The successful development of micro-needles can help transport drugs and vaccines both effectively and painlessly across the skin. However, not all micro-needles are strong enough to withstand the insertion forces and viscoelasticity of the skin. The work here focuses on the micro-fabrication of high aspect ratio needles with careful control of needle-profile using dry etching technologies. Silicon micro-needles, 150μm in length with base-diameters ranging from 90 to 240μm have been investigated in this study. A novel, multiple-sacrificial approach has been demonstrated as suited to the fabrication of long micro-needle bodies with positive profiles. The parameters that control the isotropic etching are adjusted to control the ratio of the needle-base diameter to needle length. By careful control of geometry, the needle profile can be engineered to give a suitable tip size for penetration, as well as a broad needle base to facilitate the creation of either single or multiple-through holes. This approach allows the mechanical properties of the otherwise brittle needles to be optimized. Finite element analysis indicates that the micro-needles will fracture prematurely due to buckling, with forces ranging from 10 to 30mN.
KEYWORDS: Silver, Surface plasmons, Crystals, Plasma, Temperature metrology, Particles, Transmission electron microscopy, Scanning electron microscopy, Bacteria, Plasma systems
The spark plasma sintering behaviour of silver nanopowder prepared by the electro-explosion method was investigated. Consolidation was carried out from 50°C to 800°C for 5 mins at 34 MPa with differential scanning calorimetry indicating a sintering onset temperature as low as 160°C and an activation energy of 86±1 kJ/mol. Near full density resulted from treatment at 300°C, and at higher temperatures a normal Hall-Petch relation is obeyed. The enhancement of Vicker's hardness to 1000MPa for materials sintered at 300°C is three times greater than for silver annealed in a conventional way. While polysynthetic twinning contributes to superior hardness, the primary cause is the sub-micron grain size.
Mechanical and thermodynamic measurements are reported for specimens spark plasma sintered from tungsten nano-powder.
A maximum Vickers hardness of 3.31±0.03 GPa, at a relative density of 0.98 was achieved at sintering
temperature of 1,800°C and above. This corresponds to the value of hardness for bulk tungsten. Grain size analysis
indicated a grain growth activation energy of 160±10% KJ/mol. Sintering temperatures were found to be as much as
270°C lower than previously reported. During the Spark Plasma Sintering (SPS), a reduction in the amount of tungsten
oxide was observed, especially at temperatures above 1,500°C. A number of mechanisms are thought to be responsible
including the SPS induced decomposition of the oxide into metallic tungsten.
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