Soft Exosuits

We are developing next generation soft wearable robots that use innovative textiles to provide a more conformal, unobtrusive and compliant means to interface to the human body. These robots will augment the capabilities of healthy individuals (e.g. improved walking efficiency) in addition to assisting those with muscle weakness or patients who suffer from physical or neurological disorders. As compared to a traditional exoskeleton, these systems have several advantages: the wearer's joints are unconstrained by external rigid structures, and the worn part of the suit is extremely light.  These properties minimize the suit's unintentional interference with the body's natural biomechanics and allow for more synergistic interaction with the wearer.

Structured functional textiles

suitWe are creating innovative textiles that are inspired by an understanding of human biomechanics and anatomy. These wearable garments provide means to transmit assistive torques to a wearer’s joints without the use of rigid external structures. In order to obtain high-performance soft exosuits, some considerations should be taken into account in the design process. Exosuits should attach to the body securely and comfortably, and transmit forces over the body through beneficial paths such that biologically-appropriate moments are created at the joints.   In addition, these garments can be designed to passively (with no active power) generate assistive forces due to the natural movement of the wear for particular tasks. A key feature of exosuits is that if the actuated segments are extended, the suit length can increase so that the entire suit is slack, at which point wearing an exosuit feels like wearing a pair of pants and does not restrict the wearer whatsoever.

 

Lightweight and efficient actuation

In order to provide active assistance through the soft interface, we are developing a number of actuation platforms that can apply controlled forces to the wearer by attaching at anchoring points in the wearable garment. We are developing lightweight and fully portable systems and a key feature of our approach is that we minimize the distal mass that is attached to the wearer through more proximally mounted actuation systems and flexible transmissions that transmit power to the joints. While most of our recent work is on cable-driven electromechanical approaches, we have also pursued pneumatic based approaches. This early work with McKibbon actuators in 2013 was the first demonstration that a soft exosuit can have a positive effect on mobility. 

 

 

Wearable sensors

New sensor systems that are easy to integrate with textiles and soft components are required in order to properly control and evaluate soft exosuits. Rigid exoskeletons usually include sensors such as encoders or potentiometers in robotic joints that accurately track joint angles, but these technologies are not compatible with soft structures.  Our approach is to design new sensors to measure human kinematics and suit-human interaction forces that are robust, compliant, cost effective, and offer easy integration into wearable garments.  In addition, we use other off the shelf sensor technologies (e.g. gyro, pressure sensor, IMU) that can be used to detect key events in the gait cycle. These wearable sensors can be used as part of the control strategy for the wearable robot or alternatively to monitor and record the movement of the wearer (when wearing the exosuit or as a standalone sensor suit) for tracking changes over time or determining what activities they are performing (e.g. walking vs running).

 

Intuitive and robust control

We are also developing rapidly reconfigurable multi-actuator systems that provide more flexibility for lab-based studies. Such an approach allows us to rapidly explore the basic science around human-machine interaction with such systems that can then be used to guide the design of our portable systems. A robust, intuitive and adaptive human-machine interface is a necessary component for a wearable robot to interact synergistically with the wearer. Our focus is to provide assistance in a manner that does not disrupt the natural, passive dynamics that make walking or running so efficient. To achieve this, we develop approaches to non-invasively estimate the intent so that any actuation applied assists that from the appropriate biological muscles. A key feature of our approach is to leverage integrated sensors that monitor the wearer interaction with the compliant textile that interfaces to the body as well as other sensors that detect key moment during the gait cycle.

 

Experimental biomechanics

Our motion capture lab utilizes a Vicon T-series 9-camera system for motion capture, together with a Bertec fully instrumented split-belt treadmill to measure GRFs. By comparing the average profile and range of motion of each joint in the three conditions, we can identify how the soft exosuit itself impacts gait and how the assistance applied by the exosuit changes kinematics. Our hypothesis is that it is desirable that such changes are minimal and in any case not disruptive to natural gait. We study to what extent the active exosuit is assisting the human by analyzing gait dynamics and kinetics (joint moments, power, force delivered by the exosuit). Inverse dynamics is an effective way to determine to what degree the exosuit is augmenting the body function at a joint level. The comparison of joint moments and suit assistive forces allows us to monitor the degree of synchronicity between the user and the robot. Surface electromyography (sEMG) can be used to selectively monitor muscular activity focusing on the muscle groups that are most relevant for the task under consideration. Comparing the ensemble average profiles of sEMG activity between the unpowered, active and no suit conditions allows us to determine effects on the maximum force being delivered by each muscle (peak sEMG activation) and on the energy cost of each muscle activation (integral sEMG). We use the metabolic cost of walking as a global physiological measurement to determine to what extent the suit is assisting the wearer and if assistance offsets the weight of the device. 

 

Translational applications

In addition to our work on basic research and system development, we are highly interested in pursuing applications of our soft wearable robots. Through our DARPA funded work, we are interested in developing exosuits that can assist soldiers walking while carrying heavy loads. Our belief is we can create passive and active systems that offload the high forces in the muscles and tendons in the leg – thus reducing the risk of injury and increasing the walking efficiency of the wearer. Another translational focus of our group is on gait assistance for medical applications. We foresee soft exosuits being able to restore mobility in patients with muscle weakness (e.g. the elderly) or who suffer from a neurological disease such as multiple sclerosis or stroke. Beyond our active systems, we envision translational potential in the area of sports and recreation where fully passive soft suits with structured functional textiles can provide small amounts of assistance during walking, hiking, running and other activities.

 

Associated Papers

Stronger, Smarter, Softer: Next-Generation Wearable Robots
A. T. Asbeck, S. De Rossi, I. Galiana, Y. Ding, and C. J. Walsh, “Stronger, Smarter, Softer: Next-Generation Wearable Robots,” IEEE Robotics & Automation Magazine, vol. 21, no. 4, pp. 22-33, 2014. Publisher's VersionAbstract

Exosuits show much promise as a method for augmenting the body with lightweight, portable, and compliant wearable systems. We envision that such systems can be further refined so that they can be sufficiently low profile to fit under a wearer's existing clothing. Our focus is on creating an assistive device that provides a fraction of the nominal biological torques and does not provide external load transfer. In early work, we showed that the system can substantially maintain normal biomechanics and positively affect a wearer's metabolic rate. Many basic fundamental research and development challenges remain in actuator development, textile innovation, soft sensor development, human-machine interface (control), biomechanics, and physiology, which provides fertile ground for academic research in many disciplines. While we have focused on gait assistance thus far, numerous other applications are possible, including rehabilitation, upper body support, and assistance for other motions. We look forward to a future where wearable robots provide benefits for people across many areas of our society.

P. Malcolm, et al., “Varying negative work assistance at the ankle with a soft exosuit during loaded walking,” Journal of NeuroEngineering and Rehabilitation, vol. 14, no. 1, pp. 62, 2017. Publisher's VersionAbstract

Background
Only very recently, studies have shown that it is possible to reduce the metabolic rate of unloaded and loaded walking using robotic ankle exoskeletons. Some studies obtained this result by means of high positive work assistance while others combined negative and positive work assistance. There is no consensus about the isolated contribution of negative work assistance. Therefore, the aim of the present study is to examine the effect of varying negative work assistance at the ankle joint while maintaining a fixed level of positive work assistance with a multi-articular soft exosuit.

Methods
We tested eight participants during walking at 1.5 ms−1 with a 23-kg backpack. Participants wore a version of the exosuit that assisted plantarflexion via Bowden cables tethered to an off-board actuation platform. In four active conditions we provided different rates of exosuit bilateral ankle negative work assistance ranging from 0.015 to 0.037 W kg−1 and a fixed rate of positive work assistance of 0.19 W kg−1.

Results
All active conditions significantly reduced metabolic rate by 11 to 15% compared to a reference condition, where the participants wore the exosuit but no assistance was provided. We found no significant effect of negative work assistance. However, there was a trend (p = .08) toward greater reduction in metabolic rate with increasing negative work assistance, which could be explained by observed reductions in biological ankle and hip joint power and moment.

Conclusions
The non-significant trend of increasing negative work assistance with increasing reductions in metabolic rate motivates the value in further studies on the relative effects of negative and positive work assistance. There may be benefit in varying negative work over a greater range or in isolation from positive work assistance.

P. Malcolm, et al., “Continuous sweep versus discrete step protocols for studying effects of wearable robot assistance magnitude,” Journal of NeuroEngineering and Rehabilitation, vol. 14, no. 1, pp. 72, 2017. Publisher's VersionAbstract

Background
Different groups developed wearable robots for walking assistance, but there is still a need for methods to quickly tune actuation parameters for each robot and population or sometimes even for individual users. Protocols where parameters are held constant for multiple minutes have traditionally been used for evaluating responses to parameter changes such as metabolic rate or walking symmetry. However, these discrete protocols are time-consuming. Recently, protocols have been proposed where a parameter is changed in a continuous way. The aim of the present study was to compare effects of continuously varying assistance magnitude with a soft exosuit against discrete step conditions.

Methods
Seven participants walked on a treadmill wearing a soft exosuit that assists plantarflexion and hip flexion. In Continuous-up, peak exosuit ankle moment linearly increased from approximately 0 to 38% of biological moment over 10 min. Continuous-down was the opposite. In Discrete, participants underwent five periods of 5 min with steady peak moment levels distributed over the same range as Continuous-up and Continuous-down. We calculated metabolic rate for the entire Continuous-up and Continuous-down conditions and the last 2 min of each Discrete force level. We compared kinematics, kinetics and metabolic rate between conditions by curve fitting versus peak moment.

Results
Reduction in metabolic rate compared to Powered-off was smaller in Continuous-up than in Continuous-down at most peak moment levels, due to physiological dynamics causing metabolic measurements in Continuous-up and Continuous-down to lag behind the values expected during steady-state testing. When evaluating the average slope of metabolic reduction over the entire peak moment range there was no significant difference between Continuous-down and Discrete. Attempting to correct the lag in metabolics by taking the average of Continuous-up and Continuous-down removed all significant differences versus Discrete. For kinematic and kinetic parameters, there were no differences between all conditions.

Conclusions
The finding that there were no differences in biomechanical parameters between all conditions suggests that biomechanical parameters can be recorded with the shortest protocol condition (i.e. single Continuous directions). The shorter time and higher resolution data of continuous sweep protocols hold promise for the future study of human interaction with wearable robots.

Assistance magnitude versus metabolic cost reductions for a tethered multiarticular soft exosuit
B. T. Quinlivan, et al., “Assistance magnitude versus metabolic cost reductions for a tethered multiarticular soft exosuit,” Science Robotics, vol. 2, no. 2, pp. eaah4416, 2017. Publisher's VersionAbstract

When defining requirements for any wearable robot for walking assistance, it is important to maximize the user’s metabolic benefit resulting from the exosuit assistance while limiting the metabolic penalty of carrying the system’s mass. Thus, the aim of this study was to isolate and characterize the relationship between assistance magnitude and the metabolic cost of walking while also examining changes to the wearer’s underlying gait mechanics. The study was performed with a tethered multiarticular soft exosuit during normal walking, where assistance was directly applied at the ankle joint and indirectly at the hip due to a textile architecture. The exosuit controller was designed such that the delivered torque profile at the ankle joint approximated that of the biological torque during normal walking. Seven participants walked on a treadmill at 1.5 meters per second under one unpowered and four powered conditions, where the peak moment applied at the ankle joint was varied from about 10 to 38% of biological ankle moment (equivalent to an applied force of 18.7 to 75.0% of body weight). Results showed that, with increasing exosuit assistance, net metabolic rate continually decreased within the tested range. When maximum assistance was applied, the metabolic rate of walking was reduced by 22.83 ± 3.17% relative to the powered-off condition (mean ± SEM).

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