One of the most astounding things about the human skeletal structures is the bones’ ability to harden with repeated stress. Conversely, bones have a remarkable tendency to weaken when not put under dynamic physical stress, or in a low-to-no gravity environment. Bones are a considered a brittle material that will either break or strengthen under pressure depending on the bone density. Scientists have replicated these remarkable abilities of human bone by synthesizing a nanocomposite material made of multi-walled carbon nanotubes and a synthetic polymer.
In a paper published in ACS Nano (Brent J. Carey, Prabir K. Patra, Lijie Ci, Glaura G. Silva, Pulickel M. Ajayan. “Observation of Dynamic Strain Hardening in Polymer Nanocomposites.” ACS Nano, 2011; 110321121458018 DOI: 10.1021/nn103104g), Rice University scientists report that they have synthesized a multi-walled carbon nanotube bundle and coated it with a polymer that behaves like a human bone. This composite seems to increase in strength as the composite material is put under stress. The type of induced stress is more than a simple tensile test where a molecule is pulled apart or bent over and over. This is also not the kind of stress caused by compression, such as hammering or pressing down on the material. This stress that causes the composite material to strengthen is when a large amount of weight is pressed and then released over and over in a dynamic motion, much like a weight training regimen. After millions of cycles of pressure and release, the nanotube/polymer composite was 12% stronger than it had been before, and all factors seem to indicate that there is no limit to how much stress this nanocomposite material can endure and subsequently continue to strengthen.
The components of the nanocomposite material are strikingly similar to the architectural structure of a human bone. The multi-walled carbon nanotube is a brittle tube that has a high surface area, similar to the tubular portion of bones (diaphysis) which is composed of compact bone tissue. The nanotube is surrounded by an inert, rubbery polymer known as polydimethylsiloxane (PDMS) that has a similar structural function as the membrane that surrounds the outside of bones. Multiple tests indicate that strengthening of the nanocomposite material is not due to polymer cross-linking, nor is it due to any apparent chemical change, but seems to be due to the chemical fluidity (i.e. electrons being able to move around) of the nanotube wall and the polymer interface.
Carbon nanotubes were discovered in 1991 as another fundamental form of carbon. Graphite and diamond are the most well-known naturally occurring forms of carbon. The predecessor to carbon nanotubes is a form of carbon known as C60 or buckminsterfullerene. This “buckyball” is composed of interlocking carbon atoms in hexagonal and pentagonal ring structures, similar to a soccer ball. A tubular version of a buckyball is a carbon nanotube. Carbon nanotubes have a carbon bonding structure that is similar to graphite sheets, but because of its architecture, it is markedly more durable than graphite. Perhaps the most compelling attributes of carbon nanotubes are its conductive properties.
These nanotube/polymer composites architecturally and behaviorally mimic human bone; however, scientists are still unsure of the exact mechanism that causes this behavior. In human beings, a complexity of factors within the body contributes to increases (and decreases) in bone density. Some of these factors include the complex signaling pathways of hormones. NASA has been doing ongoing studies on bone density in order to understand how to prevent the marked bone attrition observed in astronauts. The researchers at NASA report, “Bone structure is the product of three processes — longitudinal growth, modeling, and remodeling — each following a complex sequence of steps… Normally, the breakdown of old bone mass (resorption) and the formation of new bone mass (growth) occur constantly, in a balanced cycle called remodeling.” However, the NASA scientists have found that the major factors that contribute to bone strength or attrition are gravity, physical activity and biomechanics, all of which are environmental or physical factors as opposed to biochemical factors. These nanocomposite replicas seem to be strengthening as a result of the environmental factors alone.
Incidentally, this is another example of biomimicry in action. (Several examples have been reported in Evolution News and Views here and here.) Scientists have constructed a nanocomposite material that mimics what we find in nature. Furthermore, it demonstrates an architectural system that seems to be sophisticatedly suited for structural support and yet capable of adapting to stress loads. Nanotechnology is an applicational discipline. Experiments and composites are formed with a specific applicational end in mind. In this case, these nanotube/polymer composites might be useful for any type of process that requires the materials to adapt to higher loads. It has some implications for finding new ways to deal with bone loss or injury, furthering the idea that perhaps our skeletal structure was also constructed with an end in mind.