## 6 Self-Healing Flexible Sensors
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**3D Printing Technique** & \multicolumn{2}{c}{**General Advantages**} & \multicolumn{2}{c}{**General Disadvantages**} \\ \hline Stereolithography (SLA) & \(\bullet\) & Higher resolution than FDM/FFF & \(\bullet\) & Limited to UV curable materials \\  & \(\bullet\) & Potential for multi-material printing & \(\bullet\) & Printers are more expensive that \\  & \(\bullet\) & Accurate printing & & FDM or inkjet printers \\ \hline \multirow{5}{*}{Aerosol jetting} & \(\bullet\) & Higher resolution than Inkjet printing, FDM, & \\  & Stereolithography and DIW & & \\ \cline{1-1}  & \(\bullet\) & Substrates can be polymers, ceramics or metallic & \(\bullet\) & Expensive \\ \cline{1-1}  & \(\bullet\) & Multi-material printing & & \\ \cline{1-1}  & \(\bullet\) & Curved surface printing & & \\ \cline{1-1}  & \(\bullet\) & Nano/microelectronics printing & & \\ \hline \multirow{5}{*}{Direct ink writing (DIW)} & \(\bullet\) & Higher resolution than FDM & & \\ \cline{1-1}  & \(\bullet\) & Multi-material printing & & \\ \cline{1-1}  & \(\bullet\) & Easy material customisation (e.g., adding & & \\ \cline{1-1}  & \(\bullet\) & nanomaterial into polymer matrix) & & \\ \cline{1-1}  & \(\bullet\) & Portability & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._exhibited good electrical properties with a conductivity value of 714 Scm\({}^{-1}\)[91]. The self-healing properties of PDMS increases the service life of the sensor. Human skin comprises of about 7% of total body weight and covers 100% of the human body surface [92]. The skin has amazing abilities including self-healing and sensing stimuli such as pressure (touch) and temperature (heat). The research of flexible electronic skin has recently gained much attention in fields such as robotics, health monitoring and human-machine interactions. In any case, the flexible electronic skins need to respond to electrical signals translated from physical phenomenon such as touch or temperature changes. There exist two challenges in flexible electronics skin research. One of the challenges is achieving the self-healing property of the skin and the second challenge is the ability to distinguish directions, just as the actual skin. Self-healing ability enables a stable operation and increases the life span of the system while the ability to distinguish stimuli directions enables an intelligence of the electronic skin for practical uses [93]. A research team developed an electronic skin sensor based on carbon nanotubes (CNTs), polyurethane and epoxidized natural rubber [93]. The electronic skin can detect human motion, has self-healing properties and has the ability to distinguish direction [93]. The sensor was made from a hydrogel produced by cross-linking PVA and PEI with Bn, and MXene was incorporated into the matrix to enhance electrical properties (PVA/ Bn/ PEI/MXene (PBPM)). The PBPM electronic skin can self-heal within 0.06 s, which is excellent in comparison to other reported hydrogel-based electronic skins. The skin also has an impressive response time of 0.12 s due to the interaction of the copious functional groups (e.g., -OH and -O) with the polymer matrix. The electronic skin can distinguish the direction of stimuli, such as wrist up/down and head up/down. The research team claims to be the first to report such a direction recognition ability in flexible electronic skin literature. The self-healing ability of the skin can be attributed to its supramolecular interactions and the dynamic covalent bonds that revive its electrical and mechanical properties upon damage. During the fabrication of the electronic skin, samples were characterised via SEM for morphology studies and X-ray photoelectron spectroscopy (XPS) for surface analysis and chemical composition. Functional groups were characterised via Fourier transform infrared spectroscopy (FT-IR). Electrical signals from tensile stain stimuli were captured by an electrochemical workstation. Tensile strain signals were measured at a deformation rate of 100 mm min\({}^{-1}\).