The research group of Professor Wang Qining of the Department of Advanced Manufacturing and Robotics of Peking University of Technology has made an important breakthrough in intelligent power lower li

Professor Wang Qining’s research team from the Department of Advanced Manufacturing and Robotics, School of Engineering, Peking University, has made important breakthroughs in intelligent powered lower limb prostheses, proposing an origami soft powered knee prosthesis inspired by horsetail grass, which is the world’s first soft knee prosthesis. The research provides a new idea for the design of lower limb prostheses. The research results, titled "Bioinspired Origami-based Soft Prosthetic Knee", were recently published in the important international journal Nature Communications. The first author of the paper is Gao Siyuan, a doctoral student at Peking University School of Engineering, and Professor Wang Qining is the only corresponding author.

The main goal of the design of knee joint prostheses is to not only meet the basic daily mobility needs of disabled people, such as walking on flat ground, rough roads, stairs, slopes, etc., but also ensure the comfort of users, and have features such as lightness, bionics, shock absorption, and customization. At present, knee joint prostheses are all rigid prostheses, made of high-density metal materials, and the functional mechanisms of each part are relatively independent. While improving functionality, it will increase the volume and weight of the system, and the buffering performance of the rigid frame is also very limited, making it difficult to balance functionality and comfort. In recent years, the rapid development of soft materials is expected to provide innovative solutions and functional enhancements to the field of robotics. Its inherent low density, flexibility, cushioning and ease of 3D printing make the soft robot lightweight, supple, shock-absorbing, and easy to manufacture, which is very suitable for the design requirements of knee joint prostheses for wearing comfort. However, the high flexibility of soft materials also causes unavoidable and unnecessary deformation of their structures during movement and load-bearing. On the one hand, this is not conducive to modeling calculations and motion control. On the other hand, it makes the structure difficult to bear the weight of the human body, hindering the application of soft robots in knee joint prostheses.

Proposed a bionic soft knee prosthesis based on origami for the first time

This study proposed a bionic soft knee prosthesis based on origami for the first time. As shown in Figure 1, the prosthesis includes an origami structure, surface reinforcement blocks, thigh and calf connection modules. The main body of the proposed soft prosthesis is made of thermoplastic polyurethane 3D printing, the weight of the knee joint unit is only 300g, and the height of the prosthesis is only 15cm. The origami structure is composed of a front origami structure and a back origami structure. Utilizing the large-range folding and unfolding characteristics of the origami structure, it can meet the requirements of a wide range of motion of the knee joint.

Figure 1. Overview of origami software knee joint

Bionic multi-center origami knee joint design

This study proposes an origami structure with variable rotation center properties, as shown in Figure 2, where the dotted line is the valley line, the solid line is the mountain line, and the red dotted line is the auxiliary line. Fold along the mountain line and valley line, and overlap A1B and A2B, D1C and D2C respectively, to form a 3D origami structure as shown in Figure 2(b).

Figure 2. Frontal origami structure crease design

Among them, AB and CD are defined as the thigh rod and calf rod respectively, L1, L2, α1, α2, β They are all crease design parameters. By changing these parameters, the origami configuration and the asymmetry of the upper and lower parts of the structure can be adjusted, as shown in Figure 3. This asymmetry affects the knee flexion angle change of the origami knee joint, the maximum knee flexion angle, and the curve shape of the instantaneous center of knee joint rotation. By adjusting these design parameters to fit the human knee joint rotation center, human-like multi-center knee joint rotation can be achieved.

Figure 3. Changes of ABE12 and DCE12 during the folding process of 3D origami structure and α1, α2, β The relationship between the Some plants can grow up to 5 meters high, but are only 2 centimeters thick. As shown in Figure 4, by observing the cross-section of the stem, you can see a structure composed of a thin outer ring of reinforced tissue and a thicker inner ring of soft tissue. When the horsetail is bent, the tubular structure formed by the outer ring is squeezed, causing the internal pressure of the internal soft tissue cells to increase, thereby increasing the structural stiffness to resist bending. Further observation shows that during the growth process of the medullary cavity, a fold structure will be formed on the inner wall of the tubular structure. In the parts where the structural stress is most concentrated, the folds will be further depressed, squeezing local cells, causing the intracellular pressure to further increase, thereby improving the strength of the local structure. Therefore, this study summarized the two key mechanisms of horsetail resistance to bending, namely, fluid pressure stability and compressed concave structure.

In order to imitate the bending resistance mechanism of horsetail and improve the load-bearing capacity of the soft structure, this study designed a back origami structure, which together with the front origami structure forms a soft pneumatic cavity, using the air pressure of the air chamber to simulate the cell hydraulic pressure of horsetail. By adjusting the angle of the front and back creases relative to the direction of pressure, the front origami structure is relatively stiffer when under pressure, thereby imitating the role of the outer ring in horsetail to strengthen the tissue. As the air pressure in the air cavity increases, the overall stiffness of the soft origami structure increases, thereby resisting external pressure and maintaining structural stability. By adjusting the pre-inflation pressure of the air chamber, we can adjust the stiffness of the soft knee joint structure to cope with different levels of pressure. Furthermore, through the crease design of the back structure, combined with the synchronous movement characteristics of the two sides opening and up and down compression when the front origami structure deforms, the back origami structure can produce a movement change from convex to concave while complying with the movement of the front structure, thereby further compressing the volume of the air cavity, imitating the mechanism of the medullary cavity folds of horsetail being compressed and concave, and further enhancing the pressure resistance of the soft prosthesis.

Figure 4. Soft structure design that imitates the anti-bending mechanism of horsetail grass

This study adopts the method of reinforcing the folded part of the origami structure, which can effectively constrain the unnecessary deformation of the folded surface, thereby limiting the deformation error to the fold part. Since the fold only accounts for a small part of the origami structure, reinforcing the folded surface can effectively reduce the amount of unnecessary deformation of the entire structure and improve the movement accuracy.

Soft knee joint prosthesis test

Through the experimental bench experiment, this study successively tested the pneumatic control, instantaneous trajectory fitting degree, multi-dimensional impact absorption, load-bearing capacity, and torque output capability of the soft knee joint prosthesis. As shown in Figure 5, the results show that the origami structure can imitate the rotational center movement of the human knee joint, and the soft knee joint prosthesis can perform relatively precise movements driven by air pressure. The actual movement is close to the theoretical calculation, and the drive bandwidth meets the requirements of fast walking for disabled people. Facing external impacts from different directions, soft prostheses have stronger buffering capabilities than rigid prostheses (can absorb 11.5% to 17.3% more impact force). The proposed soft knee prosthesis can carry a weight of more than 75kg (250 times its own weight) at a low pre-inflation pressure, and can actively output a knee extension moment of more than 25Nm.

Figure 5. Experimental results on the experimental bench

In addition, this study invited three disabled subjects to participate in a series of walking experiments, such as 1 m/s and 1.25 m/s treadmill walking, going up and down 15 cm steps, up and down 10° slopes, crossing obstacles, etc. Figure 6 shows the experimental results of disabled people wearing soft prostheses and daily-use rigid prostheses walking on a treadmill at speeds of 1 m/s and 1.25 m/s. The experiment collected the joint angles, muscle activation levels, plantar pressure, and absorption of ground impact force and amplitude of the prosthetic limbs on the residual and healthy sides of the disabled people, and conducted gait symmetry analysis. The results show that although disabled people are not familiar with soft prostheses, they can still wear soft knee joint prostheses to walk at normal and faster paces, and have better gait symmetry. Disabled people not only save more effort, but also suffer less impact. They also subjectively believe that walking is more comfortable.

Figure 6. Treadmill walking experimental results for disabled people

In the multi-terrain walking experiment, this study mainly collected joint angle change data of disabled people, aiming to verify the versatility of soft prostheses. As shown in Figure 7, the experimental results show that the soft knee joint prosthesis can support disabled people to alternate legs up and down stairs/slopes, and can continuously cross obstacles. The prosthesis can recognize the movement intentions of disabled people and provide sufficient range of motion and force.

Figure 7. Experimental results of multi-terrain walking for disabled people

This study proposes the first soft knee joint prosthesis, which provides a new idea for the design of lower limb prostheses. Prosthetic limb designers and researchers can use new design methods to design prosthetics without the constraints of traditional mechanical design. The development of new materials, new drivers, and new sensors suitable for the application scenarios of soft lower limb prostheses will also inject fresh blood into the development of soft robots. The manufacturing of customized lower limb prostheses based on new manufacturing methods such as 3D printing will also become a research hotspot. Fast and low-cost manufacturing will accelerate technology iteration and lower the research and development threshold. Researchers believe that the realization of soft knee prostheses will open up a new cross-disciplinary research field.

The first author of the paper is Gao Siyuan, a doctoral student at Peking University School of Engineering, and Professor Wang Qining from Peking University School of Engineering is the only corresponding author. Collaborators include Dr. Yang Chengxu from Rehabilitation University, Master student Chen Hongting from Peking University School of Engineering, Professor He Xinqiang from Peking University School of Life Sciences, and Dr. Ruan Lecheng from Peking University School of Engineering. Relevant research was funded by the National Natural Science Foundation of China.

Source: Peking University