10.3.4 Chemical sensor devices
There are water，electrolytes，metabolites，hormones and other components in body fluid.The composition and content of these components can reflect the health of the human body.Therefore，by integrating chemical sensors in the e-skin to analyze the chemical components in body fluid，the health information can be detected at the molecular level，which plays an important role in the early diagnosis of diseases.
Chemical sensing e-skin devices can be divided into electrochemical sensors，chemical impedance sensors，and transistor sensors.When the device contacts the target compound，the potential，current，and resistance of the chemical sensor will change.By analyzing the signal change，the information about the target compound can be obtained.For example，in an electrochemical sensor，an ion-selective electrode can be designed and prepared.When it contacts the target ionic compound，measuring the potential changes can realize the identification of the chemical substance（potentiometry）.The electrode can also be fixed with oxidoreductases，when the enzymes immobilized on the electrode and the catalytic target compounds undergo an oxidation-reduction reaction，measuring the current changes through the device can detect the target compounds（amperometric method）.For chemical impedance sensors，the sensing element is usually placed between two electrodes.When the target compounds contact or react with the electrodes，its resistance changes，thus realizing chemical substance sensing.The transistor sensors are mainly composed of a semiconductor layer，a dielectric layer and three electrodes（e.g.，source，drain，and gate），which can be regarded as adding a dielectric layer and a gate electrode to the chemical impedance sensor.This kind of devices can realize the amplification of chemical sensing signals，further enhancing the sensitivity of the device.
In body fluid sensing，the monitoring and analysis of water content are very important，especially for e-skin devices，detecting the level of water content in the skin can be used to assist in the diagnosis of skin diseases，such as dermatitis，psoriasis，eczema，and itching.The skin water content can be obtained by measuring the impedance，thermal conductivity，and spectral properties of the skin.Compared with traditional methods，measuring skin water content through e-skin devices has more advantages.Figure 10-8（a）shows an e-skin device designed for measuring skin water content.The device consists of a sampling system and two electrodes.The sampling system provides alternating currents of different frequencies.The impedance of the skin changes with the water content，which causes the changes in amplitude and phase of the output current.By measuring the changes in amplitude and phase of the output current，the water content of the skin can be measured.
In practical applications，the electrolytes，metabolites，hormones and other components in human body fluid are relatively complex and have very low concentrations.Therefore，chemical sensors in e-skin need to have high selectivity to different components，as well as very low detection limits and high sensitivity.In addition，the chemical substances in body fluid reflect the health level of the human body.Therefore，the device needs to have high stability and good repeatability，thus the detection data obtained can be used for early diagnosis of diseases.
Recently，researchers have made significant progress in wearable chemical sensor devices that analyze the composition of sweat.This type of devices is mainly composed of electrochemical electrodes，and the components such as glucose，lactic acid，ethanol，pH，and electrolyte in sweat are analyzed by potentiometric method or amperometric method.
When preparing a sensor based on current measurement，people first immobilize glucose oxidase，lactate oxidase，alcohol oxidase，and other enzymes，on the electrode.These enzymes will selectively cause a redox reaction of the target chemicals.Since the current in the device has a linear relationship with the concentration of the analyte，the concentration of the target chemicals can be obtained by measuring the change in the current.For example，Mercier et al.used this method to prepare a wearable e-skin sensor.The redox reaction between lactate oxidase on the electrode and lactic acid in sweat can induce the change in current，which can be used to measure the concentration of lactic acid.(https://www.daowen.com)
In sensors based on potential measurement，the potential of the ion-selective electrode depends on the concentration of the target.By measuring the change in potential，the charged substances in sweat can be measured，such as ammonium ions，potassium ions，and sodium ions.For example，Davis et al.developed a wearable device that can simultaneously extract sweat and then detect glucose，sodium ions，and chloride ions in the sweat.Takei et al.developed a charge-coupled device for the measurement of sweat pH，which further improved the sensitivity of the device.
E-skin chemical sensors can also be used to monitor the hormone components in sweat.However，due to the very low concentration of hormones in sweat，the sensitivities of the sensors should be very high.Salleo et al.developed a method for detecting cortisol in sweat based on organic electrochemical transistors（Figure 10-8（b））.The device contained a molecularly imprinted layer，which could selectively combine with cortisol molecules，thereby limiting the movement of ions in the device.Furthermore，by measuring the change in current in the transistor，the concentration of cortisol could be calculated.They used the device on the human arm to detect the cortisol content in sweat.
Although a chemical sensor with a single detection function can detect the chemical composition of human sweat，it is difficult for a single chemical sensor to accurately detect the health of the human body.To overcome this drawback，researchers integrated multiple sensors on the same platform to detect various chemical components in sweat，human body temperature，pulse and other data at the same time.Therefore，it is more accurate to judge the health of the human body through this way.For example，Javey et al.integrated multiple sensors into wearable devices that can be used to simultaneously detect glucose，lactic acid，sodium ions，potassium ions and other components in sweat（Figure 10-8（c））.More importantly，the detected information can be directly wirelessly transmitted to mobile devices or cloud servers to achieve real-time monitoring of human health.Kim et al.integrated temperature，humidity，glucose，and pH sensors into an e-skin device to detect human body temperature，glucose content in sweat，and sweat pH.
In tissue fluid extraction and detection，e-skin device also shows very strong potential.Recently，Wang et al.designed a glucose sensor that can non-invasively extract tissue fluid and detect the glucose concentration in it.In addition，the tissue fluid sensor can also be integrated with a sweat sensor to detect the glucose level in the tissue fluid and sweat at the same time，which can more accurately reflect the level of the human blood sugar.