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

Immunomodulatory biomaterials for implant-associated infections: from conventional to advanced therapeutic strategies

Implant-associated infection (IAI) is increasingly emerging as a serious threat with the massive application of biomaterials. Bacteria attached to the surface of implants are often difficult to remove and exhibit high resistance to bactericides. In the quest for novel antimicrobial strategies, conventional antimicrobial materials often fail to exert their function because they tend to focus on direct bactericidal activity while neglecting the modulation of immune systems. The inflammatory response induced by host immune cells was thought to be a detrimental force impeding wound healing. However, the immune system has recently received increasing attention as a vital player in the host's defense against infection. Anti-infective strategies based on the modulation of host immune defenses are emerging as a field of interest. This review explains the importance of the immune system in combating infections and describes current advanced immune-enhanced anti-infection strategies. First, the characteristics of traditional/conventional implant biomaterials and the reasons for the difficulty of bacterial clearance in IAI were reviewed. Second, the importance of immune cells in the battle against bacteria is elucidated. Then, we discuss how to design biomaterials that activate the defense function of immune cells to enhance the antimicrobial potential. Based on the key premise of restoring proper host-protective immunity, varying advanced immune-enhanced antimicrobial strategies were discussed. Finally, current issues and perspectives in this field were offered. This review will provide scientific guidance to enhance the development of advanced anti-infective biomaterials.

Repeated evolution of underwater rebreathing in diving Anolis lizards

Air-based respiration limits the use of aquatic environments by ancestrally terrestrial animals. To overcome this challenge, diving arthropods have evolved to respire without resurfacing using air held between their cuticle and surrounding water.^1-4 Inspired by natural history observations in Haiti (unpublished data) and Costa Rica,^5,6 we conducted experiments documenting routine air-based underwater respiration in several distantly related semi-aquatic Anolis lizard species. Semi-aquatic anoles live along neotropical streams and frequently dive for refuge or food,^7-12 remaining underwater for up to 18 min. While submerged, these lizards iteratively expire and re-inspire narial air bubbles-underwater "rebreathing." Rebreathed air is used in respiration, as the partial pressure of oxygen in the bubbles decreases with experimental submersion time in living anoles, but not in mechanical controls. Non-aquatic anoles occasionally rebreathe when submerged but exhibit more rudimentary rebreathing behaviors. Anole rebreathing is facilitated by a thin air layer (i.e., a "plastron," sensu Brocher¹³) supported by the animal's rugose skin upon submergence. We suggest that hydrophobic skin, which we observed in all sampled anoles,^14,15 may have been exaptative, facilitating the repeated evolution of specialized rebreathing in species that regularly dive. Phylogenetic analyses strongly suggest that specialized rebreathing is adaptive for semi-aquatic habitat specialists. Air-based rebreathing may enhance dive performance by incorporating dead space air from the buccal cavity or plastron into the lungs, facilitating clearance of carbon dioxide, or allowing uptake of oxygen from surrounding water (i.e., a "physical gill" mechanism^4,16).

Skin hydrophobicity as an adaptation for self-cleaning in geckos

Hydrophobicity is common in plants and animals, typically caused by high relief microtexture functioning to keep the surface clean. Although the occurrence and physical causes of hydrophobicity are well understood, ecological factors promoting its evolution are unclear. Geckos have highly hydrophobic integuments. We predicted that, because the ground is dirty and filled with pathogens, high hydrophobicity should coevolve with terrestrial microhabitat use. Advancing contact-angle (ACA) measurements of water droplets were used to quantify hydrophobicity in 24 species of Australian gecko. We reconstructed the evolution of ACA values, in relation to microhabitat use of geckos. To determine the best set of structural characteristics associated with the evolution of hydrophobicity, we used linear models fitted using phylogenetic generalized least squares (PGLS), and then model averaging based on AICc values. All species were highly hydrophobic (ACA > 132.72°), but terrestrial species had significantly higher ACA values than arboreal ones. The evolution of longer spinules and smaller scales was correlated with high hydrophobicity. These results suggest that hydrophobicity has coevolved with terrestrial microhabitat use in Australian geckos via selection for long spinules and small scales, likely to keep their skin clean and prevent fouling and disease.

Ecological associations among epidermal microstructure and scale characteristics of Australian geckos (Squamata: Carphodactylidae and Diplodactylidae)

A first step in examining factors influencing trait evolution is demonstrating associations between traits and environmental factors. Scale microstructure is a well-studied feature of squamate reptiles (Squamata), including geckos, but few studies examine ecology the of microstructures, and those focus mainly on toe pads. In this study, the ecomorphology of cutaneous microstructures on the dorsum was described for eight Australian species of carphodactylid (Squamata: Carphodactylidae) and 19 diplodactylid (Squamata: Diplodactylidae) geckos. We examined scale dimensions, spinule and cutaneous sensilla (CS) morphology, using scanning electron microscopy, and described associations of these traits with microhabitat selection (arboreal, saxicoline or terrestrial) and relative humidity of each species' habitat (xeric, mesic or humid). We used a phylogenetic flexible discriminant analysis (pFDA) to describe relationships among all traits and then a modeling approach to examine each trait individually. Our analysis showed that terrestrial species tended to have long spinules and CS with more bristles, saxicoline species larger diameter CS and arboreal species tended to have large granule scales and small intergranule scales. There was high overlap in cutaneous microstructural morphology among species from xeric and mesic environments, whereas species from humid environments had large diameter CS and few bristles. Significant associations between epidermal morphology and environmental humidity and habitat suggest that epidermal microstructures have evolved in response to environmental variables. In summary, long spinules, which aid self-cleaning in terrestrial geckos, are consistent with greater exposure to dirt and debris in this habitat. Long spinules were not clearly correlated to environmental humidity. Finally, more complex CS (larger diameter with more bristles) may facilitate better perception of environmental variation in geckos living in drier habitats.

The atypically high modulus of pollen exine

Sporopollenin, the polymer comprising the exine (outer solid shell) of pollen, is recognized as one of the most chemically and mechanically stable naturally occurring organic substances. The elastic modulus of sporopollenin is of great importance to understanding the adhesion, transport and protective functions of pollen grains. In addition, this fundamental mechanical property is of significant interest in using pollen exine as a material for drug delivery, reinforcing fillers, sensors and adhesives. Yet, the literature reports of the elastic modulus of sporopollenin are very limited. We provide the first report of the elastic modulus of sporopollenin from direct indentation of pollen particles of three plant species: ragweed (Ambrosia artemisiifolia), pecan (Carya illinoinensis) and Kentucky bluegrass (Poa pratensis). The modulus was determined with atomic force microscopy by using direct nanomechanical mapping of the pollen shell surface. The moduli were atypically high for non-crystalline organic biomaterials, with average values of 16 ± 2.5 GPa (ragweed), 9.5 ± 2.3 GPa (pecan) and 16 ± 4.0 GPa (Kentucky bluegrass). The amorphous pollen exine has a modulus exceeding known non-crystalline biomaterials, such as lignin (6.7 GPa) and actin (1.8 GPa). In addition to native pollen, we have investigated the effects of exposure to a common preparative base-acid chemical treatment and elevated humidity on the modulus. Base-acid treatment reduced the ragweed modulus by up to 58% and water vapour exposure at 90% relative humidity reduced the modulus by 54% (pecan) and 72% (Kentucky bluegrass). These results are in agreement with recently published estimates of the modulus of base-acid-treated ragweed pollen of 8 GPa from fitting to mechanical properties of ragweed pollen-epoxy composites.

Bio-mimicking nano and micro-structured surface fabrication for antibacterial properties in medical implants

Orthopaedic and dental implants have become a staple of the medical industry and with an ageing population and growing culture for active lifestyles, this trend is forecast to continue. In accordance with the increased demand for implants, failure rates, particularly those caused by bacterial infection, need to be reduced. The past two decades have led to developments in antibiotics and antibacterial coatings to reduce revision surgery and death rates caused by infection. The limited effectiveness of these approaches has spurred research into nano-textured surfaces, designed to mimic the bactericidal properties of some animal, plant and insect species, and their topographical features. This review discusses the surface structures of cicada, dragonfly and butterfly wings, shark skin, gecko feet, taro and lotus leaves, emphasising the relationship between nano-structures and high surface contact angles on self-cleaning and bactericidal properties. Comparison of these surfaces shows large variations in structure dimension and configuration, indicating that there is no one particular surface structure that exhibits bactericidal behaviour against all types of microorganisms. Recent bio-mimicking fabrication methods are explored, finding hydrothermal synthesis to be the most commonly used technique, due to its environmentally friendly nature and relative simplicity compared to other methods. In addition, current proposed bactericidal mechanisms between bacteria cells and nano-textured surfaces are presented and discussed. These models could be improved by including additional parameters such as biological cell membrane properties, adhesion forces, bacteria dynamics and nano-structure mechanical properties. This paper lastly reviews the mechanical stability and cytotoxicity of micro and nano-structures and materials. While the future of nano-biomaterials is promising, long-term effects of micro and nano-structures in the body must be established before nano-textures can be used on orthopaedic implant surfaces as way of inhibiting bacterial adhesion.

High Quality Bioreplication of Intricate Nanostructures from a Fragile Gecko Skin Surface with Bactericidal Properties

The external epithelial surfaces of plants and animals are frequently carpeted with small micro- and nanostructures, which broadens their adaptive capabilities in challenging physical habitats. Hairs and other shaped protuberances manage with excessive water, light contaminants, predators or parasites in innovative ways. We are interested in transferring these intricate architectures onto biomedical devices and daily-life surfaces. Such a project requires a very rapid and accurate small-scale fabrication process not involving lithography. In this study, we describe a simple benchtop biotemplating method using shed gecko lizard skin that generates duplicates that closely replicate the small nanotipped hairs (spinules) that cover the original skin. Synthetic replication of the spinule arrays in popular biomaterials closely matched the natural spinules in length. More significantly, the shape, curvature and nanotips of the synthetic arrays are virtually identical to the natural ones. Despite some small differences, the synthetic gecko skin surface resisted wetting and bacterial contamination at the same level as natural shed skin templates. Such synthetic gecko skin surfaces are excellent platforms to test for bacterial control in clinical settings. We envision testing the biocidal properties of the well-matched templates for fungal spores and viral resistance in biomedicine as well as co/multi-cultures.

The nanotipped hairs of gecko skin and biotemplated replicas impair and/or kill pathogenic bacteria with high efficiency

We show that gecko microspinules (hairs) and their equivalent replicas, bearing nanoscale tips, can kill or impair surface associating oral pathogenic bacteria with high efficiency even after 7 days of repeated attacks. Scanning Electron Microscopy suggests that there is more than one mechanism contributing to cell death which appears to be related to the scaling of the bacteria type with the hair arrays and accessibility to the underlying nano-topography of the hierarchical surfaces.