Hybrid Soft-Rigid Actuators for Minimally Invasive Surgery

Design of an actuator with bionic claw hook-suction cup hybrid structure for soft robot

To improve the adaptability of soft robots to the environment and achieve reliable attachment on various surfaces such as smooth and rough, this study draws inspiration from the collaborative attachment strategy of insects, cats, and other biological claw hooks and foot pads, and designs an actuator with a bionic claw hook-suction cup hybrid structure. The rigid biomimetic pop-up claw hook linkage mechanism is combined with a flexible suction cup of a 'foot pad' to achieve a synergistic adhesion effect between claw hook locking and suction cup adhesion through the deformation control of a soft pneumatic actuator. A pop-up claw hook linkage mechanism based on the principle of cat claw movement was designed, and the attachment mechanism of the biological claw hooks and footpads was analysed. An artificial muscle-spring-reinforced flexible pneumatic actuator (SRFPA) was developed and a kinematic model of the SRFPA was established and analysed using Abaqus. Finally, a prototype of the hybrid actuator was fabricated. The kinematic and mechanical performances of the SRFPA and entire actuator were characterised, and the attachment performance of the hybrid actuator to smooth and rough surfaces was tested. The results indicate that the proposed biomimetic claw hook-suction cup hybrid structure actuator is effective for various types of surface adhesion, object grasping, and robot walking. This study provides new insights for the design of highly adaptable robots and biomimetic attachment devices.

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.

Jointless Bioinspired Soft Robotics by Harnessing Micro and Macroporosity

Although natural continuum structures, such as the boneless elephant trunk, provide inspiration for new versatile grippers, highly deformable, jointless, and multidimensional actuation has still not been achieved. The challenging pivotal requisites are to avoid sudden changes in stiffness, combined with the capability of providing reliable large deformations in different directions. This research addresses these two challenges by harnessing porosity at two levels: material and design. Based on the extraordinary extensibility and compressibility of volumetrically tessellated structures with microporous elastic polymer walls, monolithic soft actuators are fabricated by 3D printing unique polymerizable emulsions. The resulting monolithic pneumatic actuators are printed in a single process and are capable of bidirectional movements with just one actuation source. The proposed approach is demonstrated by two proof-of-concepts: a three-fingered gripper, and the first ever soft continuum actuator that encodes biaxial motion and bidirectional bending. The results open up new design paradigms for continuum soft robots with bioinspired behavior based on reliable and robust multidimensional motions.

Performance enhancement of the soft robotic segment for a trunk-like arm

Trunk-like continuum robots have wide applications in manipulation and locomotion. In particular, trunk-like soft arms exhibit high dexterity and adaptability very similar to the creatures of the natural world. However, owing to the continuum and soft bodies, their performance in payload and spatial movements is limited. In this paper, we investigate the influence of key design parameters on robotic performance. It is verified that a larger workspace, lateral stiffness, payload, and bending moment could be achieved with adjustments to soft materials' hardness, the height of module segments, and arrayed radius of actuators. Especially, a 55% increase in arrayed radius would enhance the lateral stiffness by 25% and a bending moment by 55%. An 80% increase in segment height would enlarge 112% of the elongation range and 70 % of the bending range. Around 200% and 150% increments in the segment's lateral stiffness and payload forces, respectively, could be obtained by tuning the hardness of soft materials. These relations enable the design customization of trunk-like soft arms, in which this tapering structure ensures stability via the stocky base for an impact reduction of 50% compared to that of the tip and ensures dexterity of the long tip for a relatively larger bending range of over 400% compared to that of the base. The complete methodology of the design concept, analytical models, simulation, and experiments is developed to offer comprehensive guidelines for trunk-like soft robotic design and enable high performance in robotic manipulation.

A multi-module soft robotic arm with soft actuator for minimally invasive surgery

BACKGROUND

Compared to traditional rigid robotic arms, soft robotic arms are flexible, environmentally adaptable and biocompatible. Recently, most minimally invasive cardiac procedures still rely on traditional rigid surgical tools. However, rigid tools lack sufficient bending angles, which are high-risk in terms of contact with tissues and organs.

METHODS

A soft robotic arm with multiple degrees of freedom was designed to repair atrial septal defects in cardiac surgery. The developed multi-module soft robotic arm consists of four different units, including a bending unit, a turning unit, a stretching unit and gripper units. The three movement units can reach the specified position, and the gripper units can hold a surgical tool stably, such as a suture needle in cardiac surgery.

RESULTS

A cardiac surgery to repair an atrial septal defect has been completed, validating the reliability and functionality of the developed multi-module soft robotic arm.

CONCLUSIONS

The multi-module flexible soft robotic arm for minimally invasive surgery proposed in this paper can reach the designated surgical area during surgery to repair Atrial Septal Defects. Meanwhile, the design of the actuator of the robot arm was used a completely soft silicone material replacing the rigid material, which releases the contact trauma of the organs during the surgery.

Soft Robotics: A Systematic Review and Bibliometric Analysis

In recent years, soft robotics has developed considerably, especially since the year 2018 when it became a hot field among current research topics. The attention that this field receives from researchers and the public is marked by the substantial increase in both the quantity and the quality of scientific publications. In this review, in order to create a relevant and comprehensive picture of this field both quantitatively and qualitatively, the paper approaches two directions. The first direction is centered on a bibliometric analysis focused on the period 2008-2022 with the exact expression that best characterizes this field, which is "Soft Robotics", and the data were taken from a series of multidisciplinary databases and a specialized journal. The second direction focuses on the analysis of bibliographic references that were rigorously selected following a clear methodology based on a series of inclusion and exclusion criteria. After the selection of bibliographic sources, 111 papers were part of the final analysis, which have been analyzed in detail considering three different perspectives: one related to the design principle (biologically inspired soft robotics), one related to functionality (closed/open-loop control), and one from a biomedical applications perspective.

A Hybrid Controller for a Soft Pneumatic Manipulator Based on Model Predictive Control and Iterative Learning Control

Due to the outstanding characteristics of the large structural flexibility and strong dexterity of soft robots, they have attracted great attention. However, the dynamic modeling and precise control of soft robots face huge challenges. Traditional model-based and model-free control methods find it difficult to obtain a balance between complexity and accuracy. In this paper, a dynamic model of a three-chamber continuous pneumatic manipulator is established based on the modal method. Moreover, a hybrid controller integrating model predictive control (MPC) and iterative learning control (ILC) is proposed, which can simultaneously perform model parameter learning and trajectory tracking control. Experimental results show that the proposed control method can optimize the parameters of the dynamic model in real time with less iterations than the traditional model-free method and have good control performance in trajectory tracking experiments. In the future, the proposed dynamic model and the hybrid controller should be verified on a multi-section manipulator.

Hydraulic Detrusor for Artificial Bladder Active Voiding

The gold standard treatment for bladder cancer is radical cystectomy that implies bladder removal coupled to urinary diversions. Despite the serious complications and the impossibility of controlled active voiding, bladder substitution with artificial systems is a challenge and cannot represent a real option, yet. In this article, we present hydraulic artificial detrusor prototypes to control and drive the voiding of an artificial bladder (AB). These prototypes rely on two actuator designs (origami and bellows) based either on negative or positive operating pressure, to be combined with an AB structure. Based on the bladder geometry and size, we optimized the actuators in terms of contraction/expansion performances, minimizing the liquid volume required for actuation and exploring different actuator arrangements to maximize the voiding efficiency. To operate the actuators, an ad hoc electrohydraulic circuit was developed for transferring liquid between the actuators and a reservoir, both of them intended to be implanted. The AB, actuators, and reservoir were fabricated with biocompatible flexible thermoplastic materials by a heat-sealing process. We assessed the voiding efficiency with benchtop experiments by varying the actuator type and arrangement at different simulated patient positions (horizontal, 45° tilted, and vertical) to identify the optimal configuration and actuation strategy. The most efficient solution relies on two bellows actuators anchored to the AB. This artificial detrusor design resulted in a voiding efficiency of about 99%, 99%, and 89%, in the vertical, 45° tilted, and horizontal positions, respectively. The relative voiding time was reduced by about 17, 24, and 55 s compared with the unactuated bladder.

Biomimetic Artificial Joints Based on Multi-Material Pneumatic Actuators Developed for Soft Robotic Finger Application

To precisely achieve a series of daily finger bending motions, a soft robotic finger corresponding to the anatomical range of each joint was designed in this study with multi-material pneumatic actuators. The actuator as a biomimetic artificial joint was developed on the basis of two composite materials of different shear modules, and the pneumatic bellows as expansion parts was restricted by frame that made from polydimethylsiloxane (PDMS). A simplified mathematical model was used for the bending mechanism description and provides guidance for the multi-material pneumatic actuator fabrication (e.g., stiffness and thickness) and structural design (e.g., cross length and chamber radius), as well as the control parameter optimization (e.g., the air pressure supply). An actuation pressure of over 70 kPa is required by the developed soft robotic finger to provide a full motion range (MCP = 36°, PIP = 114°, and DIP = 75°) for finger action mimicking. In conclusion, a multi-material pneumatic actuator was designed and developed for soft robotic finger application and theoretically and experimentally demonstrated its feasibility in finger action mimicking. This study explored the mechanical properties of the actuator and could provide evidence-based technical parameters for pneumatic robotic finger design and precise control of its dynamic air pressure dosages in mimicking actions. Thereby, the conclusion was supported by the results theoretically and experimentally, which also aligns with our aim to design and develop a multi-material pneumatic actuator as a biomimetic artificial joint for soft robotic finger application.

Bone-Inspired Bending Soft Robot

Bending soft robots must be structured and predictable to be used in applications such as a grasping hand. We developed soft robot fingers with embedded bones to improve the performance of a puppetry robot with haptic feedback. The manufacturing process for bone-inspired soft robots is described, and two mathematical models are reported: one to predict the stiffness and natural frequency of the robot finger and the other for trajectory planning. Experiments using different prototypes were used to set model parameters. The first model, which had a fourth-order lumped mass-spring-damper configuration, was able to predict the natural frequency of the soft robot with a maximum error of 18%. The model and the experimental data demonstrated that bone-inspired soft robots have higher natural frequency, lower phase shift, better controllability, and higher stiffness compared with traditional fiber-reinforced bending soft robots. We also showed that the dynamic performance of a bending soft robot is independent of whether water or air is used for the media and independent of the media pressure. Results from the second model showed that the path of a bone-inspired soft robot is a function of the relative lengths of the bone segments, which means that the model can be used to direct the design of the robot to achieve the desired trajectory. This model was able to correctly predict the trajectory path of the robot.

A Novel Pressure-Controlled Revolute Joint with Variable Stiffness

The compliance and deformability of soft robotics allow human-machine interactions in a safe manner without the need of sophisticated control systems inherent in rigid-body robotic devices. However, these advantages introduce controllability and predictability challenges. In this study, we propose a novel fluidic-driven variable stiffness revolute joint (VSRJ) based on hybrid soft-rigid approach to achieve adjustable compliance while addressing the abovementioned challenges. The VSRJ is composed of a silicone rubber cylinder as a pressure chamber and two identical rigid links. The soft cylinder is positioned in a fully closed compartment created by the assembly of the two rigid links, thus constraining its expansion when pressure is applied. By applying pressure, the stiffness of the joint increases accordingly for the following reasons: (1) increasing the friction force between the cylinder and the compartment walls and (2) creating a locking mechanism through the expansion of the cylinder into space between rigid links in a "bump" formation. Experimental results show that the VSRJ can achieve up to 8-fold rotational stiffness enhancement from 0 to 5 bar input pressure within -30° to +30° rotation angle. The modular design of the rigid link allows the assembly of multiple VSRJs to build a variable stiffness structure in which each VSRJ has an independent stiffness and relative position. The VSRJ was characterized in terms of repeatability, torque, and stiffness. The experimental results were validated by finite element analysis. This approach can provide opportunities for the use of this new variable stiffness concept as an efficient alternative to traditional variable-stiffness linkages.

Toward a Common Framework and Database of Materials for Soft Robotics

To advance the field of soft robotics, a unified database of material constitutive models and experimental characterizations is of paramount importance. This will facilitate the use of finite element analysis to simulate their behavior and optimize the design of soft-bodied robots. Samples from seventeen elastomers, namely Body Double™ SILK, Dragon Skin™ 10 MEDIUM, Dragon Skin 20, Dragon Skin 30, Dragon Skin FX-Pro, Dragon Skin FX-Pro + Slacker, Ecoflex™ 00-10, Ecoflex 00-30, Ecoflex 00-50, Rebound™ 25, Mold Star™ 16 FAST, Mold Star 20T, SORTA-Clear™ 40, RTV615, PlatSil® Gel-10, Psycho Paint®, and SOLOPLAST 150318, were subjected to uniaxial tensile tests according to the ASTM D412 standard. Sample preparation and tensile test parameters are described in detail. The tensile test data are used to derive parameters for hyperelastic material models using nonlinear least-squares methods, which are provided to the reader. This article presents the mechanical characterization and the resulting material properties for a wide set of commercially available hyperelastic materials, many of which are recognized and commonly applied in the field of soft robotics, together with some that have never been characterized. The experimental raw data and the algorithms used to determine material parameters are shared on the Soft Robotics Materials Database GitHub repository to enable accessibility, as well as future contributions from the soft robotics community. The presented database is aimed at aiding soft roboticists in designing and modeling soft robots while providing a starting point for future material characterizations related to soft robotics research.

A lobster-inspired bending module for compliant robotic applications

Ideally, robots may be designed to adapt to different tasks such as heavy lifting and handling delicate objects, in which the requirements in force compliance and position accuracy vary dramatically. While conventional rigid actuators are usually characterized by high precision and large force output, soft actuators are designed to be more compliant and flexible. In this paper, a lobster-inspired bending module with compliant actuation, enhanced torque output, and reconfigurability in assembling is presented. It is also capable of accurate control of its angular position with variable stiffness. Inspired by the anatomic structure of the lobster leg joint, the bending module has antagonistic soft chambers for actuation and rigid shells for structural protection and support. Theoretical models have been developed and their capability of independently adjusting both the bending angle and stiffness has been evaluated through experiments. A control strategy is constructed to realize angle control and stiffness adaptation. In order to demonstrate various applications of the proposed bending module, reconfigurable robotic fingers are assembled and shown to be capable of generating different motion profiles. In addition, robotic grippers are built for lifting both delicate and heavy objects, demonstrating applications that require both high force and compliant handling.

SoGut: A Soft Robotic Gastric Simulator

The human stomach breaks down and transports food by coordinated radial contractions of the gastric walls. The radial contractions periodically propagate through the stomach and constitute the peristaltic contractions, also called the gastric motility. The force, amplitude, and frequency of peristaltic contractions are relevant to massaging and transporting the food contents in the gastric lumen. However, existing gastric simulators have not faithfully replicated gastric motility. Herein, we report a soft robotic gastric simulator (SoGut) that emulates peristaltic contractions in an anatomically realistic way. SoGut incorporates an array of circular air chambers that generate radial contractions. The design and fabrication of SoGut leverages principles from the soft robotics field, which features compliance and adaptability. We studied the force and amplitude of the contractions when the lumen of SoGut was empty or filled with contents of different viscosity. We examined the contracting force using manometry. SoGut exhibited a similar range of contracting force as the human stomach reported in the literature. Besides, we investigated the amplitude of the contractions through videofluoroscopy where the contraction ratio was derived. The contraction ratio as a function of inflation pressure is found to match the observations of in vivo situations. We demonstrated that SoGut can achieve in vitro peristaltic contractions by coordinating the inflation sequence of multiple air chambers. It exhibited the functions to massage and transport the food contents. SoGut can simulate the physiological motions of the human stomach to advance research of digestion.

Comparative analysis of occlusion methods for artificial sphincters

An artificial sphincter is a device that replaces the function of the biological sphincter by occluding the relative biological lumen. The investigation of occlusion methods for artificial sphincters is crucial for a reliable and effective design of such devices. The compression induced onto the tissue by a certain pressure depends on the biomechanical and physiological features of the lumen and on the specific occlusion method. A numerical model and an experimental evaluation are presented here to assess the efficiency of different occlusion methods. Numerical models of circumferential occlusion and clamping occlusion methods to simulate the compression of the biological lumen were developed. Results revealed a relationship between the efficiency of the occlusion method and the physiological condition of the lumen. With differences related to the testing setup, this relationship was also confirmed experimentally by conducting tests on biological simulators. We analyzed the occlusion method to adopt as the physiological pressure (ie, leakage pressure values) changed. In particular, we focused on the urinary incontinence, which is a dysfunction involving the external sphincter surrounding the urethra. In this scenario, we demonstrated that a clamping occlusion is an efficient method to compress the urethra, whose physiological pressures range between 4 and 12 kPa. The clamping occlusion method resulted up to 35% more efficient in terms of sealing pressure than the circumferential one for a closing pressure varying between 2.3 and 11.5 kPa.

Fluid-Structure Coupling Model and Experimental Validation of Interaction Between Pneumatic Soft Actuator and Lower Limb

Pneumatic soft actuators (PSAs) are components that produce predesigned motion or force in different end-use devices. PSAs are lightweight, flexible, and compatible in human-machine interaction. The use of PSAs in compression therapy has proven promising in proactive pressure delivery with a wide range of dosages for treatment of chronic venous insufficiency and lymphedema. However, effective design and control of PSAs for dynamic pressure delivery have not been fully elaborated. The purpose of this study is to explore interactive working mechanisms between a PSA and lower limbs through establishing fluid-structure coupling models, an intermittent pneumatic compression (IPC) testing system, and conducting experimental validation. The developed IPC testing system consisted of a PSA unit (multichambered bladders laminated with an external textile shell), a pneumatic controller, and various real-time pressure monitoring sensors and accessory elements. The established coupling model characterized the dynamic response process with varying design parameters of the PSA unit, and demonstrated that the design of initial thickness, stiffness, and air mass flow of the PSA, as well as stiffness of limb tissues of the users, influenced PSA-lower limb interactions and resultant pressure dosages. The simulated results presented a favorable agreement with the experimental data collected by the IPC testing system. This study enhanced understanding of PSA-lower limb interactive working mechanisms and provided an evidence-based technical guidance for functional design of PSA. These results contribute to improving the efficacy of dynamic compression therapy for promotion of venous hemodynamics and user compliance in practice.

Design and Evaluation of Magnetic Hall Effect Tactile Sensors for Use in Sensorized Splints

Splinting techniques are widely used in medicine to inhibit the movement of arthritic joints. Studies into the effectiveness of splinting as a method of pain reduction have generally yielded positive results, however, no significant difference has been found in clinical outcomes between splinting types. Tactile sensing has shown great promise for the integration into splinting devices and may offer further information into applied forces to find the most effective methods of splinting. Hall effect-based tactile sensors are of particular interest in this application owing to their low-cost, small size, and high robustness. One complexity of the sensors is the relationship between the elastomer geometry and the measurement range. This paper investigates the design parameters of Hall effect tactile sensors for use in hand splinting. Finite element simulations are used to locate the areas in which sensitivity is high in order to optimise the deflection range of the sensor. Further simulations then investigate the mechanical response and force ranges of the elastomer layer under loading which are validated with experimental data. A 4 mm radius, 3 mm-thick sensor is identified as meeting defined sensing requirements for range and sensitivity. A prototype sensor is produced which exhibits a pressure range of 45 kPa normal and 6 kPa shear. A proof of principle prototype demonstrates how this can be integrated to form an instrumented splint with multi-axis sensing capability and has the potential to inform clinical practice for improved splinting.

TMTDyn: A Matlab package for modeling and control of hybrid rigid–continuum robots based on discretized lumped systems and reduced-order models

A reliable, accurate, and yet simple dynamic model is important to analyzing, designing, and controlling hybrid rigid–continuum robots. Such models should be fast, as simple as possible, and user-friendly to be widely accepted by the ever-growing robotics research community. In this study, we introduce two new modeling methods for continuum manipulators: a general reduced-order model (ROM) and a discretized model with absolute states and Euler–Bernoulli beam segments (EBA). In addition, a new formulation is presented for a recently introduced discretized model based on Euler–Bernoulli beam segments and relative states (EBR). We implement these models in a Matlab software package, named TMTDyn, to develop a modeling tool for hybrid rigid–continuum systems. The package features a new high-level language (HLL) text-based interface, a CAD-file import module, automatic formation of the system equation of motion (EOM) for different modeling and control tasks, implementing Matlab C-mex functionality for improved performance, and modules for static and linear modal analysis of a hybrid system. The underlying theory and software package are validated for modeling experimental results for (i) dynamics of a continuum appendage, and (ii) general deformation of a fabric sleeve worn by a rigid link pendulum. A comparison shows higher simulation accuracy (8–14% normalized error) and numerical robustness of the ROM model for a system with a small number of states, and computational efficiency of the EBA model with near real-time performances that makes it suitable for large systems. The challenges and necessary modules to further automate the design and analysis of hybrid systems with a large number of states are briefly discussed.