Browsing by Subject "stability"

Liposomes are nano-sized vesicles in which the aqueous phase is surrounded by lipid-derived bilayer. They are excellent drug vehicles for example in ocular drug delivery because they can, among other things, increase the bioavailability and stability of the drug molecules and reduce their toxicity. Liposomes are known to be safe to use, because they degrade within a certain period of time and they are biocompatible with the cells and tissues of the body. Owing to its structure, the surface of liposomes can also be easily modified and functionalized. Light-activated ICG liposomes allow drug release in a controlled manner at a given time and specific site. Their function is based on a small molecule called indocyanine green (ICG) which, after being exposed to laser light, absorbs light energy and thereby locally elevates the temperature of the lipid bilayer. As a result, the drug inside is released into the surroundings. The blood circulation time of liposomes has often been prolonged by coating the liposomes with polyethylene glycol (PEG). Although PEG is generally regarded as a safe and biocompatible polymer, it has been found to increase immunological reactions and PEG-specific antibodies upon repeated dosing. Conversely, hyaluronic acid (HA), is an endogenous polysaccharide, which is present in abundance for instance in vitreous. Thus, it could serve as a stealth coating material which extends the otherwise short half-life of liposomes. One of the main objectives of this thesis was to find out whether HA could be used to coat liposomes instead of PEG. In order to prepare HA-coated liposomes, one of the lipid bilayer phospholipids, DSPE, had to be first conjugated with HA. For the conjugation, potential synthesis protocols were sought from the literature. Ultimately two different reductive amination-based protocols were tested. Consequently, the protocol in which the conjugation was achieved via the aldehyde group of HA, proved to be working. Thereafter, HA-coated liposomes were prepared by thin film hydration from the newly synthesised conjugate as well as DPPC, DSPC and 18:0 Lyso PC. Calcein was encapsulated in the liposomes. HA-covered liposomes were then compared with uncoated and PEGylated liposomes by examining their phase transition temperatures, ICG absorbances, sizes, polydispersities, and both light and heat-induced drug releases. The aforementioned tests were also conducted when the effects of the HA and ICG doubling were examined and the possibility to manufacture HA liposomes with small size was assessed. HA-liposomes showed similar results as PEG-coated liposomes. In addition, successful extrusion of HA-liposomes through a 30 nm membrane was also demonstrated in the results. Doubling of HA did not significantly affect the results. In contrast, increasing the molar amount of ICG by double caused spontaneous calcein leakage even before any heat or light exposure. Based on these findings, HA could work as a coating material instead of PEG, yet further studies are required for ensuring this conclusion. The other key objective was to evaluate the stability of four different formulations, named as AL, AL18, AL16 and AL14, in storage and biological conditions. Based on the differences in the formulation phospholipid composition, the assumption was that AL would be the most stable of the group and that the stability would decrease so that AL18 and AL16 would be the next most stable and eventually AL14 would be the least stable formulation. As in the previous study, the liposomes were prepared by thin film hydration with calcein being encapsulated inside the liposomes. In the storage stability test, liposomes were stored in HEPES buffer at either 4 °C or at room temperature for one month. In the test conducted in physiological conditions, the liposomes were added either to porcine vitreous or fetal bovine serum (FBS) and the samples were incubated at 37 ºC for five days. Regardless of the experiment, phase transition temperatures as well as light and heat-induced drug releases were initially measured. As the test progressed, calcein release, ICG absorbance, size, and polydispersity were measured at each time point. The initial measurements confirmed the hypothesis about the stability differences of tested formulations. In the storage stability test, all formulations, except AL14, appeared to be stable throughout the study and no apparent differences between the formulations or temperatures were observed. On the other hand, the stability of liposomes stored in biological matrices varied so that the liposomes were more stable in vitreous than in FBS and the stability decreased in both media as expected.

This thesis examines the typology and stability of evidentiality in contact settings. The goal is to estimate the stability of evidentiality in different contact scenarios, and to find whether evidential structures are more likely to change in language contact situations than to remain stable. Furthermore, this study aims to develop methodology to approach the typology of evidentiality to examine its contact effects in the first place. Earlier research has described evidentiality as an unstable feature that diffuses easily in contact situations, but systematic research examining evidentiality in multiple contact settings and its stability in contact is yet lacking. Moreover, evidentiality has been studied widely, but there is no previous typological approach on evidentiality or on its contact effects that would be suitable for the purposes of this thesis. This study takes a typological approach to the study of evidentiality and language contact. The examination of contact effects is based on six sampling units of three-language sets across the globe, wherein contact effects are estimated on an external benchmark. The collected linguistic data from the sampling units was analyzed into logical outcomes of contact and turned into probability distributions. This finally resulted in the aggregated probability of convergence. The probability of convergence is contrasted to the stability of evidentiality and in that continuum this study estimates how likely it is that evidentiality has been affected due to contact. Furthermore, this thesis focuses on finding a suitable way to approach the first research goal and therefore presents a typological approach on evidentiality and defines grammatical evidentiality. The primary results of this study suggest high probability of convergence and evidentiality seems to be an unstable linguistic domain that diffuses easily. These findings were further contrasted to some other linguistic domains indicating that evidentiality is among the most unstable domains. This study also suggests that the semantic properties of evidentiality are more unstable than the morphological ones. The findings also highlight the sensitivity of the methodology, and these limitations are demonstrated and reflected upon.