Biomechanics of the sensor-tissue interface-effects of motion, pressure, and design on sensor performance and foreign body response-part II: examples and application

Abstract

This article is the second part of a two-part review in which we explore the biomechanics of the sensor-tissue interface as an important aspect of continuous glucose sensor biocompatibility. Part I, featured in this issue of Journal of Diabetes Science and Technology, describes a theoretical framework of how biomechanical factors such as motion and pressure (typically micromotion and micropressure) affect tissue physiology around a sensor and in turn, impact sensor performance. Here in Part II, a literature review is presented that summarizes examples of motion or pressure affecting sensor performance. Data are presented that show how both acute and chronic forces can impact continuous glucose monitor signals. Also presented are potential strategies for countering the ill effects of motion and pressure on glucose sensors. Improved engineering and optimized chemical biocompatibility have advanced sensor design and function, but we believe that mechanical biocompatibility, a rarely considered factor, must also be optimized in order to achieve an accurate, long-term, implantable sensor.

Figure 1

Pressure from sleeping on a sensor causes an anomalous dip in signal. PerQ sensors (FreeStyle Navigator) were inserted into the pig's flank using the sensor delivery unit provided with the sensor. When the subject slept on the sensor, the signal dropped markedly (first arrow) and no longer tracked the plasma glucose levels. When the subject rolled off the sensor, the sensor signal recovers (second arrow). Data provided by Dr. Ed Damiano, Boston University. Details are found in a conference abstract. BG = blood glucose.

Figure 2

Errant human CGM data due to compression during sleep. Human CGM and plasma glucose data from a DexCom SEVEN sensor (worn on the abdomen) collected as part of a closed-loop trial performed at the Clinical Research Center at the University of Virginia. Blue arrows denote incidences where patient rolled onto her side and compressed the sensor. When urged by a nurse to roll off the sensor, sensor compression decreased and sensor signal increased. Data provided by Dr. Kovatchev and Dr. Breton. Details of trial are found in conference abstracts.^,

Figure 3

Motion affects sensor performance during a glucose-tracking study in a human subject conducted 103 days postimplantation. The continuous, fully implanted SubQ sensor (-•-) closely tracks blood glucose reference data during a glucose bolus until a significant drop in sensor signal is noted when the patient stands (circled in red). The deviation in sensor signal was attributed to postural effects (i.e., compression of the blood supply in the tissue). However, the change in patient posture could have also caused motion of the sensor relative to the surrounding tissue, triggering changes in tissue–sensor contact and/or mixing of localized glucose concentration in capsule exudate. Figure adapted from Gilligan and colleagues.

Figure 4

Effects of sensor design on continuous monitoring of interstitial glucose. PerQ (A) bare and (B) porous-coated sensors (Medtronic MiniMed) were implanted into the dorsum of rats and data continuously monitored for up to 21 days. Porous-coated sensors initially experienced a more apparent and rapid signal reduction compared with bare sensors (slope of blue arrows). However, unlike bare sensors, sensors with porous coatings exhibited less signal fluctuation (noise between red bars) over time. Figure adapted from Koschwanez and colleagues.