# Recent Advances in Wearable Devices for Non-Invasive Sensing
## 4 Sensors
### Physical Sensors  
One of the fundamental conventional parameters of the body that have strong diagnostic significance are physical parameters such as movement-induced strain and temperature. These physical parameters vary temporally and spatially, hence the issues of specificity and accuracy are important when devising such physical sensors. In addition, such parameters are recorded primarily from the skin or they can be incorporated into clothing and artificial epidermal substrate for sensing purposes. Other research has focused on bodily kinetics and muscle activity to study the effect of various diseases that affect the movement of the body, such as cerebral palsy. The promising aspect of physical sensors is that the devices have stretchable electronics that can detect human motions and perform surface strain calculations. These devices can be incorporated intrinsically into clothing or directly onto the surface of the skin to detect the strain derived from bodily motion. [PERSON] et al. demonstrated a wearable sensor of single-walled carbon nanotubes that was capable of measuring strains up to 280% [122]. Figure 5a show a photograph of the actual device on synthetic clothes. This sensor can differentiate various aspects of human motion, such as movement, breathing, speech, and typing. This paves the way for an additional wide range of wearable electronics for non-invasive physical sensors. Another work that demonstrates the potential of detecting bodily movement using a wearable sensor was presented by [PERSON] et al. using an ionic liquid microband, as shown in Figure 5b [123]. This intrinsically-stretchable pressure sensor uses nanomaterials and a flexible semiconductor to fabricate a rubber band-like stretchable pressure sensor that is light and thermally sensitive. The use of a non-volatile material also reduces the complexity of the fabrication, and the device showed the detection of strains in the range of 0.1% to 500%, even during multiple repeating cycles. This device can be integrated seamlessly into woven commercial bracelets for the detection of hand gestures, and it even can identify wrist pulses. Work to develop epidermal electronics for a non-invasive physical sensor is a novel component of non-invasive wearable electronics. Tattoo electronics are key devices of such classes due to the seamless infrastructure when they are integrated for application. The work reported by [PERSON] et al. introduced a conformal, contact-based system that shows the great applicability of epidermal tattoo electronics [124]. When incorporated with various sensor systems, the epidermal device can detect various physical variables of the skin, such as temperature and strain. To reinforce the concept of the physical sensor, work to integrate physical sensors for the training and rehabilitation of muscles also shows signs of increasing importance. Fiber-shaped strain sensors can be integrated into woven textile materials for real-time mechanical feedback of surface strain detection. These stretchable sensors can be used to monitor sports activity when attached to an athlete's body [130]. Work to improve the usability of these surface strain sensors was developed further by integrating them into the exoskeleton. The knee extension-assisted exoskeleton developed by [PERSON] et al. shows that the device can help rehabilitate patients with cerebral palsy and weakened muscle activity [125]. With the onset of cerebral palsy commonly occurring during childhood, the importance of addressing the implication of such disease comes early. The work that has been presented shows evidence of improvement in the dynamic gait posture and the  
Figure 5: **(a)** A photograph of stretchable carbon nanotube strain sensor attached to clothing to detect physical motion (reprinted with permission from [122]. Copyright 2011 Springer Nature); **(b)** Ionic liquid-based sensor patch of cervical movement detection for directional strain calculation of skin during neck movement (reprinted with permission from [123].