Stimuli driven actuators are important in technological applications and in nature. They are used to open seed capsules and are implemented in technology to serve as gauges. Particularly, electricity-actuated piezo crystals are useful in sensing and high precision positioning. In nature, actuators are found in pine cones.
However, no ceramic moisture-driven bilayer actuators exist. Such devices would be helpful in controlling chemically reactive environments. Generally, bilayer actuators are prone to chemical attack, radiation and oxidation. This is stemming from their high metallic or polymeric loading. Therefore, ceramic actuators would be resistant to all these conditions.
Motivated by the need to fabricate ceramic bilayer actuators, researchers led by Professor Cordt Zollfrank from the Chair of Biogenic Polymers of the Technical University of Munich, managed to synthesize ceramic moisture-driven actuators through bio-templating pine cones. The authors applied their findings to existing models and the outcomes corresponded to the concept of silica based actuators, where water absorption is the predominant driving force. Their work is published in a peer-reviewed journal, Advanced Materials.
The actuation process witnessed in the pine cones was due to the swelling of the sclereid tissues. The authors found that, although the observed movements in ceramic replica and native cones were different in magnitude, the basic moisture-driven mechanism and its actuation direction was maintained. The authors found that this was the first example of angular actuation observed in porous ceramic bilayer actuators triggered by the uptake of water.
The authors observed a bilayer structuring in both the ceramic actuators and their biological pine cone templates. In the ceramic bilayers, the structuring was not as uniform as in the scales of the pine cones. The studied preparation method could be applied to other natural, or generally to polymeric, bilayer actuators, where one can choose the actuation mechanism as well as geometry of a selected product.
There has been several further exciting research recently in the field of moisture-induced actuation. For example, a published article by Boudot et al used artificially structured silica films on a polymer substrate to attain humidity-derived actuation. Due to the flexible nature of their substrate, they achieved curvatures up to 0.6 cm-1, while using an inorganic -silica- active layer. This translates to angular changes in an exemplary 3 cm long strip of approximately 100°. In contrast, Van Opdenbosch et al obtained smaller angular actuation due to the entire inorganic architecture of their replicated conifer cone materials, which, on the other hand, are resistant to temperatures of at least 500 °C, and also to corrosive environments.
Another work by Ganser et al reported the bending behavior of purely inorganic, artificially-structured silica films on silicon substrates, which led to moisture-induced bending. The 130 μm long structures attained tip deflections of up to 160 nm, which translates to a maximum bending angle of 0.07°. Considering the inorganic nature, the size of the actuator and possible applications as micro- switches or sensors, even these numerically small deflections and bending angles provide a useful technological tool.
Poppinga et al presented in January 2017 their work in Scientific Reports on the actuation of naturally fossilized conifer cones. This provided a highly interesting comparison. Notably, the naturally fossilized cones contain large portions of the original biomass, conserved by the mineralization processes. Therefore, these naturally fossilized cones show maximum actuation angles of 20°, which is more than half of those of the corresponding recent native conifer cones.
Daniel Van Opdenbosch1, Gerhard Fritz-Popovski2, Wolfgang Wagermaier3, Oskar Paris2, and Cordt Zollfrank1. Moisture-driven ceramic bilayer actuators from a bio-templating approach. Advanced materials, volume 28 (2016), pages 5235-5240.Go To Advanced Materials
- Professur für Biogene Polymere, Technische Universität München, Straubing Center of Science for Renewable Resources, Schulgasse 16, D-94315, Straubing, Germany.
- Institut für Physik, Montanuniversität Leoben, Franz-Josef-Straße 18, A-8700, Leoben, Austria.
- Max-Planck-Institut für Kolloid- und Grenzflächenforschung, Abteilung Biomaterialien, Am Mühlenberg 1, D-14476, Potsdam, Germany.
Boudot, M., Elettro, H. & Grosso, D. Converting Water Adsorption and Capillary Condensation in Usable Forces with Simple Porous Inorganic Thin Films. ACS Nano 10, 10031–10040 (2016).
Ganser, C. et al. Cantilever bending based on humidity-actuated mesoporous silica/silicon bilayers. Beilstein J. Nanotechnol. 7, 637–644 (2016).
Poppinga, S. et al. Hygroscopic motions of fossil conifer cones. Sci. Rep. 7, 40302 (2017).