The cuticle of plant leaves and petals is a multifunctional, multicomponent and hierarchically structured material with a lot of potential to act as concept generator for novel bio-inspired synthetic materials. PlaMatSu’s research achieved a fundamental understanding of the genetic and developmental factors that govern structure formation in plant cuticles. These structures lead, e.g., to structural color on petals which is an important feature of flowers like Hibiscus to attract pollinators by vivid colors. Stuctural color can also be caused by epicuticular wax platelets on plant leaves, and the optical mechanism that gives rise to the golden shine on Tradescantia leaves was revealed. PlaMatSu researches also deciphered how microscopic surface roughness develops during the ontogenetic growth of leaves of, e.g., the rubber tree Hevea brasiliensis, and how these surface structures govern the adhesion of insects on plant leaves. The cuticle of leaves furthermore acts as a barrier that tunes the evaporation of water from the leaves. PlaMatSu researchers demonstrated that olive and ivy leaf cuticles exhibit asymmetric water transport properties that are regulated by the hydration status of the outer layer of the cuticles which is rich in the natural polymer cutin.
Another fundamental process that plays a role in structure formation of natural and synthetic polymer films is dewetting, i.e. the formation of holes of defined size and shape in a polymer film when the film is in contact with a surface that it does not like to wet. PlaMatSu investigated these dewetting phenomena using synthetic polymers as model compounds on micro-fabricated pillar structures that allow to measure the involved forces by the deflection of the pillars.
The properties of plant cuticles studied by the biologists, chemists and physicists of PlaMatSu include structural color, tunability of insect adhesion, and water transport across the cuticle. These remarkable properties and the underlying structure-property relationships inspired the network to develop novel synthetic materials with similar properties. Structurally colored materials were achieved with cellulose nanocrystals that form liquid crystalline (more precisely chiral nematic) stacks. Thanks to the research in PlaMatSu, these rainbow-colored cellulose materials can now be produced as flexible films in a roll-to-roll process, i.e. on a large scale, and could find application as edible, biodegradable and environmentally friendly glitter in cosmetics, the food industry and in coatings. Other structurally colored materials developed by PlaMatSu are hybrid materials that combine silk proteins and cellulose nanocrystals, as well as cellulose films that feature disordered diffraction grating structures with a resolution of 200 nm.
PlaMatSu developed several routes to plant-inspired surfaces, including high precision replication of micro-structured plant leaf surfaces in polydimethylsiloxane (PDMS). The resulting replicas allow biomechanical experiments such as traction forces measurements of beetles on these surfaces, functional, ecological and evolutionary studies that allow to follow the formation of surface structures during ontogenetic leaf growth, and industrial applications like surfaces with improved haptic properties.
Polymerization-induced wrinkling and electrospraying of wrinkled colloids were two other methods to create biomimetic low friction surfaces that are slippery for insects. The wrinkled polymer films resemble the topography of the adaxial side of leaves of the rubber tree (Hevea brasiliensis), while the wrinkled colloids mimic the cuticle structure on the abaxial side of the leaves of the lychee tree (Litchi chinensis). These surfaces hold significant potential for applications including insect-repelling surfaces for crop protection.
Lubrication and low friction surfaces are also of importance in fine mechanics. PlaMatSu developed a novel surface modification for Teflon and stainless steel surfaces based on the amphiphilic protein hydrophobin. The protein readily adsorbs on these surfaces and hinders lubricants from spreading in an uncontrolled way, thus paving the way to biodegradable and environmentally friendly tribology surface modifications.
Many plant leaves are covered by thin hairs that play a role in moisture management and provide shadow and protection from sun light. Inspired by these biological role models, a process was developed to 3D-print artificial hairy surfaces with a two-photon lithography system and a specially developed ink that consists of photoreactive prepolymers. The diameter and length of the hairs could be precisely controlled allowing access to polymeric hairs with very high aspect ratios.
Not only the surface of plant cuticles, but also their internal structure plays a large role for their properties. An example is the abovementioned control of water permeability through the cuticle of olive and ivy leaves. Leaf-cuticle inspired membranes were prepared that feature a concentration gradient of cellulose nanocrystals across the thickness of a polymer membrane. This allowed for asymmetric permeability of water and ethanol through the material, thereby opening pathways to pervaporation membranes, i.e. membranes that allow to remove and recover ethanol from water-ethanol mixtures.
Another example for properties that arise from the internal structure of plant cuticles is the blue structural color created by lamellar structures in the cuticle of leaves of the tropical understory plant Selaginella sp.. Silk proteins are ideally suited as biologically derived building blocks for bio-inspired materials, as the protein is abundantly available and displays excellent mechanical and optical properties. Unfortunately, it cannot be readily stacked into lamellar cuticle-inspired structures because silk fibroin structures tend to detach from synthetic polymers during the stacking process. To address this issue, PlaMatSu developed a novel photo-crosslinker for silk-based materials. The crosslinker stabilizes silk protein films and simultaneously bonds them covalently to polymer surfaces, thus allowing to prepare bio-inspired protein films and lamellar protein-polymer hybrid structures.
Apart from ground breaking scientific results that advanced the knowledge on plant cuticles and their intriguing properties, and that lead to a multitude of novel bio-inspired materials and surfaces, PlaMatSu also yielded methods, materials and technologies that have the potential for real-life applications and commercialization. Some of these results were developed by PlaMatSu researchers in close collaboration with the company partners of PlaMatSu during industrial secondments of the ESRs. Examples include work on surface lubrication for fine mechanics in collaboration with the SME Dr. Tillwich GmbH Werner Stehr, or the application of natural pigments for cosmetics in collaboration with L’Oréal. The most advanced technology for exploitation are the structurally coloured cellulose films, pigments and flakes described above, which are of high appeal for cosmetics and other applications. The team around Prof. Silvia Vignolini at the University of Cambridge is pursuing this technology for commercialization.
Bio-inspired research and biomimetic technologies are ideal fields to draw the attention of the general public to the importance of STEM subjects and research on advanced materials. With the help of the Cambridge University Botanical Garden, PlaMatSu organized an exhibition in the garden that explained the fantastic properties of plant cuticles and their potential to inspire the next generation of novel materials (see Figure 3). The exhibition “Our Future is Nature Inspired”, as well as a bio-inspired themed walking trail through the garden, attracted many visitors of all ages who engaged intensively with the exhibits and the plants on display. Exhibits demonstrated, e.g., the principle of structural colors in plants and in artificial materials, and the effect of microscopic surface structures on the wettability, self-cleaning and insect adhesion properties of plants and artificial surfaces. In a computer-based gene-editing game, the public learned basic principles of modern plant science. Moreover, an animated video on the superpower of plants was shown. It is also available as a legacy of the project online. The exhibition was an ideal outreach activity to engage with a broad public on site, but also through the social media channels of PlaMatSu and through conventional media. To quote BBC’s Radio Cambridgeshire´s moderator Louise Hulland live on air while reporting about the PlaMatSu exhibition:
“Plants with superpowers: Move over Spider-Man, move over Wonder Women. Captain Pampas Grass is here!”