3D-Printed Bioinspired Cassie-Baxter Wettability for Controllable Microdroplet Manipulation

Laser Interference Additive Manufacturing: Mask Bundle Shape Bionic Shark Skin Structure

Here, we explored a new manufacturing strategy that uses the mask laser interference additive manufacturing (MLIAM) technique, which combines the respective strengths of laser interference lithography and mask lithography to efficiently fabricate across-scales three-dimensional bionic shark skin structures with superhydrophobicity and adhesive reduction. The phenomena and mechanisms of the MLIAM curing process were revealed and analyzed, showing the feasibility and flexibility. In terms of structural performance, the adhesive force on the surface can be tuned based on the growth direction of the bionic shark skin structures, where the maximum rate of the adhesive reduction reaches about 65%. Furthermore, the evolution of the directional diffusion for the water droplet, which is based on the change of the contact angle, was clearly observed, and the mechanism was also discussed by the models. Moreover, no-loss transportations were achieved successfully using the gradient adhesive force and superhydrophobicity on the surface by tuning the growth direction and modifying by fluorinated silane. Finally, this work gives a strategy for fabricating across-scale structures on micro- and nanometers, which have potential application in bioengineering, diversional targeting, and condenser surface.

Application of 3D printing for fabrication of superhydrophobic surfaces with reversible wettability

Control of surface wettability is needed in many applications. The potential use of 3D printing technology to gain control over wettability remains largely unexplored. In this paper, Fused Deposition Molding (FDM) 3D printing technology was utilized to print polylactic acid (PLA) microplate array structures to generate superhydrophobic surfaces with reversable wetting properties. This was achieved by spraying polydimethylsiloxane (PDMS) and silica (SiO2) solutions, over microplate surfaces. Anisotropic wetting properties were also achieved based on the surface structure design. Due to the shape memory properties of PLA, the morphology of the microplate arrays could be switched between the original upright shape and deformed shape. Through alternating pressing and heating treatments, the microplate arrays showed anisotropic wettability switching. The difference between the contact angle (CA) and sliding angle (SA) of water droplets on the original surface parallel to and perpendicular to the microplate array direction were ΔCA = 7° and ΔSA = 3° respectively, and those on the surface of the deformed microplate array were ΔCA = 7° and ΔSA = 21°, respectively. This process enabled reversible alteration in the wetting behavior of water droplets on the original and deformed surfaces between sliding and sticking states. PLA-based shape memory anisotropic superhydrophobic surfaces with tunable adhesion were successfully applied to rewritable platforms, micro droplet reaction platforms, and gas sensing.

Interaction between microorganisms and dental material surfaces: general concepts and research progress

Bacterial adhesion to dental materials’ surfaces is the initial cause of dental materials-related infections. Therefore, inhibiting bacterial adhesion is a critical step in preventing and controlling these infections. To this end, it is important to know how the properties of dental materials affect the interactions between microorganisms and material surfaces to produce materials without biological contamination. This manuscript reviews the mechanism of bacterial adhesion to dental materials, the relationships between their surface properties and bacterial adhesion, and the impact of bacterial adhesion on their surface properties. In addition, this paper summarizes how these surface properties impact oral biofilm formation and proposes designing intelligent dental material surfaces that can reduce biological contamination.

Development of Adjustable High- to Low-Adhesive Superhydrophobicity Using Aligned Electrospun Fibers

Superhydrophobic surfaces based on electrospun fibrous structures exhibit advantages of additive manufacturing and enable the passage of gases. Compared to randomly deposited fibers, directionally aligned fibers improve the control of surface wetting by a specified fiber orientation and predictable liquid-fiber contact interface. In this article, we create superhydrophobicity with adjustable adhesion based on the understanding of droplet wetting behavior on directionally aligned fibers. Directionally aligned polystyrene fibers with different diameters and interfiber distances (l) are produced using electrospinning with a rotating fin collector. The wetting behavior of droplets on the surfaces dressed by aligned fibers is characterized, and a thermodynamic model of wetting behavior is established to guide the experimental studies. As a result, high-adhesive superhydrophobicity is achieved on weak hydrophobic substrate surfaces dressed by aligned polystyrene fibers with a diameter of 1.8 μm and l between 5 and 130 μm. Water droplets (2 μL) exhibit a maximum contact angle of 156° and adhere to the fiber-dressed surfaces by tilting upside down. Low-adhesive superhydrophobicity is achieved by introducing an additional layer of aligned fibers to increase the transition energy barrier. On the dual-layer structure with an upper-layer l of 9 μm, droplets show a contact angle of 155° and can readily roll off the surface. Moreover, increasing the upper-layer l to 15 μm reserves the surface to high-adhesive superhydrophobicity.

Chemical Etching, Thermally Driven Combination Strategy to Fabricate Superhydrophobic Fe-Based Amorphous Coatings with Excellent Anticorrosion Property: Based on Hydroxylation Effect

Fe-based amorphous coatings are ideal materials for surface protection due to their outstanding mechanical properties and corrosion resistance. However, coating defects are inevitably formed during the preparation of coatings by thermal spray technology, which seriously affects the corrosion performance. Inspired by bionics, conceiving superhydrophobic surfaces with liquid barrier properties has become a new idea for the corrosion protection of metal surfaces. In this work, based on surface hydroxylation, we designed a superhydrophobic Fe-based amorphous coating with corrosion resistance by chemical etching combined with a thermally driven preparation strategy. The obtained superhydrophobic coatings exhibit liquid repellency (contact angle >150°) and excellent corrosion resistance (corrosion current density and passive current density reduced by 3 orders of magnitude). The results revealed that the superhydrophobic behavior stems from the construction of hydroxyl-induced surface micro-/nanomultilevel aggregates (cluster structures). The hydrophobic agent layer deposited on the surface of cluster aggregates and the nanoparticle elements that constitute the clusters dominate the corrosion resistance of the coating. This work provides an effective guide to the design of high-corrosion-resistant Fe-based amorphous alloy coatings and promotes their engineering applications.

Programmable Microfluidics Enabled by 3D Printed Bionic Janus Porous Matrics for Microfluidic Logic Chips

Numerous structures have been functionally optimized for directional liquid transport in nature. Inspired by lush trees' xylem that enable liquid directional transportation from rhizomes to the tip of trees, a new kind of programmable microfluidic porous matrices using projection micro-stereolithography (PµSL) based 3D printing technique is fabricated. Structural matrices with internal superhydrophilicity and external hydrophobicity are assembled for ultra-fast liquid rising enabled by capillary force. Moreover, the unidirectional microfluidic performance of the bionic porous matrices can be theoretically optimized by adjusting its geometric parameters. Most significantly, the successive programmable flow of liquid in a preferred direction inside the bionic porous matrices with tailored wettability is achieved, validating by a precisely printed liquid displayer and a microfluidic logic chip. The programmable and functional microfluidic matrices promise applications of patterned liquid flow, displayer, logic chip, cell screening, gas-liquid separation, and so on.

Biomimetic Superhydrophobic Materials through 3D Printing: Progress and Challenges

Superhydrophobicity, a unique natural phenomenon observed in organisms such as lotus leaves and desert beetles, has inspired extensive research on biomimetic materials. Two main superhydrophobic effects have been identified: the "lotus leaf effect" and the "rose petal effect", both showing water contact angles larger than 150°, but with differing contact angle hysteresis values. In recent years, numerous strategies have been developed to fabricate superhydrophobic materials, among which 3D printing has garnered significant attention due to its rapid, low-cost, and precise construction of complex materials in a facile way. In this minireview, we provide a comprehensive overview of biomimetic superhydrophobic materials fabricated through 3D printing, focusing on wetting regimes, fabrication techniques, including printing of diverse micro/nanostructures, post-modification, and bulk material printing, and applications ranging from liquid manipulation and oil/water separation to drag reduction. Additionally, we discuss the challenges and future research directions in this burgeoning field.

Hyper-anti-freezing bionic functional surface to -90°C

Freezing phenomenon has troubled people for centuries, and efforts have been made to lower the liquid freezing temperature, raise the surface temperature, or mechanical deicing. Inspired by the elytra of beetle, we demonstrate a novel functional surface for directional penetration of liquid to reduce icing. The bionic functional surface is fabricated by projection microstereolithography (PµSL) based three dimensional printing technique with the wettability on its two sides tailored by TiO₂ nanoparticle sizing agent. A water droplet penetrates from the hydrophobic side to the superhydrophilic side of such a bionic functional surface within 20 ms, but it is blocked in the opposite direction. Most significantly, the penetration time of a water droplet through such a bionic functional surface is much shorter than the freezing time on it, even though the temperature is as low as -90°C. This work opens a gate for the development of functional devices for liquid collection, condensation, especially for hyperantifogging/freezing.

Characterization of PDMS Microchannels Using Horizontally or Vertically Formed 3D-Printed Molds by Digital Light Projection

Three-dimensional (3D) printing is one of the promising technologies for the fabrication of microstructures due to its versatility, ease of fabrication, and low cost. However, the direct use of 3D-printed microstructure as a microchannel is still limited due to its surface property, biocompatibility, and transmittance. As an alternative, rapid prototyping of poly(dimethylsiloxane) (PDMS) from 3D-printed microstructures ensures both biocompatibility and efficient fabrication. We employed 3D-printed molds fabricated using horizontal and vertical arrangement methods with different slice thicknesses in a digital light projection (DLP)-based 3D printing process to replicate PDMS microchannels. The replicated PDMS structures were investigated to compare their optical transmittances and surface roughness. Interestingly, the optical transmittance of PDMS from the 3D-printed mold was significantly increased via bonding two single PDMS layers. To evaluate the applicability of the replicated PDMS devices from the 3D-printed mold, we performed droplet generation in the PDMS microchannels, comparing the same device from a conventional Si-wafer mold. This study provides a fundamental understanding of prototyping microstructures from the DLP-based 3D-printed mold.

Superhydrophobic/Superhydrophilic Janus Evaporator for Extreme High Salt-Resistance Solar Desalination by an Integrated 3D Printing Method

The emerging solar desalination technology has incomparable advantages for providing a clean water solution. However, the issue of salt accumulation on the solar evaporator tops during the steam generation leads to a considerable decrease in the evaporation rate. Herein, we demonstrate a superhydrophobic/superhydrophilic Janus evaporator that enables a stable solar evaporation even in saturated brine. Our Janus solar evaporator with a superhydrophobic top and a superhydrophilic bottom has been manufactured integrally, allowing for a fast steam evaporation without the impediment of the accumulated salt residues. The superhydrophobic top changes the water passageway from the center toward the edges while it allows for the vertical transport of both solar thermo and evaporated steams. Salt residues would only be deposited at the edges of the superhydrophilic bottom, allowing for a long-term stability of the evaporator for a continuous (>50 h) solar evaporation in saturated brine, which is record-breaking for salt-resistant solar evaporators. With stable and efficient evaporation performance out of high-salinity brine, this work provides a fascinating avenue for the desalination of seawater in a salt-resistant and efficient manner.

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.

Self-Powered Wireless Flexible Ionogel Wearable Devices

Ionogels are promising soft materials for flexible wearable devices because of their unique features such as ionic conductivity and thermal stability. Ionogels reported to date show excellent sensing sensitivity; however, they suffer from a complicated external power supply. Herein, we report a self-powered wearable device based on an ionogel incorporating poly(vinylidene fluoride) (PVDF). The three-dimensional (3D) printed PVDF-ionogel exhibits amazing stretchability (1500%), high conductivity (0.36 S/m at 105 Hz), and an extremely low glass transition temperature (-84 °C). Moreover, the flexible wearable devices assembled from the PVDF-ionogel can precisely detect physiological signals (e.g., wrist, gesture, running, etc.) with a self-powered supply. Most significantly, a self-powered wireless flexible wearable device based on our PVDF-ionogel achieves monitoring healthcare of a human by transmitting obtained signals with a Bluetooth module timely and accurately. This work provides a facile and efficient method for fabricating cost-effective wireless wearable devices with a self-powered supply, enabling their potential applications for healthcare, motion detection, human-machine interfaces, etc.

Large-Scale, Abrasion-Resistant, and Solvent-Free Superhydrophobic Objects Fabricated by a Selective Laser Sintering 3D Printing Strategy

Manufacturing abrasion-resistant superhydrophobic matters is challenging due to the fragile feature of the introduced micro-/nanoscale surface roughness. Besides the long-term durability, large scale at meter level, and 3D complex structures are of great importance for the superhydrophobic objects used across diverse industries. Here it is shown that abrasion-resistant, half-a-meter scaled superhydrophobic objects can be one-step realized by the selective laser sintering (SLS) 3D printing technology using hydrophobic-fumed-silica (HFS)/polymer composite grains. The HFS grains serve as the hydrophobic guests while the sintered polymeric network provides the mechanical strength, leading to low-adhesion, intrinsic superhydrophobic objects with desired 3D structures. It is found that as-printed structures remained anti-wetting capabilities even after undergoing different abrasion tests, including knife cutting test, rude file grinding test, 1000 cycles of sandpaper friction test, tape test and quicksand impacting test, illustrating their abrasion-resistant superhydrophobic stability. This strategy is applied to manufacture a shell of the unmanned aerial vehicle and an abrasion-resistant superhydrophobic shoe, showing the industrial customization of large-scale superhydrophobic objects. The findings thus provide insight for designing intrinsic superhydrophobic objects via the SLS 3D printing strategy that might find use in drag-reduce, anti-fouling, or other industrial fields in harsh operating environments.

Three-Dimensional Open Water Microchannel Transpiration Mimetics

The key problem that hinders the water transportation performance and application of microchannels is the annoying gaslock. Realizing liquid transport without the gaslock requires a specially designed pump and a channel system, as well as the reduction of gas concentration in liquids. In nature, to eat viscous nectar with high efficiency, hummingbirds use their open geometric tongue for nectar-sucking. Inspired by hummingbirds' tongue, we report a bionic open microchannel that discharges unwanted gas inside the microchannel from the opening without influencing its fluidic performance. The opening can also be used for extrusion of oil droplets in microchannels, indicating great potential applications in oil-water separation and chemical slow release, especially for bubble discharge in microchannels. Most significantly, a mimicked "leaf" with our bionic open microchannnels exhibits marvelous "transpiration" performance when irradiated by a laser. Our work provides a new strategy for the fabrication of open microchannels and sheds light on potential applications of multiphase phenomena in microchannels including oil-water separation, phase change heat and mass transfer, solar vapor generation, and precisely controllable drug delivery.

3D Printed Parahydrophobic Surfaces as Multireaction Platforms

Parahydrophobic surfaces (PHSs) composed of arrays of cubic μ-pillars with a double scale of roughness and variable wettability were systematically obtained in one step and a widely accessible stereolithographic Formlabs 3D printer. The wettability control was achieved by combining the geometrical parameters (H = height and P = pitch) and the surface modification with fluoroalkyl silane compounds. Homogeneous distribution of F and Si atoms onto the pillars was observed by XPS and SEM-EDAX. A nano-roughness on the heads of the pillars was achieved without any post-treatment. The smallest P values lead to surfaces with static contact angles (CAs) >150° regardless of the H utilized. Interestingly, the relationship 0.6 ≤ H/P ≤ 2.6 obtained here was in good agreement with the H/P values reported for nano- and submicron pillars. Furthermore, experimental CAs, advancing and receding CAs, were consistent with the theoretical prediction from the Cassie-Baxter model. Structures covered with perfluorodecyltriethoxysilane with high H and short P lead to PHSs. Conversely, structures covered with perfluorodecyltrimethoxysilane exhibited a superhydrophobic behavior. Finally, several aqueous reactions, such as precipitation, coordination complex, and nanoparticle synthesis, were carried out by placing the reactive agents as microdroplets on the parahydrophobic pillars, demonstrating the potential application as chemical multi-reaction array platforms for a large variety of relevant fields in microdroplet manipulation, microfluidics systems, and health monitoring, among others.

Adhesion behaviors of water droplets on bioinspired superhydrophobic surfaces

The adhesion behaviors of droplets on surfaces are attracting increasing attention due to their various applications. Many bioinspired superhydrophobic surfaces with different adhesion states have been constructed in order to mimic the functions of natural surfaces such as a lotus leaf, a rose petal, butterfly wings, etc. In this review, we first present a brief introduction to the fundamental theories of the adhesion behaviors of droplets on various surfaces, including low adhesion, high adhesion and anisotropic adhesion states. Then, different techniques to characterize droplet adhesion on these surfaces, including the rotating disk technique, the atomic force microscope cantilever technique, and capillary sensor-based techniques, are described. Wetting behaviors, and the switching between different adhesion states on bioinspired surfaces, are also summarized and discussed. Subsequently, the diverse applications of bioinspired surfaces, including water collection, liquid transport, drag reduction, and oil/water separation, are discussed. Finally, the challenges of using liquid adhesion behaviors on various surfaces, and future applications of these surfaces, are discussed.

Synthesis of Carbonaceous Hydrophobic Layers through a Flame Deposition Process

In this study we report the effect of fuel type (biodiesel vs. methane), flame structure and flame height (inner-cone vs. outer-cone), and the percent of oxygen content in the oxidizer stream for the formation of hydrophobic carbon layers using co-flow diffusion flames. It was found that a flame formed using a gaseous fuel (methane) over a vaporized liquid fuel, Canola Methyl Ester (CME), has significant structural differences that enable vastly different deposition behavior of soot layers on the surface of solid substrates. Due to its larger pyrolysis zone (taller inner-cone), the CH4/air flame has a smaller region that supports uniform soot deposition of hydrophobic carbon layers (C-layers) compared to the CME/air flame. When a solid substrate is placed within the pyrolysis zone (inner-cone) of a flame the resulting layer is non-uniform, hydrophilic, and consists of undeveloped soot. However, when outside the pyrolysis zone, the deposited soot tends to be uniform and mature, ultimately creating a hydrophobic C-layer consisting of the typical microscale interconnected weblike structures formed of spherical soot nanoparticles. The effect of oxygen content (35% and 50% O2) in the oxidizer stream for the formation of hydrophobic C-layers was also studied in this work. It was found that oxygen enrichment within the CME flame alters the structure of the flame, hence affecting the morphology of the formed C-layer. Under oxygen enrichment the central region of the deposited C-layer is composed of a weblike structure similar to those seen in the air flames; however, this central region is bordered by a region of densely compacted soot that shows signs of significant thermal stress. At 35% O2 the thermal stress is expressed as multiple microscale cracks while at 50% O2 this border region shows much larger cracks and macroscale layer peeling. The formed C-layers under the different flame conditions were tested for hydrophobicity by measuring the contact angle of a water droplet. The morphology of the C-layers was analyzed using scanning electron microscopy.

Underwater Unidirectional Cellular Fluidics

The unidirectional fluidics underwater promises the manipulation of gas/liquid for various significant applications. Inspired by the unique stomata on the surface of hornwort stems and leaves that enable the transport and storage of oxygen underwater, we propose a bionic cell with porous membranes fabricated by the projection microstereolithography based 3D printing technique. Different Laplace forces coming from different contact angles for the respectively superhydrophilic outside and hydrophobic inside promise unidirectional fluidic performance, which stop water flowing inside of the bionic cell while exhausting gas and liquid outside of it. In addition, geometric parameters of the bionic cell make a big difference in its unique unidirectional fluidic performance. Simultaneously, the underlying mechanisms of the unidirectional penetration of liquid in our 3D printed bionic cell are theoretically revealed. Moreover, we demonstrate potential applications of our bionic cell with underwater anaerobic chemical reactions to fully apply its outstanding unidirectional fluidics underwater. Our bionic cell opens a gate for potential applications in chemical and microfluidic engineering underwater, such as the storage of flammable materials, fast solid-liquid separations, and anaerobic chemical reactions.

Femtosecond Laser Regulated Ultrafast Growth of Mushroom-Like Architecture for Oil Repellency and Manipulation

Natural organisms can create various microstructures via a spontaneous growth mode. In contrast, artificial protruding microstructures are constructed by subtractive methods that waste materials and time or by additive methods that require additional materials. Here, we report a facile and straightforward strategy for a laser-induced self-growing mushroom-like microstructure on a flat surface. By simply controlling the localized femtosecond laser heating and ablation on the poly(ethylene terephthalate) tape/heat-shrinkable polystyrene bilayer surface, it is discovered that a mushroom-like architecture can spontaneously and rapidly grow out from the original surface within 0.36 s. The dimension of the re-entrant micropillar array (cap diameter, pillar spacing, and height) can be accurately controlled through the intentional control of laser scanning. Followed by a fluorination and spray coating, the obtained surface can realize the repellency and manipulation of oil droplets. This work provides new opportunities in the fields of microfabrication, microfluidics, microreactor engineering, and wearable antifouling electronics.

Hierarchical Colorful Structures by Three-Dimensional Printing of Inverse Opals

Structural coloration is a recurring solution in biological systems to control visible light. In nature, basic structural coloration results from light interacting with a repetitive nanopattern, but more complex interactions and striking results are achieved by organisms incorporating additional hierarchical structures. Artificial reproduction of single-level structural color has been achieved using repetitive nanostructures, with flat sheets of inverse opals being very popular because of their simple and reliable fabrication process. Here, we control photonic structures at several length scales using a combination of direct laser writing and nanosphere assembly, producing freeform hierarchical constructions of inverse opals with high-intensity structural coloration. We report the first 3D prints of stacked, overhanging and slanted microstructures of inverse opals. Among other characteristics, these hierarchical photonic structures exhibit geometrically tunable colors, focal-plane-dependent patterns, and arbitrary alignment of microstructure facet with self-assembled lattice. Based on those results, novel concepts of multilevel information encoding systems are presented.

Superhydrophobic μ-pillars via simple and scalable SLA 3D-printing: the stair-case effect and their wetting models

In nature, superhydrophobic surfaces (SHSs) exhibit microstructures with several roughness scales. Scalable fabrication and build-up along the X-Y plane represent the promise of 3D printing technology. Herein we report 3D printed microstructures with a dual roughness scale that achieves SHS using a readily available Formlabs stereolithography (SLA) printer. Pillar-like structure (PLS) arrangements with a wide range of geometrical shapes were 3D printed at three resolutions and two printing orientations. We discovered that a tilted printing direction enables a stair-case pattern on the μ-PLS surfaces, conferring them a μ-roughness that reduces the solid-liquid contact area. The programmed resolution governs the number of polymerized layers that give rise to the stepped pattern on the μ-PLS surfaces. However, this is reduced as the printing resolution increases. Also, all samples' experimental contact angles were consistent with theoretical predictions from Cassie-Baxter, Wenzel, and Nagayama wettability models. The underlying mechanisms and governing parameters were also discussed. It is believed that this work will enable scalable and high throughput roughness design in augmenting future 3D printing object applications.