Bioinspired surface topographies through sequential wrinkling pattern superposition

Abstract

Naturally occurring patterns on surfaces have evolved to form topological micro- and nanostructures with a variety of functionalities, such as drag reduction, tunable wetting, anti-microbial resistance, and optical effects, often found in insect wings and plant leaves. Surface topography has been shown to influence cell spreading, orientation and signaling. Further, topography can influence bacterial adhesion and subsequent biofi lm formation. This work investigates the controlled wrinkling of bi-layered materials as powerful patterning methodology mechanisms of formation and fabrication of bioinspired topographies which can nd use in a variety of applications such as controlled wetting, adhesion and photonics. This research project, is part of a ITN consortium, BIOCLEAN, funded by the European Union's Horizon 2020 research and innovation programme, where BioClean stands for BIOf ilm management and CLEANing by leveraging fundamental understanding of biological, chemical and physical combined approaches. Speci cally, in the context of the BioClean framework, this work focuses on the possible antifouling or antimicrobial function induced by the interaction of bacteria with patterned surfaces, essential in cleaning applications. Overall, taking advantage of simple mechanical buckling instability phenomena, common in natural and synthetic soft matter, pattern formation on elastometic substrates can be induced through plasma oxidation of PDMS and the application of mechanical strain on the resulting bilayer structure. The judicious choice and tunability of the experimental parameters provides a versatile, cost effective and large scale patterning platform, compared to more expensive lithographic techniques. In particular, inspired by naturally occurring surface topographies, such as the epicuticular structures found on cicada wings, a library of more complex 2D wrinkled topographies is achieved thorough a sequential 2D wrinkling methodology, built on the assumption of superposition of 1D waves, by replicating the oxidised pattern surface into fresh PDMS, to obtain a virgin, stress free surface able to undergo a secondary strain application and plasma oxidation. The orthogonal superposition of two 1D wrinkling waves produces therefore a checkerboard structure, characterised by a square array of hills and valleys in a process conceptually reminiscent of a linear superposition of orthogonal sinusoidal waves. By varying the process parameters, viz plasma exposure time and applied strain, conditions are established for the formation of symmetric, square, checkerboard or egg-box structures reminiscent of natural occurring surfaces. Signifi cantly, the sequential 2D wrinkling can achieve pattern homogeneity over areal coverage much greater than those typically achieved by a simultaneous 2D wrinkling step. The potential of the sequential 2D wrinkling method is further implemented exploring a non orthogonal superposition, applying therefore a variable compression angle between the two generations and opening up to the controlled fabrication of new 2D morphologies, characterised by additional undulations in the transverse plane only dependent on the imposed compression angle, which mechanistic understanding is achieved by modelling these 2D patterns as topological interference phenomena, inspired by unrelated developments in geology and shell mechanics, which can seemingly describe in biological or inanimate matter from nanoscale insect wings to layered geological formations or sand dunes patterns. As final aim of this research work, the effect of the different wrinkled morphologies on the initial attachment and orientation of a series of model bacteria and yeasts in static conditions is described, focusing on how pattern directionality an geometry exploits a displacing and delaying effect in the microorganisms motility during the initial attachment stage, although not displaying an intrinsic antimicrobial or biocidal activity. The results of this work set promising research directions for 2D wrinkled patterns in antimicrobial applications, specifically projected towards the development of higher aspect ratio patterns, to trigger the antimicrobial behaviour, exploring different mechanical instabilities outside wrinkling and different patterning routes, applying the sequential 2D wrinkling to the nanoscale or by the combination of wrinkling and imprinting techniques.