Summary

Shear stress and hydrostatic effects on the hMSCs early mechano gene response were similar. For the same magnitude gene response, the hydrostatic compression (1.5×10⁵ Pascal) is a 200000 times greater than the force exerted by shear stress (0.7 Pascal).

Summary

Coating of titanium implants with BMP-2-loaded polyelectrolyte multilayer films conferred the implant surface with osteoinductive properties which are fully preserved upon both air-dried storage and γ-sterilization.

Although BMP-2 is recognised as an important molecule for bone regeneration, its supraphysiological doses currently used in clinical practice has raised serious concerns about cost-effectiveness and safety issues. Thus, there is a strong motivation to engineer new delivery systems or to provide already approved materials with new functionalities. Immobilizing the growth factor onto the surface of implants would reduce protein diffusion and increase residence time at the implantation site. To date, modifying the surfaces of metal materials, such as titanium or titanium alloys, at the nanometer scale for achieving dependable, consistent and long-term osseointegration remains a challenging approach.

In this context, we have developed an osteoinductive coating of a porous titanium implant using biomimetic polyelectrolyte multilayer (PEM) films used as carriers of BMP-2. The PEM films were prepared by alternate deposition of 24 layer pairs of poly(L-lysine) (PLL) and hyaluronic acid (HA) layers (∼3.5 µm in thickness); such films were then cross-linked by means of a water-soluble carbodiimide (EDC) at different degrees. The amount of BMP-2 loaded in these films was tuned (ranging from 1.4 to 14.3 µg/cm²) depending on the cross-linking extent of the film and of the BMP-2 initial concentration. Because packaging, and storage of the devices are important issues that may limit a wide application of biologically functionalised materials, we assessed in the present study the osteoinductive performance of the BMP-2 loaded PEM coatings onto custom-made 3D porous scaffolds made of Ti-6Al-4V in vitro and in vivo pertinent to long-term storage in a dry state and to sterilization by gamma irradiation.

Analysis of PEM films by infrared spectroscopy evidenced that the air-dried films were stable for at least one year of storage at 4°C and they resisted exposure to γ-irradiation at clinically approved doses. The preservation of the growth factor bioactivity was evaluated both in vitro (using C2C12 cell model) and in vivo (in a rat ectopic model). In vitro, BMP-2 loaded in dried PEM films exhibited shelf-life stability at 4°C over a one-year period. However, its bioactivity decreased from 50 to 80% after γ-irradiation at 25 and 50 kGy, respectively. Remarkably, the in vivo studies showed that the amount of new bone tissue formation induced by BMP-2 contained in PEM-coated Ti implants was not affected after air-drying of the implants and sterilization at 25 kGy indicating the full preservation of the growth factor bioactivity.

Altogether, our results provided evidence of the remarkable property of PEM film coatings that both sequester BMP-2 and preserve its full in vivo osteoinductive potential upon both storage and γ-sterilization. The protective effects of PEM films on the growth factor bioactivity may be attributed to both the high water content in (PLL/HA) films (∼90%) and to their porosity, which may provide a “protein-friendly” environment similar to the natural extracellular matrix. This novel “off-the-shelf” technology of functionalised implants opens promising applications in prosthetic and tissue engineering fields.

Summary

45S5 bioactive glass combined with hMSC did not permit de novo ectopic bone formation. Such absence of osteogenicity was most likely due to the alkalinization of the 45S5 microenvironment that affects adversely the osteogenic differentiation of stem/precursor cells.

Bone marrow stromal cells (BMSCs) are capable of bone formation and can promote the repair of osseous defects when implanted in appropriate scaffolds. The most promising biomaterials for application in bone tissue engineering (TE) are hydroxyapatite (HA), tricalcium phosphate (TCP), calcium carbonate (coral) ceramics or bioactive glasses (BG) because of their osteoconductive properties and ability to enhance bone formation. However, information regarding the osteogenic potential of hBMSCs in combination with BG scaffolds is strikingly lacking in the TE field. The present study focused on evaluating the osteogenicity of bone constructs prepared from particles of 45S5 BG combined with hBMSCs in comparison with biphasic HA/TCP or coral particles, in a mouse ectopic model.

The in vivo osteogenicity was then correlated with various aspects of the effects of the scaffold materials tested on hBMSCs functions pertinent to bone tissue formation. Particular attention was given to the pH in the microenvironment where the cells reside in TE constructs and its effect on the osteoblastic differentiation of hBMSCs. In vivo experiments evidenced that 45S5 BG constructs with hBMSCs failed to form ectopic bone. In contrast, the cell constructs prepared with either HA/TCP or coral ceramics displayed great and consistent capacity for the ectopic bone formation. The cytocompatibility of hBMSCs on BG material was addressed and no differences were evidenced between HA/TCP and coral substrates related to the adhesion of hBMSCs and their proliferation in vitro. The hBMSCs viability was even higher within the 45S5 BG-containing constructs compared to the other two types of material constructs tested both in vitro and in vivo. These findings indicated that the absence of de novo bone formation in the hBMSCs-containing 45S5 BG constructs was not the result of cytotoxic effects of the BG material.

The potential of osteogenic differentiation of hBMSCs cultured on material substrates was next addressed and the ALP activity of hBMSCs was significantly diminished when these cells were cultured on 45S5 BG as compared to either HA/TCP or coral substrates. Because BG materials are well-known for causing external alkalinisation, the pH was specifically measured in TE constructs. The pH inside the cell-containing BG constructs, measured ex vivo, was 8.0 (i.e. 0.4–0.5 units more alkaline than that measured in the coral- or HA/TCP-constructs). The impact of such external alkalinisation on the osteogenic differentiation of hBMSCs was assessed by culturing the cells over a wide range of alkaline pH. The hBMSCs expression of osteogenic markers, ALP activity and mineralization were not significantly affected at moderate external alkaline pH (≤ 7.90) but were dramatically inhibited at higher pH.

Altogether, these findings provided evidence that despite 45S5 BG are reported to be good osteoconductive materials, they are not necessarily good scaffolds for TE, most likely due to the alkalinization of the 45S5 microenvironment that affects adversely the osteogenic differentiation of precursor cells. Controlling the shifts of pH in the local engineered extracellular environment is a critical issue for the development of bioactive TE scaffolds.