10.3.3 Electrophysiological sensor devices
The activities of human organs，tissues，and nerves will produce electrophysiological signals.These electrophysiological activities are closely related to diseases such as arrhythmia，myocardial infarction，neuromuscular disease，and epileptic seizures.Therefore，through the monitoring of electrophysiological signals，human vital signs can be further obtained.For example，an electrocardiogram can record heart activity，an electromyography can show the activity state of muscle，and electroencephalogram can record the activity of brain.
Commonly used electrophysiological signal sensing equipment（e.g.，electrocardiograph，electroencephalogram meter）can accurately monitor the electrophysiological signals of the human body.However，due to their large size and poor portability，it is difficult for patients to long-term monitor changes of electrophysiological signals.To overcome these shortcomings，researchers have developed a flexible，ultra-thin，ultra-light，and biocompatible sensing devices for detecting epidermal electrophysiological signals.They integrated the snake-shaped metal wires into the elastomer material，and adjusted the composition of the material to match the mechanical properties of the skin，thereby preparing an e-skin with electrophysiological signal sensing function.The devices can not only record ECG and EMG，but also can be integrated with other sensors（e.g.，strain sensors，wireless charging units，and LED displays）to obtain a multifunctional e-skin device（Figure 10-7（a））.
Since the electrophysiological signals need to be analyzed by the doctor，it is not convenient for the patients to use the devices.Therefore，researchers have developed an ECG monitoring device that can display different colors（Figure 10-7（b））.They integrated transistor amplifiers and color-adjustable organic light-emitting diodes into the ECG monitoring device.When the device displays red，the ECG is normal；when the device displays blue，it indicates abnormal cardiac activity.The device provides a visualized monitoring of cardiac activity for patients with heart disease，and is more suitable for longterm real-time monitoring of cardiac activity for patients.
To obtain electrophysiological signals more accurately，the electrodes of the device need to be in close contact with the skin.The electrodes of the traditional electrocardiograph are in close contact by applying gel between the electrodes and the skin，while in flexible e-skin devices，the device materials can be designed to achieve close contact with the skin.For example，Jeon et al.prepared an e-skin device that can monitor an electrocardiogram by mimicking the structure of gecko foot.The electrode of the device can form a close fit with the skin through electrostatic attraction and physical contact.Even when the human body is moving normally，the electrode of the device will not detach from the skin，and it can still accurately measure the electrocardiogram（Figure 10-7（c））.(https://www.daowen.com)
The stretchability of the device is also an important condition to ensure that it can maintain a close fit with the skin.Especially in irregular and frequently moving parts of the human body（e.g.，joints），the stretchability of the device allows the electrode to maintain a good fit with the skin during the repeated stretching and twisting process.Silver nanowires are generally used as stretchable electrode materials and have excellent electrical conductivity and processability.However，its biocompatibility is poor，and it is easy to oxidize and lead to poor conductivity.To solve this problem，silver-gold core-shell nanowires can be used to prepare electrodes with excellent stretchability，which have better biocompatibility and stronger oxidation resistance，and can maintain high electrical conductivity.The device exhibits excellent performance in monitoring electrophysiological signals，and can be applied not only to the surface of human skin，but also to the surface of animal heart.
Although e-skin devices have achieved rapid development in the field of electrophysiological signal sensing，they are still far from practical applications（see extended reading 3）.At present，there are still some problems in this field.For example，the analysis of signals still depends on human，and there are still big challenges in integrating the processor into the e-skin.The chemical stability and biocompatibility of electrodes and elastomer materials in e-skin devices still need to be improved.The movement of human leads to poor adhesion stability between the device and the skin，which affects the accurate sensing and stability of electrophysiological signals.The solution of these problems depends on the optimization of the device structure and the improvement of material properties.