Kinematic amplification strategies in plants and engineering

Abstract

While plants are primarily sessile at the organismal level, they do exhibit a vast array of movements at the organ or sub-organ level. These movements can occur for reasons as diverse as seed dispersal, nutrition, protection or pollination. Their advanced mechanisms generate a myriad of movement typologies, many of which are not fully understood. In recent years, there has been a renewal of interest in understanding the mechanical behavior of plants from an engineering perspective, with an interest in developing novel applications by up-sizing these mechanisms from the micro-to the macro-scale. This literature review identifies the main strategies used by plants to create and amplify movements and anatomize the most recent mechanical understanding of compliant engineering mechanics. The paper ultimately demonstrates that plant movements, rooted in compliance and multi-functionality, can effectively inspire better kinematic/adaptive structures and materials. In plants, the actuators and the deployment structures are fused into a single system. The understanding of those natural movements therefore starts with an exploration of mechanisms at the origins of movements. Plant movements, whether slow or fast, active or passive, reversible or irreversible, are presented and detailed for their mechanical significance. With a focus on displacement amplification, the most recent promising strategies for actuation and adaptive systems are examined with respect to the mechanical principles of shape morphing plant tissues.

Effect of Gravity on the Scale of Compliant Shells

“…Therefore, bending allows the shell to deform with less impact on the overall elastic strain energy compared to stretching. Since isotropic thin shells are considered, the general form for the surface strain energy density W is given by the equation [45] W = W streching + W bending (1) with the stretching and bending energy densities defined as…”

“…In the case of the plant structures, the complex nature of the plant material (referred to as plant tissue) requires further justification for being included in this study. Biological tissues that constitute the moving organs of the plants instances included in the study are a hierarchized, non-homogeneous material [1]. As a living material, not all parts of tissue perform structural functions [49].…”

“…In contrast, shells built for flexibility use geometry and inextensibility of materials to convert bending stresses into tuned, reversible large displacements. The use of shells as mechanisms is part of the broader, growing trend in compliant mechanisms to deform a large portion of a structure to produce movements (distributed compliance) [1][2][3][4][5][6] instead of lumped compliant hinges or common rigid body hinges.…”

“…As a result, micro-scale shell geometries found in biology, can be upscaled to engineered shell geometries. However, as the characteristic shell dimension increases, gravity (and its associated loading) becomes a hindrance to the adoption of thin shells as compliant mechanisms at the larger scales-the physical limit of compliance in the scaling of thin shells is found to be around 0.1 m.Biomimetics 2020, 5, 4 2 of 18 describe the large shape transitions of viruses [8], and of red blood cells [9], and are the mechanical system for some of the fastest repeatable plant movements [1]. Their flexibility allows the movement in engineered compliant structures such as hingeless joints [10], or active piezoelectric actuators [11].…”

“…Biomimetics 2020, 5, 4 2 of 18 describe the large shape transitions of viruses [8], and of red blood cells [9], and are the mechanical system for some of the fastest repeatable plant movements [1]. Their flexibility allows the movement in engineered compliant structures such as hingeless joints [10], or active piezoelectric actuators [11].…”