Kamal M. Katika

In vivo time-resolved autofluorescence measurements to test for glycation of human skin.

We present an evaluation of time-resolved fluorescence measurements on human skin for screening type 2 diabetes. In vivo human skin is excited with a pulse diode at 375 nm and pulse width of 700 ps. Fluorescence decays are recorded at four different emission wavelengths: 442, 460, 478, and 496 nm. Experiments are performed at various locations, including the palms, arms, legs, and cheeks of a healthy Caucasian subject to test single-subject variability. The fluorescence decays obtained are modeled using a three-exponential decay. The variations in the lifetimes and amplitudes from one location to another are minimal, except on the cheek. We compare the fluorescent decays of 38 diabetic subjects and 37 nondiabetic subjects, with different skin complexions and of ages ranging from 6 to 85 yr. The average lifetimes for nondiabetic subjects were 0.5, 2.6, and 9.2 ns with fractional amplitudes of 0.78, 0.18, and 0.03, respectively. The effects of average hemoglobin A1c (HbA1c) from the previous 4 yr and diabetes duration are evaluated. While no significant differences between the fluorescence lifetimes of nondiabetic and diabetic subjects are observed, two of the fractional amplitudes are statistically different. Additionally, none of the six fluorescence parameters correlated with diabetes duration or HbA1c. One of the lifetimes as well as two of the fractional amplitudes differ between diabetic subjects with foot ulcers and nondiabetic subjects.

Steady-state directional diffuse reflectance and fluorescence of human skin

We present numerical simulations predicting the directional diffuse reflectance and autofluorescence from human skin. Skin is modeled as a seven-layered medium, with each layer having its own optical properties and fluorophore concentrations. Both collimated and diffuse monochromatic excitation at 442 nm are considered. In addition, the effect of an index-matching cream used to eliminate total internal reflection within the skin is assessed. We compute the intensity distributions of the excitation and fluorescence light in the skin by solving the radiative transfer equation using the modified method of characteristics. It was found that the use of an index-matching cream reduces the directional fluorescence signal while increasing the directional diffuse reflectance from the skin for collimated excitation. On the other hand, both the fluorescence and diffuse reflectance increase for diffuse excitation with an index-matching cream. Moreover, the directional fluorescence intensity obtained by use of collimated excitation is larger than that obtained by use of diffuse excitation light. This computational tool could be valuable in designing optical devices for biomedical applications.

Modified Method of Characteristics for Simulating Microscale Energy Transport

This paper presents a new numerical scheme for simulating multidimensional transient and steady-state microscale energy transport. The new method is based on the method of characteristics that follows heat carriers along their pathline. Unlike traditional methods, it uses a fixed computational grid and follows the heat carriers backward in time. The method 1) is accurate, 2) is unconditionally stable, 3) can deal with complex geometries without a large increase in computational cost, and 4) can be used for solving coupled equations using other numerical schemes. First, the numerical scheme is described. Then, simulations for transient and steady-state phonon transport in dielectric thin films are discussed. Numerical results are compared with analytical and reported numerical solutions and good agreement is obtained. @DOI: 10.1115/1.1795233#

BACKWARD METHOD OF CHARACTERISTICS IN RADIATIVE HEAT TRANSFER

This paper presents the backward method of characteristics for simulating multidimensional transient and steady state radiative heat transfer. This method is based on the method of characteristics that follows photons along their pathlines. It makes use of a fixed grid and unlike the conventional method of characteristics, it follows the photons backward in time. The method (1) is accurate, (2) unconditionally stable, (3) can be used for solving coupled equations using other numerical schemes. Numerical results obtained for various cases solved were in good agreement with analytical and numerical solutions reported in literature. The method is particularly appropriate for simulating multidimensional, transient radiative transport, and/or coupled heat transfer problems.

Ultra-Short Pulsed Laser Transport in a Multilayered Turbid Media

This paper presents the modified method of characteristics for simulating the transient transport of light in an absorbing and scattering medium exposed to collimated light. This method is based on the method of characteristics that follows photons along their pathlines. After showing the validity of the method it is used to solve for the transient transport of light in human skin. Skin has been modelled as a seven layer medium with different scattering and absorbing coefficients and scattering asymmetry factors. The angular distribution of reflected light from skin is presented. This technique could be used as a tool for designing various biomedical applications.Copyright

Feasibility analysis of an epidermal glucose sensor based on time-resolved fluorescence.

The goal of this study is to test the feasibility of using an embedded time-resolved fluorescence sensor for monitoring glucose concentration. Skin is modeled as a multilayer medium with each layer having its own optical properties and fluorophore absorption coefficients, lifetimes, and quantum yields obtained from the literature. It is assumed that the two main fluorophores contributing to the fluorescence at these excitation and emission wavelengths are nicotinamide adenine dinucleotide (NAD)H and collagen. The intensity distributions of excitation and fluorescent light in skin are determined by solving the transient radiative transfer equation by using the modified method of characteristics. The fluorophore lifetimes are then recovered from the simulated fluorescence decays and compared with the actual lifetimes used in the simulations. Furthermore, the effect of adding Poissonian noise to the simulated decays on recovering the lifetimes was studied. For all cases, it was found that the fluorescence lifetime of NADH could not be recovered because of its negligible contribution to the overall fluorescence signal. The other lifetimes could be recovered to within 1.3% of input values. Finally, the glucose concentrations within the skin were recovered to within 13.5% of their actual values, indicating a possibility of measuring glucose concentrations by using a time-resolved fluorescence sensor.

The Effect of Nanoparticles on Thermal Conductivity of Nanocomposite Thin Films at Low Temperatures - eScholarship

This study is concerned with the prediction of the effective thermal conductivity of nanocomposite thin films consisting of nanoparticles randomly distributed in a solid matrix. Crystalline sodium chloride with embedded monodisperse silver nanoparticles is investigated as a case study for thin films where phonons are the main heat carriers. To the best of our knowledge, the equation for phonon radiative transfer is solved for the first time, with an exact scattering transport cross section of the nanoparticles as a function of frequency which was obtained from the literature. The one-dimensional equation for phonon radiative transfer based on the isotropic scaling approximation is solved on a spectral basis using the discrete ordinates method to predict the temperature profile and the heat flux across the nanocomposite thin films. The thermal conductivity is retrieved at temperatures where the effects of Umklapp and normal processes can be neglected and scattering by the particles on phonon transport domi...

The Effect of Nanoparticles on Thermal Conductivity of Nanocomposite Thin Films at Low Temperatures

This study is concerned with the prediction of the effective thermal conductivity of nanocomposite thin films consisting of nanoparticles randomly distributed in a solid matrix. Crystalline sodium chloride with embedded monodisperse silver nanoparticles is investigated as a case study for thin films where phonons are the main heat carriers. To the best of our knowledge, the equation for phonon radiative transfer is solved for the first time, with an exact scattering transport cross section of the nanoparticles as a function of frequency which was obtained from the literature. The one-dimensional equation for phonon radiative transfer based on the isotropic scaling approximation is solved on a spectral basis using the discrete ordinates method to predict the temperature profile and the heat flux across the nanocomposite thin films. The thermal conductivity is retrieved at temperatures where the effects of Umklapp and normal processes can be neglected and scattering by the particles on phonon transport domi...

The effect of nanoparticles on the thermal conductivity of crystalline thin films at low temperatures

This study is concerned with the prediction of the effective thermal conductivity of nanocomposite thin films consisting of nanoparticles randomly distributed in a solid matrix. Crystalline sodium chloride with embedded monodisperse silver nanoparticles is investigated as a case study for thin films where phonons are the main heat carriers. To the best of our knowledge, the equation for phonon radiative transfer is solved for the first time with an exact scattering transport cross-section of the nanoparticles as a function of frequency which was obtained from the literature. The one-dimensional equation for phonon radiative transfer based on the isotropic scaling approximation is solved on a spectral basis using the discrete ordinates method to predict the temperature profile and the heat flux across the nanocomposite thin films. The thermal conductivity is retrieved at temperatures where the effects of Umklapp and Normal processes can be neglected and scattering by the particles on phonon transport dominates. The method of solution and closure laws were validated with experimental data of thermal conductivity for bulk samples at 2.53, 5.94, and 10.56 K. The effects of the film thickness (1 μm to 2.5 cm), nanoparticle diameter (5 nm to 100 nm) and volume fraction (0.0001 to 0.2) on the thermal conductivity of the nanocomposite thin film are investigated. The results indicate that the thermal conductivity decreases with decreasing particle radius as well as with increasing particle concentration. Finally, a dimensionless analysis revealed a power law relationship between the dimensionless thermal conductivity and a dimensionless length of the order of the acoustic thickness of the medium. These results can be used to design nanocomposite thin films for various low temperature thermal applications by choosing optimal nanoparticle radius and volume fraction, and film thickness.Copyright

Numerical feasibility analysis of an epidermal glucose sensor based on time-resolved fluorescence

This paper presents numerical simulations predicting the time-resolved reflectance and autofluorescence of human skin exposed to a pulse of collimated light at 337 nm and pulse width of 1 ns. Moreover, the feasibility of using an embedded time-resolved fluorescence sensor for monitoring glucose concentration is also studied. Skin is modeled as a multilayer medium with each layer having its own optical properties and fluorophore absorption coefficients, lifetimes and quantum yields. The intensity distributions of excitation and fluorescent light in skin are then determined by solving the transient radiative transfer equation using the modified method of characteristics. In both cases, the fluorophore lifetimes are recovered from the simulated fluorescence decays and compared with the actual lifetimes used in the simulations. It was found that the fluorescence lifetime of the fluorophore contributing the least to the fluorescence signal could not be recovered while the other lifetimes could be recovered within 2.5% of input values. Such simulations could be valuable in interpreting data from time-resolved fluorescence experiments on healthy and diseased tissue as well as in designing and testing the feasibility of various optical sensors for biomedical diagnostics.