Biomedical Engineering Reference
In-Depth Information
7.2.2
Techniques for Applying Mechanical Loading
Applying various mechanical loading on bone cells
in vitro
are by no means trivial
since it requires elegant instrumentation designed for accurately imposing specific
force types. In recent years, various devices have been developed for mechanically
stimulating cells and tissues, as concisely described in an excellent review on
this topic [8]. In the review, the current state of the art has been addressed and
represented by several typical systems. These systems include the following:
1) Compression loading systems: either using platen displacement or hydrostatic
pressure to compress tissues.
2) Longitudinal stretch systems: either stretching an extendable substrate that
is placed in grip and which is seeded with cells or alternatively bending a
cell-cultured flexure (it is commonly known as
four-point bending
)forgenerating
longitudinal force.
3) Out-of-plane distension systems: pressuring or displacing a cell-seeded circular
membrane to be deformed in out-of-plane direction.
4) In-plane substrate distension: using biaxial traction or frictionless platen to
deform a cell-cultured substrate to generate in-plane homogeneous stresses.
5) Fluid shear systems: place cultured cells on a cone and plate chamber or in a
cylindrical tube and then apply fluid flow to generate flow-induced shear stress
on the cells/tissues.
In order to enhance our understanding of how mechanical stimulation is
translated in a single cell and consequently affects the cell's function, proliferation
and differentiation, single-cell mechanics, mechanotransduction of bone cells,
has become increasingly important in exploring this new frontier now [9]. For
studying mechanotransduction of single cells, various advanced instruments that
are capable of deforming single osteoblastic cells have been developed. They have
been applied to study how mechanical forces influence the properties of the single
cells. Among these advanced techniques, the most popular modes include AFM
[10], optical tweezers [11], and nanoindentation [12, 13]. A schematic illustration of
these techniques is shown in Figure 7.2.
The AFM mainly consists of a sharp tip, which is linked with an ultrasensitive
cantilever beam, and a photo detector for measuring the deflection/displacement
of the beam subjected to external forces generated from the sharp tip inter-
acting with a cell. Optical tweezers is an instrument utilizing a highly focused
laser beam shining on a bone cell directly or a particle-attached cell which
has higher refractive index compared with the surrounding liquid, and then
an optical trap force can be generated on the cell as a mode of mechanical
stimulation. Nanoindentation or microneedle indentation is to use an indenter
to press a bone cell and simultaneously measure the force and displacement
of the deformed cell. Both AFM and nanoindentation are able to apply force
in the range of 10
−
9
10
−
11
N, while optical tweezers work in the range of
−
1-100 pN (10
−
12
N).
