Phantoms for Noninvasive Blood Glucose Sensing with Near Infrared Transmission Spectroscopy

Monitoring of n-butanol vapors biofiltration process using an electronic nose combined with calibration models

Abstract

Malodorous odors, by definition, are unpleasant, irritating smells being a mixture of volatile chemical compounds that can be sensed at low concentrations. Due to the increasing problem of odor nuisance associated with odor sensations, and thus the need to remove them from the air, deodorization techniques are commonly used. Biofiltration is one of the methods of reducing odorants in the air stream. In the paper, the possibility of using an electronic nose as an alternative method to gas chromatography for the online monitoring and evaluation of efficiency of the n-butanol vapors biofiltration process in a transient state was investigated. Three calibration models were used in the research, i.e., multiple linear regression, principal component regression, and partial least-square regression. The obtained results were compared with the theoretical values.

Graphical abstract

Toward an injectable continuous osmotic glucose sensor

BACKGROUND

The growing pandemic of diabetes mellitus places a stringent social and economic burden on the society. A tight glycemic control circumvents the detrimental effects, but the prerogative is the development of new more effective tools capable of longterm tracking of blood glucose (BG) in vivo. Such discontinuous sensor technologies will benefit from an unprecedented marked potential as well as reducing the current life expectancy gap of eight years as part of a therapeutic regime.

METHOD

A sensor technology based on osmotic pressure incorporates a reversible competitive affinity assay performing glucose-specific recognition. An absolute change in particles generates a pressure that is proportional to the glucose concentration. An integrated pressure transducer and components developed from the silicon micro- and nanofabrication industry translate this pressure into BG data.

RESULTS

An in vitro model based on a 3.6 x 8.7 mm large pill-shaped implant is equipped with a nanoporous membrane holding 4-6 nm large pores. The affinity assay offers a dynamic range of 36-720 mg/dl with a resolution of +/-16 mg/dl. An integrated 1 x 1 mm(2) large control chip samples the sensor signals for data processing and transmission back to the reader at a total power consumption of 76 microW.

CONCLUSIONS

Current studies have demonstrated the design, layout, and performance of a prototype osmotic sensor in vitro using an affinity assay solution for up to four weeks. The small physical size conforms to an injectable device, forming the basis of a conceptual monitor that offers a tight glycemic control of BG.

Near-infrared microspectroscopic analysis of rat skin tissue heterogeneity in relation to noninvasive glucose sensing

BACKGROUND

Noninvasive glucose measurements are possible by analysis of transmitted near-infrared light over the 4000- to 5000-cm(-1) spectral range. Such measurements are highly sensitive to the exact position of the fiber-optic interface on the surface of the skin sample. A critical question is the degree of heterogeneity of the major chemical components of the skin matrix in relation to the size of the fiber-optic probed used to collect noninvasive spectra. Microscopic spectral mapping is used to map the chemical distribution for a set of excised sections of rat skin.

METHOD

A Fourier transform near-infrared microspectrometer was used to collect transmission spectra from 16 tissue samples harvested from a set of four healthy Harlan-Sprague male rats. A reference point in the center of the tissue sample was probed regularly to track dehydration, changes in tissue composition, and changes in instrument performance. Amounts of the major skin constituents were determined by fitting microspectra to a set of six pure component absorbance spectra corresponding to water, type I collagen protein, keratin protein, fat, an offset term, and a slope term.

RESULTS

Microspectroscopy provides spectra with root mean square noise levels on 100% lines between 418 and 1475 microabsorbance units, which is sufficient for measuring the main chemical components of skin. The estimated spatial resolution of the microscope is 220 microm. The amounts of each tissue matrix component were determined for each 480 x 360-microm(2) location of a 4.8 x 3.6-mm(2) rectangular block of skin tissue. These spectra were used to generate two-dimensional distribution maps for each of the principal skin components.

CONCLUSIONS

Distribution of the chemical components of rat skin is significant relative to the dimensions of noninvasive glucose sensing. Chemical distribution maps reveal that variations in the chemical composition of the skin samples are on the same length scale as the fiber-optic probe used to collect noninvasive near-infrared spectra. Analysis of variance between tissue slices collected for one animal and analysis of variations between animals indicate that animal-to-animal variation for all four chemical components is significantly higher than variations between samples for a given animal. These findings justify the collection and interpretation of near-infrared microspectroscopic maps of human skin to establish chemical heterogeneity and its impact on noninvasive glucose sensing for the management of diabetes.

New methodology to obtain a calibration model for noninvasive near-infrared blood glucose monitoring

This paper reports new methodology to obtain a calibration model for noninvasive blood glucose monitoring using diffuse reflectance near-infrared (NIR) spectroscopy. Conventional studies of noninvasive blood glucose monitoring with NIR spectroscopy use a calibration model developed by in vivo experimental data sets. In order to create a calibration model, we have used a numerical simulation of light propagation in skin tissue to obtain simulated NIR diffuse reflectance spectra. The numerical simulation method enables us to design parameters affecting the prediction of blood glucose levels and their variation ranges for a data set to create a calibration model using multivariate analysis without any in vivo experiments in advance. By designing the parameters and their variation ranges appropriately, we can prevent a calibration model from chance temporal correlations that are often observed in conventional studies using NIR spectroscopy. The calibration model (regression coefficient vector) obtained by the numerical simulation has a characteristic positive peak at the wavelength around 1600 nm. This characteristic feature of the regression coefficient vector is very similar to those obtained by our previous in vitro and in vivo experimental studies. This positive peak at around 1600 nm also corresponds to the characteristic absorption band of glucose. The present study has reinforced that the characteristic absorbance of glucose at around 1600 nm is useful to predict the blood glucose level by diffuse reflectance NIR spectroscopy. We have validated this new calibration methodology using in vivo experiments. As a result, we obtained a coefficient of determination, r2, of 0.87 and a standard error of prediction (SEP) of 12.3 mg/dL between the predicted blood glucose levels and the reference blood glucose levels for all the experiments we have conducted. These results of in vivo experiments indicate that if the parameters and their vibration ranges are appropriately taken into account in a numerical simulation, the new calibration methodology provides us with a very good calibration model that can predict blood glucose levels with small errors without conducting any experiments in advance to create a calibration model for each individual patient. This new calibration methodology using numerical simulation has promising potential for NIR spectroscopy, especially for noninvasive blood glucose monitoring.

Noninvasive glucose sensing

The ability to measure glucose noninvasively in human subjects is a major objective for many research groups. Success will revolutionize the treatment of diabetes by providing a means to improve glycemic control, thereby delaying the onset of the medical complications associated with this disease. This article focuses on the current state of the art and attempts to identify the principal areas of research necessary to advance the field. Two fundamentally different approaches are identified for the development of noninvasive glucose sensing technology. The indirect approach attempts to measure glucose on the basis of its effect on a secondary process. The direct approach is based on the unique chemical structure of the glucose molecule. Advances for each approach are limited by issues of selectivity. Several critical parameters are discussed for the direct approach, including issues related to the optical path length, wavelength range, dimensionality of the multivariate calibration model, net analyte signal, spectral variance, and assessment of the chemical basis of measurement selectivity. A set of publication standards is recommended as a means to enhance progress toward a successful noninvasive monitor.

Scattering and Absorption Effects in the Determination of Glucose in Whole Blood by Near-Infrared Spectroscopy

Optical properties of whole bovine blood are examined under conditions of different glucose loadings. A strong dependency is established between the scattering properties of the whole blood matrix and the concentration of glucose. This dependency is explained in terms of variations in the refractive index mismatch between the scattering bodies (predominately red blood cells) and the surrounding plasma. Measurements in the presence of a well-known glucose transport inhibitor indicate that variations in refractive index mismatch are related to the penetration of glucose into the red blood cells and demonstrate that increased scattering involves the uptake of glucose by red blood cells. Finally, multivariate calibration models are presented for the measurement of glucose in a whole blood matrix. These models are based on near-infrared spectral data collected from 80 different samples prepared from a single whole blood matrix. Calibration studies are performed over the combination, first-overtone, and short-wavelength spectral regions. The best calibration model is generated from combination region spectra, providing a standard error of prediction (SEP) of less than 1 mM over the concentration range of 3-30 mM. The model based on the first-overtone region is slightly degraded but still provides acceptable performance (SEP = 1.20 mM). The model based on the short-wavelength region is further degraded (SEP = 2.53 mM). To rationalize these results, an analysis of the selectivity of the calibration models is performed by computing the glucose net analyte signal. It is established that the models based on the combination and first-overtone regions are dominated by glucose absorption information, while the model computed from the short-wavelength region is based primarily on scattering information. This result provides evidence that absorption information is needed in order to obtain a glucose calibration model with acceptable performance.

Fluorescence-sensing methods

Novel approaches to sensor design, based on the use of an internal standard with appropriate spectral properties, provide new possibilities for designing simple devices for fluorescence sensing. Detection of combined emission from the reference and an analyte-sensitive fluorophore has been achieved in numerous measurements in cuvettes, tissues, and high-throughput formats. These methods have been used with a long-lifetime reference to measure pH, O2, pCO2, glucose, and calcium by means of modulation-sensing methods as well as by the use of oriented films as the reference for polarization sensing of glucose, pH, oxygen, and lactate. Polarization sensing has also been developed with visual detection to measure the concentration of rhodamine B and pH. Modulation and polarization sensing was found to be effective in highly scattering media such as Intralipid or tissue. The applicability of these technologies to transdermal diagnostics depends on the availability of red fluorophores that can be used in vivo. One dye that could possibly be used is indocyanine green (IcG), which absorbs and emits at wavelengths above 700 nm. Furthermore, IcG has already been approved for use in humans for monitoring burn severity and it has been detected through the skin. It appears likely that modern optics and electronic technology will allow the development of practical devices for biomedical use as shown in Scheme 1.

Charge transfer fluorescent probes using boronic acids for monosaccharide signaling

We have developed a new series of glucose sensitive fluorophores that display shifts in emission wavelengths and/or intensity change upon the binding of monosaccharides. Complexation of glucose with the boronic acid moiety changes both its orbital hybridization and its ability to accept and donate electrons. This change results in distinct emission spectra for the fluorophores when free in solution or complexed with monosaccharide. The spectral changes upon saccharide binding can be modified by substitution of electron donor or acceptor group on the fluorophore allowing rational design of the spectral response.

Nondestructive near-infrared spectroscopic measurement of multiple analytes in undiluted samples of serum-based cell culture media

An adaptive calibration procedure is used to build selective multivariate calibration models for the measurement of glucose, lactate, glutamine, and ammonia in undiluted serum-based cell culture media. This adaptive procedure removes metabolism-induced covariance between these analytes in a series of calibration samples collected during the cultivation of PC-3 human prostate cancer cells. Partial least-squares calibration models are generated from single-beam near-infrared (NIR) spectra collected over the 4800- to 4200-cm(-1) combination spectral range. Calibration models were generated with both the full spectral range and optimized spectral ranges. In both cases, the number of model factors was optimized and model validity was determined by comparing analyte concentrations predicted from a series of independent and unaltered samples that were obtained during a subsequent cultivation of the PC-3 cells. Similar analytical performance was achieved with fewer model factors when the optimized spectral range was used. The lowest standard errors of prediction were 0.82, 0.94, 0.55, and 0.76 mM for glucose, lactate, glutamine, and ammonia, respectively. Different spectral ranges were optimal for each analyte and the optimized spectral range coincided with the distinguishing spectral features of the analyte. The results of this study demonstrate that NIR spectroscopy can be used effectively in the off-line measurement of important nutrients (glucose and glutamine) and byproducts (lactate and ammonia) in a serum-based animal cell culture medium.

Wavelength-ratiometric probes for saccharides based on donor-acceptor diphenylpolyenes

The spectroscopic and photophysical properties of two donor-acceptor derivatives of 1,4-diphenylbuta-1,3-diene and 1,6-diphenylhexa-1,3,5-diene are described. Both compound posses a dimethylamino and a boronic acid groups as electron-donor and electron-withdrawing groups, respectively. Solvent polarity effects on the steady-state and fluorescence intensity decay are presented and show the formation of an excited-state charge-transfer (CT) state for both compounds. The formation of the anionic form of the boronic acid group at high pH induces a blue shift and an increase of the intensity in the emission spectra for both compounds. These spectral changes are interpreted as the lost of the electron-withdrawing property of the anionic form of the boronic acid group. The observed pK a of both compounds is around 8.8 and decrease to ~6 and ~7 in presence of fructose and glucose, respectively. Both compounds display a decrease of the mean lifetime at higher pH. Effects of the sugars on the fluorescence spectra and fluorescence lifetimes are also presented. For both compounds, a blue shift and an increase of the intensity are observed. These spectral changes lead to a wavelength-ratiometric method for the sugar recognition and analysis. Titration curves against fructose, galactose and glucose and dissociation constants are presented. Both compounds show a higher affinity for fructose. The affinity decreases for galactose and for glucose, respectively. Sugar effects on the fluorescence intensity decay are also presented. Both compounds display a decrease of the mean lifetime after addition of sugar.

Evaluation of two synthetic glucose probes for fluorescence-lifetime-based sensing

We evaluated two anthracene derivatives with covalently attached boronic acid groups for fluorescence-lifetime-based sensing of glucose. These anthracene derivatives also contained alkyl amino groups, which quenched the anthracene emission by photo-induced electron transfer. Both anthracene derivatives displayed increased intensities and lifetime in the presence of glucose, as seen from the frequency-domain measurements. A fluorescence lifetime change from 9.8 to 12.4 and 5.7 to 11.8 ns is observed, after the addition of glucose, for the anthracene substituted with one and two boronic acid groups, respectively. This results in a change in the phase angle up to 15 degrees and 30 degrees and in the modulation up to 12 and 25% at 30 MHz for these compounds, respectively. Titration curves in the presence of BSA and micelles are also presented to show the potential interferences from biomolecules. Dissociation constants were evaluated for both compounds, and association with glucose was found to be reversible. Importantly, the apparent glucose binding constants are about 5- to 10-fold smaller with phase, modulation, or mean lifetime than with intensities measurements, shifting the glucose-sensitive range to physiological values. Combining these results and the use of a simple UV-LED as excitation source, the results show an interesting potential of these two compounds in the development of lifetime base devices using synthetic probes for glucose.

Polarization-based sensing of glucose using an oriented reference film

We describe a new approach to glucose sensing using polarization measurements in the presence of a stretch-oriented reference film. The method relies on measurement of the polarized emission from the reference film and of a fluorophore which changes intensity in response to glucose. A glucose-sensitive fluorescent signal was provided by the glucose/galactose binding protein from E. coli. This protein was labeled with an environmentally sensitive fluorophore at a single genetically inserted cysteine residue, and displayed decreased fluorescence upon glucose binding. Using the protein and the reference film we observed glucose-sensitive polarization values for micromolar glucose concentrations. This method of polarization-based sensing is generic and can be used for any sensing fluorophore which displays a change in intensity. In principle, one can construct simple and economical devices for this type of glucose measurement. © 1999 Society of Photo-Optical Instrumentation Engineers.

Evaluation of Measurement Sites for Noninvasive Blood Glucose Sensing with Near-Infrared Transmission Spectroscopy

Six putative measurement sites were evaluated for noninvasive sensing of blood glucose by first-overtone near-infrared spectroscopy. The cheek, lower lip, upper lip, nasal septum, tongue, and webbing tissue between the thumb and forefinger were examined. These sites were evaluated on the basis of their chemical and physical properties as they pertain to the noninvasive measurement of glucose. Critical features included the effective optical pathlength of aqueous material within the tissue and the percentage of body fat within the optical path. Aqueous optical paths of 5 mm are required to measure clinically relevant concentrations of glucose in the first-overtone region. All of the tested sites met this requirement. The percentage of body fat affects the signal-to-noise ratio of the measurement and must be minimized for reliable glucose sensing. The webbing tissue contains a considerable amount of fat tissue and is clearly the worse measurement site. All other sites possess substantially less fat, with the least amount of fat in tongue tissue. For this reason, the tongue provides spectra with the highest signal-to-noise ratio and is, therefore, the site of choice on the basis of spectral quality.

Glucose sensor for low-cost lifetime-based sensing using a genetically engineered protein

We describe a glucose sensor based on a mutant glucose/galactose binding protein (GGBP) and phase-modulation fluorometry. The GGBP from Escherichia coli was mutated to contain a single cysteine residue at position 26. When labeled with a sulfhydryl-reactive probe 2-(4'-iodoacetamidoanilino)naphthalene-6-sulfonic acid, the labeled protein displayed a twofold decrease in intensity in response to glucose, with a dissociation constant near 1 microM glucose. The ANS-labeled protein displayed only a modest change in lifetime, precluding lifetime-based sensing of glucose. A modulation sensor was created by combining ANS26-GGBP with a long-lifetime ruthenium (Ru) metal-ligand complex on the surface of the cuvette. Binding of glucose changed the relative intensity of ANS26-GGBP and the Ru complex, resulting in a dramatic change in modulation at a low frequency of 2.1 MHz. Modulation measurements at 2.1 MHz were shown to accurately determine the glucose concentration. These results suggest an approach to glucose sensing with simple devices.

Spectroscopic and Clinical Aspects of Noninvasive Glucose Measurements

Frequent determination of glucose concentrations in diabetic patients is an important tool for diabetes management. This requires repetitive lancing and finger bleeding. Use of noninvasive (NI) detection techniques offers several advantages, such as the absence of pain and exposure to sharp objects and biohazard materials, the potential for increased frequency of testing, and hence, tighter control of the glucose concentrations, and the potential for a closed-loop system including a monitor and an insulin pump. These potential advantages have led to considerable interest in the commercialization of NI glucose monitoring devices. Review of the scientific, patent, and commercial literature indicates that the spectroscopic basis for NI determination of glucose is not yet well established, and attempts at commercialization may be several steps ahead of our understanding the origin and characteristics of an in vivo glucose-specific or glucose-related signal. Several technologies have potential for leading to viable measuring devices, but most of the data are based on in vitro experimentation. Because of the technical complexity of in vivo glucose measurements, this review aims at discussing the gap between the established need and current technology limitations.

Phantom glucose calibration models from simulated noninvasive human near-infrared spectra

The validity of published reports claiming to have successfully measured in vivo blood glucose from noninvasive near-infrared spectra collected in a time-dependent manner is challenged on the basis of results obtained from a phantom glucose spectral data set. An in vitro model is used to simulate noninvasive human near-IR spectra. The phantom glucose data set is created by purposely omitting glucose in these modeled samples. Glucose values are then assigned to successive phantom glucose spectra, and multivariate calibration models are generated for glucose based on partial-least squares regression. As expected, calibration models are incapable of predicting glucose values when the glucose assignments are made randomly. Apparently functional models are obtained, however, when glucose assignments are made in a nonrandom, time-dependent manner. Prediction errors from these nonrandom models are essentially identical to those published by other as evidence of successful noninvasive blood glucose measurements. Chance temporal correlations between assigned glucose concentrations and some uncontrolled experimental parameter are responsible for this apparent model functionality.