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\begin{document}

\title{A Soft Gripper with Granular Jamming and Electroadhesive Properties~~~~}



\author[1]{Yegor Piskarev}%
\author[1]{Antoine Devincenti}%
\author[1]{Vivek Ramachandran}%
\author[1]{Pierre-Etienne Bourban}%
\author[1]{Michael D. Dickey}%
\author[1]{Jun Shintake}%
\author[1]{Dario Floreano}%
\affil[1]{Affiliation not available}%


\vspace{-1em}



  
  \date{May 27, 2024}


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\textbf{Abstract}

In recent years, there has been a growing interest in the development of
universal soft grippers that can handle objects of varying form factors
(including flat objects), surface condition (including moistened or oily
objects), and mechanical properties (deformable and fragile). Yet, there
is no single gripper that can gently grip objects with such a wide range
of properties. In this paper, we present a soft gripper that combines
granular jamming (GJ) and electroadhesion (EA) to gently grasp and
release a large set of diverse objects. The gripper can operate in GJ
mode only, in EA mode only, or in a combination mode that simultaneously
activates GJ and EA. In GJ mode, the gripper can grasp objects with
different surface properties, lift objects 38 times its own weight using
negative pressure, and release objects by applying positive pressure,
but has difficulty in handling flat and fragile objects. In EA mode, the
gripper can manipulate flat and fragile objects but encounters
difficulties with different surface properties such as oily or
moistened. In the combination mode, the gripper can generates grasping
forces up to 35\% higher than in the GJ mode for all object sizes and
certain shapes such as a cylinder.

\subsection*{}

{\label{introduction}}

\subsection*{Introduction}

{\label{introduction}}

The softness of the human hand is a critical factor that allows us to
hold, lift, and manipulate a variety of objects and has inspired
roboticists to incorporate softness in gripper design and materials. The
compliance of soft materials enables passive adaptation of the gripper
during grasping operations allowing manipulation of a wide range of
objects without bringing additional control
complexities.\textsuperscript{{[}1,2{]}} In recent years, there has been
a growing interest in the development of universal soft grippers that
can work with objects of different form factors, rigidity, surface
properties, and level of fragility.\textsuperscript{{[}3--6{]}} A
possible approach to create such highly versatile grippers is to combine
different gripping technologies that complement their individual
limitations.\textsuperscript{{[}1,3,7--10{]}} Yet, it is still
challenging to develop a single gripper that can grasp and release
objects of different form factor including flat objects, surface
conditions (wet, porous, oily, and powdered), and mechanical properties
(fragile and deformable).

In this paper, we present a soft gripper capable of manipulating
different objects with varying physical properties, such as shape,
surface conditions, and rigidity. The proposed gripper combines two
different technologies: granular jamming to control stiffness and
electroadhesion to control adhesion. Here we show that not only does
this combination mutually compensate for the limitations of each
individual technology, but it also makes the gripper capable of
performing multi-stage grasping tasks that consist of diverse grasping
and releasing operations on objects made of different material, surface,
and shape. The manipulation of a book is an example of multi-stage
operation that requires grasping and turning a rigid cover and flipping
through single pages.

Granular jamming (GJ) enables reversible stiffness change between soft
and rigid configurations by means of negative
pressure\textsuperscript{{[}5,11,12{]}}. High compliance in the soft
state allows a GJ gripper to envelope the manipulated object by pressing
on it. When negative pressure is applied, the gripper becomes stiff and
holds the encaged object.\textsuperscript{{[}2{]}} Variable stiffness
can also be achieved by integrating phase-change materials that vary
mechanical properties under thermal
stimulation.\textsuperscript{{[}7,13{]}} However, granular jamming
offers comparatively faster response time (\textasciitilde{}100ms),
independence from environmental temperature, higher lifting force,
easier fabrication, higher robustness, and lower
cost.\textsuperscript{{[}2,5,14,15{]}} The grasping force produced by
granular jamming is sufficient to grasp objects of different
morphologies, almost independently of the surface conditions of the
object.\textsuperscript{{[}5,11,16{]}} The grasping force of GJ grippers
can vary from 0.09 to 1.2 kN.\textsuperscript{{[}12{]}} GJ has been
combined with soft pneumatic actuators to provide more dexterous grasp
and lift heavier objects because of the enhanced holding
forces.\textsuperscript{{[}10,17{]}} However, GJ grippers cannot lift
flat objects, such as a sheet of paper. Also, the grasping performance
of delicate, fragile and easily deformable objects such as a thin layer
of cloth, an egg, or water balloons, as well as larger objects than the
active area of the granular bag can be challenging and have not been
demonstrated so far.

Electroadhesion (EA) instead is an adhesive technology that leverages
the shear force generated by electrostatic
forces.\textsuperscript{{[}18{]}} Electroadhesive pads have been
combined with different actuation technologies, such as dielectric
elastomer actuation,\textsuperscript{{[}1{]}} soft pneumatic
actuation,\textsuperscript{{[}19{]}} layer
jamming,\textsuperscript{{[}20{]}} and Fin-Ray structured
actuation\textsuperscript{{[}21{]}}. The enhanced shear force makes
EA-based grippers capable of delicately grasping both flat and fragile
objects without squeezing or breaking
them.\textsuperscript{{[}1,22--26{]}} While the adhesive force of EA
pads can be tuned by regulating electrical input, EA effectiveness is
highly dependent on the environmental and surface conditions of the
object being grasped.\textsuperscript{{[}18{]}} In particular
electroadhesion is less effective for objects that are greasy, rough, or
wet.\textsuperscript{{[}2{]}} An additional challenge of soft grippers
that rely on electroadhesion is the residual electrostatic charge that
remains for a few seconds after removing the voltage and can result in
difficult release of light objects.\textsuperscript{{[}27{]}}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{Figure 1.} Structure of the gripper. a) The gripper consists of
a silicone balloon with granules and a soft electrode layer at the
bottom. The silicone bag is connected to an external vacuum pump and
fixed by a holder. The electrode layer is plugged into an external power
supply via conductive tape. b) The electrode layer consists of a
circular interdigitated electrode structure made of conductive silicone.
c) The gripper can change the geometry of the bottom layer from flat to
hemisphere by applying a positive pressure via a vacuum pump. d) In the
soft state, the gripper can take the shape of a grasped object
(spherical object, as an example) and then maintain it after the air is
evacuated.

Here we present a new soft gripper that combines granular jamming and
electrodhesion technologies to enhance and complement the grasping
capabilities of each technology used in isolation. The soft gripper
consists of an inflatable balloon filled with granules and an integrated
electroadhesive pad with an interdigitated electrode structure
(\textbf{Figure 1a,b} ). This design enables activation of GJ and EA
independently or in combination, thus offering three operation modes. In
GJ mode, the gripper can hold and lift non-flat objects with negative
air pressure (Figure 1d). Also, in GJ mode, the gripper can release
objects by removing negative pressure or applying positive air pressure,
which produces a convex curvature of the gripper's surface (Figure 1c).
In EA mode, the gripper is capable of grasping flat and delicate objects
by applying a voltage. In combined GJ and EA mode, the gripper can grasp
and release a variety of objects with different geometry and surface
conditions (moistened, oily, porous, and powdered). Moreover, the
gripper is able to perform forced releasing of objects stuck on the
device surface (for example, sticky objects) by applying positive
pressure that make the gripper bulge outwards.

We characterize the grasping performance of the gripper for different
object sizes and analyze how grasping parameters such as bending angle,
applied force, and activation of GJ and EA influence the grasping force.
We also assess the impact of the combined mode on the grasping of
objects with different geometries and deformable objects, including flat
objects of different sizes. We characterize the gripper's performance in
picking up objects with different surface conditions determined by
moisture, oiliness, porosity, and dust. Last, we show the use of the
gripper in multi-stage grasping tasks consisting of a sequence of
different grasping and releasing operations. In one case, the gripper
opens a rigid book cover in GJ mode and then gently flip pages in EA
mode. In the other case, the gripper grasps a tea bag in EA mode, picks
up a plastic cup in GJ mode, dips the tea bag into the plastic cup in EA
mode, and finally release the used tea bag by bulging out the gripper's
surface in GJ mode.

\textbf{Results and Discussion}

\subsection*{Working principle}

{\label{working-principle}}

The working principle of the dual gripper is illustrated
in\textbf{Figure 2.} After reaching an object, one of three different
working modes can be activated (Figure 2a). Applying a high potential
difference to the interdigitated electrode structure activates the EA
gripper, causing fringing electric fields that extend from the surface
of the gripper. These fields induce electric charges on the surface of
the object (Figure 2b), resulting in the electrostatic adhesion force
between the gripper and object.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{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.\textsuperscript{{[}1,28{]}}

\subsection*{Fabrication}

{\label{fabrication}}

The fabrication procedure consists of layer-by-layer and molding
techniques, as illustrated in \textbf{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).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\textbf{Figure 3.} Fabrication process of the gripper. a)
Electroadhesive pad fabrication. A three-layer structure, which consists
of a bottom silicone layer (i), a conductive silicone layer (ii), and a
top silicone layer (iii) is blade-casted and cured in an oven one on top
of the other. The electrode pattern (ii) is engraved using
CO\textsubscript{2} ablation before the top silicone layer (iii) is
cast. b) i) The fabricated pad is placed in an empty mold. Then, liquid
silicone is poured into the mold to form the silicone bag and
simultaneously connect the bag and the pad in a monolithic structure.
ii) The silicone is solidified by thermal curing and the gripper is
removed from the mold. iii) The silicone bag is filled with coffee
granules (in brown).

\subsection*{Grasping force
characterization}

{\label{grasping-force-characterization}}

The grasping performance of the gripper in GJ mode results from three
different mechanisms: physical interlocking between the gripper and
object, static friction caused by normal stresses in the contact
regions, and suction effect due to a reduced air pressure over part of
the contact surface (the latter can only occur when the gripper
generates a vacuum seal against the
object).\textsuperscript{{[}11{]}}Static friction is the main factor
because it contributes to both interlocking and suction
modes.\textsuperscript{{[}5{]}} Thus, the characterization of the
grasping force of a GJ gripper is usually made for the static friction
mechanism, which depends on the size and shape of the manipulated
object, the contact angle, surface conditions, and applied force at the
moment of grasp.\textsuperscript{{[}5,11{]}}

\textbf{Figure 4.} Characterization of grasping performance for an
object with hemispheric shape. Each data point corresponds to the
average measurement of three grippers tested five times each (15
measurements). a) Grasping force for granular jamming and combined
(granular jamming with electroadhesion) modes as a function of object
size. Diameters of hemispheres used in these tests range from 33\% to
89\% of the gripper's active area. The gripper was pushed down and
contacted the hemisphere until it reaches 5N of applied force in all
experiments. In the combined mode, the EA pad was activated by applying
2.5 kV and GJ was triggered by the air evacuation with the pressure drop
of 84 KPa. Grasping forces increase with increasing object diameter, but
the combined mode displays 15.2\% superiority for largest object
diameter compared to granular jamming alone. b) The measured total
electroadhesion force of the gripper as a function of applied voltages
on a flat surface. The normal force scales from 0 to 0.2 N with the
voltage applied from 0 to 5 kV with a step size of 500V. c) The applied
force as a function of a grasped object size. In experiments b and c,
the gripper was pulled down to a certain absolute coordinate above all
hemispheres. The applied force increases with the growth of the object
size from 1.2 to 2.2 N. d) The contact angle as a function of a grasped
object size. The contact angle changes from 73\selectlanguage{ngerman}° to 30°.

In this study, the static friction mechanism in the combined mode is
enhanced by EA, which increases the friction forces and normal forces of
the gripper.\textsuperscript{{[}1,28{]}} Thus, to quantify the grasping
performance, we characterize the grasping force in GJ mode and in
combined mode for different manipulated object sizes (\textbf{Figure 4a}
). The gripper is connected to the tensile testing machine equipped with
a load cell to perform vertical displacement and applied force
measurements on the grasped object. The objects to be grasped are fixed
on a plate beneath the gripper and in line with its central axis. The
objects consist of a set of hemispheres printed in polylactic acid (PLA)
with the diameters from 12 to 32 mm that corresponds to 33 to 89\% of
the gripper diameter. The shape of the hemispheres does not allow
interlocking grasp. The gripper is moved down to a preprogrammed height
by the tensile machine and presses against the object (see Experimental
Section). After that, the GJ or combined mode is activated, and the
tensile machine lifts the gripper up until it releases the object. In
the combined mode, the EA pad is activated by applying 2.5 kV and GJ is
triggered by the air evacuation with a pressure drop of 84 KPa. Although
the EA pad can operate up to 5 kV (Figure 4b), the applied voltage here
is set to 2.5 kV to prevent possible breakdowns that may happen when
squeezing the EA pad during the grasping operation. The grasping force
data for three experiments is collected using the load cell of the
tensile machine.

In the GJ mode, the grasping force varies from 5.8 (standard deviation
(SD) = 0.38 N) to 10.6 N (SD = 0.62 N) for object sizes from 33 to 89\%
of the gripper diameter. Interestingly, the maximum grasping force is
achieved for the hemisphere with the size of 78\% of the gripper
diameter and then starts decreasing. In the case of combined mode, the
grasping force varies from 6.1 (SD = 0.18 N) to 12.1 N (SD = 0.83 N) for
object sizes from 33 to 89\% of the gripper diameter. In this case,
there is a continuous increase in grasping force with the size of the
hemisphere. For the hemisphere size of 89\% of the gripper diameter, the
grasping force of the combined mode is 15.2\% larger than the GJ mode's
force.

In the GJ mode, the grasping force depends not only on the size of the
object but also on the applied force with which a gripper touches the
object. This applied force determines a contact angle between the
gripper surface and the object. The contact angle and size of the object
define the contact surface i.e., amount of granules in the contact zone
between the gripper and the object. To characterize the relationship
between the applied force and the contact angle, we use the same setup
with the tensile machine, hemispheres, and an additional camera placed
in front of the gripper to capture the contact angle. The gripper is
lowered to a preprogrammed height by the tensile machine. After reaching
the hemisphere, the applied force data is collected from the load cell.

The applied force increases from 1.2 (SD = 0.03 N) to 2.2 (SD = 0.61 N)
N for the object size from 33 to 89\% of the gripper diameter (Figure
4c). The applied force grows with an increase of the object size because
larger object size results in the larger contact surface area across
which more granules can cover the target. We noticed that an increase in
the applied force correlates with a decrease in the contact angle. The
lower contact angle shows that a larger area of the hemisphere is in the
contact with the gripper causing more granules to flow towards the
surface of the hemisphere, press onto it resulting in a higher applied
force. The contact angle decreases from 73° (SD = 9.6°) to 30° (SD =
4.9°) for the object size from 33 to 89\% of the gripper diameter
(Figure 4d).

The EA mode allows grasping of flat objects, which is not possible in GJ
mode, by means of the normal force generated between the gripper and the
object when voltage is applied. The amount of normal force generated by
the interdigitated electrode structure depends on the design of
electrodes, dielectric constants of contact materials, thickness of the
dielectric layer, and effective area of contact. It also scales with the
square of the applied voltage.\textsuperscript{{[}1,29{]}} Other
physical parameters, such as surface conditions and roughness can also
influence the normal force.\textsuperscript{{[}18{]}} Thus, we performed
experimental characterization of the EA generated force (Figure 4b). The
tensile machine lowers the gripper onto the flat surface and a voltage
is applied from 0 to 5 kV with a step increment of 0.5 kV. The measured
normal force scales with the voltage as reported in the
literature.\textsuperscript{{[}1,28,30{]}} The inherent stiction between
the outer dielectric layer and the flat surface causes high standard
deviation of normal force measurements at zero voltage. Normal force
reaches 0.2 N (SD = 0.02 N) at 5kV. Normal force can also be enhanced
either by decreasing the outer dielectric layer or increasing the
density of generated fringe fields by decreasing the width of the
electrodes and the gaps between electrodes.

The grasping force of conventional GJ grippers varies up to an order of
magnitude depending on the shape of
object.\textsuperscript{{[}5{]}}Thus, we characterized the grasping
force generated by the proposed gripper in GJ mode for different object
geometries, including flat and deformable objects (\textbf{Figure 5a} ).
For these experiments, we used seven different 3D shapes with the
cross-sectional diameter of 80-90\% of the gripper's diameter, which
corresponds to maximum grasping force (Figure 4a) consisting of a
sphere, cube, pyramid, disks with 20 and 50 mm in a diameter, cylinder,
and parallelepiped. All shapes were 3D printed with the highest layer
thickness of 0.4 mm (instead of a usual 0.06 mm). The objects were fixed
at the bottom of the tensile machine, while the gripper grasped and
pulled the object up and measures the holding force. We tested three
grippers three times for each shape. We observed the highest grasping
forces of 6.1, 10.4, and 8.9 N in both modes for a sphere, cube, and
disk of 20 mm. The combined mode increases the adhesion for these
objects by 5.6\%, 3.7\%, and 0.5\%. In comparison to spheres, these
shapes allow granules to flow more easily around the target object and
envelope it completely. Yet, when the manipulated object design is less
suitable for GJ mode, the influence of EA on the resulted grasping force
is higher. We observed the highest differences (35\%, 16\%, and 15\%) in
grasping forces between granular jamming mode (0.83, 1.33, 1.35 N) and
combined mode (1.12, 1.54, 1.55 N) for a cylinder, pyramid, and
parallelepiped respectively. In the case of deformable water balloon, we
observed grasping forces of 1.8 and 2 N for the GJ and combined mode
indicating the grasping enhancement of 11\%.

\textbf{Figure 5.} Characterization of grasping forces for different
geometries and surface conditions. a) The grasping force for granular
jamming (GJ) and combined mode for different object geometries that have
the same volume: sphere, cube, pyramid, disks with diameters of 20 and
50 mm, cylinder, parallelepiped, and deformable balloon with liquid. The
flat disk with a diameter of 50 mm has a diameter larger than the
diameter of a gripper and cannot be grasped using GJ mode. The highest
grasping forces of 6.1, 10.4, and 8.9 N in both modes were observed for
a sphere, cube, and disk of 20 mm. The combined mode increases the
adhesion for these objects by 5.6\%, 3.7\%, and 0.5\%. These shapes
allow granules to flow more easily around the target object and grasp it
from all sides. The highest differences of 35\%, 16\%, and 15\% in
grasping forces between GJ mode 0.83, 1.33, 1.35 N and combined mode
1.12, 1.54, 1.55 N are observed for a cylinder, pyramid, and
parallelepiped respectively. The combined mode shows worse performance
only in the case of a disk of 50 mm (flat object). The activation of GJ
in a combined mode makes the bottom surface involved in grasping rigid,
thus, less compliant. b) Grasping forces of a flat object as a function
of four grasping modes. Electroadhesion mode (EA) shows the highest
grasping force of 0.8 N compared to combined, granular jamming, and
nature stickiness forces of 0.15, 0.27, and 0.45 N resulting in the
grasping enhancement of 430\%, 194\%, and 78\% respectively. c) The
grasping force for a combined mode for different surface conditions: dry
(grey), oily (yellow), porous (pink), moistened (red), and powdered
(brown). The hemisphere with the dimeter of 28 mm, which corresponds to
78\% of the gripper diameter was used. The same setup and testing
protocol as for Figure 4a was used. The grasping of objects with dry,
moistened, and oily surfaces resulted in four to five times higher
grasping force compared to porous and powdered surfaces due to suction
effect formation.

We also characterized the grasping force for each of the objects when
GJ, EA, and the combined mode are turned off, and the object is lifted
only due to the inherent stickiness of silicone (Figure S1, Supporting
Information). Interestingly, for a flat object larger than the gripper
size, we observed that two highest forces of 0.42 and 0.80 N are
achieved in the soft state of the gripper using only the stickiness of
silicone and EA mode, respectively (Figure 5b). In these modes, the
gripper is in the soft state and is more compliant than GJ mode,
resulting in a better contact between the gripper and the object that
leads to higher grasping performance. The modes that includes GJ, thus,
stiff state of the gripper, result in the maximal grasping force of 0.27
N, which is three times lower than EA. The activation of GJ makes the
gripper rigid and the external contact surface of the gripper becomes
rough (Figure S2, Supporting Information). The higher surface roughness
in a rigid state makes the contact area between the device and the
manipulated object lower than in a soft state.\textsuperscript{{[}18{]}}

Contrary to EA-based grippers, GJ-based grippers can grasp objects that
are powdered, moistened, and porous.\textsuperscript{{[}11{]}} In this
study, we chose different gripper design and material composition from
already existing devices.\textsuperscript{{[}31{]}} Thus, we performed
characterization of the grasping force at different surface conditions
in the combined mode (Figure 5c) as this mode shows the highest gasping
force among all modes (Figure 4a). We used the hemisphere with a dimeter
of 28 mm, which corresponds to 78\% of the gripper diameter, following
the same setup and testing protocol as for Figure 4a. We measure the
grasping force for the gripper in combined mode under different surface
conditions of the object: dry, oily, porous, moistened, and powdered.
Porous and powdered surfaces resulted in the lowest grasping forces (1.1
N and 1.9 N, respectively). Oily, moistened, and dry objects instead
yielded substantially higher forces (14.8 N, 12.9 N, 12.6 N,
respectively). The porous and powdered surfaces prevent the formation of
sealed interface between the gripper and object surfaces, eliminating
the suction effect. In the case of oily, moistened, and dry objects, the
silicone surface of the gripper fastens tightly around the object
causing a suction effect, which magnifies the grasping force. These
results are in line with the performance of a conventional GJ gripper
measured with similar surface conditions.\textsuperscript{{[}11{]}}These
results indicate that the outer surface of the gripper, which is three
times thicker than conventional GJ grippers due to the presence of the
EA electrodes, does not limit grasping capabilities for different
surface conditions.

\textbf{Grasping and manipulation of fragile, deformable, and flat
objects.}

GJ is an effective method for grasping solid objects with different
shapes.\textsuperscript{{[}5,11,12{]}} However, the grasping of
deformable, and flat objects can be challenging and is never discussed
in the literature for GJ-based grippers. Here we show that the combined
use of GJ and EA enables successful pick and place of such objects
(\textbf{Figure 6} and Video S1, Supporting Information). In a first
example, we pick up two fragile quail eggs with diameters of 27 mm and a
weight of 17 grams each. The gripper in the soft state approaches the
eggs until the granules in the gripper flow around both eggs\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{Figure 6.} Working principle and grasping demonstration. a)
Grasping is performed using a motorized linear stage, which raised and
lowered the gripper. The gripper acts as an end effector (see Video S1,
Supporting Information). (i) In the initial state, a soft silicone bag
with granules is soft. (ii) When the gripper touches the object, the
silicone bag deforms. After that, the pump and voltage are turned on.
The pump evacuates the air out of the bag triggering the granular
jamming (GJ) and making it stiff to hold the object. The applied voltage
(3 kV) triggers the generation of electroadhesion (EA) force on the
bottom surface of the gripper. (iii) The objects (two quail eggs with
the weight of 34 grams) are then picked up due to the grasping force
provided by the GJ and EA while the linear stage lifts up. Release of
the objects is achieved by a reverse of the operating procedure
explained above. b) With the same operating principle, a grasping
demonstration is performed for (i) a highly deformable water-filled
balloon (31 grams), (ii) a flat and delicate fabric (0.5 grams), and
(iii) a bottle with water (507 grams). c) The high compliance of the
gripper allows grasping objects from different sides broadening
potential grasping scenarios. The fork (32 grams) can be manipulated
from three different sides (i-iii).

(Figure 6a-I,ii); subsequently, the simultaneous activation of GJ and EA
modes allow the gripper to grasp the eggs (Figure 6a-ii) and lift them
up (Figure 6a-iii). In a second example, we pick up a highly deformable
water balloon with a size larger than the size of the gripper, a mass of
31 grams, and a membrane thickness less than 30 \selectlanguage{greek}μ\selectlanguage{english}m (Figure 6b-i). In a
third example, we pick up a flat tissue with the weight of 0.5 g and
thickness less than 60 \selectlanguage{greek}μ\selectlanguage{english}m, without deforming it (Figure 6b-ii). After
lifting the tissue, the gripper can release it by applying positive
pressure bulging out its surface (Video S1, Supporting Information). In
a fourth example, we lift a bottle of water with a mass of 507 grams,
which is 38 times heavier than the gripper (Figure 6b-iii). In addition,
the high deformability of the gripper allows it to grasp objects from
different sides broadening potential grasping scenarios. In a fifth
example, we pick up a fork of 32 grams from three different sides
(Figure 6c-i,ii,iii).

The ability to selectively operate in different modes allows the gripper
to be used for multi-stage complex operations, such as opening and
flipping through the pages of a book and making a cup of tea
(\textbf{Figure 7} and Video S2 and S3, Supporting Information). In the
first example, the book hardcover weighs \textasciitilde{}300 g
(\textasciitilde{}0.3 N) and requires GJ mode to open the book (Figure
7a-i); once the book is open, the pages can be gently flipped over in EA
mode (Figure 7a-ii). In the second the EA mode can be used to grasp
(Figure 7b-i,ii) and lift (Figure 7b-iii) a tea bag with a mass of 1.8
g. The GJ mode on the second gripper is used to grasp (Figure 7c-i,ii),
lift (Figure 7c-iii) and move (Figure 7c-iv) the plastic glass with a
mass of 8 g for the tea. Then, the tea bag is moved up and down (Figure
7d-i,ii) and changes the weight from 2 to 7.7 grams as it absorbs water.
The tea bag can be rapidly released by turning off EA and bulging out
the gripper's surface by applying positive pressure (Figure 7d-iii).

\textbf{Figure 7.} Demonstration of complex manipulation scenarios. Each
black rectangular area corresponds to the use of a specific grasping
mode: granular jamming (GJ) or electroadhesion (EA). a) The gripper can
be used for complex manipulation such as an interaction with a book. (i)
The cover page can be opened by GJ mode. (ii) Then, the pages can be
gently turned over using EA mode. b) Another example of complex
manipulation is tea making. (i) The EA mode can be used to grasp (i,ii)
and lift (iii) the teabag (1.8 grams). c) The GJ mode of the second
gripper is used to grasp (i,ii), lift (iii) and move (iv) the plastic
glass (8 grams) for the tea. (d) (i,ii) The teabag is moved up and down
using linear stage. It changes the weight from 2 to 7.7 grams when
sopping. (iii) The tea bag can be rapidly released by turning off EA and
applying positive pressure to the gripper to change the curvature of the
grasping surface and detach the object. The gripper can grasp objects
with different surface conditions such as dry, oily porous, moistened,
and powdered. The grasping force for dry, oily, and moistened objects
are in four to five times higher compared to the objects with porous and
powdered surfaces due to the formation of a tightly sealed interface
between the gripper and the object surfaces, which leads to the suction
effect.

\textbf{Conclusion}

We showed that the combination of controlled stiffness by granular
jamming (GJ) and controlled adhesion by electroadhesion (EA) can lead to
versatile grippers capable of manipulating a broader range of objects
than currently existing GJ-based or EA-based
grippers.\textsuperscript{{[}5,11,12,17{]}} The gripper is fabricated
out of commercially available and low-cost components such carbon
black-filled elastomer composite and silicone elastomer using common
fabrication techniques such as film-casting, laser ablation, and
molding. The GJ mode allows lifting objects with a weight 38 times
larger than the weight of the gripper. In addition, the application of
positive pressure bulges out the gripper's surface to facilitate object
release. The EA mode enables manipulation of flat and delicate objects,
which would not be possible with GJ alone, with a weight of 20-80 grams.
The grasping force in the combined mode, when GJ and EA are activated
simultaneously, is higher for all the diameters of manipulated objects.
The difference between combined mode of GJ \& EA versus the pure GJ mode
increases with the object sizes and achieves the maximum difference of
15.2\% at the object size of 89\% of the gripper's diameter. The
generated grasping force correlates with the applied force before the
gripper activation and inversely correlates with the grasping angle in
the same manner shown in the literatures on conventional GJ
grippers.\textsuperscript{{[}5,11{]}} The gripper is capable of
manipulating objects of various shape. The combined mode shows the
highest grasping force enhancement of 35\%, 16\%, and 15\% compared to
GJ for a cylinder, pyramid, and parallelepiped shapes. The combined mode
is especially beneficial with the shapes that do not allow granules to
flow easily around the target object and grasp it from all sides.

The separate use of an EA, GJ, or combined modes makes the gripper
suitable for complex manipulation scenarios that require manipulations
of multiple objects with different sizes, shapes, softness, and surface
conditions. This gripper can be beneficial for agricultural applications
where picking fresh vegetables and fruits without damaging them remains
challenging.\textsuperscript{{[}32{]}} Moreover, the gripper can be used
to address current challenges in industry such as the development of
adaptive industrial assembly robots capable of manipulating objects in
different ways such as twisting and re-grasping with a large variety of
different geometries.\textsuperscript{{[}33,34{]}} The fabrication
process of the gripper combines molding and layer by layer fabrication
techniques that are both scalable. Thus, the proposed fabrication
approach can easily be adapted to produce grippers with various
dimensions suitable for the aforementioned applications.

\textbf{Experimental Section}

\emph{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.\textsuperscript{{[}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 \selectlanguage{greek}μ\selectlanguage{english}m and a drawing speed of 5 mm/sec. The elastomer layer was then
cured in an oven for 30 min at 80 \selectlanguage{ngerman}°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 \selectlanguage{greek}μ\selectlanguage{english}m and a drawing speed of 15 mm/sec.
The multilayer sample was then placed place in the oven for 60 min at 80
\selectlanguage{ngerman}°C to form a thickness of conductive layer around 20 \selectlanguage{greek}μ\selectlanguage{english}m. 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 \selectlanguage{greek}μ\selectlanguage{english}m 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,\textsuperscript{{[}5,11{]}} with the diameter of 0.2 mm were
poured into already fabricated silicone bag.

\emph{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.\textsuperscript{{[}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.

\textbf{Supporting Information}

Supporting Information is available from the Wiley Online Library or
from the author.

Acknowledgments

This work was supported by the SNSF Bridge project 20B2-1 180861 and
JSPS KAKENHI Grant-in-Aid for Scientific Research on Innovative Areas
``Science of Soft Robot'' project (grant number 21H00324).

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\textbf{Table of Contents}

\textbf{A soft gripper with granular jamming and electroadhesive
properties}

\emph{Yegor Piskarev, Antoine Devincenti, Vivek Ramachandran,}

\emph{Pierre-Etienne Bourban, Michael D. Dickey, Jun Shintake, and Dario
Floreano}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}

We report here a soft gripper, which combines granular jamming (GJ) and
electroadhesion (EA) as controlled stiffness and adhesion, respectively.
The gripper can operate in GJ, EA, or combined mode that simultaneously
activates GJ and EA. The gripper is suitable for complex manipulation
scenarios that require grasping of objects with different sizes, shapes,
mechanical properties, and surface conditions.

\textbf{Keywords:} Soft robotics, soft grippers, universal grippers,
granular jamming, electroadhesion.

\textbf{Supporting Information}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image9/image9}

\end{center}
\end{figure}

\textbf{S1.} Stickiness and electroadhesion for all geometries. The
grasping forces in electroadhesion mode and using the nature stickiness
of silicone are significantly higher in the case of a flat object, which
fully covers the active area of the gripper.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image10/image10}

\end{center}
\end{figure}

\textbf{S2.} Surface of the gripper's active area. a) In the soft state,
the gripper's surface is smooth. b) After the activation of GJ triggered
by air evacuation, all the granules stuck together causing the
asperities formation on the surface of the gripper in the rigid state.

\selectlanguage{english}
\FloatBarrier
\end{document}

