From nanoscale assemblies to biomimetic membrane devices

Abstract

Water is essential for the existence of life on Earth, and access to fresh water resources is a fundamental need for human to survive. Unfortunately, only ~0.77 % of the total amount of water on the planet is available as a fresh water resource. On top of that, these fresh-water resources are under stress, induced by increased water usage, climate change, resource mismanagement and contamination from human activity. As a consequence, maintenance of fresh water resources with use of water treatment technologies is a necessity. A technology that is most promising to provide high water quality in combination with high energy efficiency and scalability of the processes with a low footprint, is the membrane technology. In 1970’s, a full-scale application of membrane technology was started, and since then it faces continuous material and process development. Nature is one of the most recent inspirations for improving the transport properties of membrane materials. Researchers are particularly intrigued by the way water is transported through the natural bilayer membranes – via the selective transmembrane protein channels – aquaporins. Synthetic membranes doped with orthodox water channel proteins Aquaporin Z (AqpZ) are repeatedly reported to obtain improved water permeability without compromising on the rejection of solutes. However, preparation of such biomimetic membranes is not a trivial task. Firstly, purified transmembrane proteins need to be reconstituted within the artificial environment that mimics the natural cell plasma membrane, such as a lipid or polymer bilayer. The role of the artificial bilayer is to provide stability, while maintaining the activity of the protein. Secondly, such artificial protein host has to be integrated within the membrane active layer (AL), most commonly made of polyamide. This work evaluates step-by-step biomimetic membrane design and is based on the three studies – presented in Chapters 3-5. Chapter 3 describes preparation of lipid and polymer-based nanoscale assemblies and their detailed characterization by batch dynamic light scattering (DLS), asymmetric flow field flow fractionation coupled with multiangle light scattering and on-line dynamic light scattering (AF4-MALS/DLS), and cryogenic transmission electron microscopy (cryoTEM). The study also evaluates and explains the limitations of batch DLS, repeatedly used for characterization of the self-assemblies in literature. Moreover, it is discussed how a stand-alone use of batch DLS can lead to inaccurate conclusions. AF4-MALS/DLS is proposed as a main analytical method for evaluation of nano assemblies’ morphology, concentration and size distribution, and at the same time providing high analytical throughput, which is necessary in the industrial process of developing a stable and optimal vesicle system. Chapter 4 focuses on the evaluation of the stability of the poly(ethylene glycol)-block-poly(ε-caprolactone) (PEG-PCL) based polymersomes exposed to three different detergents. Specifically, changes in morphology, particle size distribution and concentrations of the PEG-PCL based polymersomes, were evaluated during the titration of the detergents to the polymersome solution. These changes were assessed by optical density (OD) measurements, DLS, cryoTEM and tunable resistive pulse sensing (TRPS). The work discusses the effect of detergent features on solubilization of polymeric bilayer and compares it to the results reported in the literature for liposomes and polymersomes. The knowledge obtained from this study can be used for tuning the properties of PEG-PCL polymersomes to use for applications such as medicine (drug delivery) or protein reconstitution studies. Chapter 5 describes scalable preparation of poly(methyloxazoline)-block-poly(dimethysiloxane) (PMOXA-PDMS) polymersomes with and without AqpZ protein. Furthermore, the study shows how these polymersomes can be integrated into AL of the membrane and improve the AL separation performance, using scalable interfacial polymerization (IP) between m-phenylenediamine (MPD) and trimesoyl chloride (TMC). The study includes analysis of polymersomes size distribution with use of DLS and nanoparticle tracking analysis (NTA), and polymersomes permeability with use of stopped flow light scattering (SFLS). The membrane performance was analyzed in low pressure reverse osmosis (LPRO). The effect of polymersomes integration into the AL on the final membrane properties, such as morphology, elemental composition and surface charge was also investigated by scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS) and electrokinetic analyzer (EA), respectively.