Sea turtles are recognised as one of the ocean’s most remarkable migratory species, accomplishing journeys that cover thousands of kilometres. This fact is even more extraordinary when considering sea turtles consume mostly low-energy foods. The biology of sea turtles dominates the literature; however, the swimming strategies they employ to achieve their migratory success from a biomechanical and hydrodynamic viewpoint is relatively unexplored. In past research, the sea turtle’s upstroke has been debated among researchers as to whether it is passive or for thrust production. In this work, we recreate a model based on the green sea turtle (Chelonia mydas) and develop an ad hoc testing rig to uncover the secrets behind the sea turtle’s upstroke. Our findings suggest sea turtles utilise a passive upstroke that can substantially lower the animal’s drag coefficient to levels that cause insignificant losses in swim speed despite not developing any thrust force. This can conceivably save the animal a notable amount of energy as the upstroke is responsible for a large percentage of the overall limb beat cycle. These findings could potentially pave a path towards developing high-efficiency bioinspired underwater drone technologies.

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...