## 3 PVC Gel Lens
### Macro-Sized Circular Lens  
The earliest PVC gel lens was demonstrated using a top indium tin oxide (ITO) electrode, PVC gel, and bottom ITO electrode [25]. The structure of the lens is shown in Figure 8. The two ITO electrodes are placed together with a thin gap, as shown in Figure 8a. Only the top electrode has a through hole. A PVC gel (not shown) fully fills the hole. In the voltage-off state, the PVC gel has a convex shape in the hole. By applying a DC voltage to the electrode, for example, the top (bottom) ITO electrode connects to the anode (cathode), electrons are injected to the PVC gel from the bottom electrode. The plasticizer molecules carried the electrons to migrate toward the top electrode. Due to electrostatic repulsion,  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **PVC/DBA** & **1:1** & **1:5** & **1:9** \\ \hline Charge density near the anode (C/cm\({}^{3}\)) & \(\sim\)60 & \(\sim\)365 & \(\sim\)85 \\ Stress induced 200\% strain (kPa) & \(\sim\)3100 & \(\sim\)250 & \(\sim\)5 \\ Young’s modulus (kPa) & \(\sim\)495 & \(\sim\)30 & \(\sim\)5 \\ Electrostatiic adhesive force with 500 V on the anode (g/cm\({}^{2}\)) & \(\sim\)25 & \(\sim\)185 & 115 \\ Transmittance (\%) & Very low & \(\sim\)82 & \(\sim\)88 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Key parameters of PVC gel with different DBA concentrations.  
Figure 7: Mechanism for deforming a PVC gel sandwiched between two stripe electrodes: (a) voltage-off state and (b) voltage-on state.  
the PVC gel displays creep deformation on the top electrode, causing the surface profile of the PVC gel to change, as shown in Figure 8b.  
In this lens, the PVC gel was prepared from PVC powder, tetrahydrofuran (THF), and dibutyl phthalate (DBP). DBP is a typical plasticizer. PVC powder was dissolved in a THF/DBP mixture first, and then the mixture was used to fill the hole of the electrode. After evaporating the solvent THF, a PVC gel was formed. Although a PVC gel with different weight ratios can be prepared, the gel with ~10 wt % PVC and ~90 wt % DBP can present the highest bending displacement by voltage due to the low elastic modulus. In the PVC gel, the PVC forms rigid cross-linked chains (or polymer network) and the DBP molecules are dispersed as multi-domains in the chains. For the demonstrated lens, the diameter of the hole is 2 mm and the thickness of the top electrode is ~100 \(\upmu\)m. By applying a voltage changed from 0 to 400 V, the focal length of the PVC lens can be tuned from ~10.65 mm to ~10.77 mm. When the voltage is continuously increased, the dynamic range of the focal length can be widened. In contrast to the operating voltage for a DEA lens, the required voltage for the PVC lens is relatively low.  
Based on the structure shown in Figure 8, [PERSON] et al. (2018) prepared a PVC lens. The PVC gel is prepared using PVC powder and nontoxic DBA as the plasticizer. The weight ratio of PVC/DBA is 1:9. The diameter of the hole is ~1.5 mm, and the gap of the two substrates is ~1 mm. By increasing the voltage from 300 V to 800 V, the focal length can be tuned from ~3.8 mm to ~14.3 mm. At V = 800 V, the measured current is just ~15 \(\upmu\)A. The computed maximum power consumption is ~12 mW. In another lens demonstrated by the same group ([PERSON] et al., 2018), the PVC/acetyl tributyl citrate was prepared at a 1:10 weight ratio. The transmittance of the PVC gel was measured to be ~86.6% at \(\uplambda=550\) nm. As the operating voltage increases from 0 to 500 V, the focal length changes from ~5 mm to ~15 mm. The response time of the PVC gel lens is ~1 s.  
The above PVC lenses also exhibit similar features, such as simple fabrication, a compact structure, and a good mechanical stability. However, the dynamic range is rather limited, even if the operating voltage is high (>500 V). For the first lens, its creeping rate is slow due to some memory effect ([PERSON] et al., 2018; [PERSON] et al., 2018).