Biomedical Engineering Reference
In-Depth Information
Different microfluidic, lithographic, and microarray techniques have been proposed to
solve the intricacies in the study of cell-ECM interaction [17]. For example, substratum
elasticity can be controlled by a fabricated micropost array of PDMS with magnetic control
that allows specific elastic force to the cell at a subcellular level in an
in vitro
condition [18].
Furthermore, microarray-printing technology has evolved to test the effect of different
ECM proteins and factors on the differentiation of stem cells. For example, Flaim
et al.
developed a type of ECM matrix microarray by using a DNA spotter to probe the
differentiation of primary rat hepatocyte cells and mouse embryonic stem cells (mESCs)
towards an early hepatic phenotype [19]. For this, they have used five different ECM proteins
(collagen I, collagen III, collagen IV, laminin, and fibronectin) in 32 different combinations.
Such a variety of combinations helped to identify the role of specific ECM mixtures in
directing the mESCs into a hepatic fate.
With the advent of microcontact printing technology and subsequent improvement of
this technique, understanding of cell-ECM interactions has improved. The original method
used an elastomeric stamp to transfer different functionalized long-chain alkanethiolates
onto a gold-coated surface, where alkanethiolates organize and form a self-assembled
monolayer (SAM) with functional groups exposed to the solution that have different
functions [20]. The hydrophobic SAMs helped to absorb the protein on the surface, whereas
the ethylene glycol functionalized ends helped to resist protein adsorption. Further,
improvement in microcontact printing technology allowed cell patterning by directly
stamping ECM proteins onto regular cell culture substrates, including glass, silicone rubber,
and polystyrene.
For a cell to interact with the ECM, the integrin molecules have to interact and in fact
bind to the ECM proteins, and this binding depends on the elastic modulus of the ECM
along with the local forces generated at the cell adhesion sites (or focal adhesions, FAs),
regardless of their generation sites, which can either be generated internally by intracellular
cytoskeletal contractility or externally by mechanical forces. To provide a more quantitative
control over integrin ligation and to control stem-cell adhesion, investigators have devel-
oped methods to produce alkanethiolates terminated with different adhesive peptides, such
as Arg-Gly-Asp (RGD) [21].
Nanotopography of the ECM also has a great influence on cell adhesion. In this
regard, in addition to the scale of the topography (5 nm to micrometer scale), the pattern
of ordered topography also can modulate the stem-cell behavior. The different patterned
topography includes ridges, steps, grooves, pillars, and pits, and the different topogra-
phies produce a variety of traction forces on the cells. For example, Ruiz and Chen
designed micropatterned fibronectin in a range of geometries (circle, square, rectangle,
ellipses, etc.), and measured the tractive force experienced by hMSCs grown on these
fibronectin patterns [22]. From thier results it was concluded that cells grown on
the concave surface experience greater tractive forces than those grown on convex
surfaces, and the greater forces direct the cells into an osteogenic lineage instead of an
adipogenic lineage.
Furthermore, recent progress in ECM biology has confirmed the difference in cellular
behavior when cultured on ECM-coated two-dimensional surfaces compared with cells cul-
tured in a native three-dimensional environment. In addition, in three-dimensional cultures
like collagen gels or Matrigel, the different mechanical cues like topography and rigidity also
have to be considered. In this regard, electrospinning allows configuration of nanothreads
with different ECM proteins, or with additional polymers that are coated with various ECM
proteins, which results in a more natural three-dimensional microenvironment for the culture
of stem cells than is possible with other techniques.
