Biodegradable electrohydraulic actuators for sustainable soft robots

Combating environmental pollution demands a focus on sustainability, in particular from rapidly advancing technologies that are poised to be ubiquitous in modern societies. Among these, soft robotics promises to replace conventional rigid machines for applications requiring adaptability and dexterity. For key components of soft robots, such as soft actuators, it is thus important to explore sustainable options like bioderived and biodegradable materials. We introduce systematically determined compatible materials systems for the creation of fully biodegradable, high-performance electrohydraulic soft actuators, based on various biodegradable polymer films, ester-based liquid dielectric, and NaCl-infused gelatin hydrogel. We demonstrate that these biodegradable actuators reliably operate up to high electric fields of 200 V/μm, show performance comparable to nonbiodegradable counterparts, and survive more than 100,000 actuation cycles. Furthermore, we build a robotic gripper based on biodegradable soft actuators that is readily compatible with commercial robot arms, encouraging wider use of biodegradable materials systems in soft robotics.


Fig. S2. Schematic of the biodegradation setup.
Dumbbell-shaped BioPolyester samples (same geometry as for tensile tests) were exposed to water-saturated compost soil at a fixed temperature of 58 o C in a sealed chamber exposed to humidified air.

Fig. S3. BioPolyester film in (A) virgin conditions and (B) after submersion in aerated de-ionized water for 50 days at 58 o C.
There is no visible difference between the two samples, confirming that films require industrial composting conditions under the presence of microorganisms and enzymes to rapidly biodegrade.

Fig. S4. Uniaxial tensile tests of biodegradable polymer films.
Dumbbell-shaped polymer film samples were laser cut in two orthogonal directions, labeled as "1" and "2". (Directionality of fabrication was not provided by the manufacturer).  The dielectric constants of biodegradable films for calculations of Maxwell stress were taken at a frequency of 1 Hz. BOPP was measured as the reference non-biodegradable film.  (D) Height differences of film surfaces visualized using data from profilometry. Data was extracted parallel to y-axis from the plots of (C).

Fig. S9. Fabrication of NaCl-infused gelatin hydrogel electrodes.
Polymer films were taped to a flat surface. An electrode geometry was laser-cut onto a 225-µm thick mask made from screen protector plastic, which was pressed onto the flattened polymer film. Pre-heated gelatin was then cast directly onto the mask. Excess was quickly scraped away with a flat-edged scraper and the mask was promptly removed, allowing the resulting electrode to cure overnight before it was used for testing.

Fig. S10. Relative weight of gelatin hydrogel electrodes at different ambient conditions.
Data shows typical weight change, normalized by the initial weight, of gelatin hydrogel stored at 23°C and 20%, 40% and 60% relative humidity over a period of 8 hours.  Hydrogel is cast into the mold on top of the films and covered with another layer of PET. All mold parts except for base 2 were removed before testing. (B) The peel test setup consists of a movable platform that adjusts according to the travel distance of the clamp. The sample is secured on top of the movable platform and the hydrogel material is put in the clamps in a 90° angle with respect to the biodegradable film. (C) Debond energy is calculated from the peeling force of gelatin from biodegradable film samples, for conditions where gelatin is immediately covered with PET after casting or exposed for 24 hours to the ambient environment before PET application. (D) Adhesion forces that correspond to data from (B).

Fig. S13. Fabrication of single-pouch Peano-HASEL actuators.
Two polymer films are heat sealed into a pouch geometry with an opening, using a soldering tip mounted to a pre-programmed CNC bed. Speed and temperature conditions are set for the specific film. Once sealed, gelatin electrodes are applied as depicted in Fig. S9 onto both sides of the actuator, with a sheet of electrode-shaped film laid over the hydrogel as a protective layer. The following day, the pouch is filled with FR3 by inserting a syringe into the pouch opening, which is later sealed off with a soldering iron to complete the fabrication process.

Fig. S14. Geometry of Peano-HASEL actuators. (A)
Peano-HASEL single pouch actuators were heat sealed with 60-mm widths and 20-mm heights. Gelatin hydrogel electrodes had 58-mm widths and 9-mm heights. Ruffles on the bottom half of the heat seal pattern relieved the actuated region of mechanical constraints, which has shown to improve performance in past work. (B) Laser-cut 1.5-mm acrylic mounts were mounted 2 mm from the edge of the top and bottom of actuator pouches and secured with screws for actuator characterizations. Hanging masses were loaded onto mounts, which served to distribute load evenly across actuators. A backing was incorporated onto the top mount for securely attaching wires to electrode leads, to avoid ripping hydrogels from the actuator. For Fig. 4, we demonstrated the use of wooden mounts and dowels in substitute of screws.   Pincers were subject to a voltage signal that ramped up to maximum voltage in 100 ms and held the maximum voltage for 4 seconds before ramping back down and repeating twice more under alternating polarity voltage conditions. Each data point is the result of averaging the force exerted during the three voltage cycles. See Fig. S17 and Movie S4 for visualizing the test setup. Table S1. Standardized measurements of dielectric strengths, dielectric constants, and mechanical properties of four biodegradable films. BOPP serves as a reference non-biodegradable film. "n" denotes the number of independently measured samples. Table S2. Environment-dependent dielectric strength of BOPLA and BioPolyester films. "n" denotes the number of independently measured samples. † = shape parameter, * = characteristic breakdown strength of the material, taken as the breakdown field where 63% of polymer samples experienced failure.
Movie S1. Robustness of adhesion between gelatin hydrogel and biodegradable polymer films under repetitive bending stress.

Movie S3.
Demonstration of a biodegradable SES gripper.

Movie S4.
Setup for measuring the normal force exerted by pincer tips comprising biodegradable SES grippers.