Soft Wearable Rehabilitation Robots with Artificial Muscles based on Smart Materials: A Review

Abstract

During the past decade, wearable devices such as exoskeletons have gained popularity [1] in different fields, such as the military, [2] industry, [3] and rehabilitation. [4] In the latter, rehabilitation exoskeletons are used to restore or maintain the functionality and mobility of people with physical disabilities. [5] These are receiving greater attention as the number of people with disabilities affecting physical performance will increase in the following decades as the population ages and individuals live longer with noncommunicable conditions (e.g., cerebral palsy, stroke, acquired brain injuries, and muscular dystrophies). [6][7][8] Most current exoskeletons use electric motors and rigid links to realize actuation and often have heavy and bulky designs that are difficult to safely wear outside clinical facilities. [9] Hence, researchers in this area are working to develop soft wearable rehabilitation robots (SWRRs), featuring soft actuators that are agreeable to the users as they have increased compliance, adaptability, comfort, safety, and less weight. [10][11][12] Currently, SWRR relies mainly upon two soft robotic technologies, cable-driven and fluidic actuators. For a cable-driven system, the wire is embedded into clothes or tubes and attached to an anchor point. The other side of the wire is connected to an electric motor to generate the desired movement and force by pulling the cable [13,14] (Figure 1a). Alternatively, in fluidic actuation (Hydraulic/Pneumatic), a pressurized fluid is inserted into a chamber made of highly deformable material to generate a displacement [11,15,16] (Figure 1b). However, these require large and heavy external pumps and valves to compress the fluid. [17] Unfortunately, cable-driven and fluidic actuation need cumbersome components (e.g., electric motors, pumps, and valves) to work, compromising the portability of the systems when used in daily life. [18] Therefore, in recent years, research has been committed to developing new actuator technologies that can overcome the drawbacks of the current actuators used in SWRR. These technologies include artificial muscles based on smart materials (AMSMs). AMSMs are soft actuators composed primarily of material with a low Young's modulus similar to that of soft biological materials (10 ^4-10 ^9 Pa) that can sense and directly convert physical stimulus (e.g., light, electrical, heat) into physical displacement. [19][20][21][22][23] Some examples of smart materials are shape-memory alloy (SMA), [24] dielectric elastomer actuators (DEAs), [25] ionic polymer-metal composites (IPMC), [26] shapememory polymers (SMPs), [27] and twisted and coiled polymer actuators (TCPs). [28] Due to their inherent properties and manufacturing processes, AMSM can be fabricated in various shapes, allowing them to be embedded into flexible and deformable wearable devices. [29][30][31][32] Furthermore, it is possible to fabricate robots with a relatively small weight and volume as they present a power density comparable to the skeletal muscles. [33] Neverthe...

Multimodal Soft Robotic Actuation and Locomotion

“…[61,62] Fluidic actuators gained popularity in the early stages of soft robotic applications due to their high energy efficiency, significant deformation capability, and safe operation. [63,64] For instance, wearable pneumatic actuators that incorporate stretchable bidirectional solid-state pumps have functioned as a highly deforming fluidic muscle with a high bending angle (Figure 2c). [65] Moreover, 3D-printed modular pneumatic grippers, in conjunction with mechanical meta-materials, have demonstrated excellent conformal grasping of soft objects in various dynamic conditions.…”

Paediatric ankle rehabilitation system based on twisted and coiled polymer actuators

“…These activities typically occur at a frequency of around 1.75 Hz. However, it is important to note that during rehabilitation sessions, these movements are often executed at much lower frequencies, usually falling below 0.2 Hz [10,36,37].…”

“…To overcome these limitations, researchers are now focusing on developing soft wearable rehabilitation robots (SWRRs) that incorporate artificial muscles based on smart materials (AMSMs). AMSM provides increased compliance, adaptability, comfort, safety, and reduced weight [10].…”

“…SMAs present the advantages of power density and stress, these being as high as 50 W g −1 and 200 MPa, respectively [22,25]. These advantages have made them the most popular option on SWRR prototypes [10]. However, they have limitations to be used on SWRR, as they have low efficiency, a small strain range (up to 8%) and high thermal hysteresis making them difficult to control [20,22,23].…”

Improved performance in temperature and speed of TCP artificial muscles for soft wearables robots by length modification

“…For SWRR applications, there are biomechanical considerations that need to be fulfilled, like torque/force, range of motion/displacement, and velocity/frequency [17,18]. Due to their advantages, TCPs have gained attention in the SWRR area, as they present high forces and strains.…”