Kinematics, dynamics and control of stiffness-tunable soft robots

Modeling and control methods for stiffness-tunable soft robots (STSRs) have received less attention compared to standard soft robots. A major challenge in controlling STSRs is their infinite degrees of freedom, similar to standard soft robots. In this paper, demonstrate a novel STSR by combing a soft-rigid hybrid spine-mimicking actuator with a stiffness-tunable module. Additionally, we introduce a new kinematic and dynamic modeling methodology for the proposed STSR. Based on the STSR characteristics, we model it as a series of PRP segments, each composed of two prismatic joints(P) and one revolute joint(R). This method is simpler, more generalizable, and more computationally efficient than existing approaches. We also design a multi-input multi-output (MIMO) controller that directly adjusts the pressure of the STSR's three pneumatic chambers to precisely control its posture. Both the novel modeling methodology and MIMO control system are implemented and validated on the proposed STSR prototype.

Stiffness-Tuneable Segment for Continuum Soft Robots with Vertebrae

In addition to high compliance to unstructured environments, soft robots can be further improved to gain the advantages of rigid robots by increasing stiffness. Indeed, realizing the adjustable stiffness of soft continuum robots can provide safer interactions with objects and greatly expand their application range. To address the above situation, we propose a tubular stiffening segment based on layer jamming. It can temporarily increase the stiffness of the soft robot in a desired configuration. Furthermore, we also present a spine-inspired soft robot that can provide support in tubular segments to prevent buckling. Theoretical analysis was conducted to predict the stiffness variation of the robot at different vacuum levels. Finally, we integrated the spine-inspired soft robot and tubular stiffening segment to obtain the tuneable-stiffness soft continuum robot (TSCR). Experimental tests were performed to evaluate the robot’s shape control and stiffness tuning effectiveness. Experimental results showed that the bending stiffness of the initial TSCR increased by more than 15× at 0°, 30× at 90°, and 60× in compressive stiffness.

A Continuum Over Tube with Variable Stiffness for Transrectal Notes

Background: The continuum over tube applied to natural orifice transluminal endoscopic surgery (NOTES) should be flexible during insertion and provide sufficient stiffness upon arrival at the lesion. In order to satisfy these two properties, a variable stiffness continuum over tube was proposed.

Method: The over tube realizes stiffness variation through the heating and cooling of filling materials. Polyethylene glycol, cocoa butter and gallium metal are selected as filling materials. Several experiments were designed to test its rigid and response characteristic and suitable material was selected. The risk management test was carried out, and the frequency and risk level of the risk point was recorded.

Results: According to experimental data, the polyethylene glycol and gallium shows better variable stiffness performance, with the 20 times stiffness variation range, and polyethylene glycol was selected as suitable materials for its low cost and non-toxic characteristic. The average heating time and cooling time of polyethylene glycol-filled over tube are 52.3s and 36s respectively. Moreover, three kinds of high frequency risk points including smoke, thread ejection and uneven distribution of the material were found, and corresponding design improvement and use principle are proposed.

Conclusion: The proposed continuum over tube can satisfy the requirement of colorectal NOTES, and the safety of the instrument can be improved by controlling high frequency risk points with relevant methods.

Thermoset Shape Memory Polymer Variable Stiffness 4D Robotic Catheters

Variable stiffness catheters are typically composed of an encapsulated core. The core is usually composed of a low melting point alloy (LMPA) or a thermoplastic polymer (TP). In both cases, there is a need to encapsulate the core with an elastic material. This imposes a limit to the volume of variable stiffness (VS) material and limits miniaturization. This paper proposes a new approach that relies on the use of thermosetting materials. The variable stiffness catheter (VSC) proposed in this work eliminates the necessity for an encapsulation layer and is made of a unique biocompatible thermoset polymer with an embedded heating system. This significantly reduces the final diameter, improves manufacturability, and increases safety in the event of complications. The device can be scaled to sub-millimeter dimensions, while maintaining a high stiffness change. In addition, integration into a magnetic actuation system allows for precise actuation of one or multiple tools.

Electrothermally Driven Hydrogel-on-Flex-Circuit Actuator for Smart Steerable Catheters

This paper reports an active catheter-tip device functionalized by integrating a temperature-responsive smart polymer onto a microfabricated flexible heater strip, targeting at enabling the controlled steering of catheters through complex vascular networks. A bimorph-like strip structure is enabled by photo-polymerizing a layer of poly(N-isopropylacrylamide) hydrogel (PNIPAM), on top of a 20 × 3.5 mm² flexible polyimide film that embeds a micropatterned heater fabricated using a low-cost flex-circuit manufacturing process. The heater activation stimulates the PNIPAM layer to shrink and bend the tip structure. The bending angle is shown to be adjustable with the amount of power fed to the device, proving the device’s feasibility to provide the integrated catheter with a controlled steering ability for a wide range of navigation angles. The powered device exhibits uniform heat distribution across the entire PNIPAM layer, with a temperature variation of <2 °C. The operation of fabricated prototypes assembled on commercial catheter tubes demonstrates their bending angles of up to 200°, significantly larger than those reported with other smart-material-based steerable catheters. The temporal responses and bending forces of their actuations are also characterized to reveal consistent and reproducible behaviors. This proof-of-concept study verifies the promising features of the prototyped approach to the targeted application area.

A foldable manipulator with tunable stiffness based on braided structure

Minimally invasive surgery (MIS) has recently seen a surge in clinical applications due to its potential benefits over open surgery. In MIS, a long manipulator is placed through a tortuous human orifice to create a channel for surgical tools and provide support when they are operated. Currently the relative large profile and low stiffness of the manipulators limit the effectiveness and accuracy of MIS. Here we propose a new foldable manipulator with tunable stiffness. The manipulator takes a braided skeleton to enable radial folding, whereas membrane is used to seal the skeleton so as to adjust stiffness through creating negative pressure. We demonstrated experimentally, numerically, and analytically that, a flexible and a rigid state were obtained, and the ratio of bending stiffness in the rigid state to that in the flexible state reached 6.85. In addition, the manipulator achieved a radial folding ratio of 1.95. The proposed manipulator shows great potential in the design of surgical robots for MIS. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part B, 2019.

Stiffening Sheaths for Continuum Robots

Added to their high dexterity and ability to conform to complex shapes, continuum robots can be further improved to provide safer interaction with their environment. Indeed, controlling their stiffness is one of the most challenging yet promising research topics. We propose a tubular stiffening sheath as a replaceable cover for small-diameter continuum robots to temporarily increase the stiffness in a certain configuration. In this article, we assess and compare performances of two different stiffening modalities: granular and layer jamming, provide arguments for material selection and experimental results for stiffness with respect to lateral and axial applied forces. Furthermore, we detected empirically additional effects relating sheath stiffness to material parameters and added to recent investigations in the state of the art, which are based exclusively on material roughness. Finally, we integrated the selected layer jamming material in a miniaturized sheath (13 mm outer diameter, 2.5 mm wall thickness) and covered a tendon-actuated continuum robot with it. Experimental characterization of the behavior with respect to applied external forces was performed via stiffness measurements and proved that the initial tendon-actuated continuum robot stiffness can be improved by a factor up to 24.

Active Stiffness Tuning of a Spring-based Continuum Robot for MRI-Guided Neurosurgery

Deep intracranial tumor removal can be achieved if the neurosurgical robot has sufficient flexibility and stability. Towards achieving this goal, we have developed a spring-based continuum robot, namely a Minimally Invasive Neurosurgical Intracranial Robot (MINIR-II) with novel tendon routing and tunable stiffness for use in a magnetic resonance imaging (MRI) environment. The robot consists of a pair of springs in parallel, i.e., an inner inter-connected spring that promotes flexibility with decoupled segment motion and an outer spring that maintains its smooth curved shape during its interaction with the tissue. We propose a shape memory alloy (SMA) spring backbone that provides local stiffness control and a tendon routing configuration that enables independent segment locking. In this work, we also present a detailed local stiffness analysis of the SMA backbone and model the relationship between the resistive force at the robot tip and the tension in the tendon. We also demonstrate through experiments, the validity of our local stiffness model of the SMA backbone and the correlation between the tendon tension and the resistive force. We also performed MRI compatibility studies of the 3-segment MINIR-II robot by attaching it to a robotic platform that consists of SMA spring actuators with integrated water cooling modules.

Flexible Gastro-intestinal Endoscopy - Clinical Challenges and Technical Achievements

Flexible gastro-intestinal (GI) endoscopy is an integral diagnostic and therapeutic tool in clinical gastroenterology. High quality standards for safety, patients' comfort, and efficiency have already been achieved. Clinical challenges and technical approaches are discussed in this short review.

Image enhanced endoscopy for further characterization of mucosal and vascular patterns includes dye-spray or virtual chromoendoscopy. For confocal laser endoscopy, endocytoscopy, and autofluorescence clinical value has not yet been finally evaluated. An extended viewing field provided by additional cameras in new endoscopes can augment detection of polyps behind folds. Attachable caps, flaps, or balloons can be used to flatten colonic folds for better visualization and stable position.

Variable stiffness endoscopes, radiation-free visualization of endoscope position, and different overtube devices help reducing painful loop formation in clinical routine. Computer assisted and super flexible self-propelled colonoscopes for painless sedation-free endoscopy need further research. Single-use devices might minimize the risk of infection transmission in the future.

Various exchangeable accessories are available for resection, dissection, tunneling, hemostasis, treatment of stenosis and closure of defects, including dedicated suturing devices. Multiple arm flexible devices controlled via robotic platforms for complex intraluminal and transmural endoscopic procedures require further improvement.

Development of a Variable Stiffness Over Tube Based on Low-Melting-Point-Alloy for Endoscopic Surgery

Instruments used in endoscopic surgery (colonoscopy surgery or natural orifice transluminal endoscopic surgery (NOTES)) are flexible to be advanced in human body. However, when the end of the instrument reaches the target, the instrument should be rigid enough to hold its shape against external forces for better surgical accuracy. In order to obtain these two properties, a variable stiffness over tube based on low-melting-point-alloy (LMPA) is proposed in this paper. The structure exploits the phase transformation property of the LMPA which enables the stiffness change of the over tube by heating and cooling. A prototype was fabricated using a special molding method, and experiments were carried out to evaluate its variable stiffness property and response characteristics. According to experimental results, it costs 17 s to make the over tube transform from rigid state to flexible state and 18 s to make the over tube transform from flexible state to rigid state. The experimental results also indicated that the over tube is very rigid in rigid state and flexible in compliant state. A heat insulation layer was assembled to prevent human tissue from thermal damage. The temperature of the outer wall of the over tube was 42.5 °C when hot water of 80 °C was pumped into the over tube continually with the help of the heat insulation layer.

Current state of micro-robots/devices as substitutes for screening colonoscopy: assessment based on technology readiness levels

BACKGROUND

Previous reports have described several candidates, which have the potential to replace colonoscopy, but to date, there is still no device capable of fully replacing flexible colonoscopy in the management of colonic disorders and for mass adult population screening for asymptomatic colorectal cancer.

MATERIALS AND METHODS

NASA developed the TRL methodology to describe and define the stages of development before use and marketing of any device. The definitions of the TRLS used in the present review are those formulated by "The US Department of Defense Technology Readiness Assessment Guidance" but adapted to micro-robots for colonoscopy. All the devices included are reported in scientific literature. They were identified by a systematic search in Web of Science, PubMed and IEEE Xplore amongst other sources. Devices that clearly lack the potential for full replacement of flexible colonoscopy were excluded.

ASSESSMENT OF THE CURRENT SITUATION

The technological salient features of all the devices included for assessment are described briefly, with particular focus on device propulsion. The devices are classified according to the TRL criteria based on the reported information. An analysis is next undertaken of the characteristics and salient features of the devices included in the review: wireless/tethered devices, data storage-transmission and navigation, additional functionality, residual technology challenges and clinical and socio-economical needs.

CONCLUSIONS

Few devices currently possess the required functionality and performance to replace the conventional colonoscopy. The requirements, including functionalities which favour the development of a micro-robot platform to replace colonoscopy, are highlighted.