Putative functions and functional efficiency of ordered cuticular nanoarrays on insect wings

Nanoscale mesh acts as anti-adhesive surface against particulate contamination in eyes of whiteflies

In many insects the surface of the eye is nanostructured by arrays of protuberances termed ommatidial gratings which provide the cuticle with anti-reflective, anti-wetting and self-cleaning properties. The hypothesised anti-contamination role of the gratings against dust and pollen results from theoretical predictions on grating geometry and experiments on synthetic replicas of ommatidia surfaces but has not yet been proven in an animal. Whiteflies are biological test beds for anti-contamination surfaces because they deliberately distribute wax particles extruded from abdominal plates over their entire bodies. The numerous particles protect the animal against water evaporation and radiation, but may severely impair vision. Using scanning electron microscopy (SEM) and CryoSEM, we here show that the cornea of whiteflies exhibits ~ 220 nm wide mesh-like structures forming hexagonal gratings with thin ~ 40 nm connecting walls. Quantitative measurements of wax particles on the eye show that the nanostructures reduce particle contamination by more than ~ 96% compared to other areas of the cuticle. Altogether, our study is the first description of a predicted optimized grating geometry for anti-contamination in an arthropod. The findings serve as evidence of the high effectiveness of nanostructured surfaces for reducing contact area and thus adhesion forces between biological surfaces and contaminating particles.

Multi-functional approach in the design of smart surfaces to mitigate bacterial infections: a review

Advancements in biomedical devices are ingenious and indispensable in health care to save millions of lives. However, microbial contamination paves the way for biofilm colonisation on medical devices leading to device-associated infections with high morbidity and mortality. The biofilms elude antibiotics facilitating antimicrobial resistance (AMR) and the persistence of infections. This review explores nature-inspired concepts and multi-functional approaches for tuning in next-generation devices with antibacterial surfaces to mitigate resistant bacterial infections. Direct implementation of natural inspirations, like nanostructures on insect wings, shark skin, and lotus leaves, has proved promising in developing antibacterial, antiadhesive, and self-cleaning surfaces, including impressive SLIPS with broad-spectrum antibacterial properties. Effective antimicrobial touch surfaces, photocatalytic coatings on medical devices, and conventional self-polishing coatings are also reviewed to develop multi-functional antibacterial surfaces to mitigate healthcare-associated infections (HAIs).

Staying Dry and Clean: An Insect's Guide to Hydrophobicity

Insects demonstrate a wide diversity of microscopic cuticular and extra-cuticular features. These features often produce multifunctional surfaces which are greatly desired in engineering and material science fields. Among these functionalities, hydrophobicity is of particular interest and has gained recent attention as it often results in other properties such as self-cleaning, anti-biofouling, and anti-corrosion. We reviewed the historical and contemporary scientific literature to create an extensive review of known hydrophobic and superhydrophobic structures in insects. We found that numerous insects across at least fourteen taxonomic orders possess a wide variety of cuticular surface chemicals and physical structures that promote hydrophobicity. We discuss a few bioinspired design examples of how insects have already inspired new technologies. Moving forward, the use of a bioinspiration framework will help us gain insight into how and why these systems work in nature. Undoubtedly, our fundamental understanding of the physical and chemical principles that result in functional insect surfaces will continue to facilitate the design and production of novel materials.

How much biology is in the product? Role and relevance of biological evolution and function for bio-inspired design

Bio-inspired design (BID) means the concept of transferring functional principles from biology to technology. The core idea driving BID-related work is that evolution has shaped functional attributes, which are termed “adaptations” in biology, to a high functional performance by relentless selective pressure. For current methods and tools, such as data bases, it is implicitly supposed that the considered biological models are adaptations and their functions already clarified. Often, however, the identification of adaptations and their functional features is a difficult task which is not yet accomplished for numerous biological structures, as happens to be the case also for various organismic features from which successful BID developments were derived. This appears to question the relevance of the much stressed importance of evolution for BID. While it is obviously possible to derive an attractive technical principle from an observed biological effect without knowing its original functionality, this kind of BID (“analog BID”) has no further ties to biology. In contrast, a BID based on an adaptation and its function (“homolog BID”) is deeply embedded in biology. It is suggested that a serious and honest clarification of the functional background of a biological structure is an essential first step in devising a BID project, to recognize possible problems and pitfalls as well as to evaluate the need for further biological analysis.

Keeping a Clean Surface under Water: Nanoscale Nipple Array Decreases Surface Adsorption and Adhesion Forces

While nanoscale nipple arrays are expected to reduce light reflection and/or dust contamination in some insects, similar structures have been reported in various marine invertebrates. To evaluate the anti-contamination property of the structure in aquatic regimes, we measured the adsorption and adhesion forces on the flat surface and MOSMITE™ (Mitsubishi Chemical Corporation, Tokyo, Japan), a synthetic material mimicking the nipple array, under water. A small force toward the surface occurred when the probe approached the substrate surface. This adsorption force was significantly smaller on MOSMITE™ than on the flat surface. The adhesion force toward the surface occurred when the probe was detached from the surface, and it was also significantly smaller on MOSMITE™ than on the flat surface. The adhesion force in the air was much greater than the force under water, and the force was also significantly smaller on MOSMITE™ than on the flat surface. In the aquatic regime, the nipple array provides less adsorption/adhesion properties for the surface and thus, the organisms would have less contamination of microparticles on their body surface. As the adsorption and adhesion forces are also involved in the attachment of cells, tissue, and larvae, less adhesive body surfaces should be beneficial for survival in aquatic environments, as well as land environments.

Substrate Selection of Ascidian Larva: Wettability and Nano-Structures

Ascidians are marine sessile chordates that comprise one of the major benthic animal groups in marine ecosystems. They sometimes cause biofouling problems on artificial structures underwater, and non-indigenous, invasive ascidian species can potentially and seriously alter native faunal communities. Ascidian larvae are usually tadpole-shaped, negatively phototactic, and adhere on substrates by secreting a glue from their adhesive organs. Although larvae often prefer hydrophobic surfaces, such as a silicone rubber, for settlement, hydrophobic materials are often used to reduce occurrence of fouling organisms on artificial structures. This inconsistency may indicate that an attractive surface for larvae is not always suitable for settlement. Micro-scale structures or roughness may enhance the settlement of ascidian larvae, but settlement is significantly reduced by a nano-scale nipple array (or moth-eye structure), suggesting functional properties of similar structures found on the body surfaces of various invertebrates. The substrate preferences of larvae should be one of the important bases in considering measures against biofouling, and this review also discusses the potential uses of materials to safely reduce the impacts of invasive species.

Wing wettability gradient in a damselfly Lestes sponsa (Odonata: Lestidae) reflects the submergence behaviour during underwater oviposition

The phenomenon of hydrophobicity of insect cuticles has received great attention from technical fields due to its wide applicability to industry or medicine. However, in an ecological/evolutionary context such studies remain scarce. We measured spatial differences in wing wettability in Lestes sponsa (Odonata: Lestidae), a damselfly species that can submerge during oviposition, and discussed the possible functional significance. Using dynamic contact angle (CA) measurements together with scanning electron microscopy (SEM), we investigated differences in wettability among distal, middle and proximal wing regions, and in surface nanostructures potentially responsible for observed differences. As we moved from distal towards more proximal parts, mean values of advancing and receding CAs gradually increased from 104° to 149°, and from 67° to 123°, respectively, indicating that wing tips were significantly less hydrophobic than more proximal parts. Moreover, values of CA hysteresis for the respective wing parts decreased from 38° to 26°, suggesting greater instability of the structure of the wing tips. Accordingly, compared with more proximal parts, SEM revealed higher damage of the wax nanostructures at the distal region. The observed wettability gradient is well explained by the submergence behaviour of L. sponsa during underwater oviposition. Our study thus proposed the existence of species-dependent hydrophobicity gradient on odonate wings caused by different ovipositional strategies.

Dissolvable Template Nanoimprint Lithography: A Facile and Versatile Nanoscale Replication Technique

Nanoimprinting lithography (NIL) is a next-generation nanofabrication method, capable of replicating nanostructures from original master surfaces. Here, we develop highly scalable, simple, and nondestructive NIL using a dissolvable template. Termed dissolvable template nanoimprinting lithography (DT-NIL), our method utilizes an economic thermoplastic resin to fabricate nanoimprinting templates, which can be easily dissolved in simple organic solvents. We used the DT-NIL method to replicate cicada wings which have surface nanofeatures of ∼100 nm in height. The master, template, and replica surfaces showed a >∼94% similarity based on the measured diameter and height of the nanofeatures. The versatility of DT-NIL was also demonstrated with the replication of re-entrant, multiscale, and hierarchical features on fly wings, as well as hard silicon wafer-based artificial nanostructures. The DT-NIL method can be performed under ambient conditions with inexpensive materials and equipment. Our work opens the door to opportunities for economical and high-throughput nanofabrication processes.

Structural coloration predicts the outcome of male contests in the Amazonian damselfly Chalcopteryx scintillans (Odonata: Polythoridae)

Iridescence is an optical effect that produces angle dependent coloration in animals. Recently, studies have attempted to unveil structures behind such elaborated visual signals and associated behaviors in Odonata. Here, we studied males of the Amazonian damselfly Chalcopteryx scintillans, which have hindwings that exhibit pronounced iridescence. This optical feature is used by the damselflies for intra-specific communication during territorial fights and courtship. The main question we addressed was whether male wing structural coloration may predict the outcome of male-male contests. We also studied the wing ultrastructure, in order to reveal the mechanisms that are responsible for wing coloration. Using various microscopal and spectroscopal techniques, we demonstrate that hindwing coloration is derived from two main effects: (1) light interference in the cuticle multilayer and (2) a specific angle dependent light scattering and antireflective properties of the epicuticular wax coverage. The results of our field experiment show that wing pigmentation and the hue of the dorsal surface of the hindwings is correlated with the outcome of territorial contests. This is one of the first studies showing that structural coloration derived from multilayer interference may influence the outcome of intrasexual agonistic interactions. This indicates that multicomponent structural coloration in visually guided insects may be under selective forces of male-male competition for resources and females.

The putative chitin deacetylases Serpentine and Vermiform have non-redundant functions during Drosophila wing development

The chitin modifying deacetylases (CDA) CDA1 and CDA2 have been reported to play partially redundant roles during insect cuticle formation and molting and tracheal morphogenesis in various insect species. In order to distinguish possible functional differences between these two enzymes, we analyzed their function during wing development in the fruit fly Drosophila melanogaster. In tissue-specific RNA interference experiments, we demonstrate that DmCDA1 (Serpentine, Serp) and DmCDA2 (Vermiform, Verm) have distinct functions during Drosophila adult wing cuticle differentiation. Chitosan staining revealed that Serp is the major enzyme responsible for chitin deacetylation during wing cuticle formation, while Verm does not seem to be needed for this process. Indeed, it is questionable whether Verm is a chitin deacetylase at all. Atomic force microscopy suggested that Serp and Verm have distinct roles in establishing the shape of nanoscale bumps at the wing surface. Moreover, our data indicate that Verm but not Serp is required for the laminar arrangement of chitin. Both enzymes participate in the establishment of the cuticular inward barrier against penetration of xenobiotics. Taken together, correct differentiation of the wing cuticle involves both Serp and Verm in parallel in largely non-overlapping functions.

Differences in Nanostructure and Hydrophobicity of Cicada (Cryptotympana atrata) Forewing Surface with the Distribution of Precipitation

Although the cicada wing has a variety of functions and the nanostructure and surface properties of many species have been extensively investigated, there are no reports investigating diversity of nanostructures and wetting properties within a single species collected at locations with different rainfall conditions. In this study, the hydrophobicity and nanostructure dimensions of the forewing surface of Cryptotympana atrata were measured, based on specimens collected from 12 distributions with varying precipitation averages in China and Japan. The relationships among hydrophobicity, nanostructures, and precipitation were analyzed, and the adaption of hydrophobic nanostructures under different wet environments is discussed. The precipitation of locations in the years the samples of C. atrata were collected only has an effect on the diameter and spacing of wing surface nanostructure, and the multiple years of precipitation may have an influence on the basic diameter and spacing, as well as the height of protrusions. The rougher the wing surface, the stronger the hydrophobicity which was observed from samples taken where the rainfall conditions of the collection years are high. To our knowledge, this is one special example providing evidence of hydrophobic nanostructures found on a biological surface of a single species which shows adaption for specific wet environments.

Microstructures and grease layer of water strider and its influence on superhydrophobicity

In this paper, the microstructures of the water strider surface were observed by scanning electron microscopy. The surfaces, including wings, legs, back and abdomen, all exhibited various compound microstructures, which are the important factors influencing the superhydrophobicity with a contact angle (CA) of up to 153°. Furthermore, the grease of the water strider on different substrates was studied by self-assembly of the grease for 14 d. Images of the grease indicated that the morphology and spatial orientation of the grease depend on the substrate chosen. The grease decreased the substrate wettability by approximately 30° on highly ordered pyrolytic graphite and about 23° on the silicon substrates. Gas chromatography combined with mass spectrometry was also used to study the chemical composition of the grease layer of water strider surface. The grease layer is composed of a mixture of aliphatic compounds, which possess hydrophobicity based on the chemical structure. Grease protects the microstructures of water strider surfaces and thus results in higher CAs and hydrophobic properties. The microstructure and the grease of the water strider jointly render the hydrophobic properties of the water strider surface.

Nanofabrication of mechano-bactericidal surfaces

The search for alternatives to the standard methods of preventing bacterial adhesion and biofilm formation on biotic and abiotic surfaces alike has led to the use of biomimetics to reinvent through nanofabrication methods, surfaces, whereby the nanostructured topography is directly responsible for bacterial inactivation through physico-mechanical means. Plant leaves, insect wings, and animal skin have been used to inspire the fabrication of synthetic high-aspect-ratio nanopillared surfaces, which can resist bacterial colonisation. The adaptation of bacteria to survive in the presence of antibiotics and their ability to form biofilms on conventional antibacterial surfaces has led to an increase in persistent infections caused by resistant strains of bacteria. This presents a worldwide health epidemic that can only be mitigated through the search for a new generation of biomaterials.

Rapid fabrication of bio-inspired nanostructure with hydrophobicity and antireflectivity on polystyrene surface replicating from cicada wings

The fine nanostructure on the cicada wing of Cryptotympana atrata fabricius, which exhibits hydrophobicity and antireflectivity, is carefully examined. A promising strategy is proposed for facilely and successively replicating the natural functional nanostructure of the cicada wing onto polystyrene (PS) surfaces. First, a nickel replica with tapered nanopores is fabricated by combining electroless plating and subsequent electroplating with the natural cicada wing as an original template. Then, using microinjection compression molding, with the nickel replica as a template, the tapered nanopores are transcribed onto the PS surface, resulting in orderly and densely arranged nanopillars with a mean diameter of about 156 nm and a mean pitch of about 180 nm. The natural cicada wing and fabricated nickel replica are reusable. Interestingly, the PS replica surface exhibits a water contact angle of 143° ± 2° and a reflectance of about 4% in the wavelength range of 400-1000 nm. These results mean that the bionic PS replica not only inherits the nanostructure of the natural wing, but also its hydrophobic and antireflective properties. The mechanisms for the hydrophobic and antireflective properties are revealed via composite wetting interface and effective medium layer on the replica surface, respectively. The proposed fast and efficient replication strategy can be an excellent candidate for mimicking bio-inspired functional micro/nanostructures without complicated procedures and expensive materials.

Exploring the Role of Habitat on the Wettability of Cicada Wings

Evolutionary pressure has pushed many extant species to develop micro/nanostructures that can significantly affect wettability and enable functionalities such as droplet jumping, self-cleaning, antifogging, antimicrobial, and antireflectivity. In particular, significant effort is underway to understand the insect wing surface structure to establish rational design tools for the development of novel engineered materials. Most studies, however, have focused on superhydrophobic wings obtained from a single insect species, in particular, the Psaltoda claripennis cicada. Here, we investigate the relationship between the spatially dependent wing wettability, topology, and droplet jumping behavior of multiple cicada species and their habitat, lifecycle, and interspecies relatedness. We focus on cicada wings of four different species: Neotibicen pruinosus, N. tibicen, Megatibicen dorsatus, and Magicicada septendecim and take a comparative approach. Using spatially resolved microgoniometry, scanning electron microscopy, atomic force microscopy, and high speed optical microscopy, we show that within cicada species, the wettability of wings is spatially homogeneous across wing cells. All four species were shown to have truncated conical pillars with widely varying length scales ranging from 50 to 400 nm in height. Comparison of the wettability revealed three cicada species with wings that are superhydrophobic (>150°) with low contact angle hysteresis (<5°), resulting in stable droplet jumping behavior. The fourth, more distantly related species (Ma. septendecim) showed only moderate hydrophobic behavior, eliminating some of the beneficial surface functional aspects for this cicada. Correlation between cicada habitat and wing wettability yielded little connection as wetter, swampy environments do not necessarily equate to higher measured wing hydrophobicity. The results, however, do point to species relatedness and reproductive strategy as a closer proxy for predicting wettability and surface structure and resultant enhanced wing surface functionality. This work not only elucidates the differences between inter- and intraspecies cicada wing topology, wettability, and water shedding behavior but also enables the development of rational design tools for the manufacture of artificial surfaces for energy and water applications.

Insect Analogue to the Lotus Leaf: A Planthopper Wing Membrane Incorporating a Low-Adhesion, Nonwetting, Superhydrophobic, Bactericidal, and Biocompatible Surface

Nature has produced many intriguing and spectacular surfaces at the micro- and nanoscales. These small surface decorations act for a singular or, in most cases, a range of functions. The minute landscape found on the lotus leaf is one such example, displaying antiwetting behavior and low adhesion with foreign particulate matter. Indeed the lotus leaf has often been considered the "benchmark" for such properties. One could expect that there are animal counterparts of this self-drying and self-cleaning surface system. In this study, we show that the planthopper insect wing (Desudaba danae) exhibits a remarkable architectural similarity to the lotus leaf surface. Not only does the wing demonstrate a topographical likeness, but some surface properties are also expressed, such as nonwetting behavior and low adhering forces with contaminants. In addition, the insect-wing cuticle exhibits an antibacterial property in which Gram-negative bacteria (Porphyromonas gingivalis) are killed over many consecutive waves of attacks over 7 days. In contrast, eukaryote cell associations, upon contact with the insect membrane, lead to a formation of integrated cell sheets (e.g., among human stem cells (SHED-MSC) and human dermal fibroblasts (HDF)). The multifunctional features of the insect membrane provide a potential natural template for man-made applications in which specific control of liquid, solid, and biological contacts is desired and required. Moreover, the planthopper wing cuticle provides a "new" natural surface with which numerous interfacial properties can be explored for a range of comparative studies with both natural and man-made materials.

Wettability gradient on the elytra in the aquatic beetle Cybister chinensis and its role in angular position of the beetle at water-air interface

The surface of the elytra in some species of aquatic beetles displays relatively low contact angles (CAs), even showing hydrophilic properties. In this study, we report on an observation that both sexes of Cybister chinensis beetle fresh elytral surface do not exhibit uniform CA, but rather a wettability gradient along the longitudinal axis in posterior direction. The wettability is very different between females and males due to the presence (female) or absence (male) of channels on the elytral surface. When a small drop of water touches the elytra surface, it tends to slide towards the anterior having a lower CA on the elytra. This gradient presumably supports a breathing-associated behavior of beetles in which they cause the tip of their abdomen to protrude into the surface of the water in order to collect an air bubble for oxygen uptake and, when floating on the surface, to keep the body inclined at a small angle to the water's surface with their heads immersed.

STATEMENT OF SIGNIFICANCE

Hydrophobicity on surfaces is a fundamental property which has attracted great interest across all scientific disciplines, here we have demonstrated that the gradually changing chemistry of the elytral surface facilitates the tilted beetle posture on the water's surface. The mechanism of water interacting with the elytra demonstrated the most energetically favorable posture in the diving beetles. Surfaces with directional wetting properties that promote droplet drainage are of significant practical importance in many fields. The anisotropic topography and wetting properties of the elytra may inspire microfluidic devices for medical and robotic applications.

Diversity of Cuticular Micro- and Nanostructures on Insects: Properties, Functions, and Potential Applications

Insects exhibit a fascinating and diverse range of micro- and nanoarchitectures on their cuticle. Beyond the spectacular beauty of such minute structures lie surfaces evolutionarily modified to act as multifunctional interfaces that must contend with a hostile, challenging environment, driving adaption so that these can then become favorable. Numerous cuticular structures have been discovered this century; and of equal importance are the properties, functions, and potential applications that have been a key focus in many recent studies. The vast range of insect structuring, from the most simplistic topographies to the most elegant and geometrically complex forms, affords us with an exhaustive library of natural templates and free technologies to borrow, replicate, and employ for a range of applications. Of particular importance are structures that imbue cuticle with antiwetting properties, self-cleaning abilities, antireflection, enhanced color, adhesion, and antimicrobial and specific cell-attachment properties.

Confinement of water droplets on rectangular micro/nano-arrayed surfaces

Micro-patterned surfaces with alternate hydrophilic and hydrophobic rectangular areas effectively confine water droplets down to attolitre volumes. The contact angle, volume, and geometry of the confined droplets as a function of the geometry and physico-chemical properties of the confining surfaces have been determined by phenomenological simulations, validated by atomic force microscopy measurements. The combination between experiments and simulations can be used for the purposeful design of arrays with surface-addressable hydrophobicity employed in digital microfluidics and high-throughput screening nanoarrays.

Cicada Wing Surface Topography: An Investigation into the Bactericidal Properties of Nanostructural Features

Recently, the surface of the wings of the Psaltoda claripennis cicada species has been shown to possess bactericidal properties and it has been suggested that the nanostructure present on the wings was responsible for the bacterial death. We have studied the surface-based nanostructure and bactericidal activity of the wings of three different cicadas (Megapomponia intermedia, Ayuthia spectabile and Cryptotympana aguila) in order to correlate the relationship between the observed surface topographical features and their bactericidal properties. Atomic force microscopy and scanning electron microscopy performed in this study revealed that the tested wing species contained a highly uniform, nanopillar structure on the surface. The bactericidal properties of the cicada wings were investigated by assessing the viability of autofluorescent Pseudomonas fluorescens cells following static adhesion assays and targeted dead/live fluorescence staining through direct microscopic counting methods. These experiments revealed a 20-25% bacterial surface coverage on all tested wing species; however, significant bactericidal properties were observed in the M. intermedia and C. aguila species as revealed by the high dead:live cell ratio on their surfaces. The combined results suggest a strong correlation between the bactericidal properties of the wings and the scale of the nanotopography present on the different wing surfaces.

Contaminant adhesion (aerial/ground biofouling) on the skin of a gecko

In this study, we have investigated the micro- and nano-structuring and contaminant adhesional forces of the outer skin layer of the ground dwelling gecko--Lucasium steindachneri. The lizard's skin displayed a high density of hairs with lengths up to 4 μm which were spherically capped with a radius of curvature typically less than 30 nm. The adhesion of artificial hydrophilic (silica) and hydrophobic (C18) spherical particles and natural pollen grains were measured by atomic force microscopy and demonstrated extremely low values comparable to those recorded on superhydrophobic insects. The lizard scales which exhibited a three-tier hierarchical architecture demonstrated higher adhesion than the trough regions between scales. The two-tier roughness of the troughs comprising folding of the skin (wrinkling) limits the number of contacting hairs with particles of the dimensions used in our study. The gecko skin architecture on both the dorsal and trough regions demonstrates an optimized topography for minimizing solid-solid and solid-liquid particle contact area, as well as facilitating a variety of particulate removal mechanisms including water-assisted processes. These contrasting skin topographies may also be optimized for other functions such as increased structural integrity, levels of wear protection and flexibility of skin for movement and growth. While single hair adhesion is low, contributions of many thousands of individual hairs (especially on the abdominal scale surface and if deformation occurs) may potentially aid in providing additional adhesional capabilities (sticking ability) for some gecko species when interacting with environmental substrates such as rocks, foliage and even man-made structuring.

Removal mechanisms of dew via self-propulsion off the gecko skin

Condensation resulting in the formation of water films or droplets is an unavoidable process on the cuticle or skin of many organisms. This process generally occurs under humid conditions when the temperature drops below the dew point. In this study, we have investigated dew conditions on the skin of the gecko Lucasium steindachneri. When condensation occurs, we show that small dew drops, as opposed to a thin film, form on the lizard's scales. As the droplets grow in size and merge, they can undergo self-propulsion off the skin and in the process can be carried away a sufficient distance to freely engage with external forces. We show that factors such as gravity, wind and fog provide mechanisms to remove these small droplets off the gecko skin surface. The formation of small droplets and subsequent removal from the skin may aid in reducing microbial contact (e.g. bacteria, fungi) and limit conducive growth conditions under humid environments. As well as providing an inhospitable microclimate for microorganisms, the formation and removal of small droplets may also potentially aid in other areas such as reduction and cleaning of some surface contaminants consisting of single or multiple aggregates of particles.

Diverse set of Turing nanopatterns coat corneae across insect lineages

Nipple-like nanostructures covering the corneal surfaces of moths, butterflies, and Drosophila have been studied by electron and atomic force microscopy, and their antireflective properties have been described. In contrast, corneal nanostructures of the majority of other insect orders have either been unexamined or examined by methods that did not allow precise morphological characterization. Here we provide a comprehensive analysis of corneal surfaces in 23 insect orders, revealing a rich diversity of insect corneal nanocoatings. These nanocoatings are categorized into four major morphological patterns and various transitions between them, many, to our knowledge, never described before. Remarkably, this unexpectedly diverse range of the corneal nanostructures replicates the complete set of Turing patterns, thus likely being a result of processes similar to those modeled by Alan Turing in his famous reaction-diffusion system. These findings reveal a beautiful diversity of insect corneal nanostructures and shed light on their molecular origin and evolutionary diversification. They may also be the first-ever biological example of Turing nanopatterns.

A gecko skin micro/nano structure - A low adhesion, superhydrophobic, anti-wetting, self-cleaning, biocompatible, antibacterial surface

Geckos, and specifically their feet, have attracted significant attention in recent times with the focus centred around their remarkable adhesional properties. Little attention however has been dedicated to the other remaining regions of the lizard body. In this paper we present preliminary investigations into a number of notable interfacial properties of the gecko skin focusing on solid and aqueous interactions. We show that the skin of the box-patterned gecko (Lucasium sp.) consists of dome shaped scales arranged in a hexagonal patterning. The scales comprise of spinules (hairs), from several hundred nanometres to several microns in length, with a sub-micron spacing and a small radius of curvature typically from 10 to 20 nm. This micro and nano structure of the skin exhibited ultralow adhesion with contaminating particles. The topography also provides a superhydrophobic, anti-wetting barrier which can self clean by the action of low velocity rolling or impacting droplets of various size ranges from microns to several millimetres. Water droplets which are sufficiently small (10-100 μm) can easily access valleys between the scales for efficient self-cleaning and due to their dimensions can self-propel off the surface enhancing their mobility and cleaning effect. In addition, we demonstrate that the gecko skin has an antibacterial action where Gram-negative bacteria (Porphyromonas gingivalis) are killed when exposed to the surface however eukaryotic cell compatibility (with human stem cells) is demonstrated. The multifunctional features of the gecko skin provide a potential natural template for man-made applications where specific control of liquid, solid and biological contacts is required.

Origin of order in bionanostructures

Analysis of corneal nanocoatings across insect species provides clues to the origin of order in the bionanoworld.

Under- and over-water halves of Gyrinidae beetle eyes harbor different corneal nanocoatings providing adaptation to the water and air environments

Whirligig beetles (Gyrinidae) inhabit water surfaces and possess unique eyes which are split into the overwater and underwater parts. In this study we analyze the micro- and nanostructure of the split eyes of two Gyrinidae beetles genera, Gyrinus and Orectochilus. We find that corneae of the overwater ommatidia are covered with maze-like nanostructures, while the corneal surface of the underwater eyes is smooth. We further show that the overwater nanostructures possess no anti-wetting, but the anti-reflective properties with the spectral preference in the range of 450-600 nm. These findings illustrate the adaptation of the corneal nanocoating of the two halves of an insect's eye to two different environments. The novel natural anti-reflective nanocoating we describe may find future technological applications.

Microstructure and Wettability on the Elytral Surface of Aquatic Beetle

The microstructures on elytral surface of aquatic beetles belonging to Hydrophilidae and Dytiscidae were observed under an environment scanning microscope, and the wettabilities were determined with an optical contact angle meter. The results show the elytral surfaces are relatively smooth compared to the structures of other insects such as the butterfly wing scales or cicada wing protrusions. They exhibit a polygonal structuring with grooves and pores being the main constituent units. The contact angles (CAs) range from 47.1oto 82.1o. The advancing and receding angles were measured by injecting into and withdrawing a small amount of water on the most hydrophilic (with a contact angle of 47.1o) and hydrophobic (with a contact angle of 82.1o) elytral surfaces, which illustrates the vital role of three-phase contact line (TCL) in the wetting mechanism of aquatic beetle elytral surfaces.

Surface reconstruction by a "grafting through" approach: polyacrylamide grafted onto chitosan film

Grafted polymers and polymer brushes in particular have attracted significant attention in the last 2 decades as a way to alter and control interfacial properties. In the case of polymer brushes on solid substrates, a high grafting density of polymer chains results in stretching of the polymer coils normal to the substrate surface due to the effect of excluded volume. In this study, polyacrylamide is grafted to the surface of relatively soft thin films of chitosan. The "grafting through" approach is used by introducing double C═C bonds to amino groups of chitosan. The acquired kinetic data of grafting along with AFM and ellipsometry characterization suggest that the chitosan substrate undergoes surface reconstruction during the grafting of PAAm and simultaneously induces PAAm growth inside the soft substrate. As a result, much higher amounts of grafted polymer are achieved in comparison to traditional hard substrates like silicon or glass. Additionally, selective plasma etching of PAAm reveals filament-like structures oriented normal to the surface.

High-spatial-resolution mapping of superhydrophobic cicada wing surface chemistry using infrared microspectroscopy and infrared imaging at two synchrotron beamlines

The wings of some insects, such as cicadae, have been reported to possess a number of interesting and unusual qualities such as superhydrophobicity, anisotropic wetting and antibacterial properties. Here, the chemical composition of the wings of the Clanger cicada (Psaltoda claripennis) were characterized using infrared (IR) microspectroscopy. In addition, the data generated from two separate synchrotron IR facilities, the Australian Synchrotron Infrared Microspectroscopy beamline (AS-IRM) and the Synchrotron Radiation Center (SRC), University of Wisconsin-Madison, IRENI beamline, were analysed and compared. Characteristic peaks in the IR spectra of the wings were assigned primarily to aliphatic hydrocarbon and amide functionalities, which were considered to be an indication of the presence of waxy and proteinaceous components, respectively, in good agreement with the literature. Chemical distribution maps showed that, while the protein component was homogeneously distributed, a significant degree of heterogeneity was observed in the distribution of the waxy component, which may contribute to the self-cleaning and aerodynamic properties of the cicada wing. When comparing the data generated from the two beamlines, it was determined that the SRC IRENI beamline was capable of producing higher-spatial-resolution distribution images in a shorter time than was achievable at the AS-IRM beamline, but that spectral noise levels per pixel were considerably lower on the AS-IRM beamline, resulting in more favourable data where the detection of weak absorbances is required. The data generated by the two complementary synchrotron IR methods on the chemical composition of cicada wings will be immensely useful in understanding their unusual properties with a view to reproducing their characteristics in, for example, industry applications.

Self-cleaning of superhydrophobic surfaces by self-propelled jumping condensate

The self-cleaning function of superhydrophobic surfaces is conventionally attributed to the removal of contaminating particles by impacting or rolling water droplets, which implies the action of external forces such as gravity. Here, we demonstrate a unique self-cleaning mechanism whereby the contaminated superhydrophobic surface is exposed to condensing water vapor, and the contaminants are autonomously removed by the self-propelled jumping motion of the resulting liquid condensate, which partially covers or fully encloses the contaminating particles. The jumping motion off the superhydrophobic surface is powered by the surface energy released upon coalescence of the condensed water phase around the contaminants. The jumping-condensate mechanism is shown to spontaneously clean superhydrophobic cicada wings, where the contaminating particles cannot be removed by gravity, wing vibration, or wind flow. Our findings offer insights for the development of self-cleaning materials.

Spatial variations and temporal metastability of the self-cleaning and superhydrophobic properties of damselfly wings

Self-cleaning surfaces found in nature show great potential for application in many fields, ranging from industry to medicine. The ability for a surface to self-clean is intimately related to the wetting properties of the surface; for a surface to possess self-cleaning ability it must exhibit extremely high water contact angles and low water adhesion. While investigating the self-cleaning properties of damselfly wings, significant spatial variations in surface wettability were observed. Within an area of 100 μm × 100 μm of the wing surface the water contact angle was found to vary up to 17.8°, while remaining consistently superhydrophobic. The contributions of both surface chemistry and topography to the hydrophobicity of the wings were assessed in an effort to explain these variations. Synchrotron-sourced Fourier-transform infrared microspectroscopy revealed that some of the major components of the wing were aliphatic hydrocarbons and esters, which are attributable to epicuticular lipids. The wing topography, as determined by optical profilometry and atomic force microscopy (AFM), also showed only minor levels of heterogeneity arising from irregular ordering of surface nanostructures. The measured contact angle of a single droplet of water was also found to decrease over time as it evaporated, reaching a minimum of 107°. This is well below the threshold value for superhydrophobicity (i.e., 150°), demonstrating that when the surface is in contact with water for a prolonged period, the damselfly wings lose their superhydrophobicity and subsequently their ability to self-clean. This decrease in hydrophobicity over time can be attributed to the surface undergoing a transition from the Cassie-Baxter wettability state toward the Wenzel wettability state.

Compound microstructures and wax layer of beetle elytral surfaces and their influence on wetting properties

A beetles' first line of defense against environmental hazards is their mesothoracic elytra--rigid, protective forewings. In order to study the interaction of these wings with water, the surface microstructures of various beetles' elytra were observed by Environment Scanning Electron Microscopy (ESEM) and Atomic Force Microscopy (AFM). Chemistry components were ascertained using X-ray photoelectron spectroscopy (XPS). All the beetles of various habitats (including desert, plant, dung, land and water) exhibited compound microstructures on their elytra. The wetting properties of these elytra were identified using an optical contact angle meter. In general the native elytra exhibited hydrophilic or weak hydrophobic properties with contact angles (CAs) ranging from 47.5° to 109.1°. After treatment with chloroform, the CAs all increased on the rougher elytral surfaces. The presence of wax is not the only determinant of hydrophobic properties, but rather a combination with microscopic structures found on the surfaces. Irregularities and the presence or absence of tiny cracks, hairs (or setae), pores and protrusions are important factors which influence the wetting properties. Rougher elytral surfaces tended to present a stronger hydrophobicity. Effects on hydrophobicity, such as surface microstructures, chemistry, environment and aging (referring to the time after emergence), are also included and discussed. Our results also provide insights into the motion of water droplets when in contact with beetle elytra.

Natural bactericidal surfaces: mechanical rupture of Pseudomonas aeruginosa cells by cicada wings

Natural superhydrophobic surfaces are often thought to have antibiofouling potential due to their self-cleaning properties. However, when incubated on cicada wings, Pseudomonas aeruginosa cells are not repelled; instead they are penetrated by the nanopillar arrays present on the wing surface, resulting in bacterial cell death. Cicada wings are effective antibacterial, as opposed to antibiofouling, surfaces.

Influence of cuticle nanostructuring on the wetting behaviour/states on cicada wings

The nanoscale protrusions of different morphologies on wing surfaces of four cicada species were examined under an environmental scanning electron microscope (ESEM). The water contact angles (CAs) of the wing surfaces were measured along with droplet adhesion values using a high-sensitivity microelectromechanical balance system. The water CA and adhesive force measurements obtained were found to relate to the nanostructuring differences of the four species. The adhesive forces in combination with the Cassie-Baxter and Wenzel approximations were used to predict wetting states of the insect wing cuticles. The more disordered and inhomogeneous surface of the species Leptopsalta bifuscata demonstrated a Wenzel type wetting state or an intermediate state of spreading and imbibition with a CA of 81.3° and high adhesive force of 149.5 µN. Three other species (Cryptotympana atrata, Meimuna opalifer and Aola bindusara) exhibited nanostructuring of the form of conically shaped protrusions, which were spherically capped. These surfaces presented a range of high adhesional values; however, the CAs were highly hydrophobic (C. atrata and A. bindusara) and in some cases close to superhydrophobic (M. opalifer). The wetting states of A. bindusara, C. atrata and M. opalifer (based on adhesion and CAs) are most likely represented by the transitional region between the Cassie-Baxter and Wenzel approximations to varying degrees.

New biomimetic directions in regenerative ophthalmology

One of the most complete and permanent ways of treating many causes of visual impairment and blindness is to replace the entire affected tissue with pre-cultured ocular tissues supported and maintained on biomaterial frameworks. One direction towards enhancing ocular tissue regeneration on biomaterials, in the laboratory is by applying biomimicry. Specifically to engineer biomaterials with important functional elements of the native extracellular matrices, such as topography, that support and organise cells into coherent tissues. Further problems in regenerative ophthalmology can be potentially solved through application of biomimicry. They include, more efficient ways of moving and transplanting cultivated tissues into correct therapeutic locations inside the eye and scar-less, non-destructive healing of surgical incisions and wounds, to repair structural integrity of tissues at the ocular surface. Two examples are given to show this potential for redeveloping an ocular epithelium onto a nanostructured insect wing surface and producing an origami membrane modelled on deployable structures in nature. Efforts to harness natural innovation will eventually provide unique designs and structures that cannot for now be made synthetically, for regeneration of clinically acceptable ocular tissues.

Contrasting micro/nano architecture on termite wings: two divergent strategies for optimising success of colonisation flights

Many termite species typically fly during or shortly after rain periods. Local precipitation will ensure water will be present when establishing a new colony after the initial flight. Here we show how different species of termite utilise two distinct and contrasting strategies for optimising the success of the colonisation flight. Nasutitermes sp. and Microcerotermes sp. fly during rain periods and adopt hydrophobic structuring/‘technologies’ on their wings to contend with a moving canvas of droplets in daylight hours. Schedorhinotermes sp. fly after rain periods (typically at night) and thus do not come into contact with mobile droplets. These termites, in contrast, display hydrophilic structuring on their wings with a small scale roughness which is not dimensionally sufficient to introduce an increase in hydrophobicity. The lack of hydrophobicity allows the termite to be hydrophilicly captured at locations where water may be present in large quantities; sufficient for the initial colonization period. The high wettability of the termite cuticle (Schedorhinotermes sp.) indicates that the membrane has a high surface energy and thus will also have strong attractions with solid particles. To investigate this the termite wings were also interacted with both artificial and natural contaminants in the form of hydrophilic silicon beads of various sizes, 4 µm C₁₈ beads and three differently structured pollens. These were compared to the superhydrophobic surface of the planthopper (Desudaba psittacus) and a native Si wafer surface. The termite cuticle demonstrated higher adhesive interactions with all particles in comparison to those measured on the plant hopper.

Non-wetting wings and legs of the cranefly aided by fine structures of the cuticle

Non-wetting surfaces are imperative to the survival of terrestrial and semi-aquatic insects as they afford resistance to wetting by rain and other liquid surfaces that insects may encounter. Thus, there is an evolutionary pay-off for these insects to adopt hydrophobic technologies, especially on contacting surfaces such as legs and wings. The cranefly is a weak flier, with many species typically found in wet/moist environments where they lay eggs. Water droplets placed on this insect's wings will spontaneously roll off the surface. In addition, the insect can stand on water bodies without its legs penetrating the water surface. The legs and wings of this insect possess thousands of tiny hairs with intricate surface topographies comprising a series of ridges running longitudinally along the long axis of the hair fibre. Here we demonstrate that this fine hair structure enhances the ability of the hairs to resist penetration into water bodies.

A study of the anti-reflection efficiency of natural nano-arrays of varying sizes

The dependence of optical reflectivity and wettability on the surface topography of 32 species of cicada wing membranes has been investigated using UV-visible spectrophotometry, contact angle measurements and environmental scanning electron microscopy. The nanoscale hexagonally close packed protrusions have been shown to exhibit an anti-reflection and in some cases an anti-wetting function. The parameters of the structures were measured to be 77-148 nm in diameter, 44-117 nm in spacing and 159-481 nm in height. The transmittance spectrum and static contact angles were measured. At a wavelength range of 500-2500 nm, only minor differences in the anti-reflection performance were observed for each cicada species ascribed to the mechanism of impedance matching between cuticle and air. The transmittance properties of cicada wings were altered successfully through the scanning probe microscope-based manipulation by reducing the protrusion height via the contact mode. A near linear dependence was found between a decrease in protuberance height and a resulting increase in reflectance intensity. A diversity of wettability was observed with contact angles varying from 56.5° to 146.0°. Both effects of anti-reflection and wettability are dependent on the height of protrusions. The anti-reflection is insensitive when the wavelength is larger than the lateral feature size of the nanostructure. The stronger hydrophobic properties are generally associated with a larger diameter, closer spacing and greater height of protrusions when the wing membrane is intact.

A dual layer hair array of the brown lacewing: repelling water at different length scales

Additional weight due to contamination (water and/or contaminating particles) can potentially have a detrimental effect on the flight capabilities of large winged insects such as butterflies and dragonflies. Insects where the wing surface area-body mass ratio is very high will be even more susceptible to these effects. Water droplets tend to move spontaneously off the wing surface of these insects. In the case of the brown lacewing, the drops effectively encounter a dual bed of hair springs with a topographical structure which aids in the hairs resisting penetration into water bodies. In this article, we demonstrate experimentally how this protective defense system employed by the brown lacewing (Micromus tasmaniae) aids in resisting contamination from water and how the micro- and nanostructures found on these hairs are responsible for quickly shedding water from the wing which demonstrates an active liquid-repelling surface.

Bacterial retention on superhydrophobic titanium surfaces fabricated by femtosecond laser ablation

Two-tier micro- and nanoscale quasi-periodic self-organized structures, mimicking the surface of a lotus Nelumbo nucifera leaf, were fabricated on titanium surfaces using femtosecond laser ablation. The first tier consisted of large grainlike convex features between 10 and 20 μm in size. The second tier existed on the surface of these grains, where 200 nm (or less) wide irregular undulations were present. The introduction of the biomimetic surface patterns significantly transformed the surface wettabilty of the titanium surface. The original surface possessed a water contact angle of θ(W) 73 ± 3°, whereas the laser-treated titanium surface became superhydrophobic, with a water contact angle of θ(W) 166 ± 4°. Investigations of the interaction of S. aureus and P. aeruginosa with these superhydrophobic surfaces at the surface-liquid interface revealed a highly selective retention pattern for two pathogenic bacteria. While S. aureus cells were able to successfully colonize the superhydrophobic titanium surfaces, no P. aeruginosa cells were able to attach to the surface (i.e., any attached bacterial cells were below the estimated lower detection limit).

Crystalline wax coverage of the cuticle in easy bleeding sawfly larvae

The larvae of some sawfly species belonging to the family Tenthredinidae (Hymenoptera) are capable of 'easy bleeding', an anti-predator defence strategy based on a specialised cuticle that can readily break, which frees droplets of distasteful haemolymph. Using high-resolution cryo-scanning electron microscopy, we compared the cuticle surface between easy bleeder (Rhadinoceraea micans, Phymatocera aterrima, Aneugmenus padi) and non-easy bleeder (Strongylogaster multifasciata, Nematus pavidus, Arge pagana) sawfly species. We detected crystalline waxes only on the cuticle surface of easy bleeders. Wax crystals varied in shape and dimension depending on species. We assume the reduction of surface wettability by oozed haemolymph to be the primary function of the wax crystal coverage in the easy bleeding defence strategy.

Always on the bright side of life: anti-adhesive properties of insect ommatidia grating

The surface of some insect eyes consists of arrays of cuticular protuberances, which are 50-300 nm in diameter, and are termed corneal nipples or ommatidia gratings. They were widely reported to reduce the reflectance for normally incident light, contributing to camouflage by reducing glare to predators, while furthermore enhancing the intake of light, which is especially important for nocturnal insects. Our preliminary observations suggest a third function: in contrast to the rest of the body, ommatidia of various insects remain clean, even in a heavy contaminated environment. In order to prove such an anti-contamination hypothesis of these structures, we measured the adhesive properties of polymer moulds of insect ommatidia, and compared these data with control surfaces having the same curvature radii but lacking such a nanostructure. A scanning electron microscope (SEM) study and force measurements using an atomic force microscope (AFM) on the eye surfaces of three different insect species, dragonfly Aeshna mixta (Odonata), moth Laothoe populi (Lepidoptera) and fly Volucella pellucens (Diptera), were undertaken. We revealed that adhesion is greatly reduced by corneal grating in L. populi and V. pellucens when compared with their smooth controls. The smooth cornea of A. mixta showed no statistically significant difference to its control. We assume that this anti-adhesive phenomenon is due to a decrease in the real contact area between contaminating particles and the eye's surface. Such a combination of three functions in one nanostructure can be interesting for the development of industrial multifunctional surfaces capable of enhancing light harvesting while reducing light reflection and adhesion.

Fouling of nanostructured insect cuticle: adhesion of natural and artificial contaminants

The adhesional properties of contaminating particles of scales of various lengths were investigated for a wide range of micro- and nanostructured insect wing cuticles. The contaminating particles consisted of artificial hydrophilic (silica) and spherical hydrophobic (C(18)) particles, and natural pollen grains. Insect wing cuticle architectures with an open micro-/nanostructure framework demonstrated topographies for minimising solid-solid and solid-liquid contact areas. Such structuring of the wing membranes allows for a variety of removal mechanisms to contend with particle contact, such as wind and self-cleaning droplet interactions. Cuticles exhibiting high contact angles showed considerably lower particle adhesional forces than more hydrophilic insect surfaces. Values as low as 3 nN were recorded in air for silica of ~28 nm in diameter and <25 nN for silica particles 30 μm in diameter. A similar adhesional trend was also observed for contact with pollen particles.

Experimental determination of the efficiency of nanostructuring on non-wetting legs of the water strider

Water striders demonstrate an amazing talent which enables them to effectively "row" across water surfaces without immobilization. This ability has previously been ascribed to the wax-like chemistry of the small hairs (setae) found on the legs, and theoretically attributed to the nano/microscaled hierarchical architecture of individual seta using the Cassie-Baxter equations. Here we show experimentally the strength of the contribution of the seta surface architecture to superhydrophobicity by maintaining identical surface chemistry (thin and thick coating of the setae with polydimethylsiloxane). Atomic force microscopy-based force and adhesion measurements of single uncoated and coated seta interacting with water quantitatively demonstrate the efficiency of the topographical component of the setae for repelling water.

How micro/nanoarchitecture facilitates anti-wetting: an elegant hierarchical design on the termite wing

The termite is an insect which is a weak flier, has a large wing area in relation to its body mass, and many species typically fly during rain or storm periods. Water droplets placed on these insects' wings will spontaneously roll off the surface. Here we show how the intricate hierarchical array design of these insect wings achieves anti-wetting properties with water bodies of various sizes by reducing contact area and thus adhesion. To repel large droplets, the termite uses an array of hairs with a specially designed nanoarchitecture, which we demonstrate is critical for this function. By coating single hairs with a polymer of varying thicknesses (with a similar hydrophobicity to insect cuticle), we demonstrate that hairs of the same chemistry and with the complete nanoarchitecture show the greatest resistance to penetrating water bodies. The wings also consist of an underlying non-wetting membrane substructure comprising an array of star-shaped microstructures which minimize interaction with micro-sized droplets of water. The sophisticated micro/nanostructured hierarchy on the termite wing membrane not only results in non-wetting at different length scales but also demonstrates a design for weight and material minimization while achieving this state. Elucidating the function of such structures has implications for understanding insect biology and the evolution of wings.