Modulation of Cellular Colonization of Porous Polyurethane Scaffolds via the Control of Pore Interconnection Size and Nanoscale Surface Modifications

Full-scale cell penetration within porous scaffolds is required to obtain functional connective tissue components in tissue engineering applications. For this aim, we produced porous polyurethane structures with well-controlled pore and interconnection sizes. Although the influence of the pore size on cellular behavior is widely studied, we focused on the impact of the size of the interconnections on the colonization by NIH 3T3 fibroblasts and Wharton's jelly-derived mesenchymal stem cells (WJMSCs). To render the material hydrophilic and allow good material wettability, we treated the material either by plasma or by polydopamine (PDA) coating. We show that cells weakly adhere on these surfaces. Keeping the average pore diameter constant at 133 μm, we compare two structures, one with LARGE (52 μm) and one with SMALL (27 μm) interconnection diameters. DNA quantification and extracellular matrix (ECM) production reveal that larger interconnections is more suitable for cells to move across the scaffold and form a three-dimensional cellular network. We argue that LARGE interconnections favor cell communication between different pores, which then favors the production of the ECM. Moreover, PDA treatment shows a truly beneficial effect on fibroblast viability and on matrix production, whereas plasma treatment shows the same effect for WJMSCs. We, therefore, claim that both pore interconnection size and surface treatment play a significant role to improve the quality of integration of tissue engineering scaffolds.

Step-by-step build-up of covalent poly(ethylene oxide) nanogel films

Hydrogels based on poly(ethylene glycol) (PEG) are commonly used for studies related to cell fate and tissue engineering. Here we present a new covalent layer-by-layer build-up process leading to PEG coatings of nanometer size called "nanogel films". Compared to macroscopic hydrogels, such nanogels should provide a fine control over the structure and the thickness of the coating. Alternated deposition of bifunctional and tetra functional PEG molecules reacting through thiol/maleimide click chemistry is evaluated by quartz crystal microbalance. We first study parameters influencing the build-up process of such coatings and demonstrate the importance of (i) the nature of the first deposited layer, (ii) the PEG concentrations and (iii) the length of the PEG chains that appears to be the most significant parameter influencing film growth. The build-up process can be extended to a large variety of substrates like SiO₂ or polymers by using an appropriate anchoring layer. Covalent functionalization of these nanogel films by proteins or enzymes is suited by modifying the biomolecules with thiol or maleimide groups and immobilizing them during the build-up process. Activity of the embedded enzymes can be maintained. Moreover ligands like biotin can be incorporated into the film and recognition by streptavidin can be modulated by playing with the number of PEG layers covering biotin. Compared to well-known PEG hydrogels, these new coatings are promising as they allow to (i) build thin nanometric coatings, (ii) finely control the amount of deposited PEG and (iii) organize the position of the embedded biomolecules inside the film layers.

Bioinspired Nanofeatured Substrates: Suitable Environment for Bone Regeneration

Bone mimicking coatings provide a complex microenvironment in which material, through its inherent properties (such as nanostructure and composition), affects the commitment of stem cells into bone lineage and the production of bone tissue regulating factors required for bone healing and regeneration. Herein, a bioactive mineral/biopolymer composite made of calcium phosphate/chitosan and hyaluronic acid (CaP-CHI-HA) was elaborated using a versatile simultaneous spray coating of interacting species. The resulting CaP-CHI-HA coating was mainly constituted of bioactive, carbonated and crystalline hydroxyapatite with 277 ± 98 nm of roughness, 1 μm of thickness, and 2.3 ± 1 GPa of stiffness. After five days of culture, CaP-CHI-HA suggested a synergistic effect of intrinsic biophysical features and biopolymers on stem cell mechanobiology and nuclear organization, leading to the expression of an early osteoblast-like phenotype and the production of bone tissue regulating factors such as osteoprotegerin and vascular endothelial growth factor. More interestingly, amalgamation with biopolymers conferred to the mineral a bacterial antiadhesive property. These significant data shed light on the potential regenerative application of CaP-CHI-HA bioinspired coating in providing a suitable environment for stem cell bone regeneration and an ideal strategy to prevent implant-associated infections.

Harnessing Wharton's jelly stem cell differentiation into bone-like nodule on calcium phosphate substrate without osteoinductive factors

An important aim of bone regenerative medicine is to design biomaterials with controlled chemical and topographical features to guide stem cell fate towards osteoblasts without addition of specific osteogenic factors. Herein, we find that sprayed bioactive and biocompatible calcium phosphate substrates (CaP) with controlled topography induce, in a well-orchestrated manner, Wharton's jelly stem cells (WJ-SCs) differentiation into osteoblastic lineage without any osteogenic supplements. The resulting WJ-SCs commitment exhibits features of native bone, through the formation of three-dimensional bone-like nodule with osteocyte-like cells embedded into a mineralized type I collagen. To our knowledge, these results present the first observation of a whole differentiation process from stem cell to osteocytes-like on a synthetic material. This suggests a great potential of sprayed CaP and WJ-SCs in bone tissue engineering. These unique features may facilitate the transition from bench to bedside and the development of successful engineered bone.

STATEMENT OF SIGNIFICANCE

Designing materials to direct stem cell fate has a relevant impact on stem cell biology and provides insights facilitating their clinical application in regenerative medicine. Inspired by natural bone compositions, a friendly automated spray-assisted system was used to build calcium phosphate substrate (CaP). Sprayed biomimetic solutions using mild conditions led to the formation of CaP with controlled physical properties, good bioactivity and biocompatibility. Herein, we show that via optimization of physical properties, CaP substrate induce osteogenic differentiation of Wharton's jelly stem cells (WJ-SCs) without adding osteogenic supplement factors. These results suggest a great potential of sprayed CaP and WJ-SCs in bone tissue engineering and may facilitate the transition from bench to beside and the development of clinically successful engineered bone.

Enzymatic Catalysis Combining the Breath Figures and Layer-by-Layer Techniques: Toward the Design of Microreactors

Herein, we report the fabrication of microstructured porous surfaces with controlled enzymatic activity by combining the breath figures and the layer-by-layer techniques. Two different types of porous surfaces were designed based on fluorinated and carboxylated copolymers in combination with PS, using poly(2,3,4,5,6-pentafluorostyrene)-b-polystyrene (PS5F31-b-PS21) and polystyrene-b-poly(acrylic acid) (PS19-b-PAA10) block copolymers, respectively. For comparative purposes, flat surfaces having similar chemistry were obtained by spin-coating. Poly(sodium 4-styrenesulfonate)/poly(allylamine hydrochloride) (PSS/PAH) multilayers incorporating alkaline phosphatase (ALP) were built on these porous surfaces to localize the enzyme both inside and outside of the pores using PS/PS5F31-b-PS21 surfaces and only inside the pores on PS/PS19-b-PAA10 surfaces. A higher catalytic activity of ALP (about three times) was obtained with porous surfaces compared to the flat ones. The catalysis happens specifically inside the holes of PS/PS19-b-PAA10surfaces, where ALP is located. This opens the route for applications in microreactors.

Surface confined self-assembly of polyampholytes generated from charge-shifting polymers

Polyampholyte-based films can be efficiently self-assembled onto a surface in a one-pot manner by using a charge-shifting polyelectrolyte transformed into a polyampholyte by an electrogenerated gradient of protons.

Polysaccharide films built by simultaneous or alternate spray: a rapid way to engineer biomaterial surfaces

We investigated polysaccharide films obtained by simultaneous and alternate spraying of a chitosan (CHI) solution as polycation and hyaluronic acid (HA), alginate (ALG), and chondroitin sulfate (CS) solutions as polyanions. For simultaneous spraying, the film thickness increases linearly with the cumulative spraying time and passes through a maximum for polyanion/CHI molar charge ratios lying between 0.6 and 1.2. The size of polyanion/CHI complexes formed in solution was compared with the simultaneously sprayed film growth rate as a function of the polyanion/CHI molar charge ratio. A good correlation was found. This suggests the importance of polyanion/polycation complexation in the simultaneous spraying process. Depending on the system, the film topography is either liquid-like or granular. Film biocompatibility was evaluated using human gingival fibroblasts. A small or no difference is observed in cell viability and adhesion between the two deposition processes. The CHI/HA system appears to be the best for cell adhesion inducing the clustering of CD44, a cell surface HA receptor, at the membrane of cells. Simultaneous or alternate spraying of CHI/HA appears thus to be a convenient and fast procedure for biomaterial surface modifications.

Simultaneous spray coating of interacting species: general rules governing the poly(styrene sulfonate)/poly(allylamine) system

Simultaneous spraying of two solutions of interacting species onto a substrate held vertically leads to the formation of nanometer-sized coatings. Here we investigate the simultaneous spraying of poly(styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) solutions leading to the formation of a film composed of PSS/PAH complexes. The thickness of this film increases linearly with the cumulative spraying time. For a given spraying rate of PAH (respectively PSS), the growth rate of the film depends strongly upon the PSS/PAH ratio and passes through a maximum for a PSS/PAH ratio lying between 0.55 and 0.8. For a PSS/PAH ratio that is maintained constant, the growth speed of the film increases linearly with the spraying rate of polyelectrolyte of both solutions. Using X-ray photoelectron spectroscopy, we find that the film composition is almost independent of the PSS/PAH (spayed) ratio, with composition very close to 1:1 in PSS:PAH film. The 1:1 PSS:PAH composition is explained by the fact that the simultaneous spraying experiments are carried out with salt-free solutions; thus, electroneutrality in the film requires exact matching of the charges carried by the polyanions and the polycations. Zeta potential measurements reveal that, depending on whether the PSS/PAH spraying rate ratio lies below or above the optimal spraying rate ratio, the film acquires a positive or a negative excess charge. We also find that the overall film morphology, investigated by AFM, is independent of the spraying rate ratio and appears to be composed of nanometer-sized grains which are typically in the 100 nm range.

Water-soluble pegylated quantum dots: from a composite hexagonal phase to isolated micelles

We present a simple method based on the dispersion of fluorescent quantum dots (QD) into a liquid crystal phase that provides either nanostructured material or isolated QD micelles depending on water concentration. The liquid-crystal phase was obtained by using a gallate amphiphile with a poly(ethylene glycol) chain as the polar headgroup, named I. The hydration of QD/I mixtures resulted in the formation of a composite hexagonal phase identified by small-angle X-ray scattering and by polarized light and fluorescence optical microscopy, showing a homogeneous distribution of fluorescence within hexagonal phase. This composite mesophase can be converted into isolated QD-I micelles by dilution in water. The fluorescent QD-I micelles, purified by size exclusion chromatography, are well monodisperse with a hydrodynamic diameter of 20-30 nm. Moreover, these QD do not show any nonspecific adsorption on lipid or cell membranes. By simply adjusting the water content, the PEG gallate amphiphile I provides a simple method to prepare a self-organized composite phase or pegylated water soluble QD micelles for biological applications.

Polyelectrolyte multilayer films with pegylated polypeptides as a new type of anti-microbial protection for biomaterials

Adhesion of bacteria at the surface of implanted materials is the first step in microbial infection, leading to post-surgical complications. In order to reduce this adhesion, we show that poly(L-lysine)/poly(L-glutamic acid) (PLL/PGA) multilayers ending by several PLL/PGA-g-PEG bilayers can be used, PGA-g-PEG corresponding to PGA grafted by poly(ethylene glycol). Streaming potential and quartz crystal microbalance-dissipation measurements were used to characterize the buildup of these films. The multilayer films terminated by PGA and PGA-g-PEG were found to adsorb an extremely small amount of serum proteins as compared to a bare silica surface but the PGA ending films do not reduce bacterial adhesion. On the other hand, the adhesion of Escherichia coli bacteria is reduced by 72% on films ending by one (PLL/PGA-g-PEG) bilayer and by 92% for films ending by three (PLL/PGA-g-PEG) bilayers compared to bare substrate. Thus, our results show the ability of PGA-g-PEG to be inserted into multilayer films and to drastically reduce both protein adsorption and bacterial adhesion. This kind of anti-adhesive films represents a new and very simple method to coat any type of biomaterials for protection against bacterial adhesion and therefore limiting its pathological consequences.