## 3 Results and Discussion  
The EA behavior of the insulating material was dependent on its material type and molecular structure. The EA force is a result of polarization \(P_{m}\) induced by an electric field \(E\)[15]  
\[P_{m}=kEe^{-t/\tau} \tag{1}\]  
where \(k\) is a constant related to dielectric loss factors, \(t\) is time and \(\tau\) is dielectric relaxation time. The polarization describes the separated charges, and the cross product of polarization and applied electric field presents the prehension pressure \(\sigma\)[15],  
\[\overrightarrow{\sigma}=\overrightarrow{P_{m}}\,\times\,\frac{\overrightarrow {E}}{E} \tag{2}\]  
Figure 3: Schematic diagrams of experimental procedure; an exposed-electrode EA pad was mounted on a linear rail; (**a**) the EA pad approached to a substrate and a 3.5 kV voltage was applied, holding for 5 s; (**b**) the EA pad was lifted; (**c**) voltage was removed and the release timing began; (**d**) the substrate released from the EA pad and release time is measured. Figure 2: (**a**) The EA electrodes follow a concentric-comb pattern. It has an effective diameter of 56 mm with an electrode width and gap of 4 mm. (**b–g**) fabricated EA grippers with different base materials: (**b**) acrylic; (**c**) medium-density fibreboard (MDF); (**d**) silicone film; (**e**) polyacrylate elastomer (VHB) film; (**f**) Mylar plastic film; and (**g**) Styrofoam. The dielectric relaxation time is correlated with the molar polarizability, \(P\), which can be written as [16]  
\[P=\frac{\varepsilon_{r}-1}{\varepsilon_{r}+2}\cdot\frac{M}{\rho} \tag{3}\]  
where \(\varepsilon_{r}\) is the dielectric constant, \(M\) is the molecular weight of a repeat unit, \(\rho\) is the density. A higher molar polarizability indicates a faster polarizing and depolarizing speed, and hence a reduced dielectric relaxation time. The polarized charges can be neutralized in the form of a current in an electrically conductive material, since these charges could migrate freely. However, they can barely migrate through insulators and, as a result, cause residual charge. In our experiments, when the electric field was removed, residual charges caused a residual EA force which prevented the object from falling under gravity, demonstrating a slow de-electroadhesion behavior. This is believed to be related to the dielectric relaxation time and the dielectric permittivity of the insulating material [15]. In this work, the de-electroadhesion behavior is characterized by the release time, which is the period between the time when the electric field is removed and the final release. The experimental results are shown in Figure 4 and detailed comparisons are made in Figure 5, with raw data available in Supplementary Table S2 and a video demonstrating the release process available in Supplementary Video S1. As shown in Figure 4a,b, the de-electroadhesion behavior was strongly dependent on the material type of the objects. The release time results can be divided into three different time categories: over 10 s (including Mylar and PVC plastic film); between 1 s and 10 s (including Polyethylene (PE) foam and silicone film); and under 1s (including balsa wood, paper sheet and card board). It is clear that plastic-based materials exhibited a longer release period (Mylar shows the worst release behavior) than foam, silicone, and cellulose-based materials, which had the shortest release periods. Note that despite the similar mass, Mylar plastic sheet had an average release period 119 times longer than paper. It is also interesting to note that the lightest object, tissue, needed less than 3 s to release. The key findings are summarized as follows. Figure 4: (**a**) Comparison of the release time six EA pads on eight objects. (**b**) Expanded comparison of objects with release time less than 4 s. (**c**) Performance scores of six EA pads based on the release speed of the eight objects. 1. _The effect of the dielectric constant of the object_.