Plenarvorträge - DPG-Tagungen

Arbeitskreis Biologische Physik Freitag
tified by statistically distributed or alternating gold and silver islands. A
microscope with ATR capability allows to evaluated both, the coordinate
system and the diffusion of molecular probes in cells.
AKB 50.34 Fr 10:30 B
Elastic interactions of cells with soft materials — •Ilka Bischofs
and Ulrich Schwarz — Max Planck Institut für Kolloid- und Grenzflächenforschung,
Theorie, 14424 Potsdam
Mechanically active cells sense the mechanical properties of their environment
and respond by strenghtening their cytoskeleton in the direction
of maximal effective stiffness. Rigidity gradients, prestrain induced
by external forces, traction by other cells, and the presence of sample
boundaries locally alter the effective stiffness encountered by contractile
cells. We model cells as anisotropic force contraction dipoles and use linear
elasticity theory to calculate optimal cell organization in soft media.
Our predictions for single cells agree nicely with experiments for fibroblasts:
cells orient toward clamped and away from free surfaces and along
the direction of tensile strain. The elastic interaction between cells shows
a similar angular dependence as electrical dipoles. At low cell density,
this leads to strings of cells, while at high cell density, branching into
ring-like structures occurs.
AKB 50.35 Fr 10:30 B
Active forces of motile cells — •Claudia Brunner, Michael
Gögler, Allen Ehrlicher, and Josef Käs — a
Cellular motility is a ubiquitous component of prokaryotic and eukaryotic
cells, spanning varied functions from the immune system, to the
developing brain, to the grave invasiveness of cancer.
Various individual components, such as molecular motors and actin
filament polymerization have been explored to explain cell movement,
but the cell as a whole system is not well understood. Experiments on
living cells are necessary to understand the properties and abilities of
their polymer networks functioning together as one system.
The atomic force microscope is an excellent tool to determine the
mechanical forces which are responsible for cell movement. With a
polystyrene bead glued on a commercial AFM-tip, living cells can be
safely measured. We have probed fast moving keratocytes and directly
measured the cell extension forces, allowing us to compare extension
forces with the cells velocities and other properties.
AKB 50.36 Fr 10:30 B
Functionalization and Passivation of a Silicon-on-Insulator
based Sensor Device — •Petra A. Neff, Michael G. Nikolaides,
Stefan Rauschenbach, Simon Q. Lud, and Andreas
R. Bausch — Lehrstuhl für Biophysik - E22, Technische Universität
München, 85747 Garching
The sensitive and specific detection of biomolecular interactions relies
on the functionalization and passivation of the detecting surfaces.
Recently, a new Silicon-on-Insulator (SOI) based thin film resistor device
for chemical and biological sensor applications was introduced. It
has been shown that this sensor is highly sensitive to variations of the
surface potential evoked by the adsorption of small amounts of charged
molecules. Whereas most of the conventional detection techniques require
the labeling of molecules, the SOI based sensor allows a label-free
detection of interactions.
Here we discuss the passivation and functionalization of the bare silicon
oxide surface by physical adsorption of polyelectrolytes and the covalent
binding of silanes. Results of the adsorption of PAH/PSS multilayers are
presented and their interpretation will be discussed. First results of the
application of the sensor device towards the label-free detection of DNA
hybridization will be presented.
AKB 50.37 Fr 10:30 B
Activation of Integrin Function by Nanopatterned Adhesive Interfaces
— •Marco Arnold 1 , Elisabetta Ada Cavalcanti 1 ,
Roman Glass 1 , Jacques Blümmel 1 , Wolfgang Eck 2 , Martin
Kantlehner 3 , Horst Kessler 3 , and Joachim P. Spatz 1
— 1 University of Heidelberg, Institute for Physical Chemistry, Biophysical
Chemistry, INF 253, D-69120 Heidelberg — 2 University of Heidelberg,
Institute for Physical Chemistry, Applied Physical Chemistry, INF
253, D-69120 Heidelberg — 3 Technical University of Munich, Institute
for Organic Chemistry and Biochemistry, Lichtenbergstrasse 4, D-85747
Garching
To study the function behind molecular arrangement of single integrins
in cell adhesion, we designed a hexagonally closepacked rigid template of
cell adhesive gold nanodots by lithographic means of diblock copolymer
selfassembly. The diameter of the adhesive dots are smaller than 8nm,
which allows the binding of one integrin per dot. These dots are positioned
with high precision at 28, 58, 73 and 85nm spacing at interfaces.
A separation of more than 73nm between the dots results in limited cell
attachment and spreading and reduces the formation of focal adhesion
and actin stress fibers. We attribute these cellular responses to restricted
integrin clustering rather than insufficient number of ligand molecules
in cellmatrix interface since micronanopatterned substrates consisting of
alternating fields with dense and no nanodots support cell adhesion. We
propose that the range between 58 and 73nm is a universal length scale
for integrin clustering and activation.
AKB 50.38 Fr 10:30 B
Coherent Scatter Computed Tomography — •Johannes Delfs 1 ,
Jens-Peter Schlomka 1 , and Robert L. Johnson 2 — 1 Philips Research
Laboratories Hamburg, Röntgenstrasse 24-26, 22335 Hamburg,
Germany — 2 Institut für Experimentalphysik, Universität Hamburg, Luruper
Chaussee 149, 22761 Hamburg, Germany
The dominant component of low-angle scatter in the energy regime
of diagnostic radiology ( 60 keV) is coherent scatter. This leads to the
phenomenon of x-ray diffraction, which is widely used for determining
atomic structure in material science.
A reconstructive tomographic imaging technique (”Coherent Scatter
Computed Tomography (CSCT) ”) is presented for the spatially-resolved
measurement of the coherent scatter cross-section in extended objects.
These diffraction patterns supplement conventional CT images in material
discrimination.The technique is based on the energy analysis, at
known angle, of coherent x-ray scatter by polychromatic radiation employing
a novel energy-resolving CdZnTe detector array. Images of test
objects are shown to illustrate the feasibility of the technique and potential
applications in medicine and industrial material inspection.
AKB 50.39 Fr 10:30 B
An individual-based Model to Tumor Growth in Vitro —
•Drasdo Dirk 1 and Hoehme Stefan 2 — 1 MPI fuer Mathematik in
den Naturwissenschaften, Inselstr. 22-26,Leipzig — 2 Interdisciplinary
Inst. for Bioinformatics, Kreuzstr. 7b, Leipzig
We present a mathematical model to study the spatio-temporal growth
dynamics of two-dimensional tumor monolayers and three-dimensional
tumor spheroids as a complementary tool to in-vitro experiments. Within
our model each cell is represented as an individual object and parameterized
by cell-biophysical and cell-kinetic parameters that can all be experimentally
determined. Hence our modeling strategy in principle provides
a link between mechanisms on the microscopic level of individual cells
and the macroscopic properties of a growing tumor. We find a remarkable
robustness of the growth kinetics and patterns in early growth stages
although their mechanistic origin can be different. Quantitative comparisons
of computer simulations with our model to published experimental
observations on monolayer cultures suggest a biomechanically-mediated
form of growth inhibition during the experimentally observed transition
from exponential to sub-exponential growth at sufficiently large tumor
sizes. Our simulations show that for avascular tumor spheroids this transition
can usually be explained by a nutrient or oxygen depletion, and
predict that it becomes biomechanically triggered at a sufficiently large
nutrient and oxygen supply.
AKB 50.40 Fr 10:30 B
Dynamics of Tension in Semiflexible Polymers — •Oskar Hallatschek
1 , Erwin Frey 1,2 , and Klaus Kroy 1 — 1 Abteilung Theorie,
Hahn-Meitner Institut, Glienicker Str. 100, 14109 Berlin, Germany
— 2 Fachbereich Physik, Freie Universität,14195 Berlin, Germany
We present a general discussion of non-equilibrium tension propagation
through a semiflexible polymer in various situations of experimental
interest. Typical situations include the response of the polymer’s conformation
to a given longitudinal force at one end, or the relaxation of the
polymer after it has been equilibrated under a certain applied tension.
In both cases the propagation of tension exhibits power law behavior,
but with distinct dynamic exponents. Whereas the initial exponent always
equals 1/8, the exponent for late time propagation depends on the
experimental situation. Based on a set of coarse-grained equations of motion
we give a unified theoretical explanation of the various scenarios. In
addition, we discuss the behavior of end grafted chains in an external
field, and analyze the problem of a polymer that undergoes a sudden
temperature change.