Mechanical properties of dispersed ceramic nanoparticles in polymer composites for orthopedic applications

Abstract

Ceramic/polymer composites have been considered as third-generation orthopedic biomaterials due to their ability to closely match properties (such as surface, chemistry, biological, and mechanical) of natural bone. It has already been shown that the addition of nanophase compared with conventional (or micron-scale) ceramics to polymers enhances bone cell functions. However, in order to fully take advantage of the promising nanometer size effects that nanoceramics can provide when added to polymers, it is critical to uniformly disperse them in a polymer matrix. This is critical since ceramic nanoparticles inherently have a strong tendency to form larger agglomerates in a polymer matrix which may compromise their properties. Therefore, in this study, model ceramic nanoparticles, specifically titania and hydroxyapatite (HA), were dispersed in a model polymer (PLGA, poly-lactic-co-glycolic acid) using high-power ultrasonic energy. The mechanical properties of the resulting PLGA composites with well-dispersed ceramic (either titania or HA) nanoparticles were investigated and compared with composites with agglomerated ceramic nanoparticles. Results demonstrated that well-dispersed ceramic nanoparticles (titania or HA) in PLGA improved mechanical properties compared with agglomerated ceramic nanoparticles even though the weight percentage of the ceramics was the same. Specifically, well-dispersed nanoceramics in PLGA enhanced the tensile modulus, tensile strength at yield, ultimate tensile strength, and compressive modulus compared with the more agglomerated nanoceramics in PLGA. In summary, supplemented by previous studies that demonstrated greater osteoblast (bone-forming cell) functions on well-dispersed nanophase ceramics in polymers, the present study demonstrated that the combination of PLGA with well-dispersed nanoceramics enhanced mechanical properties necessary for load-bearing orthopedic/dental applications.

Keywords: PLGA; agglomeration; biodegradable polymer; ceramic nanoparticles; dispersion; hydroxyapatite nanoparticles; mechanical properties; nanocomposites; orthopedic/dental applications; titania nanoparticles.

Figure 1

The mold-cast tensile specimens of PLGA, agglomerated nano-titania in PLGA composites (PTCa), well-dispersed nano-titania in PLGA composites (PTCd), agglomerated nano-HA in PLGA composites (PHAa), well-dispersed nano-HA in PLGA composites (PHAd) and the casting mold for tensile specimens. The tensile specimen gage length × width × thickness = 25 × 10 × 0.5 mm. The depth of the casting mold was designed as 10 mm.

Figure 2

SEM micrographs of nano-titania and nano-titania/PLGA composites: (a) nano-titania, (b,c) PTCa (the agglomerated nano-titania in PLGA composites), (d,e) PTCd (the well-dispersed nano-titania in PLGA composites). (b,d) the top surface, (c,e) the bottom surface. Magnification bars: 1 μm.

Figure 3

a SEM micrograph of particulate nano-HA synthesized by the wet chemistry method. (b,e) SEM micrographs of nano-HA/PLGA composites: (b,c) PHAa (the agglomerated nano-HA in PLGA composites); (d,e) PHAd (the well-dispersed nano-HA in PLGA composites). (b,d) the top surface, (c,e) the bottom surface. Original magnification: 100 kX. Magnification bars: 200 nm.

Figure 4

(a) The typical tensile stress-strain curves of PLGA, PTCa (the agglomerated nano-titania in PLGA composites) and PTCd (the well-dispersed nano-titania in PLGA composites) that were calculated from the load-extension data of tensile tests. (b) The tensile moduli of PLGA, PTCa and PTCd. (c) The tensile strength at yield and the ultimate tensile strength (UTS) of PLGA, PTCa and PTCd. (d) The elongation (unitless) at yield and (e) the elongation at break of PLGA, PTCa and PTCd. (f) The compressive moduli of PLGA, PTCa and PTCd. Values are mean ± SEM; n = 3; *p < 0.05 compared with PLGA; and **p < 0.05 compared with PTCa.

Figure 5

The typical tensile stress-strain curves of PLGA, PHAa (the agglomerated nano-HA in PLGA composites) and PHAd (the well-dispersed nano-HA in PLGA composites) that were calculated from the load-extension data of tensile tests.

Figure 6

(a) The tensile moduli of PLGA, PHAa (the agglomerated nano-HA in PLGA composites) and PHAd (the well-dispersed nano-HA in PLGA composites). (b) The tensile strength at yield and the ultimate tensile strength (UTS) of PLGA, PHAa and PHAd. (c) The elongation (unitless) at yield and (d) the elongation at break of PLGA, PHAa and PHAd. (e) The compressive moduli of PLGA, PHAa, and PHAd. Values are mean ± SEM; n = 3; *p < 0.05 compared with PLGA; and **p < 0.05 compared with PHAa.

Figure 7

Microscopic fracture appearances of PLGA after tensile tests. Original magnifications are 1 kX for (a,b), 5 kX for (c,d) and 50 kX for (e,f). Magnification bars are 10 μm for (a,b), 2 μm for (c,d) and 200 nm for (e,f). F shows the direction of the load.

Figure 8

Microscopic fracture appearances of PTCa (agglomerated nano-titania in PLGA composites) after tensile tests. The fracture cross-section is shown in (a). The top surfaces of PTCa near the fracture cross-section are shown in (b,c,d). Original magnifications are 1 kX for (a), 5 kX for (b), 20 kX for c and 50 kX for (d). Magnification bars are 10 μm for (a), 2 μm for (b), 1 μm for (c) and 200 nm for (d). F shows the direction of the load.

Figure 9

Microscopic fracture appearances of PTCa (agglomerated nano-titania in PLGA composites) after tensile tests. The bottom surfaces of the PTCa near the fracture cross-sections. Original magnifications are 10 kX for (a,b,c) and 50 kX for (d). Magnification bars are 2 μm for (a), 1 μm for (b,c) and 200 nm for (d). F shows the direction of the load.

Figure 10

Microscopic fracture appearances of PTCd (well-dispersed nano-titania in PLGA composites) after tensile tests. The fracture cross-section is shown in (a). The top surfaces of PTCd near the fracture cross-section are shown in (b,c,d). Original magnifications are 400 X for (a) and 50 kX for (b,c,d). Magnification bars are 100 μm for (a) and 200 nm for (b,c,d).