Researchers from Linköping in Sweden and Okayama in Japan have created a material that grows like bone.

The bio-inspired component shifts from soft to stiff — an achievement that is inspiring new ideas for microrobots and bone repair.

The bone-like material contains what is known as a variable stiffness actuator: a mechanism that changes in stiffness once heat, light, or another stimulus is applied.

Some "VSAs" are reversible, but the team from Sweden and Japan wants their actuator to stay in the rigid state, especially if those actuators are featured on an implant.

Or an implanted robot.

"Soft microrobots could be injected into the body through a thin syringe, and then they would unfold and develop their own rigid bones," said Edwin Jager, associate professor at the Department of Physics, Chemistry and Biology (IFM) at Linköping University in a recent news release .

A slowly strengthening robot offers potential for targeted bone repair.

"You want the material to be soft when injecting and to harden thereafter to repair the fracture," Prof. Jager told Tech Briefs in a Q&A below.

Prof. Edwin Jager

How to Build Bone

Jager and the team's actuator, in fact, "creates its own bone," according to a recent study, published in Advanced Materials .

The self-generating bone begins with a gel material called alginate. One side of the gel contains an electroactive polymer material. When a low voltage is applied to the polymer, the material bends in a specified direction.

On the other side of the gel, the researchers attached biomolecules that allow the soft gel material to harden. The biomolecules are extracted from the membrane of a cell that is critical to bone development.

When the material is incubated, or immersed, in a cell culture medium — an environment that resembles the body and contains calcium and phosphor — the biomolecules and calcium ions mineralize the gel, and harden it like bone.

After incubation, the Young’s modulus — a measurement of stiffness in the material — increases by a factor of 4, so the gel becomes more rigid.

"The longer we incubate, the more rigid," Jager told Tech Briefs.

The idea for a morphing material began during a research visit in Japan, when Jager met Hiroshi Kamioka and Emilio Hara, two researchers of bones. Kamioka and Hara had discovered a kind of biomolecule that could quickly stimulate bone growth. Jager and the team investigated the possibility of adding the biomolecule to materials.

Moving Forward with the Material

A soft material, powered by the electroactive polymer, still has to get around, to maneuver and expand into spaces like complicated bone fractures.

By building patterns into the gel, the researchers determined how a simple "microrobot" would bend upon application of some voltage. Perpendicular lines on the surface of the material made the simple machine bend in a semicircle, while diagonal lines initiated a corkscrew motion.

“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," said Jager in the news release. "We can embed these movements into the material’s structure, making complex programs for steering these robots unnecessary."

In an edited Q&A with Tech Briefs below, the professor talks about the team's next big design challenge, as well as other ways to get microrobots moving.

Tech Briefs: What is the most clear application for something like this, where a material needs to have different properties at different points in time?

Prof. Edwin Jager: There are many applications where materials need to change their properties. For instance, in medical devices like guidewires and catheters that need different properties during a surgical procedure. During insertion they should be flexible, so that they can move through the arteries with no or minimum damage, but once at the proper site, they need to be more stiff to achieve better “pushability.” Or in bone repair, you want the material to be soft when injecting and to harden thereafter to repair the fracture.

Tech Briefs: Is the change from softness to stiffness completely spontaneous?

Prof. Edwin Jager: The material change of the gel, going from soft to hard, is spontaneous and caused by the cell membrane nanofragments that we have embedded in the gel just for that purpose. It results in the same bone formation as in the body.

Tech Briefs: Why apply the voltage?

Prof. Edwin Jager: The applied voltage is used to activate the electroactive polymer that we have laminated to the gel to make the gel layer into a variable stiffness actuator.

Tech Briefs: What was the biggest lesson learned from your first research visit in Japan?

Prof. Edwin Jager: I would not say a biggest lesson learned, but more a confirmation that one should have an open and curious mind. This idea came up by discussing with colleagues from different disciplines: Japanese researchers who study the fundamentals of bone formation. Interdisciplinary research leads to new concepts, new materials, and new products.

The black material is an electroactive polymer, the volume of which changes when the researchers apply a low voltage, which makes this simple “microrobot” bend. On the other side of the material, you can see the gel to which the researchers have attached biomolecules that allow the soft gel material to harden like a bone. (Photo: Olov Planthaber/LiU)

Tech Briefs: What are you working on now, in relation to the material? What are your biggest challenges?

Prof. Edwin Jager: We are currently working on how to make more complex microrobots and are also looking into the repair of bone fractures. In addition, we will do more fundamental studies of the bone formation in the gels.

Our biggest challenge would probably be to grow really thick bone, but we do not know, since we haven’t tried to grow thick bone yet.

The research was carried out with financial support from organizations including the Japanese Society for the Promotion of Science (JSPS) Bridge Fellowship program and KAKENHI, the Swedish Research Council, Promobilia and STINT (Swedish Foundation for International Cooperation in Research and Higher Education).

What do you think? Share your questions and comments below.