Bone growth has inspired “microrobots” that can create their own bones

The black material is an electrically active polymer, which changes size when researchers apply a low electrical voltage, making this “mini-robot” a simple curve. On the other side of the material, you can see the gel to which the researchers attached the biomolecules that allow the soft gel to harden like bone. Credit: Olof Plantaber/Leo

Inspired by bone growth in the skeleton, researchers at Linköping Universities in Sweden and Okayama in Japan have developed a range of materials that can transform into different shapes before they harden. The material is initially soft, but later hardens through a bone growth process that uses the same materials found in the skeleton.

When we are born, we have indentations in our skulls covered with pieces of soft connective tissue called fontanelles. Thanks to the fontanelle, our skulls can be deformed during childbirth and successfully pass through the birth canal. After birth, the tissue of the fontanelle gradually changes to solid bone. Now, researchers have combined materials that together resemble this natural process.

“We want to use this for applications where materials need different properties at different points in time. First, the material is soft and flexible, and then it’s held in place when it hardens. These materials can be used, for example, in complex bone fractures. It can also be used in precision robotics. These tiny robots can be injected into the body through a thin syringe, then unfold and develop their solid bone,” says Edwin Geiger, associate professor in the Department of Physics, Chemistry and Biology (IFM) at Linköping University.

The idea emerged during a research visit in Japan when materials scientist Edwin Jagger met Hiroshi Kamioka and Emilio Hara, who had been conducting research in bones. Japanese researchers have discovered a type of biomolecule that can stimulate bone growth in a short period of time. Will it be possible to combine this biomolecule with Jäger’s materials research, to develop new materials with variable stiffness?

It inspired bone growth

When effort is applied, the material (left) slowly bends toward the chicken bone (white). If the fluid contains the minerals needed for bone growth, the material, within a few days, will begin to build an artificial bone that attaches itself to the chicken bone. Credit: Olof Plantaber/Leo

In the study that followed, published in advanced materialsThe researchers have built a kind of simple “microrobots”, which can take different shapes and vary the stiffness. The researchers started with a gel called alginate. On one side of the gel is implanted a polymer material. This material is electrically active, changing its size when a low voltage is applied, causing the tiny robot to bend in a specific direction. On the other side of the gel, the researchers enclosed the biomolecules that allow the soft gel to solidify. These biomolecules are extracted from the cell membrane of a type of cell important for bone growth. When the material is immersed in a cell culture medium — a body-like environment containing calcium and phosphorous — the biomolecules cause the gel to mineralize and harden like bone.

One potential application of interest to researchers is bone healing. The idea is that the soft material, supported by an electroactive polymer, would be able to maneuver into the voids in complex and expanding bone fractures. When the material then hardens, it can form the basis for building new bone. In their study, the researchers showed that the material can wrap around chicken bones, and that artificial bone that grows later grows alongside chicken bones.

By making patterns in the gel, the researchers can determine how the simple little robot will bend when effort is applied. Vertical lines on the surface of the material make the robot bend in a semicircle, while diagonal lines make it bend like a switch.

It inspired bone growth

Danfeng Cao, a doctoral student at Linköping University, collects a piece of bone around which a “microbot” wraps and grows together. Credit: Olof Plantaber/Leo

“By controlling how the material rotates, we can make the microrobot move in different ways, as well as influence how the material spreads in the broken bone. We can embed these movements into the material’s structure, making complex software to guide these robots unnecessary,” says Edwin Jagger. .

In order to learn more about the biocompatibility of this group of materials, researchers are now looking further into how its properties work with living cells.

Technology accelerates the thermal operation of soft robots

more information:
Danfeng Cao et al., Bio-hybrid variant, self-hardening smooth motors, osteogenesis, advanced materials (2021). DOI: 10.1002 / adma.202107345

Provided by Linköping University

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