## 6 Self-Healing Flexible Sensors
Recyclability remains an issue in the fabrication of flexible electronic skins. Hydrogels are often used in the construction of these skins due to their ability to self-heal, however most of these skins can be re-used up to 3-5 times [94]. During recycling, electrical conductivity of the electronic skin can decrease slightly due to evaporation of a small amount of water in the hydrogel. Hydrogels can be easily moulded into various shapes which brings flexibility in design. A PVA/ Bn/PEI/MXene electronic sensor showed impeccable recyclability of up to 30 cycles without significant degradation in mechanical properties and or need for treatment [93]. A self-healing polymer matrix composed of dynamic Diels-Alder (DA) adducts with CaCu\({}_{3}\)Ti\({}_{4}\)O\({}_{12}\) (S-CCTO) nanoparticles incorporated into the polymer matrix were used in fabricating a self-healing motion sensor [95]. The motion sensor can self-heal from a cut by a blade within 30 min by heating to a temperature of 105 \({}^{\circ}\)C. CaCu\({}_{3}\)Ti\({}_{4}\)O\({}_{12}\) has gained considerable attention in literature owing to its huge dielectric permittivity and thermal range from 100 to 500 K [96]. The sensor has potential use in rehabilitation, sports performance measurements and in the entertainment industry. These sensors can be incorporated in clothing or on human skin, offering real time monitoring. Naturally, the sensor is subject to bending, stretching and cuts, therefore a self-healing element is imperative to ensure reliability and reduction in safety hazards. Furthermore, a finger motion sensor was developed using the same materials for the electrodes which were then spray-coated on all surfaces with SWCNTs [95]. The SWCNTs showed homogeneity on the surface of the electrode, according to SEM and TEM measurements, which led to the required conductivity of the electrodes. After being subjected to damage, the self-healing composite layer moves, which leads to the separated SWCNTs to re-join and construct conductive paths. The electrode was placed in an LED circuit to test its electrical and self-healing abilities. The electrode allowed enough current to pass through to light up the LED which demonstrates it has functional conductive properties. The electrode was then cut with a blade, leaving a micro-gap (50 \(\upmu\)m wide) in the circuit. The LED turned off due to the broken circuit and recovered after 30 min of heating at 105 \({}^{\circ}\)C. The healing process is presented in Figure 8a. The electrodes can be used as human finger motion detectors by measuring the changes in capacitance due to bending/stretching. The sensor can distinguish various finger motions due to the fact that each figure motion exhibits a different capacitance value as shown in Figure 8b. The sensor can still retain its original capacitance properties after bending/stretching, demonstrating its excellent mechanical properties. The sensor shows promising use in human motion detection, however the recovery temperature of 105 \({}^{\circ}\)C is too high for human interaction and the self-healing time of 30 min is too high for practical uses. The recovery temperature needs to be close to room temperature, such that the device does not harm or cause discomfort to the user. Ideally, the healing time needs to be under one second to ensure safety of the device and reduce lag time. This can be achieved by co-polymerisation, which involves incorporating polymers/hydrogels with low transition temperatures and fast recovery times. Overall, the sensor showed excellent properties including good recyclability, even after the 10 th cut--healing process, the modulus recovered to 0.51 MPa (91%) and maximum elongation decreased by only 19% from the original 105%. Self-healable sensors and their characteristics are summarised in Table 4.  
Figure 8: (**a**) The self-healing process of an electrode based on SWCNT’s spread on a flexible, self-healing substrate.