Opto-thermal Transient Emission Radiometry Research Articles

Photothermal Radiometry Data Analysis by Using Machine Learning.

Photothermal techniques are infrared remote sensing techniques that have been used for biomedical applications, as well as industrial non-destructive testing (NDT). Machine learning is a branch of artificial intelligence, which includes a set of algorithms for learning from past data and analyzing new data, without being explicitly programmed to do so. In this paper, we first review the latest development of machine learning and its applications in photothermal techniques. Next, we present our latest work on machine learning for data analysis in opto-thermal transient emission radiometry (OTTER), which is a type of photothermal technique that has been extensively used in skin hydration, skin hydration depth profiles, skin pigments, as well as topically applied substances and skin penetration measurements. We have investigated different algorithms, such as random forest regression, gradient boosting regression, support vector machine (SVM) regression, and partial least squares regression, as well as deep learning neural network regression. We first introduce the theoretical background, then illustrate its applications with experimental results.

Opto-thermal Radiometry for Detection of Surface Contamination

The aim of this work is to conduct a proof-of-concept study on detection of surface contamination by biological and chemical agents using opto-thermal transient emission radiometry (OTTER). OTTER is an infrared remote sensing technology that has been developed in our research group for skin measurements. It is based on the measurement of laser-induced transient thermal emission spectra in the infrared fingerprint region, and has advantages of non-contact, non-invasive, spectroscopic in nature, and can work on arbitrary surfaces of any materials. In this paper, two types of experiments were conducted, different ingredients on the surface of a sample material and different ingredients on the skin surface. The results show that by measuring the OTTER signal at different detection wavelengths, e.g., 6.05 μm, 6.48 μm, 7.79 μm, 8.5 μm, 9.5 μm, 11.5 μm, 13.1 μm, it is possible to differentiate from skin and skin with different ingredients on the surface. The next step is to develop mathematical models and analytical algorithms to quantify the ingredient concentration on the surface.

In Vivo Skin Characterizations by Using Opto-Thermal Depth-Resolved Detection Spectra

OTTER (opto-thermal transient emission radiometry) is an infrared remote sensing technology that has been extensively used in skin measurements. It is non-contact, non-invasive, and has a unique depth profiling capability. By selecting different detection wavelengths, OTTER can be used for different types of skin measurements, such as skin hydration measurements and skin topically applied substance measurements, etc. By plotting the results at different detection wavelengths, we can have an opto-thermal detection spectrum. Combining with OTTER’s unique depth profiling capability, we can get a depth-resolved opto-thermal detection spectrum. This is a powerful tool that can be used for many skin studies. Here we will present our latest study with details on the apparatus setup, theoretical background, as well as experimental results.

In Vivo Human Hair Hydration Measurements by Using Opto-Thermal Radiometry

The water content in human hair is very important for its cosmetic properties and general health. However, to measure water content in hair, especially in vivo hair, is very difficult. Opto-thermal transient emission radiometry (OTTER) is a promising, infrared remote sensing technology that can be used for this kind of measurements. It can not only measure the water content but also the water depth profile in hair. By measuring the water content and its depth profile in hair, we can understand the status of hair, i.e., healthy or damaged, etc. In this paper, we will present our latest study on in vivo human hair hydration measurements by using OTTER. We will first present the theoretical background and then show the experimental results. We will also compare the OTTER hair results with other established measurement technique results, such as condenser trans-epidermal water loss method.

In Vivo Skin Solvent Penetration Measurements Using Opto-thermal Radiometry and Fingerprint Sensor

This latest study on in vivo transdermal drug delivery by using opto-thermal radiometry and a capacitance-based fingerprint sensor is presented. A small amount of solvent was applied on the test sites of a volar forearm for a few minutes; opto-thermal measurements and fingerprint sensor measurements were performed both before the solvent application and periodically after. The results showed that, by selecting different detection wavelengths, opto-thermal radiometry could give the information either on the water concentration within skin or the solvent concentration within skin. The capacitance-based fingerprint sensor could clearly visualize solvent penetration through in vivo human skin, as it generated dynamic two-dimensional (2D) images of solvent distribution within skin, and combining with tape stripping, it was also possible to get solvent 3D depth profiles within skin. The correlation between opto-thermal transient emission radiometry and fingerprint sensor measurements was also evaluated.

In vivo Stratum Corneum Over‐hydration and Water Diffusion Coefficient Measurements Using Opto‐Thermal Radiometry and TEWL Instruments

Skin over-hydration is a common problem that affects many people who wear incontinence pads or diapers. The aim of this study is to develop a new method for stratum corneum (SC) over-hydration and SC water diffusion coefficient measurements using opto-thermal transient emission radiometry (OTTER) and evaporimetry. With OTTER, we can measure the SC surface hydration and hydration gradient. With evaporimetry, we can measure the time-dependent evaporative drying curves of water vapour flux density (WVFD). The combination of hydration results and WVFD results can yield information on the SC water diffusion coefficient and how it depends on the SC surface hydration level. The results show that SC water diffusion coefficient is non-linearly proportional to the SC surface hydration level. The results also show strong correlations between evaporative drying flux measured using the Evaporimeter and surface hydration estimated from OTTER measurements.

Opto-Thermal Transient Emission Radiometry (OTTER) to image diffusion in nails in vivo

This work describes the first application of Opto-Thermal Transient Emission Radiometry (OTTER), an infrared remote sensing technique, to probe the extent to which solvents permeate the human nail in vivo. Decanol, glycerol and butyl acetate were selected as model solvents. After application of the solvents, individually, to human volunteers, OTTER was used to depth profile the solvents. The permeation rate of the solvents was ranked as glycerol > decanol > butyl acetate. It is possible that some of the butyl acetate may have evaporated during the experiment. The ability of decanol to extract lipids from biological tissue is also considered. These preliminary results demonstrate the potential of OTTER as a tool to identify optimal excipients with which to target drugs to the nail.

Opto-thermal radiometry for in-vivo nail measurements

We have developed a new method for in-vivo human nail characterization by using opto-thermal transient emission radiometry (OTTER) and condenser-chamber TEWL (trans-dermal water loss) method – AquaFlux. With OTTER, we can measure nail water content, nail water concentration depth profiles, as well as topically applied solvent penetration through nail. With AquaFlux, we can measure nail transonychial water loss (TOWL). Combining the water content results with TOWL results, we can get the water diffusion coefficient of nail. Measuring the water diffusion coefficients of nail at different nail water concentration levels can also yield information on how nail diffusion coefficients change with water content. We will present the theoretical background, and experimental results on water concentration depth profile in nail, as well as topically applied solvent penetration through nail.

Opto-thermal Measurement of Stratum Corneum Renewal Time

We report a new method of measuring stratum corneum renewal time, based on opto-thermal transient emission radiometry, together with the safe natural dye henna. Although henna absorbs throughout the visible spectral region, the blue radiation at 440nm wavelength from a Q-switched OPO laser was used to monitor its disappearance, since this wavelength was found to give optimum decay-time contrast between stained and unstained SC. The most notable feature of the new method is its clearly defined end point, which gives a more precise determination of renewal time than standard methods.

Opto-Thermal Mathematical Modeling and Inverse Depth Profiling Using Genetic Algorithm

The use of opto-thermal transient emission radiometry (OTTER) for non-invasive measurements of water concentration depth profiles in human skin is important for developing an understanding of its barrier function. In this paper, a new inverse method for analyzing opto-thermal data to yield optical depth profiles, which is based on a new multilayer mathematical model designed for opto-thermal skin data analysis, is presented. This has been combined with a novel inverse analysis technique using a genetic algorithm. The performance of the new approach is tested on both simulated data and in vivo experimental skin data. The theoretical background is presented, and the analysis of typical measurements using the new approach is compared with conventional analyses.

Optothermal transient emission radiometry for studying the changes in epidermal hydration induced during ripening of tomato fruit mutants

Optothermal transient emission radiometry (OTTER) was used to determine the mean surface hydration and the hydration profile of three mutants (beefsteak, slicing and salad) of harvested tomatoes (Lycopersicon esculentum) that were kept under ambient conditions for as long as 51 days. Maximal sensitivity of OTTER to water in the samples was achieved by using 2.94 μm and 13.1 μm as excitation and emission wavelengths, respectively. The surface hydration increases rapidly and reaches a constant level during the remaining period. The hydrolysis of pectic substances that occur in tomatoes while ripening might be a possible cause for the observed change in hydration.

Spectral depth profiling of arbitrary surfaces by thermal emission decay-Fourier transform infrared spectroscopy.

We report a new spectroscopic technique that combines step-scanning Fourier transform infrared spectroscopy with opto-thermal transient emission radiometry (OTTER) in order to provide near-surface depth-resolved spectra in the range 700-1800 cm(-1). It works nondestructively, without contact, with samples of arbitrary shape and size, without requiring prior preparation. The depth of surface probed depends on the thermal diffusivity of the sample; for organic materials it is approximately 10 microm. With homogeneous samples, absolute absorption coefficients can be measured. With two-layered samples, the technique proved able to distinguish between the spectral properties of the top layer and the substrate and to estimate the thickness of the top layer. We present a theoretical analysis with the main design features of the instrumentation and software, together with studies of homogeneous and layered samples, to validate the methods and illustrate the potential of the technique for practical applications.

Optothermal in vitro diffusion measurements through silicone membranes

We report the development of a new method for measuring diffusion rates of surface-applied chemicals through polymer membranes such as polydimethylsiloxane (PDMS). An important feature of the approach is the use of optothermal transient emission radiometry to sense diffusant concentration in a noncontacting, noninvasive way. This allows the method to be adapted to perform similar measurements on human skin in vivo, thus providing a way of cross-verifying in vivo and in vitro measurements. The correlation between in vitro and in vivo diffusion measurements is also important for developing credible alternatives to in vivo testing, for use with toxic chemicals or animal substitution. We present the results of experiments with several polyols diffusing through PDMS membranes of thickness 125 or 250 μm, describing the experimental details, the measurement protocol, the data analysis methods, and a study of measurement errors.

Spectroscopic Study of Water-Keratin Interactions in Stratum Corneum

Excitation spectroscopy with Opto-Thermal Transient Emission Radiometry (OTTER) was used to measure waterkeratin interaction energies in excised stratum corneum (SC) under controlled ambient relative humidity (RH) conditions. Such interactions lead to small but measurable wavelength shifts in the peaks of the absorption spectra of the interacting molecules. In the present study, an OPO laser with tuneable output radiation in the wavelength range 2.4-3.4µm was used to measure spectral peak shifts of the prominent absorption band of water near 3µm wavelength. A thermal emission wavelength around 13.1µm was selected by means of an interference filter, to reject emissions not emanating from water. Peak shifts within three types of SC samples, namely untreated, delipidized and delipidized & rinsed, with RH values in the range 3%-100% were measured. This new method of measuring interaction energies is unique in its potential for non-invasive in-vivo and in-situ measurements of water binding within the stratum corneum.

In-vivo Transdermal Drug Diffusion Depth Profiling-A New Approach to Opto-Thermal Signal Analysis

To measure the percutaneous penetration of the externally applied chemicals into human skin is very important for understanding how drugs can be delivered through the skin. Opto-thermal transient emission radiometry, or OTTER, is a new approach to such measurements which has been demonstrated to be suitable for measuring diffusion coefficients and partition constants in-vivo and in-vitro. We present a novel technique for analyzing opto-thermal signals to yield depth profiles of externally applied substances diffusing through skin. The mathematical background of the new technique, together with penetration depth profiles of different solvents into in-vivo human skin are presented.

Optothermal measurement of water distribution within the stratum corneum.

Opto-thermal transient emission radiometry (OTTER) is a novel infrared remote sensing technology that can be used for water distribution within stratum corneum measurements. Comparing with other skin water content measurement techniques, OTTER has the advantages of non-contact, non-destructive, spectroscopic in nature, and can work on unprepared, arbitrary surfaces. Here we report our work on optothermal stratum corneum water measurements. We will first introduce the OTTER apparatus, then show the opto-thermal measurement results on stratum corneum surface hydration and hydration gradient, stratum corneum thickness, stratum corneum water diffusion coefficients, and stratum corneum hydration depth profiles.

Thermal diffusivity measurement of thermally conducting films on insulating substrates

We report thermal diffusivity measurements of thin Niobium films on glass substrates, using OptoThermal Transient Emission Radiometry (OTTER). The result is a thermal diffusivity value ofD=(2.79±0.36)×10−5 m2/s, which is within 17% of the accepted value for multicrystaline bulk niobium. The technique is remote sensing, non-destructive, and the measurements depend only on the thickness of the film and the thermal properties of the substrate.

Optothermal Study of Hydrogenated Amorphous Silicon- Sulfur Alloys

The optothermal properties of doped and undoped amorphous silicon- sulfur alloys have been studied using the optothermal transient emission radiometry (OTTER) technique. The optothermal decay signal was found to depend on the concentration of incorporated sulfur atoms in the material. The results have been correlated with the density of gap- state defects induced by sulfur atoms as determined from subgap absorption measurements. The OTTER technique has proved to be a viable alternative method for measuring the relative densities of defect states in the band gaps of amorphous silicon- sulfur alloys.

Opto-thermal technique for in-vivo stratum corneum hydration measurement

A new method for measuring stratum corneum hydration in-vivo, using opto-thermal transient emission radiometry with 2.94µm Er : YAG excitation and wavelength selective detection, is described. Results on skin recovery following exogenous hydration, skin stripping with Sellotape and wound healing are presented.

Improvement of an opto-thermal technique for wood

The opto-thermal transient emission radiometry measuring system has been improved by including a microscope as part of the signal detection system to achieve a higher degree of spatial resolution. This is illustrated with measurement of decay time of optothermal signal observed from the transverse cut of pine wood samples. Good correlations were found between opto-thermal decay time and colour within the pine growth rings.