Mid-Infrared Emission Study Proposes New Principle for Noninvasive Blood Sugar Measurement

Introduction: A Non-Invasive Goal in Diabetes Care

Diabetes, defined by fasting plasma glucose concentrations of 126 mg/dL or higher, is a growing global health challenge. Patients face risks ranging from neuropathy and nephropathy to atherosclerosis, leading to cardiovascular complications. Yet the daily reality for many involves painful finger-stick devices to measure glucose levels (1).

Mid-Infrared Emission for Noninvasive Blood Sugar Measurement © RZ-chronicles-stock.adobe.com

In response, researchers have explored non-invasive optical methods. Near-infrared (NIR), infrared, and Raman spectroscopic methods have been widely explored (1–4), but noninvasive glucose detection is limited by differences in patient skin tissue, overlapping water absorption with glucose absorption, and a weak glucose signal (1–4). Mid-infrared spectroscopy, however, offers unique absorption features in the so-called fingerprint region near 10 µm. The challenge: mid-infrared light does not penetrate deeply into the skin (1).

A team led by Daigo Anabuki, Shun Tahara, Hiroto Yano, Akihiro Nishiyama, Keisuke Wada, Atsushi Nishimura, and Itsuo Ishimaru sought to address this issue by focusing on emission rather than transmission. Their work at Kagawa University and collaborating institutions proposes a physical principle to explain how glucose becomes detectable in the mid-infrared range (1).

The Emission Integral Effect

Living tissue emits mid-infrared radiation according to body temperature, as described by Planck’s law. Past studies showed correlations between emitted light intensity at the wrist and blood glucose concentration, but no model explained why glucose specifically could be identified (1).

In their article, “Emission Integral Effect on Non-Invasive Blood Glucose Measurements Made Using Mid-Infrared Passive Spectroscopic Imaging,” the researchers propose the emission integral effect. This principle describes how emission intensity changes depending on both the thickness of a material and its absorption coefficient. Unlike conventional transmission-based spectroscopy, emission-based detection is not distorted by Fresnel surface reflections (1).

Using mid-infrared passive spectroscopic imaging, the team showed that glucose emissions can be detected in the 8–14 µm wavelength range (1250 to 714 cm^-1), where characteristic glucose-related absorption features occur (1).

To explain how glucose becomes identifiable in emission measurements, they introduced the emission integral effect, which describes how emission intensity changes with both material thickness and absorption properties. Laboratory simulations with polypropylene samples, used as an optical or tissue phantom, confirmed the accuracy of this model, showing strong correlations between experimental and simulated results. The findings suggest that this principle could support practical applications in wrist-based blood glucose monitoring, offering a potential alternative to invasive finger-stick testing (1).

Laboratory Verification with Polypropylene

To test the model, the team measured spectral radiance from polypropylene samples of varying thicknesses using mid-infrared passive spectroscopic imaging. They then compared these results with simulations at specific emission peaks: 8.59, 10.02, 10.26, and 11.88 µm (1).

The correlation between experiment and simulation was striking—0.99 at 8.59 µm, 0.97 at 10.02 µm, 0.96 at 10.26 µm, and 0.95 at 11.88 µm. These values validated the emission integral effect as a working model for understanding how thickness influences emission intensity in mid-infrared studies (1).

Implications for Blood Glucose Monitoring

The researchers extended their analysis to simulate glucose emission at the wrist, where dermal tissue is about 1–4 mm thick and contains dense networks of capillaries. Using Fourier-transform infrared (FT-IR) spectroscopy to determine glucose absorption coefficients, they modeled emission intensity changes at peak glucose wavelengths of 9.25 and 9.65 µm (1).

The results showed that glucose emissions increased consistently with thickness and did not display saturation-like behavior. Importantly, the emission integral effect suggested that dilute components such as glucose are actually more detectable than high-concentration substances in emission measurements. This observation may explain why mid-infrared passive spectroscopic imaging successfully correlates with blood glucose despite glucose’s relatively low concentration in blood (1).

Looking Ahead

The study highlights both potential and limitations. While the emission integral effect provides a physical explanation for glucose detection, factors such as individual variation in skin thickness and tissue structure remain challenges. The authors note that differences observed in wrist measurements could be compensated by analyzing intensity ratios of glucose emission peaks and applying corrections based on the proposed model (1).

Future research will need to consider multi-layer tissue models, temperature gradients, and other physiological variables. Nonetheless, the principle may enable expansion of mid-infrared emission methods not only for glucose but also for other analytes measurable at a distance (1).

References
(1) Anabuki, D.; Tahara, S.; Yano, H.; Nishiyama, A.; Wada, K.; Nishimura, A.; Ishimaru, I. Emission Integral Effect on Non-Invasive Blood Glucose Measurements Made Using Mid-Infrared Passive Spectroscopic Imaging. Sensors 2025, 25 (6), 1674. DOI: 10.3390/s25061674

(2) Fathimal, S.; Kumar, J. S.; Selvaraj, J.; SP, A. K. Potential of Near-Infrared Optical Techniques for Non-Invasive Blood Glucose Measurement: A Pilot Study. IRBM 2025, 46 (1), 100870. DOI: 10.1016/j.irbm.2024.100870

(3) Song, L.; Han, Z.; Shum, P. W.; Lau, W. M. Enhancing the Accuracy of Blood-Glucose Tests by Upgrading FTIR with Multiple-Reflections, Quantum Cascade Laser, Two-Dimensional Correlation Spectroscopy and Machine Learning. Spectrochim. Acta A Mol. Biomol. Spectrosc. 2025, 327, 125400. DOI: 10.1016/j.saa.2024.125400

(4) Azimzadeh Andarabi, E.; Norouzian-Alam, S.; Shayganmanesh, M.; Haji Abdolvahab, M. Analysis of Glucose Concentrations in Blood Solutions Using FTIR and Raman Spectroscopy Methods. Biomed. Opt. Express 2025, 16 (7), 2631–2662. DOI: 10.1364/BOE.561552