Page 434 - Handbook of Biomechatronics

428 Graham Brooker

3.5.5 Photoacoustic Spectroscopy
This is a method based on pressure variations generated in tissue when
heated by a laser pulse. Selective detection of blood glucose uses specific laser
frequencies. It is not affected by water because of its poor photoacoustic
response of the liquid and a wide range of frequencies from the IR through
to the UV that can be used. It is used by the Aprise system for glucose mon-
itoring (Vashist, 2012).

3.5.6 Raman Spectroscopy
This form of spectroscopy measures the characteristics of scattered light at a
higher frequency and lower intensity than the incident source. Water has
weak scattering indices so does not interfere with the measurements. It is
generally used to measure for glucose in the aqueous humor of the eye
(Vashist, 2012).

3.5.7 Ocular Spectroscopy
Another spectroscopy method that measures glucose concentration in tears
using a boric acid derivative hydrogel wafer bound to a contact lens. The
boric acid derivatives form reversible covalent bonds with the glucose in
tears resulting in a change in frequency of light reflected from the lens that
is identified using a spectrometer. Issues with this technique include signif-
icant lag and poor correlation between glucose in tears and that in blood
(Vashist, 2012).

3.5.8 Fluorescence
This uses UV excitation of primed tissues. Currently, polymerized crystal-
line colloidal arrays that respond to glucose concentration by altering their
refractive index are used. The latest research focuses on the development of
contact lenses that change color in response to changes in glucose concen-
tration (Vashist, 2012).

3.5.9 Polarimetry
This technique has been used in industry for decades and so is well
researched as a method of measuring glucose concentration. The technique
relies on the rotation of linearly polarized light as it passes through a glucose
solution. The aqueous humor of the eye, being clear, is an ideal measure-
ment site. A path length of 10mm gives 10mdeg of rotation for a glucose
concentration 10mmol/L at a wavelength of 670nm. Unfortunately its