# A State-of-the-Art Review on Robots and Medical Devices Using Smart Fluids and Shape Memory Alloys
## 2 Robots Using ERF and MRF
### MRF-Based Robots
One potential application of MRF technology in the robotic area is in rehabilitation elements or components. Over the last decade, many studies on lower-limb rehabilitation, utilizing MR mechanisms such as clutches or brakes, were undertaken. Specifically, research work on lower-limb robotic rehabilitation related to various gait motions was performed by [PERSON] et al. [PERSON] et al. (2018). It is known that many patients with cerebral lesions have a spastic movement disorder, which slows voluntary limb movement. This has led to much research on lower-limb rehabilitation using servo-motors. However, it is challenging to guarantee both stability, and powerful motion, by employing a servo-motor-based leg system. [PERSON] et al. [PERSON] et al. (2018, 2019) proposed a haptic controllable leg-shaped robot (Leg-Robot) to be used by patients with spastic ankle joints. The Leg-Robot was made with an MR clutch, which provided a safety mechanism, and a haptic generator of the system. In the design process of the Leg-Robot, they considered mass characteristics at a level approximating that of elderly people (over 75-year-old males). Figure 3a shows the basic structure of the Leg-Robot, in which the MR clutch is built in the knee joint. It was demonstrated that the desired torque levels, corresponding to the spasticity mode and the ankle-clone mode, could be successfully achieved by controlling the MR brake. [PERSON] et al. [PERSON] et al. (2019) proposed a robot leg comprising a rotational MR damper, a torsional spring, a steel plate, and a curved part of the foot that makes contact with the ground, as shown in Figure 3b. As it walks, the foot rotates to form an angle between the core and outer cylinder of the MR damper, while the forces acting on the foot vary according to the synthesized effect of the driving torque of the motor on the axle, and to the dynamic force of the terrain acting on the foot. It was demonstrated that the vertical forces at different walking speeds can be controlled in real time to adapt to the surroundings by controlling the magnetic field applied to the MR damper. In this case, a higher angular speed than other tunable legs were achieved, due to the guaranteed stability associated with the semi-active control of the MR damper. There are several types of collaborative robot systems for robot-to-robot or robot-to-human interactions. Specifically, a robot sharing its working space with humans has been actively researched for many purposes such as carrying heavy objects in factories. [PERSON] and [PERSON] [PERSON] and [PERSON] (2018) proposed a  
Figure 3: Leg-Robot using MR devices; **(a)** MR fluid clutch [PERSON] et al. (2018, 2019); **(b)** MR fluid damper [PERSON] et al. (2019). MR-clutch type safety device for human-collaborative robots. In this work, an MR clutch comprising magnetic circuits associated with the permanent magnet and electromagnetic coil, yoke, rotor, and driving shaft was developed to achieve high levels of output torque. After verifying the safety function of the MR clutch, the transmitted torque of two different types, single- and triple-rotor, were evaluated as a function of the input current (magnetic field). They concluded that, in order to build a safety device capable of controlling the torque output of the robot joint axis and securing the holding torque in an emergency case, the proposed MR clutch was a promising candidate for practical implementation. [PERSON] et al. [24; 25] developed a robot-arm system featuring an MR clutch/brake to accomplish a safe human-robot interaction, which is a type of compliance control. [PERSON] et al. [26] proposed a dual-differential rheological actuator (DDRA) to investigate the feasibility of high-performance robotic interactions, in which safety, robustness, and versatility are paramount. The DDRA was manufactured based on the synergistic use of two differentially coupled MR brakes, and an electromagnetic (EM) motor.