Soft Robotic Technologies Research Articles

Soft Upper‐Limb Wearable Robotic Devices: Technology and Applications

One of the practical applications in the field of soft robotics involves the development of soft robotic wearable devices. These devices make use of their intrinsically compliant structures to interact safely and harmoniously with the human body. While soft wearable robots demonstrate their utility in lower‐limb applications for locomotion, the upper‐limb domain offers significant prospects in a wide range of applications that soft robotic technology can address. In this review, the current state of technology in the field of soft wearable upper limbs is systematically analyzed and categorized. Categorizations are made based on their applications in rehabilitation, activities of daily living support, and human augmentation. Furthermore, in this study, also contemporary technological aspects, encompassing sensing technology and control systems, are explored. Despite exciting potential in this domain, several limitations from existing devices inherently impede widespread adoption and thus hinder further progress in the field. In this study, also an overview of the different facets of the domain is provided and key considerations for the advancement of soft wearable robotic devices intended for upper‐limb applications are prescribed.

A Bioinspired Robotic Finger for Multimodal Tactile Sensing Powered by Fiber Optic Sensors

The rapid advancement of soft robotic technology emphasizes the growing importance of tactile perception. Soft grippers, equipped with tactile sensing, can gather interactive information crucial for safe human–robot interaction, wearable devices, and dexterous manipulation. However, most soft grippers with tactile sensing abilities have limited modes of tactile perception, restricting their dexterity and safety. In addition, existing tactile systems are often complicated, leading to unstable perception signals. Inspired by various organisms, a novel multimodal tactile‐sensing soft robotic finger is proposed. This finger, based on a modified fin ray structure, integrates a distributed fiber optic sensing system as part of its tactile sensory neural system. It replicates human finger capabilities, discerning contact forces as low as 0.01 N with exceptional sensitivity (106.96 mN nm−1). Through training neural networks models, the finger achieves an accuracy exceeding 96% in recognizing roughness, material stiffness, and finger pad position. Assembled into two‐finger parallel gripper, it demonstrates precise manipulation capabilities for fragile items like strawberries and potato chips. Moreover, through synergistic interplay of multimodal tactile sensing, this finger can successfully grasp an underwater transparent sphere, mitigating limitations of visual perception. The developed soft finger holds promise in various scenarios including hazardous environment detection and specialized grasping tasks.

An assessment of the suitability of a Liebau pump in biomedical applications

Impedance pumping and asymmetric pumping are the main mechanisms of the Liebau effect, a type of valveless pumping. The primary objective of this study is to enhance the current understanding of the mechanics and functionality of a Liebau pump. To achieve this, a prototype pump is constructed, incorporating several novel features: a pinch frequency control system to optimize pump performance, a planar actuator developed using soft robotic technology, and a compliant chamber designed to mitigate flow fluctuations and prevent reverse flow, a common feature of this pumping method. Experimental tests conducted with this prototype demonstrate that the Liebau pump holds potential as a viable technology for fluid pumping in biomedical applications, especially when pulsatile flow is essential or when hemolysis needs to be minimized.

A 3D culture method based on interpenetrating hydrogels induced by cells into skeletal muscle tissue in vitro

The combination of muscle cells and soft robotics technology has led to the rapid development of biohybrid robots in recent years. Muscle-driven systems had advantages such as joint flexibility, self-repair, and multi-signal perception, demonstrating unique advantages as an effective technological solution for robot drive systems. Current research has many shortcomings in material properties and muscle stability in 3D muscle tissue cultured in vitro, making it difficult to successfully apply the technology. In this study, we developed a new type of interpenetrating hydrogel network that, combined with 3D cell culture and tissue-induced culture, enabled 3D muscle tissue culture in a hydrogel environment and induced differentiation in muscle tissue. The results of this research provided a foundation for future studies on in vitro muscle tissue culture and the implementation of robots in this field.

A sparse-dense hierarchical structured conductive elastomer composites for highly sensitive and stretchable strain sensor

Flexible strain sensor using conductive elastomer (CE) have fostered hope for the development of healthcare, human–machine interfaces, and soft robotics technologies. However, it is still challenging to obtain CE based strain sensor with excellent stretchability, wide strain range, high sensitivity coupled with outstanding stability via facile preparation process. Herein, combining the advantages of sparse and dense conductive network, a sparse-dense hierarchical structured conductive elastomer composites (CECs) based on natural rubber (NR) and modified MWCNTs (m-MWCNTs) is developed through facile one-pot synthesis. By means of the low cloud point of nonionic surfactant Triton x-100, the thick-sparse and thin-dense conductive layers as well as the excellent interface between hierarchical structure are formed by controlling the content of Triton x-100 and dispersed temperature. The severe slippage of sparse conductive network at small deformation and the avulsion of dense conductive network at large strain could synergistically enhance mechanical property and sensing performance of NR/m-MWCNTs CECs. Consequently, the designed stain sensor can achieve high sensitivity and linearity at a strain range of 0–200 % and 200–423 %, which are superior to most other state-of-the-art CECs strain sensors. Meanwhile, a wide range of detection (1 %-423 %), a fast response time (0.23 s), excellent repeatability, and stability are also obtained. Besides, our developed strain sensor was successfully applied in elbow pad and knee pad for monitoring human motion. In addition, NR/m-MWCNTs CECs exhibit the superior electric heating performance. The above satisfactory results reveal that our developed sparse-dense hierarchical CECs has the potential for applications in smart wearable electronics.

Toward Development of Novel Remote Ultrasound Robotic System Using Soft Robotics Technology.

This paper reports on the development of a novel soft robotic system for remote ultrasound applications. Direct contact of the ultrasound probe with the patient's body represents a safety risk and therefore control of the probe's positioning and applied force is a crucial task. The proposed robot uses a passive control system that provides safe interaction between the robot and the patient by leveraging soft robotics technology. The soft robot's structure can be considered as a nonlinear spring which can be designed to exert a safe force within the robot's workspace to guarantee the safety of human-robot interaction. The literature suggests that effective ultrasound imaging of both the heart and abdomen requires six degrees-of-freedom. These degrees-of-freedom consist of three translational motions, which are achieved using a novel hybrid soft cable-driven parallel robot, and three wrist motions, which is based on a universal joint design. The experimental results show that the robot can achieve all these six degrees-of-freedom, and its blocking force can be engineered to generate a uniform force within the workspace.

Perceptual Soft End-Effectors for Future Unmanned Agriculture.

As consumers demand ever-higher quality standards for agricultural products, the inspection of such goods has become an integral component of the agricultural production process. Unfortunately, traditional testing methods necessitate the deployment of numerous bulky machines and cannot accurately determine the quality of produce prior to harvest. In recent years, with the advancement of soft robot technology, stretchable electronic technology, and material science, integrating flexible plant wearable sensors on soft end-effectors has been considered an attractive solution to these problems. This paper critically reviews soft end-effectors, selecting the appropriate drive mode according to the challenges and application scenarios in agriculture: electrically driven, fluid power, and smart material actuators. In addition, a presentation of various sensors installed on soft end-effectors specifically designed for agricultural applications is provided. These sensors include strain, temperature, humidity, and chemical sensors. Lastly, an in-depth analysis is conducted on the significance of implementing soft end-effectors in agriculture as well as the potential opportunities and challenges that will arise in the future.

Light-Driven Self-Oscillation of Thermoplasmonic Nanocolloids.

Self-oscillation-the periodic change of a system under a non-periodic stimulus-is vital for creating low-maintenance autonomous devices in soft robotics technologies. Soft composites of macroscopic dimensions are often doped with plasmonic nanoparticles to enhance energy dissipation and generate periodic response. However, while it is still unknown whether a dispersion of photonic nanocrystals may respond to light as a soft actuator, a dynamic analysis of nanocolloids self-oscillating in a liquid is also lacking. This study presents a new self-oscillator model for illuminated colloidal systems. It predicts that the surface temperature of thermoplasmonic nanoparticles and the number density of their clusters jointly oscillate at frequencies ranging from infrasonic to acoustic values. New experiments with spontaneously clustering gold nanorods, where the photothermal effect alters the interplay of light (stimulus) with the disperse system on a macroscopic scale, strongly support thetheory. These findings enlarge the current view on self-oscillation phenomena and anticipate the colloidal state of matter to be a suitable host for accommodating light-propelled machineries. In broad terms, a complex system behavior is observed, which goes from periodic solutions (Hopf-Poincaré-Andronov bifurcation) to a new dynamic attractor driven by nanoparticle interactions, linking thermoplasmonics to nonlinearity andchaos.

Mechanoresponsive Drug Release from a Flexible, Tissue-Adherent, Hybrid Hydrogel Actuator.

Soft robotic technologies for therapeutic biomedical applications require conformal and atraumatic tissue coupling that is amenable to dynamic loading for effective drug delivery or tissue stimulation. This intimate and sustained contact offers vast therapeutic opportunities for localized drug release. Herein, a new class of hybrid hydrogel actuator (HHA) that facilitates enhanced drug delivery is introduced. The multi-material soft actuator can elicit a tunable mechanoresponsive release of charged drug from its alginate/acrylamide hydrogel layer with temporal control. Dosing control parameters include actuation magnitude, frequency, and duration. The actuator can safely adhere to tissue via a flexible, drug-permeable adhesive bond that can withstand dynamic device actuation. Conformal adhesion of the hybrid hydrogel actuator to tissue leads to improved mechanoresponsive spatial delivery of the drug. Future integration of this hybrid hydrogel actuator with other soft robotic assistive technologies can enable a synergistic, multi-pronged treatment approach for the treatment of disease.

Lab2Field transfer of a robotic raspberry harvester enabled by a soft sensorized physical twin

Robotic fruit harvesting requires dexterity to handle delicate crops and development relying upon field testing possible only during the harvesting season. Here we focus on raspberry crops, and explore how the research methodology of harvesting robots can be accelerated through soft robotic technologies. We propose and demonstrate a physical twin of the harvesting environment: a sensorized physical simulator of a raspberry plant with tunable properties, used to train a robotic harvester in the laboratory regardless of season. The sensors on the twin allow for direct comparison with human demonstrations, used to tune the robot controllers. In early field demonstrations, an 80% harvesting success rate was achieved without any modifications on the lab trained robot.

Experimental characterization of an asymmetric valveless pump based on soft robotics technology

Asymmetric pumping can be achieved by periodically compressing a flexible tube in its plane of symmetry using an actuator, as long as the rigid pipes connected to its ends are asymmetric. This mechanism, together with impedance pumping, composes the Liebau effect. While there have been numerous studies on impedance pumping, there is a lack of available research on asymmetric pumping. The aim of this study is to examine the influence of key parameters on the performance of this type of pump. In addition, this study implements an actuator based on soft robotics technology in asymmetric valveless pumping for the first time. The pump developed in this study can be applied in different areas involving the pumping of special fluids, including biomedical applications.

Learning-Based Control Strategies for Soft Robots: Theory, Achievements, and Future Challenges

In the last few decades, soft robotics technologies have challenged conventional approaches by introducing new, compliant bodies to the world of rigid robots. These technologies and systems may enable a wide range of applications, including human–robot interaction and dealing with complex environments. Soft bodies can adapt their shape to contact surfaces, distribute stress over a larger area, and increase the contact surface area, thus reducing impact forces.

Design and Modeling of Fabric-Shelled Pneumatic Bending Soft Actuators

In recent years, the use of fabric to manufacture and integrate soft actuators and sensors into wearable robots elucidates the benefits of soft robotic technology, closing the gap between humans and machines. However, many limitations in terms of ease of manufacturing, customization, modeling, output force and control remain unsolved and reduce the number of practical applications of fabric-based soft robotic systems. To address these limitations, we propose the use of laser-cut fabric shells laminated onto thermoplastic polyurethane as a pneumatic bending soft actuator. In this paper the design, fabrication, modeling, and control of a fabric shelled bending actuator is presented. Our experiments suggest that the fabric-shelled actuators are highly tunable and usable for high force applications. Moreover, the finite element model developed can predict the behavior of different fabric shelled actuator designs. Finally, we show how precise control of the actuators can be achieved by the implementation of a feedback curvature control system.

3D printable resin/carbon nanotube composites for wearable strain sensors: Enhancing mechanical and electrical properties

Electrically conductive composite materials are widely used for several applications, particularly flexible strain sensors for wearable electronic devices to detect human motions or apply to soft robotic technologies. Among the carbon nanotube (CNT) composites for strain sensors, the CNT composites with polypyrrole (PPy) have been scarcely found for electromechanical property improvement via digital light processing (DLP) 3D printing. Thus, this research presents a 3D printing fabrication of flexible polyurethane/CNT/PPy (FPU/CNT/PPy) composites for high complexity and model specifications, particularly strain sensing. The FPU/CNT/PPy composites were prepared as photopolymer resin with various proportions of the three components. The composites successfully achieved a gauge factor of 91.9 at 20 percent strain with high durability in 100 stretching-releasing cycles. More than 85% of printing accuracy was also found under the DLP 3D printing method. The small deformation of the composites could be detected during the stretching-releasing process during time-dependent testing. Examples of simple human activity and complex spring motion could be successfully detected due to the high responsibility and reproducibility of the finger motion. The composites mentioned in this study could reach the field of strain sensing and can further be applied to other fields such as healthcare and soft robotics.

Characteristics and Emerging Trends in Research on Rehabilitation Robots from 2001 to 2020: Bibliometric Study.

The past 2 decades have seen rapid development in the use of robots for rehabilitation. Research on rehabilitation robots involves interdisciplinary activities, making it a great challenge to obtain comprehensive insights in this research field. We performed a bibliometric study to understand the characteristics of research on rehabilitation robots and emerging trends in this field in the last 2 decades. Reports on the topic of rehabilitation robots published from January 1, 2001, to December 31, 2020, were retrieved from the Web of Science Core Collection on July 28, 2022. Document types were limited to "article" and "meeting" (excluding the "review" type), to ensure that our analysis of the evolution over time of this research had high validity. We used CiteSpace to conduct a co-occurrence and co-citation analysis and to visualize the characteristics of this research field and emerging trends. Landmark publications were identified using metrics such as betweenness centrality and burst strength. Through data retrieval, cleaning, and deduplication, we retrieved 9287 publications and 110,619 references cited in these publications that were on the topic of rehabilitation robots and were published between 2001 and 2020. Results of the Mann-Kendall test indicated that the numbers of both publications (P<.001; St=175.0) and citations (P<.001; St=188.0) related to rehabilitation robots exhibited a significantly increasing yearly trend. The co-occurrence results revealed 120 categories connected with research on rehabilitation robots; we used these categories to determine research relationships. The co-citation results identified 169 co-citation clusters characterizing this research field and emerging trends in it. The most prominent label was "soft robotic technology" (the burst strength was 79.07), which has become a topic of great interest in rehabilitative recovery for both the upper and lower limbs. Additionally, task-oriented upper-limb training, control strategies for robot-assisted lower limb rehabilitation, and power in exoskeleton robots were topics of great interest in current research. Our work provides insights into research on rehabilitation robots, including its characteristics and emerging trends during the last 2 decades, providing a comprehensive understanding of this research field.

Soft Robotics Enables Neuroprosthetic Hand Design.

Development and implementation of neuroprosthetic hands is a multidisciplinary field at the interface between humans and artificial robotic systems, which aims at replacing the sensorimotor function of the upper-limb amputees as their own. Although prosthetic hand devices with myoelectric control can be dated back to more than 70 years ago, their applications with anthropomorphic robotic mechanisms and sensory feedback functions are still at a relatively preliminary and laboratory stage. Nevertheless, a recent series of proof-of-concept studies suggest that soft robotics technology may be promising and useful in alleviating the design complexity of the dexterous mechanism and integration difficulty of multifunctional artificial skins, in particular, in the context of personalized applications. Here, we review the evolution of neuroprosthetic hands with the emerging and cutting-edge soft robotics, covering the soft and anthropomorphic prosthetic hand design and relating bidirectional neural interactions with myoelectric control and sensory feedback. We further discuss future opportunities on revolutionized mechanisms, high-performance soft sensors, and compliant neural-interaction interfaces for the next generation of neuroprosthetic hands.

Acceptability of Automated Robotic Clinical Breast Examination: Survey Study

In the United Kingdom, women aged 50 to 70 years are invited to undergo mammography. However, 10% of invasive breast cancers occur in women aged ≤45 years, representing an unmet need for young women. Identifying a suitable screening modality for this population is challenging; mammography is insufficiently sensitive, whereas alternative diagnostic methods are invasive or costly. Robotic clinical breast examination (R-CBE)-using soft robotic technology and machine learning for fully automated clinical breast examination-is a theoretically promising screening modality with early prototypes under development. Understanding the perspectives of potential users and partnering with patients in the design process from the outset is essential for ensuring the patient-centered design and implementation of this technology. This study investigated the attitudes and perspectives of women regarding the use of soft robotics and intelligent systems in breast cancer screening. It aimed to determine whether such technology is theoretically acceptable to potential users and identify aspects of the technology and implementation system that are priorities for patients, allowing these to be integrated into technology design. This study used a mixed methods design. We conducted a 30-minute web-based survey with 155 women in the United Kingdom. The survey comprised an overview of the proposed concept followed by 5 open-ended questions and 17 closed questions. Respondents were recruited through a web-based survey linked to the Cancer Research United Kingdom patient involvement opportunities web page and distributed through research networks' mailing lists. Qualitative data generated via the open-ended questions were analyzed using thematic analysis. Quantitative data were analyzed using 2-sample Kolmogorov-Smirnov tests, 1-tailed t tests, and Pearson coefficients. Most respondents (143/155, 92.3%) indicated that they would definitely or probably use R-CBE, with 82.6% (128/155) willing to be examined for up to 15 minutes. The most popular location for R-CBE was at a primary care setting, whereas the most accepted method for receiving the results was an on-screen display (with an option to print information) immediately after the examination. Thematic analysis of free-text responses identified the following 7 themes: women perceive that R-CBE has the potential to address limitations in current screening services; R-CBE may facilitate increased user choice and autonomy; ethical motivations for supporting R-CBE development; accuracy (and users' perceptions of accuracy) is essential; results management with clear communication is a priority for users; device usability is important; and integration with health services is key. There is a high potential for the acceptance of R-CBE in its target user group and a high concordance between user expectations and technological feasibility. Early patient participation in the design process allowed the authors to identify key development priorities for ensuring that this new technology meets the needs of users. Ongoing patient and public involvement at each development stage is essential.

Soft robotics towards sustainable development goals and climate actions.

Soft robotics technology can aid in achieving United Nations' Sustainable Development Goals (SDGs) and the Paris Climate Agreement through development of autonomous, environmentally responsible machines powered by renewable energy. By utilizing soft robotics, we can mitigate the detrimental effects of climate change on human society and the natural world through fostering adaptation, restoration, and remediation. Moreover, the implementation of soft robotics can lead to groundbreaking discoveries in material science, biology, control systems, energy efficiency, and sustainable manufacturing processes. However, to achieve these goals, we need further improvements in understanding biological principles at the basis of embodied and physical intelligence, environment-friendly materials, and energy-saving strategies to design and manufacture self-piloting and field-ready soft robots. This paper provides insights on how soft robotics can address the pressing issue of environmental sustainability. Sustainable manufacturing of soft robots at a large scale, exploring the potential of biodegradable and bioinspired materials, and integrating onboard renewable energy sources to promote autonomy and intelligence are some of the urgent challenges of this field that we discuss in this paper. Specifically, we will present field-ready soft robots that address targeted productive applications in urban farming, healthcare, land and ocean preservation, disaster remediation, and clean and affordable energy, thus supporting some of the SDGs. By embracing soft robotics as a solution, we can concretely support economic growth and sustainable industry, drive solutions for environment protection and clean energy, and improve overall health and well-being.

Opportunities and Challenges in Space Robotics

Robotic technology has the potential to play a major part in the quest to uncover the origins of life and investigate the feasibility of long-term human habitation outside of Earth. However, the harsh conditions present in outer space, including radiation, vacuum, temperature fluctuations, and complex, unpredictable terrain, pose significant obstacles to the development and operation of robotic systems. Although a variety of robotic platforms have been created for exploration, construction, and maintenance in space, their capacity to address fundamental questions remains largely untapped. In particular, the design, construction, and control of robots that can withstand the rigors of extreme space environments remain a significant challenge. In this special section, which is building on a workshop co-organized by the guest editor at the 2020 IEEE International Conference on Robotics and Automation, we focus on the recent advancements of space robots, as well as opportunities and critical challenges of this field. In particular, we cover topics of significant interest to the field including mobility on granular terrain and bio-inspired and soft robots for extraterrestrial applications. Because research pertaining to space robotics is generally interdisciplinary in nature, we include articles not only from roboticists but also from physicists and planetary scientists. This special section includes two research articles (2100040, 2100125), two review articles (2100063, 2200071), and two perspectives (2100106, 2100195). The research article by Khajenejad et al., presents an alternative set-valued or set-membership estimation framework for estimation of unknown planetary terrain parameters based on how they affect rover motion (2100040). The other research article by Green et al., presents CASPER, a novel screw-propelled excavation system for planetary exploration and excavation. This platform employs screw propellers for both mobility and excavation and demonstrates significant excavation capability with low mass and power requirements (2100125). The two review papers in this special section discuss robotic attachment mechanisms (2100063) and soft robots (2200071) for space applications. In particular, the article by Spenko, reviews robotic attachment mechanisms for extraterrestrial applications. Specifically, bio-inspired fibrillar dry adhesives for smooth surfaces, electrostatic adhesives for smooth and rougher surfaces, and microspines for rocky and rough surfaces are discussed (2100063). The article by Zhang et al., reviews soft robotics technologies for space applications. Particularly, advantages of soft robots for such applications and potential design, modeling, fabrication, sensing, and control improvements for adapting to space environments are described (2200071). Finally, there are two perspective articles included in this special section that provide insight on soft robots (2100106), and bio-inspired multi-legged and limbless robots (2100195) for mobility on extra-terrestrial terrain. Ng and Lum elaborate on key advantages and potential changes to actuation, fabrication, and locomotion of soft robots for future planetary explorations (2100106). Li and Lewis discuss using terradynamics insights for developing bio-inspired ground robots that are effective in maneuvering on granular and rocky planetary terrain (2100195). The guest editor extends heartfelt gratitude to the authors, reviewers, and the editorial team for their great contributions to this special section. We hope this collection of articles will inspire researchers currently working in space robotics, and also encourage other researchers to engage this exciting and challenging field. Hamid Marvi

Early career scientists converse on the future of soft robotics.

During the recent decade, we have witnessed an extraordinary flourishing of soft robotics. Rekindled interest in soft robots is partially associated with the advances in manufacturing techniques that enable the fabrication of sophisticated multi-material robotic bodies with dimensions ranging across multiple length scales. In recent manuscripts, a reader might find peculiar-looking soft robots capable of grasping, walking, or swimming. However, the growth in publication numbers does not always reflect the real progress in the field since many manuscripts employ very similar ideas and just tweak soft body geometries. Therefore, we unreservedly agree with the sentiment that future research must move beyond "soft for soft's sake." Soft robotics is an undoubtedly fascinating field, but it requires a critical assessment of the limitations and challenges, enabling us to spotlight the areas and directions where soft robots will have the best leverage over their traditional counterparts. In this perspective paper, we discuss the current state of robotic research related to such important aspects as energy autonomy, electronic-free logic, and sustainability. The goal is to critically look at perspectives of soft robotics from two opposite points of view provided by early career researchers and highlight the most promising future direction, that is, in our opinion, the employment of soft robotic technologies for soft bio-inspired artificial organs.