# Current State of Robotics in Hand Rehabilitation after Stroke: A Systematic Review
## 2 Materials and Methods
### The Development of Hand Rehabilitation Robot
#### 2.1.1 The Exoskeleton Hand Rehabilitation Robot  
Exoskeleton hand rehabilitation robots are similar to human limbs because they are connected to the patient at multiple points and their joint axes match those of human joints [26; 27]. It is possible to train specific muscles by controlling joint movements under computational torque. Most exoskeletal robots are still based on rigid linkages. Existing hand rehabilitation robots enable finger flexion and extension movements, differing in that some designs do not enable independent movements of each finger, the choice of control signals varies, and the range of motion of the affected limb varies between rehabilitation robots. After nearly 30 years of development, the technology has become more mature, and in recent years there are still many researchers at home and abroad in innovative research, and several newer rigid exoskeleton-based hand rehabilitation robots will be introduced below. In 2016, the exoskeletal rehabilitation hand HES, produced by 3D printing at the University of Florence [28; 29] uses a cord for the transmission of driving forces and a linkage mechanism as a finger actuator while limiting joint movements to ensure safety. The HES is intended for patients with severe hand contractures to train them in hand extension, while hand flexion is performed by the patient. The maximum trajectory error after wear is 5 mm. In 2017, a wire-controlled hand exoskeleton was designed by the Ulsan University of Science and Technology, South Korea [30; 31], where each finger is driven by a separate linear motor, and each exoskeleton joint uses a four-link mechanism to measure the force on the finger and the joint movement angle using a force measuring sensor and a rotary potentiometer to limit the exoskeleton hand using the stroke of the linear motor. [PERSON] et al. [32] in Italy used springs as power transmission to reduce the risk of hand-filling blood pressure injuries during motion compared to traditional linkage rigid structural members. [PERSON] et al. [33] designed an underdriven exoskeleton hand with a modular design of the mechanism to select different models of exoskeleton hand for wearing according to different hand sizes. Each finger is driven by a separate geared motor, and the force is transmitted to the linkage on the finger by a cord, and the movement angles of the MCP and PIP joints can reach 62\({}^{\circ}\) and 88\({}^{\circ}\), respectively. [PERSON] et al. [34] designed an exoskeleton hand with two linear motors driving the thumb and four fingers and used the fixed stroke of the linear motors to limit the motion of the exoskeleton hand. The finger actuators consisted of two sets of four-linked rods, preserving the natural motion of the DIP joints. The overall structure of the device is 3D printed. However, the motors are mounted on the back of the hand, which poses a risk of a crush injury to the affected hand. In 2018, [PERSON] et al. [35] from Italy invented an underdriven exoskeleton FEX with a \"finger spine\", which is driven by a single motor with four fingers and can be adapted to different hand sizes and can be well matched to the manual motion joints, while the MCP joints can be moved inward and outward, and the driving device is placed on the patient's small arm. The actuator is placed on the patient's small arm, connected and driven by Bowden wires, and each \"spine\" has a certain joint angle restriction to protect the affected hand within a normal joint angle. However, most of the weight of the device is located on the back of the affected finger, which increases the patient's sense of weight-bearing after being worn for a long time. In recent years, domestic research has also made some progress, and many universities have researched hand rehabilitation robots.