# Advancements in Glucose Monitoring: From Traditional Methods to Wearable Sensors
## 4 Non-Invasive Methods
### Optical Coherence Tomography (OCT)  
Optical coherence tomography (OCT) is a technique that uses an interferometer with low coherence light to measure tissue characteristics. Typically, OCT uses near-infrared (NIR) light, benefiting from relatively affordable and compact components, and making it suitable for the 3D imaging of biological tissues [94]. Figure 10a illustrates the OCT system, which includes the interferometer optics and the photodetector/camera setup. The light source is split into two beams--one is scattered back from the tissue, and the other is reflected from the reference mirror towards the beam splitter. The interference of the light from both the tissue sample and the reference mirror creates the interferometric signal at the beam splitter. The photodetector then captures the interference pattern, and the resulting intensity varies depending on the glucose concentration at various tissue depths, up to 1.6 mm [94]. Although OCT has shown promise in non-invasive glucose monitoring, research is still in its early stages. For instance, [PERSON] et al. demonstrated that OCT could correlate well with blood glucose levels in vivo (R = 0.88), though motion artifacts and skin temperature significantly impacted the results [95; 96]. In the late 2000s, GlucoLight developed a glucose-monitoring device based on OCT [97]. In 2008, a small pilot study was conducted involving 27 diabetic patients, including 15 with Type 2 Diabetes (T2D) and 12 with Type 1 Diabetes (T1D) [98]. After two hours of measurements for each participant, the device demonstrated a mean absolute relative difference (ARD) of 11.5%. However, the study did not include measurements in the hypoglycemic range, and there has been no further published research on the device since then. Recently, [PERSON] et al. introduced a three-dimensional (3D) correlation technique that takes full advantage of the OCT system's 3D imaging capability [99]. This method enhances the accuracy of BGC detection by identifying and eliminating scattering data from non-correlating or low-correlating regions during data processing, leading to significantly more accurate results compared to the previous 1D correlation method and the 2D correlation approach demonstrated earlier. Additionally, they developed a wristband-style optical probe that reduces motion artifacts caused by shaking and ensures stable contact pressure between the probe and the skin. Figure 10b shows the OCT system paired with wristband-style optical probe. Clinical experiments, as represented by [PERSON]'s error grid model, show that 88.4% of the data points fall within Zone A, indicating a high level of accuracy for this approach (Figure 10c) [99]. Despite the potential, progress in optical glucose-monitoring methods has been slow, with many systems still in the prototype phase. Key challenges include miniaturization, high component costs, signal drift, and the variability between subjects [100]. Additionally, OCT is sensitive to motion artifacts and environmental factors like skin temperature and humidity, making accurate glucose detection challenging [100]. The small changes in the scattering coefficient and the influence of other physiological compounds complicate the application of OCT in glucose sensing. The future of OCT-based transdermal glucose sensing lies in reducing biological interference, enhancing sensitivity and resolution, and improving optical probes for more accurate and reliable measurements.  
Figure 10: (**a**) Schematic illustration demonstrating optical coherence tomography (OCT) process. Adapted from Ref. [94]. (**b**) Schematic illustration demonstrating OCT process. (**b**) Depiction of OCT  
system paired with wristband optical probe. Reprinted with permission from [99]. **(c)** The Clarke error grid for OCT-measured-style blood glucose indicates that 84.6% of the data points fall within Zone A, which represents accurate glucose measurements. Reprinted with permission from [99].