Bio-Inspired Soft Robotic Actuators for Minimally Invasive Surgery

More than 130,000 people have strokes each year in the United States [1]. Of these victims, 76% are left with disabilities that cost the nation over $54 billion in lost work and medical fees. One prominent disability is upper extremity hemiplegia, which occurs among approximately 50% of stroke sufferers [2]. Robotic technology has the potential to provide an automated platform for controlled rehabilitation and assisted, task-oriented therapy. Several systems have been designed to assist in patient articulation of an impaired hand using rigid robotic components. While these products have been successful in articulating the pure bending motions of the four fingers, the limited capabilities of rigid technologies fail to reproduce the complicated motion path [5] of the thumb during opposition grasp (see Figure 1). This is the most important articulation for normal hand function and specifically for picking up everyday objects. To date, robotic systems for thumb rehabilitation have not been widely investigated [3] apart from some recent work [4] that used a multi-joint rigid robot.

The benefits of soft robotic actuation have been recognized and soft actuators are increasingly used. Soft robots are generally lightweight, inexpensive, and highly compliant. They are often used for interaction with small objects which only require relatively low force output. Nevertheless, large-scale applications can still benefit from the advantages of soft actuators if force output is sufficiently large. Some soft actuation has been employed in large-scale applications, but the area remains greatly underexplored. This article presents the Soft Torque-generating Actuation System (STAS) a lean soft actuation architecture capable of producing relatively large torque output while maintaining compliant interaction with the environment. Design and fabrication of the system are described, and a theoretical model is introduced for predicting torque output. The model is validated against measured torque output, and the system is further characterized in terms of power output. The model is shown to adequately predict torque output at low actuator angles. The system is shown to require only 30.0 kPa to achieve a large torque output of 10 N-m. The system also reaches a peak power output of 1.29 Watts. Finally, a versatile application of STAS is shown through a wearable joint-assistive device. The STAS is a promising architecture for a new type of high force actuation using soft actuators.

Soft robots are inherently compliant and adaptive, therefore they are promising candidates for interacting with humans. However robotic manipulators utilizing soft actuators are often constrained by a series of actuator performance limitations. In this work we design a novel linear soft robotic actuator with significantly improved performances over the existing products, achieving 300% deformation ratio, quasi-constant output force over a wide motion range, while maintaining passive compliance and adaptability. Moreover, the novel actuator is less prone to friction, and could be fabricated using inject molding and 3D printing, hence having high repeatability at very low cost. An analytical model was developed to characterize the actuator behavior and provide a guideline for actuator design according to performance specifications. A 6 DOF soft manipulator was designed and fabricated utilizing the novel soft actuator. The manipulator arm had a serial kinematic structure with a biomimetic wrist and was driven by 12 soft actuators mounted onto the arm links. With 1.2m workspace radius and 1kg payload, the working air pressure could be as low as 1bar. Preliminary results have shown the validity of the novel soft actuator and manipulator designs, as well as the strong potential of soft robots in human-oriented applications.

Continued technological progress in robotic systems has led to more applications where robots and humans operate in close proximity and even physical contact in some cases. Soft robots, which are made of highly compliant and deformable materials, provide inherent safety features unlike conventional robots that are made of stiff and rigid components. Soft robotics is a rapidly developing field exploiting biomimetic design principles, novel sensor and actuation concepts, and advanced manufacturing techniques. In this study, we propose novel 3D printable soft vacuum actuators that are inspired by the sporangium of fern trees. These actuators that are directly manufactured using commercial and affordable fused deposition modeling 3D printers offer many advantages such as high actuation speed (5.54 Hz), long lifetime (123,000 cycles), large payload to weight ratio (∼26), and significant output forces (∼16 N). The behavior of these actuators is accurately predicted, and their performance is optimized using finite element modeling. Furthermore, diverse robotic applications such as locomotion robots (a walking robot moving with an average forward speed of vf = 3.54 cm/s, and a hopping robot called Gongaroo hopping with an average speed of vf = 3.75 cm/s), grippers, and artificial muscles have been established and activated using the new soft actuation concept. Finally, to demonstrate the modularity of the proposed actuation concept, soft actuators with multiple degrees of freedom and variable length are established using a series of 3D printable vacuum hinges.

This paper reports on design and fabrication of a novel soft fish robot. Application of soft actuators for the fish tail will generates continuum bending motion which resembles the natural motion of the fish. However, most soft actuator mechanisms are complex and have low efficiency. Thus, to address this issue we have developed a 3D printed soft bending actuator which can be actuated with an electromotor. The basic design idea of the soft bending actuator is explained, and iteration of the design showed to create the desired motion for the soft tail. The soft actuator has been successfully integrated with fish body and it has been shown that the fish can swim.

The soft robotics field investigates biological bodies’ movements to develop soft actuation with similar behavior seen in nature. The skeletal muscles’ movements are vastly mimicked in soft actuators; however, the contractions of smooth muscles are overlooked in soft robotics. Smooth muscles provide peristaltic contractions to the digestive system, such as the stomach, and their design challenges the limits of soft robot development that simulates digestive organs. In this article, we present a novel, bellows-driven soft pneumatic actuator (SPA) with a self-sensing capability that generates linear displacement to simulate a smooth muscle segment in a stomach. This SPA is proposed as a solution for applications such as stomach simulators. The SPA concept, design, fabrication, creep model, and experimental validation are presented in this article. A lumped viscoelastic model presents the actuator's modeling to predict the SPA's displacement from a known applied pressure. The model is successfully validated, which resulted in a maximum displacement of 20.0 mm with a displacement error of 3%. A sensory system embedded in the SPA measures the displacement, and it is found that a force of 0–8.0 N is produced. A polynomial equation calibrates the sensor readings, with a displacement error of 5%. The results show the SPA's capability as a linear soft actuator that can generate a deformation like a smooth muscle segment. To simulate a circular contraction from a stomach with the SPA, we install multi-SPAs in a ring frame that performs contractions. Although the proposed SPA is specifically designed to mimic the smooth muscle, the ring actuator's contractions can be used for other applications such as gripping soft objects.

Success of emerging field of soft robotics relies on the development of efficient soft actuators. Most of these actuators suffers from disadvantages such as limited blocking force, lifetime, high actuation voltage, and slow response time. Swimming robots in particular utilizes single or multiple soft actuators to mimic the dynamic of a fish during motion. In this study we used 3D printing to fabricate soft electromagnetic actuators based swimming robot. The mechanism consists of soft actuator legs, compliant paddle and floatation. We designed a hybrid propulsion mechanism by using double soft leg actuators as caudal fins and side paddles as pectoral fins. This increases the thrust and efficiency to overcome the water drag as well as providing stability. We 3D printed the soft actuator using thermoplastic polyurethane (TPU) filament to reduce the manufacturing cost as well as to simplify the process. The main body is also 3D printed using polylactic acid (PLA). The infill percentage of the soft body is adjusted to increase the bending performance without yielding under actuation. The prototype of the swimming robot was tested in water. The body velocity of the robot is measured as 0.106 BL/s. Motion analysis was made MSC Adams by simulating the deformation of flexible beams.

Introduction: Minimally Invasive Surgery (MIS) offers targeted surgical access with reduced invasiveness; however, the maneuverability challenges of traditional instruments in this domain underscore the need for innovative solutions. Soft actuators activated by fluids or gases present a promising strategy for augmenting endoscopic capabilities, thereby enhancing the surgical precision in MIS. This study aimed to explore the intricate dynamics of the interactions between soft actuators and endoscopes, with an emphasis on the pivotal role of cross-sectional chamber shapes. While previous studies have touched on the influence of chamber shapes on bending properties, we provide a comprehensive exploration. We explore how these shapes modulate friction forces, which in turn influence the interactions governing bending, response, and stiffness adjustability, all of which are essential for enhancing endoscope maneuverability in MIS contexts. Methods: A novel bilateral symmetrical air chamber design was adopted to investigate various chamber shapes. We employed finite element analysis (FEA) simulations followed by prototype testing to evaluate the interactions driven by these chamber shapes and to discern their impact on actuator properties. Recognizing the pivotal role of friction in these interactions, we conducted dedicated friction experiments. These experiments further deepened our understanding of the relationship between chamber shape and friction, and how this synergy influences the properties of the actuator. Results: Our findings showed that actuators with wider chambers generate larger friction forces, thereby enhancing the interaction and improving the bending, response, and stiffness adjustability. Additionally, the soft actuator significantly improved the maneuverability and bending radius of the endoscope, demonstrating enhanced navigation capabilities in complex environments. Discussion: The shape of a cross-sectional chamber plays a pivotal role in designing soft actuators for MIS applications. Our research emphasizes the importance of this design component, offering key insights for the development of endoscope-supporting soft actuators that can effectively handle intricate actuator-endoscope interactions, thereby enhancing surgical outcomes.

Due to their light weight, flexibility, and low energy consumption, ionic electroactive polymers have become a hotspot for bionic soft robotics and are ideal materials for the preparation of soft actuators. Because the traditional ionic electroactive polymers, such as ionic polymer-metal composites (IPMCs), contain water ions, a soft actuator does not work properly upon the evaporation of water ions. An ionic liquid polymer gel is a new type of ionic electroactive polymer that does not contain water ions, and ionic liquids are more thermally and electrochemically stable than water. These liquids, with a low melting point and a high ionic conductivity, can be used in ionic electroactive polymer soft actuators. An ionic liquid gel (ILG), a new type of soft actuator material, was obtained by mixing 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4), hydroxyethyl methacrylate (HEMA), diethoxyacetophenone (DEAP) and ZrO2 and then polymerizing this mixture into a gel state under ultraviolet (UV) light irradiation. An ILG soft actuator was designed, the material preparation principle was expounded, and the design method of the soft robot mechanism was discussed. Based on nonlinear finite element theory, the deformation mechanism of the ILG actuator was deeply analyzed and the deformation of the soft robot when grabbing an object was also analyzed. A soft robot was designed with the soft actuator as the basic module. The experimental results show that the ILG soft robot has good driving performance, and the soft robot can grab a 105 mg object at an input voltage of 3.5 V.

A new design and rapid prototyping approach for hydraulic soft actuators was developed and used to create bio-inspired underwater soft actuators that were tested and applied in the field. Underwater soft-bodied animals demonstrate complex approaches to move around the ocean and sophisticated methods of physically interacting with other organism and objects. Studying the functional design and kinematics of these animals can provide insight and solutions for complex tasks and locomotion, and soft robotics provides pathways to create complex actuators that imitate or are inspired by these animals. However, creating a complex soft actuator is currently an arduous and time-consuming process that often requires several iterations to both confirm the manufacturing process and the actuator’s performance. The work conducted in this dissertation explores ways to expedite this process and demonstrate the effectiveness of bio-inspired systems in a predictable manner. To simplify and reduce the design process, 3D printing methods and Finite Element Analysis (FEA) simulations were explored and tested to create customizable bellow-style hydraulic soft actuators. Numerous versions of the soft actuator were produced and tested with both processes to demonstrate the reliability and accuracy of using both methods. Using this approach, customized versions of the soft actuator were easily and quickly manufactured and insight into how parameter changes influence its performance were obtained. The actuator was then further developed to provide controlled drag force to a small free-falling benthic lander system. Taking inspiration from how pelagic animals that live near the seafloor are able to reduce their descent speed, the soft actuators act in a similar manner by increasing the amount of surface area of the lander thus increasing the amount of drag force acting upon it. This allows the lander to reduce its descent speed before landing on the seafloor. This concept was successfully tested in the deep-sea, with terminal velocity changes reduced by 45% of their original speed. This dissertation describes in detail the process of rapid prototyping soft actuators in a consistent manner (Chapter 1), the integration of FEA-informed simulations on testing design variables of soft actuators prior to physical production (Chapter 2), and the combined design and prototyping workflow to produce bioinspired soft actuator arrays for the controlled descent of deep-sea vehicles (Chapter 3).

Soft actuators are versatile and can readily perform various functions and interact safely with humans and the environment owing to their deformability. Shape memory alloys (SMAs) have advantages of high power-to-weight ratio, silent operation, and high response speed, which make them suitable for fabricating soft, compact, and muscle-like soft actuators with various transformation capabilities. However, it is challenging to precisely control SMA-based soft actuators and robots because of the lack of explicit dynamics-based control method. In this article, a linear phase transition model of SMA is derived to express the dynamic model of an SMA planar actuator (SPA) in an explicit form. Then, a model-based feedback controller considering constraints of strain of the SMA, temperature increment of the SPA, and load increment was built. Strain gauges are used to obtain the bending angle of the SPA as the feedback signal for the controller. Various capabilities of the SPA, such as step response, position tracking with constant angular speed, and motion control with load, were experimentally evaluated. The developed method is meaningful for modeling and control of various SMA-based actuators and robots.

Extraterrestrial exploration presents unique challenges for robotic systems, where traditional rigid rovers face limitations in stowage volume, traction on unpredictable terrain, and susceptibility to damage. Soft robotics offers promising solutions through bio-inspired designs that can mimic natural locomotion mechanisms. We present an optimised soft spider-inspired jumping robot for extraterrestrial exploration that addresses key challenges in soft robotics: actuation efficiency, controllability, and deployment. Drawing inspiration from spider physiology, particularly their hydraulic extension mechanism, we develop a lightweight limb capable of multi-modal behaviour with significantly reduced energy requirements. Our 3D-printed soft actuator leverages pressure-driven collapse for efficient retraction and pressure-enhanced rapid extension, achieving a power-to-weight ratio of 249 W/kg. The integration of a non-backdriveable clutch mechanism enables the system to hold positions with zero energy expenditure—a critical feature for space applications. Experimental characterisation and a subsequent optimisation methodology across various materials, dimensions, and pressures reveals that the robot can achieve jumping heights of up to 0.92 body lengths. The collapsible nature of the soft limb enables efficient stowage during spacecraft transit, while the integrated pumping system facilitates self-deployment upon arrival. This work demonstrates how biologically-inspired design principles can be effectively applied to develop versatile robotic systems optimised for the unique constraints of extraterrestrial exploration.

Fabrication and assembly of flexible sensors and soft actuators play important roles in improving the performance of wearable electronics and soft robots. Traditional manufacturing techniques have limitations in controlling the geometry and architecture of many soft actuation and sensing systems, which compromise their performance as well as applications. With the emergence of 3D printing, directly transforming 3D models into real objects becomes possible. In particular, vat photopolymerization techniques, represented by stereolithography, are capable of printing all‐in‐one and lab‐on‐a‐chip devices with fast speed and high accuracy. Novel polymer formulations, including functional resins, hydrogels, elastomers, polymer blends, composites, and biological materials, have been developed for vat photopolymerization 3D printing to produce highly stable architectures, which prompts a remarkable revolution in mechanics and materials for soft electronics. This review looks into the recent developments of vat photopolymerization 3D printing technology for lightweight engineering, personalized electronics, and soft robots.

As robots more frequently fraternize with humans in everyday life, aspects such as safety, flexibility of tasks, and appearance become increasingly important. Soft robotics is attractive for new human‐close applications, but soft actuators constitute a major challenge both in terms of actuation force and speed, and in terms of control and accuracy of the deformable soft actuator body. Herein, several of these challenges are addressed by developing versatile ultrasoft electromagnetic actuators that operate in absence of external magnetic fields, while simultaneously monitoring their states by internal strain sensors. The versatile actuators can compress to less than 50% of their initial length with strain‐independent contraction force and operate in both contraction and expansion modes up to 200 Hz frequency while conforming to curved surfaces. The soft multilayer conductive cellulose‐based foams are lightweight (3 mg cm−3) and provide internal strain‐sensing capability and structural support, thereby improving the monitoring and controllability of the actuators while maintaining an axial softness of 0.6 kPa. It is believed that the concept of soft versatile electromagnetic actuators with integrated lightweight strain‐sensing foams is promising for a wide range of applications within soft robotics.

Soft bimorph actuator with real-time multiplex motion perception