Scientists from the US Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) are looking deep into the ocean for ways of building stronger artificial bones.
The team of Materials Sciences researchers, Antoni Tomsia Sylvain Deville, Eduardo Saiz, and Ravi Nalla, have been working for years on creating a composite so similar to bone it will not trigger immune reactions and can change to meet the body’s requirements. They were already investigating a composite similar in structure to nacre, a finely layered substance found in some mollusc shells. However, replicating the complex natural architecture of varying length scales proved difficult using a synthetic substance.
The group turned back to the sea for their solution. Seawater can freeze like a layered material forming crystals of pure ice layers, while impurities such as salt and microorganisms are expelled from the forming ice and entrapped in channels between the ice crystals. The result is a layered structure that roughly resembles nacre’s wafer-like construction.
The Berkeley Lab team believed this same freezing process could be used to cast a layered material that mimics nacre’s toughness and lightness. They froze a watery suspension of hydroxyapatite, which is the mineral component of bone. Just like the impurities in sea ice, the hydroxyapatite concentrates in the space between the ice crystals, creating layers of nacre-like material.
They also found that the faster the suspension was frozen, the smaller the scale of the resulting structure. They obtained a microstructure measuring one micron, compared with nacre’s natural structure which measures half of a micron.
After the ice is removed via sublimation (changing from solid to gas without passing through a liquid state), the result is a porous hydroxyapatite scaffolding similar to nacre’s multilayered structure. Like nacre, the surface of each layer is rough, helping the layers lock in place with whatever substance fills the space between them. Some bridges also form between the layers, which are believed to increase fracture resistance.
In the future, the Berkeley Lab scientists hope to tailor the scaffolding so that it fosters bone tissue regeneration. To do this, the space between the scaffolding’s layers can be filled with an organic polymer that degrades over the span of several weeks, releasing antibiotics and compounds that stimulate bone growth.
The idea is to place this dense hydroxyapatite-polymer composite in the body where new bone needs grow. Over time, as the polymer degrades, the scaffolding becomes more porous and the growth factor activates, prompting bone cells to invade the newly created pores.
An increasingly aging population means the demand for artificial joints grows year on year, and this breakthrough could make replacement joints stronger and less likely to be rejected. Variations of this substance could also be used in a myriad of applications in which strength and lightness are imperative, such as dental implants, airplane manufacturing and computer hardware.