Influence of Hydroxyapatite Surface Functionalization on Thermal and Biological Properties of Poly(l-Lactide)- and Poly(l-Lactide-co-Glycolide)-Based Composites

Abstract

Novel biocomposites of poly(L-lactide) (PLLA) and poly(l-lactide-co-glycolide) (PLLGA) with 10 wt.% of surface-modified hydroxyapatite particles, designed for applications in bone tissue engineering, are presented in this paper. The surface of hydroxyapatite (HAP) was modified with polyethylene glycol by using l-lysine as a linker molecule. The modification strategy fulfilled two important goals: improvement of the adhesion between the HAP surface and PLLA and PLLGA matrices, and enhancement of the osteological bioactivity of the composites. The surface modifications of HAP were confirmed by attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), TGA, and elemental composition analysis. The influence of hydroxyapatite surface functionalization on the thermal and in vitro biological properties of PLLA- and PLLGA-based composites was investigated. Due to HAP modification with polyethylene glycol, the glass transition temperature of PLLA was reduced by about 24.5 °C, and melt and cold crystallization abilities were significantly improved. These achievements were scored based on respective shifting of onset of melt and cold crystallization temperatures and 1.6 times higher melt crystallization enthalpy compared with neat PLLA. The results showed that the surface-modified HAP particles were multifunctional and can act as nucleating agents, plasticizers, and bioactive moieties. Moreover, due to the presented surface modification of HAP, the crystallinity degree of PLLA and PLLGA and the polymorphic form of PLLA, the most important factors affecting mechanical properties and degradation behaviors, can be controlled.

Keywords: biocomposites; biosafety; hydroxyapatite surface biofunctionalization; l-lysine; monocytes.

Figure 1

The attenuated total reflectance-Fourier transform infrared (FTIR-ATR) spectra of unmodified hydroxyapatite (HAP), HAP modified with l-lysine, and HAP after polyethylene glycol (PEG) grafting.

Figure 2

TGA curves of hydroxyapatite, hydroxyapatite modified with l-lysine and PEG600 (HAP-Lys/PEG600), and hydroxyapatite modified with l-lysine and PEG2100 (HAP-Lys/PEG2100).

Figure 3

SEM images of (A) HAP, (B) HAP-Lys, (C) HAP-Lys/PEG600, and (D) HAP-Lys/PEG2100.

Figure 4

Optical images of composite films.

Figure 5

Water contact angle of poly(l-lactide) (PLLA), poly(l-lactide-co-glycolide) (PLLGA), and composite films.

Figure 6

The cooling and second heating DSC curves of (A,B) PLLA and PLLA-based composites and (C,D) PLLGA and PLLGA-based composites.

Figure 6

The cooling and second heating DSC curves of (A,B) PLLA and PLLA-based composites and (C,D) PLLGA and PLLGA-based composites.

Figure 7

Viability of (A,C) murine fibroblasts L929 and (B,D) human osteoblasts hFOB 1.19, incubated for 24 h with (A,B) PLLA- or (C,D) PLLGA-based composites, evaluated using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) reduction assay according to ISO-10993-5:2009. The cells incubated without composites served as a positive control of viability (100%). Data are presented as mean ± SD of three separate experiments (six replicates for each assay ).

Figure 8

The activation of NF-κB transcription factor in THP1-Blue™ in monocytes incubated for 24 h with (A) PLLA- and (B) PLLGA-based composites in comparison with monocytes stimulated with lipopolysaccharide (LPS) of Escherichia coli (positive control) or cell culture medium (negative control). The secreted embryonic alkaline phosphatase release following toll-like receptor stimulation and NF-κB induction was quantified spectrophotometrically (OD = 650 nm) after enzymatic substrate conversion. Data are presented as mean ± SD of three separate experiments (six replicates of each assay).