R&D PULSE
Cephalopod-Inspired Jet-Propulsion Robot
Researchers have created a fully soft jet propulsion unit actuated by coiled nickel-titanium shape memory alloys.
Written by Mark Crawford
Soft robotics research is focused on making robots that adapt to complex, unstructured environments—such as deep-water exploration—using flexible, deformable materials. A related challenge is developing advanced actuation technologies that enhance configurability and the safe interaction with humans and the environment.
Traditional actuators such as electric motors, pneumatics, and hydraulics are often bulky, noisy, and incompatible with compact, silent operation in underwater settings. In response, smart materials have emerged as promising alternatives, including shape memory alloys (SMAs).
SMAs—especially when coiled, exhibit high strain, compact integration, and improved thermal performance—are ideal for marine actuation.
The researchers proposed several soft jet designs. Image: Kitone, et al.
To take a deeper dive into SMA-driven jet propulsion, Angel Kitone, a graduate mechanical engineering student at University of Michigan, Flint, oversaw the design, fabrication, and evaluation of a fully soft jet propulsion unit actuated by coiled NiTi SMAs, activated through Joule heating.
“Our propulsion system is designed to replicate the biological locomotion of cephalopod species,” Kitone said. “We investigated the effects of key design parameters such as the location and number of embedded SMA actuators, input power, and actuation frequency on the swimming performance of the soft robotic system.”
The ultimate goal, she noted, is making a scalable, fully soft jet propulsion system powered by embedded coiled SMAs, eliminating all rigid actuators and support structures.
An experimental vortex visualization of jet unit with multiple SMA configurations. Video: University of Michigan, Flint
Building the Robot
The first research step assessed actuation strategies by comparing radial and linear SMA configurations. Both used thermal contraction to drive jet propulsion and passive relaxation for energy recovery. The radial layout produced more efficient thrust by better leveraging symmetric deformation and resonance effects.
The soft jet propulsion unit was designed to be compact, modular, and fully compliant to facilitate underwater navigation, with minimal mechanical complexity. “The body was cast from soft silicone using a 3D-printed mold designed to ensure uniform wall thickness and accurate placement of embedded SMA coils within an ellipsoidal geometry,” Kitone said. “During actuation, an electric current applied to the SMA through thin copper wires caused them to contract and locally deform the soft body.”
The fabrication process was carefully designed to preserve geometric accuracy and support repeatable integration of embedded actuators. A wax core was first molded to form the internal cavity of the jet body. The geometry of the wax was defined using a 3D-printed mold, which matched the internal dimensions of the final silicone body.
Coiled SMA actuators were selected for their high strain output, compact coiling geometry, and favorable convective behavior in underwater environments. Kitone noted the SMA-mounted wax core was then placed inside the main silicone mold assembly. Platinum-cured EcoFlex 00-10, known for its high elasticity, was selected as the structural material due to its excellent balance between flexibility and mechanical durability.
All experiments were conducted in a transparent rectangular water tank.
“Each jet propulsion unit was actuated using coiled SMAs connected to a programmable power supply via thin insulated copper wires,” Kitone said. A digital camera recorded the robot’s swimming motion and deformation during actuation.
Three experimental protocols evaluated different performance characteristics of the soft jet propulsion unit: a vortex visualization test, a free swimming performance test, and a life cycle durability test.
Experimental results demonstrated that the jet propulsion unit achieved forward locomotion at a rate of 14 centimeters per pulse, corresponding to 3.2 centimeters per second (0.22 body lengths per second).
Additionally, “we explored the scalability of the system for practical deployment in confined spaces by introducing a 4-centimeter-long mini jet, which achieves an average swimming speed of 5.5 centimeters per second (1.37 body lengths per second).
“The system demonstrates efficient pulsed-jet swimming without any rigid components and highlights how actuator placement and body geometry influence thrust generation, energy efficiency, and scalability,” Kitone said. “Experimental validation across multiple prototypes confirms the platform’s viability for compact, low-power underwater locomotion in fragile or deep-sea environments.”
Soft jet unit pulsed swimming performance under various conditions. Video: University of Michigan, Flint
This soft robot features a fully flexible jet structure. Image: University of Michigan, Flint
Future Work and Broader Impact
Ongoing research will focus on expanding the functionality and autonomy of the soft-jet system. The team is looking at integrating sensing and feedback control, particularly real-time temperature sensing of the SMA for closed-loop actuation, as well as pressure tolerance studies to assess structural and actuation performance under varying ocean depths.
Other work will focus on evaluating payload capacity, including the incorporation of cameras, sensors, and buoyancy control modules, and the potential of modular multiunit systems, where jet modules work cooperatively to enable directional control, turning, or load sharing. Computational fluid dynamics simulations could also provide deeper insight into how actuator behavior influences jet-propulsion efficiency.
Future research will also focus on thermal management considerations. “For example, the use of SMA materials with lower thermal activation points to improve performance and energy efficiency in colder aquatic environments and expand the system’s operational temperature range,” Kitone said.
Mark Crawford is a technology writer in Corrales, N.M.
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