Plant Movements as Concept Generators for the Development of Biomimetic Compliant Mechanisms

Pinecone-Inspired Humidity-Responsive Paper Actuators with Bilayer Structure

Many plant materials in nature have the ability to change their shape to respond to external stimuli, such as humidity or moisture, to ensure their survival or safe seed release. A well-known example for this phenomenon is the pinecone, which is able to open its scales at low humidity due to the specific bilayer structures of the scale. Inspired by this, we developed a novel humidity-driven actuator based on paper. This was realized by the lamination of untreated paper made from eucalyptus fibers to a paper-carboxymethyl cellulose (CMC) composite. As observed, the hygroexpansion of the composite can be easily controlled by the amount of CMC in the impregnated paper sheet, which, thus, controls the morphologic deformation of the paper bilayer. For a more detailed understanding of these novel paper soft robots, we also studied the dynamic water vapor adsorption, polymer distribution and hygroexpansion of the paper-polymer composites. Finally, we applied a geometrically nonlinear finite element model to predict the bending behavior of paper bilayers and compared the results to experimental data. From this, we conclude that due to the complexity of structure of the paper composite, a universal prediction of the hygromorphic behavior is not a trivial matter.

The Plant Leaf: A Biomimetic Resource for Multifunctional and Economic Design

As organs of photosynthesis, leaves are of vital importance for plants and a source of inspiration for biomimetic developments. Leaves are composed of interconnected functional elements that evolved in concert under high selective pressure, directed toward strategies for improving productivity with limited resources. In this paper, selected basic components of the leaf are described together with biomimetic examples derived from them. The epidermis (the "skin" of leaves) protects the leaf from uncontrolled desiccation and carries functional surface structures such as wax crystals and hairs. The epidermis is pierced by micropore apparatuses, stomata, which allow for regulated gas exchange. Photosynthesis takes place in the internal leaf tissue, while the venation system supplies the leaf with water and nutrients and exports the products of photosynthesis. Identifying the selective forces as well as functional limitations of the single components requires understanding the leaf as an integrated system that was shaped by evolution to maximize carbon gain from limited resource availability. These economic aspects of leaf function manifest themselves as trade-off solutions. Biomimetics is expected to benefit from a more holistic perspective on adaptive strategies and functional contexts of leaf structures.

A perspective on plant robotics: from bioinspiration to hybrid systems

As miscellaneous as the Plant Kingdom is, correspondingly diverse are the opportunities for taking inspiration from plants for innovations in science and engineering. Especially in robotics, properties like growth, adaptation to environments, ingenious materials, sustainability, and energy-effectiveness of plants provide an extremely rich source of inspiration to develop new technologies-and many of them are still in the beginning of being discovered. In the last decade, researchers have begun to reproduce complex plant functions leading to functionality that goes far beyond conventional robotics and this includes sustainability, resource saving, and eco-friendliness. This perspective drawn by specialists in different related disciplines provides a snapshot from the last decade of research in the field and draws conclusions on the current challenges, unanswered questions on plant functions, plant-inspired robots, bioinspired materials, and plant-hybrid systems looking ahead to the future of these research fields.

Methods for numerical simulation of knit based morphable structures: knitmorphs

Shape morphing behavior has applications in many fields such as soft robotics, actuators and sensors, solar cells, tight packaging, flexible electronics, and biomedicine. The most common approach to achieve shape morphing structures is through shape memory alloys or hydrogels. These two materials undergo differential strains which generate a variety of shapes. In this work, we demonstrate the novel concept that 2D knits comprising of yarns from different materials can be morphed into different three-dimensional shapes thereby forming a bridge between traditional knitting and shape changing structures. This concept is referred to as Knitmorphs. Our computational analysis acts as the proof of concept revealing that knitted patterns of varying materials morph into complex shapes, such as saddle, axisymmetric cup, and a plate with waves when subjected to thermal loads. Two-dimensional circular models of plain and rib developed on CAD packages are imported to the finite element analysis software Abaqus, followed by post-processing into wires and assigning fiber material properties of different thermal coefficients of expansion and stiffness. We also propose potential applications for the concept of programmable knits for developing robots based upon jellyfish like locomotion, and complex structures similar to wind turbine blades. This novel concept is meant to introduce a new field for design when considering morphable structures.

Self-Actuated Paper and Wood Models: Low-Cost Handcrafted Biomimetic Compliant Systems for Research and Teaching

The abstraction and implementation of plant movement principles into biomimetic compliant systems are of increasing interest for technical applications, e.g., in architecture, medicine, and soft robotics. Within the respective research and development approaches, advanced methods such as 4D printing or 3D-braiding pultrusion are typically used to generate proof-of-concept demonstrators at the laboratory or demonstrator scale. However, such techniques are generally time-consuming, complicated, and cost-intensive, which often impede the rapid realization of a sufficient number of demonstrators for testing or teaching. Therefore, we have produced comparable simple handcrafted compliant systems based on paper, wood, plastic foil, and/or glue as construction materials. A variety of complex plant movement principles have been transferred into these low-cost physical demonstrators, which are self-actuated by shrinking processes induced by the anisotropic hygroscopic properties of wood or paper. The developed systems have a high potential for fast, precise, and low-cost abstraction and transfer processes in biomimetic approaches and for the "hands-on understanding" of plant movements in applied university and school courses.

Polylactic Acid (PLA) Biocomposite: Processing, Additive Manufacturing and Advanced Applications

Over recent years, enthusiasm towards the manufacturing of biopolymers has attracted considerable attention due to the rising concern about depleting resources and worsening pollution. Among the biopolymers available in the world, polylactic acid (PLA) is one of the highest biopolymers produced globally and thus, making it suitable for product commercialisation. Therefore, the effectiveness of natural fibre reinforced PLA composite as an alternative material to substitute the non-renewable petroleum-based materials has been examined by researchers. The type of fibre used in fibre/matrix adhesion is very important because it influences the biocomposites' mechanical properties. Besides that, an outline of the present circumstance of natural fibre-reinforced PLA 3D printing, as well as its functions in 4D printing for applications of stimuli-responsive polymers were also discussed. This research paper aims to present the development and conducted studies on PLA-based natural fibre bio-composites over the last decade. This work reviews recent PLA-derived bio-composite research related to PLA synthesis and biodegradation, its properties, processes, challenges and prospects.

Form, Structure, and Function: How Plants vs. Animals Solve Physical Problems

Plants and animals have evolved solutions for a wide range of mechanical problems, such as adhesion and dispersal. Several of these solutions have been sources for bio-inspiration, like the Lotus Effect for self-cleaning surfaces or Velcro for adhesion. This symposium brought together plant and animal biomechanics researchers who tackle similar problems in different systems under the unifying theme of structure-function relations with relevance to bio-inspiration. For both communities it holds true that the structural systems, which have evolved in the respective organisms to address the mechanical challenges mentioned above, are often highly complex. This requires interdisciplinary research involving "classical" experimental biology approaches in combination with advanced imaging methods and computational modeling. The transfer of such systems into biomimetic technical materials and structures comes with even more challenges, like scalability issues and applicability. Having brought all these topics under one umbrella, this symposium presented the forefront of biophysical basic and application-oriented international research with the goal of facilitation knowledge transfer across systems and disciplines.