Positron annihilation lifetime spectroscopy of nano/macroporous bioactive glasses
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Adam Ingram Department of Physics of Opole University of Technology, Opole, PL-45370, Poland (Received 25 April 2012; accepted 20 June 2012)
Positron annihilation lifetime measurements are performed for sol–gel-derived 70 mol% SiO2–30 mol% CaO bioactive glass. Strong positronium formation processes are shown to be an inherent feature for these kinds of materials. Observed orthopositronium (o-Ps) lifetimes show a three-modal distribution with lifetime values weighed at ;2, ;18, and ;70 ns. The exposure of the investigated sol–gel-derived bioactive glasses to water vapor significantly modifies o-Ps lifetime distribution due to the penetration of water molecules into the nanopores, indicating high ratio of their interconnectivity. Classic Tao–Eldrup equation is used to relate the o-Ps lifetimes with the size of nanopores, whose distribution is verified by nitrogen adsorption porosimetry.
I. INTRODUCTION
Sol–gel-derived CaO–SiO2 bioactive glasses (BG) have generated significant interest as biomaterials for hard tissue replacement and regeneration, as they encourage the precipitation of an apatite-like surface layer when soaked in simulated body fluid (SBF),1 promote proliferation in vitro of bone-forming cells2,3 and stimulate in vivo tissue growth.4 In addition, having intrinsic nanoporosity and high specific surface area, they show significant biodegradability and, therefore could be promising materials for bioscaffold in tissue engineering applications.5,6 On the other hand, interconnected macropores (at least 100 lm) are required for bioscaffold to allow new tissue ingrowth and blood vessel formation.7 The newly proposed nano/macro dual porous structure has been achieved by various fabrication methods for bioinert as well as bioactive glasses. For example, nano/macroporous (with dimensions ;2–10 nm and ;0.1–40 lm, respectively) samples can be fabricated by polymerizationinduced phase separation in various silica sol–gel systems using different polymers, such as poly(ethylene oxide) (PEO), poly(ethylene glycol) (PEG), or poly(acrylic acid) (PAA)8,9; foaming sol–gel systems where a surfactant (e.g., Teepol) is added to a sol prepared from a mixture of alkoxides, deionized water and HNO35,10; Chmelka’s method—the process, where macroporous templates such a)
Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/jmr-editor-manuscripts/ DOI: 10.1557/jmr.2012.252 J. Mater. Res., Vol. 27, No. 19, Oct 14, 2012
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as a polystyrene foam or an oil-in-water emulsion are used11; or salt sintering process, where a glass prepared by melt quenching is crushed and mixed with sodium chloride, followed by heat treatment and final immersion into water to promote dissolution of the salt.12,13 More recently, hierarchically three-dimensional (3D) nano/macroporous BGs were prepa
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