Drug delivery with microsecond laser pulses into gelatin

Photo acoustic drug delivery is a technique for localized drug delivery by laser-induced hydrodynamic pressure following cavitation bubble expansion and collapse. Photoacoustic drug delivery was investigated on gelatin-based thrombus models with planar and cylindrical geometries by use of one microsecond laser pulses. Solutions of a hydrophobic dye in mineral oil permitted monitoring of delivered colored oil into clear gelatin-based thrombus models. Cavitation bubble development and photoacoustic drug delivery were visualized with flash photography. This study demonstrated that cavitation is the governing mechanism for photoacoustic drug delivery, and the deepest penetration of colored oil in gels followed the bubble collapse. Spatial distribution measurements revealed that colored oil could be driven a few millimeters into the gels in both axial and radial directions, and the penetration was less than 500 μm when the gelatin structure was not fractured.     

脉冲激光烧蚀过程中阻尼系数的角度分布

采用脉冲激光烧蚀(PLA)技术,在半圆环衬底上制备了含有纳米晶粒的硅(Si)晶薄膜。分析了纳米Si晶粒尺寸和阻尼系数随角度和压强的变化关系。使用扫描电子显微镜(SEM)、X射线衍射仪(XRD)和拉曼光谱仪(Raman)对其表面形貌和结构进行了表征。结果表明,在压强一定的情况下,纳米Si晶粒的尺寸和阻尼系数均相对于轴向呈对称分布,并随着偏离轴向角度的增加而减小;同时随着压强增大,晶粒尺寸和阻尼系数在各个角度处的值均增大。     

Microstructural development in microsecond pulsed laser melted polycrystalline nickel

The microsecond Nd glass pulsed laser interaction with polycrystalline nickel was studied with particular emphasis on melting and the nature of the microstructural development following irradiation. The change in the melt pool shape and the melting efficiency as a function of the total laser power has been determined. Evidence is presented to show that for low energy pulse, the microstructural development accompanies fresh nucleation of grains and leads to a finer grain size than that of the substrate. At a higher energy level, growth competition leads to a coarser grain size. We also report a banded feature roughly parallel to the melt substrate interface that develops in the resolidified region during high energy interaction. These results are discussed in the light of our present understanding of rapid solidification processes.     

Nano-and microsecond laser pulse confinement in compensated gallium arsenide

Experimental data on the nano-and microsecond pulsed laser radiation (λ=1.315 μm) confinement in compensated GaAs are reported. There are three regions of confinement, which are controlled by radiation selfdefocusing due to single-photon absorption by deep impurity levels, self-defocusing due to two-photon absorption, and self-focusing caused by thermal dynamic lens formation.     

Film formation during pulsed laser ablation of nickel-palladium composite targets

The films of Pd, Ni, and a Ni-Pd alloy have been obtained by pulsed laser ablation of Ni-Pd composite targets and characterized by electron diffraction, transmission electron microscopy, and vibrating-sample magnetometry techniques. It is established that a nonmagnetic metastable hexagonal phase is formed in Ni and Ni-Pd films. Upon annealing, these films acquire an equilibrium cubic structure and exhibit hysteresis during the magnetization reversal. The dependence of the solid-solution lattice constant on the concentration exhibits a positive deviation from Vegard’s law, which is characteristic of alloys with concave liquidus curves.     

Analysis of the PMMA and cornea temperature rise during excimer laser ablation

A general method to analyze the ablation temperature for different materials (in particular in the human cornea and poly-methyl-methacrylate (PMMA)) is provided. The model is comprehensive and provides directly laser beam characteristics and ablative spot properties. The model further provides a method to convert the temperature rise during ablation observed in PMMA to equivalent temperature rises in the cornea. The proposed model can be used for calibration, verification and validation purposes of laser systems used for ablation processes at relatively low cost and would directly improve the quality of results.     

Microstructural evolution of carbon fiber reinforced SiC-based matrix composites during laser ablation process

In this work, four different carbon fiber reinforced SiC-based matrix composites (C/SiC) were prepared, and microstructure evolution during laser ablation process was characterized. Laser irradiation provided a special high-temperature environment up to 3500 °C. For all four composites, different morphologies can be obtained in the transition region due to the oxidation of different matrices. While only needle-shaped carbon fiber and nanolayered carbon without any matrix remained in the central region, indicating that graphitization process occurred in the center, resulting from the high-temperature and low-oxygen environment in the laser process. Therefore, the laser ablation of C/SiC composites is controlled by chemical and physical erosion, and mainly by the physical erosion in the center.     

Detailed mass spectrometric studies during laser ablation of yttrium target in high vacuum

Time-integrated and-resolved mass spectrometric investigations were carried out during the laser ablation of yttrium target in high vacuum. The main aim of these experiments was to study the implications of the target surface chemistry and of the residual gas on the deposition process quality. The effect of the laser energy density on the ionic species formation during the ablation process was investigated. It was found that at a moderate laser fluence of 4.5 J/cm² none hydrides, oxides nor nitrides of yttrium were observed; on the contrary, at a higher laser fluence of 7.6 J/cm², the above chemical species were observed. During and after the laser ablation process, a getter effect was observed due to the absorption of the residual gas by the Y layer deposited on the internal wall of the vacuum system. The results of these mass spectrometric analyses will be useful to improve the performances of photocathodes based on Y thin films grown by pulsed laser ablation technique.     

Metal nanoparticles generated by laser ablation

We study a new method for producing ultrafine metal particles (nanopartides) that employs Laser Ablation of Microparticles (LAM). Pulsed excimer laser radiation at 248 nm wavelength was used to ablate ~2 μm feedstock of silver, gold, andpermalloy (Ni₈₁%:Fe_19%) under both normal atmospheric conditions and in other gases and pressures. A model for nanoparticle formation by LAM is proposed that includes plasma breakdown and shock-wave propagation through the initial microparticle. Behind the shock a large fraction of the original microparticle mass is converted to nanoparticles that diffuse to silicon substrates and TEM grids for collection and analysis. Nanoparticle morphologies are spherical except for gold nanoparticles >100 nm that are generally cubes. Electron micrographs of the samples were analyzed by computer-aided image processing to determine the effect of irradiation conditions on the nanoparticle size distribution. The results showed that mean particle diameters were normally in the range from 10 to 100 nm and that the particle size distributions were generally log-normal, with dispersion (diameter/standard deviation) ranging from 0.2 to 0.5. For metallic microparticle feedstock, the mean size of the produced nanoparticles generally increased with increasing laser fluence and were smallest for fluences not too far above the breakdown threshold.     

Low-power resonant laser ablation of copper

We emphasize two points: (l) the properties and mechanisms of very low-fluence ablation of copper surfaces and (2) the sensitivity and selectivity of resonant laser ablation (RLA). We present results for ablation of bulk copper and copper thin films; spot-size effects; the effects of surface-sample preparation and beam polarization; and an accurate measurement of material removal rates, typically ≤ 10(-3) ? at 35 mJ/cm(2). Velocity distributions were Maxwellian, with peak velocities ≈ 1-2 × 10(5) cm/s. In addition, we discuss the production of diffractionlike surface features, and the probable participation of nonthermal desorption mechanisms. RLA is shown to be a sensitive and useful diagnostic for studies of low-fluence laser-material interactions.     

Carbon nanofoam formed by laser ablation

Foam-like carbon (carbon nanofoam, CNF) which belongs to the porous carbon family is formed by pulsed laser ablation of graphite in liquid nitrogen. Each bubble is about 3-10 nm in size and has a layered structure with typically one to four graphene layers. The CNF forms nanoparticles of about 100 nm in size. CNF encapsulating platinum nanoparticles (Pt@CNF) is formed when a mixture of graphite, platinum, and hexadecanoic acid is used as a target. Each bubble encapsulating a platinum nanoparticle is approximately 15 +/- 4 nm in diameter and contains typically 6 +/- 3 graphene walls. The platinum nanoparticles in the CNF are 9 +/- 4 nm in diameter. The annealing of the Pt@CNF at 300 degrees C for a week in vacuum reveals that the CNF effectively prevents the platinum nanoparticles from aggregating.     

Existence of phase explosion during laser ablation and its effects on inductively coupled plasma-mass spectroscopy

A sudden increase in crater depth was observed during high irradiance (> 10(10) W/cm2) laser ablation of silicon, and it is attributed to the phenomenon of phase explosion. The threshold irradiance for phase explosion showed a dependence on two laser parameters: laser beam spot size and wavelength. For a larger beam size and longer incident wavelength, a higher laser irradiance was required to generate phase explosion. The rapid increase of crater depth above the phase explosion threshold irradiance correlated with a significant increase in the ICPMS signal intensity. The ratio of crater volume to ICPMS intensity, which represents entrainment efficiency, remained the lowest at laser irradiances slightly above the phase explosion threshold. However, this ratio increased at irradiances well above the threshold (> 10(11) W/cm2). Chemical analysis using laser ablation at irradiance above 10(11) W/cm2 provides increased sensitivity via improved entrainment and transport efficiency and increased ablation rate.     

Plasmon-enhanced sub-wavelength laser ablation: plasmonic nanojets

In response to the incident light's electric field, the electron density oscillates in the plasmonic hotspots producing an electric current. Associated Ohmic losses raise the temperature of the material within the plasmonic hotspot above the melting point. A nanojet and nanosphere ejection can then be observed precisely from the plasmonic hotspots.     

Investigation of laser ablation loading of polymers

For polyethylene and polytetrafluoroethylene targets irradiated by laser pulses with energy densities within 100–1000 J/cm², the ablation loading coefficients amount to 8–11% and 8–10%, respectively. These values satisfactorily agree with theoretical estimates of the ablation loading, which are about 6–8%.     

Chemically assisted laser ablation ICP mass spectrometry

A new laser ablation technique combined with a chemical evaporation reaction has been developed for elemental ratio analysis of solid samples using an inductively coupled plasma mass spectrometer (ICPMS). Using a chemically assisted laser ablation (CIA) technique developed in this study, analytical repeatability of the elemental ratio measurement was successively improved. To evaluate the reliability of the CLA-ICPMS technique, Pb/U isotopic ratios were determined for zircon samples that have previously been analyzed by other techniques. Conventional laser ablation for Pb/U shows a serious elemental fractionation during ablation mainly due to the large difference in elemental volatility between Pb and U. In the case of Pb/U ratio measurement, a Freon R-134a gas (1,1,1,2-tetrafluoroethane) was introduced into the laser cell as a fluorination reactant. The Freon gas introduced into the laser cell reacts with the ablated sample U, and refractory U compounds are converted to a volatile U fluoride compound (UF6) under the high-temperature condition at the ablation site. This avoids the redeposition of U around the ablation pits. Although not all the U is reacted with Freon, formation of volatile UF compounds improves the transmission efficiency of U. Typical precision of the 206Pb/238U ratio measurement is 3-5% (2sigma) for NIST SRM 610 and Nancy 91500 zircon standard, and the U-Pb age data obtained here show good agreement within analytical uncertainties with the previously reported values. Since the observed Pb/U ratio for solid samples is relatively insensitive to laser power and ablation time, optimization of ablation conditions or acquisition parameters no longer needs to be performed on a sample-to-sample basis.     

Percolation in laser ablation of binary mixtures

We have studied the optical emission of plasma expanding from a target made of a binary alloy or a pressed powder mixture and ablated by nanosecond laser pulses of moderate power. The intensity of spectral lines has been studied for the first time as dependent on the target composition. It is established that a three-dimensional percolation takes place in the plasma, with a percolation threshold determined by the atomic density of a metal component in the target composition.     

激光法制备碳质纳米材料

通过介绍在气体和液体介质中激光与固体材料相互作用的过程,评述了激光在不同介质中发生物理化学现象的差异.与气相中相比,激光冲蚀液体中固体材料产生的气态等离子区受到了液体限制,在该区域会产生更高的气态密度、温度和压力,适合于亚稳相纳米晶的合成.同时评述了激光制备碳基纳米材料的进展.激光在气相和液相中均可制得碳纳米管,气相中适于制备结构完整的碳纳米管,而液相中有利于纳米金刚石的合成.激光冲蚀液体中的石墨靶制备的纳米金刚石粒径较大,辐照石墨悬浮液工艺不仅可以获得超细的纳米金刚石还可以获得线型碳.激光法制备的碳基纳米材料具有尺寸小、纯度高和形状多样性,在未来有着广泛的潜在应用价值.     

A finite element model to predict the ablation depth in pulsed laser ablation

This work presents development of a two-dimensional finite element model to predict temperature distribution and ablation depth in a laser ablation process. The model considers a number of aspects of the process, which hitherto have been considered independently in the literature. The aspects considered include: temperature dependent material properties of the target material, effect of plasma shielding on the incident laser flux, and temperature dependent absorptivity and absorption coefficient of the target. It was evident that these considerations have resulted in a significant improvement in the ability of the model to predict the ablation depth. Finally, the predicted ablation depth was found to match extremely well with experimental results at lower laser fluences, though at higher fluences there is a marginal overestimation.     

Zinc nanoparticles in solution by laser ablation technique

Colloidal zinc metallic nanoparticles are synthesized using pulsed laser ablation of metal plate in an aqueous solution of suitable surfactant to prevent aggregation. UV-visible absorption, TEM, small angle X-ray diffraction and wide-angle X-ray diffraction are used for the characterization of colloidal zinc metallic nanoparticles. Colloidal nanoparticles are found highly stable for a long time.