MorphGI: A Self-Propelling Soft Robotic Endoscope Through Morphing Shape

Abstract

Colonoscopy is currently the best method for detecting bowel cancer, but fundamental design and construction have not changed significantly in decades. Conventional colonoscope (CC) is difficult to maneuver and can lead to pain with a risk of damaging the bowel due to its rigidity. We present the MorphGI, a robotic endoscope system that is self-propelling and made of soft material, thus easy to operate and inherently safe to patient. After verifying kinematic control of the distal bending segment, the system was evaluated in: a benchtop colon simulator, using multiple colon configurations; a colon simulator with force sensors; and surgically removed pig colon tissue. In the colon simulator, the MorphGI completed a colonoscopy in an average of 10.84 min. The MorphGI showed an average of 77% and 62% reduction in peak forces compared to a CC in high- and low-stiffness modes, respectively. Self-propulsion was demonstrated in the excised tissue test but not in the live pig test, due to anatomical differences between pig and human colons. This work demonstrates the core features of MorphGI.

Introduction

Colorectal (bowel) cancer alone accounts for ∼10% of all cancer cases worldwide. Early detection carries a 5-year survival rate of around 90%, however, late detection carries a survival rate of only 5%.¹ Regular colonoscopy is the gold standard for both detection and treatment of bowel cancer.² Approximately 16 million colonoscopies are undertaken in the United States and United Kingdom combined per year.³

Conventional colonoscope (CC) is a long flexible tube with camera and lighting mounted at the end. It is inserted into the patient through the anus to examine the colon, detect abnormalities, perform biopsies, and remove polyps. Due to inability of CC to passively morph its shape intraoperative, there is risk of significantly stretching the colon and surrounding organs. This can lead to pain and colon wall scaring or perforation.^4,5 As a result, the patient might be sedated and endoscopists undergo a lot of training to maneuver the colonoscope through the highly torturous, narrow, and compliant bowel. Studies have referenced fear of pain as a major patient concern.⁶

Fundamental colonoscope design has not changed significantly in decades.⁷ To solve these problems, many companies and researchers have looked to change the tool with which colonoscopy is performed.^8–20 Two major areas have seen development recently: designing ultraflexible colonoscopes using compliant materials²¹ such as silicone and imbuing them with bioinspired self-propulsion technology.^22–49

Self-propulsion is desirable in long and ultraflexible probes since external manual force transmission is limited due to buckling tendency. Regarding safety, self-propulsion colonoscope could lower the forces exerted on the colon wall.⁸ As a result, perforation is even less likely. Perforations occur between 0.045% and 0.070% of diagnostic colonoscopies^4,50,51 and the ramifications can be serious, with over 50% of perforation cases requiring surgery.⁵¹ Given the scale of colonoscopy screening programs, reducing such complications could obviate thousands of surgeries annually. In addition, a self-propelled colonoscope facilitates easier maneuverability, requiring less clinician skill, and expanding colonoscopy access globally, including in developing regions.

There are several examples of systems that rely on bioinspired motion to achieve self-propulsion. Peristaltic motion of earth-^52,53 or inch-worms^8,32,54–61 is among the most popular locomotion technique used for colonoscope.⁶² It relies on sections of the device alternatingly being anchored, through large contact friction areas, while other sections move relative to these anchoring points. A large contact area reduces contact forces, thus, intubation can be achieved without significant stretching of the colon, that normally causes damage or pain to patients.

Earth-worm endoscopes achieve peristaltic motion through unique actuation mechanisms, such as balloons,^57,63–65 clamping,^9,66 suction,⁶⁴ or linkages,^67,68 that are sequentially controlled. This requirement increases the controller size, complexity, and overall manufacturing cost.⁶² On the contrary, inch-worm endoscopes achieve anisotropic friction using their main body parts, which leads to simple sequential control and more cost-effective design. One approach studied by researchers is using multiple specially designed segments to mimic the “Ω” deformation of the inchworm.^69–71 Omega-anchoring locomotion, however, highly relies on the careful design of rear and front foot. As the “Ω” deformation is achieved, the contact surface of rear and front foot with environment is reduced increasing probability of back-slippage. It is for these reasons that dimensions of rear and front foot are either larger than the main body or require unique mechanisms to enhance friction. In addition, bending of such segments can only be induced in 1 degree-of-freedom (DOF).

Compared to numerous actuation principles,⁷² for instance Tendon-driven^73,74 or Smart Materials (shape memory alloys),⁷⁵ fluidic pressure-drive actuators have been proven to be the best candidate with the highest versatility in the medical field.⁷⁶ Advances in fluidic actuators research can allow MRI-compatible endoscopes to be developed, which can solve the endoscope localization problem in area such as neurosurgery.⁷⁷ Several pneumatic robots have been developed for the use in colonoscopy procedures using peristaltic motion.^69,71 However, compressibility of air makes control difficult, limits bending, and carries a risk of damaging tissue should the device fail.

To address the above challenges, we propose, an inchworm-like, self-propelled, hydraulically actuated endoscope named MorphGI (Fig. 1 and Supplementary Video S1 in Supplementary Data). The design incorporates an instrument channel, an essential aspect in colonoscopy, which is currently missing from existing inchworm-like colonoscopy robots, due to lack of sufficient hollow spaces.⁷³

FIG. 1.

Concept illustration of the MorphGI colonoscopy system. This illustrates the fundamental components of the MorphGI system. The probe is a passive device with an end-mounted camera and serves the same function as a conventional colonoscope. The probe is connected to the control unit, which processes signals from the camera and actuates the probe by injecting liquid through tubes into chambers within the probe's structure. A clinician controls these movements using a handheld controller and monitors the cameras output on the monitor.

The main contributions of MorphGI to the field of soft robotics and colonoscopy are summarized below: 1.

A novel bending-anchoring method which provides a dual-functionality to the front segment. To control direction of the tip of the soft endoscope and to effectively anchor to the colon by morphing its shape to gently “pitch” the colon wall, without the need of any unique parts (balloons, clamping, or suction mechanisms). This method reduces cost, manufacturing, and operational complexity (see Supplementary Notes S4 and S5).

Controlling multiple soft actuators to bend, extent, or contract in a particular pattern using fluidic pressure-drive can effectively achieve self-propulsion with 62–77% reduction of the interaction force to the colon wall. This demonstrates the potential clinical benefit for reducing perforation rate of colonoscopy.

A novel steerable soft segment design by placing inextensible chord in the central conduit to limit extension and increase bending range of the segment by ≃16%. A maximum bending angle of 210° can be achieved, which is higher than existing conventional designs.⁷⁸

Materials and Methods

The MorphGI system

The system proposed here consists of three individual segments: a front and rear segment which can bend in 3 DOF and one drive segment which can only extend/contract (1 DOF). The complaint probe is equipped with a Ø 3.2 mm instrument channel which is compatible with standard endoscopic biopsy tools (Fig. 2A–C).

FIG. 2.

Overview of the MorphGI prototype and its functionality. (A) Overview of the MorphGI probe, with inset describing the design of the tip. (B) Demonstration of the flexibility of the probe. (C) Demonstration of the high angulation capabilities of the probe. (D) Diagram of the self-propulsion method. (E) Pictures taken of the prototype during self-propulsion. (F) The prototype control unit. (G) Hydraulic circuit of single actuation chamber.

Locomotion is achieved by a technique known as bending-anchoring inspired by inchworms (Fig. 2D–E). This involves bending the front or rear segment to such a degree that it either wedges itself in place between two opposing surfaces, omega-anchoring, or grips onto an edge (hook-anchoring). The drive segment then extends or contracts to move the rest of the probe relative to this anchor point. The primary advantage of this is that the front segment can be used to perform two functions: to perform anchoring or control the direction of the tip using the handheld-controller.

The hydraulic circuit and electrical components of a single actuation stage of MorphGI is depicted in Figure 2F and G. The syringe pump stimulates the segment by injecting liquid, via route 1-Figure 2G. Route 2-Figure 2G, is used to initially fill in the empty syringes with distilled water and to degas air from the system during the “flushing state.” Our laboratory's degassing technique stimulates the segment with square-pattern pressure-pulses of liquid. Due to air's buoyancy, any trapped air flows back to the syringe. After each cycle, the syringe's content will be dispensed into the water reservoir via route 2, releasing air to the environment and refilling syringes with distilled water. Water is filtered through a 15 μm strainer (Masterflex, Strainer Gasket), to avoid debris going into the system that will affect control models and patient safety. More details in Supplementary Note 7.

All national and institutional guidelines for the care and use of animal parts were followed.

Probe design

Based on the colon morphology^79,80 of an average human, colonoscopes should have total length around 1.6 m and maximum diameter of 20 mm and limit force exertion to enable comfortable steering and passage of total colon lumen length.⁷ The MorphGI probe is manufactured from medical grade soft material, has a total length of 1.534 m, and 17.5 mm outer diameter.

A single segment is constructed using silicone rubber with rigid plastic parts attached on either end (Fig. 3). Shore A-20 silicone was selected as it is a widely used rubber in soft robotics.⁸¹ Its mechanical properties promise durability and require lower operating pressure levels. Comparative studies of more silicone rubber agents can be found here.⁸² The molds for all segments have the same structural design except for the shell length (Supplementary Fig. S3).

FIG. 3.

Probe design. (A) A computer-generated model of the design of a single front segment. The cut-away view shows how springs are used to support the four conduits and how the outer constraint is embedded within the silicone rubber body. The cross-sectional view shows the total outer diameter and how the actuation chambers and conduits are arranged. (B) An illustration of the completed probe. All units are in millimeters.

Cavities are integrated into the rubber core of segments that serve as both actuation chambers and conduits. The three actuation chambers and 3-out-of-4 conduits are symmetrically aligned in a circular pattern across the geometrical center in an off-axis distance. Conduits allow passage of tubes, cables, and an instrument channel. To prevent radial collapse during actuation, steel wound springs were inserted into these conduits postmolding. Thus, the complete MorphGI probe consists of nine individual actuation chambers.

A structural constraint which prevents radial expansion of the actuation chambers and the elastic material is critical to the performance of the device. A bellows-like structure was integrated into the rubber core, allowing for a smooth outer surface, fabricated from a PET mesh sleeve woven because of its favorable stiffness properties.⁸³ More details in Supplementary Note S1.

Kinematics

To control the probe, two sets of kinematics are required: extension and bending as shown in Figure 4. If the actuation chambers are infused with equal volume of fluid $(V_{w})$ , the segment elongates. Alternatively, if infused with different V_w, the segment bends. An analytical approach can be used to predict the relationship of segment's length (S) against V_w. The model assumes that (1) volume within the mesh sleeve is conserved. The thin layer of rubber outside the mesh sleeve is neglected from the analytical model because it is not technically constrained, as its thickness can vary during extension. (2) The diameter of mesh sleeve $(D_{c})$ inserted into conduits will be fixed and (3) silicone is an incompressible material. Thus, volume of a segment for any S can be written as:

FIG. 4.

Kinematics of the MorphGI segment. (A) Extension kinematics. (B) (Left) This represents a cross-sectional view taken along the bending plane B ′. (Centre) General case of the segment bending about an eccentric bending axis. (Right) The special case where bending occurs along φ = 0.

V_{r} + l \cdot \sum a_{c} + V_{w} = \frac{π}{4} \cdot D_{c}^{2} \cdot l,

(1)

where V_r is rubber's volume and $\sum a_{c}$ is the sum of the conduits area. Elongation ( $Δ S$ ) can be obtained by differentiating [Eq. (1)] with respect to V_w and rearranging leading to: $Δ S = \frac{1}{\frac{π}{4} \cdot D_{c}^{2} - \sum a_{c}} \cdot Δ V_{w} = \frac{1}{A_{x s}} \cdot Δ V_{w},$ (2)

According to kinematic for continuum robots,^84,85 the pose of the front segment is described using two variables: $φ$ and $θ$ , indicating the rotation and bending angle, respectively (Fig. 4B). Rotation angle can be calculated as: $φ = a t a n 2 [\sqrt{3} \cdot (Δ V_{w, 2} - Δ V_{w, 3}), (2 Δ V_{w, 1} - Δ V_{w, 2} - Δ V_{w, 3})],$ (4)

where $Δ V_{w, i}$ represents the volume of fluid injected into the ith-actuation chamber. For the bending angle, tailored kinematics are required since the actuation method is only capable of actively elongating a chamber and must passively “allow” it to shorten. Analogs to cantilever beam mechanics, the location, and length of the neutral axis arc can be assumed to remain constant $(S_{N A})$ . There are two additional key arc lengths: the length of the ith-actuation chamber centerline S_i and the segment's geometric centerline (S′). There are then two radial distances: $d_{i}^{'}$ and e, which defined the separation between $S_{i} - S'$ and $S' - S_{N A}$ , respectively. Thus, S_i can be written as: $S_{i} = θ d_{i}^{'} + θ e + S_{N A},$ (3)

with the general expression for $d_{i}^{'} = d cos (φ - δ_{i})$ where $δ_{i}$ = [0, $\frac{2 π}{3}$ , $\frac{4 π}{3}$ ] rad represent the location for the three actuation chambers and d the $d_{i}^{'}$ at ϕ = 0. Thus, if the required change in length to achieve bending is $Δ S = S_{i} - S_{N A}$ , substituting Equation (2) into (3) and rearranging leads to

More details in Supplementary Note S3.

Experiments

The MorphGI system is evaluated by using multiple experimental setups, including buckling experiments (Supplementary Note S7), benchtop simulators, and porcine ex vivo tests. This was necessary because short of a human trial, there is no perfect model for the human colon. To validate kinematic model, the segment was infused with fluid while extension or bending was recorded. For the axial kinematics, a total of V_w = 12 mL was injected in all three actuation chambers resulting in a total segment length of 140 mm ( $Δ S$ = 70 mm). For bending, deflection angle was recorded while infusing fluid in one or two actuation chambers. Benchtop simulators allow investigation of how different configurations of the colon may influence intubation time for the device. To evaluate the safety of the device, the forces exerted by the device on a simulated colon will also be measured and compared to measurements taken with CCs.

Results

Kinematic validation

The results of kinematic validation experiments are shown in Figure 5A–C. Initially, the rate of axial extension per unit of injected fluid is lower, attributed to compressibility of trapped air within the actuation system damping response. As fluid continues to be infused to build up pressure, the rate of air's volume asymptotically approaches zero making the extension response faster and linear. As a result, trapped air manifests as a volumetric offset $(V_{o f f, e x t} = 2.5 m L)$ in the extension data and is considered as preactuation stage, which is excluded from the curve fitting attempt. Thus, $Δ V_{w}$ in Equations (2) and (4) become $Δ V_{w} = (V_{w} - V_{o f f, e x t})$ . The axial kinematics model produces a root-mean-squared error (RMSE) of 1.923 mm, or 2.67% of the full-scale extension.

FIG. 5.

Experimental validation. (A) Elongation of segment as a function of injected volume. (B) Measured bending angles as a function of injected volume in a single or two chambers with kinematic model prediction. Data for the preactuation state are omitted. (C) Images of the segment under various bending conditions with either no constraint added or the 93 mm constraint.

The front segment can reach large bending angles of around θ = 180°, Figure 5B and C, and the model of Equation (4) result in RMSE of 1.387°. There is some over prediction of the bending angle when only a single chamber is actuated. To limit elongation and further promote bending a flexible but inextensible plastic chord of 93 mm length was installed in the central conduit (Fig. 4C) (Supplementary Note S2). This increased the maximum bending angle to θ = 210° as shown in Figure 5E.

Colon simulator

Benchtop evaluations were performed using a colonoscopy simulator (Kyoto Kagaku, Japan) with propylene glycol-based lubrication. This consists of a rigid plastic torso with a Velcro-lined base and a rubber colon. Small loops are available which allow fixturing of the colon against the base. Experimental setup is illustrated in Supplementary Figure S8.

Colon tortuosity is cited as a reason for incomplete colonoscopy⁸⁶ and accounted for 30% of all incomplete examinations in one study.⁸⁷ For any self-propelling colonoscope, this varying difficulty should be addressed. Only one example of existing work has attempted to evaluate a self-propelling colonoscope in different configurations.⁸⁸ The configurations were based on the supplied manual for the colonoscopy simulator, aimed at inducing specific colonoscope configurations.

For our case, CT colonography scans of 43 patients in the supine position (available online: Cancer Imaging Archive⁸⁹) were manually segmented using open-source software (https://www.slicer.org/), leading to four distinct types of colon configurations (Fig. 6A) (see Supplementary Note S8). Our categorization attempt matches other colon categorization efforts of colorectal morphometrics.⁷⁹ MorphGI was evaluated in Types I–III. Type IV was omitted because it was difficult to reproduce in the simulator (Fig. 6B).

FIG. 6.

Results of the colon simulator experiments. (A) Representative CT scans of the four identified configuration types. (B) Examples of how the colon simulator was arranged to replicate each of the three colon configurations. Water-filled bags are used to simulate surrounding organs and tissue, such as the small intestine. (C) Results of the simulated colon experiments. The red line indicates the median, and the blue box represents the interquartile range.

In total, 19 separate procedures were performed, and cecal intubation time (CIT) was recorded. Results are found in Figure 6C and Supplementary Table S1. The Type I configuration was the quickest, taking a mean of 8.66 min to complete. This was followed by the redundant sigmoid, at a mean of 10.60 min and the redundant transverse, at 13.64 min. Overall, the mean CIT for MorphGI was 10.84 min.

Force sensing colon simulator

A significant advantage of using a soft robotic colonoscope with self-propulsion is to potentially reduce the forces exerted on the colon. To validate this, a force sensing colon simulator (FSCS) was arranged in the Type I colon configuration, as shown in Figure 7A.

FIG. 7.

Results from the FSCS. (A) The FSCS. (B) Illustration of the various phases used to analyze the force data. (C) Raw force data from the sigmoid and descending colon force sensors. (D, E) Charts of the mean peak forces observed during each phase for each colonoscope type in the sigmoid (D) and descending colon (E) sensors. (F) A detailed view of the results of the sigmoid forces during phase 3. (G) A detailed view of the descending forces during phase 4. In all of the above, error bars represent the standard deviation. CC-HS, conventional colonoscope—high stiffness; CC-LS, conventional colonoscope—low stiffness; FSCS, force sensing colon simulator.

The FSCS was equipped with force sensors at five landmarks of colon anatomy: the sigmoid, descending colon, splenic flexure, hepatic flexure, and ascending colon. The sensors were connected to the colon via an elastic chord, to mimic the mesentery - the connective tissue that binds the colon together.⁹⁰ The colon's mobility ensures force registration only when elastic chords are under tension. Colonoscopies were carried out using a CC and the MorphGI by Dr. Bu Hayee.

Three devices were tested: CC in high stiffness mode (CC-HS); CC in low stiffness mode (CC-LS); and the MorphGI probe. The CC used was a FUJIFILM EC-760ZP-V/L with outer diameter Ø 11.8 mm. The stiffness mode of CC can be adjusted using a switch onside. For each device the colonoscopy procedure was repeated three times.

To facilitate comparison between force measurements from different experimental procedures, the colonoscopy was divided into five discreet phases (Fig. 7B), corresponding to the location of the scope's tip. Phase 1-tip is traversing the rectum and sigmoid. Phases 2–4 correspond to the descending, transverse, and ascending colon, respectively, and phase 5 represents withdrawal. In addition, since CIT was not constant, the force measurements' time component was normalized to a range between 0 (start of phase 1) and 1 (end of phase 5).

Some force measurement results are presented in Figure 7C. The sensors at the splenic flexure, hepatic flexures, and ascending colon did not register any significant forces (Supplementary Fig. S1). The peak forces recorded for each device were averaged across all test repetitions, looking at each phase individually. Force data from sensors located at the sigmoid and descending colons are presented in Figure 7D and E, respectively.

Two notable phases showed statistically significant differences in mean peak forces: phase 3 from the sigmoid sensor and phase 4 from the descending colon sensor. These results were examined in greater detail in Figure 7F and G, respectively (with the p-values⁹¹ provided in Supplementary Table S3). Statistically significant differences were present in other phases, but the force levels were <2N. In the sigmoid during phase 3, the MorphGI probe and CC-HS both demonstrated a statistical difference in mean peak force. However, the forces were both very low. In phase 4, the difference between all three devices was statistically significant in the descending colon. Furthermore, the peak forces exerted by CC-HS were around 8N. Finally, during withdrawal, all three devices exerted low forces on all sensors.

As indicated in Figure 7E, the peak forces recorded from sensor in the descending colon consistently occur during phase 4 of the examination. This is true for all devices tested. In general, the recorded force profile at different sections of colon during the study agrees with other tests in the literature, which reported a similar distribution of forces.^92,93

Calculating the percentage difference on average peak forces, MorphGI produced on average a 77% reduction in average peak force compared to CC-HS and a 62% reduction compared to CC-LS.

Ex vivo animal tissue test

To evaluate self-propulsion in more realistic environments, a section of porcine colon was arranged into the rough shape of a human colon (Fig. 8A), equivalent to the Type I colon configuration. In the test, the MorphGI probe was first manually inserted through the opening at the base of the case. The controllable part of MorphGI probe was advanced manually into the colon (Supplementary Video S2). Self-propulsion was then initiated using the hand-held controller. The intestinal folds (haustra) were present in this section of the pig's colon (Fig. 8B). As a result, the MorphGI anchoring mechanism was able to find purchase and successfully reached the end of the excised tissue in 9.42 min.

FIG. 8.

Results from the ex vivo experiments. (A) The MorphGI system having reached the end of the ex vivo pig colon. (B) Image of the MorphGI camera view during the ex vivo test. Haustral folds are clearly visible.

Discussion

Improving kinematics

Trapped air in microfluidic systems significantly hinders accuracy and performance.⁹⁴ Our laboratory technique used to degas segments is imperfect given the tube's inner diameter causing high friction between liquid and tube wall.⁹⁵ Incorporating fluid mechanics modeling will allow inaccuracies from trapped air to be predicted. A hardware solution might be to equip the water reservoir with a vacuum pump to attract more air bubbles.

Overprediction of model's bending angle when a single chamber is actuated is due to lower actuation forces. The axial kinematics were developed using data when all three actuation chambers were activated. Stress-strain behavior varies depending on the number of actuation chambers and of course the use of the inextensible chord. Thus, mechanical modeling of segments and chord is important for further improving kinematics, bending performance, allow force sensing, and optimizing the probe design. Anchoring could be achieved solely by the front segment.

Colon simulator and ex vivo test

MorphGI successfully traversed all three colon simulator configurations with a mean CIT of 10.84 min. Dr. Bu CIT were on average 2 min using both CC-HS and CC-LS, comparable to CIT reported in similar test environment and colon configurations.⁹⁶ Experience of Dr Bu coupled with external visual feedback and simplicity of colon configuration suffice the near five times difference.

Results of MorphGI's CIT are comparable with mean CIT reported in literature (9.8 ± 6.8 min),⁹⁷ illustrating its feasibility to replace CC. However, given the many factors affecting real-life CIT, such as age, body mass index, and bowel cleansing,^97,98 it is difficult to make a one-to-one comparison with a simulator or ex vivo study. This is reinforced by variations of our CIT based on colon configuration. Notably, the analyzed CT images did not show commonly encountered colonoscopy “loops” like sigmoid alpha/reverse alpha and transverse gamma loops.⁹⁹ This suggests that these loops are likely induced by colonoscope insertion rather than being naturally occurring. To enhance the classification method, consider incorporating factors like colorectal anatomy range, tortuosity, and section length. Relevant investigation studies provide methods for this analysis.^79,100

Finally, since the probe is self-propelled, we hypothesized that the experience of clinician will not adversely affect CIT. The inverse correlation between clinician's experience and CIT is reported in literature,^97,101 so tests will be performed in the future from clinicians with different experience levels.

Experiments encountered two issues: insufficient progression of the front segment during extension and slipping backward during contraction of the drive segment. The first problem occurred at sharp bends, particularly the hepatic and splenic flexures. The operator overcame it by twisting the tether slightly to change the probe orientation. Given that both are made of rubber, friction between the probe and the colon simulator was a significant factor despite the presence of lubrication. Friction during ex vivo tissue test was lower, favoring MorphGI self-propulsion. Cadaver trials are more suitable for evaluating self-propulsion effectiveness.¹⁰²

The second issue arose in the ascending colon and cecum, where the front segment slipped backward between haustral folds during drive segment's contraction. As the cecum is the point of deepest insertion, increased friction on the probe's body is experienced due to the increased surface area in contact with the colon. This friction surpassed the maximum anchoring force of the front segment due to mechanical interaction with the colon. Local geometry and mechanical properties of the colon likely influenced this limit. While studies have shown little variation in elastic modulus of human rectal tissue,¹⁰³ tensile properties of human colon tissue variation remain unexplored. Further research can focus on understanding how local factors impact anchoring performance to optimize front segment design and anchoring pose.

Force sensing colon simulator

The minimum ultimate tensile strength of the human colon is about 0.66 MPa.¹⁰³ Considering a contacting area of 100 mm² from the ≃240 mm² of MorphGI's tip (safety factor: 2.4), the threshold force to induce perforation can be calculated as ≃66 N. The MorphGI's average peak forces remained both well below this threshold and relatively consistent throughout all phases. Similar distribution of forces with no significant forces at the flexures and in the ascending colon was reported in similar experiments.⁹³ This is attributed to its self-propulsion system, generating substantial contact areas. The distance between the colonoscope tip and the point of applied force (i.e., the rear anchor) remains relatively small throughout the procedure, avoiding buckling.

In addition, the significant reduction in forces even during anchoring is expected to significantly decrease further harmful events related to colonoscopy, such as bleeding due to mucosa damage.¹⁰⁴ To guarantee this, a pressure-based intrinsic force sensing model will be attempted in the future.¹⁰⁵

The increased friction observed in the colon simulator was believed to have had an influence on all three sets of experiments. As the conditions were the same across all three experiment groups, however, it is not believed that any bias was present in favor of any one device. If anything, the high-quality coating applied to the CCs would have produced less friction than the MorphGI.

The study did not examine the influence of different users performing the same task. Other studies with robotic colonoscopy platforms have done so and found that the user's experience level plays a significant part in performance.¹⁰⁶ Taking a broader view, a future study could endeavor to combine the two colon simulator experiments to monitor both CITs and force exertion simultaneously.

Conclusion

This work has attempted to provide new solutions to the aim of safe and effective colonoscopy. With respect to the soft robot design, contributions were made surrounding the development of a novel inchworm inspired propulsion method. Compared to many other works^{57,63,64,66–68,73} the proposed method is significantly simpler and easier to implement. The effectiveness of this method has been evaluated on several different colon configurations in a simulated rubber colon and in ex vivo animal tests. A comparative study of insertion forces for both different CC stiffness and a self-propelling colonoscope has been conducted and demonstrated the effectiveness of the proposed method.

Footnotes

Acknowledgment

The authors acknowledge the Griffin Institute (NPIMR, Northwick Park) for the use of their animal testing facilities.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

This research was supported by Engineering and Physical Sciences Research Council (EPSRC) under Grant No. EP/R013977/1 (H.L. and B.H.) and InnoHK programme.

Supplementary Material

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