Predicting Biodegradable Soft Actuator Performance Under Degradation
Biodegradability offers a compelling pathway toward environmentally responsible robotics, particularly in soft robotic systems designed for deployment in natural environments. The inherent compliance of soft robots provides mechanical robustness, safety in human interaction, and adaptability to unstructured surroundings. Yet, when such robots are used in field operations—such as search and rescue or environmental exploration—loss or damage can leave behind waste. Employing biodegradable materials ensures that these devices can return harmlessly to the soil, aligning with sustainability goals and international initiatives.
Researchers have advanced this concept by developing pneumatic soft actuators from a gelatin–glycerol mixture, a biodegradable material with natural origins. This choice allowed accelerated degradation experiments while retaining mechanical properties suitable for actuation. Notably, the mechanical characteristics of biodegradable materials evolve during microbial degradation, altering actuator performance over time. Outputs may vary even under identical inputs, making it essential to incorporate these property changes into design and control strategies.
The study established a framework to characterize mechanical property changes as a function of degradation rate and to predict actuation behavior through finite element simulations. Gelatin and glycerol were mixed in a 2:1:8 mass ratio with distilled water, processed at elevated temperature, poured into molds, and cured into thin films. These films were subjected to controlled biodegradation in soil maintained at 28°C, an environment conducive to aerobic microbial activity. After set intervals, samples were sterilized, dried, cleaned, and stored under regulated humidity before tensile testing.
Mechanical characterization followed standardized dumbbell-shaped specimens, with thickness measured by laser displacement sensors. Tensile tests yielded stress–strain curves from which Young’s modulus, elongation at break, and Yeoh hyperelastic model constants (C1, C2, C3) were extracted. Results showed a marked increase in stiffness and a decrease in elongation as degradation progressed—at 25% degradation, modulus rose to 5.5 times its initial value, while elongation dropped to one-quarter. These changes were attributed to microbial cleavage of gelatin polymer chains, collapsing the network structure and reducing flexibility.
Determining degradation rate required a robust metric. Direct mass loss measurements were unreliable due to soil adhesion. Instead, the team quantified changes in moisture content, drawing on prior observations that biodegradation reduces water retention capacity. A linear correlation between moisture loss and mass loss was validated using bromelain enzyme degradation tests, confirming moisture content as a suitable proxy.
Material property data were implemented in ABAQUS finite element simulations of a pneumatic bending actuator. The actuator geometry was modeled in SOLIDWORKS, with Yeoh model constants parameterized by degradation rate. Scaling adjustments accounted for volume changes during curing and conditioning. The physical actuator was fabricated in a 3D-printed mold, cured, and fitted with a paper strain-limiting layer to ensure bending under pressurization.
Experimental validation involved pressurizing actuators to 240 kPa in 20 kPa increments, capturing deformation with high-resolution imaging, and measuring curvature via image analysis software. Tests covered degradation rates from 0% to 25%. Both simulations and experiments revealed that actuation curvature increased with pressure but decreased with higher degradation rates. At the highest degradation, reduced elongation led to premature rupture in some samples.
Agreement between simulated and physical results was strong, with R² values up to 0.997. Minor discrepancies were linked to unavoidable variations in actual degradation rates, possible inhomogeneities in microbial activity, and subtle humidity fluctuations affecting compliance. The validated model demonstrated that actuator performance under biodegradation can be reliably predicted, enabling informed design and control.
This framework extends beyond gelatin–glycerol systems. By characterizing material property evolution under realistic environmental conditions and integrating those functions into simulation, engineers can tailor biodegradable actuators for specific missions and lifespans. Future work will incorporate additional mechanical parameters—such as viscoelasticity and multiaxial failure limits—and explore diverse environmental factors, degradation methods, and biodegradable mechatronic components. Such advances promise to enhance the functionality and sustainability of soft robotic systems, contributing to the emerging field of green robotics.
