Worm-Like Soft Robot for Complicated Tubular Environments

Abstract

This article describes a worm-like soft robot capable of operating in complicated tubular environments, such as the complex pipeline with different diameters, water, oil, and gas environments, or the clinical application in natural orifice transluminal endoscopic surgery. The robot is completely soft and robust, and consists of one multidegree of freedom (DoF) extension module and two clampers for locomotion and steering. The multi-DoF extension module is able to adjust the heading direction in the three-dimensional space. The clamper has a basic expansion module structure and detachable sucking module structure. The combined clamping principle for sticking to the inner wall can be reconfigurable to adapt the tubes with multiple tubular scales and super elastic materials. For fabrication of the mechanical structure, a low-cost and time-efficient method is proposed in this article. Based on our proposed robot, a series of phantom and application experiments are performed. The results demonstrate that the soft robot can freely bend and elongate with the entire soft body, and pass through tubes with changing diameters or branches, dry tubes, liquid environments, hard surfaces, and even soft deformable tubes. It has the ability to remove a load of >10 times its own weight. In addition, an additional visualization unit, biopsy, and electromagnetic sensor are mounted on the robot tip for the real-time image inspection, manipulation, and robot tracking. The proposed worm-like soft robot is compact, flexible-actuated, and sufficiently safe, as well as extensible. Its ability to move in the complex unstructured environment shows a great potential for search and medical applications.

Introduction

Typical tubular environments, such as pipelines for transporting water, oil, or gas, have variable diameters and several branches and curves. As a point-to-point transportation system, pipeline transportation such as water, oil, or gas has fixed routes, and is usually used for a long time. Therefore, the pipeline system needs to be inspected regularly to ensure its safety. The inspecting devices must be able to move deep inside the pipe. It is obviously impractical to use a traditional industrial robot arm or pipe endoscope since long, complicated, and multibranch pipelines require a higher degree of freedom (DoF). Although some flexible in-pipe robots with different locomotion mechanisms such as the wheel type,^1,2 walking type,^3,4 worm-like type,^5–7 screw type,⁸ and snake type⁹ have been proposed, most are developed using rigid structures. Such structures have difficulty entering complex narrow pipelines, and are not suitable for slippery or compliant environments. Generally, the inspection process begins after the pipe grid is turned off. However, sometimes the media in pipelines cannot be completely discharged. The inspecting device must be able to work in the liquid, slippery, or combustible gas environments. Currently, no practical robotic technologies can maintain or examine the water/oil pipes without first draining them, in extreme applications, such as gas pipes, traditional robotic technologies cannot satisfy the safety requirements because such environments require no potential electrical spark.

Another typical application of the tubular environment is a colonoscopy procedure. The conventional colonoscope enters through an external pushing and dragging action from the doctors. There are two main problems in the conventional colonoscopy procedure: a high interaction force and inadequate bending freedom in the acute bends of the tortuous intestine. To reduce the patient's pain and achieve a convenient endoscope insertion process, a robotic semiautonomous soft body with more DoFs has been explored to improve flexibility. The natural orifice transluminal endoscopic surgery robot is one of the most typical robotic technologies for improving the flexibility of the manipulator's tip.^10–14 The flexible robotic instruments have a large number of DoFs and can perform snake-like movements in the constrained surgical area. Although the robotic technology enables endoscopy with flexible manipulation, the insertion process is also limited by the tortuous structure of human intestine. To solve such limitations, self-propelled robots have been proposed for colonoscopy procedure. Different locomotion mechanisms and their corresponding robotic systems have been developed^15–18 for medical applications. Self-propelled robots provide a new solution for endoscopic diagnosis and therapy, but their softness, flexibility, and safety still need to be improved.

With the development of materials, biomechanics, and manufacturing technologies, soft robotic technology has generated an increasing number of concerns.¹⁹ The emerging field of soft robotics offers promising alternatives to current systems and shows great potential for application in various complicated tubular environments.^20–23 Compared with hard robots, robots using soft robotic technology have a safer operation, stronger environmental adaptability, and can more easily pass through the complex constrained environment. Inspired by the earthworm or inchworm locomotion patterns, many groups have researched the worm-like robot with different structures since the worm-like mechanism is a very simple and effective locomotive mechanism for tubular environments. However, most of the structures have a relatively large size. The scopes are mainly focused on exploring the locomotion principle and bionic research.^24,25 Although they can achieve a worm-like locomotion and a higher interaction force, the flexibility and size are difficult to apply in the small-sized, slippery, or compliant environments. To adapt to a constrained or narrow space, some soft robots with compact structure have been developed.^26–28 Most of the compact soft robots are smart memory alloy (SMA) actuated. The SMA springs as the “muscle actuators” are encapsulated in a soft silicone body. However, the actuation of the SMA spring is controlled by the heating temperature, which is unstable and has a low efficiency. Moreover, the bending scale is limited because every SMA actuator has a limited shape state (the original shape and predeformed shape). Some studies on pneumatic-driven worm-like robots have been conducted and have shown a good performance in tubular environments. In previous work, the standard structure of such worm-like robots has three parts: two clampers for anchoring and one drive actuator for locomotion. Most of the drive actuator is a linear actuator that elongates and retracts the soft body. However, the linear actuator only has one freedom.^29–33 The robots cannot adjust the heading directions flexibly and actively. To address these challenges, prototypes with a bendable drive actuator have been developed.^34,35 The bendable actuators are designed to mimic the inchworm's omega deformation of the soft body. However, the bendable actuator proposed in a few studies^34,35 can only bend in a two-dimensional plane. Such structures are still insufficient for multibranch or cured tubular environments. In addition, the clamper's structures of the existing worm-like robots are just the single function (expansion adhesion or vacuum suction adhesion), which cannot satisfy the requirements of moving in various complicated tubular environments. For example, clampers using the expansion adhesion principle have a low efficiency in compliant environments due to the stroke loss generated by the large deformation of tubes during the elongation and retraction process.³⁶ With higher requirements for moving in tubular environments with various types of surfaces or elasticity, it is essential to explore a reconfigurable clamper with the combined adhesion principle.

Our research focuses on exploring a flexible robotic design that is capable of moving through complicated or unstructured tubular environments, such as tubes or pipelines with multiple diameters, rough or uneven surfaces, different media (gas, oil, water), or tubes with high elasticity and softness. To address the challenges and satisfy the complicated tubular applications, we propose a worm-like robot that consists of one multi-DoF extension module and two extensible clampers. It is completely soft and robust. The locomotion is driven by using pneumatics rather than electrical components, which enables the locomotion process to have quick response. The main contributions of our work in this article are as follows:

A multi-DoF extension module enables the worm-like robot to have the ability of flexibly adjusting the heading direction in three-dimensional (3D) space and elongating with the entire soft body, which is compact and flexible, so that the robot is more adaptable to the complicated 3D or multibranch tubular environments.

The clamper with a basic expansion module structure and a detachable sucking module structure has the combined adhesion principle, which can be reconfigurable to adapt to the tubes with multiple diameters, uneven or slippery surfaces and super elastic materials.

System verification of the robot and practical applications in different tubular environments with the proposed robot are performed to illustrate the capability and efficiency.

It should be noted that the soft body with pneumatic actuation is safer and more compliant. As the main body of the soft robot does not contain any electric components, it is not affected by electric and magnet noise. Furthermore, the locomotion of the worm-like soft robot is smoother and more robust compared with the SMA actuation and motor actuation. The multi-DoF adjustment ability is more flexible compared with the former soft tube-climbing robots. With this design, the robot is compact and highly robust for complex and narrow-curved tubular environments, especially when performed in the colon environment.

Materials and Methods

Design concept and structure of the robot

This work is inspired by inchworm. This animal has unique flexibility and compliant softness. They can move forward and adjust their heading direction by extending or twisting their soft main body.³⁷ As shown in Figure 1, the locomotion process can be typically divided into three steps. In the first step, the end section turns to anchor the surrounding wall, and the soft body adjusts the heading direction in the 3D space. Then, in the second step, the entire soft body elongates to extension. In the third step, the head section contacts with the surrounding wall as an anchor and contracts the whole body to undergo a nonsymmetric deformation. Such three steps alternate in sequence to achieve the locomotion.

FIG. 1.

Simplified motion pattern of the inchworm. Color images are available online.

Taking inspiration from the extraordinary characteristics of inchworm locomotion, we develop a soft robot that has great potential to be capable for a variety of complicated tubular environments. Figure 2 illustrates the design concept of our worm-like robot. Generally, the elementary structure of the worm-like robot is composed of one extension module and two clampers, and the extension module is equipped between the two clampers.^29–35 In our proposed design, the basic structure is composed of two expansion modules at both tips (head expansion module and end expansion module) as the legs and one multi-DoF extension module as the inchworm soft body. The detachable sucking module structure is also proposed and can be conveniently mounted on the corresponding expansion modules. A central channel is reserved for inserting air tubes, manipulation tools, or inspection devices. More importantly, different from the traditional worm-like robot, our robot has the capability to flexibly adjust the heading direction in the pipes with multiple branches or in complicated 3D space. The clamping principle can be reconfigurable to adapt the tubes with multiple tubular scales and super elastic materials.

FIG. 2.

Design concept of the worm-like soft robot. The basic structure is composed of a multi-DoF extension module and two expansion modules. Air chambers are illustrated with different highlighted colors (end expansion module: indigo, chamber1: blue, chamber2: red, chamber3: green and head expansion module: yellow). A central channel is reserved for integrating more sensors or manipulation devices. The function can be enriched by integrating more sensors or sucking modules. DoF, degree of freedom. Color images are available online.

To enable the robot to have the ability of adjusting the heading direction in 3D space and extending its entire soft body, the multi-DoF extension module must have two degrees of bending freedom and one stretching freedom. To achieve the two DoF bending motion, the structure mainly consists of three or more independent air chambers, which are equally spaced along the soft body. Since three is the least number of chambers possible to achieve bending in the 3D space, we consider a triangular configuration of parallel pneumatic chambers as the basic structure of the multi-DoF extension module. Air chambers of this module are equally spaced at 120° along the cylinder. When a chamber is pressurized, it expands while the others maintain the initial length. Consequently, the soft cylinder body bends in the direction opposite to this chamber. As the pressure in the three chambers can be controlled independently, the bending motion in any direction is achieved thanks to the internal chambers being differently pressurized. When the pneumatic pressure in three chambers is equally increased, the module can stretch in the axial direction. Thus, the basic structure has three DoFs: pitch, yaw, and stretch. However, the excessive radial expansion is the side effect of the chamber's inflation, which will cause extra friction and pressure onto adjacent environments. To eliminate the radical deformation, various constrained layer structures, such as the soft sleeve,³⁸ reinforced fiber,^{29,30,32,33,37–39} or elastomers with different stiffnesses,^40,41 are mainly used to create different shear modules on different parts of the soft actuator. In this article, we designed an embedded constrained layer using nylon fiber. The fiber is wounded symmetrically around the cylinder silicone body, and gives the module a maximum axial extension. Thus, the extension module can simultaneously adjust the heading direction in all directions and elongate the entire soft body by controlling three air chambers. Compared with previous worm-like robots, the multi-DoF extension module is more compact and flexible, so that the worm-like robot is more adaptable to the complicated 3D or multibranch tubular environments.

The main function of the clampers is to stick to the inner wall of the tube and actuate alternately. The adhesion to the tube's inner surface is achieved through two major principles: radical expansion^29–31 and sucking adhesion.^32–34,42 The radical expansion principle is that the clamper expands under the pneumatic pressure, and then firmly attaches to the inner wall of the pipe. Afterward, the radial expansion is constrained by the pipe's diameter. The wider the clamper's diameter is, the stronger the clamping force against the pipe wall will be. The expansion principle can generate a high clamping force. However, this clamping method is not suitable for super elastic tubular environments. The stroke loss would occur as the clamper could not provide sufficient clamping force when the elastic pipe has a large deformation and expands with the clamper. The sucking adhesion principle is to generate the negative pressure between the cavity inside the suckers and outer environment for achieving adhesion. This method can achieve strong and stable adhesion on various types of surfaces, especially the wet, slippery surfaces or high-elastic surfaces. To adapt to various complicated tubular environments, it is essential to explore the reconfigurable clamper with the combined adhesion principles. Thus, we design a clamper with a fixed basic expansion module and a detachable sucking module. The expansion modules of both clampers are fixedly connected on both ends of the extension module. Each expansion module has one air chamber that can achieve radial expansion and adjusts the clampers' diameter to adapt different tubes with variable diameters. The modular sucking module can be conveniently attached to the corresponding expansion module and detached. This module is designed as a hollow structure. Six suckers and six tube passages are alternatively distributed around the soft silicone body. The diameter of the tube passages is larger than the air tube of the suckers. Thus, the air tubes of the suckers on the front can be conveniently deployed through the tube passages. When the sucking module and the expansion module simultaneously get actuated, it may be effective to fix the robot against the inner wall with slippery or soft surfaces.

Furthermore, a central channel is designed throughout the entire soft body. Based on different future application requirements, the central channel is reserved for the extended inspection or manipulation functions, where air tubes, microendoscopes, sensors, or manipulation tools can be deployed.

Fabrication method

There are many methods for fabricating soft robotics.⁴² The major approach is soft lithography and/or shape deposition manufacturing.⁴⁴ The primary drawback of this method is that the robot is often constrained to a simple channel structure or a planar configuration. It is difficult to manufacture the complicated 3D multicavity structure using the conventional fabrication method. The elementary design of the proposed worm-like robot is composed of a multi-DoF extension module and two expansion modules. The inner structure has many closed inner chambers. Therefore, we promoted the traditional method and developed the retractable combined casting (RCC) method for casting the sophisticated structure of the robot.

In our fabrication method, the structure segmentation and casting sequence are the key points. The molds are designed based on the structure segmentation. In the casting process of each segment, the retractable inner molds can be taken out without deteriorating the final silicone structure after the silicone rubber is cured, and then the closed multicavity inner structure can be obtained. In the molding process, each segment is sequentially molded on the former casted silicone structure, and air tubes are sequentially embedded. As shown in Figure 3, the molds (Fig. 3I-A, II-A) are printed using a high-precision 3D printer. The basic structure of our soft robot is divided into three segments: a head expansion module, an extension module, and an end expansion module. Each segment will be sequentially fabricated. First, the extension module is independently fabricated in Step I1–Step I3, and then the end expansion module is fabricated on the extension module in Step I4. The head expansion module is poured on the other side of the extension module in Step I5.

FIG. 3.

Illustration of the fabrication process: I: Fabrication of the worm-like basic structure with a extension module and two expansion modules: the molds printed by 3D printer (I-A), casting the expansion module with spiral pattern (Step I1) and the fabrication result (I-B), winding the reinforced fiber around the silicone body cylinder (Step I2) and the result (I-C), pouring the silicone rubber after assembling the outer molds to embed the reinforced fiber (Step I3) and the demolding result (I-D), adding the air tubes and cast the end clamper (Step I4) and the demolding result (I-E), casting the head clamper (Step I5) and the demolding result (I-F), the whole worm-like soft robot with visualization unit (I-G). II: Fabrication of the modular sucking module: the molds (II-A), assembling the outer molds and inner molds (Step II1) and the assembled mold (II-B), assembling the complete outer molds to extrude the extra liquid silicone rubber and locating the inner mold (Step II2) and the result (II-C), attaching air tubes (Step II3) and the fabricated sucking module (II-D). 3D, three dimensional. Color images are available online.

Specifically, in Step I1, the outer molds with a spiral pattern and the inner molds are assembled together. We pour the liquid silicone rubber (Ecoflex 00-50, Smooth-on) into the mold softly, and then insert the cap to extrude the extra liquid silicone rubber. Once the silicone is cured, we detach the outer molds and cap. The cured rubber body with a spiral pattern is shown in Figure 3I-B. In Step I2, we wind the nylon fiber with a symmetric angle of 8°. The symmetric angle mainly contributes to the axial elongation effect during air inflation.⁴⁵ The silicone rubber with a fiber along the length of the module is shown in Figure 3I-C. In Step I3, we assemble the outer molds and pour silicone again to encapsulate the reinforced fiber. After the outer and inner molds are removed, the extension module shown in Figure 3I-D can be obtained. Then, the expansion modules of clampers are sequentially fabricated on the extension module by using the clamper molds. The assembled mold has four cavities: N1 and N3 for the end expansion module, N2 and N4 for the head expansion module. In Step I4, the mixed liquid silicone rubber is poured into the assembled mold cavity. The end expansion module is divided into two segments: the sidewall casted by using the cavity N1 and the cap casted by using the cavity N3. The air tubes are threaded through the mold. The sidewall, the cap, and the extension module are sequentially bonded together. The demolding results of the end expansion module are shown in Figure 3I-E. The head expansion module is casted using a similar molding process in Step I5. The air tube of the head expansion module is deployed through the central channel. After that, the soft robotic structure is finished. The prototype is shown in Figure 3I-F. After fabricating the prototype of the robot, a visualization unit with a micro CCD and lighting device is deployed through the central channel (2.5 mm) and mounted on the tip to achieve real-time inspection. The eventually assembled soft robot is shown in Figure 3I-G. The weight of the robot with a visualization unit is 14.5 g. The diameter is 15 mm, and the entire length is 80 mm.

The sucking module is an independent structure, which can be conveniently attached to or detached from the main body of the soft robot. As shown in Figure 3II-A, the metal bars have two different sizes (diameter: 2 and 2.5 mm). They are alternatively distributed around the inner mold. The fabrication process of the modular sucking module can be divided into three steps. In Step II1, we first assemble the outer molds and inner molds. The metal bars are fixed on the corresponding location. The assembled mold is shown in Figure 3II-B. In Step II2, we pour the liquid silicone (Ecoflex 00-30, Smooth-on) into the mold softly until the silicone evenly spreads out. As shown in Figure 3II-C, we assemble the complete outer molds to extrude the extra liquid silicone rubber and locate the inner mold. In Step II3, Six air tubes with diameter of 2 mm are glued to the fabricated silicone body. As shown in Figure 3II-D, the sucking module is obtained. The outer diameter is 25 mm and inner diameter is 15 mm. Thus, the sucking module can be sheathed outside the expansion module of both clampers. Air tubes of the suckers on the head expansion module can be conveniently deployed through the tube passage on the end expansion module as the diameter of tube passages (2.5 mm) is larger than that of the air tube of the suckers (2 mm).

Experiments and Results

Robotic system configuration

We develop a robotic actuation system for achieving the precise control of pressure. The configuration of the actuation system and schematic overview of the system are shown in Figure 4A. The basic structure of our soft robot has five DoFs, consisting of three DoF extension modules and two expansion modules. These air chambers are driven pneumatically by positive pressure, and each air chamber can be controlled independently. Specifically, in each air supply branch, the air pump (KVP04-1.1-12, Kamoer) connects with the corresponding solenoid valves. Each branch is further connected with one pressure sensor (XGZP6847-100-KP-G, CFSensor) and one air channel of the soft robot. The control strategy is shown in Figure 4B. The pneumatic flow is controlled through the microcontroller (Arduino Mega 1280) and computer. The computer receives the command from operators, processes the recording data, and communicates with the microcontroller. The control signal from the microcontroller and pressure feedback from sensors form a close-loop control. At last, the actual inner pressure is measured and dynamically regulated to reach the target pressure.

FIG. 4.

(A) The configuration of the robotic actuation system. (B) Block diagram of the control loop associated with a generic air supply branch. Color images are available online.

The sucking module can switch between the sucking state and release state by controlling the negative pressure. After two sucking modules (head sucking module and end sucking module) are mounted on the robot, the clamping capability of the robot can be enhanced when the module is triggered to suck the inner wall. The vacuum pump is used to generate the vacuum pressure and connects with the corresponding air branch of the sucking module. When the valve is close, the corresponding air branch is connected with the vacuum pump, and the module turns to sucking state. Analogously, when the valve is open, the corresponding air branch is connected with the atmosphere and the module reaches the atmospheric pressure. Thus, the head sucking module and the end sucking module can be alternately actuated.

Locomotion principle

Figure 5 shows the locomotion principle of the worm-like robot. The robot can move along the pipe by actuating the corresponding modules in a specific sequence. As shown in Figure 5, a branched pipe model is adopted to illustrate the flexibility. There are six phases in a complete motion sequence. Respectively, the internal pressure of air chambers is continuously monitored and used as the parameter to trigger the next phase. Two conditions are defined for each module as the actuation state (ON) or relaxing state (OFF). The modules are triggered sequentially from Phase (1) to Phase (6). Specific actuation step and time series are as follows:

FIG. 5.

Locomotion principle of the worm-like robot and the sequence of actuating the corresponding module. Color images are available online.

Phase (1): From T₀ to T₁, the end expansion module keeps the expansion state, the head expansion module is relaxed.

Phase (2): From T₁ to T₂, the end expansion module keeps the expansion state, the extension module bends and elongates simultaneously to adjust the desired heading direction and generate an extension stride.

Phase (3): From T₂ to T₃, the head expansion module expands to anchor the inner wall.

Phase (4): From T₃ to T₄, the head expansion module keeps the expansion state, the end expansion module is relaxed.

Phase (5): From T₄ to T₅, the extension module returns to the normal state.

Phase (6): From T₅ to T₆, the head expansion module keeps the expansion state, the end expansion module expands to anchor the inner wall.

The robot can move forward in tubular environments by repeating the Phase (1)∼Phase (6). In case of moving backward, the motion is reversed as Phase (6)∼Phase (1). When both sucking modules are mounted on the robot, they cooperate with the expansion module to suck the inner wall when the corresponding expansion module expands and release the inner wall when the corresponding expansion module is relaxed. Compared with the structure and locomotion principle of previous pneumatic worm-like robots,^29–36 our robot has two remarkable features. One is the multi-DoF extension module that enables the robot to adjust the heading direction actively and flexibly. Another is that the clamper with the combined adhesion principle enables the robot to be more adaptable. Moreover, multiple sensors or visualization unit can be deployed through the central channel. Operators can manipulate the robot through the actuation system under the real-time image navigation or position tracking.

Motion characteristics and force experiments

After prototype fabrication, we performed a pilot test on the bending, extension, and clamping motions. The results show that every single fluid tube can be controlled successfully, and the robot achieves bending motion and elongation freely in the 3D space. The safe air pressure is <80 kPa for the extension module and 50 kPa for the expansion modules.

The bending and elongation capability is the most important feature of the extension module. To evaluate how the robot responds to input air pressure, we record its posture through the electromagnetic (EM)-tracking system (3D Guidance tracking STAR, Ascension. Corp). The experimental setup is shown in Figure 6A, where the end of the robot is fixed on the platform. The EM sensor is placed on a cap, which connects with the robot's tip. The pressure of three chambers is controlled using the robotic actuation system. With different initial pressure combinations in the other two chambers (chamber2 and chamber3: 0, 20, 40 kPa), bending angle with chamber1 inflating is tested under pressures ranging from 0 to 80 kPa. Five trials are conducted to determine the repeatability. The elongation characteristic is also evaluated using the experimental setup when the three chambers are actuated under equal pressure changes.

FIG. 6.

Experimental setup of motion characteristics test. (A) The bending and elongation evaluation setup. (B) The expansion module expansion evaluation setup. Color images are available online.

The expansion module can dynamically adjust their radial dimensions by controlling the air pressure. In this way, two expansion modules grip the inner wall in the process of pushing forward and handle changes in the tube's diameter. Since the two expansion modules have similar characteristics, we measured the end expansion module to illustrate how the clamping diameter changes with air pressure. Figure 6B shows the experimental setup, where the robot is fixed vertically downward on the platform. A digital camera is set directly under the robot to capture the module inflation. Five trials are conducted to determine the repeatability. The diameter of the module is calculated in the captured image under different air pressures.

Figure 7A shows the nonlinear relationship between the bending angle and air pressure. We set the bending direction actuated by chamber1 as a positive value. The bending angle can reach 120°, which means that the robot can pass through tubes with a large elbow angle. In addition, since we apply the initial air pressures in two chambers (chamber2 and chamber3), the soft silicone body bends in the negative direction and elongates simultaneously. The bending angle reaches ∼0° when the pressure in chamber1 increases to the initial pressure. Although there is difference between the bending curves, the curves increase in a similar trend. The soft body of the robot elongates after applying the initial pressure, which changes the bending radius and causes difference between bending curves.

FIG. 7.

Experimental data for motion characteristics and clamping diameter. (A) The relationship of bending angle with air pressure. (B) The elongation performance when three chambers with the equal inner pressure. (C) The relationship between diameter of the expansion module and air pressure. Color images are available online.

The elongation motion in the axis direction is achieved as the internal pressures in three chambers are equally increased. As shown in Figure 7B, the extension module can elongate by ∼9 mm under 70 kPa.

The relation between the diameter of the expansion module and the air pressure is shown in Figure 7C. The result shows that the maximum expansion diameter under a pressure of 45 kPa is ∼28 mm. Considering the safety and the climbing efficiency, we set the pressure limitation, so that the expansion modules' diameter can increase up to 25 mm.

To enable the robot to successfully travel through the pipe, the force must be sufficiently strong to resist the friction force. Particularly, if it moves vertically with workloads, the generated force must be stronger than the weight of the robot, the friction force, and workloads. The clamping and the extension force of the extension module together determine the maximum workloads for the system.

We measured the typical extension force with the end expansion module under a clamping condition. Figure 8 shows the experimental setup to illustrate the extension force parameter. Specifically, the end expansion module is clamped to a pipe 17 mm in diameter. The extension force is measured using a force sensor, which is placed on the platform and connected with the robot tip. In this way, the robot extension is constrained. The generated force is measured with one chamber, two chambers, and three chambers inflation under different air pressures from 0 to 80 kPa.

FIG. 8.

Experimental setup for evaluating the generated force using the force sensor. Color images are available online.

Figure 9 shows the extension force in different pressure conditions. The measured force also represents how much of the workloads the robot can overcome in the moving process. One single chamber can generate a maximum of 3.5 N in relation to the input pressure. By actuating two and three chambers, the force reaches ∼4 and 4.5 N. We can control the air pressure to generate enough force for the robot to propel itself in different tubular environments.

FIG. 9.

The extension force measured by actuating one, two, and three chambers of the robot when the end expansion module is clamped. Color images are available online.

Verification of the robotic system

In the current prototype, the clampers with the basic expansion structure can stick to the pipes' inner wall varying from 15 to 25 mm by adjusting the radial expansion scales. We test our robotic system using a tubular phantom to illustrate its multi-DoF adjusting ability and feasibility. In the experiment, we use a multibranch tubular phantom with 17 mm diameter to evaluate the ability of our proposed soft robot. As shown in Figure 10A, the multibranch tubular phantom is composed of a straight PMMA tube and a 3D-printed disk with four tubular branches. Each branch is labeled on the end (U represents the upward branch, L represents the left branch, D represents the downward branch, and R represents the right branch). As illustrated above, the highlighted capabilities of moving forward and adjusting the heading direction are tested. As shown in Figures 10B and C, the robot can propel itself freely by clamping and extending sequentially. In Figure 10D and Supplementary Video S1, the robot can freely adjust the heading direction in 3D space by applying various pressure combinations in the extension module. Moreover, by using the visualization unit integrated with the soft robot, we can inspect the interior of the pipes and monitor the moving process.

FIG. 10.

Soft robot passing from the straight pipe to a multibranch tube. (A) The multibranch tubular phantom. (B) The sequence of end expansion module clamping, extension module extending, and head expansion module clamping. (C) The sequence of moving forward. (D) Adjusting the heading direction in the 3D space and the image inspection using visualization unit. Color images are available online.

Despite the expansion scale and stride length not being large, the design concept in this article provides a universal recipe for designing and fabricating the worm-like soft robot with more flexibility and capability of operating in the complicated tubular environments. According to different application requirements, it is convenient to adjust the specific parameters under the design concept and locomotion principle to adapt the environments. The inherent deformable and compatible nature of the soft robot gives itself an edge for complicated tasks that may be difficult for robots containing hard components.

Application of robot in multiple complicated tubular environments

The proposed robot might be robust for many applications, including inspecting pipes with multiple diameters and branches, multiple inner surfaces (dry, oil, or liquid-filled) and shapes (circular tube or square tube), as well as different applied fields (search, inspection, or medical field). Eventually, we tested our proposed robot in the application experiments.

To evaluate the robot's capability of propelling itself in pipes, multiple circular tubes with different diameters (17 and 20 mm) and tubes with elbow in the vertical plane are used. Figure 11A and Supplementary Video S2 show the moving process. When the robot works in pipes under 25 mm, the expansion module can efficiently stick to the inner wall. Figure 11B and Supplementary Video S3 show a demonstration of the robot turning a corner. The high clamping force enables our robot to move forward stably and efficiently for any placement angles.

FIG. 11.

Demonstrations of the worm-like robot in multiple tubular enviroments and applications. (A) The robot moves forward in pipes with different diameters. (B) The robot turns around a corner. (C) The robot moves vertically in the multibranch pipe. Color images are available online.

Some applications of this robot (e.g., searching or inspection) might be complex branches. The robot with a multi-DoF extension module has great adaptability to travel through pipes with multiple branches. By integrating the visualization unit, the real-time image in the moving process can be captured and visualized on the computer screen. The viewing spot of the visualization unit follows the robot tip, which provides the local internal information of the pipes. Figure 11C and Supplementary Video S4 show that the proposed robot can travel through the branched pipe. Inspectors can see the inner condition of the pipe on the screen and manipulate the robot by adjusting the heading directions to choose the desired branches under the real-time image guidance.

As illustrated in the force experiments, the robot has the ability to lift barriers. Figure 12A and Supplementary Video S5 show the pilot test to operate under a load of 157.3 g, which is >10 times its own weight (14.5 g). In the climbing process, the speed with the workload is slower than the speed when there is no workload.

FIG. 12.

Demonstrations of the worm-like robot in multiple tubular enviroments and applications. (A) The robot moves vertically with a workload of ∼157 g. (B) The robot moves vertically in oil condition and underwater condition. (C) The robot moves in the noncircular tubes under the image inspection using visualization unit on the tip. Color images are available online.

The robot can also work in liquid-filled tubes, oil conditions, or submerged underwater condition. Different tubular environments are used: tubes with a water column, an oil tube, and a tube submerged underwater. Figure 12B and Supplementary Video S6 show the perfect performance in such demanding environments. The robot can overcome the resistance of water when it moves in the water condition. The moving speed and performance are apparently not affected. If the tube is coated in oil, the robot can move forward with a lower speed. The robot suffers from the slipping phenomenon in the moving process. The oil film between the clampers and tube affects the clamping force and reduces the load-bearing capacity.

In addition, this kind of robot is compatible for noncircular tubes. As is shown in Figure 12C and Supplementary Video S7, the movement experiment is tested using a square tube, a D-shaped tube, and a Δ-shaped tube, which are printed using a 3D printer. It is very effective to illustrate the climbing ability of our proposed robot.

The robot can stably and flexibly move through various complicated pipes. Through the phantom experiments using PMMA/PVC pipes, the soft robot concept proposed in this article has potential in a variety of applications. The key point is that the bending and adjustment in the tube are more flexible and robust since the robot is completely soft. It would be more difficult to achieve if rigid mechanisms were used.

Application of robot with multiple sensors integration

The experiments above have illustrated that the worm-like robot can pass through various complicated tubular environments. In practical applications, in general, operators would suffer from occlusion region problems when the robot moves deep into the pipeline. Despite the visualization unit can be mounted on the robot tip to capture the inside image of the pipes, however, the image is just a local information. Especially when the robot works in the multibranch pipe, it is hard for operators to identify the robot location according to the local image information and very easy to get lost. To address this challenge, the tracking sensor can be integrated on the soft robot to track the position of robot.

In this experiment, a hybrid optical and EM-tracking method is explored. Based on the proposed structure and fabrication process, we fabricate a new prototype with a larger central channel to integrate the tracking devices. As shown in Figure 13, a visualization unit and a 6DoF EM sensor are mounted on the robot tip. A multibranched phantom is used to illustrate the effectiveness of integrating multiple sensors for intuitive robot tracking. We also establish a navigation system to track the locomotion process of the robot. A navigation software is developed to visualize the virtual image of the navigation process onto a screen. The real-time locomotion and navigation process are shown in Figure 13 and Supplementary Video S8. The robot is tracked after the sensor is registered. In the locomotion process, the visualization unit can obtain the real-time local image of inner pipe, and the EM sensor can track the global position information of the robot. This hybrid navigation method can provide the global and local information, which provides more navigation information to operators. Thus, operators can manipulate the robot to the desired branch under the navigation information. The navigation system enables the robot to have a better adaptability to travel through the complicated pipes with branches. Moreover, the quantified data recorded by the visualization unit or other sensors are the basis of realizing automatic robot navigation in the future.

FIG. 13.

Application of robot with an EM-tracking probe integration and the locomotion under the EM-tracking system and image guidance. EM, electromagnetic. Color images are available online.

Application of robot with the sucking module integration

As mentioned above, the stroke loss would be generated by the deformation of the elastic materials or the low friction coefficient of the slippery surface. The expansion clamping principle is not applicable when the robot moves in pipes with wet, slippery, or high-elastic surfaces. To address this challenge, the clamper with the combined adhesion principle is proposed. The sucking module can generate a vacuum pressure to suck the inner wall when the expansion module expands and release from the surface when the expansion module is relaxed. In this way, the clamping force can be strengthened to avoid the stroke loss. As shown in Figure 14 and Supplementary Video S9, we attach the sucking module on both expansion modules. For realizing a practical manipulation application, a visualization unit is mounted on the robot tip, and a biopsy forceps is deployed through the central channel of the robot. A high-elastic silicone phantom is used in this experiment. Experimental result confirms that the combined adhesion principle can overcome the stroke loss and reduce the deformation of soft materials. Moreover, the locomotion process and manipulation process can be guided by the real-time image of the visualization unit. This shows the potential for application in the colonoscopy procedure.

FIG. 14.

The robot with the sucking module moves in the super elastic tube and manipulates under the image inspection using visualization unit on the tip. Color images are available online.

Conclusion and Future Work

This article proposed the design concept of a flexible and robust worm-like soft robot for complicated tubular environments. Due to the complicated inner structure, the RCC method using retractable pin and sequent casting molds is applied. We characterized the soft robot and verified the whole system. The motion characteristics and force experiments show that it can simultaneously bend freely in the 3D workspace and elongate its soft body. The generated force will be sufficient for lifting the weight or moving over obstacles. Various applications illustrate that our proposed robot prototype will be more flexible and adaptable for multiple unstructured tubular environments.

More importantly, it should be pointed out that the robot described here has the following potential characteristics: (1) the entire soft body is fabricated using soft materials, which is entirely soft and lightweight (thus it can move smoothly. For medical application, it can reduce harm to the intestine and can improve the compatibility with the human body by changing suitable materials); (2) the robot can tolerate changes in the tubes diameter, tubes direction, and even complicated tube branches (how tolerant it is depends on the detailed parameter design and elasticity of materials. The motion characteristics and generated force can be further improved by optimizing the structure design); (3) it is adaptable to the complicated 3D or multibranch tubular environments by designing the multi-DoF extension module; and (4) it is compatible with multiple inner surfaces and shapes (e.g., air tubes, oil tubes, liquid-filled tubes, square tubes, or soft tubes). The good performance of real-time image navigation, the combined adhesion principle, and multi-DoF adjusting ability, as well as the workloads resisting capability, are essential for future applications (such as the search, inspection, and medical fields).

In the future, we will overcome the challenges above for a more practical robot. It is necessary to integrate more sensors or manipulation modules such as CCD, forceps, or laser fiber to enrich the sensing and manipulation abilities. In addition, in practical applications, the air pressure would be influenced by many factors, and is not enough to estimate the clamping force or bending characteristics. The soft sensors using electrical skin materials, such as liquid metal, semiconductors, or carbon nanotubes, will be explored and integrated to directly evaluate the force and bending characteristics. Furthermore, by integrating more than one extension modules in series between two clampers or integrating more clampers and extension modules, the flexibility and generated force will be further improved. The safety and sterilization of the robot should be additionally considered in clinical applications. The soft protection sheath will be applied to overlay the robot for protecting the air champers. The sheath would also be convenient for sterilization in the form of a disposable sterilized sheath.

Footnotes

Acknowledgments

The authors acknowledge support from National Key Research and Development Program of China (2017YFC0108000), National Natural Science Foundation of China (81427803, 81771940), Beijing Municipal Science & Technology Commission (Z151100003915079), and Beijing National Science Foundation (7172122, L172003).

Author Disclosure Statement

No competing financial interests exist.

Supplementary Material

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Supplementary Material

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