16 March 2022
by Alex Brinded

Soft microbots make hard bone

A Swedish and Japanese collaboration has found an innovative material combination that could promote bone healing.

The electroactive polymer makes this simple ‘microbot’ bend. On the other side, alginate gel with biomolecules allows it to harden like bone
The electroactive polymer makes this simple ‘microbot’ bend. On the other side, alginate gel with biomolecules allows it to harden like bone © Olov Planthaber/LiU

The material is soft and flexible but then hardens once positioned into place, using the same materials found in the human skeleton. The process could be used for complicated bone fractures or with ‘microbots’ that could be injected into the body through a thin syringe, before unfolding and developing their own rigid bones.

The researchers at Linköping University were inspired by a trip to a Japanese laboratory where a biomolecule had been discovered that can stimulate bone growth quickly.

The scientists then made a ‘microbot’ that can change shapes and stiffness. Using a gel material called alginate, they grew a polymer material on one side. This electroactive material changes volume when a low voltage is applied, causing the microbot to bend in a specific direction. Nanofragments from the membrane of a cell that are important for bone growth are then attached to the other side to allow it to harden.

Lead author Edwin Jager explains, ‘We synthesised the electroactive polymer polypyrrole on one side in and on the alginate to make a so-called bilayer actuator, that is a soft, ‘artificial muscle’. On the other side, we immobilised the cell membrane nanofragments that would initiate the bone formation that switches the actuator from being soft and able to move back and forth, to become stiff, rigid and immobile.’

When immersed in a cell culture medium that contains calcium and phosphor – the same minerals as in bone – the gel mineralises and hardens to form bone-like structures. The mineralisation takes two to five days, and although the layers are relatively thin, they are strong enough to block the movement.

One application area could be bone healing. The soft material could be manoeuvred into complicated bone fractures before expanding. When it hardens, it can form the foundation for new bone construction.

The team has demonstrated that the material can wrap itself around chicken bones, and the artificial bone that develops grows with the animal bone.

By making patterns in the gel, the researchers can determine how the microbot will bend when a voltage is turned on.

‘In principle any shapes or line can be drawn,’ Jager says. ‘We used these lines to programme the actuators with predetermined bending patterns. Perpendicular lines make the material bend into a semi-circle and 45° lines in a corkscrew.

In previous work, we have even put them parallel along the major axis and that will result in forming a tubular structure.’

With regards to the kind of fractures this process can be used for, Jager admits, ‘We don’t know, and this is something that we will look into in the future. So far, we have made gel layers of several square centimetres, but only a few hundred micrometres thick.’

There are potentially other biomolecules that could induce mineralisation which the team want to investigate. The focus will be testing with living cells. Jager adds, ‘We will investigate cell viability of these new materials with the cells. Although all various components are known to have shown good cell viability, this specific combination needs to be tested too. We have seen in the past that surprises can occur even when combining two or more well-known materials.’

So long as there is a source of calcium and phosphates present, he is hopeful this material can work in humans or animals, which is why they are mimicking in vivo processes in in vitro experiments.

Authors

Alex Brinded

Staff Writer