Introduction

Soft actuators and robotic devices can be considered as soft systems that aim to use highly compliant materials with elastic moduli that are comparable to soft biological materials and human tissues (kPa–MPa). Such systems are highly compliant and able to perform a range of natural and flexible movements and are generally more adaptable and robust due to their high degrees of freedom (DoF) and flexible bodies. Soft materials employed in soft actuators and devices are often fabricated using low-temperature processes, thereby providing ease of processing and low-cost fabrication and their viscoelastic properties are able to dissipate energy from impact and damping oscillations to smooth out discontinuous motions and forces. These unique soft features provide advantages that have attracted increasing interest in the development of soft robotic devices for rehabilitation and assistance where safety during human-machine interaction is of primary importance.
While soft, cost-efficient, safe, and easy-to-use rehabilitation and assistive devices are highly desirable, the current state-of-the-art rehabilitation and assistive devices rely on the combination and control of rigid actuators, links, and joints to realise a range of motions. Despite the precise nature of rigid actuators, the range of available motion can be limited in terms of their DoF. This results in rigid systems being less efficient in terms of being able to adapt to a variety of operating conditions for different users. For example, a combination of a variety of electric motors may be needed to achieve a twisting motion. In contrast, a single well-designed soft actuator can readily realise both linear and twisting motions, providing the advantages of adaptability, a lightweight nature, ease-of-manufacture and cost-efficiency. Soft rehabilitation and assistive devices can therefore perform a complex range of motions by exploiting the potentially infinite DoF provided by soft actuators. However, for soft actuators and robotic devices to achieve their potential in rehabilitation and assistance, the underpinning technologies of sensing, actuation, control, and power supply must be fully integrated and operate cooperatively.
While there are a number of excellent reviews on soft robots [1-6], soft electronic-skins [7-9], flexible wearable devices [10-12], rehabilitation and assistive exoskeletons [13-16], soft systems in health care [17-20] and soft textile Exosuits [21], there is currently no review that has a focus on soft actuators and robotic devices specifically developed for soft rehabilitation and assistance. The aim of this review is to investigate the current state-of-the-art technologies in this area to demonstrate the nexus between materials, mechanisms, actuation, and applications. The range of available actuation mechanisms are reviewed and discussed; these include pneumatic and hydraulic fluid-powered actuation, electrical motor actuation, chemical reactions and soft active material-based actuation which includes dielectric elastomers, shape memory alloys, magnetoactive elastomers, liquid crystalline elastomers, and piezoelectric materials. Figure 1 summarises these areas and their contribution, in percent, to the existing academic literature. Fluid-powered actuation is one of the most popular methods for developing soft rehabilitation and assistive devices. Approximately 33% of the work in the field is based on this actuation technology as a result of its high power density, reliability, and controllability. The applications are widely spread and range from upper limb assistance (fingers, hands, arms) devices to lower limb rehabilitation (legs, knees, ankles, feet) devices. Magnetoactive elastomers (MAEs), liquid crystal elastomers (LCEs) and chemical reactions represent relatively new areas of research and, together, they represent approximately 11% of the work in the field, and the devices are mainly at the concept stage. The advantages, current limitations, and directions for future development of soft actuators and robotic devices for rehabilitation and assistance will be outlined in the final section of this review.

Rehabilitation and assistance enabled by fluid-powered actuation

Fluid power actuation systems, which use fluids under pressure to generate, control, and transmit power, are classified into two categories: pneumatic fluid power and hydraulic fluid power. Pneumatic systems use gas as a medium for power transmission, while hydraulic systems normally use a liquid, such as mineral oil or water. Such systems are able to produce high power (in the order of kW) and high forces in small volumes, compared with electrically driven systems. For example, high fluid power of about 40 kW can be generated using a typical hydraulic gear pump operating at a pressure of 250 bar and a flow rate of 100 L/min, with a pump efficiency of 95%. The rehabilitation and assistive devices enabled by pneumatic and hydraulic fluid-powered are overviewed in this section. Table 1 summarizes the details of the devices, including actuation mechanism, functionalities, key parameters, control strategies, advantages, and challenges.