Soft Bidirectional Nitinol Actuator for Robotic Catheters
Designed and fabricated a catheter-scale soft actuator that achieves bidirectional bending from a single thermally controlled Nitinol wire, replacing the usual antagonistic-wire architecture with a simpler, lower complexity mechanism. The system combined a custom trained U-shaped Nitinol wire, a pre-stretched elastic band, and a molded silicone body to produce 120° total bending while keeping peak surface temperature to 46°C. The actuator was then integrated into a steerable catheter and demonstrated in a 3D-printed aortic phantom, where it was guided into multiple arterial branches within 30 seconds.
Problem
Steerable catheter tips need large bending range, compact packaging, and simple control. Many Nitinol catheter concepts achieve bidirectional motion by using two opposing Nitinol elements, but that doubles wiring, increases integration difficulty, and adds control complexity. Single wire designs are simpler, but they usually bend in only one direction or produce asymmetric motion that is harder to use in a practical catheter. This project focused on achieving advanced bidirectional motion with a single input variable, temperature, while preserving the small footprint needed for catheter integration.
Catheter concept showing the soft bidirectional Nitinol actuator integrated at the distal tip.
Solution
The actuator uses a single U-shaped Nitinol wire working against a pre-stretched elastic band inside a soft silicone body. At low temperature, the elastic band dominates and bends the actuator in one direction. As the Nitinol heats through its phase transformation, it recovers its trained shape and bends the actuator in the opposite direction. This created a bidirectional steering mechanism driven by one thermal control channel rather than two separately coordinated actuators. To improve motion stability, the design used two parallel Nitinol segments and a flat elastic band, which increased resistance to twisting and out-of-plane bending while keeping both electrical terminals on the same side for simpler packaging.
The elastic band bends the actuator one way when cool, while the trained Nitinol wire bends it the opposite way when heated.
Design and fabrication
The actuator was built around a 0.5 mm diameter, 130 mm long pre-trained Nitinol wire that originally memorized a straight shape. To create the final geometry, the wire was manually formed on a machined stainless-steel training plate with a circular screw pattern, heat treated at 525°C for 25 minutes, and then quenched in ice water to lock in the curved U-shape. This produced a single continuous wire with two parallel active segments, which simplified wiring and ensured both sides of the actuator were integrated uniformly into the soft body.
Exploded view and prototype of the actuator showing the U-shaped Nitinol wire, pre-stretched elastic band, silicone body, and end caps.
The soft structure was assembled using custom 3D-printed molds, end caps, and a tension jig. An elastic band was fixed into the printed end caps, coated to improve bonding, and then stretched from 16 mm to 36 mm before molding so it would provide the restoring force for reverse bending. A micro temperature sensor was tied directly to the Nitinol wire and coupled with thermal paste for feedback control. The fully tensioned assembly was then encapsulated in Dragon Skin 10 silicone, cured, removed from the mold, and finished with end molds that covered the exposed turnaround and connection regions. The final actuator package measured about 50 mm long with a 5 mm width and 3.6 mm height, placing it in a catheter relevant sub-5 mm form factor while still delivering roughly ±60° bidirectional motion.
Custom 3D-printed jigs and molds were used to tension the elastic band, place the Nitinol wire and sensor, and cast the final silicone actuator.
The bending angle of the actuator was mapped to Nitinol temperature so the motion could be predicted and commanded with a single control input. Actuator was characterized quasi-statically by stepping the Nitinol temperature from 20°C to 70°C in 5°C increments, then cooling it back down with the same step size. At each temperature step, the average bending angle was measured after the response settled. This produced a repeatable temperature-to-angle map for both heating and cooling, capturing the actuator’s hysteretic behavior across the full operating range.
Quasi-static characterization mapped bending angle to Nitinol temperature.
That characterization showed a bending range of about -70° to +71°, with cycle-to-cycle variation limited to 3.7° during heating and 3.1° during cooling. A quasi-static model was then fit to the data by coupling Nitinol thermomechanics, elastic-band mechanics, and silicone-body bending, resulting in R² = 0.995. In practice, this meant the actuator motion could be treated as a predictable function of temperature, which is what made a single thermal input viable as the steering command.
The actuator was also evaluated for cyclic durability and surface temperature. Over 150 actuation cycles, the maximum bend in each direction dropped early and then stabilized, reaching -59.7° and 59.4° at 100 cycles. Surface temperature was measured with two sensors placed on the actuator body, and the highest measured surface temperature was about 46°C when the Nitinol reached 70°C, keeping the exterior below the stated 50°C safety threshold.
Cyclic testing showed early settling followed by stable bidirectional bending over 150 actuation cycles.
To validate the design in a realistic use case, the soft actuator was integrated with a braided catheter shaft using heat shrink tubing, creating a steerable catheter robot. The device was then tested in a 3D-printed Type 1 aortic arch phantom. Starting from the descending aorta, the catheter was guided into the left subclavian, left common carotid, and brachiocephalic arteries by combining base translation and rotation with temperature driven tip steering.
Surface temperature testing confirmed the actuator exterior stayed below 46°C during thermal actuation.
The phantom trials showed how the temperature map translated into functional steering. The tip was first heated to about 46°C to remain nearly straight in the aorta. It was then heated to 51°C to reach -36° in 2 seconds for the left subclavian artery, or to 54°C to reach -47° in 2.2 seconds for the other branches. As the tip advanced into a branch, the Nitinol was allowed to cool so the distal section regained compliance and conformed to the vessel path. All three demonstrations were completed within 30 seconds, showing that the actuator could be used for anatomically relevant navigation.
Temperature-controlled steering guided the catheter into three target branches of a 3D-printed aortic phantom in under 30 seconds.
Related Publication
Soft bidirectional shape memory alloy actuators for robotic catheters.
Manian, V., Oda, K., AfshariNejad, P., Lee, K., Akbari, A., & Sheng, J.
IEEE Robotics and Automation Letters. (2025).