Development of Novel In Situ ESEM Techniques for the Study of Water Interaction with Soft Materials

Licentiate thesis, 2013

The transport of water is central to many applications of soft biomaterials; for example water management in personal care products, controlled drug release in pharmaceuticals and, on a fundamental level, the activities of living cells. Water transport through a material may occur in both liquid and gas phase, and is highly dependent on the microstructure. Understanding the relationship between structure and transport properties is the basis for predictive models and may aid the design of new functional materials. The environmental scanning electron microscope (ESEM) is a powerful tool when it comes to visualising the effects of hydration or dehydration of a specimen, facilitating the connection between microstructure and properties. However, there are still limitations when it comes to studying the transport of water through materials. The aim of this work was to develop new ESEM-based methods that enable the in situ study of water transport in a controlled manner. A new experimental platform was created based on the combination of a nanomanipulator and a solution for local cooling of a surface in the sample chamber of the ESEM. Two different applications were chosen to test and evaluate the new technique, and to demonstrate its possibilities. The first application was the in situ controlled wetting of individual cellulose fibres. The goal was to capture the interaction between a fibre and water through simultaneous manipulation and imaging. A cellulose fibre was brought in contact with a water droplet situated on the cooled surface in the ESEM using the nanomanipulator. For the first time, the transport of liquid water by an individual cellulose fibre was imaged. Repeated experiments with different fibres showed a marked variation in absorptive capacity. The second application was the study of the osmotic response of individual yeast cells, which is intimately connected with water transport across the cell membrane. The nanomanipulator was coupled with a piezoresistive AFM sensor to enable the measurement of forces with high accuracy and a temporal resolution in the millisecond range. The relative humidity around the cells was rapidly increased. This resulted in cell expansion, analogous to a hyposmotic shock. The expansion could be followed in real time and cell size changes in the nanometre range were recorded using the AFM sensor. This method is highly interesting as a tool for single cell characterisation with focus on cellular water transport. The results demonstrate the possibilities of the developed technique, but the flexibility of the setup allows the approach to be extended to other materials where the interaction with water and other fluids is of interest. Our work has advanced the field of in situ ESEM by enabling more controlled experiments with respect to liquid water absorption and transport as well as water vapour sorption and swelling.