A nanophenomenon that triggers the bone-repair process

The researcher Amir Abdollahi of the UPC’s Numerical Calculus Laboratory (LaCàn) participated in a study that found that flexoelectricity may be responsible for regenerating bone tissue damaged by microfractures.

Feb 21, 2018

Researchers at the Catalan Institute of Nanoscience and Nanotechnology (ICN2), a Severo Ochoa centre located on the campus of the Universitat Autònoma de Barcelona and a member of the Barcelona Institute of Science and Technology (BIST), have discovered that bone is flexoelectric, positing that flexoelectricity may have a role in the regeneration of bone tissue affected by the daily microfractures suffered by bones. This research has potential implications for the prosthetics industry and the development of biomimetic self-healing materials. The researchers Fabian Vasquez-Sancho, Amir Abdollahi, Dragan Damjanovic and Gustau Catalan (leader of the ICN2’s Oxide Nanophysics group) have published their work in the reference journal Advanced Materials.

Bones were already known to generate electricity under pressure, stimulating self-repair and remodelling. First reported in the late 1950s, this phenomenon was initially attributed to the piezoelectricity of bone’s organic component, collagen. However, studies have since observed markers of bone repair in the absence of collagen, suggesting that other effects are at play. The ICN2’s researchers have now revealed just such an effect: the flexoelectricity of bone’s mineral component.

Flexoelectricity is an electromechanical phenomenon that occurs at nanoscale. It is a property of some materials that causes them to emit a small voltage upon application of a non-uniform pressure. This response is extremely localised, becoming weaker as the distance from the point of maximum stress along a strain gradient increases. In microfractures it is localised to the leading edge or tip of the crack, an atomically small site that, by definition, concentrates the maximum strain a material is able to withstand before full rupture. The result is a an electric field of a magnitude that, at this local level, eclipses any background collagen piezoelectric effect.

By studying strain gradients in bones and pure bone mineral (hydroxyapatite), the researchers were able to calculate the precise magnitude of this flexoelectric field. The findings show that the field is sufficiently large within the required 50 microns of the crack tip to be sensed by the cells responsible for bone repair, which would directly implicate flexoelectricity in this process.

These results hold promise for the prosthetics industry, in which new materials that reproduce or amplify this flexoelectric effect could be used to guide tissue regeneration and enable a more successful assimilation of implants.

The study was funded by a European Research Council grant and the collaborators included the UPC’s Lacàn, the Materials Science and Engineering Research Centre at the Universidad de Costa Rica and the École Politechnique Federale de Lausanne (EPFL), Switzerland.

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Reference article:

F. Vasquez-Sancho, A. Abdollahi, D. Damjanovic, G. Catalan. Flexoelectricity in bones. Advanced Materials, 2018. DOI: 10.1002/adma.201705316