Figure 2. The gripper’s schematic structure and operating
principle of the gripper. a) The blue silicone balloon is held using an
orange holder. Granules are kept inside the balloon. The electroadhesive
pad is located on the bottom surface of the gripper and is connected to
the ground and high voltage electrodes. The gripper is lowered to the
flat surface (in grey) of a grasping object. b) The applied potential
difference causes the generation of electrostatic forces attraction
between the bottom surface of the gripper and the substrate surface of a
grasping object. c-e) The working principle of a granular jamming mode.
c) The gripper is lowered on an object. d)The gripper deforms and takes
shape of the manipulated object. e) The air is evacuated by turning on
the vacuum pump. f) The object can be released by applying positive
pressure and changing the curvature of the bottom layer of the gripper.
The GJ mode relies on a bag filled with the granules, which is soft when
the gripper approaches the target object (Figure 2c). As the gripper
lowers against the object, the soft bag deforms and takes the shape of
the object (Figure 2d); at this point, the bag is stiffened by
evacuating the air and jamming the tightly packed granules (Figure 2e).
This stiff state fixes the object against the bag such that it can be
lifted. Pumping air back into the bag makes it soft by enabling movement
of the granules, thereby causing the release of the object (Figure 2f).
In the combined mode, the simultaneous activation of GJ and EA enhances
the grasping force of the gripper. In the soft state, granules flow
around the object increasing the contact surface between the object and
the gripper. After activation GJ and EA, EA enhances the adhesive forces
of the contact surface.[1,28]
Fabrication
The fabrication procedure consists of layer-by-layer and molding
techniques, as illustrated in Figure 3 . First, an EA pad was
fabricated by encapsulating a conductive silicone layer with a
laser-engraved interdigitated electrode in-between two dielectric layers
made of silicone (Figure 3a-i,ii,iii). All the layers were fabricated
using a blade-casting technique and cured onto each other. Then, the
fabricated EA pad was placed in an empty mold, which consists of an
external, internal, and bottom part (Figure 3b-i). Liquid silicone was
poured into the mold to form the silicone bag and connect the EA pad,
resulting in a monolithic structure. After curing the silicone, the
gripper was removed from the mold (Figure 3b-ii). Finally, the silicone
bag was filled with coffee granules with diameters of 0.2 mm (Figure
3b-iii). A detailed explanation of the fabrication process is available
in the Experimental Methods section. The resulting gripper had a weight
of 13 g and the diameter of the grasping surface was 36 mm (the diameter
of the interdigitated electrode structure was 30 mm).