Fabrication of a superhydrophobic surface with underwater air-retaining properties by electrostatic flocking

Materials Selection for Antifouling Systems in Marine Structures

Fouling is the accumulation of unwanted substances, such as proteins, organisms, and inorganic molecules, on marine infrastructure such as pylons, boats, or pipes due to exposure to their environment. As fouling accumulates, it can have many adverse effects, including increasing drag, reducing the maximum speed of a ship and increasing fuel consumption, weakening supports on oil rigs and reducing the functionality of many sensors. In this review, the history and recent progress of techniques and strategies that are employed to inhibit fouling are highlighted, including traditional biocide antifouling systems, biomimicry, micro-texture and natural components systems, superhydrophobic, hydrophilic or amphiphilic systems, hybrid systems and active cleaning systems. This review highlights important considerations, such as accounting for the effects that antifouling strategies have on the sensing mechanism employed by the sensors. Additionally, due to the specialised requirements of many sensors, often a bespoke and tailored solution is preferential to general coatings or paints. A description of how both fouling and antifouling techniques affect maritime sensors, specifically acoustic sensors, is given.

Understanding and utilizing textile-based electrostatic flocking for biomedical applications

Electrostatic flocking immobilizes electrical charges to the surface of microfibers from a high voltage-connected electrode and utilizes Coulombic forces to propel microfibers toward an adhesive-coated substrate, leaving a forest of aligned fibers. This traditional textile engineering technique has been used to modify surfaces or to create standalone anisotropic structures. Notably, a small body of evidence validating the use of electrostatic flocking for biomedical applications has emerged over the past several years. Noting the growing interest in utilizing electrostatic flocking in biomedical research, we aim to provide an overview of electrostatic flocking, including the principle, setups, and general and biomedical considerations, and propose a variety of biomedical applications. We begin with an introduction to the development and general applications of electrostatic flocking. Additionally, we introduce and review some of the flocking physics and mathematical considerations. We then discuss how to select, synthesize, and tune the main components (flocking fibers, adhesives, substrates) of electrostatic flocking for biomedical applications. After reviewing the considerations necessary for applying flocking toward biomedical research, we introduce a variety of proposed use cases including bone and skin tissue engineering, wound healing and wound management, and specimen swabbing. Finally, we presented the industrial comments followed by conclusions and future directions. We hope this review article inspires a broad audience of biomedical, material, and physics researchers to apply electrostatic flocking technology to solve a variety of biomedical and materials science problems.

Electrostatic Flocking of Insulative and Biodegradable Polymer Microfibers for Biomedical Applications

Electrostatic flocking, a textile engineering technique, uses Coulombic driving forces to propel conductive microfibers toward an adhesive-coated substrate, leaving a forest of aligned fibers. Though an easy way to induce anisotropy along a surface, this technique is limited to microfibers capable of accumulating charge. This study reports a novel method, utilizing principles from the percolation theory to make electrically insulative polymeric microfibers flockable. A variety of well-mixed, conductive materials are added to multiple insulative and biodegradable polymer microfibers during wet spinning, which enables nearly all types of polymer microfibers to accumulate sufficient charges required for flocking. Biphasic, biodegradable scaffolds are fabricated by flocking silver nanoparticle (AgNP)-filled poly(ε-caprolactone) (PCL) microfibers onto substrates made from 3D printing, electrospinning, and thin-film casting. The incorporation of AgNP into PCL fibers and use of chitosan-based adhesive enables antimicrobial activity against methicillin-resistant Staphylococcus aureus. The fabricated scaffolds demonstrate both favorable in vitro cell response and new tissue formation after subcutaneous implantation in rats, as evident by newly formed blood vessels and infiltrated cells. This technology opens the door for using previously unflockable polymer microfibers as surface modifiers or standalone structures in various engineering fields.

Bioinspired surfaces with special micro-structures and wettability for drag reduction: which surface design will be a better choice?

Human beings learn from creatures in nature and imitate them to solve challenges in daily life. Thus, the use of bioinspired surfaces for drag reduction has attracted extensive attention in recent years due to their important applications in many fields, such as pipeline systems, maritime transportation, and military weapons. Herein, we introduce some typical plants and animals with low drag surfaces that exist in nature, focusing on their drag reduction patterns. There are two main mechanisms to explain how surfaces reduce frictional drag, where one is to design a suitable surface geometry to change the flow distribution of surrounding fluid and the other is to introduce a low friction lubricating layer (usually air or non-toxic silicone oil) to partially or completely replace the solid-liquid interface. Hence, by mimicking these organisms, some surfaces have been fabricated to reduce frictional drag, including riblets, superhydrophobic surfaces, and slippery liquid-infused porous surfaces. With the increasing research on drag-reducing surfaces, the drag reduction rate of different types of surface designs has greatly improved in recent years. This review provides a holistic overview that facilitates direct comparisons between these surface types. To select an optimal surface for drag reduction in practical applications, the merits and deficiencies of different surface designs are analysed and compared. Finally, based on the current challenges, we present some future prospects for the application of bioinspired surfaces in drag reduction.

Air Retention under Water by the Floating Fern Salvinia: The Crucial Role of a Trapped Air Layer as a Pneumatic Spring

The ability of floating ferns Salvinia to keep a permanent layer of air under water is of great interest, e.g., for drag-reducing ship coatings. The air-retaining hairs are superhydrophobic, but have hydrophilic tips at their ends, pinning the air-water interface. Here, experimental and theoretical approaches are used to examine the contribution of this pinning effect for air-layer stability under pressure changes. By applying the capillary adhesion technique, the adhesion forces of individual hairs to the water surface is determined to be about 20 µN per hair. Using confocal microscopy and fluorescence labeling, it is found that the leaves maintain a stable air layer up to an underpressure of 65 mbar. Combining both results, overall pinning forces are obtained, which account for only about 1% of the total air-retaining force. It is suggested that the restoring force of the entrapped air layer is responsible for the remaining 99%. This model of the entrapped air acting is verified as a pneumatic spring ("air-spring") by an experiment shortcircuiting the air layer, which results in immediate air loss. Thus, the plant enhances its air-layer stability against pressure fluctuations by a factor of 100 by utilizing the entrapped air volume as an elastic spring.