Page 427 - Handbook of Biomechatronics
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422 Graham Brooker
electronics capable of running from 3.6V down to 2.2V for the lithium coin
cell or 1.8V for the alkaline AAAs is needed. A boost switching regulator is
required in meters with more complex displays that require higher voltages
Finally, all meters must meet electrostatic discharge (ESD) specifications
in their country of use. Electronics with built-in ESD protection or the addi-
tion of ESD line protectors to exposed traces can help meet this requirement
(DiCristina, 2017).
3.4 Continuous Glucose Monitoring Systems
Continuous glucose monitoring systems (CGMSs) are reagentless and based
on the direct transfer between the enzyme and the electrode without a medi-
ator stage. This is advantageous as, by removing potentially toxic mediators,
the electrode performs direct electron transfers using the charge transfer
complexes of conductive organic materials. This has facilitated the develop-
ment of implantable needle-based devices for continuous in vivo glucose
monitoring. Conducting organic salts, such as tetrathiafulvalene-
tetracyanoquinodimethane mediate the electrochemistry of pyrrole-
quinolinequinone enzymes as well as of flavoproteins including GOx. As
a bonus, the absence of mediators also provides the biosensors with superior
selectivity. However, only a few enzymes including peroxidases have been
proved to exhibit direct electron transfer at normal electrode surfaces.
CGMSs can be subcutaneous or blood based. However because of prob-
lems with surface contamination of electrodes by blood proteins and the
possibility of embolisms due to blood coagulation, few measure blood glu-
cose directly, but rather sample glucose levels in interstitial fluid. These are
known as subcutaneous glucose monitoring systems (SCGMSs), a schematic
example of which is shown in Fig. 8. The first commercial SCGMS was
marketed by MiniMed. It did not provide a local readout but 3 days of log-
ged data could be downloaded by the health-care professional for analysis.
At present SCGMS devices including the MiniMed Guardian REAL-
Time system by Medtronic, the SEVEN by Dexcom and the Freestyle Nav-
igator by Abbott are most widely used monitors on the market. These
devices display updated real-time glucose concentrations every few minutes
using a disposable sensor with a lifespan of between 3 and 7 days. They all
consist of flexible strips implanted in the hypodermis, but are neither non-
invasive, nor painless, as their insertion requires a large bore hypodermic
needle. The smallest sensor strips on the market are 3mm long and about