# Recent Advances in Wearable Devices for Non-Invasive Sensing
## 4 Sensors
### Electrophysiological Sensors  
Another variety of non-invasive sensors are the electrophysiological sensors that collect bodily electrical signals, such as the ECG [131, 132], electroencephalogram (EEG) [133, 134], and electromyography (EMG) [135, 136]. The capability of continuous physiological monitoring is needed for the treatment and diagnosis of medical conditions. The issue related to the acquisition, filtering, amplification, and radio frequency (RF) transmission of data must be optimized for precise electrophysiological characterization. Hence, epidermal sensors have been developed that can seamlessly access the physiological data of the body. These sensors usually are composed of a soft, stretchable substrate with a biocompatible electrode that has a curvilinear structure.  
The study of the bioelectrical signals of the body has been extremely important in the early diagnosis of disease and its symptoms. In the case of cardiovascular diseases, the abrupt and random occurrences of heart attacks may require constant recording and monitoring of electrophysiological signals. Most commercial devices require hospitalization and the use of an invasive diagnostics device that can cause secondary complications. [PERSON] et al. developed a thin electronic device that, when placed on the epidermis, is capable of self-adhesion [137]. This device is integrated with commercial electronic components and uses carbon nanotubes to retain conformal contact with the wrinkles of the skin without any motion artifacts. The use of carbon nanotubes has resulted in a drastic lowering of the overall modulus of the device. This enhanced flexibility of the device enables the collection of reliable ECG data, and it has an overall performance that is superior to that of the rigid, capacitive-type electrode. [PERSON] et al. also demonstrated the use of a low-modulus, elastic, epidermal sensor for recording ECGs [138]. They used controlled mechanical buckling and soft microfluidics to fabricate ultra-low modulus devices that are intrinsically stretchable. They also demonstrated the concept of collecting data wirelessly, which is very important for the inconspicuous operation of epidermal electrophysiological sensors.  
Recent advances in non-invasive EEG devices have shown long-term operation and minimal loading contact. EEG devices, in particular, are mounted onto the auric and mastoid surface, which requires minimal thermal, mechanical, and electrical damage to the skin. In addition, the high irregularity of the surface of the auric requires detailed structural planning for the bending and stretching of these EEG devices. The non-invasive epidermal EEG developed by [PERSON] et al. has a wearable EEG sensor that remains mounted conformally during vigorous exercise [126]. Figure 5e shows the conformal attachment of the device to the skin when the device is mounted. The device has shown the features of non-invasive interfaces to the skin that show reliable EEG recording and the possibility of wireless integration. Admittedly, there is room for the amplification of the signal and improved accuracy since EEG signals usually are small and difficult to trace. To detect detection the electrical activity produced by a skeletal muscle, EMG signals usually are recorded on the surface of the skin directly above the targeted muscle. [PERSON] et al. showed an example of an EMG skin device that used silk protein substrate for a soft and stretchable sensor device that could be integrated seamlessly, as shown in Figure 5f [127]. The device had high electrical performance and conformality when it was used as an epidermal electronic sensor.