Page 121 - Glucose Monitoring Devices

122 CHAPTER 6 CGM sensor technology

functions have been developed that convert raw measurements into glucose values.
A sequence of the glucose values can then be utilized for the calculation of glucose
trends. These compensation techniques also look to overcome any differences in
glucose concentrations that may exist between blood glucose levels and interstitial
glucose levels.
The time delay between a blood glucose reading and the value displayed by a
continuous glucose monitor consists of the sum of the time lag between ISF and
plasma glucose, in addition to the inherent electrical/chemical sensor delay due to
the reaction process and any front-end signal-processing delays required to produce
smooth traces [22]. Lag time can be caused by both physiologic and technologic
issues. Physiologic lag results from the difference in glucose concentration in blood
and ISF, which can increase or decrease before the blood glucose changes. Other
physiologic interference can come from inside the body, such as blood clots or other
biofouling, or be introduced from outside the body, such as medications or dietary
factors. Technologic lags result from the time required for the sensor to analyze the
sample and from the necessity of applying signal processing algorithms to filter noise
or to average a series of readings to create a weighted glucose level over time [9].
Glucose sensor calibration methodologies are utilized to ensure that the system
maintains its performance for the full duration of use. These methodologies range
from systems that are in place to characterize individual sensors or sensor lots during
manufacturing as well as systems that utilize a calibration update after insertion.
Systems look to build in the transducer’s fundamental glucose response characteris-
tics [3,40] as well as in vivo distortion and noise compensation [14]. These advances
in calibration and modeling have enabled system systems to have factory calibration
[19]and accuracy that enables diabetes treatment decisions based on the CGM
measurements [24].

Skin interface for the wearable transmitter
Continuous skin contact is critical to the successful use of a CGM system, so man-
ufacturers must ensure that their products stay in place to avoid damage to the sensor
or disruption of data collection. CGM manufacturers continue to face the challenge
of finding an adhesive to use that will be strong enough to keep the transmitter
attached to the skin during normal wear while also not irritating the wearer’s skin.
This can be particularly difficult in young patients whose smaller body surface
area and active lifestyles can present a unique set of issues to overcome [13].
Dexcom describes their adhesive as “a pressure-sensitive acrylic adhesive coated
on top of a polyester spunlace fabric” [41], while Senseonics uses a silicon-based
adhesive. Manufacturers also provide information to help with proper wearing of
a device, such as Medtronic’s guide, “Tape Tips and Site Management.” [28].
Some commonly used adhesives, such as acrylates, colophony, and Mastisol,
have been found to cause allergy symptoms in wearers [36]. In a study of CGM
users, Jadviscokova et al. found that minor local adverse effects, including