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.