Biocompatibility of electrochemical glucose sensors implanted in the subcutis of pigs

Gastro-intestinal lesions are not relatable to diarrhoea or specific pathogens in post-weaning diarrhoea (PWD) in pigs

BACKGROUND

Post-weaning diarrhoea (PWD) is a multifactorial condition and the most well documented infectious cause is enterotoxigenic Escherichia coli. The objective of the study was to investigate possible associations between pathological manifestations and pathogens in pigs with and without PWD. The study was conducted as a case-control study and included a total of 173 pigs from 9 different commercial intensive indoor production herds in eastern Denmark.

RESULTS

Based on clinical examination, a total of 89 piglets with PWD (cases) and 84 piglets without PWD (controls) were included. Most of the pigs (n = 105/173) presented gastric lesions, which were more frequently observed in the control group. The odds of gastric ulcers were lower among pigs with PWD compared to pigs without PWD with an odds ratio (OR) of 0.2 (0.0; 0.7). Abnormal content in the colon was associated with PWD, with an OR of 6.5 (3.2; 14.3). No apparent association was found between lesions and the various pathogens or a combination of these. The odds of neutrophilic granulocyte infiltration were lower in the jejunum among pigs with PWD (OR 0.3 [0.1; 0.6]) compared to pigs without PWD. The association between neutrophilic granulocyte infiltration in jejunum and PWD differed between the herds (P = 0.03). Furthermore, the associations between PWD and hyperleukocytosis (P = 0.04) or infiltration of eosinophilic granulocytes (P = 0.04) in ileum were also herd dependent. Histopathology revealed several lesions not relatable to PWD.

CONCLUSION

The association between lesions and specific pathogens or PWD is more complex than anticipated.

Closed-loop neuromodulation will increase the utility of mouse models in Bioelectronic Medicine

Mouse models have been of tremendous benefit to medical science for the better part of a century, yet bioelectronic medicine research using mice has been limited to mostly acute studies because of a lack of tools for chronic stimulation and sensing. A wireless neuromodulation platform small enough for implantation in mice will significantly increase the utility of mouse models in bioelectronic medicine. This perspective examines the necessary functionality of such a system and the technical challenges needed to be overcome for its development. Recent progress is examined and the outlook for the future of implantable devices for mice is discussed.

A porcine model of subcutaneous Staphylococcus aureus infection: a pilot study

In this descriptive pilot study, we aim to establish a porcine Staphylococcus aureus skin infection model by subcutaneous injection (s.c.) of the porcine S54F9 S. aureus strain in the groin area. Six pigs were used in the study: Five pigs were injected with S. aureus, inocula ranging from 7 × 10³ to 5 × 10⁷ colony-forming units per kg bodyweight; one pig was injected with saline exclusively. Lesions were recorded up to 6 days postinoculation using clinical evaluation, ultrasound evaluation, microbiology, flow cytometry, and pathology. Inoculation gave rise to lesions ranging from localized skin infection, that is, minute histological changes, intracellular infection, and macroscopic abscess formation with sequestration of soft tissue, to generalized infection and development of disseminated intravascular coagulation necessitating euthanasia only 10 h after inoculation. Ultrasound assessment of maximum width and characteristics was not able to disclose the progress of the local infection. Flow cytometry and immunohistochemistry revealed the participation of γδT cells in the immune response. In conclusion, we did see a graded inflammatory response associated with the dose of s.c. inoculated bacteria, which may be useful for studying, in particular, the interaction of bacteria and inflammatory mononuclear cell populations. It needs to be investigated if the model is discriminatory and robust.

Review of the Long-Term Implantable Senseonics Continuous Glucose Monitoring System and Other Continuous Glucose Monitoring Systems

The article published by Kevin Cowart in this issue of the Journal of Diabetes Science and Technology (JDST) is a detailed overview of the clinical trial data and analysis used to demonstrate the safety and effectiveness of the Eversense continuous glucose monitoring (CGM) System for regulatory approval and clinical acceptance. The article describes the published study results for safety, accuracy, reliability, ease of insertion/removal, adverse events, and ease of diabetes patient-use for controlling their glucose levels short and long term. The author nicely compares Eversense CGM System safety and performance with the short-term subcutaneous tissue CGM systems being commercialized by Dexcom, Medtronic Diabetes, and Abbott Diabetes. This comparison may help the clinician define which type of patient with diabetes might benefit the most from the long-term implantable CGM system. The majority of studied patients describe a positive experience managing their diabetes with the Eversense CGM System and request implantation of a new sensor 90 or 180 days later.

Impact of CCL2 and CCR2 chemokine/receptor deficiencies on macrophage recruitment and continuous glucose monitoring in vivo

The accumulation of macrophages (MΦ) at the sensor-tissue interface is thought to be a major player in controlling tissue reactions and sensor performance in vivo. Nevertheless until recently no direct demonstration of the causal relationship between MΦ aggregation and loss of sensor function existed. Using a Continuous Glucose Monitoring (CGM) murine model we previously demonstrated that genetic deficiencies of MΦ or depletion of MΦ decreased MΦ accumulation at sensor implantation sites, which led to significantly enhanced CGM performance, when compared to normal mice. Additional studies in our laboratories have also demonstrated that MΦ can act as "metabolic sinks" by depleting glucose levels at the implanted sensors in vitro and in vivo. In the present study we extended these observations by demonstrating that MΦ chemokine (CCL2) and receptor (CCR2) knockout mice displayed a decrease in inflammation and MΦ recruitment at sensor implantation sites, when compared to normal mice. This decreased MΦ recruitment significantly enhanced CGM performance when compared to control mice. These studies demonstrated the importance of the CCL2 family of chemokines and related receptors in MΦ recruitment and sensor performance and suggest chemokine targets for enhancing CGM in vivo.

Biomolecular strategies to modulate the macrophage response to implanted materials

The material-induced foreign body response is a major challenge for implanted medical devices. This review highlights recent developments in biomimetic approaches to create biomaterials that mitigate the host response to biomaterials. Specifically, we will describe strategies in which biomaterials are decorated with endogenously expressed biomolecules that naturally modulate the function of immune cells. These include molecules that directly bind to and interact with immune cells, as well as molecules that control complement activation or thrombosis and indirectly modulate immune cell function. We provide perspective on how these approaches may impact the design of materials for medical devices and tissue engineering.

Basement Membrane-Based Glucose Sensor Coatings Enhance Continuous Glucose Monitoring in Vivo

BACKGROUND

Implantable glucose sensors demonstrate a rapid decline in function that is likely due to biofouling of the sensor. Previous efforts directed at overcoming this issue has generally focused on the use of synthetic polymer coatings, with little apparent effect in vivo, clearly a novel approach is required. We believe that the key to extending sensor life span in vivo is the development of biocompatible basement membrane (BM) based bio-hydrogels as coatings for glucose sensors.

METHOD

BM based bio-hydrogel sensor coatings were developed using purified BM preparations (ie, Cultrex from Trevigen Inc). Modified Abbott sensors were coated with Cultrex BM extracts. Sensor performance was evaluated for the impact of these coatings in vitro and in vivo in a continuous glucose monitoring (CGM) mouse model. In vivo sensor function was assessed over a 28-day time period expressed as mean absolute relative difference (MARD) values. Tissue reactivity of both Cultrex coated and uncoated glucose sensors was evaluated at 7, 14, 21 and 28 days post-sensor implantation with standard histological techniques.

RESULTS

The data demonstrate that Cultrex-based sensor coatings had no effect on glucose sensor function in vitro. In vivo glucose sensor performance was enhanced following BM coating as determined by MARD analysis, particularly in weeks 2 and 3. In vivo studies also demonstrated that Cultrex coatings significantly decreased sensor-induced tissue reactions at the sensor implantation sites.

CONCLUSION

Basement-membrane-based sensor coatings enhance glucose sensor function in vivo, by minimizing or preventing sensor-induced tissues reactions.

Evaluation of glucose response to 3 types of insulin using a continuous glucose monitoring system in healthy alpacas

Background

Persistent hyperglycemia is common in alpacas and typically requires insulin administration for resolution; however, little is known about alpacas’ response to different insulin formulations.

Objectives

To evaluate the effects of 3 insulin formulations on blood glucose concentrations and the use of a continuous glucose monitoring (CGM) system in alpacas.

Animals

Six healthy alpacas.

Methods

The CGM was installed in the left paralumbar fossa at the start of this crossover study and recorded data every 5 minutes. Regular insulin, NPH insulin, insulin glargine, and dextrose were administered to each alpaca over a 2‐week period. Blood samples were collected for glucose testing at 0, 1, 2, 4, 6, 8, and 12 hours, and then every 6 hours after each administration of insulin or dextrose. Data were compared by using method comparison techniques, error grid plots, and ANOVA.

Results

Blood glucose concentrations decreased most rapidly after regular insulin administration when administered IV or SC as compared to the other formulations. The NPH insulin produced the longest suppression of blood glucose. The mean CGM interstitial compartment glucose concentrations were typically lower than the intravascular compartment glucose concentrations. The alpacas had no adverse reactions to the different insulin formulations.

Conclusions and Clinical Importance

The NPH insulin might be more appropriate for long‐term use in hyperglycemic alpacas because of its extended duration of action. A CGM is useful in monitoring glucose trends and reducing blood collection events, but it should not be the sole method for determining treatment protocols.

Modulation of the foreign body response to implanted sensor models through device-based delivery of the tyrosine kinase inhibitor, masitinib

The host foreign body response (FBR) adversely effects the performance of numerous implanted biomaterials especially biosensors, including clinically popular glucose-monitoring sensors. Reactive formation of a fibrous capsule around implanted sensors hinders the transport of essential analytes to the sensor from the surrounding tissue, resulting in loss of glucose response sensitivity and eventual sensor failure. Several strategies have sought to mitigate the foreign body response's effects on CGM sensors through the use of local delivery of pharmaceuticals and biomolecules with limited success. This study describes release of a tyrosine kinase inhibitor - masitinib - from the sensor implant to target tissue resident mast cells as key mediators of the FBR. Model implants are coated with a composite polymer hydrophilic matrix that rapidly dissolves upon tissue implantation to deposit slower-degrading polymer microparticles containing masitinib. Matrix dissolution limits coating interference with sensor function while establishing a local controlled-release delivery depot formulation to alter implant tissue pharmacology and addressing the FBR. Drug efficacy was evaluated in a murine subcutaneous pocket implant model. Drug release extends to more than 30 days in vitro. The resulting FBR in vivo, evaluated by implant capsule thickness and inflammatory cell densities at 14, 21, and 28 days, displays statistically significant reduction in capsule thickness around masitinib-releasing implant sites compared to control implant sites.

Detection of porcine circovirus type 2 and viral replication by in situ hybridization in primary lymphoid organs from naturally and experimentally infected pigs

Porcine circovirus type 2 (PCV2) infection is the cause of postweaning multisystemic wasting syndrome (PMWS). It has been speculated whether cell types permissive of replication are found in the primary lymphoid organs and whether infection of these tissues has an important role in the pathogenesis of PMWS. The aim of this study was to determine if primary lymphoid organ cells support viral replication during PCV2 infection. This was done by histopathological examination of thymus and bone marrow from pigs experimentally inoculated with PCV2 (n = 24), mock-infected pigs (n = 12), pigs naturally affected by PMWS (n = 33), and age-matched healthy control animals (n = 29). In situ hybridization (ISH) techniques were used to detect PCV2 nucleic acid irrespective of replicative status (complementary probe, CP) or to detect only the replicative form of the virus (replicative form probe, RFP). PCV2 was not detected in the experimentally PCV2-inoculated pigs or the control animals. Among the PMWS-affected pigs, 19 of 20 (95%) thymuses were positive for PCV2 by CP ISH, and 7 of 19 (37%) of these also supported viral replication. By CP ISH, PCV2 was detected in 16 of 33 (48%) bone marrow samples, and 5 of 16 (31%) of these also supported replication. The 2 ISH probes labeled the same cell types, which were histiocytes in both organs and lymphocytes in thymus. The RFP labeled fewer cells than the CP. Thus, PCV2 nucleic acids and replication were found in bone marrow and thymus of PMWS-affected pigs, but there was no evidence that primary lymphoid organ cells are major supporters of PCV2 replication.

A promising solution to enhance the sensocompatibility of biosensors in continuous glucose monitoring systems

BACKGROUND

Continuous glucose monitors (CGMs) measure glucose in real time, making it possible to improve glycemic control. A promising technique involves glucose sensors implanted in subcutaneous tissue measuring glucose concentration in interstitial fluid. A major drawback of this technique is sensor bioinstability, which can lead to unpredictable drift and reproducibility. The bioinstability is partly due to sensor design but is also affected by naturally occurring subcutaneous inflammations. Applying a nonbiofouling coating to the sensor membrane could be a means to enhancing sensocompatibility.

METHODS

This study evaluates the suitability of a polyethylene-glycol-based coating on sensors in CGMs. Methods used include cross hatch, wet paper rub, paper double rub, bending, hydrophilicity, protein adsorption, bio-compatibility, hemocompatibility, and glucose/oxygen permeability testing.

RESULTS

Results demonstrate that coating homogeneity, adhesion, integrity, and scratch resistance are good. The coating repels lysozyme and bovine serum albumin, and only a low level of fibrin and blood platelet adsorption to the coating was recorded when testing in whole human blood. Cytotoxicity, irritation, sensitization, and hemolysis were assessed, and levels suggested good biocompatibility of the coating in subcutaneous tissue. Finally, it was shown that the coating can be applied to cellulose acetate membranes of different porosity without changing their permeability for glucose and oxygen.

CONCLUSIONS

These results suggest that the mechanical properties of the coating are sufficient for the given application, that the coating is effective in preventing protein adsorption and blood clot formation on the sensor surface, and that the coating can be applied to membranes without hindering their glucose and oxygen transport.

Materials for diabetes therapeutics

This review is focused on the materials and methods used to fabricate closed-loop systems for type 1 diabetes therapy. Herein, we give a brief overview of current methods used for patient care and discuss two types of possible treatments and the materials used for these therapies-(i) artificial pancreases, comprised of insulin producing cells embedded in a polymeric biomaterial, and (ii) totally synthetic pancreases formulated by integrating continuous glucose monitors with controlled insulin release through degradable polymers and glucose-responsive polymer systems. Both the artificial and the completely synthetic pancreas have two major design requirements: the device must be both biocompatible and be permeable to small molecules and proteins, such as insulin. Several polymers and fabrication methods of artificial pancreases are discussed: microencapsulation, conformal coatings, and planar sheets. We also review the two components of a completely synthetic pancreas. Several types of glucose sensing systems (including materials used for electrochemical, optical, and chemical sensing platforms) are discussed, in addition to various polymer-based release systems (including ethylene-vinyl acetate, polyanhydrides, and phenylboronic acid containing hydrogels).

The impact of Staphylococcus aureus concentration on the development of pulmonary lesions and cytokine expression after intravenous inoculation of pigs

Acute respiratory distress syndrome is a common complication in severe sepsis. In pigs, the lungs play an important role in clearing systemic bacterial infections due to pulmonary intravascular macrophages found specifically in pigs. However, this increases the exposure of the porcine lungs to pathogens and potential injury. The authors propose that increasing the concentration of the inoculum without changing the bacterial dose will lead to severe sepsis with pronounced pulmonary lesions. This could potentially create a risk of cytokine spillover to the circulation, leading to an increased systemic response. Eight Danish Landrace pigs, approximately 10 weeks old, were inoculated twice with a low or once with a high concentration of Staphylococcus aureus. Three pigs were sham-inoculated. The animals were grouped based on macro- and microscopic lung lesions. The mRNA expression of local pulmonary inflammatory markers was compared to protein levels of systemic inflammatory markers. The most severe pulmonary lesions were observed in animals receiving the high S. aureus concentration, indicating that severity of lesions is dependent on inoculum concentration rather than total numbers of bacteria. Furthermore, local mRNA expression of inflammatory cytokines appeared to be dependent on the magnitude and severity of tissue destruction, including the ability to confine the lesions. Increasing mRNA levels of serum amyloid A could be a confident marker of severity of pulmonary lesions. Since no correlation was observed between local and systemic levels of inflammatory cytokines, this finding could indicate an ability of the porcine lung to compartmentalize the local inflammatory response and thus restrict systemic contribution.

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.

Spatiotemporal effects of a controlled-release anti-inflammatory drug on the cellular dynamics of host response

In general, biomaterials induce a non-specific host response when implanted in the body. This reaction has the potential to interfere with the function of the implanted materials. One method for controlling the host response is through local, controlled-release of anti-inflammatory agents. Herein, we investigate the spatial and temporal effects of an anti-inflammatory drug on the cellular dynamics of the innate immune response to subcutaneously implanted poly(lactic-co-glycolic) microparticles. Noninvasive fluorescence imaging was used to investigate the influence of dexamethasone drug loading and release kinetics on the local and systemic inhibition of inflammatory cellular activities. Temporal monitoring of host response showed that inhibition of inflammatory proteases in the early phase was correlated with decreased cellular infiltration in the later phase of the foreign body response. We believe that using controlled-release anti-inflammatory platforms to modulate early cellular dynamics will be useful in reducing the foreign body response to implanted biomaterials and medical devices.

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

BACKGROUND

Tight glycemic control has the potential to reduce long- and short-term effects of diabetes mellitus. New and improved glucose sensors for short-term implantation in the subcutis offer an alternative to the classical self-monitored blood glucose concentration in the management of diabetes. The use of glucose sensors has an advantage over the latter due to its capacity to obtain continuous glucose measurements. However, instability of in vivo glucose sensor measurements has been reported, and this bioinstability is likely to be influenced by the inflammatory reaction to the implanted sensor. Gene expression analysis is now performed in an existing porcine model of subcutaneous glucose sensor implantation to investigate the time course of inflammation from a new perspective.

METHODS

Tissue surrounding glucose sensors was sampled to different time points (2 h, 24 h, 3 days, and 7 days) after implantation in the subcutis of pigs. From the tissue RNA was extracted, cDNA was synthesized, and real-time reverse transcription-polymerase chain reaction (RT-PCR) was performed for the quantification of immunoregulatory genes.

RESULTS

Genes coding for adhesion molecules, chemokines, cytokines, CD markers, and antigen presentation molecules were differentially expressed over time. Most of the investigated genes were significantly up-regulated 24 h and 7 days after implantation.

CONCLUSIONS

The present study demonstrated that the technology for targeted multiple-gene expression by real-time RT-PCR is useful in the evaluation of the immune response to subcutaneously implanted glucose sensors and that the expression levels also seemed to correspond to the histomorphological observations over time.

Bioluminescence imaging of glucose in tissue surrounding polyurethane and glucose sensor implants

BACKGROUND

The bioluminescence technique was used to quantify the local glucose concentration in the tissue surrounding subcutaneously implanted polyurethane material and surrounding glucose sensors. In addition, some implants were coated with a single layer of adipose-derived stromal cells (ASCs) because these cells improve the wound-healing response around biomaterials.

METHODS

Control and ASC-coated implants were implanted subcutaneously in rats for 1 or 8 weeks (polyurethane) or for 1 week only (glucose sensors). Tissue biopsies adjacent to the implant were immediately frozen at the time of explant. Cryosections were assayed for glucose concentration profile using the bioluminescence technique.

RESULTS

For the polyurethane samples, no significant differences in glucose concentration within 100 μm of the implant surface were found between bare and ASC-coated implants at 1 or 8 weeks. A glucose concentration gradient was demonstrated around the glucose sensors. For all sensors, the minimum glucose concentration of approximately 4 mM was found at the implant surface and increased with distance from the sensor surface until the glucose concentration peaked at approximately 7 mM at 100 μm. Then the glucose concentration decreased to 5.5-6.5 mM more than 100 μmm from the surface.

CONCLUSIONS

The ASC attachment to polyurethane and to glucose sensors did not change the glucose profiles in the tissue surrounding the implants. Although most glucose sensors incorporate a diffusion barrier to reduce the gradient of glucose and oxygen in the tissue, it is typically assumed that there is no steep glucose gradient around the sensors. However, a glucose gradient was observed around the sensors. A more complete understanding of glucose transport and concentration gradients around sensors is critical.

Rapid biocompatibility analysis of materials via in vivo fluorescence imaging of mouse models

BACKGROUND

Many materials are unsuitable for medical use because of poor biocompatibility. Recently, advances in the high throughput synthesis of biomaterials has significantly increased the number of potential biomaterials, however current biocompatibility analysis methods are slow and require histological analysis.

METHODOLOGY/PRINCIPAL FINDINGS

Here we develop rapid, non-invasive methods for in vivo quantification of the inflammatory response to implanted biomaterials. Materials were placed subcutaneously in an array format and monitored for host responses as per ISO 10993-6: 2001. Host cell activity in response to these materials was imaged kinetically, in vivo using fluorescent whole animal imaging. Data captured using whole animal imaging displayed similar temporal trends in cellular recruitment of phagocytes to the biomaterials compared to histological analysis.

CONCLUSIONS/SIGNIFICANCE

Histological analysis similarity validates this technique as a novel, rapid approach for screening biocompatibility of implanted materials. Through this technique there exists the possibility to rapidly screen large libraries of polymers in vivo.

Breakthrough Aspergillus fumigatus and Candida albicans double infection during caspofungin treatment: laboratory characteristics and implication for susceptibility testing

Caspofungin is used for the treatment of acute invasive candidiasis and as salvage treatment for invasive aspergillosis. We report characteristics of isolates of Candida albicans and Aspergillus fumigatus detected in a patient with breakthrough infection complicating severe gastrointestinal surgery and evaluate the capability of susceptibility methods to identify candin resistance. The susceptibility of C. albicans to caspofungin and anidulafungin was investigated by Etest, microdilution (European Committee on Antibiotic Susceptibility Testing [EUCAST] and CLSI), disk diffusion, agar dilution, and FKS1 sequencing and in a mouse model. Tissue was examined by immunohistochemistry, PCR, and sequencing for the presence of A. fumigatus and resistance mutations. The MICs for the C. albicans isolate were as follows: >32 microg/ml caspofungin and 0.5 microg/ml anidulafungin by Etest, 2 microg/ml caspofungin and 0.125 microg/ml anidulafungin by EUCAST methods, and 1 microg/ml caspofungin and 0.5 microg/ml anidulafungin by CLSI methods. Sequencing of the FKS1 gene revealed a mutation leading to an S645P substitution. Caspofungin and anidulafungin failed to reduce kidney CFU counts in animals inoculated with this isolate (P > 0.05 compared to untreated control animals), while both candins completely sterilized the kidneys in animals infected with a control isolate. Disk diffusion and agar dilution methods clearly separated the two isolates. Immunohistochemistry and sequencing confirmed the presence of A. fumigatus without FSK1 resistance mutations in liver and lung tissues. Breakthrough disseminated aspergillosis and candidiasis developed despite an absence of characteristic FKS1 resistance mutations in the Aspergillus isolates. EUCAST and CLSI methodology did not separate the candin-resistant clinical isolate from the sensitive control isolate as well as did the Etest and agar methods.

A review of the foreign-body response to subcutaneously-implanted devices: the role of macrophages and cytokines in biofouling and fibrosis

The biological response to implanted biomaterials in mammals is a complex series of events that involves many biochemical pathways. Shortly after implantation, fibrinogen and other proteins bind to the device surface, a process known as biofouling. Macrophages then bind to receptors on the proteins, join into multinucleated giant cells, and release transforming growth factor beta and other inflammatory cytokines. In response to these signals, quiescent fibroblasts are transformed into myofibroblasts, which synthesize procollagen via activation of Smad mediators. The procollagen becomes crosslinked after secretion into the extracellular space. Mature crosslinked collagen and other extracellular matrix proteins gradually contribute to formation of a hypocellular dense fibrous capsule that becomes impermeable or hypopermeable to many compounds. Porous substrates and angiogenic growth factors can stimulate formation of microvessels, which to some extent can maintain analyte delivery to implanted sensors. However, stimulation by vascular endothelial growth factor alone may lead to formation of leaky, thin-walled, immature vessels. Other growth factors are most probably needed to act upon these immature structures to create more robust vessels.During implantation of foreign bodies, the foreign-body response is difficult to overcome, and thousands of biomaterials have been tested. Biomimicry (i.e., creating membranes whose chemical structure mimics natural cellular compounds) may diminish the response, but as of this writing, it has not been possible to create a stealth material that circumvents the ability of the mammalian surveillance systems to distinguish foreign from self.

Electrochemical Sensors for Clinic Analysis

Demanded by modern medical diagnosis, advances in microfabrication technology have led to the development of fast, sensitive and selective electrochemical sensors for clinic analysis. This review addresses the principles behind electrochemical sensor design and fabrication, and introduces recent progress in the application of electrochemical sensors to analysis of clinical chemicals such as blood gases, electrolytes, metabolites, DNA and antibodies, including basic and applied research. Miniaturized commercial electrochemical biosensors will form the basis of inexpensive and easy to use devices for acquiring chemical information to bring sophisticated analytical capabilities to the non-specialist and general public alike in the future.

Actinobacillus equuli subsp. equuli associated with equine valvular endocarditis

Microbiological and pathological data from a case of equine valvular endocarditis are reported. Limited information is available on the pathogenic potential of equine Actinobacillus species as several strains originate from apparently healthy horses. After the establishment of two subspecies within this species, this seems to be the first report of an etiological association between A. equuli subsp. equuli and equine endocarditis. Furthermore, new information on some phenotypical characteristics of this subspecies is reported, compared to previous findings.

Recent advances in continuous glucose monitoring: biocompatibility of glucose sensors for implantation in subcutis

Tight glycemic control slows or prevents the development of short- and long-term complications of diabetes mellitus. Continuous glucose measurements provide improved glycemic control and potentially prevent these diabetic complications. Glucose sensors, especially implantable devices, offer an alternative to classical self-monitored blood glucose levels and have shown promising glucose-sensing properties. However, the ultimate goal of implementing the glucose sensor as the glucose-sensing part of a closed loop system (artificial pancreas) is still years ahead because of malfunctions of the implanted sensor. The malfunction is partly a consequence of the subcutaneous inflammatory reaction caused by the implanted sensor. In order to improve sensor measurements and thereby close the loop, it is crucial to understand what happens at the tissue-sensor interface.

Recent progress in analytical instrumentation for glycemic control in diabetic and critically ill patients

Implementing strict glycemic control can reduce the risk of serious complications in both diabetic and critically ill patients. For this reason, many different analytical, mainly electrochemical and optical sensor approaches for glucose measurements have been developed. Self-monitoring of blood glucose (SMBG) has been recognised as being an indispensable tool for intensive diabetes therapy. Recent progress in analytical instrumentation, allowing submicroliter samples of blood, alternative site testing, reduced test time, autocalibration, and improved precision, is comprehensively described in this review. Continuous blood glucose monitoring techniques and insulin infusion strategies, developmental steps towards the realization of the dream of an artificial pancreas under closed loop control, are presented. Progress in glucose sensing and glycemic control for both patient groups is discussed by assessing recent published literature (up to 2006). The state-of-the-art and trends in analytical techniques (either episodic, intermittent or continuous, minimal-invasive, or noninvasive) detailed in this review will provide researchers, health professionals and the diabetic community with a comprehensive overview of the potential of next-generation instrumentation suited to either short- and long-term implantation or ex vivo measurement in combination with appropriate body interfaces such as microdialysis catheters.

Biocompatibility of an enzyme-based, electrochemical glucose sensor for short-term implantation in the subcutis

BACKGROUND

Continuous glucose measurements provide improved glycemic control and may prevent hypoglycemia and long-term complications of diabetes. One of the most promising techniques is the short-term implantation of electrochemical glucose sensors in subcutis. However, the inflammatory reaction to these sensors may lead to bioinstability of sensor measurements. The purpose of the present investigation was to examine factors contributing to the observed subcutaneous inflammatory reaction to an enzyme-based electrochemical glucose sensor for continuous glucose measurements. The sensor biocompatibility was assessed in vitro and in vivo.

METHODS

A toxicological assessment was performed on sensor materials and leachables, and the endotoxin content of sensors was determined by a Limulus amoebocyte lysate (LAL) test. Moreover, as a consequence of permanent penetration of the skin by the sensor the role of bacterial migration to the tissue was investigated. In vivo biocompatibility was investigated through histological examination of implanted sensor membranes for 3 days in pigs. Additionally, the effect of needle size and type (normal vs. inserter needle) on tissue trauma at sensor insertion was evaluated, and the healing of subcutis was assessed histologically from 3 to 14 days after removal of sensors.

RESULTS

The toxicological assessment and the LAL test showed no concerns in a 3-day implantation scenario, and bacterial migration to the subcutis could not be detected. The histological examination showed that a reduction in needle size reduced the extent of inflammation to very low levels, and that the different sensor membranes showed similar extent and type of inflammation. Additionally, the extent of subcutaneous tissue reaction after removal of sensors declined gradually over time and returned to near-normal levels after 2 weeks.

CONCLUSION

The electrochemical enzyme-based glucose sensor for continuous glucose measurements in subcutis is acceptable from a biocompatibility point of view. Reducing the inserter needle in size reduces the trauma induced at sensor implantation to neglible levels. Furthermore, the tissue reaction to the sensor returns to near-normal 2 weeks after the sensor has been removed following a 3-day implantation period.