This work reports on the behavior of mesenchymal stem cells on anodic ZrO(2) nanotube layers grown by a self-ordering process on zirconium with defined diameters between 10 and 50 nm. It is demonstrated that mesenchymal stem cells show a size-specific reaction to these nanoscale patterned surfaces. We compare the behavior on these ZrO(2) nanotubes to findings on TiO(2) nanotubes of different diameters. For both nanotube materials, TiO(2) and ZrO(2), cell adhesion and spreading are enhanced for nanotube diameters of approximately 15-30 nm, while a strong decay in cell activity is observed for diameters >50 nm. Focal complex formation on adherent cells is selectively modulated by the specific nanoscale. Moreover, even if the surface chemistry of the nanotubes is completely modified with a dense AuPd coating onto the formed nanotube layers, or the length of the nanotubes is varied, the observed nano size effects still prevail. This demonstrates how strong the pure geometric diameter dependence in the range between 15 and 100 nm dominates over other possible effects on cell activity.
Highly self-ordered arrays of anodic nanopores or nanotubes have attracted tremendous scientific interest within the last few years. Starting with Al in 1995 [1], today a large number of valve metals like Ti [2,3], Zr [4,5], W [6], Hf [7], Ta [8,9], Nb [10] and alloys (TiNb [11], TiZr [12], TiAl [13]) can be anodically nanostructured using fluoride containing electrolytes. Besides of widening the spectrum of materials also extensive research is carried out to improve the self-ordering [14] and growth [15] of the nanostructured oxide films. One of the most investigated systems is nanotubular TiO 2 due to its unique properties with a vast number of possible applications [3]. Another highly prosperous material is zirconium. ZrO 2 nanotubes have been shown to grow under various conditions in several water-based electrolytes containing fluorides [4,5,16,17]. Numerous possible applications for nanotubular ZrO 2 exist such as for catalyst support structures [18,19], for sensing [20] or for applications as a solid state electrolyte [21][22][23]. However, many of these applications require a further optimization of the growth morphology (particularly order) of the ZrO 2 nanotube arrays.In the present work we report on the drastic optimization of tube morphology for tube growth in aqueous and organic fluoride containing electrolytes by means of two different surface pretreatments. The first investigated pretreatment is so-called dip-etching (1 s) of the Zr metal in a solution containing HF/HNO 3 /H 2 O (1:4:2) prior to anodization [24]. This pretreatment has been selected as the production process (i.e. rolling of the Zr sheet) leads to a damage zone on the surface. This damage zone was anticipated to have a strong effect on the tube initiation process and thus could affect the resulting tube geometry. The second pretreatment investigated is a two-step anodization process to pre-texture the Zr surface with a suitable pattern -serving as a geometrical guide for self-organization. Such pretexturing has been shown to be highly effective to increase ordering in the case of Al [25] and Ti [14]. For this procedure a first anodization was conducted in 1 M (NH 4 ) 2 SO 4 electrolyte containing 0.75 M NH 4 F at 20 V for 30 min. The obtained layers were then removed through ultrasonication in ethanol. The removal of the first nanotubular oxide layer results in a surface showing ordered dimples of regular size and distribution. Anodization of 0.1 mm thick zirconium foil (99.8% purity, Goodfellow, England) and sample cleaning were carried out as described previously [3]. The electrolytes used within this work were 1 M (NH 4 ) 2 SO 4 containing 0.15 M NH 4 F (= aqueous/inorganic electrolyte) and ethylene glycol/glycerol (50:50) mixed electrolyte containing 0.3 M NH 4 F and 4 vol% H 2 O (= organic electrolyte). Figure 1 shows SEM (Hitachi FE-SEM 4800) images of ZrO 2 nanotube layers obtained, grown on the different This work reports on the optimized growth of self-ordered ZrO 2 nanotubes in inorganic water-based and organic electrolyte ...
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