Computerized Cuff Pressure Algometry as Guidance for Circumferential Tissue Compression for Wearable Soft Robotic Applications: A Systematic Review

Investigating change of discomfort during repetitive force exertion though an exoskeleton cuff

This article investigates discomfort development for forces exerted repetitively and for extended durations through a rigid cuff. Three force patterns, chosen to mimic exoskeleton use, were applied to the thigh of 15 healthy participants for 30 min. Changes in perceived comfort and skin effects were recorded. Discomfort was detected at normal forces ranging from 40 to >230 N. Repetitive force application triggered discomfort after a median of 4.1 min (normal force only) and 5.4 min (normal and shear force) respectively. Discomfort increased over time but the repetitive force applications did generally not result in pain and there were no significant differences between repetitive loading patterns. Exoskeleton design and use should be informed by comfort thresholds specific to prolonged repetitive loading. Large interindividual differences in perception of discomfort limit the possibilities for generally applicable comfort thresholds. Further research is needed to investigate how patient groups perceive such repetitive loading.

Lightweight Pneumatically Elastic Backbone Structure with Modular Construction and Nonlinear Interaction for Soft Actuators

There has been a growing need for soft robots operating various force-sensitive tasks due to their environmental adaptability, satisfactory controllability, and nonlinear mobility unique from rigid robots. It is of desire to further study the system instability and strongly nonlinear interaction phenomenon that are the main influence factors to the actuations of lightweight soft actuators. In this study, we present a design principle on lightweight pneumatically elastic backbone structure (PEBS) with the modular construction for soft actuators, which contains a backbone printed as one piece and a common strip balloon. We build a prototype of a lightweight (<80 g) soft actuator, which can perform bending motions with satisfactory output forces (∼20 times self-weight). Experiments are conducted on the bending effects generated by interactions between the hyperelastic inner balloon and the elastic backbone. We investigated the nonlinear interaction and system instability experimentally, numerically, and parametrically. To overcome them, we further derived a theoretical nonlinear model and a numerical model. Satisfactory agreements are obtained between the numerical, theoretical, and experimental results. The accuracy of the numerical model is fully validated. Parametric studies are conducted on the backbone geometry and stiffness, balloon stiffness, thickness, and diameter. The accurate controllability, operation safety, modularization ability, and collaborative ability of the PEBS are validated by designing PEBS into a soft laryngoscope, a modularized PEBS library for a robotic arm, and a PEBS system that can operate remote surgery. The reported work provides a further applicability potential of soft robotics studies.

Tonic cuff pressure pain sensitivity in chronic pain patients and its relation to self-reported physical activity

OBJECTIVES

Physical inactivity is a global health concern and a significant problem among chronic pain patients. They often experience pain flare-ups when they try to increase their physical activity level. Most research on the relationship between pain sensitivity and physical activity has been on healthy participants. Data on chronic pain patients are lacking. Using cuff pressure algometry, this study investigated tonic cuff pressure pain sensitivity and its associations to self-reported physical activity and other patient-reported outcomes in chronic pain patients.

METHODS

Chronic pain patients (n=78) were compared to healthy controls (n=98). Multivariate data analysis was used to investigate the associations between tonic cuff pressure pain sensitivity, physical activity, and other patient-reported outcome measures.

RESULTS

The three most important variables for group discrimination were perceived health status (EQVAS: p(corr)=-0.85, i.e., lower in patients), depression (HADS-D: p(corr)=0.81, i.e., higher in patients), and the tonic cuff pressure pain sensitivity variable maximum pain intensity (VAS-peak-arm: p(corr)=0.75, i.e., higher in patients). In patients, the most important predictors for high VAS-peak-arm were female sex (p(corr)=-0.75), higher number of painful regions (p(corr)=0.72), higher pain intensity (p(corr)=0.55), followed by lower level of self-reported physical activity (p(corr)=-0.39). VAS-peak-arm in patients correlated negatively with self-reported physical activity (rho=-0.28, p=0.018).

CONCLUSIONS

Physical activity may be the most important patient-changeable variable correlating to pain sensitivity. This study highlights the importance of more research to further understand how increased physical activity may decrease pain sensitivity in chronic pain patients.

Human-Robot Joint Misalignment, Physical Interaction, and Gait Kinematic Assessment in Ankle-Foot Orthoses

Lower limb exoskeletons and orthoses have been increasingly used to assist the user during gait rehabilitation through torque transmission and motor stability. However, the physical human-robot interface (HRi) has not been properly addressed. Current orthoses lead to spurious forces at the HRi that cause adverse effects and high abandonment rates. This study aims to assess and compare, in a holistic approach, human-robot joint misalignment and gait kinematics in three fixation designs of ankle-foot orthoses (AFOs). These are AFOs with a frontal shin guard (F-AFO), lateral shin guard (L-AFO), and the ankle modulus of the H2 exoskeleton (H2-AFO). An experimental protocol was implemented to assess misalignment, fixation displacement, pressure interactions, user-perceived comfort, and gait kinematics during walking with the three AFOs. The F-AFO showed reduced vertical misalignment (peak of 1.37 ± 0.90 cm, p-value < 0.05), interactions (median pressures of 0.39-3.12 kPa), and higher user-perceived comfort (p-value < 0.05) when compared to H2-AFO (peak misalignment of 2.95 ± 0.64 and pressures ranging from 3.19 to 19.78 kPa). F-AFO also improves the L-AFO in pressure (median pressures ranging from 8.64 to 10.83 kPa) and comfort (p-value < 0.05). All AFOs significantly modified hip joint angle regarding control gait (p-value < 0.01), while the H2-AFO also affected knee joint angle (p-value < 0.01) and gait spatiotemporal parameters (p-value < 0.05). Overall, findings indicate that an AFO with a frontal shin guard and a sports shoe is effective at reducing misalignment and pressure at the HRI, increasing comfort with slight changes in gait kinematics.

Evaluation Methods and Measurement Challenges for Industrial Exoskeletons

In recent years, exoskeleton test methods for industrial exoskeletons have evolved to include simulated laboratory and field environments. Physiological, kinematic, and kinetic metrics, as well as subjective surveys, are used to evaluate exoskeleton usability. In particular, exoskeleton fit and usability can also impact the safety of exoskeletons and their effectiveness at reducing musculoskeletal injuries. This paper surveys the state of the art in measurement methods applied to exoskeleton evaluation. A notional classification of the metrics based on exoskeleton fit, task efficiency, comfort, mobility, and balance is proposed. In addition, the paper describes the test and measurement methods used in supporting the development of exoskeleton and exosuit evaluation methods to assess their fit, usability, and effectiveness in industrial tasks such as peg in hole, load align, and applied force. Finally, the paper includes a discussion of how the metrics can be applied towards a systematic evaluation of industrial exoskeletons, current measurement challenges, and future research directions.

A Trade-Off between Complexity and Interaction Quality for Upper Limb Exoskeleton Interfaces

Exoskeletons are among the most promising devices dedicated to assisting human movement during reeducation protocols and preventing musculoskeletal disorders at work. However, their potential is currently limited, partially because of a fundamental contradiction impacting their design. Indeed, increasing the interaction quality often requires the inclusion of passive degrees of freedom in the design of human-exoskeleton interfaces, which increases the exoskeleton's inertia and complexity. Thus, its control also becomes more complex, and unwanted interaction efforts can become important. In the present paper, we investigate the influence of two passive rotations in the forearm interface on sagittal plane reaching movements while keeping the arm interface unchanged (i.e., without passive degrees of freedom). Such a proposal represents a possible compromise between conflicting design constraints. The in-depth investigations carried out here in terms of interaction efforts, kinematics, electromyographic signals, and subjective feedback of participants all underscored the benefits of such a design. Therefore, the proposed compromise appears to be suitable for rehabilitation sessions, specific tasks at work, and future investigations into human movement using exoskeletons.

Ischemic preconditioning and exercise performance: are the psychophysiological responses underestimated?

The findings of the ischemic preconditioning (IPC) on exercise performance are mixed regarding types of exercise, protocols and participants' training status. Additionally, studies comparing IPC with sham (i.e., low-pressure cuff) and/or control (i.e., no cuff) interventions are contentious. While studies comparing IPC versus a control group generally show an IPC significant effect on performance, sham interventions show the same performance improvement. Thus, the controversy over IPC ergogenic effect may be due to limited discussion on the psychophysiological mechanisms underlying cuff maneuvers. Psychophysiology is the study of the interrelationships between mind, body and behavior, and mental processes are the result of the architecture of the nervous system and voluntary exercise is a behavior controlled by the central command modulated by sensory inputs. Therefore, this narrative review aims to associate potential IPC-induced positive effects on performance with sensorimotor pathways (e.g., sham influencing bidirectional body-brain integration), hemodynamic and metabolic changes (i.e., blood flow occlusion reperfusion cycles). Overall, IPC and sham-induced mechanisms on exercise performance may be due to a bidirectional body-brain integration of muscle sensory feedback to the central command resulting in delayed time to exhaustion, alterations on perceptions and behavior. Additionally, hemodynamic responses and higher muscle oxygen extraction may justify the benefits of IPC on muscle contractile function.

Quantification of Comfort for the Development of Binding Parts in a Standing Rehabilitation Robot

Human-machine interfaces (HMI) refer to the physical interaction between a user and rehabilitation robots. A persisting excessive load leads to soft tissue damage, such as pressure ulcers. Therefore, it is necessary to define a comfortable binding part for a rehabilitation robot with the subject in a standing posture. The purpose of this study was to quantify the comfort at the binding parts of the standing rehabilitation robot. In Experiment 1, cuff pressures of 10-40 kPa were applied to the thigh, shank, and knee of standing subjects, and the interface pressure and pain scale were obtained. In Experiment 2, cuff pressures of 10-20 kPa were applied to the thigh, and the tissue oxygen saturation and the skin temperature were measured. Questionnaire responses regarding comfort during compression were obtained from the subjects using the visual analog scale and the Likert scale. The greatest pain was perceived in the thigh. The musculoskeletal configuration affected the pressure distribution. The interface pressure distribution by the binding part showed higher pressure at the intermuscular septum. Tissue oxygen saturation (StO₂) increased to 111.9 ± 6.7% when a cuff pressure of 10 kPa was applied and decreased to 92.2 ± 16.9% for a cuff pressure of 20 kPa. A skin temperature variation greater than 0.2 °C occurred in the compressed leg. These findings would help evaluate and improve the comfort of rehabilitation robots.

Exoskeletons: A challenge for development

The development of exoskeletons is currently a lengthy process full of challenges. We are proposing a framework to accelerate the process and make the resulting exoskeletons more user-centered. The needed accomplishments in science are described in an effort to lay the foundation for future research projects. Since the early 2000s, exoskeletons have been discussed as an emerging technology in industrial, medical, or military applications. Those systems are designed to support people during manual tasks. At first, those systems lacked broad acceptance. Many models found their niches in ongoing developments and more diverse systems entering the market. There are still applications that are in dire need of such assistance. Due to the lack of experience with body-worn robotics, the development of such systems has been shaped by trial and error. The lack of legacy products results in longer development times. In this paper, a process to generate a framework is presented to display the required research to enable future exoskeleton designers. Owing to their proximity to the user's body, exoskeletons are highly complex systems that need sophisticated subsystems, such as kinematic, control, interaction design, or actuators, to be accepted by users. Due to the wide variety of fields and high user demands, a synchronized multidisciplinary effort is necessary. To achieve this, a process to develop a modular framework for exoskeleton design is proposed. It focuses on user- and use-case-centered solutions for matching kinematics, actuation, and control. To ensure the usefulness of the framework, an evaluation of the incorporated solutions is required.

Characterization and Evaluation of Human–Exoskeleton Interaction Dynamics: A Review

Exoskeletons and exosuits have witnessed unprecedented growth in recent years, especially in the medical and industrial sectors. In order to be successfully integrated into the current society, these devices must comply with several commercialization rules and safety standards. Due to their intrinsic coupling with human limbs, one of the main challenges is to test and prove the quality of physical interaction with humans. However, the study of physical human–exoskeleton interactions (pHEI) has been poorly addressed in the literature. Understanding and identifying the technological ways to assess pHEI is necessary for the future acceptance and large-scale use of these devices. The harmonization of these evaluation processes represents a key factor in building a still missing accepted framework to inform human–device contact safety. In this review, we identify, analyze, and discuss the metrics, testing procedures, and measurement devices used to assess pHEI in the last ten years. Furthermore, we discuss the role of pHEI in safety contact evaluation. We found a very heterogeneous panorama in terms of sensors and testing methods, which are still far from considering realistic conditions and use-cases. We identified the main gaps and drawbacks of current approaches, pointing towards a number of promising research directions. This review aspires to help the wearable robotics community find agreements on interaction quality and safety assessment testing procedures.

Industrial exoskeletons for overhead work: Circumferential pressures on the upper arm caused by the physical human-machine-interface

This study investigated the pressures occurring within the arm human-machine-interfaces (HMI) of four different exoskeletons that support static and dynamic work at or above head level, and the effects of the HMI on neurovascular supply of the upper extremity using an orthopedic provocation maneuver with raised arms with and without the exoskeletons. Decreased time in the provocation maneuver with exoskeletons indicated a negative effect of the HMIs on the vascular and neural supply of the arm. Average pressure in the static situation was 3.2 ± 0.7 kPa and 4.4 ± 0.4 kPa with regular peak values of 6.5 ± 0.5 kPa in the dynamic task. These pressures were significantly higher than the pressure values that guarantee adequate tissue oxygenation. It remains unknown whether the way exoskeletons apply pressure affects vascular and neural supply to the arms, or whether the regular unloading during dynamic activity has a neutralizing effect.

Challenges and solutions for application and wider adoption of wearable robots

The science and technology of wearable robots are steadily advancing, and the use of such robots in our everyday life appears to be within reach. Nevertheless, widespread adoption of wearable robots should not be taken for granted, especially since many recent attempts to bring them to real-life applications resulted in mixed outcomes. The aim of this article is to address the current challenges that are limiting the application and wider adoption of wearable robots that are typically worn over the human body. We categorized the challenges into mechanical layout, actuation, sensing, body interface, control, human-robot interfacing and coadaptation, and benchmarking. For each category, we discuss specific challenges and the rationale for why solving them is important, followed by an overview of relevant recent works. We conclude with an opinion that summarizes possible solutions that could contribute to the wider adoption of wearable robots.

Safety Assessment of Rehabilitation Robots: A Review Identifying Safety Skills and Current Knowledge Gaps

The assessment of rehabilitation robot safety is a vital aspect of the development process, which is often experienced as difficult. There are gaps in best practices and knowledge to ensure safe usage of rehabilitation robots. Currently, safety is commonly assessed by monitoring adverse events occurrence. The aim of this article is to explore how safety of rehabilitation robots can be assessed early in the development phase, before they are used with patients. We are suggesting a uniform approach for safety validation of robots closely interacting with humans, based on safety skills and validation protocols. Safety skills are an abstract representation of the ability of a robot to reduce a specific risk or deal with a specific hazard. They can be implemented in various ways, depending on the application requirements, which enables the use of a single safety skill across a wide range of applications and domains. Safety validation protocols have been developed that correspond to these skills and consider domain-specific conditions. This gives robot users and developers concise testing procedures to prove the mechanical safety of their robotic system, even when the applications are in domains with a lack of standards and best practices such as the healthcare domain. Based on knowledge about adverse events occurring in rehabilitation robot use, we identified multi-directional excessive forces on the soft tissue level and musculoskeletal level as most relevant hazards for rehabilitation robots and related them to four safety skills, providing a concrete starting point for safety assessment of rehabilitation robots. We further identified a number of gaps which need to be addressed in the future to pave the way for more comprehensive guidelines for rehabilitation robot safety assessments. Predominantly, besides new developments of safety by design features, there is a strong need for reliable measurement methods as well as acceptable limit values for human-robot interaction forces both on skin and joint level.

Integration of 3D Printed Flexible Pressure Sensors into Physical Interfaces for Wearable Robots

Sensing pressure at the physical interface between the robot and the human has important implications for wearable robots. On the one hand, monitoring pressure distribution can give valuable benefits on the aspects of comfortability and safety of such devices. Additionally, on the other hand, they can be used as a rich sensory input to high level interaction controllers. However, a problem is that the commercial availability of this technology is mostly limited to either low-cost solutions with poor performance or expensive options, limiting the possibilities for iterative designs. As an alternative, in this manuscript we present a three-dimensional (3D) printed flexible capacitive pressure sensor that allows seamless integration for wearable robotic applications. The sensors are manufactured using additive manufacturing techniques, which provides benefits in terms of versatility of design and implementation. In this study, a characterization of the 3D printed sensors in a test-bench is presented after which the sensors are integrated in an upper arm interface. A human-in-the-loop calibration of the sensors is then shown, allowing to estimate the external force and pressure distribution that is acting on the upper arm of seven human subjects while performing a dynamic task. The validation of the method is achieved by means of a collaborative robot for precise force interaction measurements. The results indicate that the proposed sensors are a potential solution for further implementation in human–robot interfaces.

Design and testing of a soft parallel robot based on pneumatic artificial muscles for wrist rehabilitation

Wrist rehabilitation is needed to help post-stroke and post-surgery patients recover from wrist fracture or injury. Traditional rehabilitation training is conducted by a therapist in a hospital, which hinders timely treatment due to the corresponding time and space constraints. This paper presents the design and implementation of a soft parallel robot for automated wrist rehabilitation. The presented wrist rehabilitation robot integrates the advantages of both soft robot and parallel robot structures. Unlike traditional rigid-body based rehabilitation robots, this soft parallel robot exhibits a compact structure, which is highly secure, adaptable, and flexible and thus a low-cost solution for personalized treatment. The proposed soft wrist-rehabilitation robot is driven by six evenly distributed linear actuators using pneumatic artificial muscles and one central linear electric motor. The introduced parallel-kinematic mechanism design enables the enhancement of the output stiffness of the soft robot for practical use. An electromyography sensor is adopted to provide feedback signals for evaluating the rehabilitation training process. A kinematic model of the designed robot is derived, and a prototype is fabricated for experimental testing. The results demonstrate that the developed soft rehabilitation robot can assist the wrist to realize all the required training motions, including abduction-adduction, flexion-extension, and supination-pronation. The compact and lightweight structure of this novel robot makes it convenient to use, and suitable rehabilitation training modes can be chosen for tailored rehabilitation at home or in a hospital.

Exoskeleton kinematic design robustness: An assessment method to account for human variability

Exoskeletons are wearable devices intended to physically assist one or multiple human joints in executing certain activities. From a mechanical point of view, they are kinematic structures arranged in parallel to the biological joints. In order to allow the users to move while assisted, it is crucial to avoid mobility restrictions introduced by the exoskeleton's kinematics. Passive degrees of freedom and other self-alignment mechanisms are a common option to avoid any restrictions. However, the literature lacks a systematic method to account for large inter- and intra-subject variability in designing and assessing kinematic chains. To this end, we introduce a model-based method to assess the kinematics of exoskeletons by representing restrictions in mobility as disturbances and undesired forces at the anchor points. The method makes use of robotic kinematic tools and generates useful insights to support the design process. Though an application on a back-support exoskeleton designed for lifting tasks is illustrated, the method can describe any type of rigid exoskeleton. A qualitative pilot trial is conducted to assess the kinematic model that proved to predict kinematic configurations associated to rising undesired forces recorded at the anchor points, that give rise to mobility restrictions and discomfort on the users.

Circumferential tissue compression at the lower limb during walking, and its effect on discomfort, pain and tissue oxygenation: Application to soft exoskeleton design

Soft exoskeletons apply compressive forces at the limbs via connection cuffs to actuate movement or stabilise joints. To avoid excessive mechanical loading, the interface with the wearer's body needs to be carefully designed. The purpose of this study was to establish the magnitude of circumferential compression at the lower limb during walking that causes discomfort/pain. It was hypothesized that the thresholds differ from those during standing. A cohort of 21 healthy participants were tested using two sizes of pneumatic cuffs, inflated at the thigh and calf in a tonic or phasic manner. The results showed lower inflation pressures triggering discomfort/pain at the thigh, with tonic compression, and wider pneumatic cuffs. The thresholds were lower during walking than standing still. Deep tissue oxygenation increased during phasic compression and decreased during tonic compression. According to the findings, circumferential compression by soft exoskeletons is preferably applied at anatomical sites with smaller volumes of soft tissue, using narrow connection cuffs and inflation pressures below 14 kPa.

The effect of simulated circumferential soft exoskeleton compression at the knee on discomfort and pain

There is a lack of data and guidance on soft exoskeleton pressure contact with the body. The purpose of this research was to study the relationship between circumferential loading at the knee and discomfort/pain, to inform the design of soft exoskeletons/exosuits. The development of discomfort and pain was studied during standing and walking with circumferential compression using a pneumatic cuff. Our results show higher tolerance for intermittent than continuous compression during standing. Discomfort was triggered at pressures ranging from 13.7 kPa (continuous compression) to 30.4 kPa (intermittent compression), and pain at 52.9 kPa (continuous compression) to 60.6 kPa (intermittent compression). During walking, cyclic compression caused an increase in discomfort with time. Higher cuff inflation pressures caused an earlier onset and higher end intensities of discomfort than lower pressures. Cyclic cuff inflation of 10 kPa and 20 kPa was reasonably well tolerated. Practitioner summary Soft exoskeleton compression of the knee was simulated during static and dynamic compression cycles. The results can be used to understand how users tolerate pressure at the knee, and also to understand the levels at which discomfort and pain are experienced. Abbreviations: BMI: body mass index; DDT: discomfort detection threshold; EndVAS: end of experiment rating on visual analog discomfort scale; PDT: pain detection threshold; SD: standard deviation; SE: standard error; TSP: temporal summation of pain; VAS: visual analogue scale.

Discomfort/Pain and Tissue Oxygenation at the Lower Limb During Circumferential Compression: Application to Soft Exoskeleton Design

OBJECTIVE

To establish the relationship between circumferential compression on the lower limb during simulated ramp and staircase profile loading, and the resultant relationship with discomfort/pain and tissue oxygenation.

BACKGROUND

Excessive mechanical loading by exoskeletons on the body can lead to pressure-related soft tissue injury. Potential tissue damage is associated with objective oxygen deprivation and accompanied by subjective perception of pain and discomfort.

METHOD

Three widths of pneumatic cuffs were inflated at the dominant thigh and calf of healthy participants using two inflation patterns (ramp and staircase), using a computer-controlled pneumatic rig. Participants rated discomfort on an electronic visual analog scale and deep tissue oxygenation was monitored using near infrared spectroscopy.

RESULTS

Circumferential compression with pneumatic cuffs triggered discomfort and pain at lower pressures at the thigh, with wider cuffs, and with a ramp inflation pattern. Staircase profile compression caused an increase in deep tissue oxygenation, whereas the ramp profile compression decreased it.

CONCLUSION

Discomfort and pain during circumferential compression at the lower limb is related to the width of pneumatic cuffs, the inflation pattern, and the volume of soft tissue at the assessment site. The occurrence of pain is also possibly related to the decrease in deep tissue oxygenation during compression.

APPLICATION

Our findings can be used to inform safe and comfortable design of soft exoskeletons to avoid discomfort and possible soft tissue injury.

Soft Rod-Climbing Robot Inspired by Winding Locomotion of Snake

Soft climbing robots have attracted much attention of researchers for their potential applications on the wall or inside the tube. However, making a soft robot climb on the outer surface of a rod or tube by agile and efficient motion has long been a challenge. Inspired by the winding climbing locomotion of arboreal snakes, a tethered pneumatic-actuated winding-styled soft rod-climbing robot that consists of two winding actuators and a telescopic actuator is proposed in this work. Based on constant curvature assumption, we develop a theoretical model to analyze the linear and bending motion of the actuators. We demonstrate that our robot can perform climbing locomotion similar to snakes, including turning around a corner along a rod, climbing a vertical rod with a maximum speed of 30.85 mm/s (0.193 body length/s), and carrying a larger payload (weight, 500 g, more than 25 times its self-weight) than existing soft climbing robots do on a vertical surface. In addition, the experimental tests exhibit the potential applications of the robot in special environments such as high-voltage cables, nuclear power plants, and underwater sites.