Mechanisms of Foreign Body Response Mitigation by Nitric Oxide Release.

Implantable glucose biosensors provide real-time information about blood glucose fluctuations, but their utility and accuracy are time-limited due to the foreign body response (FBR) following their insertion beneath the skin. The slow release of nitric oxide (NO), a gasotransmitter with inflammation regulatory properties, from a sensor surface has been shown to dramatically improve sensors' analytical biocompatibility by reducing the overall FBR response. Indeed, work in a porcine model suggests that as long as the implants (sensors) continue to release NO, even at low levels, the inflammatory cell infiltration and resulting collagen density are lessened. While these studies strongly support the benefits of NO release in mitigating the FBR, the mechanisms through which exogenous NO acts on the surrounding tissue, especially under the condition of hyperglycemia, remain vague. Such knowledge would inform strategies to refine appropriate NO dosage and release kinetics for optimal therapeutic activity. In this study, we evaluated mediator, immune cell, and mRNA expression profiles in the local tissue microenvironment surrounding implanted sensors as a function of NO release, diabetes, and implantation duration. A custom porcine wound healing-centric multiplex gene array was developed for nanoString barcoding analysis. Tissues adjacent to sensors with sustained NO release abrogated the implant-induced acute and chronic FBR through modulation of the tissue-specific immune chemokine and cytokine microenvironment, resulting in decreased cellular recruitment, proliferation, and activation at both the acute (7-d) and chronic (14-d) phases of the FBR. Further, we found that sustained NO release abrogated the implant-induced acute and chronic foreign body response through modulation of mRNA encoding for key immunological signaling molecules and pathways, including STAT1 and multiple STAT1 targets including MAPK14, IRAK4, MMP2, and CXCL10. The condition of diabetes promoted a more robust FBR to the implants, which was also controlled by sustained NO release.

Combination of Nitric Oxide Release and Surface Texture for Mitigating the Foreign Body Response.

The tissue response to polyurethane (PU)-coated implants employing active and/or passive FBR mitigation techniques was evaluated over a 28 day study in a diabetic swine model. Active FBR mitigation was achieved through the sustained release of nitric oxide (NO) from a mesoporous silica nanoparticle-doped PU coating. Passive FBR mitigation was achieved through the application of a foam- or fiber-based topcoat. These topcoats were designed to possess topographical features known to promote tissue integration with foam-coated implants having pore sizes of approximately 50 µm and fiber-coated implants consisting of fiber diameters of less than 1 µm. Nitric oxide-release profiles were minimally impacted by the presence of either topcoat. Inflammatory cell density and collagen density at the implant-tissue interface were assessed at 7, 14, 21, and 28 days following implantation. Nitric oxide-releasing implants had significantly lower inflammatory cell density and collagen density than non-NO-releasing controls. The presence of a topcoat did not significantly impact inflammatory cell density, though top-coated textured implants resulted in significantly lower collagen density, irrespective of NO release. Overall, coatings that combine NO release with surface texture demonstrated the greatest potential for tissue-based biomedical device applications.

Extended Nitric Oxide-Releasing Polyurethanes via S-Nitrosothiol-Modified Mesoporous Silica Nanoparticles.

S-Nitrosothiol (RSNO)-modified mesoporous silica nanoparticles (MSNs) were doped into polyurethane (PU) to achieve extended NO-releasing coatings. Parameters influencing the synthesis of RSNO-functionalized nitric oxide (NO)-releasing MSNs were evaluated to elucidate the impact of pore structure on NO release characteristics. The porous particles were characterized as having larger NO payloads and longer NO release durations than those of nonporous particles, a feature attributed to the recombination of the NO radical in confined intraporous microenvironments. NO release kinetics, particle leaching, and thermal stability of the RSNO-modified MSNs dispersed in PU were evaluated as a function of PU structure to determine the feasibility of preparing a range of NO-releasing polymers for biomedical device-coating applications. The NO release kinetics from the PUs proved to be highly extended (>30 d) and consistent over a range of PU properties. Furthermore, RSNO-modified MSN leaching was not observed from the PUs. The NO release payloads were also maintained for 4 days for polymers stored at 0 °C.

Nitric Oxide Therapy for Diabetic Wound Healing.

Nitric oxide (NO) represents a potential wound therapeutic agent due to its ability to regulate inflammation and eradicate bacterial infections. Two broad strategies exist to utilize NO for wound healing; liberating NO from endogenous reservoirs, and supplementing NO from exogenous sources. This progress report examines the efficacy of a variety of NO-based methods to improve wound outcomes, with particular attention given to diabetes-associated chronic wounds.

Long-Term Accurate Continuous Glucose Biosensors via Extended Nitric Oxide Release.

Analytical performance and tissue interactions of nitric oxide (NO)-releasing continuous glucose sensors were evaluated over a 28 d study in a diabetic swine model. Interstitial glucose was detected using an implanted needle-type amperometric glucose sensor. Two NO-release durations from the sensor surface were achieved by doping the membranes with nonporous (14 d release) or porous (30 d release) S-nitrosothiol-functionalized silica nanoparticles. Numerical and clinical accuracy of the sensors were assessed at time points (1, 7, 14, 21, and 28 d) following implantation. Nitric oxide-releasing sensors demonstrated accurate glucose detection over a time period directly correlated with the active release of NO. Silica particle-doped sensors that released NO for 30 d showed standard-compliant accuracy (i.e., mean absolute relative difference ≤ 15%) for >3 weeks post-implantation. Histological staining for inflammatory biomarkers suggested that the observed performance improvement was the result of decreased inflammatory cell count and a lower density collagen capsule.

Design Considerations for Silica-Particle-Doped Nitric-Oxide-Releasing Polyurethane Glucose Biosensor Membranes.

Nitric oxide (NO)-releasing polymers have proven useful for improving the biocompatibility of in vivo glucose biosensors. Unfortunately, leaching of the NO donor from the polymer matrix remains a critical design flaw of NO-releasing membranes. Herein, a toolbox of NO-releasing silica nanoparticles (SNPs) was utilized to systematically evaluate SNP leaching from a diverse selection of biomedical-grade polyurethane sensor membranes. Glucose sensor analytical performance and NO-release kinetics from the sensor membranes were also evaluated as a function of particle and polyurethane (PU) chemistries. Particles modified with N-diazeniumdiolate NO donors were prone to leaching from PU membranes due to the zwitterionic nature of the NO donor modification. Leaching was minimized (<5% of the entrapped silica over 1 month) in low water uptake PUs. However, SNP modification with neutral S-nitrosothiol (RSNO) NO donors lead to biphasic leaching behavior. Particles with low alkanethiol content (<3.0 wt % sulfur) leached excessively from a hydrogel PU formulation (HP-93A-100 PU), while particles with greater degrees of thiol modification did not leach from any of the PUs tested. A functional glucose sensor was developed using an optimized HP-93A-100 PU membrane doped with RSNO-modified SNPs as the outer, glucose diffusion-limiting layer. The realized sensor design responded linearly to physiological concentrations of glucose (minimum 1-21 mM) over 2 weeks incubation in PBS and released NO at >0.8 pmol cm-2 s-1 for up to 6 days with no detectable (<0.6%) particle leaching.