Biology Reference
In-Depth Information
elasticity [ 9 - 11 ]. Micro-indentation techniques are situated in
between, measuring both the effects of turgor pressure, cell wall
in-plane elasticity, and local compression of the cell wall [ 12 - 19 ].
The cell geometry and tip shape also have an infl uence on cell
deformation and the force measured. Mechanical models are
needed in order to interpret the data and untangle the contribution
of different factors on the force-indentation curves [ 13 , 15 - 18 ].
Cellular force microscopy (CFM) has been designed for force-
indentation measurements on living plant tissues [ 15 , 16 , 20 ].
CFM can be used to measure a large range of forces (sub to hun-
dreds of microNewtons), can make large displacements (up to cen-
timeters), and is well adapted to the high forces involved in the
measurement of stiffness on turgid plant cells. The CFM can also
be used as a plant microsurgery device by applying forces suffi cient
to puncture the cell wall. The robot consists of a capacitive force
sensor mounted on a 3-axis piezo positioner. The system is auto-
mated and offers a high resolution in position (nanometer scale)
and force (sub microNewton). CFM can be used to scan the sam-
ple surface and extract its shape, as well as measure stiffness from
the analysis of force-indentation curves (Fig. 1 ). The setup can
easily be integrated with light microscopy for simultaneous imag-
ing during the measurements. Large probe length (1-2 mm) allows
measurement in liquids and increases probe access on curved sam-
ples. The robot can also be used to mechanically stimulate the cells
by applying a controlled force or deformation for a certain period
of time. Here we will focus on stiffness measurements in single
cells and tissues in liquid.
During a typical measurement with CFM, the sample surface
position is fi rst detected, and then a force-indentation curve is
acquired for the stiffness measurement. CFM offers two possibili-
ties for detection of sample surface: it may be based on a change in
force or stiffness. The force criterion assumes that the force is close
to zero before touching the sample and rises only if there is contact
with the surface. For measurements in liquid, however, the surface
tension of water creates a force on the sensor tip, up to several
microNewtons. In this case, a stiffness threshold can be used, with
the stiffness coming from surface tension usually in the range of
0.01-0.1 N/m. This mode of surface detection can also be used to
avoid issues due to sensor voltage drift caused by light, tempera-
ture, and other environmental infl uences. Once the surface has
been detected, the offset on the force before contact (e.g., due to
liquid surface tension) can be computed and used to correct the
force readout. Force-indentation data is then acquired by slowly
indenting the probe into the sample until a predefi ned force thresh-
old or indentation depth is reached.
The robotic positioner has two modes of actuation, stepping
mode and scanning mode. Stepping mode involves a stick-slip
movement and is used for large displacements, but it is jittery due to
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