Ultrasonic-assisted point-one-shot mid-infrared Fourier spectroscopy for realization of ear-clip type non-invasive blood glucose sensors: Ultrasonic-assisted method for production of a reflection planenear the skin surface and detection of the blood glucose absorption peak in mice (2nd report)

摘要

For monitoring and prevention of lifestyle diseases, we have investigated development of non-invasive ear-clip typeblood glucose sensors that can be used by individuals in daily life. Because mid-infrared light (λ = 8–14 μm) is absorbedstrongly by water, it is difficult to detect transmitted light from human bodies. We propose that an ultrasonic-assistedmethod, which can actively produce a reflection plane at a blood vessel (depth of around 100 μm), could be used todetect internal reflected light from biological tissues. An ultrasonic standing wave produced near the skin surface by anultrasonic vibrator can form refractive index boundaries between areas of high and low density. Additionally, we can usea low ultrasonic frequency (1 MHz) that does not suffer from heavy damping by parametric standing waves, whichappear in non-rigid samples and produce reflection planes at the same depth when using a high frequency (10 MHz). Atthe refractive index boundary nearest the skin surface, the light from inside biological tissues is reflected and can be usedto obtain information of biological components. The optical path length can be set by changing the measurement depthby altering the ultrasonication frequency. We measured internal reflected light from the ears of mice (blood glucose level= 120 mg/dL) non-invasively using our unique mid-infrared spectroscopic imager and obtained absorption peaks forblood glucose (λ = 9.3 μm and 9.7 μm). Mid-infrared spectroscopy can be applied to measurements in samples with highwater contents.