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.

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.