Poly(vinylidene) fluoride membranes coated by heparin/collagen layer-by-layer, smart biomimetic approaches for mesenchymal stem cell culture.

The use of piezoelectric materials in tissue engineering has grown considerably since inherent bone piezoelectricity was discovered. Combinations of piezoelectric polymers with magnetostrictive nanoparticles (MNP) can be used to magnetoelectrically stimulate cells by applying an external magnetic field which deforms the magnetostrictive nanoparticles in the polymer matrix, deforming the polymer itself, which varies the surface charge due to the piezoelectric effect. Poly(vinylidene) fluoride (PVDF) is the piezoelectric polymer with the largest piezoelectric coefficients, being a perfect candidate for osteogenic differentiation. As a first approach, in this paper, we propose PVDF membranes containing magnetostrictive nanoparticles and a biomimetic heparin/collagen layer-by-layer (LbL) coating for mesenchymal stem cell culture. PVDF membranes 20% (w/v) with and without cobalt ferrite oxide (PVDF-CFO) 10% (w/w) were produced by non-solvent induced phase separation (NIPS). These membranes were found to be asymmetric, with a smooth surface, crystallinity ranging from 65% to 61%, and an electroactive ß-phase content of 51.8% and 55.6% for PVDF and PVDF-CFO, respectively. Amine groups were grafted onto the membrane surface by an alkali treatment, confirmed by ninhydrin test and X-ray photoelectron spectroscopy (XPS), providing positive charges for the assembly of heparin/collagen layers by the LbL technique. Five layers of each polyelectrolyte were deposited, ending with collagen. Human mesenchymal stem cells (hMSC) were used to test cell response in a short-term culture (1, 3 and 7 days). Nucleus cell counting showed that LbL favored cell proliferation in PVDF-CFO over non-coated membranes.

The kinetics of the structural relaxation process in PHEMA-silica nanocomposites based on an equation for the configurational entropy.

The enthalpy relaxation of polymer-silica nanocomposites prepared by simultaneous polymerization of poly(2-hydroxyethyl methacrylate) (PHEMA) and tetraethyloxysilane, TEOS, a silica precursor, is investigated. Both the glass transition temperature, Tg, and the temperature interval of the glass transition, DeltaTg , increase as the silica content in the sample does. Structural relaxation experiments show that the temperature interval in which conformational motions take place broadens as the silica content in the hybrid increases. A phenomenological model based on the evolution of the configurational entropy during the structural relaxation process, the SC model, has been used for determining the temperature dependence of the relaxation times during the process. The results show an increase of the fragility of the polymer as the silica content increases, a feature that can be related to the broadening of the distribution of relaxation times characterized by the beta parameter of the stretched exponential distribution. On another hand the silica content increase produces a significant change of the relaxation times in the glassy state.