Authors:
Jiayi Jin, Zhizhen Wei
1. Summary
This project aims to develop a fused deposition modeling (FDM)-based 3D printing approach for fabricating fish-inspired caudal fin models with spatially varying local stiffness. During the manufacturing process, by implementing fine tuning the ratio of flexible-rigid material (e.g. TPU-PLA) proportion and tailoring the geometry design, the natural non-uniform stiffness found in biological fins is properly mimicked to improve swimming performance of the robot.
2. Background
The ability of fish to modulate body and fin stiffness is a key factor in swimming efficiency and agility. Numerous studies have proven that non-uniform stiffness distribution, particularly in caudal fins, plays a vital role in enhancing thrust, speed, and energy efficiency. However, in-vivo stiffness variation study in fish is experiment-wise challenging, due to limited control over live fish and lack of measurement techniques.
To overcome these barriers, physical models and robotic analogs are used to replicate and investigate stiffness effects. Additive manufacturing (AM) offers a powerful alternative for creating more biomimetic and customizable stiffness profiles.
This project seeks to leverage AM—specifically in-place dual-material FDM printing—to fabricate caudal fins with controllable and spatially tunable stiffness by adjusting the layout and ratio of TPU (flexible) and PLA (rigid) materials. Unlike traditional multi-step assembly, this method allows continuous geometry with functional gradation, better resembling biological fin structure and material properties.
3. Impact
If successful, this project will enable more realistic and efficient robotic fin designs, with implications for underwater vehicle propulsion, soft robotics, and bioinspired engineering. This method allows for precise control of spatial stiffness variation within a single print—something impossible for conventional manufacturing techniques.
In the broader field of biomimetic robotics and marine bioengineering, our work presents a fabrication strategy that supports the experimental investigation of how different stiffness distributions would affect locomotion. By producing fins with graded stiffness more efficiently and with high reproducibility, this approach could be applied to design optimal stiffness profiles for any specific swimming mode.
4. Methods/Approach
We propose an in-place dual-material FDM 3D printing approach to fabricate a fish caudal fin model with non-uniform stiffness. The primary materials are selected as TPU and PLA for flexible and rigid sections, respectively. Stiffness tuning is achieved through adjusting the geometry and printing parameters of each material component.
Steps:
- CAD Modeling: A segmented caudal fin geometry with tunable zones will be designed in CAD software.
- Multi-Material Slicing: Use slicing software that allows precise control over assigning multiple materials to distinct regions.
- Printing Trials: Calibrate nozzle temperature, bonding conditions, and print speed to ensure adhesion between PLA and TPU.
Novelty:
Unlike most robotic fin designs that use discrete elements or homogeneous materials, our approach allows for continuous and customizable stiffness profiles within a single printed component.
Advantages:
- Eliminates assembly steps.
- Improves biomimetic fidelity.
- Enables rapid iteration of stiffness profiles for design exploration.
Risks:
- Material bonding: PLA and TPU may not adhere reliably. Mitigation: Experiment with thermal and surface treatments, optimize overlap geometries.
- Stiffness control precision: Actual mechanical properties may differ from simulation. Mitigation: Empirical calibration and mechanical testing after each design iteration.
5. Future Plan
Upon successful fabrication of a functionally graded fin, future work will include:
- Hydrodynamic Testing: Mount the fin to a robotic fish and evaluate thrust, speed, and energy efficiency.
- Dynamic Model: Use finite element analysis (FEA) or analytical models to refine and predict performance before fabrication.
- Expanded Materials and Actuation: Incorporate smart materials for actively tunable stiffness and test responsive fins under varying flow conditions.
Ultimately, this framework could serve as a design tool for optimizing flexible robotic components beyond caudal fins, such as flippers, wings, or tentacles.