Bionic Skin Research Articles

Multimode Visualization of Electronic Skin from Bioinspired Colorimetric Sensor

Bioskins possess a great ability to detect and deliver external mechanical or temperature stimuli into identifiable signals such as color changes. However, the integration of visualization with simultaneous detection of multiple complex external stimuli in a single biosensor device remains a challenge. Here we propose an all-solution-processed bioinspired stretchable electronic skin with interactive color changes and four-mode sensing properties. The fabricated biosensor demonstrates sensitive responses to various stimuli including pressure, strain, voltage, and temperature. Sensing visualization is realized by color changes of the e-skin from brown to green and finally bright yellow as a response to intensified external stimuli, suggesting great application potential in military defense, healthcare monitoring, and smart bionic skin.

High-sensitivity tactile sensor based on Ti2C-PDMS sponge for wireless human–computer interaction

Tremendous attention has been paid to high-performance flexible tactile sensors owing to their potential applications in bionic skin, wearable electronics, the Internet of Things, etc. However, the majority of pressure sensors require an intricately designed nanostructure requiring a high-cost complex manufacturing process. Therefore, the high-throughput and low-cost technology to produce high-sensitivity, flexible, pressure-sensitive materials with a large responding range is urgently needed. Herein, a novel flexible piezoresistive tactile sensor is fabricated based on the Ti2C-PDMS sponge as the conductive elastomer. The sensor exhibits a high sensitivity of 279 kPa−1 in a wide pressure range (0–34.4 kPa). The response time is as fast as 0.45 s with excellent durability over 4,000 cycles. Moreover, a 16-pixel wireless sensor system is fabricated and a series of applications have been demonstrated, including real-time force perception and pressure morphology feedback, which promote the potential applications in the visualizing of pressure distribution, human–machine communication and wearable devices.

The composite hydrogel with “2D flexible crosslinking point” of reduced graphene oxide for strain sensor

AbstractIntelligent hydrogels with excellent flexibility, biocompatibility, and stimulus responsivity can mimic the functions of the skin to detect human motions. However, the low mechanical strength limits its application in the field of biomimetic materials. In this work, polyacrylamide‐reduced graphene oxide (PAM‐rGO) composite hydrogels were prepared by the combination of PAM and partially rGO, and their biomimetic strain sensors were studied. The rGO played the role of “2D flexible crosslinking point” in the composite hydrogel. Through the H‐bonds between rGO and hydrogels, the toughness and strength of the composite hydrogel were enhanced. The maximum strain of the hydrogel changed from 751% to 1097%, and the maximum stress changed from 0.065 to 0.20 MPa. On the other hand, the interaction between the PAM backbone and the rGO provided a credible resistance response to the stimulation of strain. The better linear relationship between resistance and length was built, with R2 of 0.992. Furthermore, the composite hydrogels were assembled into wearable devices to monitor human‐motion, including fingers bending, elbows bending and walking. The experimental results showed that the PAM‐rGO composite hydrogel had great potential in the field of bionic skin.

Analysis of Mechanical Characteristics of Bionic Artificial Skin Using Different Suturing Patterns.

Artificial bionic skin material is playing an increasingly important role in the field of medicine and bionic engineering and becoming a research hotspot in many disciplines in recent years. In this work, the digital moiré method was used to measure the mechanical field of the bionic skin material under different suturing conditions. Through the digital image process, the deformation characteristics and the stress distribution near the contact area between the bionic skin material and the suture were obtained and discussed. The different healing effects caused by suturing mode were further explored, which can provide mechanical guidance for wound suturing in clinical medicine.

Bioinspired soft caterpillar robot with ultra-stretchable bionic sensors based on functional liquid metal

Soft robots have significant advantages in terms of flexibility and adaptability, leading to potential applications in the bionics field. Inspired by the caterpillars in nature, this work proposed a soft caterpillar robot (SCR) by integrating two types of ultra-stretchable bionic sensors on a dual air-chamber pneumatic network structure. In order to realize self-powered tactile sensing, four triboelectric nanogenerator tactile sensors (TTSs) based on functional liquid metal (FLM) with thorny-structured bionic whiskers are developed and attached on the SCR. Meanwhile, two ultra-stretchable resistive strain sensors (RSSs) by using FLM are covered as the bionic skin to sense self-body deformation of the SCR. The TTS has a fast response time of 0.03 s and a minimum perception of 0.05 kPa, which can be very sensitive to the unknown stimulus of various materials. The RSS with a relatively high sensitivity of 2.94 and small hysteresis of 1.42% possess the ultra-stretchable ability of 180% strain, which helps to adapt and adjust its own body bending and crawling. The biological perception capabilities of the SCR play a crucial role in mimicking bionic actions and response in an unknown environment, such as escaping from unexpected attacks as well as adaptive crawling through an unknown tunnel environment. Soft robots have significant advantages in terms of flexibility and adaptability, leading to potential applications in the bionics field. Inspired by the caterpillars in nature, this work proposed a soft caterpillar robot (SCR) by integrating two types of ultra-stretchable bionic sensors on a dual air-chambers of pneumatic network structure. In order to sense self-body deformation of the SCR, two ultra-stretchable resistive strain sensors (RSSs) by using functional liquid metal (FLM) are covered as the bionic skin. Meanwhile, four triboelectric tactile sensors (TTSs) based on FLM with thorny-structured bionic whiskers are developed and attached on the head, tail and body of the soft caterpillar, respectively, to realize self-powered tactile sensing. The biological perception capabilities of the SCR play a crucial role to mimic bionic actions and response in an unknown environment, such as escaping from unexpected attacks as well as adaptive crawling through an unknown tunnel environment. • The ultra-stretchable bionic sensors based on FLM were integrated on a SCR to realize bionic sensing. • The TTS was sensitive to the unknown stimulus of various materials. The RSS was sensitive to the bending of SCR. • Closed-loop control enabled SCR to achieve intelligent sensing and automatic motion.

Cephalopod-Inspired Chromotropic Ionic Skin with Rapid Visual Sensing Capabilities to Multiple Stimuli.

Biological skin systems can perceive various external stimuli through ion transduction. Especially, the skin of some advanced organisms such as cephalopods can further promptly change body color by manipulating photonic nanostructures. However, the current skin-inspired soft iontronics lack the rapid full-color switching ability to respond to multiple stimuli including tension, pressure, and temperature. Here, an intelligent chromotropic iontronics with these fascinating functions is developed by constructing a biomimetic ultrastructure with anisotropic electrostatic repulsion. This skin-like chromotropic iontronics can synchronously realize electrical response and optical visualization to mechanical strain and tactile sensation by adjusting the ultrastructure in cooperation with ionic mechanotransduction. Notably, it can perform instantaneous geometric changes to thermal stimuli via an anisotropic electrostatic repulsion interior. Such a capability allows bionic skin to transduce temperature or infrared light into ionic signals and color changes in real time. The design of anisotropic photonic nanostructures expands the intelligent application for soft iontronics at higher levels, providing a concise, multifunctional, interactive sensing platform that dynamically displays stimuli information on its body.

Investigating the Electrochemical Performance of Smart Self‐Powered Bionic Skin Fragment Based on Bioelectricity Generation

AbstractHuman skin, with beneficial elasticity and reparability, could sense the changes of the external environment through different receptors. Imitating these mechanical properties and perception of bionic skin with power supply function is an enormous challenge in the field of Internet of things and artificial intelligence. Herein, the neural signal transmission of human skin is imitated to create a smart self‐powered bionic skin fragment integrating skin and power supply functions. Unlike the traditional bionic skin in essence, it can intelligently perceive the outside world by using anion‐selective and cation‐selective gels to control exchangeable anions and cations to realize the change of resting current and action current, and it can maintain the relatively stable self‐powered current of 0.5 µA for nearly 2.2 h. Moreover, its mechanisms of current and voltage changes are systematically investigated. These results reveal that it can be applied to the synchronous transmission of signals for the next‐generation neurologically integrated soft engineering systems such as bionic sensors, or prosthetic devices in hybrids of living and nonliving systems.

Integrated flexible piezoresistive pressure sensor based on CB/CNTs/SR composite with SR buffer layer for wide sensing range

In the present study, an integrated flexible piezoresistive pressure sensor with a silicon rubber (SR) buffer layer is proposed to optimize the workable pressure range of flexible pressure sensor. The sensitive unit of the integrated flexible pressure sensor is made of composite elastic dielectric SR filled with carbon black (CB) and carbon nanotubes (CNTs). This paper introduces the preparation process, working principle and structure design of the CB/CNTs/SR composite with an SR buffer layer. In addition, the influence of electrode structure and buffer layer is studied. A dispersible interdigital electrode structure is selected to prevent the decline of pressure sensing sensitivity in a small range. The proposed flexible pressure sensor achieves a wide pressure range of 0–120 N. Facile fabrication, high repeatability, high stability and low cost are also excellent characteristics of the sensors investigated in this paper. In this study, a design proposal for wearable equipment and wearable bionic skin study is provided.

Transparent, highly-stretchable, adhesive, and ionic conductive composite hydrogel for biomimetic skin

Flexible, self-healing hydrogels with a variety of desirable properties are required for artificial skin applications. Here, we report a novel ionic conductive composite hydrogel consisting of polyacrylamide (PAM), a silk sericin (SS) network and NaCl for use on bionic skin. The use of NaCl made the hydrogels good ionic conductors. Intrinsic interactions between the PAM and SS endowed the hydrogel with good mechanical properties, including a high tensile strength of 0.36 MPa and a compressive strength of 0.38 MPa. The ionic hydrogel showed a high transparency of 93%, a high adhesion strength of 52 kPa, and good conductivity. After being fractured, two pieces of the ionic hydrogel were re-bonded without external stimulation, which simulated the self-healing properties of skin after injury. The hydrogel displayed moisturizing and anti-mildew properties after being stored at room temperature for 2 months. These ionic hydrogels may find applications in artificial skin, human–machine interfaces and soft robotics.

Conductive cold-resistant and elastic hydrogel: A potential bionic skin for human-machine interaction control over artificial limbs

This work enables conductive cold-resistant and elastic hydrogel for a new application in electric steering engine control, demonstrating a great potential as bionic skin for goal-directed control over artificial limbs. The photocrosslinking of acrylamide provides an elastic matrix that is functionally improved by physical crosslinking of K2CO3, carrageenan and locust bean gum. The hydrogel is highly elastic with the stress of 0.04 MPa at 650% strain, and can recover immediately after being stretched to 500% or pressed to 10% of its original length. It can keep its softness and conductivity at a low temperature of −10 °C. When an intermittent bending force is applied, it synchronously outputs the resistance variation signals between 1.1 and 0.9 times the original value. We take advantages of all these physical features to create a soft controller for the convenient manipulation of electric steering engine. We translate the bending-induced resistance variation signals into digital data of between 0 and 1023, which are then encoded to direct the electric steering engine for various controlled motility. A simple bending of hydrogel can activate the electric steering engine in 0.18 s, showing a highly-synchronous control over the dynamic response of electric steering engine in many models.

New direction's piezoelectricity and new applications of two-dimensional group V-IV-III-VI films: A theoretical study

Abstract It is challenging to realize the piezoelectricity of 2D films by exerting the stress along the direction perpendicular to the film plane. The ability to transfer the mechanical stress perpendicular to the films into electric energy can significantly enhance the transfer efficiency and have more broad application prospects, such as medical device (sphygmomanometer) and ultrathin nano-sensor devices (tactile sensor as the bionic skin of robot). Then the out-of-plane piezoelectric coefficients e33 and d33 of film have the special importance. We investigate the lattice and piezoelectricity properties of group V-IV-III-VI films (SnN–InO, GeN-GaO, and SiN–AlO) using first-principles calculations. It is found that they have the promising piezoelectric coefficients e33 and d33 due to their broken inversion symmetry along the direction perpendicular to the film plane. The e33 (38.69 × 10−10 C m−1) of SiN–AlO is the largest, which is larger than that (5.20 × 10−10 C m−1) of gallium nitride nanowires' surface layers by 7.4 times, while the out-of-plane piezoelectric strain coefficient d33 (5.29 × 103 pm V−1) of SnN–InO is the largest, which is one order of magnitude larger than that (0.42 × 103 pm V−1) of the Y-doped ZnO nano sheets. The special direction's piezoelectricity of the SiN–AlO and SnN–InO films indicates that they have the new applications in nanoscale energy harvesting devices, nano-sensor and other micro-electromechanical devices, which can transfer the mechanical stress perpendicular to the films into electric energy.

Multimodal Capacitive and Piezoresistive Sensor for Simultaneous Measurement of Multiple Forces.

Quantitative information on the magnitudes and directions of multiple contacting forces is crucial for a wide range of applications including human-robot interaction, prosthetics, and bionic hands. Herein we report a highly stretchable sensor integrating capacitive and piezoresistive mechanisms that can simultaneously determine multiple forces. The sensor consists of three layers in a sandwich design. The two facesheets serve as both piezoresistive sensors and electrodes for the capacitive sensor, with the core being a porous structure made by using a simple sugar particle template technique to give them high stretchability. The two facesheets contain segregated conductive networks of silver nanowires (AgNWs) and carbon nanofibers (CNFs). By measuring the changes in the electrical resistance of the facesheets and the capacitance between the facesheets, three separate mechanical stimuli can be determined, including normal pressure, in-plane stretch, and transverse shear force. The newly developed multidirectional sensor offers a significant opportunity for the next generation of wearable sensors for human health monitoring and bionic skin for robots.

Realization of flexible pressure sensor based on conductive polymer composite via using electrical impedance tomography

Efficient and accurate detection of pressure applied on diverse surfaces is essential for pressure sensors. In this paper, a flexible pressure sensor based on thermoplastic polyurethanes (TPU)-carbon black (CB) conductive composite and electrical impedance tomography (EIT) is introduced. The conductivity of the composite is sensitive and linearly responsive to external pressure signal, which is feasible for reconstructing pressure distribution by using EIT technique that measures voltage on the boundary of the composite and then reconstructs conductivity changes in the composite. The pressure sensor is fabricated by screen printing TPU-CB composite and Ag electrode layer on flexible Polyethylene terephthalate (PET) sheet, which shows excellent portability and full flexibility. Meanwhile, it requires few sensing units and internal transmission wires, beneficial for nondestructive measurement towards targets. We successfully use the pressure sensor to estimate the location and value of pressure applied on single or multiple regions, and further detect daily objects for practical applications, demonstrating the sensing ability of the pressure sensor. Our sensor has bright prospect in the fields of bionic skin in intelligent robots, wearable electrical devices, motion monitoring systems and touch-control instruments.

Strain-Sensitive Fluorescence from Two-Dimensional Organic Crystal.

Strain-sensitive fluorescence materials have great potential in sensing applications owing to their low cost, intuitive signal, and user friendliness. Organic crystals are one of the most developed fluorescence materials. However, modulation of the fluorescence by strain is still a challenge. Here, for the first time, we investigate the strain-sensitive fluorescence of the two-dimensional (2D) organic crystal. Without interlayer interactions, the molecular arrangement in a 2D crystal can be easily tuned, which results in photoluminescence transformation between monomer emission and excimer emission. The 2D organic crystal has higher sensitivity under strain, compared with bulk organic crystals, showing great potential in practical applications such as tactile monitors, chameleon bionic skin, and visible leakage alarms.

A bionic tactile plastic hydrogel-based electronic skin constructed by a nerve-like nanonetwork combining stretchable, compliant, and self-healing properties

To completely mimic the tactile sensing of natural skin, flexible conductive hydrogels (CHs) have been assembled into bionic skin. However, most CHs cannot perfectly rebuild the feeling of human skin due to their highly linear structure. In addition, CHs are difficult to meet the super-stretchable, rapid self-healing properties at the same time. Here, we innovatively incorporated a proanthocyanins/reduced graphene oxide (PC/rGO) composite with a nerve-like nanonetwork into a glycerol-plasticized polyvinyl alcohol-borax (PVA-borax) hydrogel system to obtain a bionic tactile PC/rGO/PVA hydrogel-based electronic skin, which perfectly simulates the tactual sensation of human skin and integrates excellent stretchability (>5000%), compliance (1 mm), self-healing (3 s, 95.73%) ability for the first time. Due to its unique structure and mechanical properties, this electronic skin has remarkable wearable and strain-sensitive (GF = 14.14) properties, which can mimic and detect some real skin epidermis movements such as finger bending, facial expression changes, and throat vocalization. Interestingly, the hydrogel can also be used as an adhesive electrode for the accurate detection of electrocardiograph (ECG) and electromyography (EMG) signals. More importantly, this work provides a new route in mimicking the natural skin’s tactile ability through a hierarchical design of hydrogel networks.

Sunlight-driven photochromic hydrogel based on silver bromide with antibacterial property and non-cytotoxicity

A novel photochromic hydrogel system was designed and fabricated based on silver bromide and cuprous bromide in poly(hydroxyethyl acrylate-co-polyacrylamide) hydrogel matrix. The first time we overcome the atoms side reaction and migration of halides in opening hydrogel system to realize photochromism. Here, polyvinyl pyrrolidone (PVP) and glycerol were employed as a surfactant and sustainable co-solvent, respectively, whose synergistic effect make AgBr endows the hydrogel with photochromic stability and repeatability. The photochromic hydrogel can be changed reversibly between light-blue and brown color for least 15 times in air under a simulated solar light irradiation. Both UV–vis absorption (approximately 408 nm) and the valence states of the metal ions were studied and verified the conversion between Ag+ and Ag0. The process of color changing was studied and the fabricated hydrogel with a half-life fading period (t1/2) was 6.78 min under a simulated solar light irradiation. Reasonably, the as-fabricated photochromic hydrogel displayed remarkable antibacterial ability to both Gram-positive- and Gram-negative bacterium with the Ag ions in the system. Interestingly, due to the little dosage of AgBr and CuBr in the hydrogel, it also possessed excellent non-cytotoxicity property to L929 Mouse Fibroblast cells. Since its excellent photochromic and biological properties, the hydrogel has potential applications in the field of smart materials such as photochromic contact lenses, sensors and artificial intelligent devices or bionic skin.

Progress in Triboelectric Materials: Toward High Performance and Widespread Applications

AbstractTriboelectric phenomena can be observed everywhere; however, they are consistently omitted from applications. Although almost all substances exhibit a triboelectrification effect in daily life, chemists as well as materials scientists have performed extensive investigations in both the aspects of basic science and practical applications to promote the development of triboelectric nanogenerators (TENGs). Here, a detailed survey of materials engineering for high triboelectric performance and multifunctional materials toward specific applications is summarized, including constructing micro/nanostructures, chemically modifying the frication surface, modulating bulk friction materials, the mechanism for improved performance, and preparing materials for implantable medical devices, bionic skin, and wearable electronic devices. Moreover, an in depth discussion of the current challenges and future efforts for strengthening the performance of TENGs is elaborated in detail, which will better guide new researchers toward a deeper understanding of and explorations about TENGs.

3D Printed Stretchable Tactile Sensors.

The development of methods for the 3D printing of multifunctional devices could impact areas ranging from wearable electronics and energy harvesting devices to smart prosthetics and human-machine interfaces. Recently, the development of stretchable electronic devices has accelerated, concomitant with advances in functional materials and fabrication processes. In particular, novel strategies have been developed to enable the intimate biointegration of wearable electronic devices with human skin in ways that bypass the mechanical and thermal restrictions of traditional microfabrication technologies. Here, a multimaterial, multiscale, and multifunctional 3D printing approach is employed to fabricate 3D tactile sensors under ambient conditions conformally onto freeform surfaces. The customized sensor is demonstrated with the capabilities of detecting and differentiating human movements, including pulse monitoring and finger motions. The custom 3D printing of functional materials and devices opens new routes for the biointegration of various sensors in wearable electronics systems, and toward advanced bionic skin applications.

Building bionic skin

One decade ago, my research group at the University of Tokyo created a flexible electronic mesh and wrapped it around the mechanical bones of a robotic hand. We had dreamed of making an electronic skin, embedded with temperature and pressure sensors, that could be worn by a robot. If a robotic health aide shook hands with a human patient, we thought, this sensor-clad e-skin would be able to measure some of the person's vital signs at the same time. Today we're still working intensively on e-skin, but our focus is now on applying it directly to the human body. Such a bionic skin could be used to monitor medical conditions or to provide more sensitive and lifelike prosthetics.

Effect of phase-change material on energy consumption of intelligent thermal-protective clothing

This article reports a study on the impact of phase-change materials (PCM) on intelligent thermal-protective clothing. The experiments were conducted in a climatic chamber in which the temperature was −15.0 °C. In the experiments, a device called a bionic skin model was used. The temperature at the simulated skin surface was controlled at 33.0 °C. Four clothing assemblies with the same structure and with/without PCM were studied. When the temperature of the second layer was lower than 27.0 °C, the conductive fabric was powered automatically on; when the temperature was higher than 29.0 °C, the conductive fabric was switched off. The results show that conductive fabric can significantly change the temperature distributions in clothing assemblies to increase the thermal-protective performance of the clothing, and that clothing assemblies with PCM can save around 30% of energy in the temperature control process.