Abstract
Biodegradable materials decompose and return to nature. This
functionality can be applied to derive robotic systems that are
environmentally friendly. This study presents a fully biodegradable soft
actuator, which is one of the key elements in “green” soft robotics.
The working of the actuator is based on an electrohydraulic principle,
which is similar to that of hydraulically amplified self-healing
electrostatic actuators. The actuator developed in this study consists
of a dielectric film made of polylactic acid (PLA) and polybutylene
adipate-co-terephthalate (PBAT), with soybean oil as the dielectric
liquid and electrodes made from a mixture of gelatin, glycerol, and
sodium chloride (NaCl). The synthesized biodegradable electrode material
exhibits a Young’s modulus of 0.06 MPa and resistivity of 258 Ω·m when
the mass fraction of NaCl relative to the amount of gelatin and glycerol
is 10 wt%. The softness and conductivity of the electrode material
results in actuation strain values of 3.4% (at 1 kV, corresponding to
1.2 kV/mm) and 18.6% (at 10 kV corresponding to 9.6 kV/mm) for the
linear-type and circular-type actuator, respectively. These values
obtained for the biodegradable electrohydraulic soft actuators are
comparable to those of non-biodegradable actuators of the same type,
representing the successful implementation of the concept.
1. Introduction
Soft robotics has a high potential owing to the high compliance from
which a wide variety of functional robots and applications can be
derived.[1–8] Synthetic polymers such as silicone
rubbers are the most widespread materials used in soft robotics. They
are low cost,[9] easy to
handle,[10] and compatible with various
fabrication methods, such as casting, molding, and
printing.[11] Synthetic polymers are also
chemically stable, making them suitable for soft robots operated in
diverse situations and environments, such as on the
ground,[12] underwater,[13]in snowstorms,[14] and even in radiation
environments.[15] On the contrary, their stable
nature and irreversible synthetic process like
thermoset[16,17] make them non-biodegradable,
which may lead to environmental destruction; this can particularly occur
when the robots performing tasks in natural fields are discarded as the
result of malfunctions or accidents. In addition, polymeric materials
used in soft robotics are mostly difficult to recycle and have a high
environmental impact. Considering these perspectives, it is important to
incorporate biodegradability into soft robots.
Researchers have demonstrated biodegradable soft robotic elements that
are focused on actuators. Their working principle includes pneumatic
actuation,[18–24]piezoelectricity,[25] ion
migration,[26–30] and
swelling.[31,32] Pneumatic actuators are
relatively easy to fabricate and can provide large outputs; however,
their performance is dependent on bulky external pumps and compressors,
which can lead to difficulty in constructing robots according to their
types and specifications. From a system perspective, actuators based on
piezoelectricity and ion migration have been driven electrically using a
portable power source. However, actuation strain generated by
piezoelectricity tends to be small (4%[33]) and
the actuation speed achieved with ion migration is normally low
(2.3%/s[34]), thus limiting the actuation
performance. Similarly, actuation based on swelling has a limitation on
speed (over 6 h required for achieving a fully swelled
state[31]) and controllability of actuated
deformation because its working principle requires material
injection[31] and cannot perform multiple
actuations[32].
In recent years, electrohydraulic
soft actuators, also known as hydraulically amplified self-healing
electrostatic (HASEL) actuators, are emerging.[35]This type of actuators consists of a pair of opposing electrodes
covering a portion of the surface of a flexible pouch encapsulating a
dielectric liquid. When a high voltage is applied, electrostatic forces
between the electrodes squeeze the pouch, causing the local position of
the liquid to change, resulting in a hydraulic deformation of the entire
structure as actuation. Electrohydraulic soft actuators exhibit large
actuation strain (107% linear strain[36]) and
force (actuation stress of ~114
kPa[36]), high power density (358
W/kg[36]), and high speed (strain rate of
900%/s[37]). Their structure is simple, allowing
to tailor them in various shapes.
In this paper, we present a biodegradable soft actuator based on the
electrohydraulic principle. This type of actuation principle requires
compliant and conductive electrodes. First, we investigated the
mechanical and electrical properties of the electrode for different
compositions. Then, we fabricated and characterized two types of
actuators that have linear and circular shapes to study the effect of
incorporating biodegradable materials into the existing actuation
principle and to validate our hypothesis.