Sep 23, 2015

Biomaterials review

My friend sent me an article, “Growing shoes and furniture: a design-led biomaterial revolution” by Liat Clark, about an exhibit showcasing biomimicry in materials development.
Clark writes, “The natural world has, over millions of years, evolved countless way to ensure its survival. The industrial revolution, in contrast, has given us just a couple hundred years to play catch-up using technology. And while we’ve been busily degrading the Earth since that revolution, nature continues to outdo us in the engineering of materials that are stronger, tougher and multipurpose.”
Spider silk, for example, is ten times tougher than steel and more durable than Kevlar, she continues.

Why isn’t the biomaterials revolution here?

The fundamental tenet of materials science is “structure determines properties.” We use this principle to engineer the performance of materials. Let’s look at the structure of aluminum, for example. Aluminum, like other metals, is made of atoms arranged into a repeating unit cell pattern, called a crystal structure. The unit cell of aluminum looks like this:
The unit cell of aluminum: face-centered cubic. Wikipedia.
Aluminum alloys engineered for high strength contain tiny additions of impurities, such as zinc. The zinc atom is a tiny bit larger than an aluminum atom. Its atomic radius is 142 pm, and aluminum’s atomic radius is 118 pm. When a zinc atom is substituted for an aluminum atom in an aluminum alloy, that unit cell is a tiny bit more densely-packed, making the alloy just a tiny bit harder and stronger than pure aluminum.

Structure determines properties.

If we want to obtain the excellent properties of natural materials, we have to recreate their structure. Mimicking the organically developed structure of living materials is the focus for many biomaterials researchers. However, these organic structures are a lot more complex than the crystal structures discussed above. Let’s take a look at articular cartilage, the connective tissue in your knee and other joints. Articular cartilage is made of collagen. It has five different functions: shear resistance, toughness, compression resistance, containment and anchoring. How can articular cartilage fulfill these five functions using only collagen as a building block? The answer is structure! In the superficial zone, the collagen fibrils are tangled, to prevent them from sliding apart under shear stress. In this zone, a neat repeating unit would result in disaster. The cartilage would tear under minimal shear. The transitional zone contains elbow-shaped collagen fibrils arranged obliquely, for toughness. The radial zone consists of vertically-aligned collagen for compression resistance. The tide mark is a densely-packed region of that acts as a containment barrier, keeping nutrients in and invading cells out. Finally, the calcified zone holds the cartilage to the bone.

Unfortunately, it can be difficult to mimic complex natural structures with existing manufacturing processes, which were designed to create materials with neat repeating structural units. There are no easy answers. The researchers and designers profiled in Clark’s article used a combination of methods to grow their materials on molds of the desired shape, but some of the products took months to create. There are intermediate approaches between a slow growth process and a too-perfect mass-produced material. Natural papers are made by hand in small batches. These papers contain pulp fibers that are messily arranged, rather than perfectly aligned, imbuing the papers with superior strength and tear-resistance. This process incorporates enough industrial innovation that it’s not nearly as slow as a growth process. If we wanted to add another dimension to the manufacturing, we might apply various pressures to the materials as they form, resulting in different structures depending on where the pressure was applied. Similarly, temperature or chemical variations could change the function; this principle is applied when growing hydrangeas in a desired color by varying soil acidity.

Biomimicry is an extremely complex field. Current attempts to deepen our understanding of biomaterials will result in outsized returns in the future. As Craig Vierra, Professor and Assistant Chair, Biological Sciences at University of the Pacific, is quoted, “Ultimately, this will affect the way humans live and operate in society.”