Assistive devices

Most developments on wearables have been focused on rehabilitation devices [1,2], addressing effects of impairments that can potentially be reduced with time and training. Another branch of developments focused on assistive devices for individuals where full recovery is not likely, e.g., stroke survivors after reaching the recovery plateau [3] or those with degenerative diseases such as Amyotrophic lateral sclerosis.

These devices primarily rely on rigid linkages and conventional electromechanical motors to exert forces on the body and may not conform to the body of the user. These devices can become heavy, and often require usage in a stationary setting like in a rehabilitation clinic with additional assistance [4,5].

Several groups have been working on portable, soft, lightweight, and frameless wearables that take advantage of the skeletal structure and joints of the body [6,7]. This approach uses actuation mechanisms and enables more optimal placement of heavy or rigid components of the device on the body. Some of these systems use electromechanical actuators coupled to compliant transmission mechanisms. Specifically, Bowden cable drives and twisted string actuators have been used to assist with walking [8,9] and upper limb movement [10–12]. The controlling and predicting the performance of the final wearable robot on the body can be a challenge due to the properties of the soft tissue and the lack of fixed attachment points [13,14]. In cable-based shoulder robots [14–17], the large reaction loads that needs to be resisted by anchors can be an issue unless an external structure is introduced to improve such effects. On the other hand, traditional force sensing can be used in these designs to control the force, and therefore, modulating assistance to the wearer [14–17].

When cable-based actuation is not feasible, soft fluidic actuators offer a suitable alternative. These actuators are lightweight, can be controlled remotely, and exhibit natural compliance, especially when air or similar gases are used as the fluid. Typically, they are made from a combination of fibers and elastomers, with the fibers controlling the strain directions within the elastomer [18,19].

Pneumatic artificial muscles (PAMs) are a widely used type of soft fluidic actuator that generate linear tensile force when pressurized [20,21]. They are popular in various wearable robots due to their efficiency and adaptability. The modeling and control of PAMs have been thoroughly explored, underscoring their significance in robotics and wearable technology [22–24].

Fluidic actuators that do not utilize pneumatic artificial muscle designs, known as non-PAM, are also explored. Predicting the behavior of these non-PAM fluidic actuators has proved a challenge due to the large multidimensional deformations that the actuators typically undergo in addition to nonlinear material properties. Despite these difficulties, recent advancements by various research groups [19,25,26] have successfully modeled fiber-reinforced elastomeric bending actuators, proving their effectiveness in applications like soft robotic gloves [27–29]. Soft fluidic actuators traditionally use elastomeric materials. These elastomers require thick walls due to low specific stiffness, i.e., stiffness to weight ratio, which limits the minimum undeformed volume and maximum power density of the actuators.

Adding fibers can enhance power densities [28,30] resulting in reducing the wall thickness and weight. Using textiles, improves the power density of the fluidic actuators. Textiles enable inflatable soft robots to be strong yet lightweight. Textile-based fluidic actuators, which bend and rotate have been used in a number of wearable robots [31–33], particularly in glove applications [34,35] where the low weight of the actuators minimizes is a significant factor when the device is on the hand. Textile actuators in glove applications have used bending modes [36–38] while the ankle and shoulder applications [39,40] used rotary modes. The bending actuators were often constructed of knit materials and utilize differential stretching of the textile to create bending motions.

Understanding the deformation modes of textiles can be a challenge, which is a necessary step in the design of textile-based assistive devices. These materials show nonlinear behavior that depends on the intricate structures of the fabric. In addition, the stretching of textiles in one direction more than the others may pose a challenge for higher force applications. Such stretches would result in excessive extension of the textile at higher fluid pressures that may lead to failure, where inextensible woven textiles, or non-crimp fabrics, can be a viable option. The ability to turn a 2D structure into a desired complex 3D structure, with desired spatially engineered behavior is fascinating, leaving exploration of textile-based designs in the realm of assistive devices an active area.

References:

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