In vitro microdialysis recoveries of glucose were evaluated and optimised by varying the inner ca... more In vitro microdialysis recoveries of glucose were evaluated and optimised by varying the inner cannula dimensions for a concentric type microdialysis probe. The recovery was shown to be significantly dependent on the outer radius of the inner cannula (r alpha). Increasing r alpha by 50% while holding all other microdialysis variables constant caused a 26% increase in recovery under the conditions tested. Mass transport modeling of microdialysis sampling showed that the magnitude of the recovery change in response to changes in r alpha may depend on other experimental variables such as effective dialysis length, membrane dimensions, membrane type and medium composition. These data indicate that microdialysis recovery may be substantially improved by optimising the outer diameter of the inner cannula. These data also suggest that in order to fully characterise and compare data from microdialysis experiments, r alpha should become one of the reported variables.
Journal of Diabetes Science and Technology, May 1, 2011
Nearly all commercially available glucose sensors share the subcutaneous interstitial fluid (ISF)... more Nearly all commercially available glucose sensors share the subcutaneous interstitial fluid (ISF) compartment as their preferred implantation site. However, ISF physiology as it relates to glucose sensors is not well understood. This special symposium titled “Interstitial Fluid Physiology as It Relates to Glucose Monitoring Technologies” is intended to help to bridge the gap in our understanding. This symposium is intended to foster a greater understanding of biological factors that impact the success of implantable glucose monitors and to inspire additional research in the area of ISF physiology as it relates to glucose sensing. Recognition that sensor designers need to have an intimate understanding of the biological environment in which their sensor will reside is emphasized. The symposium is published in two parts, with part I published in September 2010 and part II published in May 2011. All articles published in this symposium are summarized herein.
The rat is commonly used to evaluate physiological responses of subcutaneous tissue to implanted ... more The rat is commonly used to evaluate physiological responses of subcutaneous tissue to implanted devices. In vivo longevity of various devices and the biocompatibility of biomaterials de pend on how adjacent tissue interacts. How closely the rat model predicts the human response has not been well characterized. The objective of this study was to compare rat and human subcutaneous foreign body responses by monitoring the biochemical environment at a polymer-tissue interface over eight days using microdialysis. Polyamide microdialysis probes were implanted subcutaneously in humans and rats (n=12). Daily microdialysis samples were analyzed for glucose, lactate, pyruvate, glycerol and urea. Blood glucose was also monitored. Analyte concentrations differed significantly between rats and humans at the implant-tissue interface. There were also qualitative differences in the eightday trends. For example, over eight days, microdialysate glu cose increased 2 -4 fold in humans, but decreased in rats (p<0.001). This study reveals profound physiological differences at material-tissue interfaces in rats and humans and highlights the need for caution when extrapolating subcutaneous rat biocompatibility data to humans.
American journal of physiology. Endocrinology and metabolism, 2002
The rat is commonly used to evaluate physiological responses of subcutaneous tissue to implanted ... more The rat is commonly used to evaluate physiological responses of subcutaneous tissue to implanted devices. In vivo longevity of various devices and the biocompatibility of biomaterials depend on how adjacent tissue interacts. How closely the rat model predicts the human response has not been well characterized. The objective of this study was to compare rat and human subcutaneous foreign body responses by monitoring the biochemical environment at a polymer-tissue interface over 8 days using microdialysis. Polyamide microdialysis probes were implanted subcutaneously in humans and rats (n = 12). Daily microdialysis samples were analyzed for glucose, lactate, pyruvate, glycerol, and urea. Blood glucose was also monitored. Analyte concentrations differed significantly between rats and humans at the implant-tissue interface. There were also qualitative differences in the 8-day trends. For example, over 8 days, microdialysate glucose increased two- to fourfold in humans but decreased in ra...
Journal of biomedical materials research, Jan 15, 2001
Membrane biofouling and tissue changes in the foreign body response are known to cause detrimenta... more Membrane biofouling and tissue changes in the foreign body response are known to cause detrimental reductions of analyte transport into implanted biosensors. The relative contribution of each phenomenon is unknown. Hollow fiber microdialysis probes were employed to assess the effect of subcutaneous implantation on glucose flux through polymeric membranes in rats over 8 days and to differentiate the transport effects of biofouling versus tissue changes. Three commercially available membranes were examined: poly(ether sulfone) (PES), polyacrylonitrile (PAN), and polycarbonate (PC). As measured by glucose recovery (the ratio of microdialysis glucose to blood glucose concentrations), transport through PES membranes was significantly less on day 2 than day 0 (39% decrease, p < 0.05) whereas PAN and PC showed no significant decreases in flux until day 8 (42 and 43%, respectively). Application of a transport model to glucose recovery data obtained before implantation in vivo and after e...
The material-tissue interaction that results from sensor implantation is one of the major obstacl... more The material-tissue interaction that results from sensor implantation is one of the major obstacles in developing viable, long-term implantable biosensors. Strategies useful for the characterization and modification of sensor biocompatibility are widely scattered in the literature, and there are many peripheral studies from which useful information can be gleaned. The current paper reviews strategies suitable for addressing biofouling, one aspect of biosensor biocompatibility. Specifically, this paper addresses the effect of membrane biofouling on sensor sensitivity from the standpoint of glucose transport limitations. Part I discusses the in vivo and in vitro methods used to characterize biofouling and the effects of biofouling on sensor performance, while Part II presents techniques intended to improve biosensor biocompatibility.
A microdialysis method for monitoring permeability changes of implanted biosensor membranes was d... more A microdialysis method for monitoring permeability changes of implanted biosensor membranes was demonstrated in vivo. Glucose diffusion through three different hollow fiber membranes (polyethersulfone (PES), polyacrylonitrile (PAN), polycarbonate (PC)) was assessed over 8 days in rat subcutaneous tissue. There was a 77% decrease of transport through the implanted PES membranes after two days; whereas decreases in PAN and PC were
ABSTRACT Purpose: To maximize mass transport into biomaterial scaffolds for regenerative medicine... more ABSTRACT Purpose: To maximize mass transport into biomaterial scaffolds for regenerative medicine, in vivo diagnostics, therapeutics, and cell delivery. We utilized matrix morphology to encourage long-term tissue integration. Methods: We implanted 1cm-diameter poly-hydroxyethylmethacrylate (pHEMA) disks with 40 and 80µm nominal interconnected pores into rat subcutis. Solid pHEMA, silicone, and cotton disks were also implanted. Microvessel density and total cellularity were histologically quantified in 50µm-wide zones both into the implants and into the adjacent tissues. Results: One week after implantation, the microvessel density closest to the interface was 74±8/mm2 (mean±SEM) for the 80µm pHEMA and 28±8/mm2 for the 40µm pHEMA. The rate of vascularization was greater in 80µm pHEMA, with higher vascular density in the material and adjacent tissues one week and one month post-implantation (p&lt;0.001). After two months, vascular ingrowth was similar for both 40 and 80µm pHEMA (maximum = 155±32/mm2 and 182±43/mm2, respectively). Solid pHEMA and silicone exhibited no vascular ingrowth. At two months, the microvessel density in the 0-50µm adjacent tissue was 203±11/mm2, compared to 101±15/mm2 for the 40 µm pHEMA; 60±16/mm2 for the solid pHEMA; and 24±7/mm2 for silicone. All materials except 80µm pHEMA showed a narrow margin of significantly reduced vascularity at the implant-tissue interface. Total cellularity in the 0-50µm tissue adjacent to 80µm pHEMA was 7736±1011/mm2 after one week; 7582±1129/mm2 after one month; and 4498±497/mm2 after two months. Using real-time ultrasonic microscopy, the microshearing that can disrupt microvessel formation was observed to decrease as tissue integrated into the pores. Conclusion: Mass transport is key to successful tissue engineered platforms. Robust tissue integration, particularly at the critical implant-tissue interface, makes open interconnected-pore pHEMA an excellent morphology for scaffolds in regenerative medicine. Grant Funding Source: Supported by DARPA grants W911NF-11-1-0119 and HR0011-13-2-0003 and Duke Univ Photonics Institute
Continuous glucose monitoring is crucial to developing a successful artificial pancreas. However,... more Continuous glucose monitoring is crucial to developing a successful artificial pancreas. However, biofouling and host response make in vivo sensor performance difficult to predict. We investigated changes in glucose diffusivity and sensor response of optical enzymatic glucose sensors due to biological exposure. Three hydrogel materials, poly(2-hydroxyethyl methacrylate) (pHEMA), poly(acrylamide) (pAM), and poly(2-hydroxyethyl methacrylate)-co-poly(acrylamide) (p(HEMA-co-AM)), were tested for glucose diffusivity before and after exposure to serum or implantation in rats for 1 month. Luminescent sensors based on these materials were measured to compare the response to glucose before and after serum exposure. Glucose diffusivity through the pHEMA [(8.1 ± 0.38) × 10(-8) cm(2)/s] slabs was much lower than diffusivity through pAM [(2.7 ± 0.15) × 10(-6) cm(2)/s] and p(HEMA-co-AM) [(2.5 ± 0.08) × 10(-6)]. As expected from these differences, sensor response was highly dependent on material type. The pHEMA sensors had a maximum sensitivity of 2.5%/(mg/dl) and an analytical range of 4.2-356 mg/dl, while the p(HEMA-co-AM) sensors had a higher sensitivity [14.9%/(mg/dl)] and a narrower analytical range (17.6-70.5 mg/dl). After serum exposure, the pHEMA sensors were unaffected, whereas the p(HEMA-co-AM) sensors exhibited significantly decreased sensitivity and increased analytical range. Decreases in glucose diffusivity in the polymers resulting from in vitro serum exposure and residence in vivo were shown to be similar, suggesting that serum incubation was a reasonable approximation of in vivo fouling. While biofouling is expected to affect the response of flux-based sensors, we have shown that this depended on the type of sensor and matrix used. Therefore, proper design and materials selection may minimize response alterations occurring upon implantation.
Abbreviation: (ANOVA) analysis of variance, (BMI) body mass index, (DM) diabetes mellitus, (GLUT)... more Abbreviation: (ANOVA) analysis of variance, (BMI) body mass index, (DM) diabetes mellitus, (GLUT) glucose transporter, (sc) subcutaneous, (SEM) standard error of the mean, (T1DM) type 1 diabetes mellitus, (T2DM) type 2 diabetes mellitus Abstract Objective:
Poly (acrylic acid) is considered to be an important mucoadhesive for controlled release applicat... more Poly (acrylic acid) is considered to be an important mucoadhesive for controlled release applications. Yet, the specific mechanisms responsible for its bioadhesive behavior are not well understood. Attenuated total reflection infrared spectroscopy ( ATR-FTlR) was developed for investigation of chain interpenetration at a poly (acrylic acid) (PAA) and mucin interface. A thin film of acrylic acid, polymerized below the gelation point, was cross-linked directly on an ATR crystal. The cross-linked PAA film was contacted with a buffered mucin solution and the ATR-FTIR spectrum was collected in situ as a function of time. The experimental results show evidence in support of chain interpenetration at the PAA/mucin interface. The experimental results indicate that as PAA and mucin are compatible in the 5-7 pH range, an important mechanism of mucoadhesion is the swelling of PAA by mucin and adhesion is limited by the extent of chain interpenetration across the biointerface.
Membrane biofouling and tissue changes in the foreign body response are known to cause detrimenta... more Membrane biofouling and tissue changes in the foreign body response are known to cause detrimental reductions of analyte transport into implanted biosensors. The relative contribution of each phenomenon is unknown. Hollow fiber microdialysis probes were employed to assess the effect of subcutaneous implantation on glucose flux through polymeric membranes in rats over 8 days and to differentiate the transport effects of biofouling versus tissue changes. Three commercially available membranes were examined: poly(ether sulfone) (PES), polyacrylonitrile (PAN), and polycarbonate (PC). As measured by glucose recovery (the ratio of microdialysis glucose to blood glucose concentrations), transport through PES membranes was significantly less on day 2 than day 0 (39% decrease, p < 0.05) whereas PAN and PC showed no significant decreases in flux until day 8 (42 and 43%, respectively). Application of a transport model to glucose recovery data obtained before implantation in vivo and after explantation indicated that mass transport resistances originating from biofouling and tissue compartments increased between days 0 and 8. However, on average the biofouling layer adherent to the probe created substantially less resistance to glucose transport (12-24% of total) than did the tissue that surrounded the probe. These results suggested that future material developments for biosensors should be directed at understanding and modifying transport properties of tissues at the implant site.
In vitro microdialysis recoveries of glucose were evaluated and optimised by varying the inner ca... more In vitro microdialysis recoveries of glucose were evaluated and optimised by varying the inner cannula dimensions for a concentric type microdialysis probe. The recovery was shown to be significantly dependent on the outer radius of the inner cannula (r alpha). Increasing r alpha by 50% while holding all other microdialysis variables constant caused a 26% increase in recovery under the conditions tested. Mass transport modeling of microdialysis sampling showed that the magnitude of the recovery change in response to changes in r alpha may depend on other experimental variables such as effective dialysis length, membrane dimensions, membrane type and medium composition. These data indicate that microdialysis recovery may be substantially improved by optimising the outer diameter of the inner cannula. These data also suggest that in order to fully characterise and compare data from microdialysis experiments, r alpha should become one of the reported variables.
Journal of Diabetes Science and Technology, May 1, 2011
Nearly all commercially available glucose sensors share the subcutaneous interstitial fluid (ISF)... more Nearly all commercially available glucose sensors share the subcutaneous interstitial fluid (ISF) compartment as their preferred implantation site. However, ISF physiology as it relates to glucose sensors is not well understood. This special symposium titled “Interstitial Fluid Physiology as It Relates to Glucose Monitoring Technologies” is intended to help to bridge the gap in our understanding. This symposium is intended to foster a greater understanding of biological factors that impact the success of implantable glucose monitors and to inspire additional research in the area of ISF physiology as it relates to glucose sensing. Recognition that sensor designers need to have an intimate understanding of the biological environment in which their sensor will reside is emphasized. The symposium is published in two parts, with part I published in September 2010 and part II published in May 2011. All articles published in this symposium are summarized herein.
The rat is commonly used to evaluate physiological responses of subcutaneous tissue to implanted ... more The rat is commonly used to evaluate physiological responses of subcutaneous tissue to implanted devices. In vivo longevity of various devices and the biocompatibility of biomaterials de pend on how adjacent tissue interacts. How closely the rat model predicts the human response has not been well characterized. The objective of this study was to compare rat and human subcutaneous foreign body responses by monitoring the biochemical environment at a polymer-tissue interface over eight days using microdialysis. Polyamide microdialysis probes were implanted subcutaneously in humans and rats (n=12). Daily microdialysis samples were analyzed for glucose, lactate, pyruvate, glycerol and urea. Blood glucose was also monitored. Analyte concentrations differed significantly between rats and humans at the implant-tissue interface. There were also qualitative differences in the eightday trends. For example, over eight days, microdialysate glu cose increased 2 -4 fold in humans, but decreased in rats (p<0.001). This study reveals profound physiological differences at material-tissue interfaces in rats and humans and highlights the need for caution when extrapolating subcutaneous rat biocompatibility data to humans.
American journal of physiology. Endocrinology and metabolism, 2002
The rat is commonly used to evaluate physiological responses of subcutaneous tissue to implanted ... more The rat is commonly used to evaluate physiological responses of subcutaneous tissue to implanted devices. In vivo longevity of various devices and the biocompatibility of biomaterials depend on how adjacent tissue interacts. How closely the rat model predicts the human response has not been well characterized. The objective of this study was to compare rat and human subcutaneous foreign body responses by monitoring the biochemical environment at a polymer-tissue interface over 8 days using microdialysis. Polyamide microdialysis probes were implanted subcutaneously in humans and rats (n = 12). Daily microdialysis samples were analyzed for glucose, lactate, pyruvate, glycerol, and urea. Blood glucose was also monitored. Analyte concentrations differed significantly between rats and humans at the implant-tissue interface. There were also qualitative differences in the 8-day trends. For example, over 8 days, microdialysate glucose increased two- to fourfold in humans but decreased in ra...
Journal of biomedical materials research, Jan 15, 2001
Membrane biofouling and tissue changes in the foreign body response are known to cause detrimenta... more Membrane biofouling and tissue changes in the foreign body response are known to cause detrimental reductions of analyte transport into implanted biosensors. The relative contribution of each phenomenon is unknown. Hollow fiber microdialysis probes were employed to assess the effect of subcutaneous implantation on glucose flux through polymeric membranes in rats over 8 days and to differentiate the transport effects of biofouling versus tissue changes. Three commercially available membranes were examined: poly(ether sulfone) (PES), polyacrylonitrile (PAN), and polycarbonate (PC). As measured by glucose recovery (the ratio of microdialysis glucose to blood glucose concentrations), transport through PES membranes was significantly less on day 2 than day 0 (39% decrease, p < 0.05) whereas PAN and PC showed no significant decreases in flux until day 8 (42 and 43%, respectively). Application of a transport model to glucose recovery data obtained before implantation in vivo and after e...
The material-tissue interaction that results from sensor implantation is one of the major obstacl... more The material-tissue interaction that results from sensor implantation is one of the major obstacles in developing viable, long-term implantable biosensors. Strategies useful for the characterization and modification of sensor biocompatibility are widely scattered in the literature, and there are many peripheral studies from which useful information can be gleaned. The current paper reviews strategies suitable for addressing biofouling, one aspect of biosensor biocompatibility. Specifically, this paper addresses the effect of membrane biofouling on sensor sensitivity from the standpoint of glucose transport limitations. Part I discusses the in vivo and in vitro methods used to characterize biofouling and the effects of biofouling on sensor performance, while Part II presents techniques intended to improve biosensor biocompatibility.
A microdialysis method for monitoring permeability changes of implanted biosensor membranes was d... more A microdialysis method for monitoring permeability changes of implanted biosensor membranes was demonstrated in vivo. Glucose diffusion through three different hollow fiber membranes (polyethersulfone (PES), polyacrylonitrile (PAN), polycarbonate (PC)) was assessed over 8 days in rat subcutaneous tissue. There was a 77% decrease of transport through the implanted PES membranes after two days; whereas decreases in PAN and PC were
ABSTRACT Purpose: To maximize mass transport into biomaterial scaffolds for regenerative medicine... more ABSTRACT Purpose: To maximize mass transport into biomaterial scaffolds for regenerative medicine, in vivo diagnostics, therapeutics, and cell delivery. We utilized matrix morphology to encourage long-term tissue integration. Methods: We implanted 1cm-diameter poly-hydroxyethylmethacrylate (pHEMA) disks with 40 and 80µm nominal interconnected pores into rat subcutis. Solid pHEMA, silicone, and cotton disks were also implanted. Microvessel density and total cellularity were histologically quantified in 50µm-wide zones both into the implants and into the adjacent tissues. Results: One week after implantation, the microvessel density closest to the interface was 74±8/mm2 (mean±SEM) for the 80µm pHEMA and 28±8/mm2 for the 40µm pHEMA. The rate of vascularization was greater in 80µm pHEMA, with higher vascular density in the material and adjacent tissues one week and one month post-implantation (p&lt;0.001). After two months, vascular ingrowth was similar for both 40 and 80µm pHEMA (maximum = 155±32/mm2 and 182±43/mm2, respectively). Solid pHEMA and silicone exhibited no vascular ingrowth. At two months, the microvessel density in the 0-50µm adjacent tissue was 203±11/mm2, compared to 101±15/mm2 for the 40 µm pHEMA; 60±16/mm2 for the solid pHEMA; and 24±7/mm2 for silicone. All materials except 80µm pHEMA showed a narrow margin of significantly reduced vascularity at the implant-tissue interface. Total cellularity in the 0-50µm tissue adjacent to 80µm pHEMA was 7736±1011/mm2 after one week; 7582±1129/mm2 after one month; and 4498±497/mm2 after two months. Using real-time ultrasonic microscopy, the microshearing that can disrupt microvessel formation was observed to decrease as tissue integrated into the pores. Conclusion: Mass transport is key to successful tissue engineered platforms. Robust tissue integration, particularly at the critical implant-tissue interface, makes open interconnected-pore pHEMA an excellent morphology for scaffolds in regenerative medicine. Grant Funding Source: Supported by DARPA grants W911NF-11-1-0119 and HR0011-13-2-0003 and Duke Univ Photonics Institute
Continuous glucose monitoring is crucial to developing a successful artificial pancreas. However,... more Continuous glucose monitoring is crucial to developing a successful artificial pancreas. However, biofouling and host response make in vivo sensor performance difficult to predict. We investigated changes in glucose diffusivity and sensor response of optical enzymatic glucose sensors due to biological exposure. Three hydrogel materials, poly(2-hydroxyethyl methacrylate) (pHEMA), poly(acrylamide) (pAM), and poly(2-hydroxyethyl methacrylate)-co-poly(acrylamide) (p(HEMA-co-AM)), were tested for glucose diffusivity before and after exposure to serum or implantation in rats for 1 month. Luminescent sensors based on these materials were measured to compare the response to glucose before and after serum exposure. Glucose diffusivity through the pHEMA [(8.1 ± 0.38) × 10(-8) cm(2)/s] slabs was much lower than diffusivity through pAM [(2.7 ± 0.15) × 10(-6) cm(2)/s] and p(HEMA-co-AM) [(2.5 ± 0.08) × 10(-6)]. As expected from these differences, sensor response was highly dependent on material type. The pHEMA sensors had a maximum sensitivity of 2.5%/(mg/dl) and an analytical range of 4.2-356 mg/dl, while the p(HEMA-co-AM) sensors had a higher sensitivity [14.9%/(mg/dl)] and a narrower analytical range (17.6-70.5 mg/dl). After serum exposure, the pHEMA sensors were unaffected, whereas the p(HEMA-co-AM) sensors exhibited significantly decreased sensitivity and increased analytical range. Decreases in glucose diffusivity in the polymers resulting from in vitro serum exposure and residence in vivo were shown to be similar, suggesting that serum incubation was a reasonable approximation of in vivo fouling. While biofouling is expected to affect the response of flux-based sensors, we have shown that this depended on the type of sensor and matrix used. Therefore, proper design and materials selection may minimize response alterations occurring upon implantation.
Abbreviation: (ANOVA) analysis of variance, (BMI) body mass index, (DM) diabetes mellitus, (GLUT)... more Abbreviation: (ANOVA) analysis of variance, (BMI) body mass index, (DM) diabetes mellitus, (GLUT) glucose transporter, (sc) subcutaneous, (SEM) standard error of the mean, (T1DM) type 1 diabetes mellitus, (T2DM) type 2 diabetes mellitus Abstract Objective:
Poly (acrylic acid) is considered to be an important mucoadhesive for controlled release applicat... more Poly (acrylic acid) is considered to be an important mucoadhesive for controlled release applications. Yet, the specific mechanisms responsible for its bioadhesive behavior are not well understood. Attenuated total reflection infrared spectroscopy ( ATR-FTlR) was developed for investigation of chain interpenetration at a poly (acrylic acid) (PAA) and mucin interface. A thin film of acrylic acid, polymerized below the gelation point, was cross-linked directly on an ATR crystal. The cross-linked PAA film was contacted with a buffered mucin solution and the ATR-FTIR spectrum was collected in situ as a function of time. The experimental results show evidence in support of chain interpenetration at the PAA/mucin interface. The experimental results indicate that as PAA and mucin are compatible in the 5-7 pH range, an important mechanism of mucoadhesion is the swelling of PAA by mucin and adhesion is limited by the extent of chain interpenetration across the biointerface.
Membrane biofouling and tissue changes in the foreign body response are known to cause detrimenta... more Membrane biofouling and tissue changes in the foreign body response are known to cause detrimental reductions of analyte transport into implanted biosensors. The relative contribution of each phenomenon is unknown. Hollow fiber microdialysis probes were employed to assess the effect of subcutaneous implantation on glucose flux through polymeric membranes in rats over 8 days and to differentiate the transport effects of biofouling versus tissue changes. Three commercially available membranes were examined: poly(ether sulfone) (PES), polyacrylonitrile (PAN), and polycarbonate (PC). As measured by glucose recovery (the ratio of microdialysis glucose to blood glucose concentrations), transport through PES membranes was significantly less on day 2 than day 0 (39% decrease, p < 0.05) whereas PAN and PC showed no significant decreases in flux until day 8 (42 and 43%, respectively). Application of a transport model to glucose recovery data obtained before implantation in vivo and after explantation indicated that mass transport resistances originating from biofouling and tissue compartments increased between days 0 and 8. However, on average the biofouling layer adherent to the probe created substantially less resistance to glucose transport (12-24% of total) than did the tissue that surrounded the probe. These results suggested that future material developments for biosensors should be directed at understanding and modifying transport properties of tissues at the implant site.
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Papers by N. Wisniewski