Doctoral Dissertations

Date of Award

5-2016

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Biomedical Engineering

Major Professor

Stephen A. Sarles

Committee Members

Eric Boder, Paul Dalhaimer, C. Patrick Collier, Francisco Barrera

Abstract

This work describes research aimed at improving the droplet interface bilayer (DIB) platform for creating and characterizing biologically relevant model cell membranes. Improvements are made possible in part through the development of a portable, compact platform for controlling temperature with DIBs. Feedback-controlled heating allows studies to be conducted across a range of temperatures, from ambient up to at least 80°C, and also provides new understanding of methods to form DIBs using mixtures of total lipids extracted from bacterial and eukaryotic cells. The membranes formed from total lipid extracts (TLE) are introduced along with evidence that model membranes formed using lipid compositions mimicking natural biological membranes (here using total lipids extracted from Escherichia coli and from porcine brain) behave significantly different than simple single-component membranes. Results provided herein indicate TLE DIBs display high sensitivity to antimicrobial peptide (alamethicin) insertion at room temperature which is explained by evidence of thermotropic phase behavior not encountered with single-lipid DIBs. These results highlight the importance of considering lipid composition when using lipid bilayers as models of cell membranes, and new techniques are described that facilitate the study of composition-dependent phase transitions in model membranes.

The proposed research is also aimed at developing new non-ionic methods for characterizing membranes and the interactions of membrane-active molecules that do not necessarily affect membrane conductance and are thus difficult to study. A new tool is developed that utilizes the Young-Lippmann and Young-Dupre relations to enable measurement of membrane specific capacitance and surface tension in a single DIB experiment. The method is introduced, applied, and validated through studies of DPhPC DIBs incorporating various amounts of cholesterol and solvent molecules. The new method is also applied to provide insight toward the interactions of cell-penetrating mixed-monolayer protected nanoparticles in lipid bilayers. Lastly, the method is proven useful for tracking transitions in monolayer and bilayer surface tension that result from thermotropic phase transitions in total lipid extract DIBs.

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