We investigate the effect of shear flow applied during the drying of aqueous suspension of cellulose nanocrystals on optical reflective properties and structural characteristics of the resulting ...solidified films. Shear flow can significantly improve internal structural homogeneity of the films, while its effect on optical reflective properties is relatively minor. The measured width of the selective reflection peak is an order of magnitude larger than expected for an ideal helically modulated structure, which reflects a distribution of pitch values and possibly also of regimes of distorted helical modulation. We attribute these imperfections to the broad size distribution of the cellulose nanocrystals.
The chemical stability of upconverting, lanthanide-doped, fluoride nanoparticles in aqueous media, which shows great potential in bioimaging, was studied. The partial dissolution of selected ...nanoparticles (NaYF 4 , LaF 3 and GdF 3 ) co-doped with Yb 3+ and Tm 3+ was detected and compared with respect to their size, chemical composition and structure.
Pulsed photothermal radiometry (PPTR) allows for noninvasive determination of the laser-induced temperature depth profile in strongly scattering samples, including human skin. In a recent ...experimental study, we have demonstrated that such information can be used to derive rather accurate predictions of the maximal safe radiant exposure on an individual patient basis. This has important implications for efficacy and safety of several laser applications in dermatology and aesthetic surgery, which are often compromised by risk of adverse side effects (e.g., scarring, and dyspigmentation) resulting from nonselective absorption of strong laser light in epidermal melanin. In this study, the differences between the individual maximal safe radiant exposure
(
H
max
)
values as predicted from PPTR temperature depth profiling performed using a commercial mid-IR thermal camera (as used to acquire the original patient data) and our customized PPTR setup are analyzed. To this end, the latter has been used to acquire 17 PPTR records from three healthy volunteers, using 1 ms laser irradiation at 532 nm and a signal sampling rate of 20 000
s
−
1
. The laser-induced temperature profiles are reconstructed first from the intact PPTR signals, and then by binning the data to imitate the lower sampling rate of the IR camera (1000 fps). Using either the initial temperature profile in a dedicated numerical model of heat transfer or protein denaturation dynamics, the predicted levels of epidermal thermal damage
(
Ω
)
and the corresponding
H
max
are compared. A similar analysis is performed also with regard to the differences between noise characteristics of the two PPTR setups.
Pulsed photothermal profiling involves reconstruction of temperature depth profile induced in a layered sample by single-pulse laser exposure, based on transient change in mid-infrared (IR) emission ...from its surface. Earlier studies have indicated that in watery tissues, featuring a pronounced spectral variation of mid-IR absorption coefficient, analysis of broadband radiometric signals within the customary monochromatic approximation adversely affects profiling accuracy. We present here an experimental comparison of pulsed photothermal profiling in layered agar gel samples utilizing a spectrally composite kernel matrix vs. the customary approach. By utilizing a custom reconstruction code, the augmented approach reduces broadening of individual temperature peaks to 14% of the absorber depth, in contrast to 21% obtained with the customary approach.
Characteristics of thermo-mechanical laser ablation process are investigated using an original numerical model. In contrast with previous models, it is based on a microscopic physical model of the ...micro-explosion process, which combines thermodynamic behavior of tissue water with elastic response of the solid tissue components. Diffusion of dissipated heat is treated in one dimension, and the amount of thermal damage is assessed using the Arrhenius model of the protein denaturation kinetics. Influence of the pulse fluence and duration on temperature profile development, ablation threshold, and depth of thermal damage is analyzed for the case of Er:YAG laser irradiation of human skin. Influence of mechanical properties on the ablation threshold of soft tissue is predicted theoretically for the first time. In addition, feasibility of deep tissue coagulation with a repetitively pulsed Er:YAG laser is indicated from the model.
Cryogen spray cooling (CSC) has been used for selective epidermal cooling of human skin during laser therapy of patients with port wine stain (PWS) birthmarks. Unfortunately, current commercial CSC ...devices do not provide optimal cooling selectivity and, therefore, provide insufficient epidermal protection for some PWS patients. To assist in the development of improved atomizing nozzle designs, a reliable method to quantify the CSC heat flux is needed. We introduce a novel method to determine the heat flux (q/sub s/) and heat transfer coefficient (h) at the surface of a sprayed object, based on measurements of steady-state temperature gradients along a thin copper rod during continuous cryogen spraying. For an atomizing nozzle of inner diameter d/sub N/ = 0.7 mm, we found that q/sub s/ varies from 15 to 130 W/cm/sup 2/ and h increases nonlinearly from 15000 to 35000 W/m/sup 2/.K in the explored range of surface temperatures (T/sub s/, from -32 to -7/spl deg/C). Values of q/sub s/ obtained with a wider diameter nozzle (d/sub N/ = 1.4 mm) are approximately twice as large than those of the narrow nozzle. The corresponding values of h are significantly higher (32000-40000 W/m/sup 2/.K) and almost independent of T/sub s/ within the same temperature range. When combined with fast flashlamp photography (FFLP) of spray shapes and sprayed surfaces, the results demonstrate that the liquid cryogen layer, as deposited by finely atomized sprays from narrower nozzles, can significantly impair q/sub s/. In contrast, the higher-momentum impact of coarser sprays from wider nozzles reduces the thickness of the liquid layer in the impact area and/or enhances convection within it, yielding a larger q/sub s/.
Cryogenic sprays are used for cooling human skin during laser dermatologic surgery. In this paper, six straight-tube nozzles are characterized by photographs of cryogenic spray shapes, as well as ...measurements of average droplet diameter, velocity, and temperature. A single-droplet evaporation model to predict average spray droplet diameter and temperature is tested using the experimental data presented here. The results show two distinct spray patterns-sprays for 1.4-mm-diameter nozzles ( wide nozzles) show significantly larger average droplet diameters and higher temperatures as a function of distance from the nozzle compared with those for 0.5-0.8-mm-diameter nozzles (narrow nozzles). These results complement and support previously reported studies, indicating that wide nozzles induce more efficient heat extraction than the narrow nozzles.
The influence of heat diffusion on mid-infrared laser ablation of hard organic tissues was investigated by applying Er:YAG laser pulses with pulse lengths from 100 mu s to 1 ms and of varying energy ...to dentin and enamel slices without water-spray cooling. At short pulse durations, the onset of ablation is abrupt in both tissues under test, and the crater depths increase quasi-logarithmically with applied laser fluence as a result of debris screening of the crater bottom. In a quantitative analysis of this effect, the observed variation of crater diameters with the applied laser fluence must be taken into account. The influence of the pulse duration on the debris screening effect is demonstrated by an improved analytical model of the process. At longer pulse durations, the ablation efficiency is significantly reduced at fluences close to the ablation threshold, which results in a double-threshold character of the ablation process. This effect is attributed to diffusion of heat from the interaction region during the laser pulse. A simple analytical model of heat diffusion and ablation front dynamics is presented that predicts the fluence interval where this effect can be expected, depending on pulse duration and properties of the treated tissue.
Pulsed photothermal radiometry (PPTR) allows noninvasive measurement of laser-induced temperature depth profiles, providing useful information on depth distribution of specific absorbers in optically ...scattering biological tissues. In the present study, PPTR profiling is combined with numerical modeling of light transport in human skin to analyze hemoglobin dynamics in traumatic bruises. Specifically, the influence of regularization degree, applied in iterative reconstruction of temperature depth profiles from PPTR signals measured in bruised volunteers, is studied. The results show that selection between two plausible reconstruction results does not significantly affect the assessed values of key bruise evolution parameters, i.e., hemoglobin mass diffusion and characteristic decomposition time.
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