10.2.2 Realization of device stretchability
Human skin has excellent stretchability.Therefore，e-skin needs to have stretchability similar to that of human skin in order to maintain a good fit with the skin during use.In order to achieve excellent stretchability，on the one hand，it is necessary to develop materials with excellent elasticity.On the other hand，it is necessary to design the alignment structure of the materials in the devices.The previous section mainly introduced the selection of flexible and stretchable materials，mainly focused on the inherent stretchability of the materials.This section will focus on the integrated structure of the materials in the devices，and introduce methods for enhancing the stretchability of the devices.
Traditional inorganic semiconductor/conductor materials usually have high modulus，high brittleness，and poor stretchability.They are subject to many restrictions in the application of flexible devices.In order to solve these problems，researchers have developed a series of methods for preparing materials with excellent stretchability by designing the geometric configuration of the materials.
The first method is to create cracks in the continuous film by stretching.Firstly，a rigid continuous conductor/semiconductor film is placed on an elastic substrate，and then the substrate is unidirectionally or bidirectionally stretched.As the film is brittle，it breaks to form cracks when the substrate is stretched.As the degree of stretching increases，the cracks become wider and the number of cracks increases.Consequently，the material obtains good stretchability.By controlling the degree of stretching，the film can be cracked while maintaining a conductive path（Figure 10-2（a））.For example，Graz et al.used this strategy to integrate a gold film on an elastic substrate，which can still maintain good conductivity when the strain reaches 100％.
The second method is to spread the one-dimensional fibrous material on the substrate.Due to the random orientation of the one-dimensional material，a conductive network path can be formed on the substrate.When the substrate is stretched，the material can still maintain direct contact with the substrate，and the conductive network can deform.Even when the deformation is large，the conductive network will not be destroyed（Figure 10-2（b））.For example，Lee and Ko's team used silver nanowire as conductive material to make electrode material with high stretchability and high conductivity.The strain of the electrode could reach 460％，and the resistance was still low when stretched.(https://www.daowen.com)
The third method is to use micro-fabrication technique to pattern the conductive materials（Figure 10-2（c））into a curved or spiral structure，and then put the conductive materials on an elastic substrate.The conductive materials with a spiral structure can undergo very large deformation with the stretching of the substrate，thereby giving the composite materials good stretchability.Gonzalez et al.studied the effect of various patterned structures of conductive materials on the stretchability of the device.They found that when the conductive materials were placed in the substrate in a serpentine structure or a horseshoe shape，the stretchability of the materials was the best，which could still maintain good conductivity when the strain reaches 100％.The stretchability could be further improved by stacking multiple serpentine structures.
The fourth method is to composite a high modulus conductive material with a prestretched elastic substrate.When the substrate shrinks，wrinkles on the conductive material appear.Unidirectional pre-stretching can produce linear folds（Figure 10-2（d）），and bidirectional stretching can produce herringbone folds（Figure 10-2（e））.In addition，it is also possible to construct a wrinkle structure on the surface of the elastic substrate in advance by a micro-fabrication method，and then put the conductive material on the surface of wrinkle.The pleated structure prepared by this method also has good deformability.This strategy is applicable to a variety of materials，including inorganic semiconductors，metals，composite materials，graphene，carbon nanotubes，etc.Due to the existence of wrinkles，when the flexible device is stretched，the deformation of the conductive materials therein is mainly derived from the change of its geometric structure，and the material itself does not produce large deformation.The use of the wrinkle strategy can not only give the device good stretchability，but also avoid the repeated deformation of the material itself，which is beneficial for prolonging the service time of the device.