Probing electromechanical coupling in collagen at the nanoscale via scanning probe microscopy

Electromechanical coupling is ubiquitous in nature and is a functional characteristic in a large range of inorganic and organic materials, including collagen type I - a fibrous protein abundant in mammals. Understanding the biofunctionality of electromechanical coupling in its linear form - piezoelectricity, has been a topic of research spanning over seven decades and yet many questions still remain unanswered. Piezoelectricity in bone and connective tissues such as tendon has been investigated at the macroscopic scale since the discovery of piezoelectricity in bone in 1957 and induced currents via the piezoelectric effect have been shown to activate the healing process in tissues under tension. Biological systems consist of complex hierarchical structures which results from a high degree of organization from the macroscale down to the nanoscale. These complex structures, however, make quantitative piezoelectric measurements difficult. Therefore, there exists a need to understand these processes at the individual protein level - i.e. at the nanoscale. In this thesis, a voltage-modulated form of atomic force microscopy called piezoresponse force microscopy is utilized to investigate the counterpart which is responsible for piezoelectricity in bone and connective tissues - collagen. The polar properties of collagen were revealed at the nanoscale and were shown to result in a highly complex polar architecture in natural tissue, which is important for understanding tissue development. Shear piezoelectricity was discovered to persist in engineered collagen hydrogels, a study intended to highlight the importance of replicating both structural and functional properties in replacement tissues. The electromechanical properties of collagen type II were investigated which were previously unknown. Collagen type II was shown to be a shear piezoelectric, exhibiting an angle dependence of the piezoelectric signal with cantilever-fibril angle. In addition, the piezoelectric tensor of collagen type I was determined at the nanoscale. Most piezoelectric coefficients measured were higher than those previously reported at the macroscopic scale. The new local tensor here will be useful for future studies which are concerned with the biofunctional implications of piezoelectrically-induced charges in collagen at the nanoscale.