Background
1. Structure-Function patterns in nature
Nature forms and structures provide a wide range of properties with minimal costs. They have been studied by researchers from various disciplines that acknowledged the appearance of structural and form patterns in nature. These patterns are forms and structures in which some features recur, identically or similarly in places that apparently have nothing in common, at the nanoscale or at the macroscale, and provide benefits and efficiencies, irrespective of their size or material. The idea that 'Structures and forms fits functions' is one of the basic design principles in nature and is well accepted both in biology and in design literature. On a meta-level, structure–function relationships may be abstracted to some structural patterns that repeat in nature and relate to some generic functions. From a biomimetic perspective, these patterns are the reusable solutions used in nature to achieve basic functions. Therefore, such a framework can be useful for bridging biology and technology through the biomimetic design processes, and used mainly during the searching and abstraction stages of the biomimetic design process.
Structural functions in nature could be divided into Engines & Brakes. Engines in nature are structures that exploit the potential propulsion properties of forces in space as clean and renewable energy sources for the purpose of performing a motion. Their correlated main functions are dynamic and are related to motion: to move, to attach, to detach, to channel, and to change. Brakes in nature are structures that inhibit undesired motion resulting from forces in space. They sustain different loads by absorbing them, pushing them back, or changing their direction. Their correlated main functions are static and are related to states of no motion: to stop, to protect, to defend, to stabilize.
2. Engine structures
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Repeated Protrusions
This structure is identified with repeated protrusions of different materials and shapes. Protrusions may be diverse including hairs, denticles, bristles, epidermal protrusions, or scales.
These protrusions are associated with generic functions “Attach” or “Detach” to different target objects such as dirt particles, bacteria, and water droplets. They are also associated with attachment to different sorts of surfaces like walls, sand, soil, ice, or fur.
Image by William Thielicke, Wikimedia
The Lotus leaf (Nelumno nucifera) epidermal protrusions are related to the dirt removal mechanism.
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Repeated Tubes/Channels/Tunnels.
This structure is identified with tubes, channels or tunnels with or without valves. The structure is associated with generic functions “Channel” if it has no valves or to the function “Regulate” if it has valves.
Image Copyright Peter Fratzl Regul
Wood is composed of parallel hollow tubes formed by the wood cells shape. These tubes are responsible for leading water and nutrients from the ground toward the tree’s organs.
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Asymmetry
Geometric asymmetry is defined when the structure has at least one dimension without symmetry.
The basking shark asymmetric stretched jaw creates a passive flow of water through the shark’s gills due to the pressure gradient between the bottom and the upper parts of the jaw.
Material asymmetry is defined when the material properties of a structure are distributed asymmetrically in time or space.
Image by Fir0002, Wikimedia
Pine scales are combined of two tissues with different alignment and different coefficient of hygroscopic expansion of fibers.
Time asymmetry is defined when the structure demonstrates different appearances at different times.
Image by Brocken InaGlory, Wikimedia
Puffer fish is able to inflate its body by swallowing water.
Asymmetry creates various reactions to external factors. Asymmetric structures are associated with the generic function “Change.”
3. Brakes structures
Mechanical Structures (structures 4-7)
The following structures are associated with the generic function “Protect” or “Defend” from mechanical loads. Some of these structures may be effective also in the exposure to thermal or chemical loads. Each one of them is adjusted to different types and directions of loads.
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Repeated Layers - Sandwich Structure
Sandwich structures are composites combined of several parallel alternating layers of different materials with different properties. These structures resist bending in one particular direction and are strong, stable, tensile, durable, and temperature resistant.
Image Copyright Paul Hansma.
The shell of the abalone combines alternating hard and soft layers.
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Intersected Layers - Crisscrossed Structure
This structure consists of a crosslinking of fibers that creates a crisscrossed structure. In two dimensions it looks like a network, in three dimensions it may look like foam, a cellular structure or honeycomb structure. This structure provides an elastic foundation with high resistance to flexure loading.
Image Copyright 2005 Meyer’s Group
The toucan beak is constructed of internal fibrous network of a closed cell foam-like structure made of struts.
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Hollow Cylinders - (Tubular) Structure
The structure is characterized with a hollow cylinder, rod, or tube. The hollow cylinder provides stability against bending and buckling and is adjusted to resist bending in all directions. We note that in the case of repeated tubes structure, each tube in the arrangement functions as a single tube. A tube exposes an interesting duality as it is an engine structure when placed in the flow orientation and a brake structure when it resists mechanical loads that may cause deformations.
Image Copyright Peter Fratzl
A human bone is a hollow tube which resists the tendency to break.
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Helical Structure
The structure is characterized by a helical form in three dimensions. The helical structure is associated with mechanical efficiency. It stores and absorbs energy, provides flexibility and stability, prevents wrinkling, and resists circumferential and longitudinal stresses.
Image Copyright Deborah Spurlock
The intervertebral disk is composed of helical collagen fibers that work like shock absorbers and protect the disk.
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Streamlined Structures/Shapes.
Streamlined shapes are contours designed to minimize resistance to motion through a fluid (such as air). Streamlined geometries are associated with efficient flow. They minimize heat loss from friction associated with kinetic energy; they change the direction of the flow and help maintain laminar instead of turbulent flow around them. They are associated with the generic function of stabilizing (“Stabilize”) an object in the presence of fluids flows.
Image By Andrew Butko, Wikimedia
One common streamline structure in nature is the spiral
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Container Structure
Container structure is an enclosed cavity encompassed by an external cover. The cavity may be full of gases, liquids, or solids. Its generic function is to “Contain.” It is adjusted to sustain the load of gravity or other mechanical loads by exerting a resistance to the load, thus inhibiting the motion of the contained object.
Carnivorous plants cups
For further details and examples, read the paper "Biomimetics: Structure–Function Patterns Approach".
4. Integration of structures
Biological systems usually integrate several structure-function patterns that create complex functional mechanisms. An example for intertwined patterns in the same system is blood arteries. Blood arteries have a tubular structure that channels the blood, whereas the walls are made of elastic helical collagen that protects the system from blood pressure. Nature’s structure-functions patterns may be the basic 'words' of nature structural language. Their integration creates 'sentences' of more complicated functional mechanisms.