E-skin Sensor Research Articles

Toward an AI Era: Advances in Electronic Skins.

Electronic skins (e-skins) have seen intense research and rapid development in the past two decades. To mimic the capabilities of human skin, a multitude of flexible/stretchable sensors that detect physiological and environmental signals have been designed and integrated into functional systems. Recently, researchers have increasingly deployed machine learning and other artificial intelligence (AI) technologies to mimic the human neural system for the processing and analysis of sensory data collected by e-skins. Integrating AI has the potential to enable advanced applications in robotics, healthcare, and human-machine interfaces but also presents challenges such as data diversity and AI model robustness. In this review, we first summarize the functions and features of e-skins, followed by feature extraction of sensory data and different AI models. Next, we discuss the utilization of AI in the design of e-skin sensors and address the key topic of AI implementation in data processing and analysis of e-skins to accomplish a range of different tasks. Subsequently, we explore hardware-layer in-skin intelligence before concluding with an analysis of the challenges and opportunities in the various aspects of AI-enabled e-skins.

Electronic Skin as Human–Robot Interface for Human Grasp Recognition in Home-Care Robots: Introducing a Complete Set of Resources

Population aging has highlighted the significance of developing home-care robots to assist human life. However, traditional home-care robots work according to fixed procedures, which lack human–robot interaction to mimic human hand action. Human–robot interfaces provide an opportunity for interactions between robots and users, which play an important role in home-care robots. Traditional human–robot interfaces are based on rigid, cumbersome, and high-priced machines, which prevent their wide application in performing home-care tasks. An e-skin sensor with 11 stretchable resistors evenly distributed on the back of the human hand, which could be applied to monitor human finger motion and control robotic fingers to dexterously grasp diverse objects, is presented in this article. A 3D printer is employed to pattern high-performance wires into structurally soft and stretchable filamentary serpentines. The stretchable resistors are fabricated by a thin sensing film of mixed multiwalled carbon nanotubes (MWCNTs) and silicone elastomer. The e-skin sensor is applied in a hand-in-the-loop system for controlling a robotic hand to help grasp an object synchronously. A robot system is developed to connect the e-skin with the robotic hand to perform grasp actions with a success rate of more than 80%. By analyzing the experimental results of the e-skin sensor and robotic hand grasp, this work obtains insights showing that the skin sensor can help engineers design a home-care assistant plan for patients according to monitoring value or a human–robot interaction mode by robotic fingers mimicking human finger actions.

Biometric-Tuned E-Skin Sensor with Real Fingerprints Provides Insights on Tactile Perception: Rosa Parks Had Better Surface Vibrational Sensation than Richard Nixon.

The dense mechanoreceptors in human fingertips enable texture discrimination. Recent advances in flexible electronics have created tactile sensors that effectively replicate slowly adapting (SA) and rapidly adapting (RA) mechanoreceptors. However, the influence of dermatoglyphic structures on tactile signal transmission, such as the effect of fingerprint ridge filtering on friction-induced vibration frequencies, remains unexplored. A novel multi-layer flexible sensor with an artificially synthesized skin surface capable of replicating arbitrary fingerprints is developed. This sensor simultaneously detects pressure (SA response) and vibration (RA response), enabling texture recognition. Fingerprint ridge patterns from notable historical figures - Rosa Parks, Richard Nixon, Martin Luther King Jr., and Ronald Reagan - are fabricated on the sensor surface. Vibration frequency responses to assorted fabric textures are measured and compared between fingerprint replicas. Results demonstrate that fingerprint topography substantially impacts skin-surface vibrational transmission. Specifically, Parks' fingerprint structure conveyed higher frequencies more clearly than those of Nixon, King, or Reagan. This work suggests individual fingerprint ridge morphological variation influences tactile perception and can confer adaptive advantages for fine texture discrimination. The flexible bioinspired sensor provides new insights into human vibrotactile processing by modeling fingerprint-filtered mechanical signals at the finger-object interface.

Hysteresis-Free Temperature Sensing with Printable Electronic Skins Made of Liquid Polyisoprene/CNTs.

Developing an electronic skin (e-skin) is becoming popular due to its capability to mimic human skin's ability to detect various stimuli. Mostly, such skins are tactile-based sensors. However, the exploration of nontactile-based sensing capability in the e-skin is still in a nascent stage. Herein, we report an approach toward developing electrical hysteresis- and cross-interference-free nontactile e-skin using liquid polyisoprene with an ultralow concentration of multiwalled carbon nanotubes (ϕ = 0.006 volume fraction) by leveraging the stencil printing technique. The impact of cross-linking the samples was studied. Uncross-linked samples demonstrated higher electrical conductivity than the cross-linked samples. A coarse-grained phenomenological model with molecular dynamics simulation was utilized to investigate filler network formation and percolation that dictate the conductivity of uncross-linked and cross-linked samples. Simulation studies supported the fidelity of the experimental findings. The uncross-linked e-skin demonstrated a higher temperature sensitivity (-1.103%/°C) than the cross-linked e-skin (-0.320%/°C) in the thermal conduction mode. Despite the superior sensitivity of the uncross-linked e-skin, the cross-linked systems demonstrated superior cyclic stability (35 thermal cycles), ensuring reliable sensor readings over extended usage. Judicious choice of encapsulant warranted the cross-linked e-skin sensor to nullify the impact of moisture on signal output, thereby providing cross-interference-free results. The optimized e-skin sample retained a similar thermal sensitivity even when used in the nontactile mode. From the application purview, the utility of the developed sensor was tested successfully for nontactile sensing of human body temperature. Additionally, the sensor was utilized to determine the respiratory profile by integrating the developed sensor into a wearable mask. This study advances nontactile e-skin-based sensing technology and opens new avenues for creating wearable and IoT devices for healthcare and human-machine interactions.

Evaporation-induced self-assembled ultrathin AgNW networks for highly conformable wearable electronics

The flexibility and stability of transparent electrodes play a crucial role in the growing popularity of flexible devices, especially in potential wearable electronics. To date, various solution-coating techniques have been developed for fabricating silver nanowire (AgNW) flexible bioelectronics. However, achieving the orderly distributed patterns of AgNW without undesirable aggregations still poses a grand challenge. Here, an approach to realize regular patterned ultrathin AgNW networks on a freestanding electrospun PVDF-TrFE frame by evaporation-induced self-assembly is proposed. The patterning mechanism of evaporating AgNW colloidal suspension is investigated from experimental and theoretical analysis. The influence of evaporation-induced flow inside colloidal freestanding membranes on forming regular square hole-shaped arrays, selective deposition of AgNW, and aligning them along the artificial pinning array are addressed. Owing to the orderly arrangement of AgNW networks, the resultant flexible electrode achieves ultrathin thickness (about 5 μm), high optical transmittance (87.8%), and low sheet resistance (8.4 Ω·sq−1) with a relatively low dosage of AgNW (9 μg·cm−2). The electrode exhibits excellent durability during cyclic bending (50,000 times) and stretching (50% strain). The resistance remains virtually unchanged during 200 days in everyday environments. Furthermore, the excellent conformability and breathability of the flexible transparent electrode attached to the human skin demonstrates its potential application as an e-skin sensor. Our findings reliably urge a simple approach to underscore better outcomes with effective patterns by self-assembly of AgNW for highly conformal wearable electronics.

Printing paper-derived ultralight and highly sensitive E-skin for health monitoring and information encryption

Simplified fabrication technology and excellent sensing performance are highly desired for the development of flexible and user-friendly wearable sensors. However, there is usually a trade-off between the simplicity of the fabrication technology and the sensing performance of the device. Recently, paper-based wearable sensors have attracted much attention due to their excellent flexibility, simplified preparation, and low cost etc. However, the pressure sensitivity of the paper-derived sensors is far from satisfactory. In this work, a strategy of pencil graphite frottage (PGF) is reported by employing sandpaper as template and a pencil sketch technique to achieve functional printing paper as pressure-sensitive layers for electronic skin (e-skin). As a result, the e-skin prepared by our PGF technique shows excellent reproducibility and pressure sensing performance, including a maximum sensitivity of 7202.2 kPa−1, a fast response time of 25.0 ms, and an excellent durability over 20,000 loading/unloading cycles, and high ambient stability (> 6 months). For application demonstration, the e-skins are applied to monitor human wrist pulses, acoustical vibration and information encryption. Furthermore, pressure sensing arrays are developed and employed to map the spatial pressure distribution, indicating potential application in intelligent robots and prosthetics. It is believed that the fabrication strategies proposed in this work can bring insight onto the other area of bionic e-skin and multifunctional sensor devices.

Thermochromic optical/electrical hydrated ionogel with anti-freezing and self-healing ability for multimodal sensor

Hydrogels featuring with high flexibility, comparable modulus to human skin, and biocompatibility, are deemed as an appropriate material for electronic-skins (e-skins). However, irreparable physical damage and poor environmental stability of the hydrogel-based sensor limit its practical applications in reality. Herein, we developed an ionohydrogel-based e-skin through in situ polymerizing thermoresponsive poly(oligo(ethylene glycol) methylacrylate) and acrylic acid in a hydrated ionic liquid (HIL) consisting of water and IL. Diversified hydrogen bonds among the polymer network, water and ILs enable the e-skin sensor with good self-healing ability. In addition, the inclusion of IL into the ionohydrogel greatly decreases the freezing point of water, imparting the e-skin sensor with excellent anti-freezing properties. More importantly, the temperature-induced switchable hydrophilia-hydrophobicity of polymer chain and the temperature-dependent destruction and reconstitution of the hydrogen bonds endow the ionohydrogel with switchable transparency, allowing intuitive visualization of ambient temperature. Attributing to the above merits, the ionohydrogel sensor could be used to monitor various human movements and visualize the change of temperature, showing promising applications in the multifunctional flexible sensor.

A 3D cross-linked hierarchical hydrogel E-skin with sensing of touch position and pressure

Artificial hydrogel electronic skin (E-skin) sensing systems have received considerable attention due to their ability in providing tactile perception. Recent advancements in hydrogel E-skin have allowed for simultaneous sensing of touch position and pressure through construction of sensor arrays. However, the fabrication process is complex, and the sensing resolution is limited by the size and number of array cells. Here, we report a multi-modal sensory E-skin based on a polyvinyl alcohol (PVA) hydrogel embedded with carbon nano-materials, which can recognize touch position and pressure simultaneously without constructing arrays. Combining the advantages of high electron transfer efficiency of 1D nanotube and 2D graphene nanosheet, the 3D hierarchical cross-linked polydopamine (PDA) modified reduced graphene oxide (rGO)/carboxylic multiwalled carbon nanotube (MWCNT-COOH) composites were proposed as a conductive filler, which was added by nano-doping to hydrogel substrate to gain an elastic modulus and stretchability of the hydrogel similar to that of human skin. Meanwhile, a surface-capacitive touch (SCT) system and piezoresistive technology were adopted to sense touch position and pressure due to the stability of conductive networks and the piezoresistive properties of carbon nanomaterials. Our E-skin device demonstrates superior sensing performance and provides a new direction for designing novel E-skin sensor devices.

An all-nanofibrous Janus textile with directional perspiration for triboelectric nanogenerator and self-powered e-skin sensor

Due to the increased demand for health monitoring and disease alarm, the self-powered epidermal electric sensors have attracted extensive attention, such as triboelectric nanogenerator (TENG)-based electric skin (e-skin). However, the humidity accumulation at the interface between e-skin and skin will threaten the stability and reliability of the monitoring signals. Moisture absorptivity and perspiration are still great challenges that need to be addressed. Here, inspired by the internal structure of plants used to transport water, we fabricated an all-nanofiber Janus textile via a continuous electrospinning/electrospray technology, presenting a double gradient variations in the pore size and wettability along the thickness direction. Janus textile was constructed from the hydrolyzed polyacrylonitrile (HPAN)/hexagonal boron nitride nanosheets (BNNS) fibrous membrane with small pores and hydrophilicity, conducting Ag nanowire (NW)layer, and polyurethane (TPU) nanofibrous membrane with large pores and hydrophobicity. By utilizing the wicking effect of external HPAN/BNNS layer, the sweat droplet could be transported from the interface between the skin and Janus textile to the external environment, meanwhile, the returned sweat will be blocked by interior TPU layer to keep the dry state of the skin. Therefore, the directional moisture transportation was realized. As the near-skin side, TPU was used as the other triboelectric layer, the optimal triboelectrification output reached 78.10 V, 0.16 μA, and 3.31 W m−2. Moreover, the damped Janus textile also exhibited an open-circuit voltage of 30 V, a short-circuit current of 0.051 μA, and a power density of 0.14 W m−2. With the assistance of an adhesive layer, Janus textile e-skin could easily adhere to the skin surface, generating stable movement and physiological sensing signals. In short, this work presents a noval way to construct Janus e-skin patch with directional sweating and breathability for monitoring long-term real-physiological signals.

Partially insoluble regenerated silk fibroin film induced by UV irradiation for electronic skins

The regenerated silk fibroin (RSF) film has been regarded as an ideal substrate for biocompatible, flexible, and biodegradable electronic skin (e-skin) devices. However, it is still a great challenge to balance the flexibility and solubility of the RSF film by adjusting its secondary structure. Herein, a film prepared with the hydrolyzed RSF was exposed to the 254 nm ultraviolet (UV) light to prepare a crosslinked and partially water-insoluble substrate for a strain-sensing e-skin. The hydrolyzed low-molecular-weight RSF was produced by heating the LiBr-silk fibroin solution at 85 °C for a certain duration. The film cast with the hydrolyzed RSF solution could be thoroughly dissolved in water rapidly. The UV irradiation could induce the crosslinking of the low-molecular-weight RSF to form insoluble substances, thus producing a partially insoluble RSF film. After silver nanowires painting, an e-skin strain sensor was successfully constructed based on the UV-irradiated film. The sensor shows a fast response time (2.01 s), high sensitivity (GF = 1.03 within 0%–40% strain range), and good stability. The device could be tightly attached to human skin with a drop of water. The finger, wrist, elbow, and knee bending could be sensitively detected in real-time. The head nodding and mouth opening could also be sensed by sticking the e-skin at the neck and cheek, respectively. This work may provide a facile way to prepare a stretchable and stickable RSF film, which could serve as an ideal substrate of low-cost, biodegradable, direct-to-skin sensors for wearable applications.

Flexible pressure and temperature sensors towards e-skin: material, mechanism, structure and fabrication

Electronic skin (E-skin) has gained significant attention due to its potential applications in the Internet of Things (IoT), artificial intelligence (AI), and flexible multi-sensing systems. Mimicking human skin, e-skin sensing devices can be employed in various scenarios. Among the most important sensing elements for tactile e-skin sensors are pressure and temperature sensors, which have increasingly garnered research interest over the past few decades. However, the design and fabrication of advanced pressure and temperature sensors can be challenging owing to complications such as signal interference, complex mechanism integration, and structural design issues. This review provides an overview of flexible pressure and temperature sensors used in e-skin, covering four main perspectives: material selection, mechanism integration, structural design, and manufacturing methods. The materials of different elements in the entire sensing system are comprehensively discussed, along with single and compound mechanisms of pressure and temperature sensing. Pressure and temperature sensors are divided into two types based on their electric output signals, which are exemplified in detail. The manufacturing methods used to fabricate these sensors, including printing methods, are outlined. Lastly, a summary of the future challenges faced by flexible pressure and temperature sensors used in e-skin is presented.

Bilayer PVA composite film with structural color for high-performance and multifunctional sensing

Intelligent E-skins attract lots of attention for their potential in health monitoring, human-computer interaction, and soft-body robotics. In this work, a colorful multifunctional E-skin sensor with a Janus structure was constructed by solution casting of polyvinyl alcohol (PVA) with clay and carbon nanoparticles on a patterned PDMS template. The halloysite nanotubes (HNTs) and carbon nanoparticles (C) derived from Chinese ink were introduced into PVA to obtain a nano-reinforced layer (top) with structural color and multi-functional layer (bottom) for detection of pressure, temperature, and humidity. Uniformly dispersed HNTs greatly improves the mechanical properties of PVA (up to 46.8 MPa at 303.9% strain). The difference in water absorption of the two layers provides excellent sensitivity to humidity, so it can be used as contactless password unlocking, closing and blooming of bionic flowers, and humidity alarm. The device can achieve object capture and release within 12 s by turning on and off the xenon light. Furthermore, the sensor responds distinctly to pressure and can track human activities such as drinking motion and finger bending, which can also detect the temperature by resistance change. This work provides a materials design strategy for E-skin aimed to applications in robotics, artificial intelligence, prosthetics, and health monitoring.

Nature-Skin-Derived e-Skin as Versatile "Wound Therapy-Health Monitoring" Bioelectronic Skin-Scaffolds: Skin to Bio-e-Skin.

Electronic skins (e-skins) have the potential to turn into breakthroughs in biomedical applications. Herein, a novel acellular dermal matrix (ADM)-based bioelectronic skin (e-ADM) is used to fabricate versatile "wound therapy-health monitoring" tissue-nanoengineered skin scaffolds via a facile "one-pot" bio-compositing strategy to incorporate the conductive carbon nanotubes and self-assembled micro-copper oxide microspheres with a cicada-wing-like rough surface and nanocone microstructure. The e-ADM exhibits robust tensile strength (22MPa), flexibility, biodegradability, electroactivity, and antibacterial properties. Interestingly, e-ADM exhibits the pH-responsive ability for intelligent command between sterilization and wound repair . Additionally, e-ADM enables accurate real-time monitoring of human activities, providing a novel flexible e-skin sensor to record injury and motions. In vitro and in vivo experiments show that with electrical stimulation, e-ADM could prominently facilitate cell growth and proliferation and further promote full-thickness skin wound healing, providing a comprehensive therapeutic strategy for smart sensing and tissue repair, guiding the development of high-performance "wound therapy-health monitoring" bioelectronic skin-scaffolds.

Cellulose Nanocrystal-Based All-3D-Printed Pyro-Piezoelectric Nanogenerator for Hybrid Energy Harvesting and Self-Powered Cardiorespiratory Monitoring toward the Human-Machine Interface.

Biomaterials with spontaneous piezoelectric property are highly emerging in recent times for the generation of electricity from mechanical energy sources that are amply available in nature. In this context, pyroelectricity, an integral property of piezoelectric materials, might be an interesting tool in harvesting thermal energy from the fluctuations of temperature. On the other hand, respiration and heart pulse are the significant human vital signs that can be used for early detection and prevention of cardiorespiratory diseases. Here, we report an all-three-dimensional (3D)-printed pyro-piezoelectric nanogenerator (Py-PNG) based on the most abundant and completely biodegradable biopolymer on earth, i.e., cellulose nanocrystal (CNC) for hybrid (mechanical as well as thermal) energy harvesting, and interestingly, the NG could be used as an e-skin sensor for application in self-powered noninvasive cardiorespiratory monitoring for personal healthcare. Notably, the CNC-based device will be biocompatible and economically advantageous due to its biomaterial-based supremacy and huge availability. This is an original approach with 3D geometrical advancement in designing a NG/sensor, where the unique all-3D-printed manner is adopted, and certainly, it has promising potential in reducing the number of processing steps to required equipment during the multilayer fabrication. The all-3D-printed NG/sensor shows outstanding mechano-thermal energy harvesting performance along with sensitivity and is capable of accurate detection of heart pulse as well as respiration, whenever and whichever required without the need of any battery or an external power supply. In addition, we have also extended its application in demonstrating a smart mask-based breath monitoring system. Thus, the real-time cardiorespiratory monitoring provides notable and fascinating information in medical diagnosis, stepping toward biomedical device development and human-machine interface.

Composite biomaterial for mimetic electric skin generated by conductive polymer/anion synergistic effect

A versatile bionic E-skin sensor was fabricated based on an electronic/ionic-conductive hydrogel, which could perceive tiny and large strain signals in a wide sensing range.

Recent Advances and Progress of Conducting Polymer-Based Hydrogels in Strain Sensor Applications

Conducting polymer-based hydrogels (CPHs) are novel materials that take advantage of both conducting polymers and three-dimensional hydrogels, which endow them with great electrical properties and excellent mechanical features. Therefore, CPHs are considered as one of the most promising platforms for employing wearable and stretchable strain sensors in practical applications. Herein, we provide a critical review of distinct features and preparation technologies and the advancements in CPH-based strain sensors for human motion and health monitoring applications. The fundamentals, working mechanisms, and requirements for the design of CPH-based strain sensors with high performance are also summarized and discussed. Moreover, the recent progress and development strategies for the implementation of CPH-based strain sensors are pointed out and described. It has been surmised that electronic skin (e-skin) sensors are the upward tendency in the development of CPHs for wearable strain sensors and human health monitoring. This review will be important scientific evidence to formulate new approaches for the development of CPH-based strain sensors in the present and in the future.

Printed, wearable e-skin force sensor array

The need to develop electronic skin (e-skin) for robotics is essential to provide a sense of touch on a large area, the same as human skin. This paper reports the design and development of printed e-skin force sensor arrays. The performance analysis of these capacitive force sensor arrays showed a resolution of 2.5 sq. mm, localization accuracy of 99% and a sensitivity of 0.53 pF/N. The sensor patches provided significant force sensitivity and stable loading and unloading for static force. A graphical user interface is developed in open-source python software. An unsupervised machine learning algorithm trains the tactile e-skin system for different user styles to distinguish the cluster of lower and higher touch angles for improving accuracy. The proposed e-skin sensor finds application in large-area sensors and gripper pads for prosthetics in biomedical devices where the sense of touch and high resolution is needed.

A flexible wearable e-skin sensing system for robotic teleoperation

AbstractElectronic skin (e-skin) is playing an increasingly important role in health detection, robotic teleoperation, and human-machine interaction, but most e-skins currently lack the integration of on-site signal acquisition and transmission modules. In this paper, we develop a novel flexible wearable e-skin sensing system with 11 sensing channels for robotic teleoperation. The designed sensing system is mainly composed of three components: e-skin sensor, customized flexible printed circuit (FPC), and human-machine interface. The e-skin sensor has 10 stretchable resistors distributed at the proximal and metacarpal joints of each finger respectively and 1 stretchable resistor distributed at the purlicue. The e-skin sensor can be attached to the opisthenar, and thanks to its stretchability, the sensor can detect the bent angle of the finger. The customized FPC, with WiFi module, wirelessly transmits the signal to the terminal device with human-machine interface, and we design a graphical user interface based on the Qt framework for real-time signal acquisition, storage, and display. Based on this developed e-skin system and self-developed robotic multi-fingered hand, we conduct gesture recognition and robotic multi-fingered teleoperation experiments using deep learning techniques and obtain a recognition accuracy of 91.22%. The results demonstrate that the developed e-skin sensing system has great potential in human-machine interaction.

Robot-Based Calibration Procedure for Graphene Electronic Skin.

The paper describes the semi-automatised calibration procedure of an electronic skin comprising screen-printed graphene-based sensors intended to be used for robotic applications. The variability of sensitivity and load characteristics among sensors makes the practical use of the e-skin extremely difficult. As the number of active elements forming the e-skin increases, this problem becomes more significant. The article describes the calibration procedure of multiple e-skin array sensors whose parameters are not homogeneous. We describe how an industrial robot equipped with a reference force sensor can be used to automatise the e-skin calibration procedure. The proposed methodology facilitates, speeds up, and increases the repeatability of the e-skin calibration. Finally, for the chosen example of a nonhomogeneous sensor matrix, we provide details of the data preprocessing, the sensor modelling process, and a discussion of the obtained results.

Skin-Induced Motion Artifact Removal for Ambulatory Electrocardiography

Skin-induced motion artifact (SIMA) is a troublesome problem for ambulatory electrocardiography (ECG) signal acquisition by e-skin dry sensor. Reference based algorithm is a promising way to tackle it. A highly correlated reference signal is the key for this solution. In this article, by using a new synthesized reference signal for LMS algorithm, which combines both skin induced impedance behavior and skin “slope” and recovery “time constant”, the SIMA related noise is greatly reduced. Firstly, we show the phenomenon that the SIMA “slope” and “recovery time constant” have some relationships with skin induced impedance, its derivatives and specific tissue characteristics. Secondly, We do the offline “slope” and “time constant” calibration work first, then measure the real time skin induced impedance, and then combine the above information together, to form a virtual reference signal, then we show the new reference has high correlation with actual SIMA noise. Finally, considering that SIMA potential consists of fast changing part and slow changing part, we use an improved LMS algorithm to handle the contradiction between speed and accuracy, which includes fast tracking part and low convergence error part. We set up the SIMA removal system in Matlab, and use actual motion artifact contaminated ECG signal as the input, and then do offline signal processing. Results show that SIMA is improved around <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$7\sim 15$ </tex-math></inline-formula> dB typically on average. Integrating skin-induced impedance with human body related “slope” and “time constant” as a reference signal seems a feasible method to remove skin-induced motion artifact.