The next frontier in design? Hierarchical structures

"As thin as a human hair" is a trite analogy that suggests simplicity in small things. But as biologists know, the human hair is anything but simple when you take a closer look.

Hair is made of protein — keratin to be exact — and like many of the wondrous strategies nature has, it is how this protein is arrayed that gives hair its performance capabilities, allowing it to be as thin as it is. Keratin is first wound in a right-handed helix, then braided in a two-part left-handed coil, then bundled and sheathed into a microfibril. This bundling is done not once but several times as the hair reaches its ultimate width of 180 micrometers (0.00067 to 0.00709 inches). Even the outside coating of the hair, the cuticle, is complex and made of many overlain parts.

Hair is a complicated system of interrelated parts that solves functional challenges across length scales. It is what's known as a hierarchical structure, which saves material and the energy needed to make it. Another example of this is bone, which is as strong as steel but as light as aluminum thanks to its complex, hierarchical structure that, among other things, resists cracking through at least five different strategies at as many scales.

Natural hierarchical systems share some common traits that are worthy of emulation: They use a few components (like keratin) to make a wide array of different structures in controlled orientations with durable interfaces between different materials. They are dependent or sensitive to water and produced with benign chemistry. Their properties and performances can vary in response to the environment. These complex, controlled shapes are resilient and often are able to repair themselves.

Hierarchical structures in technology

Hierarchical structures are common in nature, but it's not that easy to replicate the integration of those parts in technology. Mankind has some notable but simple macroscopic examples, such as the cable suspension bridge, composites like the belted radial tire, and buildings including the Eiffel Tower or the geodesic dome. Advances in discovery of the very small and its processes, as well as methods of manufacturing, have put more complex versions within our reach. Here are two examples of significant research being done on the mechanics of materials to provide such versions.

1. Harvard research on cross bracing

Dr. Joanna Aizenberg and her colleagues at Harvard studied the cylindrical endoskeleton of the marine sponge Euplectella in a well-cited 2005 Science article that has become a model for this type of work. These investigators noted that the Venus Flower Basket achieved seven scale layers of structural strengthening in growing its silica skeleton. The organism precipitated silica out of seawater and formed this silica into nanospheres arranged in concentric layers and alternated with organic layers. These were bundled into rods; these rods were in turn ganged into composite beams and these beams formed into cage-like struts at the micron scale. These struts were then arrayed at the macroscale in a square lattice with diagonal cross-beams. 

Fascinatingly, the method of cross bracing every other square and thereby achieving the needed reinforcement without excess weight was exactly what engineers had practiced based on structural engineering calculations. Indeed, such cross bracing methods can be observed on buildings such as London's St Mary Axe (formerly the Swiss Re Tower and informally known as the Gherkin). In another optimized form, the layering of crystals and organic material in the rods was not equal. Where crack resistance was needed, toward the outside, the layers of silica were thin, and where tensile-compressive strength was needed, at the core, the layers were thicker.

Aizenberg has continued to investigate complex structures at her lab at the School of Engineering and Applied Sciences at Harvard University, where she is a professor of materials science, chemistry and chemical biology, as well as director of the Kavli Institute for Bionano Science and Technology.

Just one of the lines of investigation is that of Adaptive Hybrid Architecture (of material), where composite systems are made that can successfully bridge the differences between materials and scales. The goal is to make responsive and adaptive mechanisms that are activated by the environmental conditions themselves, something I have written about here under the slogan "Surfing for Free."

In an explanation of design principles for one project, the lab has coupled a range of environmental cues such as humidity and light intensity to a set of structural elements, including hierarchical systems. This coupling, theoretically, yields an "adaptive, integrated responsive system."

2. MIT research on "materiomics"

At Markus Buehler's Laboratory for Atomistic and Molecular Mechanics (LAMM) lab in the Civil and Environmental Engineering department at MIT, engineers are involved in failure of all kinds — structural failure that is, of biological materials. In finding out how and why these hierarchical systems fail, they hope to discover new ways to make sophisticated new structures out of cheap materials.

Dr. Buehler is particularly interested in the cross-scale and cross-material properties of these natural systems, and has even coined a new phrase for how his lab studies these multi-faceted phenomena: "materiomics." Materiomics is defined by the lab as "the study of the material properties of natural and synthetic materials by examining fundamental links between processes, structures and properties at multiple scales, from nano to macro, by using systematic experimental, theoretical or computational methods."

The lab studies the processes, structures and properties of materials from a fundamental, systematic perspective by incorporating all relevant scales, from nano to macro, in the synthesis and function of materials and structures. Thus it gains an integrated view of these interactions at all scales through some sophisticated computer modeling.

The lab examines all kinds of proteins, from those in spider silk to tendons, bones, hair and teeth. Unlike biologists, the scientists are examining these living materials using the methods and intentions of civil engineering and architecture applied to the very small. They have divided these proteins into three groups of basic structural building blocks and have looked closely at how these blocks are bonded.

Using their multi-scale modeling and confirmatory testing, they have been able to typify the structural and mechanical properties of collagen from the molecular to the tissue scale. They have discovered, for instance, that collagen maintains a maximum strength at a length of 200 to 400 nanometers, which explains why one sees only this length of collagen tissue. Collagenous materials, like bone, are typically under stress and the collagen protein provides mechanical stability, elasticity and strength to organisms. Tendons, for instance, are made predominantly from collagen. 

They have also been able to discover that spider webs owe their superior performance not just to the ultimate strength of the silk thread, but also from a nonlinear response to stress and alignment within the geometry of the web. The silk nanocrystals are a stacked arrangement with each layer dialed in a different direction. They are held together by weak hydrogen bonds that act together in the stack to resist external force. The weakly bonded array has the ability to reform easily broken bonds and to slow the rate of failure. In addition, the group found that the size of the nanocrystals was critical to performance; larger crystals that were tested failed catastrophically.

Implications of the findings

The discoveries being made at the Aizenberg and Buehler labs have broad implications for material and structure development for everything from biomedical devices to buildings. Their way of investigating these phenomena also has implications for how we think about design.

Understanding the structure of these biological systems and being able to predict their performance capabilities cannot be done by an examination of the isolated constituent parts. An integrated assessment is needed to test processes, structures and properties across scales. Complex problems sometimes require complex solutions, and understanding those problems sometimes requires complex models.

In the future, I do not believe that architects, engineers and industrial designers will be designing static objects and services. Instead they will be analyzing conditions, figuring out how to manipulate those conditions, and then designing structures to adapt and respond to (and maybe evolve with) those existing conditions or ones that have been created. It will be more like gardening than house building, but it will be based on a technical knowledge of systems, physics and biological science.

Hair photo by Africa Studio via Shutterstock