Resumen de Flexoelectricity in biomaterials
Fabián Norberto Vásquez Sancho
Flexoelectricity is the ability of materials to generate electricity upon being bent, or, more generally, upon being inhomogeneously deformed. It is a property that is allowed by symmetry in all materials and, therefore, it was in principle possible that it existed in biomaterials –one precedent existed for their observation in inner-ear stereocilia, in fact, pointing to its importance for acousto-electric transduction in mammalian hearing. In this context, this thesis investigates into the flexoelectrical properties of several biologically-produced ceramic composites, ranging from bones to coral, putting emphasis not only on measuring the flexoelectrical properties, but also in connecting them to their potential physiological role.
Chapter 1 introduces the topic of the mechanoelectric properties of piezoelectricity and flexoelectricity, and gives an overview of the biomaterials studied in this thesis.
In Chapter 2, a theoretical analysis of the mechanoelectric properties of inhomogeneous systems is developed. For biomaterials, flexoelectricity and piezoelectricity cannot be as easily separeted as in the case of crystal or ceramic samples that are regular and with defined properties. The use of biomaterials forced us to consider situations in which flexoelectricity and piezoelectricity may act together. Situations in which piezoelectricity is able to disguise itself as flexoelectricity or vice-versa are presented, with an aim to lay the conceptual framework for the electromechanical measurements and results of the following chapters.
Chapter 3 describes the characterization and analysis of macroscopic measurements of flexoelectricity in hydroxyapatite and bones. Bending-induced polarization of both kinds of samples yielded very similar results, which demonstrates that hydroxyapatite can account for most of the polarization of bones without needing to invoke collagen piezoelectricity.
Considering that flexoelectricity is more relevant at the microscale, where strain gradients are bigger, in Chapter 4, we developed a model to study flexoelectric fields around microcracks in bones. We determined that the magnitude of the electric fields generated by a loaded crack can induce apoptosis in osteocytes. Osteocyte apoptosys is known to be the first step in the bone remodeling process.
In Chapter 5, we performed in vitro experiments with osteocytes and osteoblast to probe whether flexoelectric fields are indeed able to affect cells. We observed not only that crack-generated flexoelectric fields experiments are able to induce apoptosys of cells in the short term, but in the long-term culture experiments, flexoelectricity is also able to stimulate the differentiation of cells.
In Chapter 6, we explored the mechanoelectric properties of other Ceramic-based biomaterials such as teeth, coral skeleton, and the club of a stomapod. In the case of teeth, they are composed by the same constituents as bones, and they were therefore a good material to compare with bone. Meanwhile, coral skeleton is a material commonly used as a bone graft due to the similarities with bone. By comparing flexoelectric properties of both materials, we were able to determine that they are also very similar, leading us to hypothesise that flexoelectric compatibility may be a helping factor in the good performance of coral-based bone grafts, a possibility we propose to explore in other candidates for bone grafts. Finally, the club of a stomapod has an outstanding capacity to stand stress without fracture and this phenomenon motivated us to study the mechanoelectric properties of the club, as flexoelectricity is known to affect the mechanical properties of matter.
Finally, Chapter 7 gives a personal overview of the perspectives and future lines that could derive from this research. The complete description of experimental procedures for electromechanical and biological experiments is in Appendix ), and Appendix B is the Mathematica algorithm that I programmed for calculating flexoelectric fields around cracks.
