206 6.2 Rheology and Hydrodynamics Tools
are important components of techniques that permit whole cells and tissues to be mechan
ically probed.
6.2 RHEOLOGY AND HYDRODYNAMICS TOOLS
Rheology is the study of matter flow, principally in a liquid state. For the investigation of
living matter, the liquid state is primarily concerned with water, namely, hydrodynamics
forces, especially those that operate primarily through viscous drag forces on biological
material, but is also concerned with the force response of fluid states of cellular structures.
For example, how lipid membranes in cells, which have many properties consistent with
those of liquid crystals, respond to external force and also how components inside the
cell membrane impart rheological forces on neighboring components. In this section, we
discuss a range of hydrodynamics force techniques used to study biological matter, as well
as rheological force methods for probing cellular liquid/soft-solid states. These include a
range of standard but invaluable tools (e.g., chromatography can arguably be considered
a rheological force method) and also methods that utilize centrifugation and osmosis to
characterize and/or isolate biological components. We also discussed techniques that
result in plastic/viscoelastic rheological deformation of biological soft matter in response
to applied forces.
6.2.1 CHROMATOGRAPHY TECHNIQUES
Chromatography is a standard biophysical tool used to separate components in an in vitro
biological sample on the basis of different molecular properties such as mass and charge. In
many biochemistry textbooks, this might not be considered in the context of being a “force
method”; however, it does rely on a range of cohesive forces in water in particular, and so we
discussed it here. Related methods include high-performance liquid chromatography, gas
chromatography, gel filtration, thin-layer chromatography, and even standard paper chro
matography. Molecular components bind to an immobile substrate to form a stationary
phase for a characteristic dwell time, dependent on the physical and chemical features nature
of the substrate. The mobile phase moves through the chromatography device via diffusion
often facilitated by a driving pressure gradient.
Sepharose beads (sepharose is the trade name of a type of polysaccharide sugar generic
ally called agarose, which is purified from seaweed and used for several purposes in experi
mental biology) of diameter ~40–400 μm are often used as the immobile substrate, tightly
packed into a glass column for gel filtration chromatography and related affinity chroma
tography that uses specific antibodies bound to the beads, or different bead surface charges
in ion-exchange chromatography. These factors, in addition to chemical parameters of the
media in the mobile phase such as pH and ionic strength, determine the dwell time in the
stationary phase, and thus the mean drift speed of each molecular component through
the device. The end result is to separate out different molecular components on the basis of
their relative binding strengths to the immobile substrate and of their mean speed of trans
location through the chromatography device, with emerging components often detected
using an optical absorption technique at a specific wavelength.
Size exclusion chromatography (SEC) is a chromatography method in which molecules in
solution are separated on the basis of their size, and/or molecular weight, usually applied to
large molecules such as proteins and nucleic acids. In SEC, a small molecule can penetrate
more regions of the stationary phase pore system compared to larger molecules and so will
have a slower drift speed, thus enabling larger and smaller molecules to emerge as different
fractions at the bottom of a gel filtration column.
Reversed phase chromatography uses an electrically polar aqueous mobile phase and
a hydrophobic stationary phase. Hydrophobic molecules preferentially adsorb to this sta
tionary phase, and thus hydrophilic molecules have a faster drift speed in the mobile phase