Effect of subcutaneous glucose sensor implantation on skin mRNA expression in pigs

Activation of polymorphonuclear leukocytes by candidate biomaterials for an implantable glucose sensor

BACKGROUND

Continuous monitoring of glucose by implantable microfabricated devices offers key advantages over current transcutaneous glucose sensors that limit usability due to their obtrusive nature and risk of infection. A successful sensory implant should be biocompatible and retain long-lasting function. Polymorphonuclear leukocytes (PMN) play a key role in the inflammatory system by releasing enzymes, cytokines, and reactive oxygen species, typically as a response to complement activation. The aim of this study was to perform an in vitro analysis of PMN activation as a marker for biocompatibility of materials and to evaluate the role of complement in the activation of PMN.

METHODS

Fifteen candidate materials of an implantable glucose sensor were incubated in lepirudin-anticoagulated whole blood. The cluster of differentiation molecule 11b (CD11b) expression on PMN was analyzed with flow cytometry and the myeloperoxidase (MPO) concentration in plasma was analyzed with enzyme-linked immunosorbent assay. Complement activation was prevented by the C3 inhibitor compstatin or the C5 inhibitor eculizumab.

RESULTS

Three of the biomaterials (cellulose ester, polyamide reverse osmosis membrane, and polyamide thin film membrane), all belonging to the membrane group, induced a substantial and significant increase in CD11b expression and MPO release. The changes were virtually identical for these two markers. Inhibition of complement with compstatin or eculizumab reduced the CD11b expression and MPO release dose dependently and in most cases back to baseline. The other 12 materials did not induce significant PMN activation.

CONCLUSION

Three of the 15 candidate materials triggered PMN activation in a complement-dependent manner and should therefore be avoided for implementation in implantable microsensors.

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

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.

Biomechanics of the sensor-tissue interface-effects of motion, pressure, and design on sensor performance and the foreign body response-part I: theoretical framework

The importance of biomechanics in glucose sensor function has been largely overlooked. This article is the first part of a two-part review in which we look beyond commonly recognized chemical biocompatibility to explore the biomechanics of the sensor-tissue interface as an important aspect of continuous glucose sensor biocompatibility. Part I provides a theoretical framework to describe how biomechanical factors such as motion and pressure (typically micromotion and micropressure) give rise to interfacial stresses, which affect tissue physiology around a sensor and, in turn, impact sensor performance. Three main contributors to sensor motion and pressure are explored: applied forces, sensor design, and subject/patient considerations. We describe how acute forces can temporarily impact sensor signal and how chronic forces can alter the foreign body response and inflammation around an implanted sensor, and thus impact sensor performance. The importance of sensor design (e.g., size, shape, modulus, texture) and specific implant location on the tissue response are also explored. In Part II: Examples and Application (a sister publication), examples from the literature are reviewed, and the application of biomechanical concepts to sensor design are described. We believe that adding biomechanical strategies to the arsenal of material compositions, surface modifications, drug elution, and other chemical strategies will lead to improvements in sensor biocompatibility and performance.

Effect of the sensor site-insulin injection site distance on the dynamics of local glycemia in the minipig model

BACKGROUND

Previous studies have shown that at the near steady state attained by slow insulin infusion, the local glycemia of subcutaneous fluid tracks the venous glucose concentration (i.e., it is not perturbed by the infusion of insulin). Here we test whether the subcutaneous glycemia near the site of injection of a bolus of insulin is perturbed by the injection in the minipig model without diabetes.

METHODS

A bolus of short-acting diluted insulin was administered in the animal's flank while three to five continuous glucose monitoring systems measured the subcutaneous glucose concentrations at 0.5, 1, 2, and 3 cm ("near sensors") and at 10-15 cm ("far sensors") from the injection site.

RESULTS

We found no statistically significant (P < 0.05) evidence that near and far sensors differ in response time, that is, the elapsed time to onset of signal drop or the elapsed time to minimum signal following insulin injection. We found mixed evidence that near and far sensors differ in the percentage drops at the glycemic minimum. The near versus far difference for near sensors at 0.5 and 3 cm from the injection site was statistically significant (P < 0.05): the average percentage drops for these near sensors were 3% and 11%, respectively, below those for far sensors. We did not find evidence of a difference for near sensors at 1 and 2 cm.

CONCLUSIONS

Because there is some evidence that insulin injection can cause a minor perturbation (about ≤10%) in the local glycemia, caution is warranted when co-locating glucose sensing and insulin injection sites.