Biosensor technology in the treatment of cardiovascular disease - Biomaterials and Devices for the Circulatory System - page 287

Transmitter

Glucose
sensor

11.5 Needle-type subcutaneous glucose sensor, used in Medtronic's
Guardian continuous glucose monitoring system. Transmitter
transfers data on glucose levels from sensor to monitor (image
courtesy of Medtronic Limited).

basis of point-of-care devices for home testing of glucose, which require
only small amounts of blood but involve regular fi ngerprick tests. Tighter
control of blood glucose levels could be achieved by continuous monitoring,
which is impractical with devices that require regular blood sampling. Sub-
cutaneous implanted sensors have been widely researched with a view to
continuous in vivo monitoring. Glucose levels in the interstitial fl uid of
the subcutaneous tissue can be correlated with blood glucose levels, and
there is even software available to account for the time lag between the
levels. Needle-type electrodes have been widely used in this application,
Medtronic's version of which is shown in Fig. 11.5. Such devices can provide
a large amount of information about transient glucose levels, but the elec-
trodes have to be removed and replaced every few days. Commercially
available real-time continuous glucose monitoring systems, which use
subcutaneous sensors, include DexCom's SevenPlus, Abbott's Freestyle
Navigator and Medtronic's Guardian.
Trends in glucose sensing are towards non-invasive devices, which could
measure glucose concentrations in sweat or saliva. The GlucoWatch Biog-
rapher (Cygnus Inc.) was a device approved by the US Food and Drug
Administration (FDA), which measured glucose non-invasively, by drawing
glucose across the skin by reverse iontophoresis. It was the fi rst device of
its kind, but was never widely taken up, due to problems with skin irritation
and usability. 22
Cholesterol biosensors have also increased in clinical importance, in
identifying and monitoring patients with high blood cholesterol, which is
known to be a signifi cant risk factor for atherosclerosis, coronary artery

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