A systematic approach towards biomimicry of nanopatterned cicada wings on titanium using electron beam lithography.

The interaction of bacteria on nanopatterned surfaces has caught attention since the discovery of the bactericidal property of cicada wing surfaces. While many studies focused on the inspiration of such surfaces, nanolithography-based techniques are seldom used due to the difficulties in fabricating highly dense (number of pillars per unit area), geometrical nanostructured surfaces. Here we present a systematic modelling approach for optimising the electron beam lithography parameters in order to fabricate biomimicked nanopillars of varying patterned geometries. Monte Carlo simulation was applied to optimize the beam energy and pattern design prior to the experimental study. We optimized the processing parameters such as exposure factor, write field size, pitch, the different types and thicknesses of the PMMA resist used, and the shape of the feature (circle or a dot) for the fabrication of nanopillars to achieve the best lift-off with repeatable result. Our simulation and experimental results showed that a circle design with a voltage of 30 kV and 602 nm thickness of PMMA 495 A4 as base layers and 65 nm of PMMA 950 A2 as top layer achieves the best results. The antibacterial activity was also validated on the representative fabricated titanium nanopillar surface. The surface with a base diameter of 94.4 nm, spike diameter of 12.6 nm, height of 115.6 nm, density of 43/µm2, aspect ratio of 2.16 and centre to centre distance of 165.8 nm was the optimum surface for antibacterial activity. Such a systematic design approach for fabrication of insect wing-mimicked closely packed nanopillars have not been investigated before which provides an excellent platform for biomedical Ti implants.

Multi-biofunctional properties of three species of cicada wings and biomimetic fabrication of nanopatterned titanium pillars.

Recently, multi-biofunctional properties of cicada wings have drawn keen interest for biomedical device applications due to their superhydrophobic, self-cleaning and bactericidal effects. We present a systematic evaluation of bactericidal and cytocompatible properties of cicada wings. We also present biomimetic nanofabrication of a patterned array of titanium nanopillars using electron beam lithography. We have characterized the nanoscale architecture of the wings of three different Australian species of cicadas (Psaltoda claripennis, Aleeta curvicosta and Palapsalta eyrei) using helium ion microscopy (HIM), scanning electron microscopy, atomic force measurement (AFM) and transmission electron microscopy (TEM). The chemical nature of the nanopatterned substrates was investigated using Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Pseudomonas aeruginosa and Staphylococcus aureus cells were attached to determine the bactericidal activity of the insect wings. Human osteoblast cells were attached to examine the biocompatibility of the insect wings. It was found that all the three cicada species have unique surface topography on their wing membranes and veins. The height, spacing, diameter, density and aspect ratio of the three species varied between the species and between the membrane and the veins. The density and aspect ratio of the nanopillars on the membranes were significantly higher than on the veins. Bacterial attachment investigation confirmed that P. aeruginosa cells and S. aureus cells were damaged by the nanopatterned array of pillars. A significant reduction in colonies of P. aeruginosa cells was found on the wings of the three species compared to the control after 18 hours. A significant reduction of S. aureus cells on the wings was observed at 2 and 4 hours but not at 18 hours compared to the control. The cell morphology of the human osteoblast cells appeared intact after 24 hours of attachment, indicating the biocompatibility of the insect wings. As a proof of concept, patterned nanopillars of titanium have been fabricated using the electron beam lithography technique directly inspired by the cicada wing architecture. The titanium nanopillars were observed to damage the bacterial cells of P. aeruginosa in a manner similar to the cicada wing species and remain compatible to osteoblast cells. The outcomes of this research can help to engineer an optimum nano-patterned surface to enhance the bioactivity and bactericidal effect on biomedical devices.

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.