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Bone Growth-Inspired “Microrobots” Can Create Their Own Bone

This “microrobot” can assume different shapes and change stiffness, potentially applicable for bone repair and bone tissue engineering.

An electroactive polymer that changes in volume when a low voltage is applied causing this “microrobot” to bend. On the other side of the material, there is a gel with biomolecules that allow the soft gel material to harden like a bone. Photo Credit: Olov Planthaber/LiU.

Inspired by the dynamic way in which bones grow in the skeleton, scientists at the universities of Linköping in Sweden and Okayama in Japan have created a combination of materials that can be transformed into a variety of shapes before hardening. While initially soft, the material can harden through a bone development process that uses the same materials found in the skeleton.

In medicine and tissue engineering, variable stiffness is an important factor to develop compliant hydrogels, scaffolds, and transplanted materials for reconstructing the function and mechanical properties of injured hard tissue. Low rigidity and material compliance are essential to reduce mechanical resistance or promote morphological adaptation, whereas high rigidity is needed for structural load-bearing after changes in shape are completed. For instance, when humans are born, there are gaps in our skulls that are covered by pieces of soft connective tissue called fontanelles. Fontanelles enable our skulls to deform to pass through the birth canal. After birth, when flexibility is no longer needed and a rigid protective material is instead needed to protect the brain, the fontanelle tissue gradually hardens into rigid bone.

In their recent study, the researchers have combined several materials that mimic this natural process, from which they constructed a simple “microrobot” that can assume different shapes and change stiffness.

“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. 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”, explained Edwin Jager, associate professor at the Department of Physics, Chemistry and Biology (IFM) at Linköping University, and an author of the study.

Starting with a gel material called alginate, Jager and colleagues grew a polymer material on one side of the gel. The material is electroactive and can change in volume when a low voltage is applied to it. This enables the microrobot to bend in a specified direction. On the other side of the gel, the researchers attached biomolecules, extracted from the cell membrane of a cell that is crucial for bone development, to allow the soft gel material to harden. When immersed in a cell culture medium that resembles the body’s environment and contains calcium and phosphor, the biomolecules trigger the gel to mineralise and harden like bone.

According to the researchers, their microrobot could be applied to bone healing since the soft material, powered by the electroactive polymer, would be able to manoeuvre itself into spaces in complex bone fractures and expand. As the material hardens, it can form the foundation for the construction of new bone. To demonstrate this potential application, the researchers use chicken bones and showed how the material can wrap itself around the bones, after which the artificial bone develops and grows together with the chicken bone.

By creating patterns in the gel, the scientists were also able to determine how the simple microrobot will bend when voltage is applied. Perpendicular lines on the material’s surface will cause the robot to bend in a semicircle, whereas diagonal lines make it bend like a corkscrew.

“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 programmes for steering these robots unnecessary”, said Edwin Jager.

Currently, the scientists are conducting further investigation into how the material’s properties work together with living cells to better understand its biocompatibility. With improvements, these newly developed biohybrid variable-stiffness actuators are expected to advance bone repair and bone tissue engineering. [APBN]

Source: Cao et al. (2022). Biohybrid Variable-Stiffness Soft Actuators that Self-Create Bone. Advanced Materials, 2107345.