We aim to realize non-invasive blood glucose measurements in daily life. The human body emits light with an intensity that depends on the body temperature (approximately 300 K). Mid-infrared passive spectroscopic imaging obtains component information from this radiated light. Using radiated light measurements of the arm from a distance of 600 mm, we identified the specific emission spectral peaks of glucose components at wavelengths of 9.25 μm and 9.65 μm. In addition, we determined the correlation between the intensity of radiated light at the peak wavelength and blood glucose level. From these results, we previously reported the possibility of non-invasive blood glucose measurements from a distance using mid-infrared passive spectroscopic imaging. Therefore, we developed a passive one-shot Fourier spectrometer to apply this method to wearable devices. The apparatus was designed with a numerical aperture (N.A.) of 0.77 for passive spectroscopy of the living body. The field curvature due to the increased N.A. was mitigated by combining an imaging lens with a phase shifter. Additionally, the apparatus was configured with two lenses to enhance transmission. Owing to the small object lens (diameter of 6 mm) and short optical path (axis length of approximately 14 mm), the apparatus was mountable on wearable devices. Moreover, the apparatus was equipped with a multi-slit to prevent loss of interference sharpness. The multi-slit was designed with five lines comprising aperture patterns that enabled the detection of glucose. As a result, we succeeded in detecting spectral characteristics of polypropylene using a blackbody as the background light source.
We developed a one-shot passive Fourier spectrometer to integrate with wearable devices for non-invasive blood glucose measurements. The apparatus was designed with a N.A. of 0.77 for passive spectroscopy of the living body. The field curvature due to the increased N.A. was improved by combining an imaging lens with a phase shifter. Because the apparatus was configured with two lenses, the transmission was high and the optical axis length was 14 mm. In addition, the apparatus was equipped with a multi-slit to prevent loss of interference sharpness. The multi-slit was designed with 5 lines with aperture patterns that enabled the detection of glucose. We measured a blackbody to evaluate the apparatus.
We had already reported that blood glucose levels of internal bodies were able to be measured by the proposed mid-infrared (LWIR: 8-14μm) passive spectroscopic imager from a distance. The mid-infrared passive spectroscopic imager, whose sensitivity is very high because of the proposed multi-slit array technology, is able to detect and analyze radiations from body heats. Expanding the wavelength region to 3 to 20μm will realize the measurement of lactic acids and ketone bodies. Lactic acid levels, whose emission peak are 5.65μm etc., are an indicator of fatigue. Ketone bodies, whose emission peak are 4.65μm etc., are an indicator of metabolic effects of dieting. On the other hand, transmission type lens requires AR(Anti-reflection) coating whose band pass is theoretically narrow around 8 to 14μm. But our passive spectroscopic imager is able to be constructed by the wavelength-independent reflection optics. In this report, we mentioned the broad band, whose wavelength was 3 to 20μm, reflective optics spectroscopic imager using free-form mirror lens units (provided by NALUX Co., Ltd.) with a micro-bolometer array sensor.
The proposed mid-infrared passive spectroscopic imager could measure blood glucose levels of internal bodies from a distance. In this report, we distinguished blood vessel areas and interstitial fluid areas in accordance with time-response differences. Freestyle Libre measures blood glucose levels in interstitial fluid. Thus, the measured value has time delay around 10 min compared with the conventional blood collection type. In accordance with correlation coefficients of radiation value, whose wavelength was 9.65μm, we identified these two areas at every pixel.
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