Janus Hydrogel to Mimic the Structure and Property of Articular Cartilage.

Designing hydrogels with adequate strength, remarkable swelling resistance, low friction coefficient, excellent biocompatibility, and osseointegration potential is essential for replacing articular cartilage. However, it remains challenging to integrate all these properties into one material. In this work, a Janus hydrogel was prepared from polyvinyl alcohol, chitosan, and sodium hyaluronate, followed by a one-sided dipping in situ precipitation mineralization to form a layer of hybridized hydroxyapatite (HAp), wherein the two surfaces had distinct compositions and functions. Because of the negative carboxyl groups from sodium hyaluronate, the top surface possessed a friction coefficient as low as 0.024. On account of the HAp mineralized layer, the bottom side had osteogenesis potential. Owing to the synergy of physical linkages, the hydrogel displayed compressive strength as high as 78 MPa. Furthermore, it demonstrated remarkable swelling resistance with strength retention near 100% even after soaking in PBS solution at 37 °C for 7 days. The absence of toxic chemicals maintained the merits of starting polymers and resulted in excellent biocompatibility (cell viability ≈ 100%), making it an ideal substitute for articular cartilage.

Control of leading-edge separation on bioinspired airfoil with fluttering coverts.

In this work, the aerodynamic role of the artificial covert feathers (i.e., coverts) on an airfoil is experimentally studied in a wind tunnel to investigate the flow control effect on the leading-edge separation. We apply flexible featherlike devices on a high-angle-of-attack airfoil. We use a hot-wire anemometer to measure the velocity profiles and turbulent fluctuations in the downstream wake flow. As a baseline of flow separation, a two-dimensional NACA 0018 airfoil model is set at the angle of attack of 15 ° at the chord-based Reynolds number of 1.0×10^{5}, causing strong leading-edge and trailing-edge shear layers and a low-speed wake flow area in between as large as 0.35 chord length. When deployed on the upper wing surface, the flexible coverts adaptively flutter under the influence of the local unsteady airflow. Hot-wire measurement results show that the leading-edge coverts effectively suppress the flow separation and reduce the size of the wake flow area. The change of power spectral density shows that the predominant peaks as the fundamental and harmonic frequencies are both attenuated due to the suppression of unsteady motions of the shear layers. On the other hand, the fluttering coverts at the trailing edge modify the trailing-edge shear layer by redistributing the turbulent kinetic energy to the high-frequency components. By simultaneous double-point measurement, we find that the leading-edge and trailing-edge shear layers are drawn closer to each other, and the two shear layers show an increased peak in the coherence spectrum. Further multiscale wavelet analysis shows that the perturbations at the 60% chord length increase the large-scale amplitude modulation of small-scale turbulence and therefore they stabilize the leading-edge and trailing-edge shear layers. Meanwhile, the flow intermittency outside of the wake flow area is attenuated as well. The effective flow control effects in the present work are in good agreement with the previous direct observations of bird flight in literature that the coverts on the upper wing surface play an important role in flow separation control during high-angle-of-attack flight. These findings advance the understanding of aerodynamic contribution of the covers on bird wings and reveal the engineering potential of bioinspired coverts for flow separation control of aircrafts and unmanned air vehicles.