3.2 Examples of hierarchical structures in nature

Nature's hierarchical structures are marvels of engineering. From bone to spider silk, these materials showcase intricate designs across multiple scales. This organization gives them unique properties like strength, flexibility, and toughness that far surpass their individual components.

Surface structures in nature are equally impressive. Gecko feet, lotus leaves, and butterfly wings demonstrate how micro and nanostructures can create amazing abilities. These examples inspire new materials with enhanced adhesion, self-cleaning, and optical properties.

Biological Materials

Bone Structure and Composition

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• Bone is a hierarchical composite material consisting of organic collagen fibers and inorganic hydroxyapatite crystals
• Collagen fibers provide flexibility and tensile strength while hydroxyapatite crystals contribute to bone's stiffness and compressive strength
• Bone's hierarchical structure spans multiple length scales from the nanoscale to the macroscale (collagen molecules, mineralized collagen fibrils, osteons, cortical and trabecular bone)
• Bone continuously remodels itself through the balanced actions of osteoblasts (bone-forming cells) and osteoclasts (bone-resorbing cells) to maintain its strength and adapt to mechanical loads

Spider Silk Properties and Production

• Spider silk is a protein-based fiber with exceptional mechanical properties including high tensile strength, extensibility, and toughness
• Dragline silk, used as a lifeline and for web construction, is composed of two main proteins: major ampullate spidroin 1 (MaSp1) and major ampullate spidroin 2 (MaSp2)
• These proteins contain repetitive amino acid sequences that form crystalline β-sheet structures (providing strength) and amorphous regions (providing elasticity)
• Spiders produce silk in specialized glands and spin it into fibers using a complex spinning process involving shear forces, pH changes, and ion exchange

Wood Anatomy and Mechanical Properties

• Wood is a hierarchical, anisotropic, and porous material composed of cellulose, hemicellulose, and lignin
• Cellulose microfibrils are the main load-bearing components, providing tensile strength and stiffness along the grain direction
• Hemicellulose acts as a matrix material, binding cellulose microfibrils together, while lignin provides compressive strength and resistance to decay
• Wood's cellular structure, consisting of elongated cells (tracheids in softwoods and vessels and fibers in hardwoods), is optimized for water transport and mechanical support

Abalone Shell Structure and Toughness

• Abalone shell is a hierarchical composite of calcium carbonate (aragonite) platelets bound together by a small amount of organic material (proteins and polysaccharides)
• The aragonite platelets are arranged in a brick-and-mortar structure, with the organic material acting as the mortar, providing a tough and fracture-resistant material
• The platelets are organized into layers with alternating crystal orientations, further enhancing the shell's toughness and crack resistance
• Abalone shell's hierarchical structure and composition result in a material that is 3,000 times more fracture-resistant than pure calcium carbonate

Surface Structures

Nacre (Mother-of-Pearl) Formation and Properties

• Nacre is the iridescent inner layer of mollusk shells, composed of a hierarchical arrangement of aragonite platelets and organic material (similar to abalone shell)
• The aragonite platelets are organized into layers, with each layer rotated relative to the adjacent layers, creating a highly ordered, brick-and-mortar structure
• The organic material, consisting of proteins and polysaccharides, binds the platelets together and provides a ductile interface for stress dissipation and crack deflection
• Nacre's hierarchical structure and composition result in a material with high strength, toughness, and fracture resistance, making it a model for biomimetic materials design

Gecko Foot Adhesion Mechanism

• Geckos can adhere to and climb on various surfaces using specialized adhesive pads on their toes called lamellae
• Each lamella is covered with millions of microscopic, hair-like structures called setae, which further branch into nanoscale spatula-shaped structures
• The setae and spatulae make intimate contact with surfaces, creating a large surface area for van der Waals forces to act, enabling strong adhesion
• Gecko adhesion is reversible, allowing them to easily attach and detach their feet during climbing, and is self-cleaning, as dirt particles do not adhere as strongly as the spatulae

Lotus Leaf Surface Structure and Hydrophobicity

• Lotus leaves exhibit superhydrophobicity and self-cleaning properties due to their hierarchical surface structure and chemical composition
• The leaf surface is covered with microscale papillae (bumps) and nanoscale wax crystals, creating a highly rough, low-surface-energy surface
• Water droplets maintain a nearly spherical shape on the leaf surface, minimizing contact area and allowing them to easily roll off, picking up dirt particles in the process (self-cleaning effect)
• The combination of surface roughness and low surface energy (hydrophobicity) is known as the "Lotus effect" and has inspired the development of biomimetic self-cleaning surfaces

Butterfly Wing Scale Nanostructures and Optical Properties

• Butterfly wings display a wide range of colors and patterns, which are often produced by the micro- and nanostructures of the wing scales rather than pigments
• Wing scales are composed of chitin and are arranged in a shingle-like pattern, with each scale having a complex hierarchical structure of ridges, cross-ribs, and lamellae
• The nanostructures on the wing scales interact with light through various mechanisms, such as thin-film interference, diffraction, and photonic crystals, producing iridescent and structural colors
• Examples of structural coloration in butterflies include the blue morpho butterfly (Morpho peleides), whose wing scales have a multilayer structure that reflects blue light, and the glasswing butterfly (Greta oto), whose transparent wings are due to nanopillars that minimize light reflection