Chapter 6 – Forces  255

microscope stage, different levels of force can be applied to extend the molecules. It has much

of the capabilities of vertical magnetic tweezers though cannot apply torsion but is arguably

easier to configure in high-​throughput modes.

6.7  TOOLS TO MECHANICALLY PROBE CELLS AND TISSUES

Several tissue types exhibit a range of important mechanical properties. These can be

investigated using a range of biophysical biomechanical tools. Much research in this field has

involved study of muscle tissue in particular, but several types of connective and bone tissues

in animals have also been studied, as have mechanical forces relevant to plant tissues.

6.7.1  MECHANICAL STRETCH TECHNIQUES ON MUSCLE FIBERS AND MYOFIBRILS

A variety of mechanical stretching apparatus has been developed for various tissue samples,

most especially exemplified by bundles of muscle fibers to subject them to mechanical

stretching and subsequent relaxation. For example, by attaching controllable electrical motors

conjugated to the ends of muscle fiber bundles while subjecting the muscle fibers to different

biochemical stimuli to explore the onset of active muscle contraction. Active contraction

requires the hydrolysis of ATP through the interaction of myosin and actin protein filament

systems, as well as the maintenance of passive elasticity through other muscle filaments such

as titin already discussed in this chapter.

Stretched muscle fiber bundles can also be monitored using various optical diffraction

techniques. Muscle fibers have several structural features that are spatially highly periodic,

which therefore can act as diffraction gratings for appropriate incident wavelengths of elec­

tromagnetic radiation. Visible light laser diffraction through fiber bundles can be used to

estimate the dynamic change in length of the sarcomere, the repeating structural subunit

of myofibrils from which muscle fibers are assembled. Fluorescence microscopy can also

be combined with myofibril stretching to indicate the change in position to specific parts of

filamentous molecules, for example, using fluorescently labeled antibodies that target spe­

cific locations in the giant muscle molecule titin, to explore the relative elasticity of different

regions of the titin molecule.

X-​ray diffraction (see Chapter 5) can also be used on muscle fiber bundles to investi­

gate smaller molecular length scale changes to the protein architecture during muscle con­

traction. For example, using both small-​angle x-​ray scattering to explore large length scale

changes to the sarcomere unit and higher-​angle diffraction investigates more subtle changes

to the binding of myosin to action. This has contributed to a very detailed knowledge of the

operation of molecular motors, which is now being complemented by a range of cutting-​edge

single-​molecule methods such as optical tweezers.

6.7.2  MECHANICAL STRESS TECHNIQUES ON NONMUSCLE TISSUES

Developing bone tissue has also been investigated using similar mechanical stretch appar­

atus, as has connective tissue (the tissue that connects/​separates different types of tissues/​

organs in the body), and epithelial tissue (the tissue that typically lines surface structures in

the body), including skin. Stretch-​release experiments on such tissues can also generate bulk

tissue mechanical parameters such as the Young’s modulus, which can be linked back to

biological structural details mathematical modeling such as discretized finite element ana­

lysis and biopolymer physics mesoscale modeling approaches (see Chapter 8). Other forms of

continuum mathematical modeling of elasticity, also discussed in Chapter 8, include entropic

spring approaches such as characterizing the elasticity by a freely jointed chain or worm-​like

chain in addition to modeling the viscous relaxation effects of tissues manifest as energy

losses in tissue stretch-​relaxation cycles in characteristic hysteresis loops, which again can be

linked back to specific biological structures in the tissues.