A novel multishaped variable stiffness magnetic catheter for minimally invasive surgery

Minimally invasive surgery has become an important alternative to conventional open surgery because it offers smaller incisions, faster recovery, and reduced operative risks. Flexible continuum catheters can reach deep surgical sites through natural anatomical lumens or blood vessels, providing better compliance and safety than traditional rigid instruments and showing strong potential in endoscopy, vascular intervention, and intraluminal therapy. However, current flexible catheters still face a key trade-off between miniaturization and dexterity. Tendon-driven, cable-driven, and multibackbone motorized designs can improve degrees of freedom, but they usually require internal transmission components that limit further miniaturization and may introduce friction, hysteresis, and control errors. Magnetic catheters address some of these limitations by embedding magnetic elements and using external magnetic fields to steer bending without complex internal transmission, enabling flexible and compact designs. Yet most existing magnetically controlled continuum catheters are limited to C-shaped deformation, where tip orientation is strongly coupled with catheter posture, restricting obstacle avoidance, stable tip alignment, and atraumatic navigation in complex anatomical channels. At the same time, catheters for minimally invasive surgery must remain flexible during navigation to reduce tissue injury while becoming sufficiently rigid for positioning, puncture, or distal tool operation. “Existing variable-stiffness strategies, such as particle jamming and low-melting-point alloys, still face miniaturization limits, complex encapsulation, or potential leakage risks.” said the author Xingfeng Xu, a researcher at Tianjin University, “Therefore, developing a magnetic flexible catheter that combines multishaped deformation with safe variable-stiffness control is important for improving navigation flexibility, operational stability, and clinical adaptability in minimally invasive procedures.”

This study designed and fabricated a multishaped variable-stiffness magnetic catheter based on a shape memory polymer (SMP). The catheter body used a biocompatible polyurethane SMP, with 2 independent enameled copper coils wound around it as heating elements. By regulating current amplitude and heating duration, the 2 variable-stiffness segments could be locally softened or stiffened, enabling both compliance during intraluminal navigation and structural support during interventional tasks. The catheter incorporated 2 spatially separated radially magnetized permanent magnets. The distal magnet was mounted on a torque coil tube and could be axially rotated by an external driven module, allowing the relative magnetization direction between the 2 magnets to be adjusted. Under a uniform magnetic field generated by a triaxial Helmholtz coil system, the 2 magnets experienced differential magnetic torques, enabling C-shaped, J-shaped, and S-shaped catheter configurations. The researchers established a deformation model relating magnetic torque to catheter bending and used COMSOL finite element simulations to verify the target deformation modes under different magnetic-field configurations. Experimentally, the team first tested multishaped deformation, bending responses under different stiffness states, repeatability, and hysteresis on the triaxial magnetic actuation platform. They then characterized the thermomechanical behavior and segmented variable-stiffness performance of the SMP using 3-point bending tests, uniaxial compression tests, heating/cooling measurements, and cantilever loading experiments. Finally, functional validation was conducted in obstacle and bifurcation models, gastric phantoms, biliopancreatic duct phantoms, and an ex vivo porcine stomach model. By integrating distal functional components such as a needle tip, miniature camera, and sprayer, the study evaluated the catheter’s feasibility for obstacle avoidance, submucosal injection, multicatheter cooperation, lesion visualization, and targeted delivery in minimally invasive procedures.

The results showed that the multishaped variable-stiffness magnetic catheter could achieve flexible deformation, stiffness modulation, and simulated clinical operation within a compact structure. First, under a 15 mT uniform magnetic field, the catheter generated multiple configurations, including C-shaped, S-shaped, and J-shaped forms, by adjusting the relative magnetization directions of the 2 radially magnetized magnets. It could also enter a standby mode in which opposing magnetic torques cancel each other, reducing interference with other magnetic instruments. Bending tests showed that the catheter reached maximum distal bending angles of 127°, 93°, and 80° in flexible, partially flexible, and rigid states, respectively, indicating strong large-angle steering in the flexible state and effective curvature suppression in the rigid state. The catheter also showed good repeatability and low hysteresis, with a maximum angular error of 3.9°. Thermomechanical tests further confirmed the effectiveness of the SMP-based variable-stiffness segments: the assembled catheter achieved an approximately 20.8-fold stiffness change from 30 to 70 °C, while the outer surface temperature reached about 41 °C when the internal SMP entered the flexible state, suggesting favorable thermal safety. Segmented heating experiments also demonstrated that different variable-stiffness sections could be independently controlled, thereby changing bendable length and load response. In functional validation, the catheter successfully performed obstacle avoidance and branched-path navigation, completed submucosal injection in a gastric phantom, visualized lesions in gastric and biliopancreatic duct phantoms, and entered the simulated biliopancreatic duct, with magnetically controlled operation times below 2 min. In addition, in an ex vivo porcine stomach experiment, the catheter integrated with a miniature sprayer achieved stable positioning and controllable targeted delivery without observed damage to surrounding tissues. These results indicate strong application potential for multipath navigation, stable intervention, multicatheter cooperation, and intraluminal diagnosis and therapy.

This work proposes a magnetically controlled flexible catheter that combines multishaped deformation with variable-stiffness modulation, offering a new strategy for complex intraluminal navigation and stable intervention in minimally invasive surgery. By using 2 radially magnetized magnets and an axially rotatable distal magnet, the catheter can generate C-shaped, J-shaped, and S-shaped configurations under a single uniform magnetic field, while also entering a standby mode through magnetic torque cancellation to support multicatheter cooperation. Its dual shape-memory-polymer-based variable-stiffness segments allow the catheter to remain compliant during navigation and provide support during puncture, visualization, or delivery. Phantom and ex vivo porcine stomach experiments preliminarily demonstrated its potential for gastric lesion visualization, submucosal injection, biliopancreatic duct entry, and targeted delivery. “In the future, we will further optimize cooling response, inverse kinematics control, and closed-loop feedback, and combine force sensing, lubrication improvement, miniaturization design, and in vivo verification to promote their transformation into real minimally invasive diagnostic and therapeutic applications.” said Xingfeng Xu.

Authors of the paper include Xingfeng Xu, Xinling Li, and Siyang Zuo.

This work was supported by the National Natural Science Foundation of China under grants 62573314 and 62133010.

The paper, “A Novel Multishaped Variable Stiffness Magnetic Catheter for Minimally Invasive Surgery” was published in the journal Cyborg and Bionic Systems on Sep 1, 2026, at https://doi.org/10.34133/cbsystems.0675.