3D Reticulated Actuator Inspired by Plant Up-Righting Movement Through a Cortical Fiber Network

Since most plant movements take place through an interplay of elastic deformation and strengthening tissues, they are thus ideal concept generators for biomimetic hingeless actuators. In the framework of a biomimetic biology push process, we present the transfer of the functional movement principles of hollow tubular geometries that are surrounded by a net-like structure. Our plant models are the recent genera Ochroma (balsa) and Carica (papaya) as well as the fossil seed fern Lyginopteris oldhamia, which hold a net of macroscopic fiber structures enveloping the whole trunk. Asymmetries in these fiber nets, which are specifically caused by asymmetric growth of the secondary wood, enable the up-righting of inclined Ochroma and Carica stems. In a tubular net-like structure, the fiber angles play a crucial role in stress–strain relationships. When braided tubes are subjected to internal pressure, they become shorter and thicker if the fiber angle is greater than 54.7°. However, if the fiber angle is less than 54.7°, they become longer and thinner. In this article, we use straightforward functional demonstrators to show how insights into functional principles from living nature can be transferred into plant-inspired actuators with linear or asymmetric deformation.

4D pine scale: biomimetic 4D printed autonomous scale and flap structures capable of multi-phase movement

We developed biomimetic hygro-responsive composite polymer scales inspired by the reversible shape-changes of Bhutan pine (Pinus wallichiana) cone seed scales. The synthetic kinematic response is made possible through novel four-dimensional (4D) printing techniques with anisotropic material use, namely copolymers with embedded cellulose fibrils and ABS polymer. Multi-phase motion like the subsequent transversal and longitudinal bending deformation during desiccation of a natural pinecone scale can be structurally programmed into such printed hygromorphs. Both the natural concept generator (Bhutan pinecone scale) and the biomimetic technical structure (4D printed scale) were comparatively investigated as to their displacement and strain over time via three-dimensional digital image correlation methods. Our bioinspired prototypes can be the basis for tailored autonomous and self-sufficient flap and scale structures performing complex consecutive motions for technical applications, e.g. in architecture and soft robotics. This article is part of the theme issue 'Bioinspired materials and surfaces for green science and technology (part 3)'.

A seed flying like a bullet: ballistic seed dispersal in Chinese witch-hazel (Hamamelis mollis OLIV., Hamamelidaceae)

The fruits of Chinese witch-hazel (Hamamelis mollis, Hamamelidaceae) act as 'drying squeeze catapults', shooting their seeds several metres away. During desiccation, the exocarp shrinks and splits open, and subsequent endocarp deformation is a complex three-dimensional shape change, including formation of dehiscence lines, opening of the apical part and formation of a constriction at the middle part. Owing to the constriction forming, mechanical pressure is increasingly applied on the seed until ejection. We describe a structural latch system consisting of connective cellular structures between endocarp and seed, which break with a distinct cracking sound upon ejection. A maximum seed velocity of 12.3 m s-1, maximum launch acceleration of 19 853 m s-2 (approx. 2000g) and maximum seed rotational velocity of 25 714 min-1 were measured. We argue that miniscule morphological differences between the inner endocarp surface and seed, which features a notable ridge, are responsible for putting spin on the seed. This hypothesis is further corroborated by the observation that there is no preferential seed rotation direction among fruits. Our findings show that H. mollis has evolved similar mechanisms for stabilizing a 'shot out' seed as humans use for stabilizing rifle bullets and are discussed in an ecological (dispersal biology), biomechanical (seed ballistics) and functional-morphological (fine-tuning and morphospace of functional endocarps) contexts, and promising additional aspects for future studies are proposed.

Adaptation and Evolution of Biological Materials

Research into biological materials often centers on the impressive material properties produced in Nature. In the process, however, this research often neglects the ecologies of the materials, the organismal contexts relating to how a biological material is actually used. In biology, materials are vital to organismal interactions with their environment and their physiology, and also provide records of their phylogenetic relationships and the selective pressures that drive biological novelties. With the papers in this symposium, we provide a view on cutting-edge work in biological materials science. The collected research delivers new perspectives on fundamental materials concepts, offering surprising insights into biological innovations and challenging the boundaries of materials’ characterization techniques. The topics, systems, and disciplines covered offer a glimpse into the wide range of contemporary biological materials work. They also demonstrate the need for progressive “whole organism thinking” when characterizing biological materials, and the importance of framing biological materials research in relevant, biological contexts.