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[REVIEW] Moving toward Soft Robotics: A Decade Review of the Design of Hand Exoskeletons – Full Text HTML


Soft robotics is a branch of robotics that deals with mechatronics and electromechanical systems primarily made of soft materials. This paper presents a summary of a chronicle study of various soft robotic hand exoskeletons, with different electroencephalography (EEG)- and electromyography (EMG)-based instrumentations and controls, for rehabilitation and assistance in activities of daily living. A total of 45 soft robotic hand exoskeletons are reviewed. The study follows two methodological frameworks: a systematic review and a chronological review of the exoskeletons. The first approach summarizes the designs of different soft robotic hand exoskeletons based on their mechanical, electrical and functional attributes, including the degree of freedom, number of fingers, force transmission, actuation mode and control strategy. The second approach discusses the technological trend of soft robotic hand exoskeletons in the past decade. The timeline analysis demonstrates the transformation of the exoskeletons from rigid ferrous materials to soft elastomeric materials. It uncovers recent research, development and integration of their mechanical and electrical components. It also approximates the future of the soft robotic hand exoskeletons and some of their crucial design attributes.

1. Introduction

The emerging trend of soft robotics has stimulated the interest of engineers and researchers around the world to look into various applications, ranging from biomedical and rehabilitation to grasping and manipulation [1,2]. Biomimetic and bioinspired soft robots have been among the most successful products of soft material robotics. Among others, the inspiration for these soft robots originates from examining invertebrates like caterpillars, worms and fish grubs [2]. The hydrostatic and fluid-like structure motivates researchers to look more into the use of soft materials to develop similar structures.
One of the major lessons learned from these biostructures was the ability to form and adapt to complexly shaped bodies. This led to various developments such as: (1) an octopus-like robot for flexible manipulation [2]; (2) a worm-like robot that uses a thermal shape-memory alloy (SMA) actuator to imitate the motion of its biological counterpart [3]; and (3) a caterpillar-shaped soft robot that imitates the process of translating deformation to locomotor dynamics [4].
Another important development in the emerging field of soft robotics is the use of pneumatic soft grippers for handling fragile objects such as an uncooked egg or an anesthetized mouse [5]. These devices can grip, hold and release certain complexly shaped objects. They have several fingers to hold delicate objects by intelligently adapting themselves to the shape of the object and providing maximum gripping force without damaging it.
This new trend is especially interesting for biomedical and rehabilitation engineering applications as well, with the hand exoskeleton as one of the examples. A major shift from the use of hard to soft materials can be observed in some of the latest designs of hand exoskeletons, such as the Wyss Institute glove [6,7,8], the Magnetic Resonance Compatible (MRC) glove [9,10,11,12], the National University of Singapore (NUS) glove [13,14,15,16,17] and the Seoul National University (SNU) glove [18,19,20,21,22,23]. The hand exoskeleton is an integral part of rehabilitation robotics that provides rehabilitation exercises and assistance in activities of daily living (ADL), such as gripping and grasping [24]. It is commonly recommended for patients with cerebrovascular disease [24], cerebral palsy [25] and rheumatoid arthritis [26].
Unlike a prosthetic hand, a hand exoskeleton is designed and built around the human hand; thus, it has to conform to the hand anatomy and its range of motion to minimize the wearer’s discomfort. More importantly, it has to be light and able to be put on easily so that the wearer can use it daily to perform basic activities. Figure 1a shows the natural skeletal structure of the human finger, the movement of which may be assisted. The structure consists of three joints: distal (DIP), proximal (PIP) and metacarpal (MCP) interphalangeal joints [27,28]. The finger movements are controlled through the activation of extrinsic and intrinsic muscles. The extrinsic muscles are actuated from the forearm and control the flexor and extensor muscle tendons to move the fingers. The intrinsic muscles are located within the finger, and they control the independent motion of the finger [28]. The maximum flexion for the MCP joint ranges from 70° to 95° depending on the finger orientation, while the maximum flexion for the DIP and PIP joints is about 110° and 90°, respectively.
Hand exoskeletons have endured extensive research, primarily in the field of assistive and rehabilitative robotics, and have also been discussed from various perspectives [29,30,31]. Several iterations of different hand exoskeletons indicate the growing need for a better, lighter and more practical solution. Most of the existing hand exoskeletons adopt one of the design approaches depicted in Figure 1. With the rise of soft robotics, there has been a progressive shift from the conventional rigid mechanical structure designs (Figure 1b) to designs with softer actuation (Figure 1c) and designs that closely resemble the natural finger musculoskeletal structure (Figure 1d) [27]. This study aims to review these changes in the recent decade and discuss how the adoption of soft robotics helps in designing a more compliant hand exoskeleton. The design of a hand exoskeleton can be divided into three main components: the mechanical design, the actuation unit and sensory feedback control. This work also examines how soft robotics technology has changed the architectures of these components over the years.[…]


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