Nature is filled with trade-offs. One of the more frustrating trade-offs for engineers is choosing between stiffness and toughness. A stiff material resists deformation in response to an applied force. A tough material absorbs energy and resists breaking, mostly by bending. A diamond won’t bend prior to breaking, but sapling tree will.
It is difficult to engineer a material that is both highly stiff and tough, because increasing one generally decreases the other. However, human intervertebral discs are an example of a material that has both properties. These pancake-like discs have a rigid exterior wall made of collagen, but are filled with a gel-like liquid called the nucleus pulposus.
Drawing inspiration from these discs, scientists injected a flexible elastomer structure with pockets of liquid gallium. Compared with structures that had no pockets, pockets filled with air, and pockets filled with water, the gallium-infused structure showed markedly increased stiffness and toughness. These results are due to the gallium’s viscosity, or ability to resist motion, and bulk modulus, the ability to resist compression in the face of increasing pressure.
This technique will be invaluable for material engineering, but several questions need to be addressed before products like artificial intervertebral discs can be realized. The biggest issue is that of temperature. Gallium is only liquid at room temperature and above, which means these structures demonstrate stiffness and toughness within a certain temperature range. Scientists have also compared liquids of differing viscosities, but it is unclear how different bulk modulus influence the structure. There are also open questions about the optimal size number of gallium pockets.
Acknowledgements: Many thanks to Ankita Shastri for her comments on this article. Ankita received her degree in material chemistry at Harvard and now works at a med-tech startup.
Managing Correspondent: Zane Wolf
Image Credit: Jeff Fitlow/Rice University
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