Micro-robots can form their own bone

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Published 17 January 2022 at 19.09

Science. Inspired by how bones in the skeleton are formed, Swedish and Japanese researchers have developed a combination of materials that can take different forms and harden on their own. The material is first soft and then becomes hard as bones are formed from the same minerals found in the skeleton.

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When we are born, we have gaps between the skull bones that are covered with soft connective tissue, called fontanelles. Thanks to the fontanelles, the skull can be compressed and adapted to the narrow birth canal. After birth, the tissue in the fontanelles slowly transforms into hard bone.

Now researchers have combined materials that together mimic the process of nature.

– We want to be able to use this for applications where the material should have different properties at different times. Initially, the material is soft and pliable, and then it can be locked in place. Possible uses are healing of complicated bone fractures or soft micro-robots that could be injected through a small needle as the robot unfolds and forms its bones, says Edwin Jager, senior lecturer at the Department of Physics, Chemistry and Biology, IFM, at Linköping University. p>

The idea was hatched during a research stay in Japan when materials researcher Edwin Jager met Hiroshi Kamioka and Emilio Hara, who are researching bones. The Japanese researchers had discovered a type of biomolecule that could stimulate bone growth in a short time. Would it be possible to combine the biomolecule with Hager's materials research to develop new materials whose stiffness can be varied?

In the current study, which is published in Advanced Materials, the researchers have constructed a kind of simple “micro-robot”, which can take different forms and change hardness. The researchers are based on a gel material called alginate.

On one side of the gel, a polymeric material is allowed to grow. This material is electroactive and changes volume when the researchers apply a weak voltage, which causes the micro-robot to bend in one direction. On the other side of the gel, the researchers attach biomolecules that allow the soft gel material to become hard. These biomolecules are extracted from the cell membrane of a type of cell that is important for bone formation. When the material is dipped in a cell culture fluid, which is supposed to mimic the environment in the body and contains calcium and phosphorus, the biomolecules cause the gel to mineralize and harden like bone.

One of the potential applications that researchers are interested in is bone healing. The idea is that the soft material, which is given strength and mobility by the electroactive polymer, moves into cavities in complicated bone fractures and unfolds. Once the material has hardened, it can form the basis for the formation of new bone, is the idea. In the current study, the researchers demonstrate that the material can wrap around chicken bone and that the artificial bone growth can grow together with the chicken bone.

By making patterns in the gel, the researchers can influence how the simple microrobot will bend when voltage is applied. Perpendicular lines in the surface cause the material to bend in a semicircle, while diagonal lines cause it to rotate like a corkscrew.

– By controlling how the material rotates, we can produce different movement patterns in micro-robots, but also control how the material unfolds in, for example, a broken bone. We can bake the movement into the structure of the material, so that you do not have complex control programs for the micro-robots, says Edwin Jager in a press release.

The researchers now proceed to investigate the properties of the material combination and how it works together with living cells, to learn more about its biocompatibility.