Morphological characterization and in vitro biocompatibility of a porous nickel–titanium alloy

Abstract

Disks consisting ofmacroporous nickel–titanium alloy (NiTi, Nitinols, Actipores) are used as implants in clinical surgery, e.g. for fixation of spinal dysfunctions. The morphological properties were studied by scanning electron microscopy (SEM) and by synchrotron radiation-based microtomography (SRmCT). The composition was studied by X-ray diffractometry (XRD), differential scanning calorimetry (DSC), and energy-dispersive X-ray spectroscopy (EDX). The mechanical properties were studied with temperature-dependent dynamical mechanical analysis (DMA). Studies on the biocompatibility were performed by co-incubation of porous NiTi samples with isolated peripheral blood leukocyte fractions (polymorphonuclear neutrophil granulocytes, PMN; peripheral blood mononuclear leukocytes, PBMC) in comparison with control cultures without NiTi samples. The cell adherence to the NiTi surface was analyzed by fluorescence microscopy and scanning electron microscopy. The activation of adherent leukocytes was analyzed by measurement ofthe released cytokines using enzyme-linked immunosorbent assay (ELISA). The cytokine response of PMN (analyzed by the release of IL-1ra and IL-8) was not significantly different between cell cultures with or without NiTi. There was a significant increase in the release ofIL-1ra (po0:001), IL-6 (po0:05), and IL-8 (po0:05) from PBMC in the presence of NiTi samples. In contrast, the release ofTNF- a by PBMC was not significantly elevated in the presence ofNiTi. IL-2 was released from PBMC only in the range ofthe lower detection limit in all cell cultures. The material, clearly macroporous with an interconnecting porosity, consists ofNiTi (martensite; monoclinic, and austenite; cubic) with small impurities ofNiTi 2 and possibly NiCx. The material is not superelastic upon manual compression and shows a good biocompatibility.