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Hardening through a bone development process that uses the same materials found in the skeleton, the materials were developed by researchers at the universities of Linköping in Sweden and Okayama in Japan.
Newborn babies have gaps in their skulls covered by pieces of soft connective tissue called fontanelles, allowing their skulls to be deformed during birth to fit through the birth canal. Post-birth, the fontanelle tissue gradually changes to hard bone. The researchers combined materials that can replicate this natural process.
“We want to use this for applications where materials need to have different properties at different points in time,” said Edwin Jager, associate professor at Linköping.
“Firstly, the material is soft and flexible, and it is then locked into place when it hardens. This material could be used in, for example, complicated bone fractures. It could also be used in microrobots – these soft microrobots could be injected into the body through a thin syringe, and then they would unfold and develop their own rigid bones.”
The team constructed a simple microrobot that can assume different shapes and change stiffness. They began with a gel material called alginate, and grew an electroactive polymer material on one side. The polymer changes volume when a low voltage is applied, causing the microrobot to bend in a specified direction.
On the other side of the gel, the researchers attached biomolecules extracted from cell membranes that allow the soft gel material to harden. When the material is immersed in a cell culture medium – an environment that resembles the body and contains calcium and phosphor – the biomolecules make the gel mineralise and harden like bone.
One potential application of interest is bone healing. The soft material, powered by the electroactive polymer, would manoeuvre into spaces in complicated bone fractures and expand. When the material has hardened, it could form the foundation for the construction of new bone.
The researchers demonstrated that the material can wrap itself around chicken bones, and the artificial bone that subsequently developed grows together with the chicken bone.
By making patterns in the gel, the researchers can determine how the simple microrobot will bend when voltage is applied. Perpendicular lines on the surface of the material make the robot bend in a semicircle, while diagonal lines make it bend like a corkscrew.
“By controlling how the material turns, we can make the microrobot move in different ways, and also affect how the material unfurls in broken bones. We can embed these movements into the material’s structure, making complex programs for steering these robots unnecessary,” Jager said.
In order to learn more about the biocompatibility of this combination of materials, the researchers are now looking further into how its properties work together with living cells.
The research was published in Advanced Materials.
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