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Viscoelasticity and Structure of Soft Biological Interfaces: From Artificial Models to Living Tissues

Veschgini, Mariam

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Abstract

The primary aim of this thesis is to shed a quantitative light on the mechanics of dynamic biological interfaces with different levels of structural complexities, ranging from lung surfactant models to regenerating tissues. In chapter 3, the correlation between biophysical properties and function of the native extracellular matrix (ECM), mesoglea, of the freshwater polyp Hydra was studied. In the body design of Hydra, mesoglea acts as an interlayer between external (ectodermal) and internal (endodermal) cell layers, sustaining the mechanical integrity of polyps. In this study, nano-focused grazing incidence small angle X-ray scattering on isolated mesoglea revealed that the packing order of Hydra collagen type I was comparable to its vertebrate homologue. The structure was anisotropic with respect to the oral-aboral axis, supporting the extensive extension and contractions of the body along this axis. In the next step, the spatio-temporal evolution of mesoglea mechanics was tracked ex vivo by nano-indentation using an atomic force microscope. The experimental data demonstrated that freshly detached polyps initially had a uniformly soft mesoglea, but mesoglea changed the characteristic "elasticity patterns" during the asexual reproduction. This change could be explained by a quantitative proteome analysis, implying that the mechanical remodeling of Hydra was highly correlated with protease expression activity. When the body column tissue was transformed into head tissue either by a drug or by the over-expression of β-catenin, mesoglea had low elastic moduli over the whole body. This result suggests that the spatio-temporal patterns in mesoglea mechanics is strongly correlated with the stem cell activity. In chapter 4 a highly sensitive two-fingered micro-robotic hand was used to determine the viscoelastic properties of Hydra tissue fragments (regenerates) during early stages of regeneration. Owing to the dexterous grasping motion of microobjects realized by the micro-robot, the bulk elastic modulus of Hydra regenerates could be determined by linearly compressing the tissue by keeping the strain level low. Under a constant strain, the stress relaxation behavior could be interpreted by applying the Maxwell model of viscoelastic materials, yielding the Stokes frictional coefficient and viscous modulus. Furthermore, the forces actively generated by the regenerate were measured and shown to correlate well with shape fluctuations of a freely regenerating sample. In chapter 5, lung surfactant inactivation by serum proteins during the acute respiratory distress syndrome (ARDS) was simulated. As the model of dynamic, oscillating interfaces in lung, the competitive adsorption of dipalmitoylphosphatidylcholine (DPPC) and bovine serum albumin (BSA) to the air/water interface was monitored by periodically changing the surface area. The model was used to investigate the impact of perfluorohexane (PFH) as a potential therapeutics. The lipid-protein composite films at the air/water interface in the presence and absence of PFH gas could be visualized by fluorescence microscopy, indicating an accelerated displacement of a pre-adsorbed BSA by DPPC in saturated PFH atmosphere. The acceleration of BSA-DPPC replacement under PFH atmosphere was accompanied by significant changes in viscoelasticity of the interface, suggesting the incorporation of PFH to the protein layer.

Item Type: Dissertation
Supervisor: Tanaka, Prof. Dr. Motomu
Date of thesis defense: 16 December 2016
Date Deposited: 13 Jan 2017 10:24
Date: 2016
Faculties / Institutes: Fakultät für Chemie und Geowissenschaften > Institute of Physical Chemistry
Subjects: 500 Natural sciences and mathematics
540 Chemistry and allied sciences
570 Life sciences
590 Zoological sciences
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