Metabolic fingerprinting of biofluids by infrared spectroscopy: modeling and optimization of flow rates for laminar fluid diffusion interface sample preconditioning.

The laminar fluid diffusion interface (LFDI) is a microfluidic tool that manipulates the composition of liquid mixtures by exploiting differences among diffusion coefficients of the dissolved components. One application is the preprocessing of (bio)fluids prior to spectroscopic characterization. For example, in the case of infrared (IR) spectroscopy, the technique can improve sensitivity to low-concentration serum metabolites. The practical benefit is "metabolic fingerprinting" measurements that are more sensitive to low-concentration metabolites than are the counterpart measurements for the original serum sample. Optimal use of the LFDI technique has proven elusive, since the composition of the product of interest is very sensitive to the choice of flow rates for the liquid streams entering and emerging from the LFDI channel. To provide the basis for optimal use, this study had the objective of developing a simulation package that predicts the composition of the LFDI product, given the LFDI structural and operating parameters. To demonstrate the utility of the simulations, composition of the LFDI products predicted for two illustrative sets of trials were compared with experimental data. The flow rates thus derived provided a LFDI product that is relatively rich in serum metabolites, while largely depleted of protein, and very well suited for subsequent IR spectroscopic characterization.

Toward point-of-care diagnostic metabolic fingerprinting: quantification of plasma creatinine by infrared spectroscopy of microfluidic-preprocessed samples.

Infrared (IR) spectroscopy has previously been established as a means to accurately quantify several serum and urine metabolites, based upon spectroscopy of dry films. The same technique has also provided the basis to develop certain diagnostic tests, developed in the 'metabolomics' spirit. Here, we report on the further development of an integrated microfluidic-IR technology and technique, customized with the aim of dramatically extending the capabilities of IR spectroscopy in both analytical and diagnostic (metabolomic) applications. By exploiting the laminar fluid diffusion interface (LFDI), serum specimens are processed to yield product streams that are better suited for metabolic fingerprinting; metabolites are captured within the aqueous product stream, while proteins (which otherwise dominate the spectra of films dried from serum) are present in much reduced concentration. Spectroscopy of films dried from the aqueous stream then provides enhanced diagnostic and analytical sensitivity. The manuscript introduces an LFDI card design that is customized for integration with IR spectroscopy, and details the development of a quantitative assay for serum creatinine--based upon LFDI-processed serum samples--that is substantially more accurate (standard error of calibration, SEC = 43 micromol/L) than the corresponding assay based upon unprocessed serum specimens (SEC = 138 micromol/L). Preliminary results of diffusion modeling are reported, and the prospects for further optimization of the technique, guided by accurate modeling, are discussed.

Near-IR spectroscopic imaging for skin hydration: the long and the short of it.

Near-IR spectroscopic methods have been developed to determine the degree of hydration of human skin in vivo. Noncontact reflectance spectroscopic imaging was used to investigate the distribution of skin moisture as a function of location. A human study in a clinical setting has generated quantitative data showing the effects of a drying agent and a moisturizer on delineated regions of the forearms of eight volunteers. Two digital imaging systems equipped with liquid-crystal tunable filters were used to collect stacks of monochromatic images at 10-nm intervals over the 650-1050 and 960-1700 nm wavelength bands. Synthetic images generated from measurements of water absorption band areas at three different near-IR wavelengths (970, 1200, and 1450 nm) showed obvious differences in the apparent distribution of water in the skin. Changes resulting from the skin treatments were much more evident in the long-wavelength images than in the short-wavelength ones. The variable sensitivity of the method at different wavelengths has been interpreted as being the result of different penetration depths of the IR light used in the reflectance studies.

Spectroscopic assessment of cutaneous hemodynamics in the presence of high epidermal melanin concentration.

BACKGROUND: For individuals with lightly pigmented skin, early stage pressure ulcers appear as areas of redness, which have compromised microcirculation and do not blanch in response to pressure. The lack of a visible blanch (hemodynamic response) to pressure is a convenient diagnostic test for stage I sores. However, the blanch response is not visually apparent in people with highly pigmented skin color due to the overwhelming contribution of melanin to the reflectance of skin. METHODS: A simple least squares projection operator method is described, which can separate the reflectance contributions from melanin and hemoglobin. The methodology was tested in a study population of 20 subjects with healthy skin. The study population was evenly divided into a lightly pigmented skin group (visible blanch response) and a highly pigmented skin group (no visible blanch response). RESULTS: The hemodynamic response to pressure being applied to the skin could clearly be distinguished spectroscopically in both groups at a high level of statistical significance. CONCLUSION: The specific aim of this work was directed towards developing a spectroscopic basis for distinguishing the healthy blanch response in a manner that was independent of skin pigmentation. However, the technique has a general application when optical hemodynamic measurements are being made over a diverse patient population or under conditions of varying pigmentation such as the seasonal changes in skin color.