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The process by which bones are maintained and healed is dictated by electromechanical and biochemical signals which transmit information from bone tissue to stem cells within the body. One aspect of this electromechanical signalling system is piezoelectricity, whereby bones generate electrical fields when under mechanical stress, triggering increased densification and mineralisation. However, conventional orthopaedic implants are typically comprised of titanium alloys (Ti-6Al-4V), which lack piezoelectric functionality, and this absence of electromechanical stimulation may contribute to demineralisation and loosening at the implant site. By utilising piezoceramics in orthopaedic materials, we can design implants capable of delivering the necessary electrical stimulation through their inherent functional properties—in other words, an implant capable of communicating with the same ‘electromechanical language’ as bone.

A biointerfacing material must be both safe and effective. Sodium potassium niobate ((K,Na)NbO3, KNN) systems are excellent candidates for active bioceramics, owing to their non-toxic constituent ions and favourable piezoelectric properties for orthopaedic applications. Cytocompatibility investigations with human osteoblast cells have confirmed that KNN exhibits no cytotoxic effects. Though unmodified KNN has poor stability in vitro, our work in stoichiometrically-modified KNN-based ceramics has developed KNN systems with long-term stability in aqueous environments. By suppressing the formation of hygroscopic secondary phases in synthesis, we have developed KNN-based ceramics capable of maintaining polarisation and electromechanical functionality in liquid. Moreover, the microstructure of KNN can be stoichiometrically tuned, modulating grain size and porosity to direct cellular responses and tissue infiltration.

A prominent challenge in active bioceramics is stability—the iterative nature of material-body interactions can affect both the safety and efficacy of an implant, as ions released by the ceramic influence cellular behaviour and alter the material’s structure. It is necessary to understand these effects at both the local and bulk scale, as both osteogenesis and piezoelectricity are local phenomena that manifest as bulk effects. With piezoelectric analysis and in situ piezoresponse force microscopy (PFM), we have ensured that the electromechanical functionality of these KNN-based systems is maintained whilst in liquid. By investigating the functionality from the cell-surface interface to the bulk ceramic, our work in KNN-based piezoceramics presents the exciting potential of active electromechanical communication for bone regeneration.

Caitlin M. Guzzo

Department of Materials Science and Engineering Norwegian University of Science and Technology (NTNU), Norway

https://www.ntnu.edu/ima/research/facet/biomedical

caitlin.guzzo@ntnu.no

https://www.linkedin.com/in/caitlin-guzzo-9546b467/