nanoparticles (AgNPs) was successfully synthesized. The structure of acrylic resin was designated according to the elements and groups on the surface of modified AgNPs. The content of γ-(2,3-epoxypropoxy)propytrimethoxysilane (KH-560) and conductive filler, temperature, mechanical properties, anticorrosion, and so on were investigated in order to optimize the electrical conductivity and dispersion of modified AgNPs in coating. Surface chemical structure of modified AgNPs was characterized by X-ray photoelectron spectroscopy. Morphology of conductive coating was characterized by scanning electron microscopy. Groups in modified AgNPs and acrylic resin were characterized by Fourier transform infrared spectroscopy. And surface resistivity of conductive coating was characterized by digital multimeter techniques. The optimum surface resistivity of the acquired AgNPs conductive coating could be up to 0.01 ohm/sq, reaching the level of electromagnetic interference shielding. Moreover, the conductive coating possesses good property of anti-chemical and weather corrosion resistance.

25th anniversary article: The evolution of electronic skin (e-skin): a brief history, design considerations, and recent progress

Human skin is a remarkable organ. It consists of an integrated, stretchable network of sensors that relay information about tactile and thermal stimuli to the brain, allowing us to maneuver within our environment safely and effectively. Interest in large-area networks of electronic devices inspired by human skin is motivated by the promise of creating autonomous intelligent robots and biomimetic prosthetics, among other applications. The development of electronic networks comprised of flexible, stretchable, and robust devices that are compatible with large-area implementation and integrated with multiple functionalities is a testament to the progress in developing an electronic skin (e-skin) akin to human skin. E-skins are already capable of providing augmented performance over their organic counterpart, both in superior spatial resolution and thermal sensitivity. They could be further improved through the incorporation of additional functionalities (e.g., chemical and biological sensing) and desired properties (e.g., biodegradability and self-powering). Continued rapid progress in this area is promising for the development of a fully integrated e-skin in the near future.

Planar patterned stretchable electrode arrays based on flexible printed circuits

For stretchable electronics to achieve broad industrial application, they must be reliable to manufacture and must perform robustly while undergoing large deformations. We present a new strategy for creating planar stretchable electronics and demonstrate one such device, a stretchable microelectrode array based on flex circuit technology. Stretchability is achieved through novel, rationally designed perforations that provide islands of low strain and continuous low-strain pathways for conductive traces. This approach enables the device to maintain constant electrical properties and planarity while undergoing applied strains up to 15%. Materials selection is not limited to polyimide composite devices and can potentially be implemented with either soft or hard substrates and can incorporate standard metals or new nano-engineered conductors. By using standard flex circuit technology, our planar microelectrode device achieved constant resistances for strains up to 20% with less than a 4% resistance offset over 120,000 cycles at 10% strain.

A PDMS-based integrated stretchable microelectrode array (isMEA) for neural and muscular surface interfacing

Numerous applications in neuroscience research and neural prosthetics, such as electrocorticogram (ECoG) recording and retinal prosthesis, involve electrical interactions with soft excitable tissues using a surface recording and/or stimulation approach. These applications require an interface that is capable of setting up high-throughput communications between the electrical circuit and the excitable tissue and that can dynamically conform to the shape of the soft tissue. Being a compliant material with mechanical impedance close to that of soft tissues, polydimethylsiloxane (PDMS) offers excellent potential as a substrate material for such neural interfaces. This paper describes an integrated technology for fabrication of PDMS-based stretchable microelectrode arrays (MEAs). Specifically, as an integral part of the fabrication process, a stretchable MEA is directly fabricated with a rigid substrate, such as a thin printed circuit board (PCB), through an innovative bonding technology-via-bonding-for integrated packaging. This integrated strategy overcomes the conventional challenge of high-density packaging for this type of stretchable electronics. Combined with a high-density interconnect technology developed previously, this stretchable MEA technology facilitates a high-resolution, high-density integrated system solution for neural and muscular surface interfacing. In this paper, this PDMS-based integrated stretchable MEA (isMEA) technology is demonstrated by an example design that packages a stretchable MEA with a small PCB. The resulting isMEA is assessed for its biocompatibility, surface conformability, electrode impedance spectrum, and capability to record muscle fiber activity when applied epimysially.

Encapsulating Elastically Stretchable Neural Interfaces: Yield, Resolution, and Recording/Stimulation of Neural Activity

A high resolution elastically stretchable microelectrode array (SMEA) to interface with neural tissue is described. The SMEA consists of an elastomeric substrate, such as poly(dimethylsiloxane) (PDMS), elastically stretchable gold conductors, and an electrically insulating encapsulating layer in which contact holes are opened. We demonstrate the feasibility of producing contact holes with 40 µm × 40 µm openings, show why the adhesion of the encapsulation layer to the underlying silicone substrate is weakened during contact hole fabrication, and provide remedies. These improvements result in greatly increased fabrication yield and reproducibility. An SMEA with 28 microelectrodes was fabricated. The contact holes (100 µm × 100 µm) in the encapsulation layer are only ~10% the size of the previous generation, allowing a larger number of microelectrodes per unit area, thus affording the capability to interface with a smaller neural population per electrode. This new SMEA is used to record spontaneous and evoked activity in organotypic hippocampal tissue slices at 0% strain before stretching, at 5 % and 10 % equibiaxial strain, and again at 0% strain after relaxation. The noise of the recordings increases with increasing strain. The frequency of spontaneous neural activity also increases when the SMEA is stretched. Upon relaxation, the noise returns to pre-stretch levels, while the frequency of neural activity remains elevated. Stimulus-response curves at each strain level are measured. The SMEA shows excellent biocompatibility for at least two weeks.

Electrical stimulation for epilepsy: experimental approaches

Direct electrical stimulation of the brain is an increasingly popular means of treating refractory epilepsy. Although there has been moderate success in human trials, the rate of seizure freedom does not yet compare favorably to resective surgery. It therefore remains critical to advance experimental investigations aimed toward understanding brain stimulation and its utility. This article introduces the concepts necessary for understanding these experimental studies, describing recording and stimulation technology, animal models of epilepsy, and various subcortical targets of stimulation. Bidirectional and closed-loop device technologies are also highlighted, along with the challenges presented by their experimental use.

An effective lift-off method for patterning high-density gold interconnects on an elastomeric substrate

High-resolution, high-density gold interconnects are effectively patterned on an elastomeric substrate. A 3cm cable of ten gold wires with 10μm width and 20μm pitch is achieved, successfully demonstrating density increases of more than one order of magnitude from previously established work. Many applications in the fields of stretchable electronics and conformable neural interfaces will benefit from these fabrication developments.