Biomedical Engineering Reference
In-Depth Information
accompanying upregulation in proteins related to cytoskeletal dynamics [26]. Sjostrom
et al.
showed that 15-nm-high nanopillars enhanced the spreading of human MSCs and the
formation of bone-matrix nodules, while 55-nm-high and 100-nm-high nanopillars displayed
an inhibitory effect [24]. Lim
et al.
showed that the formation of mature adhesion plaque in
osteoblasts was increased on polymer-demixed nanoprotrusions 11 nm in height [27]. Biggs
et al.
also showed that on a surface with 45-nm-high “nano-islands” created by polymer
phase separation, STRO
+
MSCs developed focal adhesions comparable to the cells cultured
on the planar substrates, while the synthesis of osteospecific proteins was upregulated [28].
Besides nanoprotrusion height, the planar size of nanoprotrusions and size of edge-edge
spacing are also important for the regulation of focal adhesion formation. For example,
Sjostrom
et al
. showed that cellular spreading and focal adhesion formation were reduced
when skeletal stem cells were cultured on nanopillars with an edge-edge spacing approach-
ing 70 nm [24]. They suggested that when the height and density of nanoprotrusions are
sufficient to prevent cell contact with the planar basal substrate, the planar size of naopro-
trusions and edge-edge spacing of the nanoprotrusions become critical in the formation of
integrin clusters and focal adhesions.
In summary, the evidence indicates that on low nanoprotrusions cell adhesion and
functions may be affected. On the other hand, high nanoprotrusions hinder the formation
of contact adhesions and cell spreading. However, studies regarding stem-cell response to
nanoprotrusions are still few, especially concerning differentiation.
Responses of Stem Cells to Nanoporous Substrates
As we discussed previously, the corneal basement membrane comprises a nanoporous struc-
ture (Figure 11.1A and 1B). Therefore, it is reasonable to study the effect of nanoporous
substrates on cell culture
in vitro
. Various artificial porous structures have been fabricated
in order to investigate their influence on cell adhesion, migration, proliferation, and
differentiation [29, 30]. Silicon, aluminium, tantalum, and titanium-based substrates have
commonly been used for fabrication of porous surfaces for biomedical application due to
their ease in creating a porous structure and having good biocompatibility. The formation
of porous and/or oxidized layers on these materials has been studied extensively, and sum-
marized in several reviews [11, 31, 32]. For example, porous silicon substrates can be readily
fabricated using electrochemical etching in hydrogen fluoride (HF)-based solution [33]. The
porous silicon topography, possessing pore sizes ranging from few to tens of nanometers
depending on the etching condition, displays topography similar to native corneal basement
membrane (Figure 11.1C).
Recent studies have shown that stem-cell behavior such as self-renewal and differentiation
are manipulated by surface porous topography
in vitro
[34, 35]. For example, Jin
et al
. showed
that cell attachment, proliferation, and differentiation of human ESCs were enhanced on
porous membrane with a pore size of 400 nm [36]. The expression of a number of ECM pro-
teins and cell-adhesion molecules (collagen type XI α
1
, catenin α
1
, β
1
, and δ
1
, and laminin α
3
and γ
1
on polyethylene terephthalate (PET) membrane; integrin α
1
, α
V
, and β
5
, collagen type XI
α
1
, type XII α
1
, and type XVI α
1
, and laminin γ
1
on polyethylene (PE) membrane) was upregu-
lated. An enhanced nuclear translocation of β-catenin was detected in cell cultures on these
porous membranes. They demonstrated that the porous membranes stimulate the binding of
Wnt ligand to its receptor, resulting in stabilization and nuclear translocation of β-catenin,
which triggers upregulation of the expression of several types of collagen and integrin and
enhances the proliferation and differentiation of human ESCs. In this section, we summarize
and discuss the effect of nanoporous surfaces on stem-cell behaviour (Table 11.1).
