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.