Optimizing interactions between biomaterials and stem
cells
An ideal surface would promote the interactions between biomaterials and
stem cells to achieve the expansion of stem cells without compromised
potency, and differentiation of stem cells with maintained
differentiated phenotypes. Niche is the native microenvironment where
stem cells residing in that regulates the behavior of stem cells
including adhesion, proliferation and differentiation through various
intrinsic signaling pathways. Recent studies also focused on
biomimicking such environment in terms of comparable mechanical and
biochemical properties via biofunctionalization of various proteins,
peptides and growth factors.
Embryonic stem cells (ESCs) are considered as pluripotent that can be
differentiated into almost all different cell lineages. Human induced
pluripotent stem cells (iPSCs) are derived from somatic cells through
reprogramming. Unlike ESCs, iPSCs originated from human autologous cells
can bypass certain ethical issues and exhibit lower immune
response[54]. However, a feeder layer is
frequently required to culture pluripotent stem cells and support their
pluripotency. Mouse embryonic fibroblasts (MEF) and Matrigel are
typically used as feeder layers; yet use of xenogeneic cell source and
mouse sarcomas derived products brings about the risk of potential
disease and pathogen transmission.
UV/ozone surface treatment has been applied to polystyrene substrates to
construct feeder layer-free system for iPSCs[55].
The polymer chains of polystyrene were decomposed into shorter fragments
through UV treatment, and formed functional carboxylic acid groups on
surface. Results showed that a more hydrophilic and cell-adhesive
surface was generated. Such changes in surface chemistry resulted in
promoted attachment and proliferation of iPSCs. The pluripotency of
iPSCs was well maintained as indicated by the comparable Nanog
expression of iPSCs cultured on UV-treated PS to those on MEF feeder
layer.
A vitronectin peptide (VN)-decorated nanofibrous niche was developed to
promote in vitro culture and osteogenic differentiation of human
iPSCs[56]. VN was immobilized to the PCL scaffolds
through an intermediate carboxymethyl chitosan (CMC) layer. Grafting of
CMC and VN tuned the initial super hydrophobic PCL surface to
hydrophilic with water contact angle changed from 122.3 ± 3.91° to 23.8
± 1.0°. The peptide-decorated nanofibrous scaffolds well supported the
proliferation of iPSCs with maintained pluripotency. Upon osteogenic
induction by adding osteoinductive medium, iPSCs showed enhanced
osteogenic differentiation in the feeder layer-free culture system.
Decoration of VN to PDA-coated tissue culture plates via CMC conjugation
not only stabilized long-term pluripotency of hESCs and hiPSCS, but
supported reprogramming of human somatic cells (human urine derived
cells and human umbilical cord blood cells) into hiPSCs under defined
conditions[57].
An iron-containing porphyrin, hemin, was dip-coated on serum albumin
(SA) electrospun scaffolds to confer conductivity resembling the
electroresponsive nature of neurons[58]. Human
iPSCs derived neural stem cells (NSCs) were cultured on surface treated
scaffolds. Hemin doped SA scaffolds exhibited higher cell attachment and
viability than non-doped scaffolds. Whereas no significant difference in
NSCs differentiation was found. The electrical stimulation of
hemin-doped scaffolds resulted in enhanced neuronal differentiation and
maturation. Fibroblast growth factors-2 (FGF-2) was non-covalently bind
to hemin-doped scaffolds. Although through a non-covalent binding, there
was a strong binding of FGF-2 to SA scaffolds with a slow release
profile. The FGF-2 incorporation led to higher cell proliferation yet
lower neuronal differentiation than other respective groups without
FGF-2. That is quite consistent with the prediction that FGF-2 mainly
functions in the proliferation of NSCs.
Adult stem cells can be harvested from various sites such as bone marrow
and adipose tissue that constitute an alternative stem cell source.
Those stem cells possess multipotency that can be differentiated to
various cell lineages unlike the pluripotency of embryonic stem cells.
Various bioactive molecules including fibronectin, collagen, RGD
peptides and designed peptide (R-peptide) were coated on glass
substrates to study their effects on cellular behavior of bone marrow
derived-MSCs[59]. R-peptide exhibits a sequence of
GRKKRRQRRRGGGRGD by linking RGD peptide with basic domain of Tat protein
(recognized as heparin binding domain). Well-established filopodia and
focal adhesions of hMSCs were found on fibronectin and R-peptide coated
substrates indicating enhanced cell attachment. There was appreciable
difference in the proliferation rate of hMSCs between R-peptide coated
substrate and other coated substrates, which suggests R-peptide as a
promising sequence for controlling proliferation and attachment of
hMSCs.
FGF-2 and chitosan were conjugated to tissue culture polystyrene after
the chemical vapor deposition (CVD) of parylene onto the surface for
ADSCs culturing[60]. The CVD copolymerization
process led to improved coating durability in terms of adhesive strength
and thermal stability; and offered functional groups including amine and
thiol groups to bind chitosan and FGF-2. Chitosan promoted the
self-assembled cellular spheroids formation; FGF-2 enhanced the
proliferation of ADSCs. Through a layer-by-layer assembly technique that
based on alternating exposure of precharged PLGA/nanoHA membrane to
polyelectrolytes, 14 layers of multipeptides can be grafted on surface
based on a 3D peptide gradient[61]. Peptides
functionalized PCL/nanoHA enhanced proliferation and osteogenic
differentiation of bone marrow derived-hMSCs; upon in vivoimplantation, the scaffolds showed enhanced osteoconductivity and
improved bone healing.
Nanopatterning of platinum bulk metallic glass (Pt-BMG) was achieved by
thermoplastic forming to study the effect of nano-topography on
differentiation of adipose derived-hMSCs[62].
Nanorods of a nominal diameter of 200 nm were patterned on the surface
by thermoplastic nanomolding. The surface roughness was significantly
increased from 14.1 ± 2.8 nm to 231.7 ± 47 nm. The elemental surface
composition and modulus remained unchanged. Results showed that
nanopatterned Pt-BMG directed adipogenic differentiation of hMSCs,
whereas flat Pt-BMG induced osteogenic differentiation. Many studies
suggested that stiffer substrate guides preferential osteogenic
differentiation. However, when increasing the stiffness of nanopatterned
Pt-BMG, no difference in osteogenic differentiation was observed
suggesting that the osteogenic differentiation of Pt-BMG was dominated
by topography. Nanotopography can influence cellular behavior by
interacting with integrin-receptors and the formation of focal adhesion.
Focal adhesions are essential in sensing the stiffness of substrates and
regulating intracellular signaling
transductions[63]. Previous studies suggested that
higher number of focal adhesions can lead to improved osteogenic
differentiation[64]. While more focal adhesions
were formed on flat Pt-BMG than nanopatterned one.
A nano-roughened PDMS surface was developed by chemical etching of a
polystyrene mold using acetone and rapid prototyping of
PDMS[65]. The surface roughness increases as
raising the acetone concentration and etching time. Whereas no defined
correlation was found between surface roughness and surface wettability.
Protein adsorption was favored on more roughened surface as indicated by
a significant increase in fibronectin adsorption on nano-roughened PDMS
than native PDMS. The surface wettability was also increased due to
fibronectin coating. The nano-roughened and protein coated PDMS enabled
adhesion and proliferation of bone marrow-derived MSCs, which makes it
potential for PDMS-based lab-on-a-chip devices.