Sunlight Recovering the Superhydrophobicity of a Femtosecond Laser-Structured Shape-Memory Polymer

Nature-inspired adhesive systems

Many organisms in nature thrive in intricate habitats through their unique bio-adhesive surfaces, facilitating tasks such as capturing prey and reproduction. It's important to note that the remarkable adhesion properties found in these natural biological surfaces primarily arise from their distinct micro- and nanostructures and/or chemical compositions. To create artificial surfaces with superior adhesion capabilities, researchers delve deeper into the underlying mechanisms of these captivating adhesion phenomena to draw inspiration. This article provides a systematic overview of various biological surfaces with different adhesion mechanisms, focusing on surface micro- and nanostructures and/or chemistry, offering design principles for their artificial counterparts. Here, the basic interactions and adhesion models of natural biological surfaces are introduced first. This will be followed by an exploration of research advancements in natural and artificial adhesive surfaces including both dry adhesive surfaces and wet/underwater adhesive surfaces, along with relevant adhesion characterization techniques. Special attention is paid to stimulus-responsive smart artificial adhesive surfaces with tunable adhesive properties. The goal is to spotlight recent advancements, identify common themes, and explore fundamental distinctions to pinpoint the present challenges and prospects in this field.

Programming Anisotropic Functionality of 3D Microdenticles by Staggered-Overlapped and Multilayered Microarchitectures

Natural sharkskin features staggered-overlapped and multilayered architectures of riblet-textured anisotropic microdenticles, exhibiting drag reduction and providing a flexible yet strong armor. However, the artificial fabrication of three-dimensional (3D) sharkskin with these unique functionalities and mechanical integrity is a challenge using conventional techniques. In this study, it is reported on the facile microfabrication of multilayered 3D sharkskin through the magnetic actuation of polymeric composites and subsequent chemical shape fixation by casting thin polymeric films. The fabricated hydrophobic sharkskin, with geometric symmetry breaking, achieves anisotropic drag reduction in frontal and backward flow directions against the riblet-textured microdenticles. For mechanical integrity, hard-on-soft multilayered mechanical properties are realized by coating the polymeric sharkskin with thin layers of zinc oxide and platinum, which have higher hardness and recovery behaviors than the polymer. This multilayered hard-on-soft sharkskin exhibits friction anisotropy, mechanical robustness, and structural recovery. Furthermore, coating the MXene nanosheets provides the fabricated sharkskin with a low electrical resistance of ≈5.3 Ω, which leads to high Joule heating (≈229.9 °C at 2.75 V). The proposed magnetomechanical actuation-assisted microfabrication strategy is expected to facilitate the development of devices requiring multifunctional microtextures.

Geometry-Gradient Magnetocontrollable Lubricant-Infused Microwall Array for Passive/Active Hybrid Bidirectional Droplet Transport

Manipulation of droplets has increasingly garnered global attention, owing to its multifarious potential applications, including microfluidics and medical diagnostic tests. To control the droplet motion, geometry-gradient-based passive transport has emerged as a well-established strategy, which induces a Laplace pressure difference based on the droplet radius differences in confined state and transport droplets with no consumption of external energy, whereas this transportation method has inevitably shown some critical limitations: unidirectionality, uncontrollability, short moving distance, and low velocity. Herein, a magnetocontrollable lubricant-infused microwall array (MLIMA) is designed as a key solution to this issue. In the absence of a magnetic field, droplets can spontaneously travel from the tip toward the root of the structure as a result of the geometry-gradient-induced Laplace pressure difference. When the subject of an external magnetic field, the microwalls bend and overlap sequentially, ultimately resulting in the formation of a continuous slippery meniscus surface. The formed meniscus surface can exert sufficient propulsive force to surmount the Laplace pressure difference of the droplet, thereby effectuating active transport. Through the continuous movement of the microwalls, droplets can be actively transported against the Laplace pressure difference from the root to the tip side of the MLIMA or continue to actively move to the root after finishing the passive self-transport. This work demonstrates passive/active hybrid bidirectional droplet transport capabilities, validates its feasibility in the accurate control of droplet manipulation, and exhibits great potential in chemical microreactions, bioassays, and the medical field.

A Review of Smart Superwetting Surfaces Based on Shape-Memory Micro/Nanostructures

Bioinspired smart superwetting surfaces with special wettability have aroused great attention from fundamental research to technological applications including self-cleaning, oil-water separation, anti-icing/corrosion/fogging, drag reduction, cell engineering, liquid manipulation, and so on. However, most of the reported smart superwetting surfaces switch their wettability by reversibly changing surface chemistry rather than surface microstructure. Compared with surface chemistry, the regulation of surface microstructure is more difficult and can bring novel functions to the surfaces. As a kind of stimulus-responsive material, shape-memory polymer (SMP) has become an excellent candidate for preparing smart superwetting surfaces owing to its unique shape transformation property. This review systematically summarizes the recent progress of smart superwetting SMP surfaces including fabrication methods, smart superwetting phenomena, and related application fields. The smart superwettabilities, such as superhydrophobicity/superomniphobicity with tunable adhesion, reversible switching between superhydrophobicity and superhydrophilicity, switchable isotropic/anisotropic wetting, slippery surface with tunable wettability, and underwater superaerophobicity/superoleophobicity with tunable adhesion, can be obtained on SMP micro/nanostructures by regulating the surface morphology. Finally, the challenges and future prospects of smart superwetting SMP surfaces are discussed.

Recent Advances on the Abrasion Resistance Enhancements and Applications of Superhydrophobic Materials

Researches on superhydrophobicity have been overwhelming and have shown great advantages in various fields. However, the abrasion resistance of superhydrophobic structures was usually poor, and they were easily damaged by external force or harsh environment, which greatly limited the applications of superhydrophobic surfaces. Much attention has been paid to improving the abrasion resistance of superhydrophobic materials by researchers. In this review, aimed at the advances on improving the abrasion resistance of superhydrophobic surfaces, it was summarized and compared three enhancement strategies including the reasonably design of micro-nano structures, the adoption of adhesives, and the preparation of self-healing surface. Finally, the applications of typical superhydrophobic materials with abrasion resistance were reviewed in various fields. In order to broaden the application fields of superhydrophobic materials, the abarasion resistance should be further improved. Therefore, we proposed the ideas for the future development of superhydrophobic materials with higher abrasion resistance. We hope that this review will provide a new approach to the preparation and development of stable superhydrophobic surfaces with higher abrasion resistance.

Programmable 4D Printing of Photoactive Shape Memory Composite Structures

4D printing is an advanced manufacturing technology combining additive manufacturing with smart materials. Based on light-active shape memory composites, smart medical structures with remote control capability, therapeutic function, and biocompatibility are hopefully fabricated by 4D printing. Here, a multifunctional composite with good mechanical properties, biocompatibility, and light-active shape memory performance is prepared by incorporating gold nanoparticles into a shape memory polyurethane matrix. The composites demonstrate a rapid and stable light-thermal effect, which can achieve localized and controlled breast tumor ablation, providing an approach to hyperthermia treatment for cancer cells. By directly bioprinting the composite melt, a series of 4D-printed structures are manufactured accurately in a convenient, clean, and safe way, which show a fast autonomous light-driven shape recovery process. The examples of a 4D-printed soft tissue scaffold and intraluminal scaffold can expand from a conveniently insertional shape to an expanded shape under light exposure. The proposed strategies provide great inspiration for customized multifunctional light-thermal therapeutic structures for minimally invasive treatment.