## 5 Applications and Challenges
### Challenges
#### 5.2.1 Material Performance Challenges
Meanwhile, the stability of the internal liquid or soft solid material when DEs are used as actuators must also be considered [99]. Material durability  
The durability of the lens mainly depends on the performance retention of the DE, and the decline of its mechanical properties during repeated driving is a problem that should not be ignored. In practical applications, tuneable lenses frequently change the focal length, requiring DE to withstand repeated mechanical effects such as stretching and compression. However, the mechanical properties of DE are relatively weak and prone to fatigue problems under repeated driving over a long period [100]. Fatigue is a phenomenon in which a DE is subjected to the mechanical action of cyclic loading. Microcracks gradually develop and expand within the material, ultimately degrading the material's properties. When a DEA drives a tuneable lens, each time a voltage is applied and removed, the dielectric elastomer undergoes a cycle of stretching and recovery. As cycles increase, microcracks within the DE accumulate, which reduces the material's strength and toughness. Studies have shown that the tensile strength of some DE may decrease after tens of thousands of cycle drives [92; 101]. This fatigue phenomenon affects the driving performance of DE. It may cause the lens to fail to accurately reach the preset focal length during the zoom process, making the picture blurred and distorted. This is an urgent problem for tuneable lens applications that require long-term stable operation. To improve the service life of DETL, it is necessary to develop DE materials with higher durability or to reduce the stresses that DE are subjected to during the driving process by improving the structural design and optimising the driving method to reduce the occurrence of the mechanical property degradation [102; 103]. Currently, commonly used DE materials, such as PDMS, generally have a low relative dielectric constant of around 2-3. This makes it often necessary to apply driving voltages in the thousands of volts to achieve the deformation required by DE to drive tuneable lenses in practical applications. Such high drive voltages pose several problems. Firstly, the generation and control of high voltages requires complex and expensive power supply equipment, which increases the cost and size of the system, limiting the use of tuneable lenses in several size- and cost-sensitive application scenarios, such as portable imaging devices,endoscopic probes, and so on. Secondly, high voltage also poses a safety hazard. When the applied voltage exceeds the breakdown field strength of the DE, a conductive channel will be formed inside the DE, leading to material damage and preventing the tuneable lens from functioning correctly, which reduces the reliability and stability of the system. At the same time, the high voltage also challenges the research of its DETL self-sensing technology. The self-sensing technology mainly predicts strain by measuring the current capacitance value of DEA, but during the measurement of DEA capacitance, the high-voltage environment makes it difficult for most capacitance sensors to be appropriately applied, and special circuits need to be prepared, which restricts the effective implementation of the relevant technical solutions [104; 105].