Biology Reference
In-Depth Information
curves contain enough points for data interpretation, and the max-
imal deformation should be chosen such that the indentation is not
too invasive for the cells, especially if the experiments have to be
repeated (time lapse study). Loading and unloading curves are
often different and show hysteresis, which can be due to viscoelas-
tic effects and/or adhesion between the tip and the sample. If load-
ing and unloading cycles are repeated, a drift in the force- indentation
curves may be due to creep (irreversible, time-dependent deforma-
tion), viscous effects, or sample movement (Fig. 3c ). Overlapping
curves, on the other hand, indicate reversible (elastic) deforma-
tions and proper stabilization of the sample (Fig. 3a ).
During data analysis, the Z position is fi rst corrected to account
for sensor defl ection under load, using the stiffness value obtained
by measuring on glass or metal [ 15 ]. Further analysis of the raw
force-indentation curves will depend on the experiment purpose,
and different methods can be applied. The contact point can be
detected based on a force or stiffness criterion or by fi tting the
entire force-indentation curve. Each method relies on the fact that
the force-indentation curve presents two different regimes, before
and after the contact point. Typically, values of force and stiffness
are low before contact and present a nearly linear increase of force
with indentation (in the order of 10 −2 -10 −1 N/m) which is due to
surface tension of the medium. After contact, the force rises sharply
with indentation depth, often in a nonlinear fashion. The contact
point can be poorly detected in very soft samples which do not
largely differ in stiffness compared to values measured in liquid
alone. Once the contact point has been detected, force-indentation
curves can be used to extract an apparent stiffness or fi t model
curves. The apparent stiffness is useful for qualitative comparison
between cells. The stiffness can be extracted as the tangent of force-
indentation curves at a particular force or indentation depth [ 15 ].
Note that in that case no particular assumption is made on the exact
shape of the force-indentation curves. In quantitative studies, the
experimental data are fi tted to simulated force-indentation curves
obtained with a mechanical model [ 4 , 5 , 7 , 8 , 16 - 18 ]. Unique
solutions for the cell wall elastic modulus or the turgor pressure can
be obtained only if enough model parameters are known, such as
the cell wall thickness, cell geometry, initial stretch ratio, and
Poisson's ratio. Depending on the timescale of indentation, it may
also be required to consider cell wall permeability to water [ 4 , 7 ].
2
Materials
The actuator robot is a modular system composed of three linear
micro-positioners (SmarAct GmbH, SLC-1780) and controller
(SmarAct GmbH, MCS-3D). The controller has knobs for the
manual positioning prior to experiments and a USB interface that
2.1 CFM Setup
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