The Role of Interfacial Mechanics in the Prediction of Global Mechanical Behavior of a Bioactive Composite: An In Vitro Study
A bioactive bone-tissue substitute, hydroxyapatite (HA)-polymethylmethacrylate (PMMA) with the addition of a copolymer coupling agent, was examined in vitro to determine the influence of the coupling agent on the local mechanical properties of the system before and after simulated biologic conditions. Nano-indentation of the cross-sectional interface between the HA and PMMA of the composite was studied. The fracture mechanism and position of each indent mark were analyzed at up to 5000× magnification under field-emission, environmental-scanning electron microscopy. The local interfacial results were compared with global quasistatic compression test results. It was found that nano-indentation of the interface could predict changes in global mechanical behavior of the composite. Both interfacial and global Young's moduli were reduced after immersion in the simulated biologic media. Although the coupling agent improved the interfacial and global mechanical properties before and after 24 hours in in vitro immersion, it did not affect the surface bioactivity of the system, as shown in the measurement of calcium and phosphate concentration uptake. Thus, nano-indentation is a sensitive technique for examining interfacial mechanics and mechanical consequences of biologic reactivity of composite materials.Abstract
Field-emission scanning electron microscope photos of the composites immersed for 72 hours with indentations at (a) the center of the hydroxyapatite (HA) particle (original magnification ×3500) and (b) the HA-polymethylmethacrylate interface (original magnification ×5000).
Calcium-to-phosphate ratios on a 100-μm2 surface area of the controls and treated composites after immersing in simulated body fluid at various immersion times.
The field-emission scanning electron microscope photos (10 keV, 3.0 spot, original magnification ×5000) of the treated hydroxyapatite particles after in vitro chemical reaction.
(a) Calcium concentration in simulated body fluid (SBF) after immersion of controls and treated composite from 1 hour to 72 hours. (b) Phosphate concentration in SBF after immersion of controls and treated composite from 1 hour to 72 hours.
(a) Global Young's modulus of the controls and treated composites before and after 24 hours of immersion in simulated body fluid (SBF). (b) Global ultimate compressive strength of the controls and treated composites before and after 24 hours of immersion in SBF.
(a) Typical load-displacement curves at the interface of the controls unimmersed and 72 hours immersed. (b) Typical load-displacement curves at the interface of the polymethylmethacrylate-methylmethacrylate–coupled composites unimmersed and at 72 hours of immersion.
(a) Interfacial Young's modulus of the controls and treated composites as a function of immersion time. (b) Interfacial hardness of the controls and treated composites as a function of immersion time.
Normalizing the global and interfacial Young's moduli of the treated composite with properties of the control.
Contributor Notes
Emily Ho, PhD, and Michele Marcolongo, PhD, are associate professors in the Materials Science and Engineering Department at Drexel University in Philadelphia, Pa. Address correspondence to Dr Marcolongo, Lebow Room 336, 32 and Chestnut Streets, Philadelphia, PA 19104.