The black material is an electroactive polymer, while on the other side is the gel with attached biomolecules that allow the material to harden like bone. Image: Olov Planthaber/LiU.
The black material is an electroactive polymer, while on the other side is the gel with attached biomolecules that allow the material to harden like bone. Image: Olov Planthaber/LiU.

Inspired by the growth of bones in the skeleton, researchers at Linköping University in Sweden and Okayama University in Japan have developed a combination of materials that can morph into various shapes before hardening. The material combination is initially soft, but later hardens through a bone development process that uses the same materials found in the skeleton.

When babies are born, they have gaps in their skulls that are covered by pieces of soft connective tissue called fontanelles. It is thanks to fontanelles that our skulls can be deformed during birth and pass successfully through the birth canal. Post-birth, the fontanelle tissue gradually changes to hard bone. Now, researchers have combined materials that can together replicate this natural process.

“We want to use this for applications where materials need to have different properties at different points in time. Firstly, the material is soft and flexible, and it is then locked into place when it hardens,” says Edwin Jager, associate professor in the Department of Physics, Chemistry and Biology (IFM) at Linköping University. “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 idea was hatched during a research visit in Japan when materials scientist Jager met Hiroshi Kamioka and Emilio Hara from Okayama University, who conduct research into bones. The Japanese researchers had discovered a biomolecule that can stimulate bone growth over a short period of time. Would it be possible to combine this biomolecule with Jager’s materials research, to develop new materials with variable stiffness?

In the study that followed, reported in a paper in Advanced Materials, the researchers constructed a kind of simple 'microrobot', which can assume different shapes and change stiffness. The researchers began with a gel material called alginate. On one side of this gel, they grew a polymer material. This material is electroactive and changes its volume when a low voltage is applied, causing the microrobot to bend in a specific direction.

On the other side of the gel, the researchers attached biomolecules that allow the soft gel material to harden. These biomolecules are extracted from the outer membrane of a cell that is important for bone development. When the soft gel material is immersed in a cell culture medium – an environment that resembles the body and contains calcium and phosphor – these biomolecules mineralize the gel, hardening it like bone.

One potential application of interest to the researchers is bone healing. The idea is that the soft material, powered by the electroactive polymer, will be able to fit into spaces in complicated bone fractures and expand. When the material subsequently hardens, it can form the foundation for the construction of new bone. In their study, the researchers demonstrate that the material can wrap itself around chicken bones, and that the resulting artificial bone grows together with the chicken bone.

By making patterns in the gel, the researchers could control 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 programmes for steering these robots unnecessary,” says Jager.

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.

This story is adapted from material from Linköping University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.