22-11-2012, 04:14 PM
Continuous Blood Glucose Monitoring Using Wireless Implantable Microsystem
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Abstract—
A remotely powered implantable microsystem for
continuous blood glucose monitoring is presented. The
microsystem consists of a microfabricated glucose biosensor
flip-chip bonded to a transponder chip. The transponder chip
is inductively powered by an external reader with a 13.56-
MHz carrier. It then measures the output signal of the glucose
biosensor and transmits the measured data back to the
external reader using load-shift keying (LSK ). The
microsystem has a volume of 32 mmı. The procedures for the
microfabrication of the glucose sensor and the assembly of the
microsystem are described along with the description of the
circuit blocks of the transponder chip. The transponder chip
has been fabricated with the TSMC 0.18- m CMOS process
and has a total area of 1.3 1.3 mm_. The chip can measure the
sensor output current ranging from 1 nA to 1 A with less than
0.3% nonlinearity error, provided that the amplitude of the
received RF signal is higher than 2.6 V; the circuit consumes
a total current of about 110 A.
MONITORING SYSTEM OVERVIEW
a simplified block diagram of the monitoring
system consisting of an implantable microsystem and an
external reader. The external reader generates a highfrequency
magnetic field in an external coil (primary coil),
through which power is inductively transferred to an internal
coil (secondary coil) which is connected to the implantable
microsystem. The microsystem measures the blood glucose
concentration and transmits the measured data to the external
reader by using load-shift keying (backscatter modulation).
The required data-transmission rate for continuous blood
glucose monitoring is very low. One measurement with 5%
accuracy [11] every 10 min [20] can adequately describe the
dynamics of blood glucose variation. As a result, our proposed
strategy is to turn on the external reader every 10 min through
which the microsystem is powered. The microsystem is kept
on for about 2 min in order for the glucose sensor to settle to
its final value. The glucose sensor used in our microsystem is
an amperometric electrochemical biosensor which generates a
current proportional to the glucose concentration. It consists of
three electrodes: 1) a working electrode (WE); 2) a reference
electrode (RE); and a counter electrode (CE).
TRANSPONDER CHIP
The transponder chip was fabricated by using TSMC’s
0.18Im- CMOS process. All of the circuit blocks in the RF
front end, excluding the band gap bias circuit, were designed
by using 3.3-V transistors which have 7-nm-thick gate oxides.
The DAQ block was designed with 1.8-V transistors which
have 4-nm-thick gate oxide. The chip occupies an area of 1.3x
1.3 mm . The chip area is limited by the required number of
pads for test purposes. The main core circuit occupies an area
of about 0.2mm. The unused active area was filled with
capacitors required for the analog and digital supplies and also
for a resonance capacitor , resonating with the internal coil
at 13.56 MHz
DISCUSSION
When a device is to be implanted in the body, some
considerations should be taken into account, such as
biocompatibility ofthe device, implantation site, the effects of
the body’s immunesystem on the device, and packaging of the
device. The packaging of our microsystem is explained in
Section IV. In this section, we briefly discuss the other
issues.We note that our sensorhas not yet been used for in-vivo
tests. We base our discussionon prior published implantation
work from other researchers.
Our sensor is biocompatible since it is very similar to the
conventional enzyme-based glucose biosensors which have
provento be biocompatible. The only new material in the
sensor isiridium oxide which is also biocompatible .
The implantation site is targeted to be just underneath
theskin in the subcutaneous tissue where the implant will be
incontact with interstitial fluid whose glucose content has
goodcorrelation with the blood glucose concentration [6].When
an electrochemical glucose sensor is implanted in thebody, the
body tries to reject the sensor as it does to other implants. Thus,
during the first few days after implantation, thesurrounding
tissue continuously changes and causes the outputsignal of the
sensor to drop or drift considerably. Therefore,
themeasurements are not very reliable during the first few days
following implantation.