Electronic skin

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Electronic skin refers to flexible, stretchable and self-healing electronics that are able to mimic functionalities of human or animal skin. [1] [2] The broad class of materials often contain sensing abilities that are intended to reproduce the capabilities of human skin to respond to environmental factors such as changes in heat and pressure. [1] [2] [3] [4]

Contents

Advances in electronic skin research focuses on designing materials that are stretchy, robust, and flexible. Research in the individual fields of flexible electronics and tactile sensing has progressed greatly; however, electronic skin design attempts to bring together advances in many areas of materials research without sacrificing individual benefits from each field. [5] The successful combination of flexible and stretchable mechanical properties with sensors and the ability to self-heal would open the door to many possible applications including soft robotics, prosthetics, artificial intelligence and health monitoring. [1] [5] [6] [7]

Recent advances in the field of electronic skin have focused on incorporating green materials ideals and environmental awareness into the design process. As one of the main challenges facing electronic skin development is the ability of the material to withstand mechanical strain and maintain sensing ability or electronic properties, recyclability and self-healing properties are especially critical in the future design of new electronic skins. [8]

Rehealable electronic skin

Self-healing abilities of electronic skin are critical to potential applications of electronic skin in fields such as soft robotics. [7] Proper design of self-healing electronic skin requires not only healing of the base substrate but also the reestablishment of any sensing functions such as tactile sensing or electrical conductivity. [7] Ideally, the self-healing process of electronic skin does not rely upon outside stimulation such as increased temperature, pressure, or solvation. [1] [7] [8] Self-healing, or rehealable, electronic skin is often achieved through a polymer-based material or a hybrid material.

Polymer-based materials

In 2018, Zou et al. published work on electronic skin that is able to reform covalent bonds when damaged. [8] The group looked at a polyimine-based crosslinked network, synthesized as seen in Figure 1. The e-skin is considered rehealable because of "reversible bond exchange," meaning that the bonds holding the network together are able to break and reform under certain conditions such as solvation and heating. The rehealable and reusable aspect of such a thermoset material is unique because many thermoset materials irreversibly form crosslinked networks through covalent bonds. [9] In the polymer network the bonds formed during the healing process are indistinguishable from the original polymer network.

Figure 1. Polymerization scheme for formation of polyimine-based self-healing electronic skin. Polymerization scheme for formation of polyimine-based self-healing e-skin.svg
Figure 1. Polymerization scheme for formation of polyimine-based self-healing electronic skin.

Dynamic non-covalent crosslinking has also been shown to form a polymer network that is rehealable. In 2016, Oh et al. looked specifically at semiconducting polymers for organic transistors. [10] They found that incorporating 2,6-pyridine dicarboxamide (PDCA) into the polymer backbone could impart self-healing abilities based on the network of hydrogen bonds formed between groups. With incorporation of PDCA in the polymer backbone, the materials was able to withstand up to 100% strain without showing signs of microscale cracking. In this example, the hydrogen bonds are available for energy dissipation as the strain increases.

Hybrid materials

Polymer networks are able to facilitate dynamic healing processes through hydrogen bonds or dynamic covalent chemistry. [8] [10] However, the incorporation of inorganic particles can greatly expand the functionality of polymer-based materials for electronic skin applications. The incorporation of micro-structured nickel particles into a polymer network (Figure 2) has been shown to maintain self-healing properties based on the reformation of hydrogen bonding networks around the inorganic particles. [7] The material is able to regain its conductivity within 15 seconds of breakage, and the mechanical properties are regained after 10 minutes at room temperature without added stimulus. This material relies on hydrogen bonds formed between urea groups when they align. The hydrogen atoms of urea functional groups are ideally situated to form a hydrogen-bonding network because they are near an electron-withdrawing carbonyl group. [11] This polymer network with embedded nickel particles demonstrates the possibility of using polymers as supramolecular hosts to develop self-healing conductive composites. [7]

Figure 2. Self-healing material based on hydrogen bonding and interactions with micro-structured nickel particles. Self-healing material based on hydrogen bonding and interactions with micro-structured nickel particles.svg
Figure 2. Self-healing material based on hydrogen bonding and interactions with micro-structured nickel particles.

Flexible and porous graphene foams that are interconnected in a 3D manner have also been shown to have self-healing properties. [4] Thin film with poly(N,N-dimethylacrylamide)-poly(vinyl alcohol) (PDMAA) and reduced graphene oxide have shown high electrical conductivity and self-healing properties. The healing abilities of the hybrid composite are suspected to be due to the hydrogen bonds between the PDMAA chains, and the healing process is able to restore initial length and recover conductive properties. [4]

Recyclable electronic skin

Zou et al. presents an interesting advance in the field of electronic skin that can be used in robotics, prosthetics, and many other applications in the form of a fully recyclable electronic skin material. [8] The e-skin developed by the group consists of a network of covalently bound polymers that are thermoset, meaning cured at a specific temperature. However, the material is also recyclable and reusable. Because the polymer network is thermoset, it is chemically and thermally stable. [9] However, at room temperature, the polyimine material, with or without silver nanoparticles, can be dissolved on the timescale of a few hours. The recycling process allows devices, which are damaged beyond self-healing capabilities, to be dissolved and formed into new devices (Figure 3). [8] This advance opens the door for lower cost production and greener approaches to e-skin development.

Figure 3. Recycling process for conductive polyimine-based e-skin. Recycling process for conductive polyimine-based e-skin.svg
Figure 3. Recycling process for conductive polyimine-based e-skin.

Flexible and stretchy electronic skin

The ability of electronic skin to withstand mechanical deformation including stretching and flexing without losing functionality is crucial for its applications as prosthetics, artificial intelligence, soft robotics, health monitoring, biocompatibility, and communication devices. [1] [3] [4] [12] Flexible electronics are often designed by depositing electronic materials on flexible polymer substrates, thereby relying on an organic substrate to impart favorable mechanical properties. [1] Stretchable e-skin materials have been approached from two directions. Hybrid materials can rely on an organic network for stretchiness while embedding inorganic particles or sensors, which are not inherently stretchable. Other research has focused on developing stretchable materials that also have favorable electronic or sensing capabilities. [1]

Zou et al. studied the inclusion of linkers that are described as "serpentine" in their polyimine matrix. [8] These linkers make the e-skin sensors able to flex with movement and distortion. The incorporation of alkyl spacers in polymer-based materials has also been shown to increase flexibility without decreasing charge transfer mobility. [10] Oh et al. developed a stretchable and flexible material based on 3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP) and non-conjugated 2,6-pyridine dicarboxamide (PDCA) as a source of hydrogen bonds (Figure 4). [10]

Figure 4. A stretchable and self-healing semiconducting polymer-based material. A stretchable and self-healing seminconducting polymer-based material.svg
Figure 4. A stretchable and self-healing semiconducting polymer-based material.

Graphene has also been shown to be a suitable material for electronic skin applications as well due to its stiffness and tensile strength. [13] Graphene is an appealing material because its synthesis to flexible substrates is scalable and cost-efficient. [13]

Mechanical Properties of Skin

Skin is composed of collagen, keratin, and elastin fibers, which provide robust mechanical strength, low modulus, tear resistance, and softness. The skin can be considered as a bilayer of epidermis and dermis. The epidermal layer has a modulus of about 140-600 kPa and a thickness of 0.05-1.5 mm. Dermis has a modulus of 2-80 kPa and a thickness of 0.3-3 mm. [14] This bilayer skin exhibits an elastic linear response for strains less than 15% and a non linear response at larger strains. To achieve conformability, it is preferable for devices to match the mechanical properties of the epidermis layer when designing skin-based stretchy electronics.

Tuning Mechanical Properties

Conventional high performance electronic devices are made of inorganic materials such as silicon, which is rigid and brittle in nature and exhibits poor biocompatibility due to mechanical mismatch between the skin and the device, making skin integrated electronics applications difficult. To solve this challenge, researchers employed the method of constructing flexible electronics in the form of ultrathin layers. The resistance to bending of a material object (Flexural rigidity) is related to the third power of the thickness, according to the Euler-Bernoulli equation for a beam [15] . It implies that objects with less thickness can bend and stretch more easily. As a result, even though the material has a relatively high Young's modulus, devices manufactured on ultrathin substrates exhibit a decrease in bending stiffness and allow bending to a small radius of curvature without fracturing. Thin devices have been developed as a result of significant advancements in the field of nanotechnology, fabrication, and manufacturing. The aforementioned approach was used to create devices composed of 100-200 nm thick Si nano membranes deposited on thin flexible polymeric substrates. [15]

Furthermore, structural design considerations can be used to tune the mechanical stability of the devices. Engineering the original surface structure allows us to soften the stiff electronics. Buckling, island connection, and the Kirigami concept have all been employed successfully to make the entire system stretchy. [16]

Mechanical buckling can be used to create wavy structures on elastomeric thin substrates. This feature improves the device's stretchability. The buckling approach was used to create Si nanoribbons from single crystal Si on an elastomeric substrate. The study demonstrated the device could bear a maximum strain of 10% when compressed and stretched. [17]

In the case of island interconnect, the rigid material connects with flexible bridges made from different geometries, such as zig-zag, serpentine-shaped structures, etc., to reduce the effective stiffness, tune the stretchability of the system, and elastically deform under applied strains in specific directions. It has been demonstrated that serpentine-shaped structures have no significant effect on the electrical characteristics of epidermal electronics. It has also been shown that the entanglement of the interconnects, which oppose the movement of the device above the substrate, causes the spiral interconnects to stretch and deform significantly more than the serpentine structures. [16] CMOS inverters constructed on a PDMS substrate employing 3D island interconnect technologies demonstrated 140% strain at stretching. [17]

Kirigami is built around the concept of folding and cutting in 2D membranes. This contributes to an increase in the tensile strength of the substrate, as well as its out-of-plane deformation and stretchability. These 2D structures can subsequently be turned to 3D structures with varied topography, shape, and size controllability via the Buckling process, resulting in interesting properties and applications. [16] [17]

Conductive electronic skin

The development of conductive electronic skin is of interest for many electrical applications. [3] [7] [18] Research into conductive electronic skin has taken two routes: conductive self-healing polymers or embedding conductive inorganic materials in non-conductive polymer networks. [1]

The self-healing conductive composite synthesized by Tee et al. (Figure 2) [7] investigated the incorporation of micro-structured nickel particles into a polymer host. The nickel particles adhere to the network though favorable interactions between the native oxide layer on the surface of the particles and the hydrogen-bonding polymer. [7]

Nanoparticles have also been studied for their ability to impart conductivity on electronic skin materials. [8] [18] Zou et al. embedded silver nanoparticles (AgNPs) into a polymer matrix, making the e-skin conductive. The healing process for this material is noteworthy because it not only restores the mechanical properties of the polymer network, but also restores the conductive properties when silver nanoparticles have been embedded in the polymer network. [8]

Sensing ability of electronic skin

Some of the challenges that face electronic skin sensing abilities include the fragility of sensors, the recovery time of sensors, repeatability, overcoming mechanical strain, and long-term stability. [5] [19]

Tactile sensors

Applied pressure can be measured by monitoring changes in resistance or capacitance. [13] Coplanar interdigitated electrodes embedded on single-layer graphene have been shown to provide pressure sensitivity for applied pressure as low as 0.11 kPa through measuring changes in capacitance. [13] Piezoresistive sensors have also shown high levels of sensitivity. [19] [20] [21]

Ultrathin molybdenum disulfide sensing arrays integrated with graphene have demonstrated promising mechanical properties capable of pressure sensing. [19] Modifications of organic field effect transistors (OFETs) have shown promise in electronic skin applications. [22] Microstructured polydimethylsiloxane thin films can elastically deform when pressure is applied. The deformation of the thin film allows for storage and release of energy. [22]

Visual representation of applied pressure has been one area of interest in development of tactile sensors. [3] [23] The Bao Group at Stanford University have designed an electrochromically active electronic skin that changes color with different amounts of applied pressure. [3] Applied pressure can also be visualized by incorporation of active-matrix organic light-emitting diode displays which emit light when pressure is applied. [23]

Prototype e-skins include a printed synaptic transistor–based electronic skin giving skin-like haptic sensations and touch/pain-sensitivity to a robotic hand, [24] [25] and a multilayer tactile sensor repairable hydrogel-based robot skin. [26] [27]

Other sensing applications

Humidity sensors have been incorporated in electronic skin design with sulfurized tungsten films. The conductivity of the film changes with different levels of humidity. [28] Silicon nanoribbons have also been studied for their application as temperature, pressure, and humidity sensors. [29] Scientists at the University of Glasgow have made inroads in developing an e-skin that feels pain real-time, with applications in prosthetics and more life-like humanoids. [30]

A system of an electronic skin and a human-machine interface that can enable remote sensed tactile perception, and wearable or robotic sensing of many hazardous substances and pathogens. [31] [32]

See also

Related Research Articles

<span class="mw-page-title-main">Organic electronics</span> Field of materials science

Organic electronics is a field of materials science concerning the design, synthesis, characterization, and application of organic molecules or polymers that show desirable electronic properties such as conductivity. Unlike conventional inorganic conductors and semiconductors, organic electronic materials are constructed from organic (carbon-based) molecules or polymers using synthetic strategies developed in the context of organic chemistry and polymer chemistry.

<span class="mw-page-title-main">Flexible electronics</span> Mounting of electronic devices on flexible plastic substrates

Flexible electronics, also known as flex circuits, is a technology for assembling electronic circuits by mounting electronic devices on flexible plastic substrates, such as polyimide, PEEK or transparent conductive polyester film. Additionally, flex circuits can be screen printed silver circuits on polyester. Flexible electronic assemblies may be manufactured using identical components used for rigid printed circuit boards, allowing the board to conform to a desired shape, or to flex during its use.

<span class="mw-page-title-main">Conductive polymer</span> Organic polymers that conduct electricity

Conductive polymers or, more precisely, intrinsically conducting polymers (ICPs) are organic polymers that conduct electricity. Such compounds may have metallic conductivity or can be semiconductors. The main advantage of conductive polymers is that they are easy to process, mainly by dispersion. Conductive polymers are generally not thermoplastics, i.e., they are not thermoformable. But, like insulating polymers, they are organic materials. They can offer high electrical conductivity but do not show similar mechanical properties to other commercially available polymers. The electrical properties can be fine-tuned using the methods of organic synthesis and by advanced dispersion techniques.

<span class="mw-page-title-main">Electroactive polymer</span>

An electroactive polymer (EAP) is a polymer that exhibits a change in size or shape when stimulated by an electric field. The most common applications of this type of material are in actuators and sensors. A typical characteristic property of an EAP is that they will undergo a large amount of deformation while sustaining large forces.

<span class="mw-page-title-main">PEDOT:PSS</span> Polymer

poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is a polymer mixture of two ionomers. One component in this mixture is made up of polystyrene sulfonate which is a sulfonated polystyrene. Part of the sulfonyl groups are deprotonated and carry a negative charge. The other component poly(3,4-ethylenedioxythiophene) (PEDOT) is a conjugated polymer and carries positive charges and is based on polythiophene. Together the charged macromolecules form a macromolecular salt.

<span class="mw-page-title-main">Printed electronics</span> Electronic devices created by various printing methods

Printed electronics is a set of printing methods used to create electrical devices on various substrates. Printing typically uses common printing equipment suitable for defining patterns on material, such as screen printing, flexography, gravure, offset lithography, and inkjet. By electronic-industry standards, these are low-cost processes. Electrically functional electronic or optical inks are deposited on the substrate, creating active or passive devices, such as thin film transistors; capacitors; coils; resistors. Some researchers expect printed electronics to facilitate widespread, very low-cost, low-performance electronics for applications such as flexible displays, smart labels, decorative and animated posters, and active clothing that do not require high performance.

<span class="mw-page-title-main">Stretchable electronics</span>

Stretchable electronics, also known as elastic electronics or elastic circuits, is a group of technologies for building electronic circuits by depositing or embedding electronic devices and circuits onto stretchable substrates such as silicones or polyurethanes, to make a completed circuit that can experience large strains without failure. In the simplest case, stretchable electronics can be made by using the same components used for rigid printed circuit boards, with the rigid substrate cut to enable in-plane stretchability. However, many researchers have also sought intrinsically stretchable conductors, such as liquid metals.

<span class="mw-page-title-main">E-textiles</span> Fabrics that incorporate electronic components

Electronic textiles or e-textiles are fabrics that enable electronic components such as batteries, lights, sensors, and microcontrollers to be embedded in them. They are not to be confused with smart textiles, which are fabrics that have been developed with new technologies that provide added value. Many smart clothing, wearable technology, and wearable computing projects involve the use of e-textiles.

<span class="mw-page-title-main">Force-sensing resistor</span> Material whose resistance changes when a force is applied

A force-sensing resistor is a material whose resistance changes when a force, pressure or mechanical stress is applied. They are also known as force-sensitive resistor and are sometimes referred to by the initialism FSR.

Quantum tunnelling composites (QTCs) are composite materials of metals and non-conducting elastomeric binder, used as pressure sensors. They use quantum tunnelling: without pressure, the conductive elements are too far apart to conduct electricity; when pressure is applied, they move closer and electrons can tunnel through the insulator. The effect is far more pronounced than would be expected from classical (non-quantum) effects alone, as classical electrical resistance is linear (proportional to distance), while quantum tunnelling is exponential with decreasing distance, allowing the resistance to change by a factor of up to 1012 between pressured and unpressured states.

<span class="mw-page-title-main">PHOSFOS</span>

PhoSFOS is a research and technology development project co-funded by the European Commission.

<span class="mw-page-title-main">Tactile sensor</span>

A tactile sensor is a device that measures information arising from physical interaction with its environment. Tactile sensors are generally modeled after the biological sense of cutaneous touch which is capable of detecting stimuli resulting from mechanical stimulation, temperature, and pain. Tactile sensors are used in robotics, computer hardware and security systems. A common application of tactile sensors is in touchscreen devices on mobile phones and computing.

A nanogenerator is a small device that converts mechanical or thermal energy into electricity. It is a energy harvesting device, where energy from external sources also known as ambient energy, is converted into electricity for use by small, wireless autonomous devices, like those used in wearable electronics, condition monitoring, and wireless sensor networks. Energy harvesters usually provide a very small amount of power for low-energy electronics. While the input fuel to some large-scale energy generation costs resources, the energy source for energy harvesters is present as ambient background. For example, temperature gradients exist from the operation of a combustion engine and in urban areas, there is a large amount of electromagnetic energy in the environment due to radio and television broadcasting, and energy can be harvesting from the vibrations during walking.

Robotic sensing is a subarea of robotics science intended to provide sensing capabilities to robots. Robotic sensing provides robots with the ability to sense their environments and is typically used as feedback to enable robots to adjust their behavior based on sensed input. Robot sensing includes the ability to see, touch, hear and move and associated algorithms to process and make use of environmental feedback and sensory data. Robot sensing is important in applications such as vehicular automation, robotic prosthetics, and for industrial, medical, entertainment and educational robots.

A conductive elastomer is a form of elastomer, often natural rubber or other rubber substitute, that is manufactured to conduct electricity. This is commonly accomplished by distributing carbon or other conductive particles throughout the raw material prior to setting it. Carbon black and silica are common additives to induce conductivity in elastomers. Silica has been studied more so than other additives due to its low cost however, its conductance is also lower. These additives can not only enable conductance but can increase the mechanical properties of the elastomer.

<span class="mw-page-title-main">Soft robotics</span> Subfield of robotics

Soft robotics is a subfield of robotics that concerns the design, control, and fabrication of robots composed of compliant materials, instead of rigid links. In contrast to rigid-bodied robots built from metals, ceramics and hard plastics, the compliance of soft robots can improve their safety when working in close contact with humans.

A stretch sensor is a sensor which can be used to measure deformation and stretching forces such as tension or bending. They are usually made from a material that is itself soft and stretchable.

<span class="mw-page-title-main">Stéphanie P. Lacour</span> French neurotechnologist

Stéphanie P. Lacour is a French neurotechnologist and full professor holding the Foundation Bertarelli Chair in Neuroprosthetic Technology at the Swiss Federal Institute of Technology in Lausanne (EPFL). Lacour is a pioneer in the field of stretchable electronics and directs a laboratory at EPFL which specializes in the development of Soft BioElectronic Interfaces to enable seamless integration of neuroprosthetic devices into human tissues. Lacour is also a co-founding member and director of the Center for Neuroprosthetics at the EPFL Satellite Campus in Geneva, Switzerland.

<span class="mw-page-title-main">Ana Claudia Arias</span> American physicist

Ana Claudia Arias is a Brazilian American physicist who is a professor of Electrical Engineering and Computer Sciences at the University of California, Berkeley. Her research considers printed electronic materials and their application in flexible electronics and wearable medical devices.

A chemical sensor array is a sensor architecture with multiple sensor components that create a pattern for analyte detection from the additive responses of individual sensor components. There exist several types of chemical sensor arrays including electronic, optical, acoustic wave, and potentiometric devices. These chemical sensor arrays can employ multiple sensor types that are cross-reactive or tuned to sense specific analytes.

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