Biological muscles exhibit a high level of integration, in which actuators, sensors and transmission elements can be included in one component. Artificial muscles or actuators refer to intelligent stimuli-responsive materials that could reversibly deform with the trigger of various external stimuli. These materials, which have attracted tremendous attention, produce natural muscle-like actuation performance and show promising applications in robotics. After an introduction of various actuator technologies that contribute to robotic applications, a comparative analysis of the main actuation parameter is provided. The comprehensive comparisons of each kind of artificial muscle are summarised, and the promising properties that are required in robotics are presented, which highlight the development of their actuation performances and the challenges that limit their further employments. Future developmental prospects and perspectives of artificial actuators are discussed.

Leg properties have been involved in the broad study of human walking from mechanical energy to motion prediction of robotics. However, the variable leg elasticities and their functions during gait have not been fully explored. This study presented that the fundamental leg properties during human walking comprise axial stiffness, rest leg length, tangential stiffness and force-free leg angles. We measured the axial force-leg length and tangential force-leg angle data in eight participants (mean ± s.d. age 24.6 ± 3.0 years, mass 68.2 ± 6.8 kg, height 177.5 ± 5.2 cm) at three self-selected walking speeds (slow: 1.25 ± 0.22, normal: 1.48 ± 0.28, fast: 1.75 ± 0.32 m/s) on two different contact conditions (fixed and moving). After obtaining these gait measurements, we extracted the linear and nonlinear leg elasticities during human walking by using a minimum root-mean-square fitting. We found that the axial stiffness of nonlinear elasticity (fixed condition: 7.1−8.0, moving condition: 21.3−22.6) is higher than that of the linear elasticity (fixed condition: 5.0−5.7, moving condition: 15.2−16.5). The tangential stiffness behaves different during four stance phases of gait, with the highest (linear: 2.52−3.72, nonlinear: 1.71−2.01, in moving condition) occurred at early stance and second highest at late stance, followed by two stiffnesses in mid-stance. For both linearity and nonlinearity, the axial stiffness and rest length are independent of walking speeds in both contact conditions, while the tangential stiffness and contact angles are independent of walking speeds only in moving condition. Regardless of walking speed, elasticity and contact condition, the force-free contact angle at mid-stance is maintained at average of 82.2°. This paper first demonstrates the mechanical walking leg property from both axial and tangential aspects. The findings provide insight into the fundamental properties including linearity and nonlinearity of human leg during locomotion for stability analysis and precise motion prediction of robotics and rehabilitation exoskeletons.

Impacts occur everywhere, and they pose a serious threat to human health and production safety. Flexible materials with efficient cushioning and energy absorption are ideal candidates to provide protection from impacts. Despite the high demand, the cushioning capacity of protective materials is still limited. In this study, an integrated bionic strategy is proposed, and a bioinspired structural composite material with highly cushioning performance is developed on the basis of this strategy. The results demonstrated that the integrated bionic material, an S‐spider web‐foam, has excellent energy storage and dissipation as well as cushioning performance. Under impact loading, S‐spider web‐foam can reduce peak impact forces by a factor of 3.5 times better than silicone foam, achieving unprecedented cushioning performance. The results of this study deepen the understanding of flexible cushioning materials and may provide new strategies and inspiration for the preparation of high‐performance flexible cushioning materials.

Traditional robotic foot and ankle have received considerable attention for adaptive locomotion on complex terrain and land buffering ability. This study aims to develop a robotic ankle based on series elastic actuator with a parallel spring (SEAPS) by mimicking the two degree-of-freedom human ankle and related muscles. A unique four-SEAPS-based spring mechanism enhances motion adaptation and landing buffer. A dynamic model based on the SEAPS drive is presented, whereas a kinematic solution is realized. The bionic ankle has a wide range of motions with 30° in both the sagittal and frontal planes, which cover most of the human ankle motions. Four experiments were conducted to thoroughly characterize the capabilities. First, the static stiffness and the 2-D/3-D trajectory tracking performance of the proposed ankle were tested. Second, the angle sensing capacity under inclined road surface and the land buffering performance from a specific height were evaluated. The results show that the maximum 3-D motion tracking error is smaller than 2.3%, and the minimum sensing error of inclined road is smaller than 0.5%. The proposed ankle can closely track the 3-D walking trajectory of natural ankle joint with relative root-mean-square error well below 1.0%. Compared with no springs, landing buffer of the ankle with SEAPS can yield remarkable reduction in both peak ground reaction force (52.2%) and joint torque (57.9%). These findings prove that the SEAPS-based bioinspired robotic ankle exhibits high land buffering performance in an unstructured environment, and also well reproduces the human-like movement in terms of complex 3-D ankle motions.

This paper aims to develop and validate a subject-specific framework for modelling the human hand. This was achieved by combining medical image-based finite element modelling, individualized muscle force and kinematic measurements. Firstly, a subject-specific human hand finite element (FE) model was developed. The geometries of the phalanges, carpal bones, wrist bones, ligaments, tendons, subcutaneous tissue and skin were all included. The material properties were derived from in-vivo and in-vitro experiment results available in the literature. The boundary and loading conditions were defined based on the kinematic data and muscle forces of a specific subject captured from the in-vivo grasping tests. The predicted contact pressure and contact area were in good agreement with the in-vivo test results of the same subject, with the relative errors for the contact pressures all being below 20%. Finally, sensitivity analysis was performed to investigate the effects of important modelling parameters on the predictions. The results showed that contact pressure and area were sensitive to the material properties and muscle forces. This FE human hand model can be used to make a detailed and quantitative evaluation into biomechanical and neurophysiological aspects of human hand contact during daily perception and manipulation. The findings can be applied to the design of the bionic hands or neuro-prosthetics in the future.