Design and characterization of protein films for modeling near-infrared spectra of human tissue

Abstract

Near-infrared (near-IR) spectroscopy has been investigated for use in direct measurements of human tissue for the purpose of quantifying clinically relevant analytes such as glucose. Spectra collected by transmitting near-IR light through human tissue reveal the presence of both aqueous components and solid structures within the optical path of the measurement. Developing technology for use in making these measurements requires either the availability of human subjects or an in vitro experimental platform that can effectively simulate the spectroscopic properties of tissue. This paper describes the preparation and testing of films of collagen and keratin, the two proteins that comprise the bulk of the solid material in the human epidermis and dermis. By placing these films in the optical path of a near-IR spectrometer together with an aqueous sample cell, a phantom can be constructed that allows experiments to be performed that simulate noninvasive measurements of tissue. In this work, both constant and variable thickness films are prepared and evaluated by use of a regression fit to spectra of human tissue. The stability and spectral reproducibility of the prepared films are also assessed. The regression fits to the human subject spectra yield values of R(2) ranging from 0.97 to 0.99 and the films are found to yield spectra that vary by less than a 2% relative standard deviation under three different reproducibility tests.

Nocturnal Hypoglycemic Alarm Based on Near-Infrared Spectroscopy: In Vivo Studies with a Rat Animal Model

“…An absorbance spectrum for human skin on the back of the hand is also displayed in Figure A and shows a similar shape to the spectrum collected from the rat skin tissue. The similarities in the shapes of the spectra indicate that the main chemical components are present in reasonably similar amounts, with fat appearing to be a larger contributor to the rat spectrum.…”

“…This diagnostic procedure has been described previously. 23 Over the 4800− 4200 cm −1 range, values of the coefficient of multiple determination (R 2 ) for the fits all exceeded 0.9997. In the fitted models, the water and collagen terms had the largest regression coefficients, with the average ± standard deviation equaling 0.738 ± 0.005 mm for water and 0.596 ± 0.003 mm for collagen.…”

“…The model also included intercept and linear slope terms to help fit the spectral baseline. This diagnostic procedure has been described previously . Over the 4800–4200 cm –1 range, values of the coefficient of multiple determination ( R 2 ) for the fits all exceeded 0.9997.…”

“…We demonstrated that we could classify spectra into groups corresponding to being above or below a concentration threshold. The second study described an effort to create in vitro samples that have a similar near-infrared spectral response to human tissue . In a follow-on study, we simulated the operation of the nocturnal alarm by use of this constructed tissue phantom …”

“…The second study described an effort to create in vitro samples that have a similar near-infrared spectral response to human tissue. 23 In a follow-on study, we simulated the operation of the nocturnal alarm by use of this constructed tissue phantom. 24 The work described in the current paper extends the testing of the nocturnal alarm methodology to the use of a rat animal model to implement true in vivo noninvasive blood glucose measurements.…”