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01/07/2016

12.00 pm, Seminar Room on the 1st Floor

Some Surprises and Open Questions in Soft and Particulate Matter

Prof. Steve Granick (IBS Center for Soft and Living Matter, South Korea)

A fundamental challenge of modern soft matter science is to form structure that is not frozen in place but instead reconfigures internally driven by energy throughput and adapts to its environment robustly. Predicated on fluorescence imaging at the single-particle level, this talk describes quantitative studies of how this can happen. With Janus colloidal clusters, we show the powerful role of synchronized motion in self-assembly.

more information in PDF format (112.86 Kb)

08/07/2016

12.00 pm, Seminar Room on the 1st Floor

To Be Announced

Dr. Daniel Ruiz

15/07/2016

12.00 pm, Seminar Room on the 1st Floor

To Be Announced

Dr. Ivan Coluzza (University of Vienna, Austria)

19/07/2016

12.00 pm, Seminar Room on the 1st Floor

Physical principles underlying structure, mechanics and dynamic re-organization of hyaluronan-rich matrices — from tissues to supramolecular models in experiment and theory

Xinyue Chen

The extracellular matrix (ECM) is the acellular structure of all tissues and essential for multicellular life. Next to biochemical signals, the physical properties of the ECM provide important signals to cells. The polysaccharide hyaluronan (HA) is ubiquitous in the extracellular space of vertebrates and an important structural component of the ECM. HA is a linear, unbranched and regular polymer of the glycosaminoglycan (GAG) family, and serves as a scaffold that responds dynamically to molecular stimuli such as HA-binding proteins, or changes in pH or ionic strength. The objective of this PhD research project is to elucidate physical principles underlying the structure, mechanics and dynamic re-organization of HA-rich matrices. To address this question, we have studied the mechanical properties and morphology of HA-rich matrices at distinct levels of complexity. On one hand, we have studied the cumulus cell-oocyte complex (COC) matrix as an example of complex, native HA-rich tissue. On the other hand, we have studied so-called HA brushes as a well-defined in vitro reconstituted model of HA-rich matrices. The HA-rich matrix in the COC forms around oocytes just before ovulation and plays vital roles in oocyte biology. We have analyzed the micromechanical response of mouse COC matrix by colloidal probe atomic force microscopy (AFM). We found the COC matrix to be extremely soft yet elastic, suggesting a stable gel-like network structure with high porosity and a large mesh size. With a Young's modulus around 1 Pa, COC matrices are among the softest elastic biological materials known to date. In addition, the elastic modulus increased progressively with indentation. Furthermore, using optical microscopy to correlate these mechanical properties with ultra-structure, we discovered that the COC is surrounded by a thick matrix shell that is essentially devoid of cumulus cells. We propose that the pronounced non-linear elastic behaviour of COC matrix is a consequence of structural heterogeneity and serves important functions in biological processes, such as oocyte transport in the oviduct and sperm penetration. To understand more comprehensively the response of HA polymers to changes in their aqueous environment, the thickness and viscoelastic properties of films of end-grafted HA (also called HA brushes) as a well defined in vitro model of HA-rich matrices were characterized by reflection interference contrast microscopy (RICM) and quartz crystal microbalance with dissipation monitoring (QCM-D) as a function of Ca2+ concentration and pH. Within the physiological range, the thickness of HA brushes decreased significantly with Ca2+ concentration but did not change with pH. By screening a large range of Ca2+ concentrations, we discovered that the effect of Ca2+ on HA brush thickness is virtually identical to the effect of Na+ at 10-fold higher concentrations. HA brushes responded only weakly to pH changes above pH 6.0, but showed a sharp collapse around pH 3. Our results provide insights into how HA matrices are affected by solution properties, which is relevant in biological systems and for the design of synthetic tissues. Finally, using theoretical computations based on self-consistent mean field theory, we elucidated how the morphology of polymer brushes is influenced by the formation of physical cross-links between polymers, such as they would occur in the presence of cross-linking proteins. We find that cross-links promote a denser and more homogeneous brush morphology. The effect of cross-links is comparable to the effect of reduced solvent quality when the density of cross-links is low, but unique features including the retention of solvent even with strong cross-links emerge at high cross-linking densities. This work provides novel insights into how the supramolecular structure and thus the mechanical properties of HA-rich matrices can be dynamically regulated by changes in microenvironmental conditions. This can be linked to different biological functions of native HA-rich extracellular matrices but is also of interest for the design of tailored, synthetic HA-based materials for applications in tissue engineering.