Abstract
Pine cones are well known natural actuators that can move their scales upon humidity gradient. The mechanism manifests itself through a displacement easily observable by the naked eye, but coupled with stress generation. In ancient Egypt, wooden wedges were used to break soft blocks of stone by the generated swelling stress. The purpose of the present study is to evaluate the ability of pine cone scales to generate forces while being wetted. In our experiments, a blocking force of around 3N is measured depending on the position on the pine cone where the scales are extracted. A fairly good agreement is obtained when theoretical results based on bimetallic strip systems are compared with experimental data, even if overestimation is observed arising from the input data considered for dry tissues. Inspired by a simplified pine cone microstructure, a biocomposite analogue is manufactured and tested. Although an adequate blocking force can be generated, it has a lower value compared to natural pine cones which benefit from optimized swelling tissue content and interfacial bond strength between them. This study provides new insights to understand the generation of force by pine cones as well as to develop novel biocomposite functionalities.
Highlights
Pine cones are well known natural actuators that can move their scales upon humidity gradient
A hierarchical plant tissue architecture constitued of single cells with a composite structure, i.e. stiff cellulose fibrils oriented in a swellable matrix[4], is required to generate anisotropic swelling
By comparing the response of biocomposite analogues to pine cone scales, it is clear that the maximum force and the time required to reach the maximum force are far below the values obtained for pine cones
Summary
Pine cones are well known natural actuators that can move their scales upon humidity gradient. Recent studies have shown that the principle of natural actuators, based on a bilayer, can be used to develop self-shaping wood and plant fibre-based biocomposite actuators[13,14] making use of the swelling ability of plant tissue such as vegetal fibres. Biocomposite analogues (e.g. vegetal fibres reinforced polymer composite) are manufactured inspired by the structure of pine cone tissues and making use of the anisotropic swelling of plant tissues. Several parameters such as fibre content and interfacial bonding strength are tested to improve our understanding of force generation mechanisms in natural actuators and to develop novel biocomposite functionalities and applications
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