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Since the development of the field effect transistor in 1920s, the electronics industry has focused on high speed and large capacity devices such as microprocessors and random access memories. However, the recent emergence of personalized and mobile electronics has diversified the research efforts from performance-oriented research to human-friendly topics. Conventional bulky and rigid electronics prevent compliant interfacing with soft human skin for health monitoring and human–machine interaction, due to the incompatible mechanical characteristics.
Biological materials with dimensions ranging from the nanoscale, e.g. deoxyribonucleic acid (DNA), to the macroscale, e.g. tissues and organs, are generally curved, soft, and elastic, with low Young’s moduli E ranging from 100 Pa for brain tissue to 10 kPa for skin. The resulting mismatch in mechanical properties at the interface of biology and traditional electronics often causes non-conformal contact, resulting in discomfort and performance degradation. Furthermore, the large mechanical discrepancy at the cell-electrode interface affects multiple aspects of the cell’s behavior, such as its growth and differentiation, resulting in significant challenges for a variety of healthcare and medical devices
To overcome the limitations, soft skin‐mountable electronics with superior mechanical softness, flexibility, and stretchability provide an effective platform for intimate interaction with humans. In addition, soft electronics offer comfortability when worn on the soft, curvilinear, and dynamic human skin. Stretchable Electronics is an umbrella term that conceals great diversity. It refers to a whole host of emerging electronic materials, components and devices that exhibit some degree of mechanical strain tolerance or “stretchability”. These include interconnects, sensors, actuators, functional films, batteries, logic, displays, etc.
Soft material-enabled electronics can address a wide range of applications by enabling a comfortable, continuous, and real-time monitoring of physiological signals via conformal, ergonomic interactions with human tissues when compared to conventional electronics based on bulky and rigid materials. Tissue-friendly, intimate lamination of soft wearable and implantable electronics allow for a long-term, high-fidelity recording of biological signals. These electronics can enhance healthcare, diagnosis, and therapeutics by offering improved biocompatibility, signal monitoring, and wearability without sacrificing patient comfort.
These biomedical devices based on soft-electronics have been developed in wearable and/or implantable forms. More specifically, they can be categorized into four different groups: 1) fabric-based wearable devices, 2) skin-mounted electronics, 3) fully implantable devices, and 4) minimally invasive surgical tools.
Stretchable electronic devices have received increasing attention from researchers globally. They can be applied in many innovative fields, such as epidermal electronic devices, biomedical engineering, healthcare monitoring, soft robotics, electronic skins, and human-machine interfaces (HMI).
Soft robots, compared to their hard counterparts, show advantages in interacting with human beings and handling fragile objects. Stretchable electronic devices are essential for the sensing and actuation of soft robots, and transform the interaction between human beings and machines. The recent development in soft electronics has played a prominent role to circumvent the mechanical mismatch dilemma between the flat, rigid nature of conventional electronic devices and the curved, deformable surface of the soft actuators, promoting the integration of functional electronic components with artificial muscles to enable interactive soft robotic systems with bidirectional functionalities of actively sensing and responding.
Soft bioelectronics that could be integrated with soft and curvilinear biological tissues/organs have attracted multidisciplinary research interest from material scientists, electronic engineers, and biomedical scientists. Scientists are now focussing on developing bioelectronic devices that are not only fast, sensitive, biocompatible, soft, and flexible, but also have long-term stability in physiological environments such as the human body.
These devices would greatly improve human health, from monitoring in-home wellness to diagnosing and treating neuropsychiatric diseases, including epilepsy and Parkinson’s disease. Skin-based electronics can continuously and non-invasively record important medical information from human body and accumulate them to generate big data. Big data analysis of the continuously monitored vital signs (i.e., activity, electroencephalogram, electrocardiogram, pulse, and body temperature), for example, predicts and/or prevents many diseases including cardiac and lifestyle illness.
Biological systems have the powerful ability to self-heal. Human skin can, for example, autonomously heal from wounds of various degrees, allowing it to restore its mechanical and electrical properties. In contrast, human-made electronic devices degrade over time due to fatigue, corrosion or damage incurred during operation, leading to device failure. Self-healing chemistry has emerged in recent years as a promising method for constructing soft electronic materials that are mechanically robust and can self-repair.
Technologies
Broadly, stretchable electronics consist of integrated circuits on pliable and stretchable material composed of elastomeric fabric. Compared with traditional printed circuit boards, stretchable electronic circuits can mechanically bend, twist, compress, stretch, and self-heal if damaged from being stretched using soft substrate materials to adapt to the attached surface’s contour. Stretchable conductors must withstand high mechanical strains greater than 50% and high electrical conductivity based on these new generation devices’ performance requirements. Because of their potential human health-related applications, soft bioelectronics require stringent demands for biocompatible components.
In addition to high electrical performances and strain-releasing designs, a tight and conformal adhesion of the devices on the dynamically contorting skin is important for the long-term monitoring of subtle physiological and electrophysiological signals. To achieve strong adhesion, various strategies have been employed such as controlling the modulus of the elastomeric substrate, employing interfacial micro-/nano-structures, and applying chemical adhesives.
Substrate made of ultrasoft elastomers, whose moduli are similar to that of the epidermis (~100 kPa), permit conformal contact of the system that maximize the Van der Waals force (Figure 3e). Bio-inspired microstructures facilitate further robust bonding to the skin. An elastomeric patch with microscale suction cup structures, for example, shows a three-fold higher adhesion compared to a plain elastomeric patch. Another bio-mimetic adhesive, known as swellable microneedles, achieved a 3.5 times greater adhesion than that of a conventional staple fixation. Sprayed chemical adhesives that are significantly thinner than the film-type adhesives, keep the device mounted for as long as two weeks under touching, washing, and deforming.
An increasing number of materials, integration designs, and fabrication technologies are being developed towards realizing soft electronics due to these advantages.
Bioelectronic devices often require advanced signal processing to implement diagnostic and therapeutic operations, from differential amplification and time division multiplexing to analog to digital conversion and high-speed digital communication. Silicon-based electronics can accomplish each of these functions, with a large repository of preexisting designs, fabrication processes, and experiential knowledge. These technologies are rigid and incompatible with ion-rich physiologic environments.
Conventional transistors are made out of silicon, which means they need to be encased in metal or plastic within the body as they cannot function in the presence of ions and water – eventually breaking down because of ion diffusion into the device. The devices are also are not effective at interacting with ionic signals, which is how the body’s cells communicate and, as a result, these properties restrict the abiotic/biotic coupling to capacitive interactions only on the surface of material, resulting in lower performance.
A wide variety of organic materials are inherently flexible, chemically inert, nontoxic, and have tunable physical properties, making them optimal for this function. In addition, conducting and semiconducting organics can form nonlinear electronic components (such as transistors and diodes) capable of biological signal sensing and transduction
A device with organic material at the abiotic/biotic interface to perform signal preprocessing and advanced silicon-based circuits for subsequent signal communication and analysis would combine the beneficial properties of both approaches. However, two substantial challenges hinder realization of these devices: (i) lack of stable, high-performance, independently addressable organic components for integrated circuits, and (ii) absence of scalable, biocompatible processes to seamlessly integrate soft, organic materials with rigid silicon-based circuits
Khodagholy’s team took advantage of both the electronic and the ionic conduction of organic materials to create ion driven transistors which they call e-IGTs, or enhancement-mode, internal ion-gated organic electrochemical transistors, that have embedded mobile ions inside their channels. Because the ions do not need to travel long distances to participate in the channel switching process, they can be switched on and off quickly and efficiently. “We’re excited about these findings,” says Gelinas. “We’ve shown that E-IGTs offer a safe, reliable, and high-performance building block for chronically implanted bioelectronics, and I am optimistic that these devices will enable us to safely expand how we use bioelectronic devices to address neurologic disease.”
Inspired by electrically active cells the team created a single material capable of performing multiple, non-linear, dynamic electronic functions just by varying the size and density of its composite mixed-conducting particles.
Researchers Patricia Jastrzebska-Perfect et al have developed an innovative, soft, biocompatible composite material (MCP) that can serve as various electronic components, depending on the size and density of constituent particles. It can be used to create different electronic components based on the size and density of its constituent particles. This approach would allow a single material to function as multiple different principle electronic components, eliminating the need for several bonded layers of patterned conducting, semiconducting, and insulating materials.The key components of MCP are conducting polymer particles and an ion-conducting scaffolding polymer matrix, enabling high conductivity and ionic interactions through de/doping. The physical processes used to prepare the MCP are scalable, solvent free and preserve the electrical properties of the conducting polymer.
Because the composite is fully biocompatible and has controllable electronic properties, MCP can non-invasively record muscle action potentials from the surface of arm and large-scale brain activity during neurosurgical procedures to implant deep brain stimulation electrodes. Khodagholy, who directs the Translational NeuroElectronics Lab at Columbia Engineering, said: “Instead of having large implants encapsulated in thick metal boxes to protect the body and electronics from each other, such as those used in pacemakers, and cochlear and brain implants, we could do so much more if our devices were smaller, flexible, and inherently compatible with our body environment.
Silk
Silk, as a kind of well-known ancient natural biopolymer, shows unique combined merits such as good biocompatibility, programmable biodegradability, processability into various material formats, and large-scale sustainable production. Such unique merits have made silk popular for intensive design and study in soft bioelectronics over the past decade. Due to the development of fabrication techniques in material processing and progress in research, silk has been engineered into a variety of advanced materials including silk fibers/textiles, nanofibers, films, hydrogels, and aerogels.
Based on commercial silk fibers/textiles and the availability of re-engineered silk materials with versatile technological formats, functional soft electronics have been explored with silk as flexible biosupports/biomatrixes or active components. These soft systems include bioresorbable electronics, ultraconformal bioelectronics, transient electronics, epidermal electronics, textile electronics, conformal biosensors, flexible transistors, and resistive switching memory devices. Silk-derived carbon materials with rationally designed morphologies and structures have also been developed as active components for wearable sensors, electronic skins, and flexible energy devices, which provide novel concepts and opportunities for soft electronics.
Nanomaterials
Stretchable electronics’ development has achieved remarkable progress based on nanomaterials’ tremendous growth and new nanofabrication technologies during the past few decades.
Nanomaterial-based soft bioelectronics have attracted great attention for healthcare applications because of their unique features including medical multifunctionality, mechanical deformability, and outstanding performances. In achieving these unique advantages, nanomaterials have played a crucial role. For example, the large surface area of mesoporous-silica nanoparticles enables an efficient drug delivery, the unique deformability of carbon nanotubes allows a stretchable electrode, and the unusual chemical structure of graphene accomplishes the high mobility in soft electronics.
It is essential to understand nanoscale mechanics, material properties, and structure-property relationships, with micro-fabrication and material processing techniques, to visualize the various forms of stretchable conductors. Everyday usage of the device, including stretching, twisting, impact, and temperature fluctuations, can lead to damage. This damage is generally invisible to the naked eye. Still, it will reduce the performance level and serviceable lifespan of soft electronics. The design of these devices must consider self-healing since, visibly, the damage is undetectable. The repair must be automatic, accurate, and able to restore the electronic component to full functionality.
Nanofabrication
To achieve the nanomaterial-based device array, their uniform and large-scale assembly is essential. Several assembly techniques such as spin casting, Langmuir Blodgett (LB) method, mechanical molding, dry transfer printing, and lithography have been used to integrate the nanomaterials into softelectronics.
To enable soft and stretchable properties in electronic devices, both design and material strategies have been investigated. In design-focused approaches, compliant two-dimensional (2D) serpentine or three-dimensional (3D) helical patterns are formed from solid metal thin films on soft substrates to endure mechanical deformation. These engineered 2D/3D network architectures can interface with biological materials over large areas. However, their limited resolution (10 μm line width for 2D serpentine patterns14 and 50 μm line width for 3D helical patterns15) is one of the drawbacks for the fabrication of submicron- or nanoscale devices.
Advanced nanoprinting techniques, such as nanotransfer printing, can create complex patterns with tens of nanometer resolution using high-resolution stamps fabricated by electron-beam lithography (EBL) or laser interference lithography (LIL). However, these printed nano patterns are not compliant structures, and the nanotransfer process is also limited to print on rigid substrates only. Moreover, the rigid metal patterns ultimately limit the strain the pattern can endure and lower the density of electronic components because of the use of space-consuming serpentine or helical wiring interconnections.
On the other hand, material-focused approaches utilize elastic conductors based on conductive nanomaterials that are either embedded into polymer matrices or dispensed directly onto a soft substrate. These printing approaches enable inexpensive fabrication processes for conductive circuits without the need of serpentine geometries, but the relatively low resolution (50–150 μm for conventional printing methods19,20,21) and low conductivity (<1 × 104 S m−1 for single-walled-carbon-nanotube-doped elastic conductors20) of these conductors still limit their usability for high-density electronics integration.
Overall, the major limitations for both approaches are patterning resolution, scalability, and resulting electronic density. In particular, scaling patterns down to submicron or even nanoscale dimensions is technically difficult using transfer printing techniques for serpentine or helical metal patterns or direct printing techniques for conductive nanomaterial networks.
Researchers Min-gu Kim, and others have proposed nanofabrication strategy to create submicron-scale, all-soft electronic devices based on eutectic gallium-indium alloy (EGaIn) using a hybrid method utilizing electron-beam lithography and soft lithography. EGaIn offers a number of advantages, including a low melting temperature (MP < 15 °C), favorable mechanical stretchability (being a liquid, the stretchability is typically limited by the mechanical properties of the encasing material), thermal conductivity (k = 26.6 W m−1 K−1), and electrical conductivity (σ = 3.4 × 106 S m−1). Under atmospheric oxygen level, a thin oxide layer (t ≈ 1–3 nm) is formed on the EGaIn surface, which allows EGaIn to be molded to elastomeric substrates.
In particular, a hybrid lithography process is introduced that combines electron-beam lithography (EBL) for nano/microstructure fabrication with soft lithography for EGaIn transfer. The hybrid lithography process is applied to a biphasic structure, comprising a metallic adhesion layer coated with EGaIn, to create soft nano/microstructures embedded in elastomeric materials. Submicron-scale EGaIn thin-film patterning with feature sizes as small as 180 nm and 1 μm line spacing was achieved, resulting in the highest resolution EGaIn patterning technique to date. The resulting soft and stretchable EGaIn patterns offer a currently unrivaled combination of resolution, electrical conductivity, and electronic/wiring density.
Soft and Stretchable electronics market
The electronic industry is in the midst of a major paradigm shift: novel form factors are emerging ranging from limited flexibility to ultra-elastic and conformable electronics. This transfiguration has been in the making for more than a decade now, but is only now beginning to make a substantial commercial impact. This is not an incremental shift along well-established industry lines. Instead, it seeks to create new functions, new applications, and new users.
This is a disruptive megatrend that will transform traditional electronics from being components-in-a-box into truly invisible electronics that are structurally integrated where needed. This is a major long-term theme that will lead to a root-and-branch change in the electronics industry including materials, components and the entire value chain. Stretchable and conformable electronics is giving shape to this megatrend. Indeed, it enables it.
Stretchable Electronics Market Valued at USD 635.6 Million in 2020 and is Expected to Reach USD 2.98 Billion by 2026 Growing with a CAGR of 25.29%
References and Resources also include:
https://www.healtheuropa.eu/developing-soft-and-flexible-biocompatible-electronic-devices/99677/
