Among the very large number of polymeric materials that have been proposed in the field of orthopedics, polyethylene terephthalate (PET) is one of the most attractive thanks to its flexibility, thermal resistance, mechanical strength and durability. Several studies were proposed that interface nano- or micro-structured surfaces with mesenchymal stromal cells (MSCs), demonstrating the potential of this technology for promoting osteogenesis. All these studies were carried out on other biomaterials than PET, which remains almost un-investigated in terms of cell shaping, alignment and differentiation. In a previous study, we developed a hot-embossing method to transfer nano-textures (down to below 100 nm lateral size) onto PET substrate, and demonstrated that PET nanogratings (NGs) can optimally stimulate hMSC mechanotransduction mechanism. Specifically, we showed that cell and nuclear morphology, and cytoskeletal components are similarly affected by NGs, and that NG ridge sizes of 500 nm and 1 μm were both effective in stimulating cell polarization, without compromising cell viability.

We study the effect of PET 350-depth nanogratings (NGs) having ridge and groove lateral size of 500 nm (T1) or 1 µm (T2), on bone-marrow human MSC (hMSC) differentiation towards the osteogenic fate. In particular, we cultured hMSCs on PET NGs having different periodicity and measured the expression of a complete set of genes characterizing osteo-differentiation, at different time-points from day 3 up to day 21. In order to evaluate how the contact interaction with PET NGs affects hMSC differentiation, the expression of a set of genes (RUNX2, COL1A1, ALPL, BMP2 and IBSP) characterizing osteogenesis was measured by RT-qPCR. BMP2 and IBSP were the most sensitive to the presence of the engineered surfaces The production of bone matrix was finally evaluated at the end of the differentiation period in terms of morphology, substrate coverage and alignment to the underlying topography.

Overall, the data show that among the tested genes, BMP2 and IBSP are the most sensitive to the presence of the engineered surfaces. Although for RUNX2, COL1A1 and ALPL we measured only small modifications, upregulation of BMP2 and IBSP was relevant, especially in case of Osteogenic Medium (OM) and for the T2 geometry. This result suggests the T2 substrate as the most promising structure for stimulating hMSCs towards osteogenic maturation

We demonstrate that these substrates, especially the T2, can promote the osteogenic phenotype more efficiently than standard flat surfaces and that this effect is more marked if cells are cultured in osteogenic medium than in basal medium. Finally, we show that the shape and disposition of calcium hydroxyapatite granules on the different substrates was influenced by the substrate symmetry, being more elongated and spatially organized on NGs than on flat surfaces.

This study demonstrates that PET nanogratings can promote osteogenic differentiation of hMSC in vitro. Since PET is an FDA approved material and we did not use any surface chemical treatment for cell adhesion and spreading, PET NGs can be considered promising for clinical translation in the field of orthopedic tissue engineering.

In view of possible clinical applications of mesenchymal stromal cells (MSCs), interesting results in repairing the Achilles tendon have been achieved in rabbit models since 1997. Histological and immunochemical studies have demonstrated the quality of repair. A basic problem in tissue repair is the way to administer stem cells. Several questions remain:

have the cells to be differentiated or not?

Attempting to cure, as a clinical model, horses with a pathological core lesion in the superficial digital flexor tendon (SDFT), MSCs were recovered from autologous bone marrow, expanded ex vivo, suspended in autologous serum and re-injected directly into the core lesion.

All 11 horses implanted with autologous MSCs exhibited no adverse reaction due to the implantation of the cells, either locally or systemically. After rehabilitation therapy nine MSC-treated animals recovered from their clinical conditions, had an excellent ultrasound image of tendons after a period ranging from 3 to 6 months, and returned to racing with good or even optimal results in the previous category of competition in 9 to 12 months without any re-injuring event. All of them are still active more than 2 years from diagnosis. One of the 2 remaining horses received less than 1×106 of MSCs, and its tendon did not heal relapsing after rehabilitation, the other was lost to follow-up. In contrast, most of horses from the control group showed tendon ultrasound images that revealed fibrosis during the healing process, and all of them were re-injured after a median time of 7 months.

The ability of tissue microenvironments to induce cell differentiation could render unnecessary a partial or total ex vivo differentiation and direct infusion of undifferentiated MSCs could represent a safe therapeutic approach to tendon repair.