Biomedical Engineering Reference
In-Depth Information
phosphate buffered saline at 22°C, at least 30 min will be required for the
immersed cantilever to achieve a stable deflection signal (position of
the reflected laser on the photodiode). Prior to that stabilization time, the
difference in temperature between the air and imaging fluid, as well as
any laser-heating of the imaging fluid, will cause the cantilever
dimensions and position to fluctuate. Ideally, the imaging fluid will be
placed in a sealed container within the AFM overnight to stabilize the
imaging fluid temperature and, during the experiment, fluid will be
added as required to minimize changes in salinity and pH that will affect
both cantilever position stability and sample degradation. To confirm the
extent and stabilization of signal drift, one can either monitor the position
of the reflected laser within the photodiode and its reported voltage, or
equivalently record the photodiode output voltage over time until a
threshold drift rate is attained. This threshold drift rate should be
considerably less than the displacements or depths expected in
mechanical analysis of the biomaterial sample.
3.2.2. Controlled gaseous atmosphere
For some commercial AFMs, it is possible to define the gaseous
environment around the AFM stage and transducers with gasketed gas
inlets. More typically, it is sufficient to control the %CO 2 and pH
required for cell survival and tissue stability via use of buffered aqueous
solvents such as HEPES or Hank's buffered saline. Whenever solvents
are used to fill the instrument or sample chamber, including gases, it is
important to confirm compatibility with the instrument materials. One
common point of concern for AFM-enabled indentation is chemical
stability of the AFM cantilever mount, which braces the chip on which
cantilevers are fabricated and also intentionally traps fluid between the
cantilever and biomaterial sample.
3.2.3. Fluid cells for hydrated biological materials
Several types of fluid cells, or confined chambers that maintain hydration
of the biomaterial sample, exist and are proprietary to each AFM. These
range from as simple as cantilever mounts fabricated from hydrophobic
Search WWH ::




Custom Search