Smart Skin Research Articles

Crack Detection of Reinforced Concrete Structure Using Smart Skin.

The availability of carbon nanotube (CNT)-based polymer composites allows the development of surface-attached self-sensing crack sensors for the structural health monitoring of reinforced concrete (RC) structures. These sensors are fabricated by integrating CNTs as conductive fillers into polymer matrices such as polyurethane (PU) and can be applied by coating on RC structures before the composite hardens. The principle of crack detection is based on the electrical change characteristics of the CNT-based polymer composites when subjected to a tensile load. In this study, the electrical conductivity and electro-mechanical/environmental characterization of smart skin fabricated with various CNT concentrations were investigated. This was performed to derive the tensile strain sensitivity of the smart skin according to different CNT contents and to verify their environmental impact. The optimal CNT concentration for the crack detection sensor was determined to be 5 wt% CNT. The smart skin was applied to an RC structure to validate its effectiveness as a crack detection sensor. It successfully detected and monitored crack formation and growth in the structure. During repeated cycles of crack width variations, the smart skin also demonstrated excellent reproducibility and electrical stability in response to the progressive occurrence of cracks, thereby reinforcing the reliability of the crack detection sensor. Overall, the presented results describe the crack detection characteristics of smart skin and demonstrate its potential as a structural health monitoring (SHM) sensor.

Possible design/fabrication of a soft biomimetic magic (flying) carpet made with Ionic Polymer Metal Composites (IPMCs)

A novel design of an artificial soft robot similar to the motions of a magic carpet is presented. Reported in the planning, designing, and fabricating of a biomimetic magic (flying) carpet made with ionic polymer metal composites (IPMCs). A new family of IPMC sensors and actuators is proposed to be used as a flexible plate and a flexible carpet to form a biomimetic flexible soft sensor and actuator system. The magic carpets made with IPMCs will be experimentally capable of bending, rolling, turning, twisting, and simultaneous sensing. However, their sensing characteristics as a soft biomimetic magic carpet feedback sensor are shown to have great potential for ubiquitous robot-human interactions (RHI). Upon various types of deformation, they are shown to generate unique output voltage signals and transient currents to be correlated to the actual magic carpet feedback force. Furthermore, a flexible sheet of IPMC can be actuated simultaneously on the fly by a small power supply embedded in the magic carpet. The magic carpet version of IPMC can be applied as smart skin to develop human-like dexterous and soft manipulation.

Overcoming Moisture‐Induced Charge Decay in Tribo‐Materials

AbstractThe paper extensively explores moisture‐induced charge decay in tribo‐materials, addressing charge generation fundamentals and overcoming strategies. Triboelectric effect and contact electrification models are discussed, with corona charging and hydro‐charging as effective charge generation methods. Moisture‐induced adverse effects, such as swelling and charge dissipation, are outlined. Electronegativity and dangling bonds' roles in charge traps are explored, along with the impact of functionalities on materials. Various strategies, including hydrophobic surfaces, crystalline phases, and water‐reactive materials, are proposed to counter moisture effects. Tribo‐materials are currently applied in energy, sensors, environment, and healthcare, with potential in smart skin sensors and implantable devices. Overcoming challenges, including high charge density and durability, can lead to breakthroughs, expanding applications to harsh environments like underwater and high temperatures.

Bio‐Inspired Camouflage Skin with Photonic Crystal Structure and Size‐Confinement Effect

AbstractCamouflage skin holds immense potential for a variety of advanced applications, such as artificial intelligence, soft robotics, and flexible/wearable electronics. Currently, most existing materials for constructing camouflage skin are mainly based on inorganic materials with an inverse opal scaffold, which significantly limits their further development because of the poor mechanical properties and environmental instability. Inspired by rana slvaticas, a new type of camouflage skin based on one‐dimentional (1D) photonic crystal (PC) gel is designed and synthesized, which combines the advantages of 1D PC structures with 3D flexible gels in a single skin device. Taking advantage of the unique structures, the resulting camouflage skins demonstrated attractive properties, including excellent mechanical performance, self‐adaptive camouflage capabilities, and long‐term stability in ambient conditions. Specifically, the camouflage skin can quickly recognize and match the background by modulating the optical signals of external stimuli. The synergistic effect of the glycerol, the dodecyl glycerol itaconate (DGI), and the nano‐space structures altogether contribute to the outstanding long‐term environmental stability of the camouflage skin. Overall, by combining the bio‐inspired concept and molecular engineering strategy, camouflage skin based on PC gels is synthesized, which greatly enriched the application and development in the smart skins field.

A Smart Skin for Hydrogels That Enables Switchable Solute Release.

Many applications of hydrogels rely on their ability to deliver encapsulated solutes, such as drugs; however, small hydrophilic solutes rapidly leak out of gels by diffusion. A need exists for a way to regulate solute release out of gels─to ensure zero release until a desired time (the OFF state) and thereafter for the release to be switched ON at a high rate. This should ideally be a repeatable switch; i.e., the gel should be cyclable repeatedly between the ON and OFF states. Such perfect, cyclical ON-OFF release of solutes from gels is demonstrated for the first time through a "smart skin" that is synthesized rapidly (in ∼10 min) around an entire gel. The thin (∼100 μm) and transparent polymer skin is endowed with redox-responsive properties through the use of urethane and acrylate monomers, one of which contains a thioether group. Initially, the skin is hydrophobic (water contact angle 102°), and it completely prevents hydrophilic solutes from leaking out of the gel. When contacted with oxidants such as hydrogen peroxide (H2O2), the thioethers are converted to sulfoxides, making the skin hydrophilic (water contact angle 42°) and thereby turning ON the release of solutes. Conversely, solute release can be turned OFF subsequently by adding a reducing agent such as vitamin C that reverts the sulfoxides to thioethers and thus returns the skin to its hydrophobic state. The release rate in the ON state can be tuned via the skin thickness as well as the oxidant concentration. The ability to regulate solute delivery from gels using smart skins is likely to prove significant in areas ranging from separations to agriculture and drug delivery.

A Review on Magnetic Smart Skin as Human–Machine Interfaces

AbstractIn recent years, there is a meteoric rise in the prevalence of electronic wearables, and flexible wearables have attracted tremendous attention in human–machine interfaces due to their high biocompatibility, functionality, conformability, and low‐cost. Flexible magnetic smart skin is part of this rapidly progressing field of flexible wearable electronics, which has paved the way for a new path for human perceptual development, as they may form a new perception, also known as magnetic sense, that is, the capacity to detect and interact with magnetic fields. In this review, the concept of the magnetic smart skin is defined: magnetoelastomers and flexible magnetic sensors. In magnetoelastomers, different sources of magnetic field, structures, and applications are summarized by recent and renowned research. In flexible magnetic sensors, different flexible substrates, sensor types, and applications are also summarized. This review further concludes by discussing the outlook and current challenges of magnetic smart skin in human–machine interfaces.

An Interfacial‐Assembled Self‐Supporting Nanofilm Induced by Strong Intermolecular Interaction between Silk Fibroin and Lysozyme

AbstractOrganic ultrathin nanofilms are increasingly favored in a wide range of applications, including flexible photonics and electronics, smart skin devices, sensing, and bioinspired designs, due to their high flexibility, chemical activity, and stimuli‐responsiveness. To create large‐area, robust, and freestanding nanofilms efficiently, a protein interfacial cooperative assembly technique that utilizes silk fibroin and lysozyme as building blocks is developed. The strong intermolecular interaction between these two proteins, coupled with the abundant intra‐ and intermolecular β‐sheets, confers remarkable mechanical robustness to the nanofilms with thicknesses as low as 50 nm. Consequently, these nanofilms exhibit the unique ability to stand freely in the air and can be conformally transferred to substrates of different types and topographies, forming stable, and versatile nanocoatings. The advantageous attributes of their ultrathinness, large‐scale homogeneity, robustness, and rapid and tunable responsiveness facilitate the construction of ultrasensitive photonic architectures and organic–inorganic hybrid multilayers with outstanding optical and mechanical properties. It is envisioned that these all‐protein‐based robust nanofilms with unique nanoscale effects can be easily integrated into various advanced technological platforms to enable adaptive, multifunctional, and intelligent systems with exceptional mechanical strength.

A Large-Scale Sensing-Monitoring Integrated Aircraft Smart Skin with Ultralow Weight and Ultralow Power Consumption for Structural Health Monitoring

A Large-Scale Sensing-Monitoring Integrated Aircraft Smart Skin with Ultralow Weight and Ultralow Power Consumption for Structural Health Monitoring

Soft Kirigami Composites for Form‐Finding of Fully Flexible Deployables

AbstractA new class of thin flexible structures is introduced that morph from flat into prescribed 3D shapes through strain mismatch between layers of a composite plate. To achieve control over the target shape, two different concepts are coupled. First, motivated by biological growth, strain mismatch is applied between the flat composite layers to transform it into a 3D shape. Depending on the amount of the applied strain mismatch, the transformation involves buckling into one of the available finite number of deformation modes. Second, inspired by kirigami, portions of the material are removed from one of the layers according to a specific pattern. This dramatically increases the number of possible 3D shapes and allows us to attain specific topologies. An experimental apparatus that allows precise control of the strain mismatch is devised. An inverse problem is posed, where starting from a given target shape, the physical parameters that make these shapes possible are determined. To show how the concept works, it focuses on circular composite plates and designs a kirigami pattern that yields a hemispherical structure. The analysis combines a theoretical approach with numerical simulations and physical experiments to understand and predict the shape transition from 2D to 3D. The tools developed here can be extended to attain arbitrary 3D shapes. The initially flat shape suggests that conventional additive manufacturing techniques can be used to functionalize the soft kirigami composite to fabricate, for example, deployable 3D structures, smart skins, and soft electromagnetic metasurfaces.

Ionic Conductivity Switchable and Shape Changeable Smart Skins with Azobenzene‐Based Ionic Reactive Mesogens

AbstractTo develop smart ionic skins capable of multi‐responsiveness and switchable ionic conductivity, a stimuli‐responsive and ionic conductive azobenzene‐based monomer is newly synthesized, uniaxially oriented, and polymerized for anisotropic liquid crystal elastomers (LCEs). Since the uniaxially oriented monodomain LCE with ionic asymmetric azobenzene monomers (i‐AAM) is prepared by thermal oligomerization, uniaxial stretching, and subsequent photopolymerization, the stimuli‐responsive i‐AAM LCE is thermally contracted by increasing the temperature above TNI as well as is bent along the aligned direction by irradiating it with UV light. In addition to the change of shape, the ionic conductivity of the LCE is reversible in response to heat and light stimuli. Polydomain i‐AAM LCE polymerized without the stretching process exhibits higher ionic conductivity than stretched monodomain LCE due to the initially formed stable ionic pathways. Ionic conductivity can also be switched simultaneously by polarity changes in response to the photoisomerization of i‐AAM, increased mobility from heat, and an elastic mechanoresponse from external stresses. These i‐AAM LCE‐based soft grippers exhibit stimuli‐responsive grasping and releasing actuation and detect ionic conductivity switches finely, suggesting potential as smart skins for advanced soft robots.

Fabrication of Laser-Induced Graphene Based Flexible Sensors Using 355 nm Ultraviolet Laser and Their Application in Human-Computer Interaction System.

In recent years, flexible sensors based on laser-induced graphene (LIG) have played an important role in areas such as smart healthcare, smart skin, and wearable devices. This paper presents the fabrication of flexible sensors based on LIG technology and their applications in human-computer interaction (HCI) systems. Firstly, LIG with a sheet resistance as low as 4.5 Ω per square was generated through direct laser interaction with commercial polyimide (PI) film. The flexible sensors were then fabricated through a one-step method using the as-prepared LIG. The applications of the flexible sensors were demonstrated by an HCI system, which was fabricated through the integration of the flexible sensors and a flexible glove. The as-prepared HCI system could detect the bending motions of different fingers and translate them into the movements of the mouse on the computer screen. At the end of the paper, a demonstration of the HCI system is presented in which words were typed on a computer screen through the bending motion of the fingers. The newly designed LIG-based flexible HCI system can be used by persons with limited mobility to control a virtual keyboard or mouse pointer, thus enhancing their accessibility and independence in the digital realm.

Structural and dimensional engineering of three-dimensional carbon nanotube/polydimethylsiloxane composite for stretchable sensor

A highly sensitive strain sensor is essential for the wearable electronic in constantly emerging applications such as human motion monitoring, personalized healthcare, human-machine interaction, soft robot and smart skin. However, most of the physical sensors currently reported are facing with challenges of non-adjustable sensing performance, unsatisfactory linearity, complicated preparation processes, and small working ranges, which greatly restricts their practical applicability. In this work, we proposal a facile technology base on the structural and dimensional engineering of carbon nanotube (CNT) conducting networks, to fabricate the flexible and stretchable strain sensor with tunable sensitivity and superior linearity in a wide sensing range. The three-dimensional CNT frameworks with adjustable density were constructed by the chemical vapor deposition method, which were used as the filler network to prepare the flexible CNT/polydimethylsiloxane (CNT/PDMS) composites. The resultant strain sensors base on the CNT/PDMS composites can realize the tunable sensitivities (gauge factor ranges from 0.7 to 233 at 100 % strain) by simply changing and regulating the structural density and macro-size of sensing strips. Meanwhile, the obtained sensors possess excellent linearity (R2 > 90 %) and repeatability on the 10 000 cycles. The technology developed in this work will offer a wide range of practical applications in wearable electronics field.

Microring structure for flexible polymer waveguide-based optical pressure sensing.

Flexible pressure sensors provide a promising platform for artificial smart skins, and photonic devices provide a new technique to fabricate pressure sensors. Here, we present a flexible waveguide-based optical pressure sensor based on a microring structure. The waveguide-based optical pressure sensor is based on a five-cascade microring array structure with a size of 1500 µm × 500 µm and uses the change in output power to linearly characterize the change in pressure acting on the device. The results show that the device has a sensing range of 0-60 kPa with a sensitivity of 23.14 µW/kPa, as well as the ability to detect pulse signals, swallowing, hand gestures, etc. The waveguide-based pressure sensors offer the advantages of good output linearity, high integration density and easy-to-build arrays.

Deep Learning-Based Subsurface Damage Localization Using Full-Field Surface Strains.

Structures in their service life are often damaged as a result of aging or extreme events such as earthquakes or storms. It is essential to detect damage in a timely fashion to ensure the safe operation of the structure. If left unchecked, subsurface damage (SSD) can cause significant internal damage and may result in premature structural failure. In this study, a Convolutional Neural Network (CNN) has been developed for SSD detection using surface strain measurements. The adopted network architecture is capable of pixel-level image segmentation, that is, it classifies each location of strain measurement as damaged or undamaged. The CNN which is fed full-field strain measurements as an input image of size 256 × 256 projects the SSD onto an output image of the same size. The data for network training is generated by numerical simulation of aluminum bars with different damage scenarios, including single damage and double damage cases at a random location, direction, length, and thickness. The trained network achieves an Intersection over Union (IoU) score of 0.790 for the validation set and 0.794 for the testing set. To check the applicability of the trained network on materials other than aluminum, testing is performed on a numerically generated steel dataset. The IoU score is 0.793, the same as the aluminum dataset, affirming the network's capability to apply to materials exhibiting a similar stress-strain relationship. To check the generalization potential of the network, it is tested on triple damage cases; the IoU score is found to be 0.764, suggesting that the network works well for unseen damage patterns as well. The network was also found to provide accurate predictions for real experimental data obtained from Strain Sensing Smart Skin (S4). This proves the efficacy of the network to work in real-life scenarios utilizing the full potential of the novel full-field strain sensing methods such as S4. The performance of the proposed network affirms that it can be used as a non-destructive testing method for subsurface crack detection and localization.

A novel material identification method for different thermal conductivities materials based on the micro thermoelectric generator

An artificial intelligence (AI) technical system is the foundation of the internet of things. While in the technical system, the material identification technology is the key point. Material identification technology can be applied to AI robots and smart skin. However, most of the traditional material identification technologies have limitations such as high cost and complicated operation, while the low-cost ones have the problem of low accuracy. To accurately, efficiently, and simply identify objects is a bottleneck that researchers urgently need to overcome. Based on the thermoelectric effect, a material identification theory is proposed and a prototype is fabricated in this paper. A series of experiments are carried out to test the performance of the prototype. According to the results, the prototype can precisely identify metal materials and nonmetal materials less than 3 s. There are many merits of the prototype, such as high identification accuracy, low cost, and low maintenance difficulty. Besides, considering complex surface of materials may decrease the identification accuracy, this paper proposes a kind of flexible thermal conductive material to improve identification accuracy. It is verified that the identification accuracy is improved with the flexible thermal conductive material.

Additively manufactured flexible on-package phased array antennas for 5G/mmWave wearable and conformal digital twin and massive MIMO applications

This paper thoroughly investigates material characterization, reliability evaluation, fabrication, and assembly processes of additively manufactured flexible packaging and reconfigurable on-package antenna arrays for next-generation 5G/mmWave wearable and conformal applications. The objective is to bridge the technology gap in current Flexible Hybrid Electronics (FHE) designs at mmWave frequencies and address the challenges of establishing future design standards for additively manufactured flexible packages and System-on-Package (SoP) integrated modules. Multiple 3D printed flexible materials have been characterized for their electrical and mechanical properties over the 5G/mmW frequency band (26–40 GHz), and the inkjet printed interconnects on 3D printed Polypropylene (PP) substrates demonstrated excellent electrical and mechanical performance during a 10,000-time cyclic bending test over typical wearable flexible radii down to 1 inch. A proof-of-concept flexible on-package phased array with an integrated microfluidic cooling channel on 3D printed substrates was fabricated and measured, demonstrating pm 37^{circ } beam steering capability with efficient cooling. The proposed reconfigurable design and low-temperature fabrication approach using additive manufacturing can be widely applied to next-generation highly-complex on-demand FHE, flexible multi-chip-module integration, and on-package phased-array modules for 5G/mmWave wearable and conformal smart skin, digital twin and massive MIMO applications.

CNT/PDMS conductive foam-based piezoresistive sensors with low detection limits, excellent durability, and multifunctional sensing capability

In this paper, a carbon nanotube (CNT)/polydimethylsiloxane (PDMS) conductive composite foam (CCF) was fabricated via a dual-solvent ice template (DSIT) process, whose structure features the conductive filler CNT "embedded" in the cell wall surface, and this CCF was applied to the field of flexible piezoresistive sensing. Benefiting from the sensitive conductive network constructed by the DSIT process, the CNT/PDMS CCF-based piezoresistive sensor can effectively detect compression strains down to 0.1% and exhibits excellent and stable response at compression strains up to 90%. In addition, the CCF shows fast response and recovery times (54 ms and 65 ms), as well as excellent durability and stability (2000 cycles). An electronic skin assembled from the CCF into 5 × 5 pixels was applied to detect the magnitude and spatial distribution of forces and strains. The CCF was also applied for roughness recognition, optical and thermal sensing responses, which shows its potential applications in personalized medical monitoring, electronic smart skin fabrication, external environment monitoring and other fields.

Smart e-Skin Cancer Care in Europe During and after the Covid-19 Pandemic: a Multidisciplinary Expert Consensus.

Melanoma is the deadliest of all the skin cancers and its incidence is increasing every year in Europe. Patients with melanoma often present late to the specialist and treatment is delayed for many reasons (delay in patient consultation, misdiagnosis by general practitioners, and/or limited access to dermatologists). Beyond this, there are significant inequalities in skin cancer between population groups within the same country and between countries across Europe. The emergence of the COVID-19 pandemic only aggravated these health deficiencies. The aim was to create an expert opinion about the challenges in skin cancer management in Europe during the post COVID-19 acute pandemic and to identify and discuss the implementation of new technologies (including e-health and artificial intelligence defined as "Smart Skin Cancer Care") to overcome them. For this purpose, an ad-hoc questionnaire with items addressing topics of skin cancer care was developed, answered independently and discussed by a multidisciplinary European panel of experts comprising dermatologists, dermato-oncologists, patient advocacy representatives, digital health technology experts, and health technology assessment experts. After all panel of experts discussions, a multidisciplinary expert opinion was created. As a conclusion, the access to dermatologists is difficult and will be aggravated in the near future. This fact, together with important differences in Skin Cancer Care in Europe, suggest the need of a new approach to skin health, prevention and disease management paradigm (focused on integration of new technologies) to minimize the impact of skin cancer and to ensure optimal quality and equity.

Construction and mechanism study of lignin-based polyurethane with high strength and high self-healing properties

Lignin is a natural polymer with abundant functional groups with great application prospects in lignin-based polyurethane elastomers with self-healing abilities. In this study, a lignin self-healing polyurethane (PUDA-L) was specially designed using lignin as the raw material of polyurethane, combining lignin with Diels-Alder (DA) bond and hydrogen bonds. The experimental results showed that PUDA-L was prepared with good thermal stability, fatigue resistance, shape memory effect, excellent mechanical strength, and self-healing ability by partially replacing the crosslinking agents with bio-based lignin and hydroxylated modified lignin to increase the hydroxyl content. Polyurethane has a tensile strength of up to 29 MPa and an elongation at break of up to 500 %. The excellent self-healing ability of PUDA-L originates from the internal DA bonds and cross-linked hydrogen bonds. After the dumbbell sample was fused and heated at 130 °C for 4 h, the elastomer could be completely healed, the tensile strength was restored to 29 MPa, and the self-healing efficiency was up to 100 %. The developed PUDA-L elastomer has promising applications in sensors and smart skins.

Autonomous Adaption of Intelligent Humidity‐Programmed Hydrogel Patches for Tunable Stiffness and Drug Release

AbstractIntelligent humidity‐programmed hydrogel patches with high stretchability and tunable water‐uptake and ‐release are prepared by copolymerization and crosslinking of N‐isopropylacrylamide and oligo(ethylene glycol) comonomers. These intelligent elastomeric patches strongly respond to different humidities and temperatures in terms of mechanical properties which makes them applicable for soft robotics and smart skin applications where autonomous adaption to environmental conditions is a key requirement. It is shown that beyond using the hydrogel in the conventional state in aqueous media, new patches can be controlled by relative humidity. This humidity programming of the patches allows to tune drug release kinetics, opening potential application fields such as skin wound therapy and personalized medication. In situ dynamic‐mechanical measurements show a huge dependence on temperature and humidity. The glass transition temperature Tg shifts from around 60 °C at dry conditions to below 0 °C for 75% r.h. and higher. The storage modulus is tunable over more than four orders of magnitude from 0.6 up to 400 MPa. Time‐temperature superposition in master curves allows to extract relaxation times over 14 orders of magnitude. With strains at break of over 200% the patches are compliant with human skin and therefore patient‐friendly in terms of adapting to movements.