Foreign body response to subcutaneous biomaterial implants in a mast cell-deficient Kitw-Sh murine model

Mast cells (MCs)_are recognized for their functional role in wound-healing and allergic and inflammatory responses – host responses that are frequently detrimental to implanted biomaterials if extended beyond acute reactivity. These tissue reactions impact especially on the performance of sensing implants such as continuous glucose monitoring (CGM) devices. Our hypothesis that effective blockade of MC activity around implants could alter the host foreign body response (FBR) and enhance the in vivo lifetime of these implantable devices motivated this study. Stem cell factor and its ligand c-KIT receptor are critically important for MC survival, differentiation and degranulation. Therefore, an MC-deficient sash mouse model was used to assess MC relationships to the in vivo performance of CGM implants. Additionally, local delivery of a tyrosine kinase inhibitor (TKI) that inhibits c-KIT activity was also used to evaluate the role of MCs in modulating the FBR. Model sensor implants comprising polyester fibers coated with a rapidly dissolving polymer coating containing drug-releasing degradable microspheres were implanted subcutaneously in sash mice for various time points, and the FBR was evaluated for chronic inflammation and fibrous capsule formation around the implants. No significant differences were observed in the foreign body capsule formation between control and drug-releasing implant groups in MC-deficient mice. However, fibrous encapsulation was significantly greater around the drug-releasing implants in sash mice compared to drug-releasing implants in wild-type (e.g. MC-competent) mice. These results provide insights into the role of MCs in the FBR, suggesting that MC deficiency provides alternative pathways for host inflammatory responses to implanted biomaterials.     

Reduced foreign body reaction to implanted biomaterials by surface treatment with oriented osteopontin

The foreign body reaction (FBR), which leads to the encapsulation of implanted biomaterials, has been implicated in the failure of many medical devices. The protein layer that is nonspecifically adsorbed onto the implant surface immediately after implantation is thought to dictate this reaction. It is hypothesized that biomaterial surfaces having specific proteins with precisely controlled orientations will decrease the FBR. Previously, we have reported that osteopontin (OPN) adsorbed on positively charged surfaces has a preferable orientation for in vitro cell adhesion and spreading as compared to negatively charged surfaces. It is expected that coating a layer of OPN in its preferred orientation on an implant surface will decrease the FBR. In this work, in vivo studies were performed to test this hypothesis. A positively charged polymer (p(HEMA-co-AEMA)) and a negatively charged polymer (p(HEMA-co-CEA)) coated with OPN were implanted subcutaneously in wild-type mice for 7 or 28 days. Uncoated polymers were used as control. For the 7-day implants, cells on OPN-coated p(HEMA-co-AEMA) spread more than cells on the other three materials. Following 28 days of implantation the implants were explanted and the capsule thickness and vascularity around the implants were characterized. Additionally, the macrophage and foreign body giant cells (FBGCs) around the implants were quantified. It was found in this study that the modification of the positively charged polymer surface with OPN in a controlled orientation led to a reduction in the foreign body reaction as determined by capsule thickness. Our finding provides valuable information for designing better biocompatible biomaterials with improved in vivo performance.     

The effect of nitric oxide surface flux on the foreign body response to subcutaneous implants

Although the release of nitric oxide (NO) from biomaterials has been shown to reduce the foreign body response (FBR), the optimal NO release kinetics and doses remain unknown. Herein, polyurethane-coated wire substrates with varying NO release properties were implanted into porcine subcutaneous tissue for 3, 7, 21 and 42 d. Histological analysis revealed that materials with short NO release durations (i.e., 24 h) were insufficient to reduce the collagen capsule thickness at 3 and 6 weeks, whereas implants with longer release durations (i.e., 3 and 14 d) and greater NO payloads significantly reduced the collagen encapsulation at both 3 and 6 weeks. The acute inflammatory response was mitigated most notably by systems with the longest duration and greatest dose of NO release, supporting the notion that these properties are most critical in circumventing the FBR for subcutaneous biomedical applications (e.g., glucose sensors).     

Porous Implants Modulate Healing and Induce Shifts in Local Macrophage Polarization in the Foreign Body Reaction

The foreign body reaction (FBR) to implanted materials is of critical importance when medical devices require biological integration and vascularization to support their proper function (e.g., transcutaneous devices, implanted drug delivery systems, tissue replacements, and sensors). One class of materials that improves FBR outcomes is made by sphere-templating, resulting in porous structures with uniform, interconnected 34 μm pores. With these materials we observe reduced fibrosis and increased vascularization. We hypothesized that improved healing is a result of a shift in macrophage polarization, often measured as the ratio of M1 pro-inflammatory cells to M2 pro-healing cells. In this study, macrophage polarity of 34 μm porous implants was compared to non-porous and 160 μm porous implants in subcutaneous mouse tissue. Immunohistochemistry revealed that macrophages in implant pores displayed a shift towards an M1 phenotype compared to externalized cells. Macrophages in 34 μm porous implants had up to 63% greater expression of M1 markers and up to 85% reduction in M2 marker expression (p < 0.05). Macrophages immediately outside the porous structure, in contrast, showed a significant enrichment in M2 phenotypic cells. This study supports a role for macrophage polarization in driving the FBR to implanted materials.     

Permeability of subcutaneous tissues surrounding long-term implants to oxygen

Certain types of implanted medical devices depend on oxygen supplied from surrounding tissues for their function. However, there is a concern that the tissue associated with the foreign body response to implants may become impermeable to oxygen over the long term and render the implant nonfunctional. We report oxygen flux recordings from electrochemical oxygen sensor devices with wireless telemetry implanted in subcutaneous porcine tissues. The devices remained implanted for up to 13 weeks and were removed with adjacent tissues at specified times for histologic examination. There are four main observations: (1) In the first few weeks after implantation, the oxygen flux to the sensors, or current density, declined to a sustained mean value, having unsynchronized cyclic variations around the mean; (2) The oxygen mass transfer resistance of the sensor membrane was negligible compared to that of the tissue, allowing for a sensitive estimate of the tissue permeability; (3) The effective diffusion coefficient of oxygen in tissues was found to be approximately one order of magnitude lower than in water; and (4) Quantitative histologic analysis of the tissues showed a mild foreign body response to the PDMS sensor membrane material, with capillaries positioned close to the implant surface. Continuous recordings of oxygen flux indicate that the tissue permeability changes predictably with time, and suggest that oxygen delivery can be sustained over the long term.     

Immunomodulation by mesenchymal stem cells combats the foreign body response to cell-laden synthetic hydrogels

The implantation of non-biological materials, including scaffolds for tissue engineering, ubiquitously leads to a foreign body response (FBR). We recently reported that this response negatively impacts fibroblasts encapsulated within a synthetic hydrogel and in turn leads to a more severe FBR, suggesting a cross-talk between encapsulated cells and inflammatory cells. Given the promise of mesenchymal stem cells (MSCs) in tissue engineering and recent evidence of their immunomodulatory properties, we hypothesized that MSCs encapsulated within poly(ethylene glycol) (PEG) hydrogels will attenuate the FBR. In vitro, murine MSCs encapsulated within PEG hydrogels attenuated classically activated primary murine macrophages by reducing gene expression and protein secretion of pro-inflammatory cytokines, most notably tumor necrosis factor-α. Using a COX2 inhibitor, prostaglandin E2 (PGE2) was identified as a mediator of MSC immunomodulation of macrophages. In vivo, hydrogels laden with MSCs, osteogenically differentiating MSCs, or no cells were implanted subcutaneously into C57BL/6 mice for 28 days to assess the impact of MSCs on the fibrotic response of the FBR. The presence of encapsulated MSCs reduced fibrous capsule thickness compared to acellular hydrogels, but this effect diminished with osteogenic differentiation. The use of MSCs prior to differentiation in tissue engineering may therefore serve as a dynamic approach, through continuous cross-talk between MSCs and the inflammatory cells, to modulate macrophage activation and attenuate the FBR to implanted synthetic scaffolds thus improving the long-term tissue engineering outcome.     

In vivo biocompatibility of collagen-poly(hydroxyethyl methacrylate) hydrogels

Collagen-p(HEMA) hydrogels were subcutaneously implanted in rats for up to 6 month and the immediate short- and long-term tissue response to these implants was studied. Histopathological data indicated that the tissue reaction at the implant site progressed from an initial acute inflammatory response characterized by the presence of eosinophils and polymorphs to a chronic response marked by few macrophages, foreign body giant cells and fibroblasts. After one month a very thin fibrous capsule (approximately 11 microns thick) was observed around the implant. Even 6 month post-implantation, the capsule thickness was maintained at about 11-12 microns. No necrosis, calcification, tumorigenesis or infection was observed at the implant site up to 6 month. Fibrous capsule analysis showed that the collagen content and the capsule thickness were well within the threshold limits. The collagen-p(HEMA) hydrogels were found to be well-tolerated, non-toxic and highly biocompatible.     

Optical imaging of fibrin deposition to elucidate participation of mast cells in foreign body responses

Mast cell activation has been shown to be an initiator and a key determinant of foreign body reactions. However, there is no non-invasive method that can quantify the degree of implant-associated mast cell activation. Taking advantage of the fact that fibrin deposition is a hallmark of mast cell activation around biomaterial implants, a near infrared probe was fabricated to have high affinity to fibrin. Subsequent in vitro testing confirmed that this probe has high affinity to fibrin. Using a subcutaneous particle implantation model, we found significant accumulation of fibrin-affinity probes at the implant sites as early as 15 min following particle implantation. The accumulation of fibrin-affinity probes at the implantation sites could also be substantially reduced if anti-coagulant – heparin was administered at the implant sites. Further studies have shown that subcutaneous administration of mast cell activator – compound 48/80 – prompted the accumulation of fibrin-affinity probes. However, implant-associated fibrin-affinity probe accumulation was substantially reduced in mice with mast cell deficiency. The results show that our fibrin-affinity probes may serve as a powerful tool to monitor and measure the extent of biomaterial-mediated fibrin deposition and mast cell activation in vivo.     

植入式葡萄糖传感器膜材料壳聚糖与Nafion的组织相容性比较

背景：壳聚糖是天然高分子多糖,可单独或者与其他材料复合制作敷料、药物、基因载体、生物涂层、组织工程支架、传感器膜材料等。 目的：了解壳聚糖作为植入式葡萄糖传感器膜材料的组织相容性,并与Nafion膜进行对比。 方法：制备壳聚糖膜并对其理化性质进行表征,比较壳聚糖膜皮下植入与肌肉植入、Nafion膜肌肉植入的生物相容性。 结果与结论：壳聚糖膜的厚度、溶胀率、表观密度等理化参数可以通过浓度、铸膜液体积来控制;壳聚糖膜能生物降解,63 d皮下植入的降解率为(17.0±9.9)%,说明壳聚糖的体内降解速度较慢。壳聚糖膜皮下植入引起的炎症反应较肌肉植入重,63 d后形成的纤维包膜比肌肉植入要厚(P < 0.05);肌肉植入Nafion与壳聚糖膜引起材料周围纤维包膜厚度差异无显著性意义(P > 0.05),两者均在15 d以后趋于稳定。证明壳聚糖膜能生物降解,与Nafion膜均有较好的组织相容性。     

Linking the foreign body response and protein adsorption to PEG-based hydrogels using proteomics

Poly(ethylene glycol) (PEG) hydrogels with their highly tunable properties are promising implantable materials, but as with all non-biological materials, they elicit a foreign body response (FBR). Recent studies, however, have shown that incorporating the oligopeptide RGD into PEG hydrogels reduces the FBR. To better understand the mechanisms involved and the role of RGD in mediating the FBR, PEG, PEG-RGD and PEG-RDG hydrogels were investigated. After a 28-day subcutaneous implantation in mice, a thinner and less dense fibrous capsule formed around PEG-RGD hydrogels, while PEG and PEG-RDG hydrogels exhibited stronger, but similar FBRs. Protein adsorption to the hydrogels, which is considered the first step in the FBR, was also characterized. In vitro experiments confirmed that serum proteins adsorbed to PEG-based hydrogels and were necessary to promote macrophage adhesion to PEG and PEG-RDG, but not PEG-RGD hydrogels. Proteins adsorbed to the hydrogels in vivo were identified using liquid chromatography-tandem mass spectrometry. The majority (245) of the total proteins (≥300) that were identified was present on all hydrogels with many proteins being associated with wounding and acute inflammation. These findings suggest that the FBR to PEG hydrogels may be mediated by the presence of inflammatory-related proteins adsorbed to the surface, but that macrophages appear to sense the underlying chemistry, which for RGD improves the FBR.     

Macroporous condensed poly(tetrafluoroethylene). I. In vivo inflammatory response and healing characteristics

This study was designed to determine whether the novel spatial geometry of macroporous condensed poly(tetrafluoroethylene) (cPTFE) favorably affects the in vivo repair process. Specifically, the macroporous surface geometry and the reduced material thickness contribute to better healing characteristics. For this purpose, three other materials used for abdominal wall repair were selected, expanded poly(tetrafluoroethylene) (ePTFE), low-weight poly(propylene) (lwPP), and high-weight poly(propylene) (hwPP). Samples of each material (1 x 2 cm, n = 4) were implanted subcutaneously in rats for 7, 28, or 56 days. After sacrificing the animals, at each time point, the tissue implant sites were subjected to morphometric analysis and evaluation of inflammatory and wound-healing tissue characteristics. Although the fibrous capsule thickness did not significantly vary among the four materials (p > 0.05), cPTFE consistently led to the most mature fibrous capsule. However, ePTFE showed the greatest tissue-material integration. Both PP materials presented various levels of tissue integration, but they were characterized by significant early inflammation and increased foreign body reaction around the mesh openings, especially for hwPP. In contrast, cPTFE did not induce extensive inflammation or elevated foreign body reaction around its mesh openings. We conclude that cPTFE combines the inherent PTFE biocompatibility with low polymer surface area (large mesh openings, reduced material thickness) leading to a better inflammatory and wound healing response compared to available materials used in abdominal wall reconstruction.     

Temporal and spatial distribution of macrophage phenotype markers in the foreign body response to glutaraldehyde-crosslinked gelatin hydrogels

Currently, it is not well understood how changes in biomaterial properties affect the foreign body response (FBR) or macrophage behavior. Because failed attempts at biomaterial degradation by macrophages have been linked to frustrated phagocytosis, a defining feature of the FBR, we hypothesized that increased hydrogel crosslinking density (and decreased degradability) would exacerbate the FBR. Gelatin hydrogels were crosslinked with glutaraldehyde (0.05, 0.1, and 0.3%) and implanted subcutaneously in C57BL/6 mice over the course of 3 weeks. Interestingly, changes in hydrogel crosslinking did not affect the thickness of the fibrous capsule surrounding the hydrogels, expression of the pan-macrophage marker F480, expression of three macrophage phenotype markers (iNOS, Arg1, CD163), or expression of the myofibroblast marker aSMA, determined using semi-quantitative immunohistochemical analysis. With respect to temporal changes, the level of expression of the M1 marker (iNOS) remained relatively constant throughout the study, while the M2 markers Arg1 and CD163 increased over time. Expression of these M2 markers was highly correlated with fibrous capsule thickness. Differences in spatial distribution of staining also were noted, with the strongest staining for iNOS at the hydrogel surface and increasing expression of the myofibroblast marker aSMA toward the outer edge of the fibrous capsule. These results confirm previous reports that macrophages in the FBR exhibit characteristics of both M1 and M2 phenotypes. Understanding the effects (or lack of effects) of biomaterial properties on the FBR and macrophage phenotype may aid in the rational design of biomaterials to integrate with surrounding tissue.     

Characterization of porous,dexamethasone-releasing polyurethane coatings for glucose sensors

Commercially available implantable needle-type glucose sensors for diabetes management are robust analytically but can be unreliable clinically primarily due to tissue–sensor interactions. Here, we present the physical, drug release and bioactivity characterization of tubular, porous dexamethasone (Dex)-releasing polyurethane coatings designed to attenuate local inflammation at the tissue–sensor interface. Porous polyurethane coatings were produced by the salt-leaching/gas-foaming method. Scanning electron microscopy and micro-computed tomography (micro-CT) showed controlled porosity and coating thickness. In vitro drug release from coatings monitored over 2 weeks presented an initial fast release followed by a slower release. Total release from coatings was highly dependent on initial drug loading amount. Functional in vitro testing of glucose sensors deployed with porous coatings against glucose standards demonstrated that highly porous coatings minimally affected signal strength and response rate. Bioactivity of the released drug was determined by monitoring Dex-mediated, dose-dependent apoptosis of human peripheral blood derived monocytes in culture. Acute animal studies were used to determine the appropriate Dex payload for the implanted porous coatings. Pilot short-term animal studies showed that Dex released from porous coatings implanted in rat subcutis attenuated the initial inflammatory response to sensor implantation. These results suggest that deploying sensors with the porous, Dex-releasing coatings is a promising strategy to improve glucose sensor performance.     

Reduced foreign body response at nitric oxide-releasing subcutaneous implants

The tissue response to nitric oxide (NO)-releasing subcutaneous implants is presented. Model implants were created by coating silicone elastomer with diazeniumdiolate-modified xerogel polymers capable of releasing NO. The host tissue response to such implants was evaluated at 1, 3, and 6 weeks and compared to that of uncoated silicone elastomer blanks and xerogel-coated controls incapable of releasing NO. Delivery of NO (approximately 1.35 micromol/cm2 of implant surface area) reduced foreign body collagen capsule ("scar tissue") thickness by >50% compared to uncoated silicone elastomer after 3 weeks. The chronic inflammatory response at the tissue/implant interface was also reduced by >30% at NO-releasing implants after 3 and 6 weeks. Additionally, CD-31 immunohistochemical staining revealed approximately 77% more blood vessels in proximity to NO-releasing implants after 1 week compared to controls. These findings suggest that conferring NO release to subcutaneous implants may promote effective device integration into healthy vascularized tissue, diminish foreign body capsule formation, and improve the performance of indwelling medical devices that require constant mass transport of analytes (e.g., implantable sensors).     

Study of the effects of tissue reactions on the function of implanted glucose sensors

The relationship between tissue reactions to a subcutaneously implanted glucose sensor and the function of the sensor was evaluated over a period of 4-weeks using tubular, porous polyvinyl alcohol (PVA) sponges implanted subcutaneously in rats. The PVA sponges were used as scaffolds in which the foreign body response could develop. Coil-type glucose sensors were then placed in the center of the PVA sponges and tested on day 3, and weekly thereafter. In the first approach, the sensors were placed in the sponges still implanted in the rats and tested. In vivo glucose sensor sensitivity peaked at day 7 and steadily decreased until day 35. In the second approach, the sensors were placed in the explanted sponges and then tested. This test showed no sensor function after day 7, indicating that functional blood vessels are critical in maintaining any function whatsoever. In both cases the sensors themselves were never implanted to eliminate any potential in vivo degradation of the sensors that could have affected the outcome of this study. Sensors were then tested in absence of sponges and found to be working properly with no change from preimplantation sensitivity. Once sensor testing was concluded, the PVA sponge/tissue samples were prepared for quantitative histological analysis. It was determined that the increase in collagen deposition within the sponge correlated with the decrease in sensor sensitivity. It was also observed that natural angiogenesis (peak at day 14) did not overcome the barrier to glucose diffusion created by the fibrous capsule.     

Sequential delivery of dexamethasone and VEGF to control local tissue response for carbon nanotube fluorescence based micro-capillary implantable sensors

In this study, we examined the in vivo pharmacological effects of the sequential delivery of dexamethasone (DX) followed by vascular endothelial growth factor (VEGF) on the immune response and localized vascular network formation around a hydrogel-coated, micro-capillary implant for single-walled carbon nanotube based fluorescence sensors. We demonstrate, for the first time, imaging of an SWNT fluorescence device implanted subcutaneously in a rat. For tissue response studies, the chick embryo chorioallantoic membrane (CAM) was used as a tissue-model for an 8-day implantation period. The average vascular density of the tissue surrounding a hydrogel-coated microdialysis capillary sensor with simultaneous, sequential, or no delivery of DX and VEGF was 1.24+/-0.35x10(-3)vessels/microm(2), 1.15+/-0.30x10(-3)vessels/microm(2) and 0.71+/-0.20x10(-3)vessels/microm(2), respectively. Calculation of the therapeutic index (vasculature/inflammation ratio), which reflects promotion of angiogenesis versus the host immune response, demonstrates that sequential DX/VEGF delivery was 60.3% and 139.3% higher than that of VEGF and DX release alone, respectively, and was also 32.1% higher when compared to simultaneous administration, proving to be a more effective strategy in utilizing the pharmacological impact of DX and VEGF around the biosensor-model implant.     

The effect of bone anchoring and micro-grooves on the soft tissue reaction to implants

Here we aimed to compare the tissue reaction to smooth and micro-grooved implants, at different implantation sites. We hypothesised that subperiosteally less mobility is to be expected between an implant and the surrounding tissue, which can lead to a more subdued tissue response. In addition, we hypothesised that a similar effect can be reached when substrata are equipped with micro-grooves. Poly-L-lactic acid smooth or micro-grooved surfaces (width 2 or 10 microm, depth 1 microm) were implanted subperiosteally on the frontal bone of the skull, or subcutaneously in the flanks of goats for 2, 4 and 12 weeks. After sacrifice, implants and surrounding tissue were histologically processed. Light microscopical and histomorphometrical evaluation of the histomorphometrical analyses, capsule thickness, capsule quality and implant-tissue interface was performed. In addition, we stained for alpha-smooth muscle actin. collagen and CD-68 expression. All implants were surrounded by a fibrous capsule. Capsules around subperiosteal implants were more matured than around subcutaneous implants. In time, capsule thickness significantly decreased around subperiosteal implants, but increased around subcutaneous implants. Also, nowhere differences were found in the presence of collagen or alpha-smooth muscle actin. The interfacial cells around all implants frequently showed staining for the monocyte-macrophage marker CD-68. We concluded that in this model, decreased mobility of an implant relative to the surrounding tissue did positively influence the peri-implant tissue response, but the applied surface topography did not.     

Localized Immunosuppressive Environment in the Foreign Body Response to Implanted Biomaterials

The implantation of synthetic biomaterials initiates the foreign body response (FBR), which is characterized by macrophage infiltration, foreign body giant cell formation, and fibrotic encapsulation of the implant. The FBR is orchestrated by a complex network of immune modulators, including diverse cell types, soluble mediators, and unique cell surface interactions. The specific tissue locations, expression patterns, and spatial distribution of these immune modulators around the site of implantation are not clear. This study describes a model for studying the FBR in vivo and specifically evaluates the spatial relationship of immune modulators. We modified a biomaterials implantation in vivo model that allowed for cross-sectional in situ analysis of the FBR. Immunohistochemical techniques were used to determine the localization of soluble mediators, ie, interleukin (IL)-4, IL-13, IL-10, IL-6, transforming growth factor-β, tumor necrosis factor-α, interferon-γ, and MCP-1; specific cell types, ie, macrophages, neutrophils, fibroblasts, and lymphocytes; and cell surface markers, ie, F4/80, CD11b, CD11c, and Ly-6C, at early, middle, and late stages of the FBR in subcutaneous implant sites. The cytokines IL-4, IL-13, IL-10, and transforming growth factor-β were localized to implant-adherent cells that included macrophages and foreign body giant cells. A better understanding of the FBR in vivo will allow the development of novel strategies to enhance biomaterial implant design to achieve better performance and safety of biomedical devices at the site of implant.It is estimated that at least 20 million people in the United States have a biomaterial device implant.¹ Implant device failure or implant-associated infections can have disastrous consequences for the implant device function and the host. Although the risk of implant-associated infections is small (1 to 7%), these infections are associated with considerable morbidity, expensive health care, and prolonged antibiotic therapy.² The medical and surgical cost of treating certain device failures or implant-associated infections can average up to $50,000 per patient.^3,4 These significant burdens and the increasing use of biomaterial implants in a myriad of medical applications warrants a clear understanding of the immune response to these materials.The foreign body response (FBR) also known as the host response to implanted biomaterials, involves a complex cascade of immune modulators, including various cell types, soluble mediators, and cellular interactions⁵ Although the FBR has proven detrimental to many implanted devices and dangerous to the patient in many cases, it has not been studied in depth. Two major problems that compromise the function of the implant are associated with the FBR; first is the fibrotic encapsulation of the device (eg, glucose sensor implant) and second is the promotion of enzymes and reactive intermediaries by activated cells in the FBR that over time are capable of degrading the biomaterials. At the present time, we do not have a clear understanding of the FBR and how to control it to avoid failure of the biomaterial implant devices. Thus, a major goal in both clinical medicine and the biomaterial industry is to obtain a better understanding of FBR response to enable further development of novel devices, materials, therapies, and treatments that can improve the safety and function of biomaterials in medicine.Previous studies have made it clear that the macrophage is the dominant cell in the FBR and both macrophages and multinucleated foreign body giant cells (FBGCs) are commonly observed in the FBR.^5,6 It is believed that macrophages adherent on the implant fuse through a complex series of events to form FBGCs.⁵ FBGCs have been characterized as cells expressing a wide variety of surface markers including CD14, F4/80, CD11b, and others, linking the origin of these cells to the fusion of monocyte-derived macrophages.⁵ The adherence of FBGCs on biomaterial surfaces is correlated to the release of enzymes (eg, esterases, lipases) and other bio-reactive intermediates that can degrade the biomaterial and cause a loss of function in the implant.^5,7Although several studies in vitro^8,9 and in vivo^{10,11,12,13,14,15,16,17,18} have implicated immunoregulatory cytokines [interleukin (IL)-4, IL-13, IL-10, and transforming growth factor (TGF-β)], inflammatory cytokines [IL-6 and tumor necrosis factor (TNF)-α], and chemokines (MCP-1) in the formation of the FBR, the spatial tissue distribution, localization, specific cell expression and proximity of these cytokines and chemokines around the implant is not clear. We believe this knowledge is essential to the development of new therapies aiming to treat the harmful effects of the FBR or in the development of new biomaterial devices that avoid the deleterious effects of the FBR.We have used a subcutaneous implantation model and in situ immunohistochemical techniques to assess the spatial relationship and expression patterns between FBR-associated cytokines, chemokines, and specific cell types known to be critical to the progression of the FBR. Our murine subcutaneous biomaterials implantation model reliably induces a characteristic FBR. Analyzing the tissue implantation site using immunohistochemical techniques at early (2 weeks), middle (4 weeks), and late (10 weeks) time points allowed for direct in situ analysis of the FBR. Results provide spatially resolved data correlating cells, cytokines, time, and tissue locations to biomaterials and the FBR. This approach should further the understanding of the FBR and the immune environment around biomaterial implants in vivo to facilitate improvements in materials and methods in biomaterials applications in medicine.     

Controlling fibrous capsule formation through long-term down-regulation of collagen type I (COL1A1) expression by nanofiber-mediated siRNA gene silencing

The foreign body reaction often interferes with the long-term functionality and performance of implanted biomedical devices through fibrous capsule formation. While many implant modification techniques have been adopted in attempts to control fibrous encapsulation, the outcomes remained sub-optimal. Nanofiber scaffold-mediated RNA interference may serve as an alternative approach through the localized and sustained delivery of siRNA at implant sites. In this study, we investigated the efficacy of siRNA–poly(caprolactone-co-ethylethylene phosphate) nanofibers in controlling fibrous capsule formation through the down-regulation of collagen type I (COL1A1) in vitro and in vivo. By encapsulating complexes of COL1A1 siRNA with a transfection reagent (Transit TKO) or the cell penetrating peptides CADY or MPG within the nanofibers (550–650 nm in diameter), a sustained release of siRNA was obtained for at least 28 days (loading efficiency ～60–67%). Scaffold-mediated transfection significantly enhanced cellular uptake of oligonucleotides and prolonged in vitro gene silencing duration by at least 2–3 times as compared to conventional bolus delivery of siRNA (14 days vs. 5–7 days by bolus delivery). In vivo subcutaneous implantation of siRNA scaffolds revealed a significant decrease in fibrous capsule thickness at weeks 2 and 4 as compared to plain nanofibers (p < 0.05). Taken together, the results demonstrated the efficacy of scaffold-mediated siRNA gene-silencing in providing effective long-term control of fibrous capsule formation.     

Bioevaluation of plasma polymerized films in skeletal muscle.

Plasma polymerized ethylene (PPE), styrene (PPS), and chlorotrifluoroethylene (PPCTFE) were synthesized by exposing the monomeric gases to an inductively coupled radio frequency "glow-discharge" field. The polymer films were deposited on poly(dimethyl) siloxane (medical grade Silastic), which was then surgically implanted in rat paravertebral muscle for periods up to 84 weeks. The biocompatibility of the plasma deposited films and uncoated Silastic was evaluated by qualitative (graded inflammatory cell response) and quantitative (connnective tissue capsule thickness) techniques as a function of time. The morphological features of the connective tissue capsule and the plasma polymerized films were examined by SEM after 75 weeks of implantation. Results showed that the acute inflammatory cell migration around PPS and PPCTFE was at a maximum in 2 weeks, decaying to control levels in 4 to 8 weeks. The PPE response was judged as less than the control response up to 4 weeks. After 8 weeks no qualitative difference could be detected between the plasma polymerized films and Silastic. On the other hand, a quantifiable change in fibrous capsule response as a function of time and material was noted until 24 weeks. From these data we conclude that these types of films do not elicit an untoward foreign body reaction at a skeletal muscle implant site in rats.