Electrochemical glucose biosensors - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 65

0.09 mm

Gold

15 m 15 m 20 m

Sensing layer
glucose oxidase 110 ng
PVI 13 -dme-Os 552 ng
peg 42 ng

Biocompatible layer
(polyethylene oxide)
PEO/TA 885 ng

Mass transport limiting layer
EAQ
PVPA
PAZ

560 ng
26 ng
62 ng

FIGURE 3.5 Design of an implantable three-layered glucose biosensor for subcutaneous monitoring
(based on [15]).

continuously functioning devices, possessing adequate ( 1 week) stability, are expected

in the near future. Such devices would enable a swift and appropriate corrective action
(through a closed-loop insulin delivery system, i.e. an artifi cial pancreas). Algorithms
correcting for the transient difference (short time lag) between blood and tissue glucose
concentrations have been developed [38]. The CGMS unit of Medtronic Minimed Inc.
(Sylmar, CA) offers a 72 h period of such subcutaneous monitoring, with measurement
of tissue glucose every 5 min and data storage in the monitor's memory [39]. After 72 h,
the sensor is removed, and the information is transferred to a computer for identify-
ing patterns of glucose variations. In addition to easily removable short-term implants,
efforts are continuing towards chronically implanted devices (aimed at functioning reli-
ably for 6-12 months). A similar system is currently being developed by Abbott Inc.
[40]. This system is based on the wired enzyme technology of Heller's group (Fig. 3.5),
an insertion of a short needle into the skin, to yield a reading every minute. Both the
Abbott and Minimed devices include a limited range transmitter that relays the sensor
data to a pager-like device that provides the necessary warnings and stores the data.
Additional devices based on patch-like sensors, nano-needles or microdialysis sampling
are currently being developed by different companies.

3.6.3 Towards non-invasive glucose monitoring
Non-invasive glucose sensing is the ultimate goal of glucose monitoring. Such a non-
invasive route for continuous glucose monitoring is expected to obviate the challenges
of implantable devices. In particular, Cygnus Inc. has developed a wearable glucose
monitor, based on the coupling of reverse iontophoretic collection of glucose and bio-
sensor functions [41]. The GlucoWatch biographer provides up to three glucose read-
ings per hour for up to 12 h (i.e. 36 readings within a 12 h period). The system has
been shown to be capable of measuring the electroosmotically extracted glucose with