Electrical stimulation: Effective cue to direct osteogenic differentiation of mesenchymal stem cells?

Mesenchymal stem cells (MSCs) play a major role in bone tissue engineering (BTE) thanks to their capacity for osteogenic differentiation and being easily available. In vivo, MSCs are exposed to an electroactive microenvironment in the bone niche, which has piezoelectric properties. The correlation between the electrically active milieu and bone's ability to adapt to mechanical stress and self-regenerate has led to using electrical stimulation (ES) as physical cue to direct MSCs differentiation towards the osteogenic lineage in BTE. This review summarizes the different techniques to electrically stimulate MSCs to induce their osteoblastogenesis in vitro, including general electrical stimulation and substrate mediated stimulation by means of conductive or piezoelectric cell culture supports. Several aspects are covered, including stimulation parameters, treatment times and cell culture media to summarize the best conditions for inducing MSCs osteogenic commitment by electrical stimulation, from a critical point of view. Electrical stimulation activates different signaling pathways, including bone morphogenetic protein (BMP) Smad-dependent or independent, regulated by mitogen activated protein kinases (MAPK), extracellular signal-regulated kinases (ERK) and p38. The roles of voltage gate calcium channels (VGCC) and integrins are also highlighted according to their application technique and parameters, mainly converging in the expression of RUNX2, the master regulator of the osteogenic differentiation pathway. Despite the evident lack of homogeneity in the approaches used, the ever-increasing scientific evidence confirms ES potential as an osteoinductive cue, mimicking aspects of the in vivo microenvironment and moving one step forward to the translation of this approach into clinic.

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.