A Preliminary Investigation into the Design of an Implantable Optical Blood Glucose Sensor

A preliminary investigation into the design of a near-infrared (NIR) optical bio-implant for accurate measurement of blood glucose level is reported. The use of an array of electrically pumped vertical-cavity surface-emitting laser (VCSEL) diodes at specific wavelengths for high-power narrow single-frequency emission leads to a high signal-to-noise ratio in the measured NIR absorption spectrum while maximizing the sensor’s sensitivity to small absorption changes. The emission wavelengths lie within the combination and first-overtone spectral bands known to be dominated by glucose absorption information. A Quantum well infrared (QWI) photodiode transducer senses the received optical power after passing through the blood sample, followed by an artificial neural network (ANN) for the measurement of glucose in a whole blood matrix. For an independent test set made with fresh bovine blood, the optimal ANN topology for processing the two selected spectral bands yielded a standard error of prediction of 0.42 mM (i.e., 7.56 mg/dl) over the glucose level range of 4−20 mM. The empirical results obtained with a prototype mounted on PCB for blood glucose monitoring are closely correlated with the absorption spectra collected on a Vertex 70 Bruker spectrometer.Development of semiconductor light-emitting devices is critical to optical blood glucose sensing using near-infrared absorption spectroscopy. Since glucose absorption spectrum has many features in the combination (2−2.5 µm) and first- overtone (1.54−1.82 µm) spectral bands, sample devices that could emit high-power narrow single-frequency light within these two wavelength ranges are required. Fortunately, InP-based VCSEL structures that could efficiently meet those requirements have recently been developed and shown to have stable emission spectra over a fairly broad wavelength range[5]. In our study, the VCSEL devices will be electrically pumped at wavelengths centered at 1.54, 1.67, 1.73, 2.10, 2.27 and 2.32 µm, respectively; where glucose has specific absorption features and water has relatively higher transmittance. The VCSEL devices are used in a round-robin sequence, from the one emitting at the shortest wavelength to the one emitting at the largest wavelength, until all devices are used.

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