Abstract

Endoluminal and endocavitary intervention via natural orifices of the body is an emerging trend in medicine, further underpinning the future of early intervention and precision surgery. This motivates the development of small continuum robots to navigate freely in confined and tortuous environment. The trade-off between a large range of motion and high precision with concomitant actuation cross-talk poses a major challenge. Here, we present a submillimeter-scale fiber robot (~1 mm) capable of decoupled macro and micro manipulations for intervention and operation. The thin optical fibers, working both as mechanical tendons and light waveguides, can be pulled/pushed to actuate the macro tendon-driven continuum robot and transmit light to actuate the liquid crystal elastomer–based micro built-in light-driven parallel robot. The combination of the decoupled macro and micro motions can accomplish accurate cross-scale motion from several millimeters down to tens of micrometers. In vivo animal studies are performed to demonstrate its positioning accuracy of precise micro operations in endoluminal or endocavitary intervention.

 

Introduction and Methods

To realize macro-micro manipulation of continuum robots for endoluminal procedures, the associated challenges include miniaturized robot size, macro-micro trade-offs in terms of speed, range of motion and accuracy, and actuation cross-talk. Avoiding actuation cross-talk between micro and macro actuators is particularly important. Previous attempts have included the use of hybrid actuation via traditional mechanical-driven robots for macro motion and compliant fluidic-actuated segments or magnetic skin for micro actuation. However, the base segment is inevitably affected by fluidic channels or tendons during micro actuation. Another method is using the equilibrium structure modulation. By changing the cross-sectional flexural rigidity, it is feasible to regulate the static equilibrium of the continuum robot to perform micro motion. In this approach, the backbones are pushed and pulled to adjust the equilibrium state, achieving micro modulation. Nevertheless, mechanical pushing and pulling with accompanied friction can introduce fluctuations to the macro structure, and therefore the macro and micro motions are not fully or thoroughly decoupled. A more fundamental problem associated with these existing approaches is that it is difficult to scale down the robot size to submillimeter scale for endoluminal intervention. Overall, to overcome these challenges, we proposed a macro-micro decoupled, optical fiber–actuated, submillimeter-scale continuum robot capable of macro-micro manipulation. In addition to the ease of miniaturization, the proposed concept uses two different modes of optical and mechanical actuations using the same fiber, with intrinsically decoupled capability to resolve the cross-talk issue.

 

The proposed configuration of the fiber robot is shown in Fig. 1 and movie S1. The intrinsically decoupled optical fiber–actuated miniature fiber robot has a serial-parallel configuration, consisting of a macro tendon-driven continuum robot (TDCR) and a micro light-driven parallel robot (LDPR). The same optical fibers are used for simultaneous mechanical and light actuations for both macro and micro motions. Here, three independent LCE fibers [outer diameter (OD) of about 300 μm] are assembled into a three degrees-of-freedom (DoF) LDPR, and three built-in optical fibers (OD of about 125 μm) provide light stimuli to the LCE fibers. Above that, a micro LDPR is mounted on the tip of a macro TDCR (OD of about 1 mm). The body of the macro TDCR is constructed by using thermally drawn multilumen styrene ethylene butylene styrene (SEBS) as the substrate to provide tendon channels and the femtosecond laser–profiled polyimide as the backbone. By controlling the output light intensity, precise motion can be achieved, and the trajectory can achieve a mean error of 4.8% over a range of 300 μm × 300 μm. Experiments are performed to verify the effectiveness and accuracy of the macro-micro fiber robot. Subsequently, animal experiments involving microneedle penetrations on the tympanic membrane (TM) of live guinea pigs are conducted for demonstrating the potential use of the macro-micro fiber robot system for navigating through the external auditory canal (EAC) and performing local microneedle positioning on the TM. Our work uses three optical fibers both as the waveguides for light transmission to the distal end for micro motion, and as the mechanical cables to drive the main body of the continuum robot for macro motion. The proposed concept with decoupled actuations lays the foundation for the macroscale, rapid maneuvers and microscale, delicate operations toward endoluminal and endocavitary intervention.

 

Key Results and Conclusions

The macro-micro manipulation of conventional continuum robots in endoluminal procedures requires a balance between the geometric size, steerability, positioning accuracy, and kinematic coupling. We developed a submillimeter-scale fiber robot for macro-micro manipulation to address three primary challenges: (i) scaling down the robot size to submillimeter dimensions to adapt to confined and tortuous environments while maintaining macro-micro characteristic; (ii) selecting appropriate ranges to balance macro and micro motions; and (iii) mitigating cross-talk level between macro and micro actuations using mechanical and light actuations. The fiber robot uses thin optical fibers for both the macro TDCR and the light-actuated micro LDPR, thereby enabling cross-scale navigation and manipulation. The small size of optical fibers, coupled with their multipurpose functionality, allows the fiber robot to be scaled down to 1 mm or submillimeter easily, thereby addressing the first challenge. The range of motion between the micro actuator and macro actuator covers several orders of magnitude, from tens of micrometers to several millimeters even centimeters, enabling rapid maneuvers and delicate operations, which addresses the second challenge. Last, the experimental validations demonstrate that the cross-talk between macro and micro motions, induced by mechanical pull/push and light activation, is negligible, which addresses the third challenge. Furthermore, the built-in stimuli also enable controllable joint contraction, reliable multi-DoF, and improved actuation performance. Demonstrations and in vivo animal studies are performed to confirm the accuracy of the fiber robot, showing its potential for delicate micro operations in endoluminal interventions. However, our optical fiber–actuated macro-micro fiber robot is still in its infancy.

 

Given the promising performance of the fiber robot, further enhancement may be considered. First, the primary control manner of the current robotic system is teleoperation. Implementing closed-loop control for the fiber robot could help enhance accuracy. Vision/medical image-based feedback or onboard sensors can be used for realizing closed-loop control. For vision/medical image-based feedback, markers can be affixed to both macro and micro actuators for pose estimation or MR image can be used for positioning and navigation, and for onboard sensors, strain sensors, such as carbon nanotube–based resistive sensors, can be used for pose estimation. Second, fiber robot with two-section macro continuum robot and Stewart-structure micro parallel robot can acquire more DoFs, enhancing navigation and positioning capabilities. This increased dexterity can be particularly beneficial for endoluminal intervention procedures. Last, for effective heat management and to prevent unintended side effects, an active cooling system or a heat-insulating shell can be used to cool the surrounding environment or prevent direct contact between the actuator and the surrounding tissues. Moreover, progress in other functional materials or actuation mechanisms is also expected to help advance the system, thus opening up a diverse range of applications.

 

Fig. 1：Submillimeter fiber robots capable of decoupled macro-micro motion for endoluminal manipulation.

The macro-micro fiber robot uses thin optical fibers for both tendon-driven macro motion and light-driven micro motion, facilitating cross-scale manipulations. (A) The controllable built-in light stimulus enabling LCE-based micro LDPR precision positioning capability. (B) The optical fibers transmitting light for micro manipulation. (C) The macro TDCR with thermally drawn multilumen matrix (SEBS fiber) and femtosecond laser–profiled PI backbone enabling macro navigation capability. (D) Pulling the optical fibers for macro manipulation through motors.

 

Fig. 2：Design and actuation performance of light-driven LCE fibers.

(A) Built-in optical fibers for light-responsive LCE fiber stimulus. (B) POM images of the stretched LCE fibers. (C) Optical image of a single LCE fiber. Scale bar, 100 μm. (D) Mechanical property of the LCE fiber after the first cross-linking stage. There is a stress plateau region corresponding to the reorientation of the mesogens and extension of the polymer chain, but the fiber stiffens when the strain reaches approximately 300%. (E) Contraction strain as a function of input power light intensity for LCE fibers with fixed length and various stretched ratios. (F) Contraction strain as a function of input power light intensity for LCE fibers with fixed stretched ratio and various lengths. (G) Diameter of stretched LCE fibers as a function of applied stretch ratio. (H) Output force of the LCE fibers versus input power density at fixed stretch ratio of five. (I) Output power intensity versus duty cycles under different input power intensity. (J) Displacement of LCE fibers as a function of duty cycle. The input power intensity is fixed and the duty cycle increases from 0 to 100% and decreases from 100 to 0% for five loops. (K) Displacement of LCE fibers over time for five cycles. The input power intensity is fixed and the duty cycle increases from 0 to 100% and decreases from 100 to 0% at the interval of 10% for five loops. (L) Repeatability performance of LCE fibers over time for five cycles.

 

Fig. 3：Actuation performance of the micro LDPR.

(A) Schematic illustration of the structure of the micro LDPR. The micro LDPR was assembled by three parallel LCE fibers. (B) Workspace of the developed micro LDPR. Blue points correspond to the discrete points in workspace. (C to E) Path following performance of the designed micro LDPR. (F) Experimental demonstration of teleoperating the micro LDPR to follow a designated path on a 3D-printed micro maze (the width of the path about 60 μm) using the developed kinematics. Scale bar, 100 μm.

 

Fig. 4：TDCR for macro manipulation.

(A) Schematic diagram of thermal drawing process. A four-channel SEBS preform with OD of 30 mm was thermally drawn into a fiber with OD of 1 mm, while the original structure was maintained. Scale bar, 100 μm. (B) Femtosecond laser–profiled polyimide tubes. The laser-profiled snake-like backbone was used to enhance the stiffness of the macro TDCR. (C) Temperature ramps from 20° to 200°C of the thermal drawing materials SEBS. The result shows that the material transitions from the solid state to the liquid state at about 136°C. The loss modulus remains greater than the storage modulus above this temperature. (D) Angular frequency ramps from 0.01 rad/s to 100 rad/s at temperature of 145°C for thermal drawing materials SEBS. The relative strength of the storage modulus and the loss modulus varies with angular frequency. (E and F) Bending performances of macro TDCR. (G) Macro manipulation of TDCR by teleoperation.

 

Fig. 5： Decoupled macro-micro manipulation capability of the fiber robot.

(A) Image of the fiber robot. (B) Experimental validation of the decoupled macro-micro motion. To observe the disturbances caused to the macro TDCR when actuating the micro LDPR, a high-resolution camera was used to monitor the pose of the macro TDCR. A marker was bonded on the distal end of the macro TDCR to calculate its pose. Scale bar, 500 μm. (C) Recording the pose of the macro TDCR and calculating the position and orientation changes of the marker. (D) Demonstration of teleoperating the macro-micro fiber robot. The fiber robot can be teleoperated to follow a designated macro-micro path on a laser-cut maze mold (overall size of the designed path is below 2.5 mm × 2.5 mm and the width of the path range from 0.06 to 0.3 mm), which exhibits a wide range and high accuracy operating capability. Scale bar, 300 μm. (E) Demonstration of teleoperating a microneedle to reach a target location. With a microneedle mounted on the tip of the fiber robot, it can be teleoperated with a joystick to reach a target location using the developed inverse kinematics. Overall size of the designed mold is under Φ 4.7 mm and the diameter of the target region is 0.06 mm. Scale bar, 400 μm.

 

Fig. 6：In vivo micro penetration on TM of living guinea pigs.

(A) Overall illustration of the micro penetration system. (B) Schematic illustration of the system for endocavitary manipulation. A fiber robot and a microcamera were assembled for in vivo operation. (C) Optical images of in vivo TM micro penetration system. (D and E) In vivo penetrating procedures on the TM of a living guinea pig. Under the guidance of the microcamera, the fiber robot can be teleoperated to navigate through EAC to approach TM and perform micro penetration along different paths (linear or circular paths).

 

 

 

https://www.science.org/doi/10.1126/sciadv.adr6428