Noninvasive and minimally-invasive optical monitoring technologies

Aptamer-Based Lateral Flow Biosensor for Rapid Detection of Salivary Cortisol

We have developed a disposable point-of-care (POC) aptamer-based biosensor for the detection of salivary cortisol. Nonstressful and noninvasive sampling of saliva compared to that of blood makes saliva an attractive biological matrix in developing POC devices for biomarker monitoring. Aptamers are attractive as recognition elements for multiple reasons, including their specific chemical synthesis, high stability, lack of immunogenicity, and cell-free evolution. A duplex aptamer conjugated to the surface of Au nanoparticles (AuNPs) by Au-S bonds is utilized as the sensor probe in a lateral flow assay (LFA) device. The addition of saliva samples containing cortisol makes the cortisol-aptamer undergo conformational changes and dissociate from the capture probe. Increasing cortisol concentration in the dispensed saliva sample results in increased dissociation and leads to increased binding of AuNP conjugate on the test line. Therefore, the color intensity of the test line on the LFA is a direct function of the concentration of cortisol in saliva. This simple and fast method provides detection in the cortisol range of ∼0.5-15 ng/mL, which is in the clinically accepted range for salivary cortisol. The limit of detection was 0.37 ng/mL, and the accuracy was confirmed by enzyme-linked immunosorbent assay (ELISA) testing results. High selectivity was observed for salivary cortisol against other closely related steroids and stress biomarkers present in saliva.

Glucose monitoring in neonates: need for accurate and non-invasive methods

Neonatal hypoglycaemia can lead to devastating consequences. Thus, constant, accurate and safe glucose monitoring is imperative in neonatal care. However, point-of-care (POC) devices for glucose testing currently used for neonates were originally designed for adults and do not address issues specific to neonates. This review will address currently available monitoring options and describe new methodologies for non-invasive glucose monitoring in newborns.

Robust calibration transfer in noninvasive ethanol measurements, Part II: Modification of instrument measurements by incorporation of expert knowledge (MIMIK)

Several calibration transfer methods require measurement of a subset of the calibration samples on each future instrument, which is impractical in some applications. Another consideration is that these methods model inter-instrument spectral differences implicitly rather than explicitly. The present work argues that explicit knowledge of the origins of inter-instrument spectral distortions can benefit calibration transfer during the fabrication and assembly of instrumentation, the formation of the multivariate regression, and its subsequent transfer to future instruments. In Part I of this work, a Fourier transform near-infrared system designed to perform noninvasive ethanol measurements was discussed and equations describing the optical distortions caused by self-apodization, retroreflector misalignment, and off-axis detector field of view were provided and examined using laboratory measurements. The spectral distortions were shown to be nonlinear in the amplitude and wavenumber domains, and thus cannot be compensated by simple wavenumber calibration procedures or background correction. Part II presents a calibration transfer method that combines in vivo data with controlled amounts of optical distortions in order to develop a multivariate regression model that is robust to instrument variation. Evaluation of the method using clinical data showed improved measurement accuracy, outlier detection, and generalization to future instruments relative to simple background correction.

Applications of ultrasensitive wavelength-modulated differential photothermal radiometry to noninvasive glucose detection in blood serum

Wavelength-Modulated Differential Laser Photothermal Radiometry (WM-DPTR) has been designed for noninvasive glucose measurements in the mid-infrared (MIR) range. Glucose measurements in human blood serum in the physiological range (20-320 mg/dl) with predicted error <10.3 mg/dl demonstrated high sensitivity and accuracy to meet wide clinical detection requirements, ranging from hypoglycemia to hyperglycemia. The glucose sensitivity and specificity of WM-DPTR stem from the subtraction of the simultaneously measured signals from two excitation laser beams at wavelengths near the peak and the baseline of the strongest interference-free glucose absorption band in the MIR range. It was found that the serum glucose sensitivity and measurement precision strongly depend on the tunability and stability of the intensity ratio and the phase shift of the two laser beams. This level of accuracy was favorably compared to other MIR techniques. WM-DPTR has shown excellent potential to be developed into a clinically viable noninvasive glucose biosensor.

Detection of trace glucose on the surface of a semipermeable membrane using a fluorescently labeled glucose-binding protein: a promising approach to noninvasive glucose monitoring

BACKGROUND

Our motivation for this study was to develop a noninvasive glucose sensor for low birth weight neonates. We hypothesized that the underdeveloped skin of neonates will allow for the diffusion of glucose to the surface where it can be sampled noninvasively. On further study, we found that measurable amounts of glucose can also be collected on the skin of adults.

METHOD

Cellulose acetate dialysis membrane was used as surrogate for preterm neonatal skin. Glucose on the surface was collected by saline-moistened swabs and analyzed with glucose-binding protein (GBP). The saline-moistened swab was also tested in the neonatal intensive care unit. Saline was directly applied on adult skin and collected for analysis with two methods: GBP and high-performance anion-exchange chromatography (HPAEC).

RESULTS

The amount of glucose on the membrane surface was found (1) to accumulate with time but gradually level off, (2) to be proportional to the swab dwell time, and (3) the concentration of the glucose solution on the opposite side of the membrane. The swab, however, failed to absorb glucose on neonatal skin. On direct application of saline onto adult skin, we were able to measure by HPAEC and GBP the amount of glucose collected on the surface. Blood glucose appears to track transdermal glucose levels.

CONCLUSIONS

We were able to measure trace amounts of glucose on the skin surface that appear to follow blood glucose levels. The present results show modest correlation with blood glucose. Nonetheless, this method may present a noninvasive alternative to tracking glucose trends.

Noninvasive glucose detection in human skin using wavelength modulated differential laser photothermal radiometry

Noninvasive glucose monitoring will greatly improve diabetes management. We applied Wavelength-Modulated Differential Laser Photothermal Radiometry (WM-DPTR) to noninvasive glucose measurements in human skin in vitro in the mid-infrared range. Glucose measurements in human blood serum diffused into a human skin sample (1 mm thickness from abdomen) in the physiological range (21-400 mg/dl) demonstrated high sensitivity and accuracy to meet wide clinical detection requirements. It was found that the glucose sensitivity could be tuned by adjusting the intensity ratio and phase difference of the two laser beams in the WM-DPTR system. The measurement results demonstrated the feasibility of the development of WM-DPTR into a clinically viable noninvasive glucose biosensor.

Non-invasive glucose monitoring technology in diabetes management: a review

The frequent monitoring of glucose is an essential part of diabetes management. Despite the fact that almost all the commercially successful blood glucose monitoring devices are invasive, there is an immense need to develop non-invasive glucose monitoring (NGM) devices that will alleviate the pain and suffering of diabetics associated with the frequent pricking of skin for taking the blood sample for glucose testing. There have been numerous developments in the field of NGM during the last decade, which stress the need for a critical review. This manuscript aims to review the various NGM techniques and devices. The challenges and future trends in NGM are also discussed.

High-efficiency optical systems for interrogation of dermally-implanted sensors

Ratiometric Luminescent microparticle sensors have been developed for sensing biochemical targets such as glucose in interstitial fluid, enabling use of dermal implants for on-demand monitoring. For these sensor systems to be deployed in vivo, a matched optoelectronic system for interrogation of dermally-implanted sensors was previously designed, constructed, and evaluated experimentally. During evaluation experiments, it revealed that the system efficiency was compromised by losses due to fiber connections, the entrance aperture, and the entrance slit of the spectrometer. In this work, two optimization methods were investigated to overcome photon loss at fiber connections and internal trade-off between resolution and input light power of the current spectrometer: 1) Replacement of the CCD spectrometer with a two-detector system, enabling extraction of key spectral information by integrating signals over two wavelength regions (reference and sensing emission peaks); and 2) Free-space coupling of the optical probe to a custom low-resolution spectrometer. Photon loss was evaluated by experiments and simulations, preliminary hardware of two-detector system was constructed, and optimization simulations were performed to explore conceptual feasibility of the free-space coupling custom-designed spectrometer.

Comparison of spectroscopically measured finger and forearm tissue ethanol concentration to blood and breath ethanol measurements

Previous works investigated a spectroscopic technique that offered a promising alternative to blood and breath assays for determining in vivo alcohol concentration. Although these prior works measured the dorsal forearm, we report the results of a 26-subject clinical study designed to evaluate the spectroscopic technique at a finger measurement site through comparison to contemporaneous forearm spectroscopic, venous blood, and breath measurements. Through both Monte Carlo simulation and experimental data, it is shown that tissue optical probe design has a substantial impact on the effective path-length of photons through the skin and the signal-to-noise ratio of the spectroscopic measurements. Comparison of the breath, blood, and tissue assays demonstrated significant differences in alcohol concentration that are attributable to both assay accuracy and alcohol pharmacokinetics. Similar to past works, a first order kinetic model is used to estimate the fraction of concentration variance explained by alcohol pharmacokinetics (72.6-86.7%). A significant outcome of this work was significantly improved pharmacokinetic agreement with breath (arterial) alcohol of the finger measurement (mean k(Art-Fin) = 0.111 min(-1)) relative to the forearm measurement (mean k(Art-For) = 0.019 min(-1)) that is likely due to the increased blood perfusion of the finger.

Iris as a reflector for differential absorption low-coherence interferometry to measure glucose level in the anterior chamber

We present a method of glucose concentration detection in the anterior chamber with a differential absorption optical low-coherent interferometry (LCI) technique. Back-reflected light from the iris, passing through the anterior chamber twice, was selectively obtained with the LCI technique. Two light sources, one centered within (1625 nm) and the other centered outside (1310 nm) of a glucose absorption band were used for differential absorption measurement. In the eye model and pig eye experiments, we obtained a resolution glucose level of 26.8 mg∕dL and 69.6 mg∕dL, respectively. This method has a potential application for noninvasive detection of glucose concentration in aqueous humor, which is related to the glucose concentration in blood.

Depth-resolved imaging and detection of micro-retroreflectors within biological tissue using Optical Coherence Tomography

A new approach to in vivo biosensor design is introduced, based on the use of an implantable micron-sized retroreflector-based platform and non-invasive imaging of its surface reflectivity by Optical Coherence Tomography (OCT). The possibility of using OCT for the depth-resolved imaging and detection of micro-retroreflectors in highly turbid media, including tissue, is demonstrated. The maximum imaging depth for the detection of the micro-retroreflector-based platform within the surrounding media was found to be 0.91 mm for porcine tissue and 1.65 mm for whole milk. With further development, it may be possible to utilize OCT and micro-retroreflectors as a tool for continuous monitoring of analytes in the subcutaneous tissue.

Three-dimensional, multiwavelength Monte Carlo simulations of dermally implantable luminescent sensors

Dermally implanted luminescent sensors have been proposed for monitoring of tissue biochemistry, which has the potential to improve treatments for conditions such as diabetes and kidney failure. Effective in vivo monitoring via noninvasive transdermal measurement of emission from injected microparticles requires a matched optoelectronic system for excitation and collection of luminescence. We applied Monte Carlo modeling to predict the characteristics of output luminescence from microparticles in skin to facilitate hardware design. Three-dimensional, multiwavelength Monte Carlo simulations were used to determine the spatial and spectral distribution of the escaping luminescence for different implantation depths, excitation light source properties, particle characteristics, and particle packing density. Results indicate that the ratio of output emission to input excitation power ranged 10(-3) to 10(-6) for sensors at the upper and lower dermal boundaries, respectively, and 95% of the escaping emission photons induced by a 10-mm-diam excitation beam were confined within an 18-mm circle. Tightly packed sensor configurations yielded higher output intensity with fewer particles, even after luminophore concentration effects were removed. Most importantly, for the visible wavelengths studied, the ability to measure spectral changes in emission due to glucose changes was not significantly affected by absorption and scattering of tissue, which supports the potential to accurately track changes in luminescence of sensor implants that respond to the biochemistry of the skin.

Glucose concentration monitoring using a small Fabry-Perot etalon

Accurate measurements of aqueous glucose concentrations have been made in a double-chamber Fabry-Perot etalon that can be miniaturized for subcutaneous implantation to determine the concentration of glucose in interstitial fluid. In general, optical approaches to glucose detection measure light intensity, which in tissue varies due to inherent scattering and absorption. In our measurements, we compare the spectral positions of transmission maximums in two adjunct sections of an etalon in order to determine the refractive index difference between these sections and therefore we can tolerate large changes in intensity. With this approach, we were able to determine aqueous glucose concentrations between 0 mg/dl and 700 mg/dl within the precision of our reference measurement (+/-2.5 mg/dl or 2% of the measurement value). The use of reference cavities eliminates interference due to temperature variations, and we show the temperature independence over a temperature range of 32 degrees C to 42 degrees C. Furthermore, external filters eliminate interference from large molecule contaminants.

Single walled carbon nanotubes as reporters for the optical detection of glucose

This article reviews current efforts to make glucose sensors based on the inherent optical properties of single walled carbon nanotubes. The advantages of single walled carbon nanotubes over traditional organic and nanoparticle fluorophores for in vivo-sensing applications are discussed. Two recent glucose sensors made by our group are described, with the first being an enzyme-based glucose sensor that couples a reaction mediator, which quenches nanotube fluorescence, on the surface of the nanotube with the reaction of the enzyme. The second sensor is based on competitive equilibrium binding between dextran-coated nanotubes and concanavalin A. The biocompatibility of a model sensor is examined using the chicken embryo chorioallantoic membrane as a tissue model. The advantages of measuring glucose concentration directly, like most optical sensors, versus measuring the flux in glucose concentration, like most electrochemical sensors, is discussed.

Experimental validation of an optical system for interrogation of dermally-implanted microparticle sensors

Dermally-implanted microparticle sensors are being developed for on-demand monitoring of blood sugar levels. For these to be deployed in vivo, a matched optoelectronic system for delivery of excitation, collection and analysis of escaping fluorescent signal is needed. Previous studies predicted the characteristics of fluorescence from microparticle sensors to facilitate design of hardware system. Based on the results of simulations, we designed and constructed the optical part of this opto-electronic system. This study experimentally verified the simulation results and tested the capability of the designed optical system. Reliable skin phantoms sufficient for future dynamic tests were developed. Skin phantoms with different thicknesses were made and the optical properties of skin phantoms were determined with an integrating sphere system and Inverse Adding-Doubling method. Measurements of sensor emission spectrum through phantoms with different thicknesses were done with the designed optical system. Simulations for the experiment situation were performed. The experimental measurements agreed well with simulations in most cases. The results of hardware experiment and validation with skin phantoms provided us with critical information for future dynamic tests and animal experiments.

Modeling of selective photon capture for collection of fluorescence emitted from dermally-implanted microparticle sensors

Fluorescence-based sensors have been developed in microsphere formats for many biochemical targets. For these to be deployed in vivo for on-demand monitoring, a matched optical system for delivery of excitation and measurement of emission is needed. To optimize excitation and collection efficiency, statistical ray-tracing may be used to model the distribution of diffusely reflected light resulting from varying input beam profiles. In this work, simulations were performed for models of microsphere fluorescent materials embedded in skin to predict the distribution of excitation and fluorescent photons escaping the skin surface. Simulations prove that the emission photons possess sufficient intensity and spectral information for quantitative analysis. This modeling approach will enable further design of intensity or lifetime instrumentation to maximize signal-to-noise for measurements from implanted sensor particles.

Optical instrument design for interrogation of dermally-implanted luminescent microparticle sensors

Luminescence-based sensors have been developed in microparticle formats for biochemical targets such as glucose, enabling use of dermal implants for on-demand monitoring. For these to be deployed and interrogated in vivo, a matched optoelectronic system for delivery of excitation, collection and analysis of luminescence response is needed. In this work, simulations based on Monte Carlo ray-tracing were performed for models of luminescent microparticle materials embedded in skin. The spectral and spatial distribution of luminescence escaping the skin was determined for different concentrations, implantation depths, and input beam sizes. Results indicate that the implant environment does not significantly alter the measured spectral intensity ratios. The escaping emission light possesses measurable power and spectral information for quantitative analysis. Using these findings, an optical system has been designed specifically for sensor interrogation and response acquisition, and is currently implemented in hardware. Following benchtop validation and signal-to-noise maximization with tissue phantoms, the instrument will be used for measurement on sensors in rat subjects.

Current development in non-invasive glucose monitoring

Painless control of blood glycemic levels could improve life quality of diabetes patients, enabling a better regulation of hyper- and hypoglycaemia episodes and thereby avoiding physiological complications. Although research groups have been trying for decades to separate non-invasive glucose information from interference compounds, none of the available commercial devices offers enough precision to replace lancet approaches. Many reviews have already been published on this topic, but the great amount of information available and the fast development of technologies require a continuous update in the research status. Besides the description of current in-vivo methods and the analysis of devices available commercially, one also explains treatment algorithms useful for multivariate analysis.

Recent progress in analytical instrumentation for glycemic control in diabetic and critically ill patients

Implementing strict glycemic control can reduce the risk of serious complications in both diabetic and critically ill patients. For this reason, many different analytical, mainly electrochemical and optical sensor approaches for glucose measurements have been developed. Self-monitoring of blood glucose (SMBG) has been recognised as being an indispensable tool for intensive diabetes therapy. Recent progress in analytical instrumentation, allowing submicroliter samples of blood, alternative site testing, reduced test time, autocalibration, and improved precision, is comprehensively described in this review. Continuous blood glucose monitoring techniques and insulin infusion strategies, developmental steps towards the realization of the dream of an artificial pancreas under closed loop control, are presented. Progress in glucose sensing and glycemic control for both patient groups is discussed by assessing recent published literature (up to 2006). The state-of-the-art and trends in analytical techniques (either episodic, intermittent or continuous, minimal-invasive, or noninvasive) detailed in this review will provide researchers, health professionals and the diabetic community with a comprehensive overview of the potential of next-generation instrumentation suited to either short- and long-term implantation or ex vivo measurement in combination with appropriate body interfaces such as microdialysis catheters.

Mid-infrared laser measurements of aqueous glucose

Mid-IR semiconductor lasers of two wavelength bands, 5.4 and 9.6 microm, are applied to measure aqueous glucose concentration ranging from 0 to 500 mgdL with Intralipid emulsion (0 to 8%) added as a fat simulator. The absorption coefficient micro(a) is found linear with respect to glucose and Intralipid concentrations, and their specific absorption coefficients are obtained via linear regression. These coefficients are subsequently used to infer the concentrations and compare with known values. The objective is to evaluate the method accuracy. Glucose concentration is determined within +/-21 mgdL with 90% confidence and +/-32 mgdL with 99% confidence, using <1-mJ laser energy. It is limited by the apparatus mechanical error and not the photometric system noise. The expected uncertainties due to photometric noise are +/-6 and +/-9 mgdL with 90 and 99% confidence, respectively. The uncertainty is fully accounted for by the system intrinsic errors, allowing rigorous inference of the confidence level. Intralipid is found to have no measurable effect on glucose determination. Further analysis suggests that a few mid-IR wavelengths may be sufficient, and that the laser technique offers advantages with regard to accuracy, speed, and sample volume, which can be small, approximately 0.4x10(-7) mL for applications such as microfluidic or microbioarray monitoring.

Evanescent cavity ring-down spectroscopy (e-CRDS) of hemoglobin absorption at the silica-water interface

The absorption of hemoglobin (Hb) from controlled urine samples was observed using the technique of evanescent cavity ring-down spectroscopy (e-CRDS). A room temperature, alexandrite laser pumped LiF:F2(+**) color-center pulsed laser was used to excite Hb at 425 nm. A minimum absorbance level of 2.57 x 10(-4) was achieved corresponding to a minimum detectable concentration of Hb in urine of 5.8 nM. These levels could have advantages in the diagnosis of hemoglobinurea. The formation of layers of Hb upon the silica surface allowed for an increased sensitivity for smaller concentrations of Hb than would be expected for only a free floating solution. The formation of the layers also suggested a higher binding constant of Hb to the silica surface than between other layers of Hb molecules. Future studies are underway to understand the effects of salinity on the observed absorption due to the competitive binding of Na+ to the surface. Absorption isotherm modeling will also be used to better understand the development of layers upon the surface.

Continuous glucose sensing with fluorescent thin-film hydrogels. 2. Fiber optic sensor fabrication and in vitro testing

BACKGROUND

There continues to be a need for better sensors for continuous glucose monitoring. We are working on a two-component sensing system based on a viologen boronic acid and a fluorescent dye, which are immobilized in a hydrogel. This system has the potential for further development into a real-time glucose-monitoring device. The current study reports the fabrication of sensors using preformed hydrogels and the first in vitro monitoring of glucose concentrations in a prototype sensor configuration.

METHODS

Glucose sensing hydrogels containing a fluorescent dye and viologen boronic acid quencher were preformed in a mold. These preformed hydrogels were then attached to the distal end of a plastic fiber optic cable using different adhesives to prepare the in vitro sensors. These sensors were connected to a flow cell and monitored using a fluorescence spectrometer. The fluorescence emitted by the hydrogel changes depending on the glucose concentration. Hydrogel components were modified in order to optimize the performance of the sensors.

RESULTS

A soft tissue adhesive used by veterinarians was found to be an effective adhesive for bonding the hydrogel to the fiber tip. This adhesive did not affect the glucose sensing ability of the hydrogels after fabrication. Several sensors were fabricated with varying composition of sensing elements, and all of them showed stable and reversible glucose response. The glucose signal was found to be stable over months on repeated testing. Glucose sensing studies using the sensors with hydrogels containing different compositions of sensing elements showed that the ratio of dye to quencher is an important parameter in determining the magnitude and linearity of glucose response in the biological range. The response time of the sensor was shown to be dependent on the hydrophilicity of the hydrogels. Modifying the hydrogels with ionic comonomers shortened the response time.

CONCLUSIONS

The combination of the anionic dye 2 and viologen-based boronic acid 1 immobilized in a 2-hydroxyethyl methacrylate hydrogel functions well in a fiber optic configuration. This preliminary study suggests that the two-component sensing system has several advantages in terms of stability and ease of fabrication. Improvement of the configuration of the sensor and further development of the sensor towards application for in vitro study are underway.

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.

Middle infrared, quantum cascade laser optoelectronic absorption system for monitoring glucose in serum

Advances in middle infrared technology are leading researchers beyond the Fourier transform infrared spectrometer and to the quantum cascade laser. While most research focuses on gas-phase detection, recent research explores its use for condensed-phase matter studies. This work investigates its use for monitoring biologically relevant samples of glucose in serum. Samples with physiological glucose concentrations were monitored with a laser at 1036 cm-1. A 0.992 R2 linearity value was observed. In addition, using another laser at 1194 cm-1 as a measure of the background spectroscopic characteristics, a linearity of 0.998 R2 was observed. The average predictive standard errors of the mean (SEM) were 32.5 and 24.7 mg/dL, respectively, for each method. Quantum cascade lasers could be used to develop middle infrared devices for uses beyond the confines of the laboratory.

Glucose determination in human aqueous humor with Raman spectroscopy

It has been suggested that spectroscopic analysis of the aqueous humor of the eye could be used to indirectly predict blood glucose levels in diabetics noninvasively. We have been investigating this potential using Raman spectroscopy in combination with partial least squares (PLS) analysis. We have determined that glucose at clinically relevant concentrations can be accurately predicted in human aqueous humor in vitro using a PLS model based on artificial aqueous humor. We have further determined that with proper instrument design, the light energy necessary to achieve clinically acceptable prediction of glucose does not damage the retinas of rabbits and can be delivered at powers below internationally acceptable safety limits. Herein we summarize our current results and address our strategies to improve instrument design.

Hemoglobin adsorption isotherm at the silica-water interface with evanescent wave cavity ring-down spectroscopy

Evanescent wave cavity ring-down spectroscopy (EW-CRDS) is used to observe the adsorption isotherm for hemoglobin (Hb) from controlled urine samples to assess the potential for rapid diagnosis in hemoglobinuria. The absorbance of Hb at 425 nm is monitored using an alexandrite laser-pumped, room temperature, LiF:F2+** color-center pulsed laser. A minimum absorbance detection level of 2.57 x 10(-4) is achieved, corresponding to a minimum detectable concentration of Hb in urea of 5.8 nM. A multilayered Hb biofilm is formed, and a minimum of eight layers are required to model the adsorption isotherm, allowing for cooperative binding within the layers and extending 56 nm into the interface. A binding constant for Hb to silica 18.23+/-7.58 x 10(6) M is derived, and a binding constant for Hb to Hb in subsequent layers is determined to be 5.631+/-0.432 x 10(5) M. Stoichiometric binding coefficients of 1.530+/-0.981 for layer one and 1.792+/-0.162 for subsequent layers suggest that cooperative binding both to the silica surface and between the layers of the biofilm is important.

Noninvasive and minimally invasive methods for transdermal glucose monitoring

Noninvasive and minimally invasive techniques for monitoring glucose via the skin are reviewed. These approaches rely either on the interaction of electromagnetic radiation with the tissue or on the extraction of fluid across the barrier. The structure and physiology of the skin make the technical realization of transdermal glucose monitoring a difficult challenge. The techniques involving transdermal fluid extraction circumvent and/or compromise the barrier function of skin's outermost and least permeable layer, the stratum corneum, by the application of physical energy. While sonophoresis and microporation methods, for example, are in relatively early-stage development, a device using reverse iontophoresis [the GlucoWatch Biographer (Cygnus, Inc., Redwood City, CA)] is already commercially available. Optical techniques to monitor glucose are truly noninvasive. The tissue is irradiated, the absorbed or scattered radiation is analyzed, and the information is processed, to provide a measure proportional to the concentration of glucose in the dermal tissue. These techniques include near-infrared and Raman spectroscopy, polarimetry, light scattering, and photoacoustic spectroscopy. By contrast, impedance spectroscopy measures changes in the dielectric properties of the tissue induced by blood glucose variation. Large-scale studies in support of efficacy of these methodologies are as yet unavailable. At present, therefore, transdermal fluid extraction technologies are offering greater promise in terms of practical and realizable devices for patient use. The truly noninvasive allure of the optical approach assures continued and intense research activity--for the moment, however, an affordable, efficient, and portable system is not on the immediate horizon.

In vivo noninvasive measurement of blood glucose by near-infrared diffuse-reflectance spectroscopy

This paper reports in situ noninvasive blood glucose monitoring by use of near-infrared (NIR) diffuse-reflectance spectroscopy. The NIR spectra of the human forearm were measured in vivo by using a pair of source and detector optical fibers separated by a distance of 0.65 mm on the skin surface. This optical geometry enables the selective measurement of dermis tissue spectra due to the skin's optical properties and reduces the interference noise arising from the stratum corneum. Oral glucose intake experiments were performed with six subjects (including a single subject with type I diabetes) whose NIR skin spectra were measured at the forearm. Partial least-squares regression (PLSR) analysis was carried out and calibration equations were obtained with each subject individually. Without exception among the six subjects, the regression coefficient vectors of their calibration models were similar to each other and had a positive peak at around 1600 nm, corresponding to the characteristic absorption peak of glucose. This result indicates that there is every possibility of glucose detection in skin tissue using our measurement system. We also found that there was a good correlation between the optically predicted values and the directly measured values of blood samples with individual subjects. The potential of noninvasive blood glucose monitoring using our methodology was demonstrated by the present study.

Metallic colloid wavelength-ratiometric scattering sensors

Gold and silver colloids display strong colors as a result of electron oscillations induced by incident light, which are referred to as the plasmon absorption. This absorption is dependent on colloid-colloid proximity, which has been the basis of absorption assays using colloids. We now describe a new approach to optical sensing using the light scattering properties of colloids. Colloid aggregation was induced by avidin-biotin interactions, which shifted the plasmon absorption to longer wavelengths. We found the spectral shift results in changes in the scattering at different incident wavelengths. By measuring the ratio of scattered intensities at two incident wavelengths, this measurement was made independent of the total colloid concentration. The high scattering efficiency of the colloids resulted in intensities equivalent to fluorescence when normalized by the optical density of the fluorophore and colloid. This approach can be used in a wide variety of assay formats, including those commonly used with fluorescence detection.

Using two discrete frequencies within the middle infrared to quantitatively determine glucose in serum

Tight glucose monitoring is essential for the reduction of diabetic complications. This research investigated the changes of absorption spectra observed in serum at three prominent glucose absorption peaks in the middle infrared using a demountable liquid, transmission cell. Two frequencies of light were used to determine the glucose absorption: one at 1193 cm(-1 ) to determine the background water absorption and the other at one of the characteristic peaks (1035, 1080, and 1109 cm(-1)). The peak at 1035 cm(-1) was best for quantitative determination with a standard of error of 20.6 mg/dl (1.1 mmol/L). While interference from other serum constituents could cause problems, urea and albumin-two constituents known to have close absorption peaks-were determined to have no effect on the ability to determine the glucose levels at 1035 cm( -1).