The change of crystal symmetry and cation ordering in Li-Mg ferrites

In this work we present results of synthesis and structural investigation of the spinels with general formula LixMg1-2xFe2+xO4, where 0.00less than or equal toxless than or equal to0.50. Ultrafine powder of MgFe2O4 (x=0.00) was prepared by thermal decomposition of a mixture of complex compounds [Mg(AA)(2)] and [Fe(AA)(3)] at 500degreesC (AA=acetylacetone). The solid solutions of LixMg1-2xFe2+xO4 (0.10less than or equal toxless than or equal to0.45) were synthesized by conventional ceramic method. The samples with xgreater than or equal to0.40 belong to ordered spinels with primitive cubic cell, space group P4(3)32 and the samples with xgreater than or equal to0.35 are spinels with face-centered cubic cell, space group Fd-3m. Crystal structures of samples with x=0.00, 0.40 and 0.45 were refined by the Rietveld method using DBWS-9411 software package. In ordered spinels (x=0.40, 0.45) Li+ ions occupy both 4b (preferentially) and 12d octahedral sites. At tetrahedral 8c positions Li+ ions were not found (within the experimental error). Mg2+ ions occupy octahedral 12d and tetrahedral 8c sites and Fe3+ ions occupy all three different cationic sites. Cation ordering in 1:3 type ordered spinel (the occupancy ratio of the Wyckoff's positions 4b and 12d) of Li+ and Fe3(+) at octahedral sites is not complete as in Li0.5Fe2.5O4, In MgFe2O4 spinel, cation distribution in the tetrahedral sites was determined and given by occupation numbers N: N(Mg-8a(2+))=0.183(7) and N(Fe-8a(3+))=0.817(7). (C) 2002 Elsevier Science BY All rights reserved

The enhancement of cartilage regeneration by use of a chitosan-based scaffold in a 3D model of microfracture in vitro : a pilot evaluation

Purpose: Even though various types of scaffolds have been used lately as a complement to microfracture, the exact mechanism of reported cartilage repair improvement when using scaffolds is still unclear. In this study, an effort has been made to identify the specific effects that scaffolds may have on the cells of reparation when using this technique. Methods: A 3-D model in vitro, representing microfracture and containing both chondrocytes and bone marrow-derived cells in different experimental conditions was made, and the cells were cultured for eight weeks. Subsequently, the constructs containing our 3-D model were removed from the cell culture medium, fixed in paraffin and analyzed with immunohistochemistry. Results: Bone marrow - derived cells migrated to the upper compartment of the construct through a perforated nylon membrane containing both enzymatically digested- and non-digested particulated cartilage. The histological sections were stained with hematoxylin, eosin, S-100, SOX-9, Gomori, and procollagen type I and II. When minced cartilage wasn't pretreated with collagenase, exclusively bone-derived cells have created new extracellular matrix as showed by the histological analysis. Conclusions: In this model of microfracture, bone-derived cells but not chondrocytes have shown to have an active role in new cartilage formation without predigestion with collagenase. Moreover, it seems that the addition of a chitosan-based scaffold may lead to the improvement of a new cartilage matrix synthesis and integration. This effect hasn't been seen without the use of scaffold or when a fibrin- or a collagen-based scaffold have been used