# State of the Art: Bipedal Robots for Lower Limb Rehabilitation
## 4 Challenges and Outlooks  
The application of bipedal robots to rehabilitation has become a research hotspot, and its application prospects are very broad. However, there is still a long way to go for the commercialization of the rehabilitation equipment. First, the rehabilitation effect of the equipment is the aspect that people most concerned. Although rehabilitation equipment can significantly reduce the workload of physical therapist and contribute to the popularity of rehabilitation training, the rehabilitation effect of equipment has not yet reached the level of physical therapist. Practicality is also one of the factors that rehabilitation equipment needs to consider. The structure of traditional rigid rehabilitation equipment is too complex and cumbersome, which is not only detrimental to wear but also not conducive to the late maintenance and repair. The new soft rehabilitation robot greatly simplifies the structure and weight of the equipment, but the auxiliary force is also reduced. The commercialization of rehabilitation  
Figure 8: Human-in-the-loop optimization of exoskeleton assistance during walking (reproduced with permission from [80], Copyright Science, 2017). equipment must take the cost into account. At present, the price of rehabilitation robot, such as HAL and ReWalk, is very high. This is not all patients can afford. In order to achieve commercialization in the future, the cost of rehabilitation equipment must be well controlled. Comprehensive research status, there are still many challenges in theoretical and technical aspects:  
* Mechanical Structure  
The rehabilitation robot is a complex system with multiple joints and redundant degrees of freedom. The complex and heavy structure is presently one of the important factors limiting the development of the rehabilitation robot. A fully functional intelligent rehabilitation robot must have a compact and disposable mechanical body. How to achieve the desired function and optimize the structure is worth exploring. The use of advanced materials, such as carbon fiber and titanium alloy, is the usual method used to reduce the weight. However, the types of advanced materials are limited and the cost is usually high, resulting in it being difficult to meet the demand. However, with the progress in processing and manufacturing, integration and modular design will be the development trend in the future. In general, the mechanical structure should be designed to meet people's requirements for flexibility, stability, and comfort. * Kinematics and Dynamics  
The control of the rehabilitation robot is performed based on kinematics and dynamics. The accuracy of the model determines the final control effect. Due to the high order, strong coupling, and non-linearity of the rehabilitation robot system, completely solving the kinematics and dynamics is difficult. Some scholars have introduced methods such as neural networks, genetic algorithms, fuzzy logic, chaos theory, and other methods. However, these methods are not yet mature and have their own advantages and disadvantages. With the introduction of new theoretical methods and the continuous improvement of computer performance, better solutions should be presented. In the future, accurate kinematics and dynamics simulation on a computer will be possible. * New Drives  
The drive is an important part of a rehabilitation robot. As performance requirements for rehabilitation equipment become increasingly higher, the drive requirements are following suit. Rehabilitation equipment must provide power to help people walk. To provide sufficient power, the drive used by the robot is often cumbersome. This increases the weight of the equipment and wastes energy. The ideal drive must be lightweight, small, and provide enough drive torque, while having good heat dissipation. The development requirements of the robot are not being met by the traditional drive method, so people are attempting to find new drives. Shape memory alloy drive and piezoelectric ceramic drive are being gradually applied to the field of robots.