Concepts found in nature have great potential for novel or improved products and technologies for humans in the daily life. The strength, toughness and biocompatibility of spider silk for protective clothing or biomedical applications, the aerodynamic body shape of the box fish for a fuel-efficient concept car or the fog-catching surface on the Namibian beetle for effective water harvesting are only a few amazing examples of such biologically inspired approaches.
In this blog, we want to introduce you to an actually new example of fascinating natural concepts: the skin patterns of springtails. Springtails, also known as Collembola, are wingless soil-arthropods that form a sister group of insects in the hexapod clade. Differing from insects, which breathe using tubes called trachea, the most springtail species respire directly through their skin. To prevent suffocating in wet conditions, the springtail skin has to possess a barrier against complete wetting, which would inevitably block the gas exchange. Concerning this matter, springtails have evolved an extraordinary skin surface that repels a variety of liquids with remarkable efficiency.
In our research group, we focus on characterizing the springtail skin and work on mimicking their robust and effectively repellent surface characteristics that may offer exciting opportunities for emerging applications. Microscopic studies revealed a hierarchically arranged and highly textured skin of springtails. As an example, Tetrodontophora bielanensis and its characteristic skin morphology is shown in the figure below. The entire body is de facto covered with nanoscopic primary granules and interconnecting ridges; together forming gas permeable nanocavities that are arranged in a comb-like pattern. Transmission electron micrographs revealed that the primary granules protrude above the ridges, thus, the nanocavities exhibit overhangs. At the microscopic scale, T. bielanensis possess papillose secondary granules that are completely decorated by primary granules excluding the smooth domes at the top of these papillae. At least, the springtail skin possesses a limited number of thin bristles as a tertiary structure.
To get a general view of the hierarchical skin morphology, we examined 35 species from 4 orders in cooperation with Senckenberg Museum of Natural History Görlitz (Germany) and classified the observed patterns concerning their preferred habitats. Interestingly, the size, shape and spacing of the nanostructure were rather similar and could be concluded as a unique surface feature for all analyzed species. In turn, the occurrence of the papillae and bristles showed a high diversity and clear ecological and taxonomic dependency.[1]
As already mentioned, the survival of springtails is dependent on environmental humidity and is particularly critical for living in temporarily rain-flooded habitats. The consequent repellence of aqueous media evolved by the springtail skin has been known for more than a half century. However, we recently found the remarkable wetting resistance of the skin surfaces even against wetting by low-surface-tension liquids such as alkanes or ethyl alcohol using simple immersion tests. In our opinion, the obtained findings may reflect the adaption to habitats where aqueous media is massively loaded with surface-active substances originating from decaying organic matter and from microorganisms, which dramatically reduce the surface tension.[2]
In order to answer the question how springtails protect themselves against wetting; we made accurate polymer-based replicas of the skin surface and compared our results with numerical simulations. We could show that the tiny overhangs of the primary granules help to trap air against the surface in the wet, providing an effective barrier against wetting.[3] We further developed model profiles with well-defined geometries to set the obtained results into context and to develop a general design principle for the most robust surface structures. Taking this approach we could show that even intrinsically wettable materials will effectively repel liquids when structured in this way.[4]
However in practice, one of the common limitations for liquid-repellent surface is often the mechanical stability of the involved nano and microstructures. In particular, its lack of resistance against shear loads by scratching can dramatically reduce the long-term durability. Being soil-dwelling arthropods, springtails are permanently exposed to granular matter that requires an adequate level of protection against abrasion damage. We tested this in our lab using sand blast experiments and found that the pronounced comb-like formation of primary granules and ridges acts as a mechanically self-supporting network. In particular, the interconnecting ridges inhibits bending of the primary granules and allows for high shear load dissipation as a prerequisite for a sufficient mechanical stability.[2]
Taken together, springtails effectively repel liquids and protect themselves against suffocation in wet conditions by a solely structural control over wetting. Our research turns out that the hierarchically arranged skin surface, i.e. covering the entire body of springtails, is the key to the success. These newly garnered understandings provide valuable insights that may help to design engineered materials with improved anti-wetting properties. In particular, overhanging cross-sections and the arrangement of the surface features into a self-supporting comb-like pattern has potential to serve a novel strategy for overcoming common limitations due to: (i) higher mechanical stability by interconnecting ridges supporting the primary granules; (ii) surface structures featuring overhangs that provide repellence even against low-surface-tension liquids such as oils or contaminated aqueous media. Hence, a feasible strategy for mimicking the effective liquid-repellent and mechanically stable springtail skin morphology into synthetic materials or coatings may offer exciting opportunities and a broad range of applications such as self-cleaning, easy-to-clean, non-fouling, anti-icing or drag-reducing purposes.
References
[1] J. Nickerl, R. Helbig, H.-J. Schulz, C. Werner and C. Neinhuis, Diversity and potential correlations to the function of Collembola cuticle structures, Zoomorphology, 2013, 132, 183-195
[2] R. Helbig, J. Nickerl, C. Neinhuis and C. Werner, Smart skin patterns protect springtails, PLoS ONE, 2011, 6, e25105
[3] R. Hensel, R. Helbig, S. Aland, A. Voigt, C. Neinhuis and C. Werner, Tunable nano-replication to explore the omniphobic characteristics of springtail skin, NPG Asia Materials, 2013, 5, e37
[4] R. Hensel*, R. Helbig*, S. Aland, H.-G. Braun, A. Voigt, C. Neinhuis and C. Werner, Wetting resistance at its topographical limit – The benefit of mushroom and serif T structures, Langmuir, 2013, 29, 1100 – 1112 (*Authors contributed equally)
Authors
René Hensel is a PhD candidate at the Institute of Biofunctional Polymer Materials within Max Bergmann Center of Biomaterials Dresden. He is a fellow of the DFG Research Training Group “Nano- and Biotechniques for Electronic Device Packaging” at Technische Universität Dresden.
Carsten Werner is head of the Institute Biofunctional Polymer Materials within the Max Bergmann Center of Biomaterials and Professor for Biofunctional Polymer Materials at the Technische Universität Dresden (Center for Regenerative Therapies Dresden and Department of Chemistry and Food Chemistry).
Contact
hensel@ipfdd.de