Electrospinning Nanofibers Based Artificial Skins: History
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Artificial skin, also known as bioinspired electronic skin (e-skin), refers to intelligent wearable electronics that imitate the tactile sensory function of human skin and identify the detected changes in external information through different electrical signals. Flexible e-skin can achieve a wide range of functions such as accurate detection and identification of pressure, strain, and temperature, which has greatly extended their application potential in the field of healthcare monitoring and human-machine interaction (HMI). Compared with other traditional electronic sensors, artificial electronic skin can meet the demand of human health monitoring and HMI when it is used in seamless and stable contact with human skin and obtains low impedance physiological signals. Therefore, it has higher requirements on material permeability, tensile resistance, and biocompatibility. Due to its high porosity, high toughness, and small mass, electrospun nanofiber-based bioinspired artificial skins with high flexibility and a three-dimensional porous mesh structure are often considered as the first choice.

  • artificial skin
  • electrospun nanofiber
  • healthcare monitoring

1. Introduction

The advancement of wearable electronics has been attracting more and more attention recently due to their ability to simulate the haptic perception of human skin to identify changes in detected external information through different electrical signals [1,2,3,4,5]. Unlike traditional rigid electronic devices that cannot maintain polymorphic contact with the human body, wearable electronic products can serve for health management or providing other smart functions, which greatly enrich people’s daily needs. Among them, bioinspired artificial skin is considered to be an important component of wearable electronic devices that can be affixed to the surface of human muscles or joints to collect physiological signals, with promising applications in the areas of real-time healthcare monitoring and human-machine interaction (HMI) [6,7,8,9,10,11,12,13]. Therefore, the design of artificial skin needs to be considered on skin-like flexible materials, mainly focusing on the stable monitoring of artificial skin in use, wearing comfort, and physical and chemical properties suitable for human skin. Electrospinning-based flexible devices provide a practical path for human skin construction based on such flexible material substrates [14,15,16,17,18].
Typically, electrospinning is a particular method of fiber manufacturing that uses a solution or melt of polymer for jet spinning under a high voltage electric field, which produces nanometer diameter polymer fibers with flexibility and continuity. The idea of electrospinning was conceived in 1600 by William Gilbert, who observed in his research that water droplets would form cones in an electric field [19]. In 1887, Charles V. Boys used a viscous liquid to pull out fibers while on the edge of an insulated dish connected to a power source, and the method of extracting fibers from a viscoelastic liquid under strong electric field conditions was first reported. In 1902, the electrospinning technology was patented by John Cooley and William Morton, respectively, and the prototype of the electrospinning device was determined [20]. From 1964–1969, a number of papers were published by Jeffrey Taylor, mathematically describing and simulating the process of changing a viscous polymer solution from a sphere to a cone at an electrospinning nozzle under the effects of a high-voltage electric field, achieving a breakthrough of electrospinning technology [21,22,23,24]. However, electrospinning technology development has stalled because of the absence of microscopic-scale characterization tools. It was not until the beginning of this century with the popularization of electron microscopy that the technology began to receive more and more attention from researchers, and the performance and applications were developed as never before. Through the process of developing new strategies to control structures and performances of electrospun nanofibers, electrospinning technology had already been used extensively in the area of bio-inspired artificial skin.
Artificial skin, as bionic human skin, needs to meet the characteristics of the high elasticity and breathability of human skin [25]. In the electrospinning process, polymer solution jets are stretched in a strong electric field to form nanofibers ranging from a few nanometers to 500 nanometers in diameter, which are then deposited on a collection plate to form a nanofiber film [26,27,28]. Compared with the thin film type flexible substrate material, the mesh structure of the nanofiber membrane makes it flexible and breathable, with a great ratio of surface area and thermal stability, which can better meet the material requirements of artificial skin [29,30,31,32,33]. The performance of electrospun nanofibers can be further enhanced by adding different nano-fillers to develop artificial skin with different structures and different functions [34,35,36,37,38,39].

2. Healthcare Monitoring

With the advancement of flexible electronics, electrospun nanofiber-based artificial skin is increasingly being used for human health monitoring [151,152,153,154,155,156,157,158,159,160,161,162]. Comfortable, accurate, and real-time collection of physiological electrical signals is important for determining human health conditions. After prolonged conformal contact with human skin or joints, the ability to avoid elevated impedance caused by sweat and to withstand repeated mechanical deformation are crucial issues that need to be addressed [163,164,165,166,167,168,169,170]. Therefore, a flexible conductor with high permeability and stretchability (liquid-metal fiber mat, LMFM) was developed by Zheng et al. in 2021 [171]. The preparation is based on the coating of liquid metal (eutectic gallium-indium alloy, EGaln) on an electrospun fiber mat (styrene-butadiene-styrene, SBS). After the pre-stretching process, the liquid metal between the elastomeric SBS nanofibers formed a lattice-like structure and the LMFM maintained a high permeability to both gases and liquids. In tensile tests, EGaIn-SBS can achieve over 1800% stretching while the conductor impedance remains at a low level without significant change during the process, showing ultra-high conductivity and electrical stability.
n process for wearable devices based on wet heterostructure electrospinning technology [172]. Electrospinning micro-pyramidal arrays (EMPAs) with unique structures were constructed using a far-field electrospinning device with a charged grounded aluminum foil with bumps as the collector. PVDF was used as the proof-of-concept material to fabricate the EMPAs-based films, and the SEM images showed a uniform planar distribution of the micro-pyramidal structure on the film, and typical features of the pyramidal structure were shown with the tilted three prongs intersecting at the apex. Since the micro-pyramid structure microfibers constructed the permeable network, the film with EMPAs was ultra-thin, ultra-light, breathable, and suitable to be adopted as the artificial skin. Therefore, a piezoelectric capacitive sensor based on EMPAs was developed to collect pulse signals in real time for human health monitoring with high permeability and sensitivity. Electrospun barium titanate/polyvinylidene fluoride (BTO/PVDF) nanofibers are modified mainly by using polydopamine (PDA). Groups of DA formed cross-links with the BTO nanoparticles due to van der Waals forces as well as attached to the PVDF polymer fibers, encasing the protruding BTO nanoparticles and making the fiber surface smooth. In addition, the piezoelectric performance had been greatly improved. 
In addition to myoelectric and pulse signals, electrospinning nanofiber-based artificial skin can also be used for several other health monitoring applications. For instance, Wang et al. developed a TENG-based nanofiber electronic skin (SANES) for respiratory monitoring and diagnosis during sleep [174], which was characterized by good permeability, high sensitivity, and was easy to wear. SANES is mainly assembled by the top encapsulation layer, middle functional layer, and bottom substrate layer, and all three nanofiber functional layers are prepared by electrospinning. The PA66 and PAN sandwiched in the middle were used as electrodes with a layer of Au of 100 nm thickness at the surface, respectively. The upper and lower parts are protected from electrode interference by PA66 and PAN as cover layers, respectively. The device is placed on the abdomen of the test subject, which monitors the occurrence of OSAHS during sleep based on the movement of the abdominal skin during breathing and records the number of apneas and hypoventilation state. Classifying or alerting according to the severity has great application prospects in the area of personal sleep health monitoring. In addition to directly collecting physiological electrical signals for real-time monitoring of the human body, electrospun nanofiber-based devices can also monitor and provide early warnings of human health and safety by establishing medical monitoring systems. In 2022, Yu et al. prepared a triboelectric energy harvesting sensor (TEHS) using triboelectric fiber films made by electrospinning technology and built a medical monitoring system by multiple TEHS devices [175].

3. Intelligent HMI

As artificial intelligence emerges and develops, artificial skin plays a crucial role not only for medical monitoring, but also for intelligent HMI [176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191]. Besides the monitoring of physiological parameters and the movement status of the human body, multifunctional artificial skin based on electrospun nanofibers can be used for mechanical control [192,193], on-demand therapy [194,195], and gesture recognition and intelligent control [196,197,198,199,200,201,202,203,204,205,206,207]. In 2022, a wearable flexible electrode (nano-liquid metal (LM)-based highly robust stretchable electrode, NHSE) that can be used for game control and thermal therapy was proposed by Li et al. [208].
Different sensors have different working mechanisms; common flexible sensors used for HMI are not only pressure sensors, but also humidity sensors. During the COVID-19 epidemic, a flexible non-contact sensing array based on humidity sensing was reported by Yang et al. [210]. The single sensor (MG/PA66 humidity sensor, MPHS) is a composite material made of two-dimensional graphene flakes embedded in an electrospun PA66 nanofiber by ultrasonic treatment. The characteristics of the electrospun nanofiber network give the composite a physical structure with a large ratio surface area, in addition to the PA66 chemical structure rich in water-absorbing functional groups, ensuring a high response to humidity. MPHS can be arranged to form a humidity sensing array for HMI in non-contact mode.
In addition to remote control, pressure detection, and game control, artificial skin made of electrospun nanofibers has extensive applications in areas such as healthcare and fire alarms. For example, Zhang et al. proposed a ventilatable artificial skin with real-time temperature monitoring and the ability to perform anti-infection heating therapy in 2019 [211]. The device consists of an electrospun moxifloxacin hydrochloride (MOX) nanofiber network with high toughness, gas permeability, and stability that can be used as a flexible heater when coated with a thermosensitive polymer film printed with a conductive pattern. When SGNI is exposed to a high temperature environment, the intelligent fire alarm system will sound an alarm and send an alarm message once the temperature reaches the alarm threshold. By using the ability of SGNI to respond to fire hazards, a concept was developed that could sense the position of the fire source and then control the robot to make evasive maneuvers in actual hazardous HMI situations. Once the location of the robot installed on the SGNI skin is close to the fire source, the SGNI can sense the fire temperature and location within 6s and send signals back to the control unit to command the robot to actively avoid hazards and make a move away from the fire source, which provides highly promising applications in the area of secure and intelligent HMI.

This entry is adapted from the peer-reviewed paper 10.3390/biomimetics8020223

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