Böhm, P.-H. (Corresponding author)

2018

Abstract: Cell adhesion is a fascinating albeit very complex process. It ismediated by the perhaps most abundant self-assembling molecularsystem in nature: the highly ordered, heterogeneous and immenselydiverse cell membrane. Lipids are the main constituents of the cellmembrane. The hydrophobic interactions between them and thesurrounding are the main driving force for the self-assembly. Thecell adhesion process and the structure and functionality of the cellmembrane are a prerequisite for life. A deeper understanding is ofhigh biological and pharmaceutical relevance. In the scope of thisthesis, the interface between solid substrates and membranes wasstudied with X-ray and neutron reflectometry.Resolving the lipid structure on graphene is a prerequisite forfuture applications of lipid decorated solution-gated graphene fieldeffecttransistor based biosensors. X-ray reflectometry measurementson lipid coated graphene were performed on a custom-builtmolybdenum-anode-based in-house reflectometer. It wasreconstructed during this thesis to meet the requirements regardingmeasurement time and signal-to-noise ratio. An around 20Å thicklipid monolayer formation was revealed. The interpretation of theresults from an electrical characterization of the lipid graphene interfaceand lipid spreading experiments correspond to the results ofthe reflectometry experiments. Solution-gated graphene field-effecttransistors were applied to monitor the formation of lipid layers andto investigate the influence of differently charged lipid layers. Insitumonitoring of the formation of lipid layers in real time wasdemonstrated.Cell adhesion on a silicon surface has been studied in neutron andX-ray reflectivity experiments. The neutron reflectometry experimentswere conducted at FRM II. Sample chambers were developed,which provide the required sterile and temperature-stable sampleenvironment for performing in-vitro reflectometry experiments oncell layers. Furthermore they allow for an exchange of the cell mediumhence ensuring the necessary medium exchange for cell culturing andcontrast variation. Neutron reflectivity experiments withconfluent layers of epithelial cells adhering to silicon in cell mediumwith different scattering contrasts were performed. This contrastvariation reduces the ambiguity of modelling the neutron data. Theaverage thickness and degree of hydration of the interface betweenthe adherent cells and the substrate were determined. A three-layeredinterfacial organization is suggested, consisting of a very dense70 - 120Å thick protein film bound to the silicon surface interface,followed by a highly hydrated, 180 - 280Å thick layer. The third,several hundred Å thick layer is attributed to the less hydratedcomposite membrane. This layer structure is in agreement with thetheory of cellular adhesion and immobile receptors. It is furthersuggested that the bottom dense protein layer is formed by the cells togenerate a bioanalogue tissue film. Additionally, X-rayreflectometry measurements were performed. The measurements didnot provide more insight, what is attributed to beam damage in thesample. Nevertheless, the prerequisites for continuative measurementswere created.

Keyword(s): Health and Life (1st) ; Biology (2nd)

1. REFSANS (REFSANS)
2. MARIA (MARIA)

1. REFSANS: Horizontal TOF Reflectometer with GISANS option (NL2b)
2. MARIA: Magnetic reflectometer with high incident angle (NL5N)