Fig. 1. Preparation of tunable microlense array by electro-wetting method. (a) Schematic of the principle of electro-wetting method (EWOD)^[22]; (b) SEM image of a concave microlens array prepared using the EWOD under the application of a certain external voltage^[27]; (c) schematic illustration of microelectrodes printed by a stabilized conical jet printing mode and droplet microlenses printed by a droplet mode of electronic jet printing^[28]; (d) normalized light intensity distribution of the focused spot of the lens in MLAs induced by the EWOD mechanism at different voltages^[28]; (e) schematic representation of the processes of wetting, merging, growth, and de-wetting of sulfur droplets on a gold electrode^[29]

Fig. 2. Force-deformed microlens array. (a) Schematic of the fabrication process of a zoom compound eye^[35]; (b) schematic of the fabrication process of a negative-pressure artificial compound eye^[36]; (c) PDMS microsphere imaging switch^[31]; (d) imaging of PDMS microbeads as an elliptical lens^[31]

Fig. 3. pH-responsive BSA compound eyes^［42］. (a)‒(d) Imaging of BSA compound eyes in different pH environments; (e) fabrication of the SU-8/BSA-based compound eye; (f)(h) 3D laser confocal microscopic image and the crosssectional profile of the SU-8 microlens; (g) (i) 3D laser confocal microscopic image and the cross-sectional profile of the SU-8/BSA-based composite compound eye

Fig. 4. Schematic and working mechanism of electrical control of electro-reconfigurable adaptive PVC gel-based microlens^[45]. (a) Composition of PVC gel lens using PCB plates; (b) schematic of the prepared donut-shaped electrode design; (c) compression of the PVC gel to develop biconvex lens; (d) 3D micro-shape of PVC gel lens when the fan-shaped electrodes are independently operated for changing the focal length and focal point; (e) surface profile of the upper side of the biconvex microlens under an each applied electric field of Fig.4 (d)

Fig. 5. Thermally tuned microlens arrays. (a)‍‒(f) Fabrication process of glycerol microlens arrays based on deposited graphene microsheets^[48]; (g) (h) schematic of an IR light-actuated tunable microlens and corresponding optical images^[49]

Fig. 6. Optofluidic microlens. (a) Schematic of the concept of optofluidic tunable microlenses. The focusing characteristics are controlled by filling the channel with different media and changing the refractive index contrast^[53]; (b) schematic of the dual-channel optofluidic microlens array^[55]; (c) schematic of the optofluidic microlens used to observe the cell flow in the microfluidic channel^[54]; (d) image of erythrocyte cells and fluorescence image of MCF-7 cells captured by the optofluidic microlens^[54]

Fig. 7. Schematic of the principle of refractive index adjustment of a liquid crystal microlens^[62]

Fig. 8. Tunable superlens. (a) Schematic of an electrically tunable polarization-multiplexed achromatic dielectric superlens^[74]; (b) schematic of a stretchable superlens with a stretching force applied along the x-axis and y-axis directions^[74]; (c) transmission spectra and the corresponding focusing intensity distributions along the z-axis (x=y=0) of superlenses with p-values of 450 nm, 550 nm, 650 nm, and 750 nm, respectively, after stretching^[74]; (d) a tunable superlens consisting of a fixed superlens and a tunable superlens consisting of a movable superlens, where, as designed, a small change in the distance between the two lenses leads to a large change in the focal length^[75]

Fig. 9. Microlens arrays in imaging systems. (a) Schematic of the key functioned structure configuration of the camera^[82]; (b) schematic and 3D schematic of a prototype endoscope with a liquid tunable-focus microlens integrated at its end and actuated through IR light^[84]

Fig. 10. 2D/3D display conversion based on liquid crystal microlenses. (a) Schematic of liquid crystal microlens realizing 2D/3D display conversion^[90]; (b) measurement results of biconvex lens refractive error with voltage change^[93]; (c) 3D scene and 2D picture display effect in 2D/3D display mode^[93]