Bio-inspired Surface Research Articles

A hybrid and scalable nanofabrication approach for bio-inspired bactericidal silicon nanospike surfaces

Insects and plants exhibit bactericidal properties through surface nanostructures, such as nanospikes, which physically kill bacteria without antibiotics or chemicals. This is a promising new avenue for achieving antibacterial surfaces. However, the existing methods for fabricating nanospikes are incapable of producing uniform nanostructures on a large scale and in a cost-effective manner. In this paper, a scalable nanofabrication method involving the application of nanosphere lithography and reactive ion etching for constructing nanospike surfaces is demonstrated. Low-cost silicon nanospikes with uniform spacing that were sized similarly to biological nanospikes on cicada wings with a 4-inch wafer scale were fabricated. The spacing, tip radius, and base diameter of the silicon nanospikes were controlled precisely by adjusting the nanosphere diameters, etching conditions, and diameter reduction. The bactericidal properties of the silicon nanospikes with 300 nm spacing were measured quantitatively using the standard viability plate count method; they killed E. coli cells with 59 % efficiency within 30 h. The antibacterial ability of the nanospike surface was further indicated by the morphological differences between bacteria observed in the scanning electron microscopic images as well as the live/dead stains of fluorescence signals. The fabrication process combined the advantages of both top-down and bottom-up methods and was a significant step toward affordable bio-inspired antibacterial surfaces.

Integrative Bioinspired Surface with Annular Pattern and Three Dimension Wettable Gradient for Enhancement of Fog Collection

AbstractA novel integrative bioinspired surface with annular pattern and 3D wettable gradient (APWG) is proposed for fog collection via a simple needle‐face anodic oxidation strategy. The fog collection behaviors are studied on APWG with different pattern types. Such a surface can not only improve the fog droplet capture performance effectively owing to annular wettable gradient pattern, but also realize the fog droplet capture and convergence on different surfaces to accelerate the surface regeneration rate, leading to a further fog collection enhancement. The results of experiment observation and underlying mechanism analysis have certified that such a design can realize a continuous 3D fog collection model to enhance the fog collection efficiency, resulting in a 1345% increase compared to the original surface.

Liquid/air dynamic behaviors and regulation mechanisms for bioinspired surface

Liquid/air accurate regulation has attracted growing attention in recent years for its diverse potential applications in bio-medicines, heat management, green energy, etc. Natural surfaces evolved innumerable hierarchical structures with exceptional functions to govern or regulate the liquid dynamic behaviors for their vital living, which have gradually been discovered as inspirations for creative design, such as fog harvesting, water fast transporting, and strong wet attachment. This review summarizes the current progress of bioinspired liquid/air regulations and their underlying mechanisms, including fast liquid/air spreading, liquid/air directional transport, and the interfacial liquid/air bridge acting forces. A fundamental understanding of both liquid/air dynamic behaviors on liquid–air–solid interfaces and their effects on the surface function has been increased with awareness of the importance of coupling effects from surface structures and material properties. The design principles and fabrication methods for bioinspired surface structure with unique liquid/air regulation are concluded, and several significant applications for electronics heat dissipation and biomedical devices are also presented. Finally, we provide new insights and future perspectives for the liquid/air regulation-based bioinspired functional materials.

Bioinspired slippery asymmetric bumps of candle soot coating for condensation and directional transport of water

Natural species use their surface structures for water harvesting. It is the dire need of the hour to learn from nature, mimic its functions, and solve the water scarcity problem through dew condensation. However, the present microscale and nanoscale approaches still suffer from complicated fabrication methods and durability failure in harsh conditions. Here, we introduce a bioinspired slippery surface of candle soot particles which uses its bumps to enhance the growth of condensed water droplets. The Namib desert beetles-inspired convex topography, cacti-inspired asymmetric slope, and Nepenthes pitcher plant-inspired slippery surface of soot particles synergistically combines the large droplet nucleation density, fast droplet coalescence, and easy removal of droplet from the surface. The hydrophilic SLIPS of soot particles harvests water at a rate of 326.4 ± 9 mg/cm2 h-1 which is 70 % high as compared to hydrophobic SLIPS of soot particles. The fluorinated SLIPS reveals 32 % high water collection rate as compared to the methyl-terminated SLIPS of candle soot particles and the results are corroborated by calculating the intermolecular hydrogen bonding energies through DFT. The programmed motion of water droplet on a slippery surface of soot particles is also observed by applying laser light, heat, and magnetic field even against gravity. The slippery surface shows long-term stability due to nano-porosity, permeation of lubricant into soot particles and lubricant’s crosslinking ability with the binder. The developed surface is biocompatible, self-cleaning, and suitable for water harvesting particularly in cold and arid regions.

Biomimetic slippery liquid-infused porous surface on the basis of hierarchical ZIF-67@Cu dendrite: Preparation and corrosion inhibition

In the harsh environment, the corrosion of Cu leads to the failure of pipelines, condenser, printed circuit board and so forth. Thus, protecting Cu from corrosion is critically important from both science and engineering backgrounds. In this report, the hierarchical structure compositing dendritic Cu and ZIF-67 is prepared onto Cu by a two-step electrodeposition protocol. After thiol modification and oil infusion, bio-inspired superhydrophobic surface (SHS) and slippery liquid-infused porous surface (SLIPS) is respectively achieved, and sophisticated techniques are used for characterizing chemical composition, morphology, topology and wettability. Scanning Kelvin probe is employed to know the surface potential distribution before and after the biomimetic transformation. Using different electrochemical approaches, the corrosion inhibition effect is elucidated. In 3.5 wt% NaCl corrosive medium, |Z|0.01Hz of SLIPS is as high as 1.35 × 108 Ω·cm2 at the initial immersion stage. The value is remarkably enhanced compared with SHS (6.76 × 104 Ω·cm2) and bare Cu (8.74 × 103 Ω·cm2). After soaking for 168 h, |Z|0.01Hz value of SHS and SLIPS is 3.84 × 104 Ω·cm2 and 4.93 × 106 Ω·cm2, illustrating that SLIPS has high resistance to protect Cu from corrosion.

Effect of bio-inspired surface texture on the resistance of 3D-printed polycarbonate bonded joints

Surface preparation before adhesive bonding is crucial to improve the resistance and durability of the joint by altering the surface properties of the adherend. The purpose of surface treatment is to clean the surface from contaminants, activate the adherend surface and create an optimal surface structure to promote adhesion mechanisms. In that context, this work aims to investigate the influence of substrate surface texturing on the resistance of adhesive joints. Two bio-inspired surface textures were investigated, Fish scale (FS) and Tree frog (TF). Polycarbonate (PC) specimens with different surface patterns were manufactured using the fused deposition modelling process. Surface morphology, such as pattern dimension (shape and depth), surface roughness (Ra), and wettability, were used to characterise the substrates. The influence of these texture patterns on the shear strength of adhesively bonded joints was evaluated through the standardised block shear test method ASTM D4501-01. Moreover, the shear strength of the structured joints was compared to the results from bonding with polished surfaces (surfaces abraded with 80, 600 and 1000 grit paper), and with as-printed surfaces. The results revealed that the FS and TF surface textures enhanced the shear strength by 242% and 283% compared to the adhesive joints with polished surfaces. It was also shown that the variation in depth of the bio-inspired surface texture has no significant impact on the joint strength. Failure analysis demonstrated that the fracture mode of bonded joints with polished surfaces was the adhesive failure while mixed failure (cohesive and adhesive) characterises the as-printed, TF and FS surfaces. Worthy results are obtained rising the effectiveness of surface texture for the PC's bonded joints. Graphical Abstract [Formula: see text] This is a graphical representation of the abstract.

Laminar drag reduction in a closed channel using bioinspired textured surfaces

Low flow drag is of great importance to a variety of engineering applications, and an effective way to achieve low drag is to use bioinspired microstructured surfaces. This work aims to reduce the skin-friction drag in closed channel flow using textured surfaces inspired by leaves of indocalamus and rice. A channel formed from a polydimethylsiloxane chunk and a silicon wafer was fabricated to study the drag reduction behavior for water or liquid paraffin oil in laminar flow. Bioinspired textures were processed on the silicon wafer surface using the deep silicon plasma etching method. The pressure drop of water or paraffin oil passing through textured channels with different velocities was measured. The maximum pressure drop reductions for the paraffin oil flow with a low velocity (Re ≈ 1) and for the water flow with a high velocity (Re < 1000) were about 5.1 and 27.3%, respectively. The contact angles of the bioinspired textured surface are also presented, and mechanisms to explain the drag reduction are proposed. Hydrophobicity leading to a change from a liquid–solid interface to a liquid–air interface is believed to provide the drag reduction for water flow, while the thin oil film formed on the textured surface due to oleophilicity helps reduce the oil flow drag.

Smart Soft Materials with Multiscale Architecture and Dynamic Surface Topographies

ConspectusSmart soft materials have one or more characteristics that can be significantly altered in convertible fashions by external stimuli, such as light, moisture, mechanical force, temperature, electric/magnetic fields, pH, and so on. These materials can lead to widespread application in multifunctional smart devices. Recently, various smart soft materials have been developed under the inspiration of the intriguing multiscale structures, adaptive mechanisms, and dynamic responses of natural life, with an aim to further design advanced material systems with novel, intriguing, and unprecedented properties.Herein, inspired by the fascinating visual display strategies and adaptive mechanisms in animals and plants, we have fabricated a series of smart soft material-based devices that can respond to external stimuli with instantaneous and reversible fashions in optical, electrical, mechanical, and/or shape deformation signals. These devices can be fabricated for widespread applications, including smart windows, encryption devices, thermal camouflage, wearable strain sensors, anticounterfeit tabs, 3D stretchable electronics, dynamic displays, rewritable media, human–machine interfaces, and so on. The key to successfully achieving those intriguing characteristics in these smart material systems lies in the function-orientated structural design, which integrates bioinspired design and surface engineering with multiscale architecture as the crucial elements. In this Account, we provide a summary, with a main focus on our own work, of the recent advances in the bioinspired smart soft materials fabricated on the basis of 2D or 3D film–substrate multilayered structures. These materials are characterized by convertible topographies like dynamic cracks, folds, stimuli-responsive wrinkles, and other analogous structures. Those topographies are responsible for the dynamic stimuli-responsive optical, electrical, and mechanical properties demonstrated in the system, such as strain-dependent light scattering effect, mechanical tunable light shielding properties, moisture/mechanically/photothermally tunable surface reflectance, moisture/mechanically responsive resistance, etc. Besides, those 2D functional materials can be further evolved into shape adaptive 3D structures via strain relaxation methods. These systems demonstrate high design flexibility, excellent reversibility, and wide applicability, which can pave new routes for designing next generation smart soft materials equipped with versatile, tunable, adaptable, and interactive stimuli-responsive properties.

Bio-inspired superhydrophobic magnesium alloy surfaces with active anti-corrosion and self-healing properties

Bio-inspired superhydrophobic magnesium alloy surfaces with active anti-corrosion and self-healing properties

Experimental investigation of flow characteristics over a surface-modified structured torpedo model

The present work aims to improve the aerodynamic performance of navigating objects using bio-inspired surface modification. Inspired by the denticles present over sharkskin, this work investigates the study of a surface-modified structured torpedo model. Different modified surface structures (riblets) are used to study their effect on the drag and flow characteristics. The modified structure placed in the shear layer modifies the flow above as well as behind the torpedo. A detailed flow field is studied using PIV (Particle Image Velocimetry) and CTA (Constant Temperature Anemometry) over the torpedo model. The experiments are performed in a subsonic wind tunnel for the Reynolds numbers (Re) range of 1.02–5.08 × 105, thus covering the laminar-turbulent transition range. The study reveals that riblets structure enhances the momentum flux, thus enabling the surface to reduce drag by suppressing the vorticity generation and reducing the turbulent dissipation. Different turbulent and wake characteristics are measured in the work. The velocity and fluctuation analysis shows the variation in wake width and wake velocity for different cases. The result showed that riblet effectively reduces drag coefficient by 11.7% using an optimal structure. The maximum reduction in formation length of 4.76% is observed at Re 3.05 × 105. The decrease in the formation length reduces the adverse pressure gradient zone and decreases the power requirement. The wake half-width at the 2D downstream location in the streamwise direction shows a reduction of 9% for the drag-reducing case at Re 3.05 × 105. The vorticity reduction is observed in the wake zone of drag-reducing riblets and suggests less dissipation of the eddy. The results show the perspective to improve the current research status by using riblet surface for marine application. The study could effectively reduce drag, thus improving the operating performance by reducing the energy consumption and increasing the speed of the navigating vessel.

Bioinspired Micro/Nanostructured Polyethylene/Poly(Ethylene Oxide)/Graphene Films with Robust Superhydrophobicity and Excellent Antireflectivity for Solar-Thermal Power Generation, Thermal Management, and Afterheat Utilization.

The rational utilization and circulation of multiple energy sources is an effective way to address the crises of energy shortages and environmental pollution. Herein, microextrusion compression molding, an industrialized polymer molding technology that combines melt blending and compression molding, is proposed for the mass production of a bioinspired micro/nanostructured polyethylene/poly(ethylene oxide)/graphene (MN-PPG) film. The MN-PPG film exhibits robust shape stability, high storage energy density, and excellent thermal management capability owing to the cocontinuous network formed by poly(ethylene oxide) and the polyethylene matrix. The MN-PPG film has sufficient photothermal property due to the uniformly dispersed graphene nanosheets and the bioinspired surface micro/nanostructures. Interestingly, the MN-PPG film surface exhibits durable superhydrophobicity, acid/alkali resistance, and active deicing performance. Further, a multifunctional energy harvesting and circulation system was established by integrating the MN-PPG film, an LED chip, and a thermoelectric module. The hybrid system produced an open-circuit voltage of 315.4 mV and power output of 2.5 W m-2 under 3 sun irradiation. Furthermore, the afterheat generated by the LED chips at night can be converted into electricity through thermoelectric conversion. The proposed method enables the large-scale fabrication of multifunctional phase change composites for energy harvesting in harsh environments.

Special Superwetting Materials from Bioinspired to Intelligent Surface for On-Demand Oil/Water Separation: A Comprehensive Review.

Since superwetting surfaces have emerged, on-demand oil/water separation materials serve as a new direction for meeting practical needs. This new separation mode uses a single porous material to allow oil-removing and water-removing to be achieved alternately. In this review, the fundamentals of wettability are systematically summarized in oil/water separation. Most importantly, the two states, bioinspired surface and intelligent surface, are summarized for on-demand oil/water separation. Specifically, bioinspired surfaces include micro/nanostructures, bioinspired chemistry, Janus-featured surfaces, and dual-superlyophobic surfaces that these superwetting materials can possess asymmetric wettability in one structure system or opposite underliquid wettability by prewetting. Furthermore, an intelligent surface can be adopted by various triggers such as pH, thermal and photo stimuli, etc., to control wettability for switchable oil/water separation reversibly, expressing a thought beyond nature to realize innovative oil/water separation by external stimuli. Remarkably, this review also discusses the advantages of all the materials mentioned above, expanding the separation scope from the on-demand oil/water mixtures to the multiphase immiscible liquid-liquid mixtures. Finally, the prospects of on-demand oil/water separation materials are also concluded.

Stretchable and Electroactive Crosslinked Gelatin for Biodevice and Cell Culture Applications.

Biomimetic substrates that incorporate functionality such as electroactivity and mechanical flexibility, find utility in a variety of biomedical applications. Toward these uses, nature-derived materials such as gelatin offer inherent biocompatibility and sustainable sourcing. However, issues such as high swelling, poor mechanical properties, and lack of stability at biological temperatures limit their use. The enzymatic crosslinking of gelatin via microbial transglutaminase (mTG) yields flexible and robust large area substrates that are stable under physiological conditions. Here, we demonstrate the fabrication and characterization of strong, stretchable, conductive mTG crosslinked gelatin thin films. Incorporation of the conductive polymer poly(3,4-ethylenedioxythiophene) polystyrene sulfonate in the gel matrix with a bioinspired polydopamine surface coating is used to enable conductivity with enhanced mechanical properties such as extensibility and flexibility, in comparison to plain gelatin or crosslinked gelatin films. The electroconductive substrates are conducive to cell growth, supporting myoblast cell adhesion, viability, and proliferation and could find use in creating active cell culture systems incorporating electrical stimulation. The substrates are responsive to motion such as stretching and bending while being extremely handleable and elastic, making them useful for applications such as electronic skin and flexible bioelectronics. Overall, this work presents facile, yet effective development of bioinspired conductive composites as substrates for bio-integrated devices and functional tissue engineering.

Bioinspired metal-organic framework-based liquid-infused surface (MOF-LIS) with corrosion and biofouling prohibition properties

Corrosion and biofouling are two main deterioration effects for shortening the service life of metal material in marine environment. In this study, targeting corrosion and biofouling of Cu, a bioinspired liquid-infused surface (LIS) coating based on metal organic frameworks (MOFs) is constructed onto Cu by sequential steps, including oxidation, coordination reaction, thiol modification and infusion process. The microstructure, chemical composition and wettability of the coating are uncovered through different characterization techniques, such as scanning electron microscopy, high-resolution transmission electron microscopy, electron diffraction, X-ray photoelectron spectroscopy and so forth. Corrosion durability test results show that MOF-LIS performs high corrosion inhibition to underneath Cu. As a representative guiding parameter, Rct of MOF-LIS reaches 5.26 × 107 Ω cm2, which is much higher than that of bare Cu (694.6 Ω cm2), illustrating a corrosion inhibition efficiency up to 99.999% for Cu metal. Using diatoms as the probe organisms, after being immersed in seawater for 8 d, the attaching diatoms density on MOF-LIS (7.33 × 104 cells/cm2) is much less than bare Cu surface (3.95 × 108 cells/cm2), illustrating the high biofouling inhibition efficiency. Therefore, the versatile properties enable MOF-LIS as a coating having potential usage for anti-corrosion and anti-biofouling in the marine environment.

Enhanced self-cleaning performance of bio-inspired micropillar-arrayed surface by shear

Inspired by the sliding behavior of gecko feet during climbing, the contribution of the shear effect to the self-cleaning performance of a bio-inspired micropillar-arrayed surface is studied through a load-shear-pull contact process. It is found that self-cleaning efficiency can be enhanced significantly by shear. The efficiency also depends on microparticle size. For the case of relatively large and small microparticles, self-cleaning efficiency increases first and then almost keeps a constant with the increase of shear distance at different preloads. For medium microparticles, shear can effectively improve self-cleaning efficiency only when the preload is small. The mechanical mechanism under such enhancement is mainly due to the varying contact states between microparticles and micropillars with the shear distance. When the shear distance is large enough, the final self-cleaning efficiency is not sensitive to shear distance anymore because the contact state reaches dynamic equilibrium. Based on such a self-cleaning mechanism of large microparticles, a simple and effective manipulator that can efficiently transfer solid particles is further proposed.

Fabrication of a superhydrophobic micron-nanoscale hierarchical structured surface for delayed icing and reduced frosting

Multifunctional superhydrophobic surfaces with dual-scale hierarchical structures have drawn considerable interest in real-world applications. The complicated fabricating process, higher costs, and poor durability limit the widespread utilization of bio-inspired superhydrophobic surfaces. Herein, we use an effective method to create a micron-nanoscale (dual-scale) hierarchical structure, which is composed of a micron-scaled rough structure constructed by a chemical-etching process and a secondary nanostructure obtained by hydrophobic nano silica (SiO2) modification. The micron-nanoscale hierarchical structured surface has superhydrophobicity with a water contact angle of 161.5°±1.9°. Compared with the micron-scaled rough structure surface, the manufactured micron-nanoscale hierarchical structured surface presented excellent anti-icing properties with a freezing delay time of 5013.6 s at -15 °C. The frost layer on the micron-nanoscale hierarchical structured surface can be rapidly melted to transform into some isolated water droplets in about 7 s due to its ultra-low water adhesion force. Additionally, the prepared superhydrophobic micron-nanoscale hierarchical structured surface has chemical stability and mechanical durability except for the delayed icing and reduced frosting performance. The construction of a micron-nanoscale hierarchical structure is expected to expand superhydrophobic surface applications in anti-icing, anti-frosting, and self-cleaning.

Bioinspired Functional Structures for Lubricant Control at Surfaces and Interfaces: Wedged-Groove with Oriented Capillary Patterns.

In this work, a design concept of bioinspired functional surfaces is proposed for lubricant control at surfaces and interfaces subjected to external thermal gradients. Inspired by the conical structures of cactus and the motion configuration of Centipedes, a bioinspired surface of wedged-groove with an oriented capillary pattern is constructed. The effect of geometrical parameters on the directional lubricant manipulation capacity and sliding anisotropy is discussed. It is found that by regulating the orientation of the capillary pattern, a controllable lubricant self-transport capacity can be achieved for varying conditions from surfaces to interfaces, with or without thermal gradients. The lubricant self-transport process is captured, and the mechanism is revealed. The design philosophy of the proposed bioinspired functional surface is believed to have potential applications for lubricant control in modern machinery and complex liquid control in lab-on-a-chip and microfluidics devices.

Bioinspired Surface Design for Magnesium Alloys with Corrosion Resistance

Magnesium alloys are regarded as potential candidates in industrial and biomedical applications because of their excellent mechanical properties and biodegradability. However, the excessive degradation rate of magnesium alloys can cause a premature disintegration of mechanical integrity, which is the main bottleneck that limits applications. Inspired by nature, various novel surface designs provide a clever strategy to regulate the corrosion behavior of magnesium alloys. This review extensively discusses bioinspired surface designs to reduce corrosion resistance and realize functionalization, so as to offer new ideas with great potential for biomedical applications. Future research on corrosion resistance is expected to benefit greatly from the bioinspired surface designs.

Anisotropic Wettability of Bioinspired Surface Characterized by Friction Force.

Bioinspired surfaces with special wettabilities attract increasing attention due to their extensive applications in many fields. However, the characterizations of surface wettability by contact angle (CA) and sliding angle (SA) have clear drawbacks. Here, by using an array of triangular micropillars (ATM) prepared by soft lithography, the merits of measuring the friction force of a water droplet on ATM over measurements of CA and SA in characterizing the surface wettability are demonstrated. The CA and SA measurements show ignorable differences in the wettabilities of ATM in opposite directions (1.13%) and that with different periodic parameters under the elongation ranging from 0 to 70%. In contrast, the friction measurement reveals a difference of > 10% in opposite directions. Moreover, the friction force shows a strong dependence on the periodic parameters which is regulated by mechanical stretching. Increasing the elongation from 0 to 50% increases the static and kinetic friction force up to 23.0% and 22.9%, respectively. Moreover, the stick-slip pattern during kinetic friction can reveal the periodic features of the measured surface. The friction force measurement is a sensitive technique that could find applications in the characterization of surface wettabilities.

Recent Developments in Multifunctional Antimicrobial Surfaces and Applications toward Advanced Nitric Oxide-Based Biomaterials.

Implant-associated infections arising from biofilm development are known to have detrimental effects with compromised quality of life for the patients, implying a progressing issue in healthcare. It has been a struggle for more than 50 years for the biomaterials field to achieve long-term success of medical implants by discouraging bacterial and protein adhesion without adversely affecting the surrounding tissue and cell functions. However, the rate of infections associated with medical devices is continuously escalating because of the intricate nature of bacterial biofilms, antibiotic resistance, and the lack of ability of monofunctional antibacterial materials to prevent the colonization of bacteria on the device surface. For this reason, many current strategies are focused on the development of novel antibacterial surfaces with dual antimicrobial functionality. These surfaces are based on the combination of two components into one system that can eradicate attached bacteria (antibiotics, peptides, nitric oxide, ammonium salts, light, etc.) and also resist or release adhesion of bacteria (hydrophilic polymers, zwitterionic, antiadhesive, topography, bioinspired surfaces, etc.). This review aims to outline the progress made in the field of biomedical engineering and biomaterials for the development of multifunctional antibacterial biomedical devices. Additionally, principles for material design and fabrication are highlighted using characteristic examples, with a special focus on combinational nitric oxide-releasing biomedical interfaces. A brief perspective on future research directions for engineering of dual-function antibacterial surfaces is also presented.