Eero Santala

The preparation of reusable magnetic and photocatalytic composite nanofibers by electrospinning and atomic layer deposition

A versatile synthesis procedure for composite nanofibers, combining electrospinning and atomic layer deposition (ALD), is presented. Both solid core/sheath nanofibers and nanoparticle loaded nanotubes can be made, depending on the order of calcination of the electrospun fiber and the ALD process. Magnetic and photocatalytic nanofibers prepared this way can be recycled readily by collecting with a magnet.

Selective-Area Atomic Layer Deposition Using Poly(vinyl pyrrolidone) as a Passivation Layer

Selective-area atomic layer deposition (ALD) was studied using poly(vinyl pyrrolidone) (PVP) films as growth-preventing mask layers. The PVP films were prepared by spin coating and patterned by UV lithography. The PVP films were tested in several ALD processes: iridium, platinum, ruthenium, Al 2 O 3 , and ZrO 2 . The deposition temperatures were 250-300°C. In general, the PVP film passivated the surface against the noble metal processes, but the oxide films grew on PVP. However, the oxide films did not grow through the PVP film on the substrate surface and, therefore, the films could still be patterned, though with more of a lift-off method rather than with pure selective-area ALD.

Reducing stiction in microelectromechanical systems by rough nanometer-scale films grown by atomic layer deposition

Stiction during device operation remains one of the mechanisms leading to permanent failure of operating silicon-based MEMS devices (MicroElectroMechanical Systems). The goal of this work was to investigate, whether stiction between parallel, smooth silicon surfaces can be decreased by thin inorganic films grown by atomic layer deposition (ALD). Test structures based on the cantilever-beam-array (CBA) method were fabricated and coated with ALD layers varying in chemical nature and roughness. Rough crystalline TiO2 decreased the adhesion energy orders of magnitude as compared to Si and other smooth films, indicating that TiO2 and other crystalline ALD films are candidates for anti-stiction layers in MEMS.

Osteoclasts in the interface with electrospun hydroxyapatite.

Electrospinning is a method to produce lightweight, resorbable and bioinspired scaffolds for tissue engineering. Here we investigated the influence of electrospun hydroxyapatite fibers (HA) on macrophages and osteoclasts. A mouse macrophage cell line (RAW 264.7) and human bone marrow derived primary osteoclasts (hOC) were cultured with electrospun HA fibers embedded in Matrigel. Cell morphology and the secretion of pro-inflammatory cytokines (IL-6 and TNF-α) were analyzed using macrophages. Both fluorescent microscopy and scanning electron microscopy indicated that the cell morphology differed on the various materials (HA fibers on Matrigel, pure Matrigel and a glass control). Control macrophages were activated with bacterial lipopolysaccharide (LPS) but electrospun HA did not provoke an inflammatory response. Cytokine secretion detected with enzyme-linked immunosorbent assay (ELISA) also supported this observation. LPS, but not HA fibers, stimulated TNF-α and IL-6 secretion by macrophages at the 2 day time point. After 4 days in culture there was an increasing trend in cytokine secretion in the HA fiber samples. Human bone marrow myeloid precursor cells were able to fuse and differentiate on the fibrous mineral scaffold to form functional multinuclear osteoclasts that were able to resorb the HA nanofibers. This indicates that osteoclasts do not necessarily need a continuous bone surface but osteoclast ruffled border membranes can form a resorption interface with a fibrous mineral scaffold.

Preparation and bioactive properties of nanocrystalline hydroxyapatite thin films obtained by conversion of atomic layer deposited calcium carbonate.

Nanocrystalline hydroxyapatite thin films were fabricated on silicon and titanium by atomic layer deposition (ALD) of CaCO3 and its subsequent conversion to hydroxyapatite by diammonium hydrogen phosphate (DAP) solution. The effects of conversion process parameters to crystallinity and morphology of the films were examined. DAP concentration was found to be critical in controlling the crystal size and homogeneity of the films. The hydroxyapatite phase was identified by XRD. ToF-elastic recoil detection analysis studies revealed that the films are calcium deficient in relation to hydroxyapatite with a Ca/P ratio of 1.39 for films converted with 0.2 M DAP at 95 °C. The coatings prepared on titanium conformally follow the rough surface topography of the substrate, verifying that the good step coverage of the ALD method was maintained in the conversion process. The dissolution tests revealed that the coating was nondissolvable in the cell culture medium. Annealing the coated sample at 700 °C for 1 h seemed to enhance its bonding properties to the substrate. Also, the biocompatibility of the coatings was confirmed by human bone marrow derived cells in vitro. The developed method provides a new possibility to produce thin film coatings on titanium implants with bone-type hydroxyapatite that is biocompatible with human osteoblasts and osteoclasts.

Electrospinning of calcium carbonate fibers and their conversion to nanocrystalline hydroxyapatite

Calcium carbonate (CaCO3) fibers were prepared by electrospinning followed by annealing. Solutions consisting of calcium nitrate tetrahydrate (Ca(NO3)2·4H2O) and polyvinylpyrrolidone (PVP) dissolved in ethanol or 2-methoxyethanol were used for the fiber preparation. By varying the precursor concentrations in the electrospinning solutions CaCO3 fibers with average diameters from 140 to 290 nm were obtained. After calcination the fibers were identified as calcite by X-ray diffraction (XRD). The calcination process was studied in detail with high temperature X-ray diffraction (HTXRD) and thermogravimetric analysis (TGA). The initially weak fiber-to-substrate adhesion was improved by adding a strengthening CaCO3 layer by spin or dip coating Ca(NO3)2/PVP precursor solution on the CaCO3 fibers followed by annealing of the gel formed inside the fiber layer. The CaCO3 fibers were converted to nanocrystalline hydroxyapatite (HA) fibers by treatment in a dilute phosphate solution. The resulting hydroxyapatite had a plate-like crystal structure with resemblance to bone mineral. The calcium carbonate and hydroxyapatite fibers are interesting materials for bone scaffolds and bioactive coatings.

Low-temperature magnetism of alabandite: Crucial role of surface oxidation

Abstract Manganese(II) monosulphide crystallizes into three different polymorphs (α-, β-, and γ-MnS). Out of these, α-MnS, also known as mineral alabandite, is considered the most stable and is widespread in terrestrial materials as well as in extraterrestrial objects such as meteorites. In this study, a low-temperature antiferromagnetic state of α-MnS was investigated using macroscopic magnetic measurements as induced and remanent field-cooled (FC) and zero-field-cooled (ZFC) magnetizations and magnetic hysteresis. Both natural alabandite and synthetic samples show: (1) Néel temperatures in a narrow temperature range around 153 K, and (2) a rapid increase of the magnetization around 40 K. An anomalous magnetic behavior taking place at about 40 K was previously ascribed to the magnetic transition from a high-temperature antiferromagnetic to a low-temperature ferromagnetic state documented for non-stoichiometric α-MnS slightly enriched in manganese. However, our detailed microscopic observations and, in particular, oxidation experiments indicate that the anomalous magnetic behavior around 40 K is caused by the presence of an oxide layer of ferrimagnetic hausmannite (Mn3O4) on the surface of α-MnS rather than being an intrinsic property of nearly stoichiometric α-MnS.

Electrospun sodium titanate fibres for fast and selective water purification

ABSTRACT From the environmental and end-users’ viewpoints, electrospun ion exchange fibres provide highly efficient and sustainable material for separation of for example trace pollutants, such as radionuclides and heavy metals. This work aimed to reduce the amount of ion exchange material needed per unit volume of raw material subjected to an ion exchange process. We present a very simple process to electrospinning of sodium titanate fibres, but also test results of ion exchange kinetics measurements. Sodium titanate fibres are very promising material and it is possible that by exploiting electrospun inorganic sub-micron fibres the ion exchanger mass required for a given capacity can be decreased significantly. GRAPHICAL ABSTRACT