Probing The Microstructure Of Biomaterials With Positrons

Abstract

Voids in biological membranes are vital to the diffusion characteristics of the membranes and as such information gained from e.g. void sizes elucidates the understanding of the functionality of the membrane. In this thesis the focus is on lipid bilayers, a type of membrane that exists in almost all organisms as a diffusion barrier surrounding the cells. The lipid bilayer consists of hydrophilic and hydrophobic parts that can form the bilayer structure in aqueous solution and the structural properties of the inner core of the bilayer (aka. hydrocarbon tail area) have a strong effect on the diffusion of particles through the bilayer. One of the key elements in the bilayer structure is the distribution and size of the free volume pockets, voids. The size of the voids depend both on the lipid type, but as well as the structural parameters of the bilayer, so called phase behaviour of the bilayer structure. Positron annihilation lifetime spectroscopy is a widely used tool to characterize atomic-scale structural properties of both solid state and soft matter and as such the correlation with positron (or to be specific positronium, the bound state of positron and electron) lifetime and the size of voids in e.g. polymers has been observed. In this thesis positron annihilation lifetime spectroscopy has been applied to probe the voids in lipid bilayers in several manners: Firstly as a proof-of-concept-type of experiment reveals the feasibility of positron annihilation lifetime spectroscopy as a tool to detect subtle differences in the void sizes depending on the lipid concentration in aqueous solution, secondly it is found possible to characterize the phase transition temperature by observing the change of void sizes below and above the phase transition temperature. In addition, the effect of dopants, namely cholesterol, on the bilayer structure is studied as a function of cholesterol concentration. The feasibility of positron annihilation lifetime spectroscopy to study intact tissues in-situ is also presented in a study where the temperature-induced structural transition of mammalian lens is observed and traced down to the lipids the lens strongly consists of.

Structural, topographical, and mechanical characteristics of purified polyhydroxyoctanoate polymer

“…While AFM and FS have been widely used to characterize surfaces of biopolymers, PAS has not been utilized as frequently to determine the internal structure of different polyhydroxyalkanoates. PAS is a nondestructive spectroscopy technique used to study open volume defects, voids, cavities, and pores in different types of materials . The high selectivity and sensitivity of PAS create additional possibilities for materials research by employing positrons as probe particles .…”

“…The high selectivity and sensitivity of PAS create additional possibilities for materials research by employing positrons as probe particles . The technique allows for deep examination of a material structure without perturbing the arrangement or destroying the sample during a measurement . PAS is an appropriate method for studying the polymer's free volume regions .…”

“…PAS is a nondestructive spectroscopy technique used to study open volume defects, voids, cavities, and pores in different types of materials. 23,24 The high selectivity and sensitivity of PAS create additional possibilities for materials research by employing positrons as probe particles. 25 The technique allows for deep examination of a material structure without perturbing the arrangement or destroying the sample during a measurement.…”

“…25 The technique allows for deep examination of a material structure without perturbing the arrangement or destroying the sample during a measurement. 24,26 PAS is an appropriate method for studying the polymer's free volume regions. 27,28 The presence of free volumes plays a crucial role in determining the macroscopic properties of polymers, namely density and thermal and mechanical properties.…”