Branched Amphiphilic Polylactides as a Polymer Matrix Component for Biodegradable Implants

Abstract

The combination of biocompatibility, biodegradability, and high mechanical strength has provided a steady growth in interest in the synthesis and application of lactic acid-based polyesters for the creation of implants. On the other hand, the hydrophobicity of polylactide limits the possibilities of its use in biomedical fields. The ring-opening polymerization of L-lactide, catalyzed by tin (II) 2-ethylhexanoate in the presence of 2,2-bis(hydroxymethyl)propionic acid, and an ester of polyethylene glycol monomethyl ester and 2,2-bis(hydroxymethyl)propionic acid accompanied by the introduction of a pool of hydrophilic groups, that reduce the contact angle, were considered. The structures of the synthesized amphiphilic branched pegylated copolylactides were characterized by 1H NMR spectroscopy and gel permeation chromatography. The resulting amphiphilic copolylactides, with a narrow MWD (1.14–1.22) and molecular weight of 5000–13,000, were used to prepare interpolymer mixtures with PLLA. Already, with the introduction of 10 wt% branched pegylated copolylactides, PLLA-based films had reduced brittleness, hydrophilicity, with a water contact angle of 71.9–88.5°, and increased water absorption. An additional decrease in the water contact angle, of 66.1°, was achieved by filling the mixed polylactide films with 20 wt% hydroxyapatite, which also led to a moderate decrease in strength and ultimate tensile elongation. At the same time, the PLLA modification did not have a significant effect on the melting point and the glass transition temperature; however, the filling with hydroxyapatite increased the thermal stability.

Preparation and Hydrolytic Degradation of Hydroxyapatite-Filled PLGA Composite Microspheres

“…Apparently, an increase in the hydrophilicity of the microsphere's surface and a decrease in the proportion of ordered regions with an increase in the proportion of glycolide units in the chain are of greater importance. According to the data presented in [59,60], the contact angle of wetting with water on the PLGA surface decreases almost linearly with an increase in the molar fraction of glycolic acid residues (Figure 6), according to Equation ( 1):…”

“…where: θ-contact angle of wetting PLGA surface with water, %; Δ -molar fraction of glycolic acid residues in PLGA. As can be seen, when the mole fraction of glycolide units is less than 10%, there obvious deviations from linear dependence (1), and the wetting angle of PLLA with wa is 93.30 [60]. Probably, deviations from Equation ( 1) are associated with a significant p portion of ordered regions in PLGA samples containing a small amount of glycolic a residues.…”

“…According to XRD data, for PLGA with a mole fraction of glycolic acid units more than 10%, there are no obvious reflections that are characteristic of the crystall phase [59]. Although, due to a large number of defects in ordered regions, the exp mental determination of PLGA crystallinity degree is obviously conditional, all availa As can be seen, when the mole fraction of glycolide units is less than 10%, there are obvious deviations from linear dependence (1), and the wetting angle of PLLA with water is 93.30 [60]. Probably, deviations from Equation ( 1) are associated with a significant proportion of ordered regions in PLGA samples containing a small amount of glycolic acid residues.…”

“…Upon transition to composite microspheres based on PLGA of various compositions filled with 30 wt% of hydroxyapatite, a sharp increase in the rate of acid-catalyzed hydrolytic degradation was observed (Figure 8). First, filling with hydroxyapatite increases the hydrophilicity of the surface [60], facilitating its wetting with water. Secondly, filling with hydroxyapatite contributes to the involvement of a larger PLGA surface in the reaction zone due to the penetration of water into microspheres in places of formation of main pores and defects caused by filling.…”

“…Figure 6. The dependence of the contact angle of wetting PLGA surface with water on the composition of the copolymers (according to the data of[59,60]). …”