Design of a Low Profile, Unpowered Ankle Exoskeleton That Fits Under Clothes: Overcoming Practical Barriers to Widespread Societal Adoption

Abstract

Here, we present the design of a novel unpowered ankle exoskeleton that is low profile, lightweight, quiet, and low cost to manufacture, intrinsically adapts to different walking speeds, and does not restrict non-sagittal joint motion; while still providing assistive ankle torque that can reduce demands on the biological calf musculature. This paper is an extension of the previously-successful ankle exoskeleton concept by Collins, Wiggin, and Sawicki. We created a device that blends the torque assistance of the prior exoskeleton with the form-factor benefits of clothing. Our design integrates a low profile under-the-foot clutch and a soft conformal shank interface, coupled by an ankle assistance spring that operates in parallel with the user's calf muscles. We fabricated and characterized technical performance of a prototype through benchtop testing and then validated device functionality in two gait analysis case studies. To our knowledge, this is the first ankle plantarflexion assistance exoskeleton that could be feasibly worn under typical daily clothing, without restricting ankle motion, and without components protruding substantially from the shoe, leg, waist, or back. Our new design highlights the potential for performance-enhancing exoskeletons that are inexpensive, unobtrusive, and can be used on a wide scale to benefit a broad range of individuals throughout society, such as the elderly, individuals with impaired plantarflexor muscle strength, or recreational users. In summary, this paper demonstrates how an unpowered ankle exoskeleton could be redesigned to more seamlessly integrate into daily life, while still providing performance benefits for common locomotion tasks.

Fig. 1.

Examples of portable exoskeletons that assist ankle plantarflexion. A) Powered exoskeleton from TU Delft uses an electric motor and leaf spring design [25], [26]. The device requires a backpack-mounted controller and battery pack (not depicted). B) Powered exoskeleton from MIT also uses an electric motor plus leaf spring design, but in a different configuration [3], [4]. A battery pack and controller unit are worn at the waist (not depicted). C) Unpowered exoskeleton from Carnegie Mellon and North Carolina State University uses a clutch and spring [2], [27]. D) Powered soft exoskeleton (exosuit) from Harvard uses motorized Bowden cables to assist ankle plantarflexion (and also dorsiflexion) [28]–[30]. A battery pack, and actuator unit are worn at the waist (not depicted). Several ankle exoskeletons have demonstrated objective benefits in terms of reducing metabolic cost or muscle activity during walking. However, existing designs tend to have mechanical elements that protrude out from the leg, foot, back or waist, which could impede widespread adoption of exoskeletal technologies in society; particularly amongst individuals who prefer to wear devices inconspicuously under their normal clothing.

Fig. 2.

New ankle exoskeleton design combines form-factor of clothing with the function and assistive benefits of a previously-successful unpowered exoskeleton [2]. This design integrates into the shoe and under clothing such that it could be worn inconspicuously in daily life. An additional hook- and-loop strap was attached at the top of the interface and used to provide light compression and distribute forces. It was not depicted to avoid visually obscuring the semi-rigid plastic component on the shank.

Fig. 3.

Exoskeleton function. During walking, the stiff assistance spring is engaged (clutched) during stance phase to assist the plantarflexors, then this spring is disengaged (unclutched) during leg swing to allow for normal, unrestricted ankle dorsiflexion. At foot contact the ankle is slightly dorsiflexed beyond neutral (foot perpendicular to shank), and the weak reset spring is in a stretched position. After foot contact, the ankle begins to plantarflex until the foot becomes flat on the ground. This motion allows the reset spring to recoil. Recoil of the reset spring then pulls the unstretched assistance spring to its default position. As the person progresses over their foot throughout stance, the center-of-pressure under the foot (i.e., location of net ground reaction force) also progresses forward. The ground reaction force compresses the gripper upon the slider, clutching it. In other words, the ground reaction force increases the friction force (between the slider and gripper), preventing further motion of the slider. Because the slider is now fixed, dorsiflexion of the ankle stretches the stiff assistance spring. Stretching of the assistance spring stores elastic energy, and also offloads the calf muscles and tendons acting in parallel. During late stance, biological ankle motion reverses and the ankle begins to plantarflex. At this time the assistance spring recoils, assisting biological ankle plantarflexion Push-off power. As the ground reaction forces underneath the foot rapidly decrease towards zero (toe-off), frictional forces between the slider and gripper also decrease, unclutching the mechanism. The slider can then translate freely, with minimal resistance due to stretching of the weak reset spring. This allows the person to dorsiflex their ankle normally during leg swing (to provide toe clearance). Upon subsequent foot contact the cycle begins again. White areas indicate approximate time of clutch engagement (similar in timing to the stance phase), and grayed areas indicate clutch is disengaged (similar in timing to the swing phase). See Figure 4 for further description of clutch design and function.

Fig. 4.

A) Under-the-foot clutch design. The friction clutch consists of 5 core elements: reset spring, slider, top gripper, bottom gripper, and spacer. Specific materials, construction details and design rationale are provided in the main text. B) Under-the-foot clutch function when disengaged. When no normal force is applied to the grippers during leg swing (and early stance), the assistance spring and slider translate as the weak reset spring stretches/shortens. C) Under-the-foot clutch function when engaged. When normal force is applied due to body weight during stance, the assistance spring can stretch/shorten, while the slider and reset spring are clutched (i.e., unable to move) due to the friction force between the slider and the grippers. See Fig. 3 for further description of clutch function.

Fig. 5.

Clutch characterization results. A) Friction force (F_f, i.e., max clutch holding force) vs. normal force (F_N), for both flat foot (teal) and heel- off (dark blue) configurations. Friction coefficient, μ, for each configuration was estimated via linear regression. B) Theoretical maximum torque (right vertical axis, teal/dark blue curve) that the exoskeleton could provide based on empirical coefficients of friction and typical ground reaction forces (left vertical axis, gray curve) during walking at 1.25 m/s (data from [52]). Inset: Exoskeleton ankle torque (in Nm/kg) for three different reset spring stiffnesses (in kN/m) when foot is unloaded (i.e., swing phase) and rotated through a full range of ankle plantarflexion (−) and dorsiflexion (+).

Fig. 6.

Case study results. A) Soleus EMG was reduced, specifically in mid-stance, when wearing the exoskeleton with various assistance springs vs. not wearing the exoskeleton (normal shoes only, Shod). EMG was normalized to maximum muscle activation and averaged across strides. B) Average soleus EMG over stride (dimensionless) was reduced by 5–17% when wearing the exoskeleton. EMG mean and standard deviation are depicted. C) Exoskeleton torque increased as spring stiffness increased. Spring stiffnesses were selected to match range of springs used in the prior study by Collins, Wiggin and Sawicki [2]. Due to larger lever arm the prior exoskeleton provided more ankle torque (shaded gray) for same spring stiffness range.

Fig. 7.

Representative example of ankle exoskeleton functioning across a range of speeds. Results from two separate trials are overlaid. When assistance spring stiffness was doubled from 17.7 kN/m (teal) to 35.4 kN/m (dark blue), the peak exoskeleton torque approximately doubled at each speed.