Cuticle-mimetic Layered Polymeric Materials
Livia Bast, University of Strathclyde/University of Fribourg
Prof. Nico Bruns, University of Strathclyde
Plant cuticles are multi-functional hybrid materials that consist of a hierarchical assembly of a variety of compounds, notably waxes, natural polymers, and polysaccharides. In some plants, like Selaginella sp., parts of the cuticle form lamellar structures that result in structural color. Waxes that permeate through the cuticle to the surface modulate the adhesion properties of plant surfaces.

The objectives of this project were to create functional hybrid materials that are inspired by the function and structure of cuticles. As most of the components of the cuticle are only available in small quantities, we chose to create hybrid materials based on proteins that are also available on an industrial scale. Silk fibroin from the silkworm Bombyx mori is such a structural protein. In order to create layered hybrid materials, a new photocrosslinker was developed for silk fibroin. One side of the crosslinker molecule forms an azo-bond to tyrosines in the protein, while the other side uses C,H-insertion chemistry to chemically crosslink the silk film and, simultaneously, covalently attach it to any other synthetic or natural polymer. This allowed to stack silk layers on synthetic polymers such as poly(dimethylsiloxane). Moreover, the silk crosslinker paves the way to a multitude of silk-based materials. The original plan was to mimic color-creating structures of cuticles by stacking layers of synthetic polymers and proteins into lamellar multilayer stacks. However, even with the crosslinker in hand this approach proofed to be very laborious. Instead, a more elegant way to produce structurally colored composite materials was developed, which consisted of infiltration of chiral nematic cellulose nanocrystal films with silk fibroin and other proteins. The infiltration method allowed to combine cellulose nanocrystals with strongly interacting proteins which previously lead to uncontrolled aggregation of the nanocrystals. Infiltration resulted in structurally colored hybrid materials. This approach enables the preparation of highly functional protein-cellulose hybrid materials that are based solely on naturally occurring macromolecules and, therefore, should be biodegradable.

Another research goal was to prepare bio-inspired lubrication surfaces. Plants, fungi and animals produce a variety of surface coatings that reduce or modify adhesion and lubricity. An example is a fungal protein called hydrophobin. Because it is industrially available, it was investigated for industrial tribology technology in collaboration with PlaMatSu’s partner Dr. Tillwich GmbH Werner Stehr. Hydrophobin coatings were prepared on Teflon and stainless steel surfaces by a simple dip-coating procedure. Measurements of friction behaviour were carried out. The protein was found to effectively inhibit the unwanted spreading of lubrication oils on the surfaces, thus paving the way to biodegradable and environmentally friendly tribology surface modifications.

Insect Repellent Wrinkly Colloids
Johannes Bergmann, University of Fribourg
Prof. Ullrich Steiner, University of Fribourg
This project focused on the preparation surfaces on which insects cannot walk. This project was inspired by the leaf surfaces of several plants, on which beetles cannot walk. The basic principle was the production of very small particles, the surfaces of which are wrinkled (similar to dried raisins, but much smaller). Spraying these particles onto substrates endows them with a wrinkly surface texture that is very similar to some of the plant leaves that beetles cannot walk on. When manufacturing the desired particles, it is important to control both the particle size and wrinkle pattern, and through optimisation, insect repellent surfaces were produced that surpass the biological original. The motivation of this project is to provide an alternative to toxic insecticides by developing a simple spray-on solution of these particles consisting of natural, bio-degradable components, such as cellulose. This project demonstrated this approach on a small scale, showing that spray-on wrinkled particles can be used to mimic biological defence mechanisms against pests. We expect that these results can be employed for applications in which inhibition of insect adhesion is important, e.g. in crop protection.

Hairy Surfaces – From Plants to Novel Technical Materials
Ha-Neul Chae, University of Freiburg
Prof. Jürgen Rühe, University of Freiburg
Hairy structures are one way that nature uses to generate superhydrophobic surfaces. Inspired by these natural examples we have developed a novel way to generate surfaces with very long hairs at high density. A writing techniques based on two photon lithography has been combined with novel photoreactive polymers to write such structures.

Industrially Producible Structurally Coloured materials
Benjamin Droguet, University of Cambridge
Prof. Silvia Vignolini, University of Cambridge
In this project, we developed a method to create large-scale structurally coloured cellulose nanocrystal films from renewable materials extracted from plants and commercially available material, using a large area manufacturing technique. Red, green and blue films were obtained by tuning either the properties of the cellulose nanocrystal suspension or the deposition and drying conditions of the suspension. The cellulose films were further processed into microparticles and we showed that these particles can retain the excellent optical properties of the films they are made from. Additional treatments allow to use these particles in various environment, from aqueous and solvent-based liquid medium to polymer formulation and in coatings.

Regulation of Cuticle Patterning
Jordan Ferria, University of Cambridge
Prof. Beverley Glover, University of Cambridge
The objectives of this research project are to explore genetic regulation of cuticular patterning, and to test the hypothesis that mechanically induced buckling generates cuticular patterning. The combined approaches adopted by Jordan will provide the first analysis of surface patterning in any organism. These analyses will feed into the work of our colleagues by providing plants with varied surface properties for optical analysis (Silvia Vignolini, Ullrich Steiner), analysis of insect interactions (Thomas Speck), and biomimetic inspiration for the materials groups. The insights into the regulation of cuticle patterning will inform design principles for all groups.

Leaf-cuticle Inspired Membranes
Aristotelis Kamtsikakis, University of Fribourg
Prof. Christoph Weder, University of Fribourg
Artificial polymer composite membranes using renewable nanomaterials based on cellulose have been developed, and their permeation properties were systematically investigated. The architecture of these artificial membranes were inspired by the structure of plant membranes (cuticles), which protect plants from dehydration. A comparison of the transport properties of the artificial and biological membranes isolated from olive and ivy leaves was conducted and we found that both plant membranes and plant inspired-membranes exhibit directional water transport properties induced by their compositionally graded architecture and humidity-dependent properties. The performance of the developed artificial membranes was assessed for the separation of challenging liquid-liquid mixtures such as ethanol and water. By modifying the membrane’s nanoparticle content and the surface chemistry of the nanoparticles, we also tailored the permeation properties of the membranes and their ethanol-recovery properties from water-ethanol mixtures.

Tuning Hierarchical Topographical and Chemical Surface Patterns
Konstantinos Roumpos, University of Freiburg
Prof. Günter Reiter, University of Freiburg
In this project, we replicated and mimicked topographical plant surface patterns by using thin polymer films. Moreover, we were able to directly measure the forces developed during this pattern forming process as well as to transform pattern shapes from circular to square by regulating the flow of polymer.

Design Spaces of Wrinkled Cuticle Surfaces
Venkata A. Surapaneni, University of Freiburg
Prof. Thomas Speck, University of Freiburg
The objectives of this research project were to study the robustness of surface-mechanical properties against changes in geometrical configurations of the microstructures like height, width and distance of cuticular folds. This not only allowed us to identify the point(s) in ontogeny at which plant surfaces gain (and maybe lose again) their full functionality but also to define the "design space" for the development of biomimetic structures. The experimental results, which describe the relationship between geometrical configuration and function of cuticular folds will serve as basis for the development of bioinspired technical surfaces with adaptive properties that are tuneable in their surface friction and other physical properties e.g. by selective swelling.

The experimental results of the growth-induced variation of leaf surface morphology and the corresponding insect attachment forces were accepted for publication in Royal Society Open Science. A theoretical model was also developed to explain the robustness of micro-scale morphology of cuticular folds on insect walking forces, and we aim to publish the results very soon. Collaborative work has also been carried out with PlaMatSu partners at the University of Fribourg (Prof. Steiner Group), and insect attachment tests were performed on artificial rough surfaces that mimic the natural leaf surfaces. These results were published in Advanced Materials Interfaces (Bergmann JB, Moatsou D, Surapaneni VA, Thielen M, Speck T, Wilts BD, Steiner U (2020): Polymerization-induced wrinkled surfaces with controlled topography as slippery surfaces for Colorado potato beetles. Advanced Materials Interfaces, 7: 2000129).

Wrinkled Cellulose Surfaces for Structural Colours
Gea van de Kerkhof, University of Cambridge
Prof. Silvia Vignolini, University of Cambridge
In this project, Gea developed a method to create non-pigmented colourful materials from a biodegradable material: cellulose. The colour on the resulting films comes from a diffraction grating that has been copied in the cellulose. Additionally, Gea studied the appearance of similar structures in nature, to expand on current knowledge of different mechanisms that can create colour. This will pave the way for future bio-inspired applications.