Electrospun polyphosphazene nanofibers for in vitro osteoblast culture

Since 20 years both natural and syntehtic materials had been studied and applied to bone tissue engineering. In this context, poly(organophosphazene)s, high molecular weight polymers with a backbone of alternating phosphorus and nitrogen atoms and two organic side groups bonded to each phosphorus atom, can represent an attractive alternative to the materials used currently. The polymer degrade in acqueous medium to nontoxic products, including ammonia, phosphate, aminoacids, and the corresponding alchol. Polyphosphazenes polymers were indeed studied for the controlled release of many drugs bath few works concerned tissue engineering applications. In this work a poly[(ethyl phenylalanato)1.4(ethyl glycinato)0.6phosphazene] (PPhe-GlyP) has been prepared and used to assemble scaffolds for bone tissue engineering purposes. Either solvent casting or electrospinning methods were employed. We have also evaluated the effects on osteoblast attachment and proliferation of PPhe-GlyP blends with two widely used polymers, poly(lactic acid) (PLA) and poly(caprolactone) (PLC). PPhe-GlyP disks, obtained by solvent casting method, presented a smooth surface with several holes whose diameter ranged from 0.5 to 2 m. The in vitro degradation carried out in phosphate buffer, pH 7.4, at 37°C, displayed a nearly constant degradation kinetics, losing approximately 20% of the disk mass in 100 days. To verify the in vivo biocompatibility, PPhe-GlyP disks were inserted into subcutaneous pocket of BALB/c mice. A thin fibrous capsule around the polymeric disk was still present until 60 days, and no cells were visible inside the implants The in vivo results confirmed the biocompatibility of the PPhe-GlyP polymer, but the solvent casting method was not suitable to obtain PPhe-GlyP scaffolds able to allow host cell ingrowth.On the contrary, PPhe-GlyP scaffolds obtained by electrospinning method showed good porosity and fiber dimensions (600±300 nm) resembling those of the natural extracellular matrix. Osteoblasts collected from bone marrow of femurs of Sprague-Dowley rats were seeded on electrospun scaffolds composed of PLA, PCL alone, or as blends with PPhe-GlyP [PLA/PPhe-GlyP 75:25 (w/w) and PCL/PPhe-GlyP 75/25 (w/w)]. Although PPhe-GlyP supports osteoblast adhesion and growth to a lesser extent than that observed for electrospun PLA, a synergic effect on cell proliferation was noted when osteoblasts were cultured on PLA/PPhe-GlyP 75/25. Since polyphosphazenes can exert a buffering effect on acidic degradation products of PLA, electrospun PPhe-GlyP/PLA blend may represent an interesting material to use for bone tissue engineering. Finally, it must be noted that the poor mechanical properties of nanofibrous scaffolds make these materials useful only to repair defects whereby limited mechanical loading occurs, such as some cranial and maxillofacial defects.