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NO-Generating Coats for Subcutaneous Glucose Sensors

These implantable sensors can be used for monitoring diabetes patients.

In a research program now in progress, polymeric materials that catalyze the generation of nitric oxide (NO) at low concentrations are being developed and tested for utility as coatings on surgically implantable amperometric glucose sensors. Heretofore, efforts to develop subcutaneous sensors for real-time clinical monitoring of glucose concentrations in diabetic patients have been stymied by inflammatory/foreign-body responses to implantation of sensors. The present research program follows from an observation, made in a related prior research program, that local generation of NO at low concentrations enhances the biocompatibility of implanted sensors by reducing inflammatory responses.

The Inflammatory Response of Rats to implantation of sham devices was quantified as the normalized value of the fraction of implanted-device area coated with inflammatory marker cells at 7 days after implantation. The values in the histogram are normalized to the inflammatory response of the devices coated with NO-generating material.

The focus of this research program is, more specifically, to develop new biomedical hydrogel and polyurethane coating materials containing immobilized copper(II) ions or other chemical species (e.g., organoselenium) that act as catalysts for in situ conversion of endogenous nitrosothiol compounds (e.g., nitrosoglutathione and nitrosocysteine) to NO, thereby enabling sustained local generation of NO at the surfaces of the implanted sensors. One of the accomplishments thus far has been the synthesis and characterization of a new material that includes a Cu(II)- cyclen compound covalently attached to a cross-linked poly (2-hydroxy- ethyl methacrylate). This material has been found to generate NO from endogenous nitrosothiols in solution at physiological pH. In preliminary in vivo studies using a lipophilic Cu(II)- cyclen compound blended into a biomedical-grade polyurethane coating material on sham subcutaneous devices implanted in rats, an apparent reduction in inflammatory response was demonstrated (see figure). However, proof that NO is the agent causing the observed improvement has yet to be obtained.

Needle-type subcutaneous glucose sensors that are to be coated with various NO-generating materials are undergoing development. At the time of reporting the information for this article, it was planned to use these sensors in experiments to determine (1) whether the NO-generating chemistry is compatible with the glucose-sensing chemistry and (2) whether the sensing function is compromised by coating with the NO-generating materials.

This work was done by Mark. E. Meyerhoff of the University of Michigan for the Army Research Laboratory.

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NO-Generating Coats for Subcutaneous Glucose Sensors

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    Defense Tech Briefs Magazine

    This article first appeared in the June, 2008 issue of Defense Tech Briefs Magazine (Vol. 2 No. 3).

    Read more articles from the archives here.

    Overview

    The document is an annual report detailing research on "Nitric Oxide Generating Polymeric Coatings for Subcutaneous Glucose Sensors," led by Dr. Mark E. Meyerhoff at the University of Michigan. The research aims to address the challenges faced in developing implantable glucose sensors for real-time monitoring in diabetic patients, particularly issues related to biocompatibility and inflammatory responses caused by sensor implantation.

    The report highlights that previous efforts to create reliable implantable chemical sensors have been hindered by the inflammatory response of surrounding tissues, which can lead to unreliable analytical results. The primary goal of this research program is to optimize the chemistries involved in fabricating amperometric glucose sensors that feature outer polymeric coatings capable of generating low levels of nitric oxide (NO). The local generation of NO is expected to enhance the biocompatibility of these sensors by reducing inflammation and promoting angiogenesis and wound healing at the implantation site.

    Key findings from the research include the successful demonstration that NO-generating polymer coatings, based on a lipophilic Cu(II)-cyclen complex embedded in medical-grade polyurethane, can modestly reduce the inflammatory response in tissue surrounding implanted devices in rat models over a seven-day period. Additionally, the report discusses the fabrication of enzyme-based needle-type glucose sensors that exhibit a linear response to glucose concentrations ranging from 0 to 30 mM.

    The report also outlines the development of new polymeric coatings, including biomedical hydrogels and polyurethanes, which contain immobilized copper ion sites. These sites serve as catalytic surfaces for the in situ conversion of endogenous nitrosothiol species (RSNO) to NO, providing a sustained local generation of NO at the sensor surface. Preliminary experiments indicate that RSNO levels in the subcutaneous fluid of rats are sufficient to generate local NO, thereby reducing the inflammatory response at the implantation site.

    The report concludes with a summary of conference presentations and reportable outcomes, emphasizing the potential of NO-generating materials in creating thromboresistive polymers and improving the performance of glucose sensors. Overall, the research represents a significant step toward enhancing the biocompatibility and functionality of implantable glucose monitoring devices for diabetic patients.

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