Experimental Section
Gripper Fabrication: The silicone elastomer and carbon black-filled elastomer composite were fabricated out of conductive nanoparticles (AkzoNobel, Ketjenblack EC-300J) and a liquid silicone elastomer (Smooth-On, Ecoflex 00–30) in a planetary centrifugal mixer (Thinky, ARE-250) following already existing manufacturing approach.[35] The fabrication of the gripper consists of six steps. At the first step, the silicone layer for electroadhesive (EA) pad was prepared. The liquid silicone was poured onto a PET sheet and spread using an applicator coater (Zehntner, ZUA2000) and a variable gap applicator (Zehntner, ZAA2300) with a gap of 100 μm and a drawing speed of 5 mm/sec. The elastomer layer was then cured in an oven for 30 min at 80 °C. Then, in the second step, the conductive silicone layer was formed by pouring conductive silicone composite onto the silicon layer and blade casting it using a variable gap applicator with the gap of 25 μm and a drawing speed of 15 mm/sec. The multilayer sample was then placed place in the oven for 60 min at 80 °C to form a thickness of conductive layer around 20 μm. In the third step, the electrode layer was ablated using a laser engraver (Trotec, Speedy 300) to obtain the interdigitated electrode structure (Figure 2a-ii), and then the sample surface was cleaned with a solvent (isopropyl alcohol) to remove etched conductive particles. Uncleaned regions with carbon particles will lead to a drastic decrease in the breakdown voltage of the pads. Conductive tape (3M, 9713) was placed onto the electrodes from both sides to connect the device with the power supply after assembly. In the fourth step, the first step was repeated to encapsulate the interdigitated electrode structure. A variable gap applicator with a gap of 1000 μm and a drawing speed of 5 mm/sec was used. In the step five, liquid silicone was poured into the mold with the pad screwed to the bottom side of the mold. Then the silicone was left at least two hours at room temperature to cure. Leaving it at room temperature also has the advantage to leave more time for the trapped air to emerge to the surface. In the sixth step, the coffee granules, which was already used in conventional GJ-based grippers,[5,11] with the diameter of 0.2 mm were poured into already fabricated silicone bag.
Grasping Parameters and Grasping Force Characterization: All the experiment the objects were 3D printed using a printer (Utimaker, S5) and polylactic acid (PLA) material (Ultimaker, Tough PLA) with the highest layer thickness of 0.4 mm (instead of a usual 0.06 mm). Porous hemisphere was made by drilling multiple through-holes to prevent airtight seal between the gripper and the manipulated object. All the objects had a screw hole on the bottom to ensure the connection between the object and the surface under the gripper in the linear stretcher (Instron, 5965). The gripper moved down with the speed of 1 mm/sec and stopped when the gripper’s surface is 10 mm lower than the highest point of the manipulated object to ensure immersion of the object into granules from all the sides. After moving the gripper down, the force data form the load cell (Instron, EX2580-500N), which corresponds to applied force, was recorded. Then, the gripper was activated and moved up, while the force data was collected from the load cell of the linear stretcher. The GJ mode was activated using vacuum pump (Thomas VTE 3), which performance was measured using a manual pressure sensor. The pump was able to generate a pressure drop of 84 KPa, thus an absolute vacuum of 16 kPa considering the ambient pressure to be 100 kPa. This is close to the datasheet value of 15 kPa for an absolute vacuum. The maximum generated force in each of the experiments was depicted and used to represent grasping force results. The grasping angle data was collected using a camcorder of the phone (IPhone, 6s). Then, the data was manually processed. In the electroadhesion (EA) characterization, a flat sheet of paper was mounted on the bottom of the linear stretcher instead of the target object. The paper was chosen because of it does not stick to the silicone bottom layer of the gripper.[1] A high voltage supply (Stanford Research Systems, PS350/5000V-25W) was used to activate the devices. A low capacity load cell (Instron, EX2580-10N) was used to measure the grasping forces.