Flexibility-Patterned Liquid-Repelling Surfaces

Hollow microneedles as a flexible dosing control solution for transdermal drug delivery

Microneedles, small needle-like structures typically less than 1000 μm in length, are effective tools for transporting substances across biological barriers via minimally invasive pathways. Various microelectromechanical system (MEMS) processes enable the production of different types of microneedles, including solid, coated, dissolving, hydrogel, and hollow microneedles, each tailored to specific drug and fluid delivery mechanisms. Among these, hollow microneedles stand out for their ability to offer flexible dosage control adaptable to varying drug formulations, making them particularly promising for transdermal drug delivery systems. This review examines the fabrication processes of hollow microneedles, highlights the advantages of their hollow structure for medical applications, and discusses the key factors influencing their performance. Finally, it proposes directions for advancing these technologies in both industrial and research settings.

3D-Printed Micro/Nano-Scaled Mechanical Metamaterials: Fundamentals, Technologies, Progress, Applications, and Challenges

Micro/nano-scaled mechanical metamaterials have attracted extensive attention in various fields attributed to their superior properties benefiting from their rationally designed micro/nano-structures. As one of the most advanced technologies in the 21st century, additive manufacturing (3D printing) opens an easier and faster path for fabricating micro/nano-scaled mechanical metamaterials with complex structures. Here, the size effect of metamaterials at micro/nano scales is introduced first. Then, the additive manufacturing technologies to fabricate mechanical metamaterials at micro/nano scales are introduced. The latest research progress on micro/nano-scaled mechanical metamaterials is also reviewed according to the type of materials. In addition, the structural and functional applications of micro/nano-scaled mechanical metamaterials are further summarized. Finally, the challenges, including advanced 3D printing technologies, novel material development, and innovative structural design, for micro/nano-scaled mechanical metamaterials are discussed, and future perspectives are provided. The review aims to provide insight into the research and development of 3D-printed micro/nano-scaled mechanical metamaterials.

Tribological Behavior of Bioinspired Surfaces

Energy losses due to various tribological phenomena pose a significant challenge to sustainable development. These energy losses also contribute toward increased emissions of greenhouse gases. Various attempts have been made to reduce energy consumption through the use of various surface engineering solutions. The bioinspired surfaces can provide a sustainable solution to address these tribological challenges by minimizing friction and wear. The current study majorly focuses on the recent advancements in the tribological behavior of bioinspired surfaces and bio-inspired materials. The miniaturization of technological devices has increased the need to understand micro- and nano-scale tribological behavior, which could significantly reduce energy wastage and material degradation. Integrating advanced research methods is crucial in developing new aspects of structures and characteristics of biological materials. Depending upon the interaction of the species with the surrounding, the present study is divided into segments depicting the tribological behavior of the biological surfaces inspired by animals and plants. The mimicking of bio-inspired surfaces resulted in significant noise, friction, and drag reduction, promoting the development of anti-wear and anti-adhesion surfaces. Along with the reduction in friction through the bioinspired surface, a few studies providing evidence for the enhancement in the frictional properties were also depicted.

Superhydrophobic Strategy for Nature-Inspired Rotating Microfliers: Enhancing Spreading, Reducing Contact Time, and Weakening Impact Force of Raindrops

Wind-dispersal of seeds is a remarkable strategy in nature, enlightening the construction of microfliers for environmental monitoring. However, the flight of these microfliers is greatly affected by climatic conditions, especially in rainy days, they suffer serious raindrop impact. Here, a hierarchical superhydrophobic surface is fabricated and a novel strategy is demonstrated that the superhydrophobic coating can enhance spreading while reduce contact time and impact force of raindrops, all of which are beneficial for the rotating microfliers. When the surface rotating speed exceeds a critical value, the effect of centrifugal force becomes considerable so that the droplet spreading is enhanced. The rotating superhydrophobic surface can rotate an impacting droplet by the tangential drag force from the air boundary layer, and the rotation of the droplet generates a negative pressure zone inside it, reducing the contact time by more than 30%. The impact force by the droplet on the rotating superhydrophobic surface also has a remarkable reduction of 53% compared to that on unprocessed hydrophilic surfaces, which helps maintain the flight stability of the microfliers. This work pioneers in revealing the droplet impact effect on rotating microflier surfaces and demonstrates the effectiveness of protecting microfliers with superhydrophobic coatings, which shall guide the manufacture and flight of microfliers in rainy conditions.

Facile fabrication of self-roughened surfaces for superhydrophobic coatings via polarity-induced phase separation strategy

Rough structures have gained increasing attention since they are essential for surfaces with special wettability, which can be used for various applications. It is still a challenge to find a low-cost and simple way to fabricate rough surfaces despite extensive efforts. Herein, we report a facile strategy to fabricate self-roughened surfaces based on polarity-induced phase separation. The strategy relies on the migration of flexible chains of the nonpolar polysiloxane to airside, driven by surface tension and polarity difference with the polar crosslinker, which forms a self-roughened surface with numerous protrusions. It is worth noting that this strategy does not require strict control of procedures, since it is insensitive to environmental changes unlike other phase separation methods, as shown by the results of systematic studies on several key parameters. Modified fabrics and coatings exhibit excellent superhydrophobicity with a water contact angle higher than 160°. Moreover, due to the strong hydrogen bonds formed by the polar urea groups of the crosslinker with substrates, the abrasion resistance of the coating is significantly enhanced. It is believed that the proposed novel and facile strategy will be a promising candidate for industrial manufacturing.

Fabrication of anisotropic superhydrophobic surface based on the Nepenthes slippery zone

Depending on its highly evolved structures that consist of microscale lunate cells and nanoscale wax coverings, the slippery zone of Nepenthes alata shows significant anisotropic superhydrophobicity, which has gradually become the biomimetic prototype for designing superhydrophobic surfaces. In this study, the authors constructed the structures of the slippery zone into equidistantly distributed greenhouses and array of cylinders, therefore obtaining a biomimetic model of an anisotropic superhydrophobic surface. The greenhouses were printed using ultraviolet-cured material, via 3D printing, and then flake graphite was selected as a substitute for the array of cylinders (wax coverings) and was absorbed onto the printed greenhouses by using high-voltage electrostatic absorption technology. The contact/sliding angle was measured to verify the anisotropic superhydrophobic effect of the fabricated sample. The contact angle increases significantly with an increase in the greenhouse density (l/L value) and achieves a value of 152.6 ± 0.6° when l/L is 0.8, and the sliding angle toward bottom and top shows values of 3.07 ± 0.26° and 5.69 ± 0.24°, respectively. These results indicate that the fabricated sample has anisotropic superhydrophobicity. Therefore, this study provides a simple and low-cost approach for the biomimetic fabrication of anisotropic superhydrophobic surfaces.