Investigating Piezoelectric And Ferroelectric-like Properties Of Biological Tissues By Scanning Probe Microscopy Methods

Since the biological piezoelectricity has been observed in biological systems,it has attracted so much attention from the researchers.All the studies suggest that piezoelectricity should be a fundamental property in biological tissues.It had reported that piezoelectricity plays an important role on tissues repair and thus has significance on physiological functions.Thanks for the scanning probe microscopy(SPM)methods,the researchers have found that biological tissues are not only piezoelectric,but also ferroelectric-like and their ferroelectricity has strong correlation with the evolution of diseases in organisms.However,the research on biological piezoelectricity and ferroelectricity is still at its early stage.There are few studies focused on the regularity of piezoelectric and ferroelectric-like properties in biological tissues.The microscopic origins of piezoelectric and ferroelectric-like properties are still unknown.Therefore,this thesis first study the piezoelectric and ferroelectric-like properties of murine artery tissues and murine lung tissues by using of conventional and newly developed SPM methods and theories.This thesis also explores the correlation between the structure and the piezoresponse of collagen fibrils in porcine artery tissues and thus investigates the origin of biological piezoelectricity.Especially,the primary content could be covered as following:(1)The study on the piezoelectricity and polarity switching behaviors of murine artery tissues.The changes of piezoelectricity and ferroelectricity in murine artery tissues are summarized.The linear and quadratic harmonic responses under both vertical piezoresponse force microscopy(VPFM)and lateral piezoresponse force microscopy(LPFM)modes are studied.The locally out-of-plane and in-plane piezoelectricity is probed.The voltage-dependence and time-dependence of polarity switching behaviors are researched.The experimental results suggest that the murine artery tissues are inherently piezoelectric and the polarization in murine artery tissues is spontaneous.The strain responses have strong dependence on the amplitudes and the periods of voltage.However,the phase-bias hysteresis loops don't have any significant changes with increasing voltages and prolonging periods.It is believed that the dependence of strain responses is related to the relaxation behavior of polarity switching.(2)The research on the piezoelectricity and polarity switching behaviors of murine lung tissues.The regularity of piezoelectric and ferroelectric-like properties in murine lung tissues is explored.This study first probes the internal structure and topography of the slices of murine lung tissues.The linear and quadratic harmonic responses as well as local piezoelectricity of murine lung tissues are studied under both VPFM and LPFM modes.The voltage-dependence and time-dependence of polarity switching behaviors are also explored.The results indicate that the lung tissues are fibrous network with randomly oriented nanofibers and indeed piezoelectric and spontaneously polarized.The polarity switching behaviors of lung tissues are voltage-dependent and time-dependent,which are believed to be related to relaxation behavior of polarity switching and thus independent to ionic motion.(3)The study of the fine piezoelectric structure of collagen fibrils in porcine artery.The newly developed sequential excitation(SE)and capacitive excitation(CE)PFM methods are employed to probe the heterogeneous piezoresponse of collagen fibrils.The intrinsic piezoresponse is obtained by principal component analysis(PCA)and simple harmonic oscillator(SHO)fitting.The results show that the piezoelectricity of collagen fibrils has strong correlation with its structure and polarization direction.The overlap region has higher piezoresponse than the one in the gap region,which is resulted from the different packing density.These results also demonstrate that SE-PFM is a powerful method to study electromechanical coupling in connective tissues,while CE-PFM is not.The observation of the piezoelectric and ferroelectric-like properties of murine artery and lung tissues as well as collagen fibrils is expected to lay the foundation of understanding the electromechanical coupling behaviors and physiological significance.