By combining rigid scales with a flexible silicone substrate, the BIS significantly improved energy absorption while maintaining high swimming performance. Experiments showed it retained 87 % of the speed of an unprotected robot and incurred only minimal extra drag, offering a practical solution for reliable operation in demanding aquatic environments.
Background
Soft robotics excel in underwater tasks due to their compliance and adaptability. However, their inherent softness makes them vulnerable to mechanical damage, limiting their use in harsh environments.
Existing research has primarily focused on armor for flat or static applications, designs that are unsuitable for soft robots, which require complex, curved geometries and dynamic movement.
The study addressed this gap by developing a rigid-flexible skin-scale system specifically for soft robotic fish. The integrated protective scales on curved surfaces did not sacrifice swimming performance, and successfully balanced the often conflicting needs of durability and flexibility for mobile underwater robots.
Materials, Fabrication, and Performance Evaluation
This BIS system was developed through computational models, advanced material fabrication, and multi-faceted testing.
Using a MATLAB model, the system was optimized for the best placement of the standardized, rigid polypropylene scales onto a three-dimensional (3D) curved surface, using parameters like overlap ratio and local curvature to ensure uniform coverage without gaps.
A multi-stage molding process was used to form the scales. A low-melting-point wax mold held laser-cut scales in place with precision. Ecoflex silicone was poured over the molds, forming mechanical interlocks with the scales. Once the wax was melted the integrated structure was cleanly released without damage.
The BIS system was assessed thoroughly via a multitude of tests. Static compression tests quantified its energy absorption capacity. Its dynamic performance was evaluated on a custom, tendon-driven robotic fish, measuring the tail-beat amplitude and swimming speed in underwater experiments.
Computational fluid dynamics (CFD) simulations analyzed the steady-state flow and drag forces around the BIS at various low speeds. This approach allowed for a comprehensive evaluation of the system's protective enhancement, its impact on locomotive performance, and its hydrodynamic properties.
Evaluation of BIS Durability and Mobility
The BIS successfully balanced mechanical protection and functional performance. Static compression tests revealed that the BIS significantly enhanced the robot's durability, absorbing at least 130 % and up to 241 % more energy than the purely flexible structure (PFS) across five different elastomer materials, proving its superior resistance to damage.
Importantly, this protection did not come at a high cost to mobility. In underwater swimming tests, the robotic fish equipped with the BIS achieved a peak speed of 12.8 cm/s, retaining 87 % of the speed of the unprotected PFS robot at a tail beat frequency of 3 Hz.
The amplitude of its tail undulation also showed a minimal deviation of less than 10 % in water, confirming that the scales did not severely restrict movement.
Furthermore, CFD simulations confirmed that the BIS did not create a significant hydrodynamic penalty. The total drag on the BIS was nearly identical to the smooth PFS at low speeds, as a slight increase in pressure drag was offset by a reduction in viscous drag.
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Enhancing Durability without Sacrificing Mobility
This study successfully demonstrated a BIS that enhances the durability of soft robotic fish without critically compromising their movement. The design, which integrates rigid scales with a flexible silicone substrate, resulted in a 3.6-fold increase in load-bearing capacity compared to a purely flexible structure.
This protective benefit came with only a minor impact on performance. The robot retained 87 % of its original swimming speed, and the tail's undulation amplitude showed minimal deviation, particularly in water at lower frequencies.
Fluid dynamics simulations further confirmed that the BIS did not significantly increase overall drag. While the BIS exhibited slightly higher pressure drag, this was nearly balanced by lower viscous drag, resulting in an overall drag difference of less than 1 %, which is within the expected range of numerical error for the CFD method used.
These combined results validated the BIS, presenting it as an effective solution for adding robust protection to soft robotic fish without substantially compromising their swimming agility or efficiency.
Conclusion
This study tackled a major challenge in soft robotics by protecting robots against vulnerability to damage. Integrating rigid scales onto a flexible silicone substrate for robotic fish, the system provides exceptional protection, absorbing up to 241 % more energy than unprotected robots. This durability does not hinder performance.
The BIS-equipped robot retains 87 % of its original swimming speed and experiences minimal extra drag. Performance varies slightly by tail beat frequency but remains high across the range tested. This innovation successfully balances the often conflicting needs of robustness and agility, offering a practical solution for the use of soft robots in demanding underwater environments.
Journal Reference
Wang, J., Li, Y., Tian, S., Zhao, Y., Ren, G., Xi, F., & Zhao, Y. (2025). Bio-inspired rigid-flexible coupled skin-scales system for enhanced protection in soft robotic fish. Npj Robotics, 3(1). DOI:10.1038/s44182-025-00052-1
https://www.nature.com/articles/s44182-025-00052-1
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