Synthesis of new PLGA based grafted polymers for formation of bioactive nanoparticles

The loss of bone tissues because of age, obesity, trauma or diseases can provoke a significant morbidity and an important socio-economical cost. During the last few decades, tissue engineering and regenerative medicine have become promising strategies for rebuilding bones. The tissue engineering of bones has for objective the functional regeneration of bones through a synergetic combination of biomaterial scaffolds, cells and active biomolecules such as proteins, peptides or growth factors. The objective of our work was to synthesize a copolymer, curdlan-graft-PLGA, in supercritical CO2 without any organic solvent. This copolymer was then used for the formulation of nanoparticles that are intended to be loaded with bioactive molecules for bone regeneration.

Starting with cyclic monomers, glycolide and lactide, poly-lactic-co-glycolic acid (PLGA) was obtained through ring-opening polymerization (ROP) in supercritical carbon dioxide (scCO2). Curdlan (1,3-β-glucan), a polysaccharide, was used as an initiator. Organic catalysers, including 1,8-diazabicyclo[5.4.0]undec-7-ene and thio-urea, were also used. The purity, composition and molecular mass of the curdlan-graft-PLGA copolymer were determined with liquid NMR.
Nanoparticles were prepared through a phase separation method. The copolymer was dissolved in dimethyl isosorbide (DMI), then an anti-solvent, an aqueous solution of glycine and NaOH, was progressively added in large excess. The average nanoparticle size was determined by dynamic light scattering. The morphology of nanoparticles was studied with scanning and transmission electronic microscopy. Finally, the zeta potential was also measured.

In this work, we used curdlan both as an initiator for the ROP and as a backbone for the copolymerization with PLGA. Thus, the resulting curdlan-graft-PLGA copolymer is composed of PLGA chains grafted on the curdlan backbone. scCO2 is considered as a “green” solvent because its critical pressure and temperature are relatively low, it is abundant, inert, nontoxic and non-flammable. Therefore, the ROP in scCO2 allows to obtain the curdlan-graft-PLGA copolymer in one step without any toxic solvent. Moreover, the synthesis parameters were optimised to diminish the amount of organocatalysts used. The rate of conversion of this process was determined to be of 98% and the molar mass of the PLGA grafts synthesised of 10 000g.mol-1.
Nanoparticles were formed using this copolymer via a method of nanoprecipitation by phase separation. This method uses the difference in hydrophilicity between PLGA and curdlan. Curdlan is more hydrophilic than PLGA, therefore it will be preferentially oriented toward the aqueous phase on the surface of the nanoparticles while the PLGA will find itself in the core. This configuration was confirmed by modifying the quantity of curdlan in the copolymer which leads to a difference in the zeta potential of nanoparticles. The particles were observed to be spherical with a diameter of 300 nm in average and a polydispersity index of 0.2.