Most materials will contract if stretched and will expand if squeezed. Think of the stretching of a rubber band and the way it narrows during the stretching process, and the way the circumference of a tennis ball bows outward when it is squeezed.
If a material shows the opposite effect, contracting when it is squeezed and expanding when it is stretched, it is said to be auxetic. (Think of a sponge).
Man-made items such as shock-absorbing and dampening materials are engineered to have this reaction, but aside from some natural sponges, this effect is fairly rare in nature. However, a research team from the University of Cambridge discovered this property in an unexpected place - in the nucleus of an embryonic stem cell. The team's work was published in a recent edition of Nature Materials.
The property is only observed during the transition period of embryonic stem cells, when they are in the process of evolving into a specific type of cell within the body such as muscle or heart tissue.
During the transition period, the researchers injected a dye into the cytoplasmic fluid around the nuclei and found that when the nucleus was stretched, the dye was absorbed - but without the stretching force, it was not. This suggests that porosity was increased by expansion of the nuclei caused during the stretching process, allowing dye to permeate into the structure.
Perhaps its auxetic properties play a role in the remarkable flexibility of embryonic stem cells to develop into any other type of cell in the body, but if so the mechanism remains to be seen. Physical forces and the surrounding environment within the body may play key roles, and further research into these mechanisms may assist in the understanding of certain disorders.
Outside of the human body, this research could lead to interesting material compositions. Understanding the mechanism of auxetism in the transitional stem cell could lead to development of new industrial materials.
Most auxetic materials have a high degree of order in their structure - a honeycomb would be a classic example. This structure allows for the distribution and dissipation of forces that are applied to the structure. However, some materials can produce this effect with a relatively random orientation, and the nuclei of transitional stem cells appear to be in that category.
Is this an evolutionary process or an inherent property of the transitional stem cell? If it has evolved, was the process originally more ordered, with the evolution providing needed structural changes?
Understanding the difference in the auxetism mechanisms between highly ordered and disordered systems could lead to technological advances, assuming the mechanism found in stem cells can be transferred to man-made materials in the outside world.
It could be the case that disordered or randomly oriented systems that have the same effect of auxetism can produce improved materials for shock absorption - imagine bulletproof vests as an example. We may find improvements in performance, cost, ease of manufacture - or all of those properties.