Nanostructured Color Filter based Wearable Optobiomedical Sensor for Non-invasive Diagnosis

Advanced developments in LED technology allow light to become a powerful tool for non-invasive and continuous medical tests. Optical methods avoid people to suffer from discomfort and painfulness during sampling or... [ view full abstract ]

Advanced developments in LED technology allow light to become a powerful tool for non-invasive and continuous medical tests. Optical methods avoid people to suffer from discomfort and painfulness during sampling or measurement. However, up to now, most of these sensors in the market have complex operation, bulky dimension, and high cost as well as provide only a few health indicators. Therefore, a small sensor system is designed aiming to capture and measure reliable relevant health data quickly. Differentiating from normal measurement device, e.g., pulse oximetry, which passes two or more wavelengths of light through our body part and detects transmitted or reflected light, our sensor system consists of an LED with wide bandwidth and an image sensor with color filter matrix.

Instead of using color filters made of pigments or dyes, which have low transmission efficiency, imperfect color purity and gradual aging, nanostructured color filters are developed in this work to comprise a color filter matrix. Every image sensor pixel is set under a matrix cell with a certain transmission peak wavelength measuring the intensity to build the spectrum.

Finite-difference time-domain (FDTD) simulations of guided-mode-resonance (GMR) color filter were performed to determine the suitable geometry for each single peak transmission wavelength. In terms of filtering effect, the GMR filter depends on its material, grating period, depth, fill factor, buffer layer, and waveguide layer thickness. Thus, all those parameters were carefully studied to optimize the proposed filters. For GMR, silver has a narrower bandwidth than other metals, because of the poor adhesion between silver and SiO₂, we use ITO instead of Ti or Cr as the adhesion layer, to ensure a higher transmission. A fill factor of 0.9 suppresses subwavelength better and can have high sufficient transmission of about 0.4 to 0.8. From the results, we could determine the metal grating depth of 50 nm, ITO adhesion layer thickness of 5 to 10 nm, buffer layer of 50 nm, waveguide layer of 100 nm, and grating period varies from 380 to 660 nm to cover 600 nm to 988 nm peak transmission wavelength.