## 3 Lens Construction
### Monolithic Lens  
Monolithic lenses facilitate compact and efficient optical tuning by connecting a dielectric elastomer driver to the lens. Ordinary monolithic tuneable lenses are classified into three types based on planar, spherical, and conical DEA drivers according to the type of DEA. 1. Planar DEA  
The three primary forms of planar actuators are ring, disc, and sector. These actuators are essential in tuneable lens design due to their mature manufacturing process and easy integration. According to the structure, planar actuator-driven DETLs are mainly nonconvex, biconvex, and droplet lenses. [PERSON] et al. [46] were the first to propose an annular DEA to drive a monoconvex silicone lens (Figure 5a), verifying this configuration's feasibility to achieve zoom. [PERSON] et al. [47] used a disc-shaped driver through structural optimisation to achieve lens zoom with the help of an internal frame fixation and an external fan-coupler driver, as shown in Figure 5b. They achieved a zoom ratio of 165.7% at a driving voltage of 5 kV. [PERSON] et al. [48] prepared a monoconvex ion eye using ion gel as both lens and electrode materials, as shown in Figure 5c. A disc-type DEA was sandwiched between the lens and electrode, and the deformation of the DEA after the voltage application drove the lens's deformation to achieve the zoom. The zoom ratio could reach -150%. [PERSON] et al. [49] innovated to divide the electrode into four sectors, as shown in Figure 5d, and induced lens deformation by asymmetric driving to achieve active regulation of aberration, which provides ideas for improving imaging quality. Biconvex lenses have attracted much attention in the field of bionic optics due to their resemblance to the human eye lens, and [PERSON] et al. [30] first proposed an 'electronic eye'  
Figure 5: Planar actuator-driven monoconvex DETL. **(a)** Driven based on a butyl rubber DEA: **(a1)** top view of the lens object; **(a2)** schematic diagram of the lens structure [46]. **(b)** Driven using a sector coupler: **(b1)** top view of the lens object; **(b2)** lens zooming effect, the upper and lower figures show the imaging effect before and after applying the voltage, respectively [47]. **(c)** Solid-state lens driven by a disc-type DEA: **(c1)** top view of the lens object; **(c2)** lens zoom schematic, the upper and lower diagrams show the shape of the lens before and after the application of voltage, respectively, ‘\(\star\)’ and ‘\(-\)’ indicate positive and negative charges respectively [48]. **(d)** Adjustable aberration lens driven by a sector-type DEA: **(d1)** top view of the lens object; **(d2)** lens aberration regulation effect, the upper and lower diagrams show the imaging effect of the lens before and after the application of voltage, respectively [49]. design that combines an adjustable corneal lens with a bionic lens. [PERSON] et al. [50] designed a biconvex liquid lens driven by a circular silicone rubber dielectric elastomer, as shown in Figure 6a. This lens features a magnification ratio exceeding 120% and a response time under 175 microseconds, which is the fastest DETL in terms of response time currently available. [PERSON] et al. [51] developed an all-solid-state adjustable soft lens, as shown in Figure 6b, with a focus variation range of up to 209%. For the performance enhancement of biconvex lenses, [PERSON] et al. [52] performed a detailed theoretical analysis based on ring driver-driven biconvex liquid lenses, explored the influence of multiple parameters on the lens performance, and solved the coupling and non-uniform deformation problems of deformable lenses. [PERSON] et al. [53] proposed a concise computational model based on the assumption of uniform deformation, applying the energy method, and matched well with the realisation results. [PERSON] et al. [54] used 3D printing technology to print transparent conductive gels as lenses on both sides of DE to prepare a biconvex solid lens, as shown in Figure 6c.