## 5 Applications and Challenges
### Challenges  
#### 5.2.1 Material Performance Challenges  
Although DE has many advantages in tuneable lens applications, current material properties still have limitations in driving performance, stability, durability, and high voltage, which restrict their further development and broad application [92]. 1. Driver performance  
At the drive performance level, existing DEA finds reconciling the dual requirements of large deformation and fast response challenging. Studies have shown that DE materials that achieve large optical zoom ratios often require properties of low modulus of elasticity and high dielectric constant. Still, the mechanical response time of such materials is usually large (>100 ms), which cannot meet the response requirements of dynamic target tracking and other scenarios. On the contrary, high-modulus materials reduce the response time, but their magnification ratio is drastically reduced, resulting in a limited optical adjustment range. This inherent contradiction in material performance essentially stems from the dynamics of the polymer chain segments. While low crosslink density materials can achieve large deformations, the viscoelastic dissipation of the molecular chain reconstruction process severely constrains the response speed [93]. Blending materials with low Young's modulus and low dielectric loss [94], along with the addition of plasticisers (such as epoxidised soybean oil) and high dielectric constant fillers, offers potential solutions to the contradiction between fast response and large strain of DE [95]. In addition, the common single-layer planar DEA has significant mechanical performance shortcomings. The equivalent driving force generated by a single-layer DEA film at the driving voltage is a few mN, which may be challenging to overcome the combined resistance generated by the interfacial tension of the liquid lens and the elastic reset mechanism when applied to tuneable lenses with an extensive adjustment range of size or focal length. Therefore, attempts can be made to enhance the driving force by stacking multilayer DEA structures to improve the lens performance [95; 96]. Figure 13: DETL applications. (**a**) Autofocus systems: (**a1**) top view of the lens object; (**a2**) schematic diagram of the lens structure; (**a3**) schematic diagram of the zoom system [85]. (**b**) Eye-tracking lenses: (**b1**) lens physical diagram; (**b2**) lens zoom effect diagram; the left and right diagrams are the imaging pictures before and after the application of voltage [66]. (**c**) Microimaging: (**c1**) system  
schematic; (**c2**) system zoom effect diagram; the upper and lower diagrams are the imaging pictures before and after the voltage change, respectively [83]. (**d**) AR displays: (**d1**) system schematic diagram; (**d2**) AR display effect diagram; the upper and lower diagrams are imaging pictures before and after voltage change, respectively (where \({}^{\prime}\)\(\mathcal{H}\)\({}^{\prime}\) is the Chinese character for light) [84]. (**e**) Infrared imaging: (**e1**) system schematic diagram; (**e2**) infrared imaging effect picture [91]. The stability of materials such as DE and drive electrodes is also a key issue. DE materials can also change the electrical and mechanical properties over time due to stress relaxation and other effects after pre-stretching, reducing the stability and reliability of tuneable lenses [97; 98]. Carbon grease is widely used as an electrode material for DEAs due to its low price and simple preparation advantages. However, carbon grease electrodes tend to solidify over time, significantly reducing the lens's zoom ratio and affecting the stability of the lens's performance. In addition, hydrogel material has become another research direction of electrode materials due to its high transparency and flexibility. Still, hydrogel has high water absorption, and by absorbing water in the air, the electrical and mechanical properties of the electrode are changed, which further affects the performance stability of the tuneable lens.