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.