Page 429 - Handbook of Biomechatronics

424 Graham Brooker

converted, a current is generated proportional to the glucose concentration
that can be sensed by applying a low voltage. An example of a microneedle
array is shown in Fig. 9 (Invernale et al., 2014).
An alternative mechanism SCGM uses microdialysis that avoids direct
contact of the interstitial fluid and the sensor. This method provides superior
precision and accuracy and lower signal drift than needle-based sensors and is
used by GlucoDay from Menarini, and SCGM from Roche. This is a closed
fluid containing system as can be seen in Fig. 10. The microdialysis probe
consists of a tubular dialysis membrane with a 20,000 molecular weight cut-
off glued to the end of a double lumen catheter. The outer in-going lumen
of the catheter is continuously perfused with Ringer’s solution at a flow rate
of 0.3mL/min by a miniature peristaltic pump. The dialysate leaves the
probe through the inner out-going lumen. The peristaltic pump operates
as both a pumping and suction unit to generate identical flow rates into
and out of the catheter. The dialysate is mixed with the GOx containing
reagent solution via the Y mixer inside the cassette. The GOx then catalyzes
the oxidation of the glucose in the dialysate to gluconolactone and H 2 O 2 in
the subsequent tube segment. To prevent an incomplete reaction caused by
an oxygen deficit when the glucose concentration is high, this segment is

Fig. 9 Image of the microneedle array. (Based on Invernale, M., Tang, B., York, R., Le, L.,
Hou, D., 2014. Microneedle electrodes toward an amperometric glucose-sensing smart
patch. Adv. Healthc. Mater. 3: 338–342.)