Page 431 - Handbook of Biomechatronics

426 Graham Brooker

transdermal extraction of interstitial fluid using vacuum methods or reverse
iontophoresis. These techniques are reviewed below, as described by
Vashist (2012).

3.5.1 Reverse Iontophoresis
This process draws glucose out through the skin using by the potential dif-
ference between two electrodes attached to the skin. It works by drawing
+

Na cations and Cl anions through the skin to the cathodic and anodic
electrodes, respectively. Some of the uncharged glucose molecules present
in the interstitial fluid are carried along with the ions and collected at the
electrodes. Glucose concentration is then measured using one of the con-
ventional methods described previously (Vashist, 2012). This technique
was used by the now discontinued GlucoWatch shown in Fig. 11.

3.5.2 Ultrasound
This method provides an alternative to reverse iontophoresis, as vibrations at
a frequency of 20kHz (beyond the range of human hearing) increases the
permeability of skin to interstitial fluids, and so glucose is transported to
the epidermis where it can be measured using conventional glucose-sensing
technology (Vashist, 2012).

3.5.3 Bioimpedance Spectroscopy
This method examines the tissue impedance at different frequencies by
introducing an alternating current in the frequency range from 100 to
100MHz. Changes in the glucose concentration in blood plasma alter the
membrane potential of red blood cells that can be inferred from changes
in the bioimpedance. However, it is susceptible to water content and dis-
eases that affect the cell membrane. This technique is used in the Pendra glu-
cose monitor (Vashist, 2012).

3.5.4 Infrared Spectroscopy
These techniques range from the thermal in which glucose absorption at 9.8
and 10.9 μm is monitored against the background thermal signature of the
body to the near and mid infrared bands where glucose affects the absorption
and scattering effects at specific frequencies. Unfortunately, the signal is
either weak compared to that of water or the penetration is poor. The prin-
ciple used is attenuated total reflection and relies on the light beam guided
through a crystal and a thin film of squalane oil in contact with the skin.
SugarTrack, Sensys, and Orsense use variations of this technique for glucose
monitoring (Vashist, 2012).