Glucose-sensitive membrane and infrared absorption spectroscopy for potential use as an implantable glucose sensor

The applicability of using infrared absorption spectroscopy for an implantable glucose sensor was tested by measuring absorbance at 9.66 microns for glucose in aqueous solution at known concentrations. A commercial Fourier transform infrared spectrometer was used with an attenuated total reflection technique (ATR). Absorbance measurements were reproducible and linearity was excellent (coefficient of correlation: 0.996), in spite of pH and temperature changes over values that span the human physiologic ranges (pH 6.8-7.8, temperature, 32-43 degrees C) and 18-34% decreases in incident infrared energy. The minimum detectable concentration of glucose, however, was 31 mmol/L (560 mg/d) and, to use infrared sources and detectors compatible with an implantable device, an absorbance peak between 1 and 3 microns must be used. To solve the detection sensitivity and wavelength problems, the ATR prism was coated with a hydrophobic liquid membrane. The absorbance of water varied at 2.95 microns in inverse linear proportion to glucose concentration between 2-31 mmol/L (35-560 mg/dL, r = -0.992). Estimated incident infrared energy at 2.95 microns (0.2 micron bandwidth) was low enough to theoretically allow the use of a lithium battery small enough for implantation. These findings support the concept of using infrared absorption spectroscopy for an implantable glucose sensor.