Page 436 - Handbook of Biomechatronics

430 Graham Brooker

compared with 4–6mmol/L in blood, as well as a poor correlation and sig-
nificant time lag linking the concentration of glucose in tears and that in
blood. Finally there are three different sorts of tears: basal tears that lubricate
the eye, tears caused by irritation, and finally those caused by emotion, each
of which could have different glucose levels relative to the blood concen-
tration (Barrettino, 2017).
The alternative according to Lucisano et al. (2017), for a continuous
glucose monitor would be one that is not percutaneous, unobtrusive,
notattachedtothe skin,abletoretainlong-term calibrationandrequires
no maintenance by the user. To that end an implantable device within a
hermetically sealed titanium puck housing the battery-powered signal
conditioning and telemetry circuitry has been developed. A ceramic insert
with a planar antenna on the inside and eight 300-μm diameter platinum
electrodes and counter electrodes on the outside forms the core of the
device. The WE are negatively polarized for oxygen reduction and cov-
ered with a thin electrolyte layer, a protective layer of pol-
ydimethylsiloxane (PDMS), and finally a further layer of PDMS with
hollows over the active electrodes filled with gel-containing immobilized
GOx and a catalyst.
In trials, these pucks were implanted in the abdominal tissue below any
subcutaneous fat with electrodes facing inward in contact with muscle tissue
but not tethered. The telemetry antenna faced outward. Tests were con-
ducted over a 6-month period, with glucose clamp studies conducted at
monthly intervals. These clamp studies involved the infusion of glucose
and insulin to achieve specified blood glucose levels in the normal, hypogly-
cemic, and hyperglycemic ranges. Venous blood was extracted and analyzed
on a regular basis and was used as a retrospective calibration reference.
Between studies, the diabetic patients managed their condition in the
normal way.
Results of this study showed excellent correlation between the calibrated
outputs of the sensor and measured glucose levels both during the clamp
studies as shown in Fig. 12, and intervening finger-prick measurements.
This allows for the construction of an accurate glucose/insulin model for
individual patients (discussed later).
Another developer of this technology now under FDA review is the
Eversense from Senseonics designed to provide continuous monitoring
for between 3 and 6 months. It is small enough to be replaceable with a sim-
ple surgical procedure doable by any trained health-care provider (Castle
et al., 2017).