Printed miniature robotic actuators with curvature-induced stiffness control inspired by the insect wing.

Stimuli-responsive actuating materials offer a promising way to power insect-scale robots, but a vast majority of these material systems are too soft for load bearing in different applications. While strategies for active stiffness control have been developed for humanoid-scale robots, for insect-scale counterparts for which compactness and functional complexity are essential requirements, these strategies are too bulky to be applicable. Here, we introduce a method whereby the same actuating material serves not only as the artificial muscles to power an insect-scale robot for load bearing, but also to increase the robot stiffness on-demand, by bending it to increase the second moment of area. This concept is biomimetically inspired by how insect wings stiffen themselves, and is realized here with manganese dioxide as a high-performing electrochemical actuating material printed on metallized polycarbonate films as the robot bodies. Using an open-electrodeposition printing method, the robots can be rapidly fabricated in one single step in ∼15 minutes, and they can be electrochemically actuated by a potential of ∼1 V to produce large bending of ∼500° in less than 5 s. With the stiffness enhancement method, fast (∼5 s) and reversible stiffness tuning with a theoretical increment by ∼4000 times is achieved in a micro-robotic arm at ultra-low potential input of ∼1 V, resulting in an improvement in load-bearing capability by about 4 times from ∼10µN to ∼41µN.

Molecular dynamics study on cold-welding of 3D nanoporous composite structures.

Nanoporous metals are a class of novel nanomaterials with potential applications in many fields. Herein, we demonstrate the cold-welding mechanism of nanoporous metals with various combinations using molecular dynamics simulations. This study shows that it is possible to cold-weld two nanoporous metals to form a novel composite material. The influence of temperature, in the range of 300-900 K, on the mechanical properties of the resultant composite material was investigated. With an increase in temperature, the weld stress and the mechanical strength of the nanoporous structures significantly decreased as an increase in disorder magnitude was observed. These results could lead to bottom-up nanofabrication and nanoassembly of combined nanoporous metals for high mechanical performance.

Nanoindentation Investigation of Temperature Effects on the Mechanical Properties of Nafion® 117.

Operating temperature can be a limiting factor in reliable applications of Proton Exchange Membrane (PEM) fuel cells. Nanoindentation tests were performed on perfluorosulfonic acid (PFSA) membranes (Nafion® 117) in order to study the influence of the temperature condition on their mechanical properties. The hardness and reduced modulus of Nafion® 117 were measured within a certain temperature range, from 10 to 70 °C. The results indicate that both hardness and elastic modulus show non-monotonic transition with the increase of the test temperature, with reaching peak values of 0.143 and 0.833 GPa at 45 °C. It also found that the membranes have a shape memory effect and a temperature dependent shape recovery ratio.

One-step spray-coating process for the fabrication of colorful superhydrophobic coatings with excellent corrosion resistance.

A simple method was used to generate colorful hydrophobic stearate particles via chemical reactions between inorganic salts and sodium stearate. Colored self-cleaning superhydrophobic coatings were prepared through a facile one-step spray-coating process by spraying the stearate particle suspensions onto stainless steel substrates. Furthermore, the colorful superhydrophobic coating maintains excellent chemical stability under both harsh acidic and alkaline circumstances. After being immersed in a 3.5 wt % NaCl aqueous solution for 1 month, the as-prepared coatings remained superhydrophobic; however, they lost their self-cleaning property with a sliding angle of about 46 ± 3°. The corrosion behavior of the superhydrophobic coatings on the Al substrate was characterized by the polarization curve and electrochemical impedance spectroscopy (EIS). The electrochemical corrosion test results indicated that the superhydrophobic coatings possessed excellent corrosion resistance, which could supply efficient and long-term preservation for the bare Al substrate.