One of the great challenges in chemistry and materials science is getting substances to harden at the right time. It may sound simple, but it’s very difficult to make something malleable that becomes rigid when you need it to and not before – particularly if it’s somewhere difficult to reach, like the interior of the human body.
A team of Japanese and Swedish researchers have created a bone-inspired substance that can change its shape with low voltages of electricity, before hardening. The material could be used to make “microrobots” that would help with bone growth and fracture healing.
“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, an associate professor at Linköping University in Sweden, and co-author on a paper describing the research, published in Advanced Materials.
“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 substance has been born from a combination of Jager’s materials science research at Linköping, and biomolecules discovered by bone growth researchers at the University of Okayama, Japan.
These biomolecules – called plasma membrane nanofragments – could provoke bone growth in a short amount of time. In a lab-based experiment, the Okayama researchers were able to stimulate bone growth with their biomolecules in a day.
In this study, the researchers constructed a microrobot out of their plasma membrane fragments and an electroactive polymer called polypyrrole.
These two substances sandwiched a gel called alginate between them. When they applied a low voltage to the material, it changed its volume and its shape. And when the material was submerged in a solution designed to encourage cell growth (a cell culture), the biomolecules made the gel mineralise, and become hard like a bone.
The researchers have tested this material as a method of healing bones. They showed in a lab that the gel could envelop and fill gaps in chicken bones, then harden to become a source of artificial bone.
They can also manipulate the way the material folds, by making patterns in the gel.
“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,” says Jager.
“We can embed these movements into the material’s structure, making complex programs for steering these robots unnecessary.”
The researchers are now investigating how compatible this material will be with living cells.