Researchers at the universities of Linköping in Sweden and Okayama in Japan have developed a combination of materials that can morph into various shapes before hardening. It is initially soft but later hardens through a bone development process that uses the same materials found in the skeleton.
The material is inspired by the fontanelle tissue that allows babies’ skulls to be soft and flexible when they’re passing through the birth canal and then gradually changes to hard bone shortly after the birth.
Researchers constructed a kind of simple microrobot, one that can assume different shapes and changes stiffness. They began with a hydrogel material called alginate. On one side of the gel, an electroactive polymer called polypyrrole (PPy) is grown. This material changes its 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 – known as cell-derived plasma membrane nanofragments (PMNFs) – that allows the soft gel material to harden like a bone. These biomolecules are extracted from the cell membrane of a kind of cell that is important for bone development. 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 mineralize and harden like 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.
One possible application of interest to researchers is bone healing. The idea is that the soft material, powered by the electroactive polymer, will be able to maneuver in spaces of complicated bone fractures and expand. When the material hardens, it can form the basis for building new bones. In their study, the researchers demonstrate that the material can wrap itself around chicken bones, and the artificial bone that develops later grows along with the animal’s bone. The developed biohybrid variable-stiffness actuators can be used in soft (micro-)robotics and as potential tools for bone repair or bone tissue engineering.
“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”, says Edwin Jager.
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.