10.3.6 Electronic skin for energy supply
With the development of e-skin devices，it will be increasingly used for long-term monitoring of human health.For most devices，they consume energy during work to realize functions such as identification and recording of physiological signals，sensing of physical and chemical stimuli，and controlled release of drugs.This requires the realization of uninterrupted energy supply of devices.To achieve this goal，researchers developed stretchable energy storage devices（e.g.，supercapacitors，lithium-ion batteries）or energy conversion devices（e.g.，solar cells，nanogenerators）which were integrated into the e-skin，further expanding the application range of e-skin devices.
Supercapacitor is a new type of component that stores energy through the double layer of the interface formed between the electrode and the electrolyte.It has the advantages of fast charging and discharging，high power density，and long working life，and has attracted widespread attention.Generally，a supercapacitor is composed of three parts：electrode，electrolyte and dielectric layer.The structure of the material and the electrochemical properties of the material significantly affect the performance of the supercapacitor.At present，a variety of stretchable materials have been used to prepare supercapacitors，including carbon nanotubes，carbon fibers，graphene，and metal nanowires.The supercapacitors can be designed in one-dimensional，two-dimensional，and three-dimensional structures.For example，Peng et al.designed and prepared one-dimensional fibrous supercapacitors.They first assembled a layer of carbon nanotubes on the surface of elastic fibers，and then assembled a layer of gel electrolyte，and finally covered a layer of carbon nanotubes on the outermost layer，to formed a coaxial fibrous supercapacitor which could be used for weaving smart fabrics and e-skin devices.Wei et al.constructed a two-dimensional supercapacitor using carbon nanotube film as electrodes，electrospun polyurethane fiber film as a dielectric layer，and PDMS as an elastic substrate.The device exhibits very good stability during repeated stretching and charging and discharging，and can be used in e-skin devices，for stably providing energy for other sensor devices.Niu et al.reported a case of three-dimensional solid-state supercapacitor based on polyaniline，carbon nanotubes and sponge electrodes in 2016.They loaded the carbon nanotubes on the sponge-like skeleton of the electrode through repeated dip-coating-drying methods，and then polymerized aniline on the surface to obtain a supercapacitor with reversible compression-recovery properties.When its strain exceeds 60％，the device can still maintain almost constant capacitance.
In addition to supercapacitors，flexible batteries can also be directly integrated into eskin devices to provide energy for them.Compared with supercapacitors，batteries have higher energy and are more suitable for e-skin devices that require long-term work.Lithiumion batteries have been widely used due to their advantages of portability and high energy.In order to meet the needs of e-skin applications，flexible lithium-ion batteries developed rapidly.At present，research focuses on the flexible design of electrolyte materials（e.g.，LiCoO2，LiMn2O4，Li4Ti5O12）and electrode materials.Similar to supercapacitors，lithiumion batteries can be designed in one-dimensional，two-dimensional，and three-dimensional structures.The team of Huang and Rogers reported the design method of a flexible lithiumion battery.The battery uses flexible silicone rubber as a substrate.When the biaxial stretching reaches 300％，its power density can still reach 1.1 mA·h·cm-2.More importantly，a flexible wireless charging system can also be integrated into the flexible lithium-ion battery，which can guarantee the long-term use of the battery，thereby providing stable energy for e-skin devices and other implantable devices.
With the rapid development of batteries，researchers have developed a variety of portable batteries，such as zinc-air batteries and aluminum-air batteries.In theory，these metal-air batteries have a higher energy density than lithium-ion batteries and are more suitable for providing energy for e-skin devices.For example，Peng et al.used carbon nanotubes to prepare a spring-like cathode and constructed a fibrous flexible zinc-air battery.The battery exhibits excellent charge and discharge performance under high current density，and is very suitable for powering small devices.The fibrous battery can be woven into fabrics，used in e-skin，smart watches，and others.
In addition，the rapid development of solar cells also provides more options for the energy supply of e-skin devices.However，the application of solar cells in e-skin is also subject to some restrictions.For example，when the e-skin is applied to the human body，the availability of light sources and the intensity of the light sources are limited.In addition，solar cell materials are mainly rigid materials，and there are a few materials that can be used for flexible solar cells.At present，using organic photovoltaic materials，researchers have developed a variety of flexible solar cells.For example，using polythiophene materials and fullerene derivatives can fabricate flexible solar cells.This type of batteries can maintain a close fit with the skin，and can still maintain a stable energy supply after repeated charging and discharging 1000 times.Similar to metal-air batteries，solar cells can also be designed in a one-dimensional fiber structure，thereby obtaining better stretchability，and various forms of fabrics can be fabricated by spinning，which greatly promotes the development of e-skin and wearable devices.(https://www.daowen.com)
In recent years，researchers have developed a variety of nano-generators to convert mechanical energy obtained from human movements（such as heartbeat，blood flow，walking，breathing，and muscle contraction）into electrical energy for wearable devices.At present，there are two main types of flexible nano-generator devices：piezoelectric nanogenerator and triboelectric nano-generator.
For piezoelectric devices，when the piezoelectric material is deformed，polarization will occur.At the same time，positive and negative charges will appear on the opposite surfaces.Baik et al.integrated a piezoelectric unit（Figure 10-11（a））on a flexible PDMS substrate to prepare a flexible piezoelectric device.They first deposited polystyrene beads on a silica substrate as a template，and then deposited piezoelectric materials（e.g.，zinc oxide，lead zirconate titanate）on the surface of the beads，and then burned to remove the polystyrene beads template.Afterwards，a hollow hemispherical piezoelectric material array is obtained，which is then embedded in the PDMS substrate to form a stretchable piezoelectric film.The output voltage of the device can reach 4 V，and the current density can reach 0.13μA·cm-2.The output voltage and current density can be further improved by stacking multiple layers of piezoelectric materials.
Triboelectric devices are based on the coupling of triboelectricity and electrostatic induction.When friction occurs between two materials，charges are generated，which are separated by external force，thereby forming a potential difference.For example，Bao and Kim's team used PDMS and single-walled carbon nanotubes to prepare a triboelectric device，as shown in Figure 10-11（b）.The device can detect resistance and capacitance change signals at the same time，to realize the sensing of the above-mentioned mechanical stimuli.This device with both energy supply and sensing performance can be used for self-powered e-skin devices，which can be used in long-term monitoring of changes in human body temperature，physiological electrical signals，and blood pressure.