Smart Skin Research Articles

Smart Skins Based on Assembled Piezoresistive Networks of Sustainable Graphene Microcapsules for High Precision Health Diagnostics

AbstractThe environmental impact of plastic waste has had a profound effect on our livelihoods and there is a need for future plastic‐based epidermal electronics to trend toward more sustainable approaches. Infusing graphene into the culinary process of seaweed spherification produces core‐shell, food‐based nanocomposites with properties exhibiting a remarkably high degree of tunability. Unusually, mechanical, electrical, and electromechanical metrics all became decoupled from one another, allowing for each to be individually tuned. This leads to the formation of a general electromechanical model which presents a universal electronic blueprint for enhanced performances. Through this model, performance optimization and system miniaturization are enabled, with gauge factors (G) >108 for capsule diameters (D) ≈290 µm and produced at a record rate of >100 samples per second. When coalesced into quasi‐2D planar networks, microcapsules form the basis of discrete, recyclable electronic smart skins with areal independent sensitives for muscular, breathing, pulse, and blood pressure measurements in real‐time.

Self-powered smart skins for multimodal tactile perception based on triboelectric and hygroelectric working principles

Human being perceives multiple tactile modalities in the process of sensation on the skin and interpretation in the brain. To date, several sensing techniques facilitate the accurate measurement of individual tactile modality, but multimodal static and dynamic sensing remain challenging. Moreover, low-cost and highly efficient interpretation techniques are still required for tactile perception. Herein, we present cost-effective and high-performing self-powered smart skins that mimic multimodal tactile perception, enabling accurate perception of pressure, vibration, and humidity in the process of sensation on the smart skin and interpretation by machine learning. The dynamic and static stimuli are encoded by triboelectric and hygroelectric principles in the smart skins, respectively, while the hygroscopic nature empowers humidity sensation capability in the smart skin with an accuracy rate as high as 84.0%−100.0%. We believe our smart skin will enable the smooth transition of e-skin into practical applications, such as robotics, prosthetics, healthcare, and intelligent industry.

Finger gesture recognition with smart skin technology and deep learning

Finger gesture recognition (FGR) was extensively studied in recent years for a wide range of human-machine interface applications. Surface electromyography (sEMG), in particular, is an attractive, enabling technique in the realm of FGR, and both low and high-density sEMG were previously studied. Despite the clear potential, cumbersome electrode wiring and electronic instrumentation render contemporary sEMG-based finger gestures recognition to be performed under unnatural conditions. Recent developments in smart skin technology provide an opportunity to collect sEMG data in more natural conditions. Here we report on a novel approach based on soft 16 electrode array, a miniature and wireless data acquisition unit and neural network analysis, in order to achieve gesture recognition under natural conditions. FGR accuracy values, as high as 93.1%, were achieved for 8 gestures when the training and test data were from the same session. For the first time, high accuracy values are also reported for training and test data from different sessions for three different hand positions. These results demonstrate an important step towards sEMG based gesture recognition in non-laboratory settings, such as in gaming or Metaverse.

Design of Active Sensing Smart Skin for Incipient Slip Detection in Robotics Applications

Tactile sensing is paramount for robots operating in human-centered environments to help understand interactions with objects. One challenging issue in tactile sensing is the slip detection problem. Currently, most slip detection methods use passive sensors, which only measure force or vibration, but not directly the contact area, which is one of the most reliable parameters for slip detection. Other methods, which do measure contact area such as optical methods often suffer from complex sensor configurations and lack of flexibility for customization. Active sensing, in contrast, has simple sensor configurations, and can also directly measure the contact area to improve the efficiency for slip detection. In this article, a novel active sensing smart skin was developed for incipient slip detection, which leverages piezoelectric transducers as actuators/sensors. First, a robotic fingertip with embedded actuator and sensor was created in which the actuator generates ultrasonic guided waves received by the sensor during a slip scenario. By analyzing the received signals using an attenuation-based method, the entire contact condition evolution can be monitored within 1 ms. Then, an optical method was used to validate the signal attenuation, which correlated consistently with the contact area. Finally, a unique robotic skin was created which demonstrated robust and sensitive response for incipient slip detection.

Chameleon-inspired design of dynamic patterns based on femtosecond laser-induced forward transfer

Chameleon can display variable body color/patterns under environmental stimulation via bioelectricity-controlled three layers of pigment cells and photonic crystals in the chameleon's skin. Inspired by color variation of chameleon, numerous “smart skins” have been achieved successfully, which possess great potential for optical information record and security. However, it remains challenging to produce full color dynamic fluorescent patterns with higher precision and flexibility to improve information capacity and anti-counterfeiting property. In this work, the micro-sized full color dynamic fluorescent patterns were successfully achieved by multiple transferring different colored quantum dots (QDs) from the carrier substrates to the receiving substrate via femtosecond laser-induced forward transfer (FsLIFT) technology using the chameleon skin as a prototype. The ability to dynamically display by facilely controlling the excitations is demonstrated, ensuring the information transformation function as chameleon skin. The resulting dynamic fluorescent patterns provide dual biomimetic of chameleon skin morphology and function. To illustrate the promising practical applications, flexible information encryption and information decryption relying on portable devices based on the fabricated dynamic fluorescent patterns have been achieved. It is anticipated that this bioinspired strategy is universal and promising for fabricating on-demand fluorescent dynamic patterns.

Active-passive compound metasurface for simultaneously manipulating radiation and scattering in a wide band

Metasurfaces, which are usually passive or non-radiating, have provided unprecedented degree of freedom especially for scattering manipulation. For the manipulating radiation, metasurfaces often serve as auxiliaries to radiators since they cannot radiate themselves. Thus, it is unfavorable for integration and miniaturization. In this work, we propose the concept of active–passive compound metasurface (APCM) and explore the possibility of utilizing meta-atoms as active radiators, so as to simultaneously achieve flexible radiation and scattering manipulations at the same time. To this end, a meta-atom structure with large reflection phase-span is delicately designed. In this way, the meta-atom can serve both as wideband radiator fed by a probe and reflection-phase modulator. Flexible scattering-pattern manipulation for one polarization state and high-efficiency radiation-pattern manipulation for the orthogonal polarization state can be simultaneously achieved within the same band from 8.0 to 12.0 GHz. As a proof of concept, two prototypes, one for scattering-cancellation and high-directive radiation while the other for deflected reflection and vortex-beam radiation, were designed, fabricated, and measured, respectively. Both the simulated and measured results verify the active–passive compound design. This work paves an alternative route to integrated multi-functional materials or interfaces and may find wide applications in next-generation communication, radar, smart skin, and others.

Flexible, monolithic piezoelectric sensors for large-area structural impact monitoring via MUSIC-assisted machine learning

Aircraft smart skin requires the integration of large-scale, ultrathin, and high-sensitive sensor network on the surface for structural health monitoring (SHM). However, it is fairly difficult to fabricate such large-area flexible sensor networks with low cost and high reliability. Although flexible and monolithic sensors are easy to be fabricated, their applications in large-area impact monitoring have yet to be developed and revealed. In this study, an impact monitoring and localization technology based on monolithic small-area sensors is proposed for large-area structures. The pivotal principle lies in the combination of the flexible piezoelectric sensor array and the Multiple Signal Classification (MUSIC)-assisted Artificial Intelligence Network (ANN) algorithm, where the key sorted feature matrix from the output signals to ANN can be captured by the MUSIC algorithm to achieve the spatial location estimation. The results show that the flexible piezoelectric sensors demonstrate excellent performance to monitor structural impacts in a region of more than 7500% of its area. Secondly, excellent conformation between the sensors and complex surfaces is achieved by the ultrathin thickness almost without affecting the surficial flow field. The overall recognition accuracy and robustness of impact location of machine learning is greatly increased by the integration of MUSIC algorithm, which is immune to the shape, material, thickness, or opening holes of the structure. Except for the impact location, impact energy, impact frequency, impact hardness and other structural health parameters can also be monitored. Therefore, a great prospect in the integrated monitoring of the aircraft’s structural and environmental parameters is expected.

Hydrogen-bonding topological remodeling modulated ultra-fine bacterial cellulose nanofibril-reinforced hydrogels for sustainable bioelectronics

Bacterial cellulose (BC) with its inherent nanofibrils framework is an attractive building block for the fabrication of sustainable bioelectronics, but there still lacks an effective and green strategy to regulate the hydrogen-bonding topological structure of BC to improve its optical transparency and mechanical stretchability. Herein, we report an ultra-fine nanofibril-reinforced composite hydrogel by utilizing gelatin and glycerol as hydrogen-bonding donor/acceptor to mediate the rearrangement of the hydrogen-bonding topological structure of BC. Attributing to the hydrogen-bonding structural transition, the ultra-fine nanofibrils were extracted from the original BC nanofibrils, which reduced the light scattering and endowed the hydrogel with high transparency. Meanwhile, the extracted nanofibrils were connected with gelatin and glycerol to establish an effective energy dissipation network, leading to an increase in stretchability and toughness of hydrogels. The hydrogel also displayed tissue-adhesiveness and long-lasting water-retaining capacity, which acted as bio-electronic skin to stably acquire the electrophysiological signals and external stimuli even after the hydrogel was exposing to air condition for 30 days. Moreover, the transparent hydrogel could also serve as a smart skin dressing for optical identification of bacterial infection and on-demand antibacterial therapy after combined with phenol red and indocyanine green. This work offers a strategy to regulate the hierarchical structure of natural materials for designing skin-like bioelectronics toward green, low cost, and sustainability.

3D Solar Harvesting and Energy Generation via Multilayers of Transparent Porphyrin and Iron Oxide Thin Films

Photovoltaic solar cells have been extensively used for various applications and are considered one of the most efficient green energy sources. However, their 2D surface area solar harvesting has limitations, and there is an increasing need to explore the possibility of multiple layer solar harvest for enhanced energy density. To address this, we have developed spectral-selective transparent thin films based on porphyrin and iron oxide compounds that allow solar light to penetrate multiple layers, significantly increasing solar harvesting surface area and energy density. These thin films are designed as photovoltaic (PV) and photothermal (PT) panels that can convert photons into either electricity or thermal energy for various green energy applications, such as smart building skins and solar desalination. The advantages of this 3D solar harvesting system include enlarged solar light collecting surface area and increased energy density. The multilayer system transforms the current 2D to 3D solar harvesting, enabling efficient energy generation. This review discusses recent developments in the synthesis and characterization of PV and PT transparent thin films for solar harvesting and energy generation using multilayers. Major applications of the 3D solar harvesting system are reviewed, including thermal energy generation, multilayered DSSC PV system, and solar desalination. Some preliminary data on transparent multilayer DSSC PVs are presented.

Wearable and Flexible Hydrogels for Strain Sensing and Wound Electrical Stimulation

Conductive hydrogels have attracted a great deal of interest in health testing and electrical stimulation therapy. To improve the inherent brittleness and stickiness of hydrogels, a multifunctional electronic skin mediated by dopamine-modified carbon nanotubes and gelatin was fabricated. In this work, a flexible, conductive, and biocompatible adhesive hydrogel was prepared directly without further processing by simply mixing dopamine-modified carbon nanotubes, gelatin, and acrylamide. Carbon nanotubes play a dominant role in the electromechanical and mechanical properties of the hydrogel. The polydopamine gives the hydrogel good and reproducible adhesion properties. At the same time, the hydrogels obtained have good sensing properties and can detect mechanically conducted signals of human motion. In addition, the hydrogels are able to act as conductive media to deliver electrical stimuli generated by poly(vinylidene fluoride-trifluoroethylene) piezoelectric membranes to accelerate wound healing. In conclusion, this work provides a feasible method to prepare hydrogel sensors with good biocompatibility, good mechanical properties, and good adhesion, which can be widely used in human motion detection, smart skin, and electrical stimulation therapy.

Subsurface damage detection via noncontact laser based surface level strain sensing smart skin with carbon nanotubes

Subsurface damage may remain undetected for long periods and accumulate until it leads to structural failure. Non-destructive measurements of surface strain can provide critical data for detecting early symptoms of hidden damage and helping to prevent serious consequences. This study investigates the capabilities of the developed method called strain-sensing smart skin (S4) to find internal damage. The S4 method uses single-wall carbon nanotubes in a thin polymer coating as optically interrogated fluorescent strain sensors to measure the strain map. A set of aluminum specimens with internal cavities representing damage were coated both with S4 films and with digital image correlation (DIC) speckle patterns. Maps of surface strain from the two methods were then compared (after the application of tensile stress to the specimens) to compute finite element method (FEM) strain maps. The strain maps obtained from S4 point-wise laser scanning agreed well with FEM predictions, while DIC strain maps were not very precise. S4 maps revealed strain map features with superior accuracy. It is concluded that S4 strain mapping can be used as a non-destructive evaluation method for detecting and visualizing subsurface damage in structural elements for structural health monitoring.

High-Sensitivity Composite Dual-Network Hydrogel Strain Sensor and Its Application in Intelligent Recognition and Motion Monitoring

Flexible electronic sensors are attracting more and more attention because of their outstanding flexibility and foldability. Based on this background, a composite hydrogel with excellent flexibility, good stability, and high sensitivity is prepared in this work. This high-performance hydrogel is composed of polyacrylamide, sodium alginate, MXene, and PEDOT:PSS through dynamic supramolecular cross-linking. The electrical conductivity is significantly enhanced by the addition of the conductive polymer PEDOT:PSS and MXene. Hydrogel conductivity increased by 150% with the addition of conductive materials at the same tensile strain. It exhibits high sensitivity (GF = 1.99) and excellent linearity over a wide operating range of 1000% of strain, which ensures the detection performance of the hydrogel sensor under large strain conditions. In addition, it has excellent repeatability (600 cycles) and fast response/recovery times (0.624 s/0.912 s). Due to this excellent performance, it was made into a smart sensor to explore its application in smart electronic skin. The hydrogel sensor and data acquisition module were integrated for real-time monitoring of pressure. We combine it with deep learning algorithms to provide a new solution for human motion recognition, such as joint flexion, facial expression change, health monitoring, and handwriting recognition.

Hydrophobic and multifunctional strain, pressure and temperature sensor based on TPU/SiO2-ILs ionogel for human motion monitoring, liquid drop monitoring, underwater applications

Ionogel-based flexible electronic devices have attracted extensive attention from industry and academia due to their unique advantages in wearable electronic devices, human motion sensing, etc. However, the internal ionic liquids (ILs) or polymers have weak resistance to liquid interference, which limits the development of ionogel in the actual environment. In this paper, TPU/SiO2-ILs (Thermoplastic polyurethane/SiO2-Ionic liquids) ionogel with micro-nano structures on the surface was constructed by sandpaper template method and further modified by 1 H, 1 H, 2 H, 2 H-perfluorooctyltrichlorosilane to enhance its hydrophobicity. The ionogel sensor has a high sensitivity, a fast strain response time (346 ms), and excellent mechanical properties. In addition, the sensor can still show a stable response signal after 1000 s of strain and compression cycle testing. The ionogel sensor’s excellent hydrophobic properties allow it to be used not only in wearable electronic devices to detect human physiological activity in dry or underwater environments and for writing recognition systems, but also to monitor tiny response signals caused by toy fish wiggling and droplet dripping. Most importantly, compared with other sensors that can only monitor a single stimulus, the TPU/SiO2-ILs ionogel sensor has a wide and multifunctional detection range for stimuli in different environmental such as strain (0.1–200%), pressure (0.6–25 kPa) and temperature (30–100 °C). Thus, the TPU/SiO2-ILs ionogel shows great potential for applications in areas such as smart electronic skin, human motion monitoring, underwater communication and underwater environmental monitoring.

Facile Transfer of a Transparent Silver Nanowire Pattern to a Soft Substrate Using Graphene Oxide as a Double-Sided Adhesion-Tuning Layer.

Silver nanowires (AgNWs) have been employed in various optoelectronic devices as transparent electrodes. However, it remains a great challenge to facilely pattern silver nanowires to realize desirable soft skin devices. Here, we develop an intact transfer method via a double-layered adhesion regulator of graphene oxide (GO) enabling complete transfer of a silver nanowire pattern from a tough substrate onto soft polydimethylsiloxane (PDMS) and flexible polyethylene (PE). We achieve positive and negative patterns simultaneously when selectively transferring silver nanowire patterns. The resulting patterned AgNW electrodes have uniform conductivity and long-term stability. The underlying mechanism of the clean transfer is thoroughly investigated via transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). GO plays a role in reducing the adhesion of AgNW to the donor tough substrate and enhancing adhesion of AgNW to the target soft substrate simultaneously. Finally, we demonstrate the utility of the patterned electrodes as transparent sensors detecting body motion. This work offers an effective solution to the challenging patterning problem of silver nanowires on a hydrophobic soft substrate, which is compatible with the soft component in emerging smart skin or wearable electronics.

Smart Skin‐Adhesive Patches: From Design to Biomedical Applications

AbstractWith the advancement of medical and digital technologies, smart skin adhesive patches have emerged as a key player for complex medical purposes. In particular, skin adhesive patches with integrated electronics have created an excellent platform for monitoring health conditions and intelligent medication. However, the efficient design of the adhesive patches is still challenging as it requires a strong combination of network structure, adhesion, physical properties, and biocompatibility. To design an assimilated device, one must have a deep knowledge of various skin adhesive patches. This article provides a comprehensive review of the recent advances in skin‐adhesive patches, including hydrogel‐based adhesive patches, transdermal patches, and electronic skin (E‐skin) patches, for various biomedical applications such as wound healing, drug delivery, biosensing, and health monitoring. Furthermore, the key challenges, implementable strategies, and future designs that can potentially provide researchers in designing innovative multipurpose smart skin patches are discussed. These advanced approaches are promising for managing the health and fitness of patients who require regular medical care.

An Overview of Flexible Sensors: Development, Application, and Challenges.

The emergence and advancement of flexible electronics have great potential to lead development trends in many fields, such as "smart electronic skin" and wearable electronics. By acting as intermediates to detect a variety of external stimuli or physiological parameters, flexible sensors are regarded as a core component of flexible electronic systems and have been extensively studied. Unlike conventional rigid sensors requiring costly instruments and complicated fabrication processes, flexible sensors can be manufactured by simple procedures with excellent production efficiency, reliable output performance, and superior adaptability to the irregular surface of the surroundings where they are applied. Here, recent studies on flexible sensors for sensing humidity and strain/pressure are outlined, emphasizing their sensory materials, working mechanisms, structures, fabrication methods, and particular applications. Furthermore, a conclusion, including future perspectives and a short overview of the market share in this field, is given for further advancing this field of research.

Chitosan-Based Hydrogels for Bioelectronic Sensing: Recent Advances and Applications in Biomedicine and Food Safety.

Due to the lack of efficient bioelectronic interfaces, the communication between biology and electronics has become a great challenge, especially in constructing bioelectronic sensing. As natural polysaccharide biomaterials, chitosan-based hydrogels exhibit the advantages of flexibility, biocompatibility, mechanical tunability, and stimuli sensitivity, and could serve as an excellent interface for bioelectronic sensors. Based on the fabrication approaches, interaction mechanisms, and bioelectronic communication modalities, this review divided chitosan-based hydrogels into four types, including electrode-based hydrogels, conductive materials conjugated hydrogels, ionically conductive hydrogels, and redox-based hydrogels. To introduce the enhanced performance of bioelectronic sensors, as a complementary alternative, the incorporation of nanoparticles and redox species in chitosan-based hydrogels was discussed. In addition, the multifunctional properties of chitosan-based composite hydrogels enable their applications in biomedicine (e.g., smart skin patches, wood healing, disease diagnosis) and food safety (e.g., electrochemical sensing, smart sensing, artificial bioelectronic tongue, fluorescence sensors, surface-enhanced Raman scattering). We believe that this review will shed light on the future development of chitosan-based biosensing hydrogels for micro-implantable devices and human-machine interactions, as well as potential applications in medicine, food, agriculture, and other fields.

Development of Multilayer Transducer and Omnidirectional Reflection Model for Active Reflection Control.

Underwater detection is accomplished using an underwater ultrasonic sensor, sound navigation and ranging (SONAR). Stealth to avoid detection by SONAR plays a major role in modern underwater warfare. In this study, we propose a smart skin that avoids detection by SONAR via controlling the signal reflected from an unmanned underwater vehicle (UUV). The smart skin is a multilayer transducer composed of an acoustic window, a double-layer receiver, and a single-layer transmitter. It separates the incident signal from the reflected signal from outside through the time-delay separation method and cancels the reflected wave from the phase-shifted transmission sound. The characteristics of the receiving and transmitting sensors were analyzed using a finite element analysis. Three types of devices were compared in the design of the sensors. Polyvinylidene fluoride (PVDF), which had little effect on the transmitted sound, was selected as the receiving sensor. A stacked piezoelectric transducer with high sensitivity compared to a cymbal transducer was used as the transmitter. The active reflection control system was modeled and verified using 2D 360° reflection experiments. The stealth effect that could be achieved by applying a smart skin to a UUV was presented through an active reflection-control omnidirectional reflection model.

Chameleon-inspired active tunable structural color based on smart skin with multi-functions of structural color, sensing and actuation.

Tunable structural color has many potential applications in artificial camouflage, mechanical sensors, etc. Despite the extensive efforts to develop efficient tunable structural color, there is still a wide gap between the existing "passive" tuning methods and the "active" strategy found on organisms such as chameleons that can change color according to the environment. Inspired by the active tunable color system of chameleons, we propose a smart skin comprising a nanoscale hole array of photonic crystals, carbon nanotube coatings, and liquid crystal elastomers, to integrate multiple functions, i.e., structural color tunability, sensing, and actuation, in one structure. The smart skin was further coupled with an image acquisition unit (which mimics eyes to obtain colors from the environment) and a controller (which mimics the brain to process the signals transmitted from the image acquisition unit to the smart skin), to construct an active tunable structural color system. The proposed system autonomously modulates the color according to the environmental color. To validate the color tuning, color scanning from red to green to blue or vice versa is demonstrated in this work, which could certainly open up new paths to create active tunable structural color systems, and thus, push the development of structural color-based devices and systems.

Positive/Negative Temperature Coefficient Behaviors of Electron Beam-Irradiated Carbon Blacks-Loaded Polyethylene Nanocomposites.

Polymer-based materials with positive temperature coefficients (PTC) are regarded as potential candidates for electrical heating elements in a wide range of applications, such as wearable electronics, soft robots, and smart skin. They offer many advantages over ceramic or metal oxide-based composites, including low resistance at room temperature, excellent flexibility and processability, and low cost. However, the electrical resistance instability and poor reproducibility have limited their use in practical applications. In this work, we prepared carbon blacks-reinforced high-density polyethylene nanocomposites (CBs-HDPE) loaded with polar additives (polyols or ionomers), which were subsequently subjected to electron beam (EB) irradiation to explore their PTC behaviors. We found that the EB-treated nanocomposites exhibited PTC behaviors, while the untreated samples showed negative temperature coefficients. Further, EB-ionomer-CBs-HDPE showed the highest PTC intensity of 3.01 Ω·cm, which was ∼35% higher than that of EB-CBs-HDPE. These results suggested that the EB irradiation enabled a specific volume expansion behavior via enhanced crosslinking among CBs, polar additives, and HDPE, inhibiting the formation of conductive networks in the nanocomposites. Thus, it can be concluded that polar additives and further EB irradiation played an important role in enhancing the PTC performances. We believe the findings provide crucial insight for designing carbon-polymer nanocomposites with PTC behaviors in various self-regulating heating devices.