M. Gerritsen

Performance of subcutaneously implanted glucose sensors for continuous monitoring

Despite a considerable amount of research attributed to the development of an implantable glucose sensor, to date there is no clinically applicable concept for continuous glucose monitoring. Investigations to validate the subcutaneous tissue for continuous glucose sensing mostly comprised short-term implantations of glucose sensors. Most implanted glucose sensors showed a significant drift in sensitivity over the implantation period. This bioinstability was not to be expected from the in vitro performance of the sensors. In this paper, the influence of possible failure mechanisms on the poor in vivo performance of subcutaneously implanted glucose sensors is reviewed.

Influence of inflammatory cells and serum on the performance of implantable glucose sensors

The objective of this investigation was to evaluate the influence of polymorphonuclear granulocytes on the performance of uncoated and cellulose acetate/Nafion coated amperometric glucose sensors in vitro. The response of these sensors was also investigated in serum. Uncoated and coated sensors showed lower sensitivities to glucose, with a significant drift in sensor output upon exposure to serum or leukocytes. Although the use of a coating resulted in higher sensitivity, the progressive loss of output was not completely prevented. Stimulated granulocytes were shown to excrete components, probably catalase and myeloperoxidase, which consumed the hydrogen peroxide formed by the oxidation of glucose. In addition, adsorbed serum proteins formed a diffusional barrier for glucose. Furthermore, serum was found to contain low-molecular weight components that alone inhibited glucose oxidase activity. Based on preliminary electrochemical results, we postulate that rabbit serum contains oxidizing substrates that compete with molecular oxygen for the acceptance of electrons from the oxidized enzyme. Consequently, future efforts should be aimed at elucidating the mechanisms involved in the interference of unknown serum components with electron transfer. In addition, further investigations have to be performed to develop an outer membrane that minimizes protein adsorption as well as the actions of inflammatory cells.

Silica-based hybrid materials as biocompatible coatings for glucose sensors

The preparation of sol–gel silica-based biocompatible coatings, which can be used for future implantable glucose sensors is described. Tetraethylorthosilicate (TEOS) was used as precursor for water-borne silicate gels of which the properties were varied by mixing the sol with polyethylene glycol (SG-PEG), heparin (SG-HEP), dextran sulfate (SG-DS), nafion (SG-NAF) or polystyrene sulfonate (SG-PSS). The toxicity of the coatings was examined in vitro using human dermal fibroblasts. All materials showed to be non-toxic and the cell proliferation rate of fibroblasts was found to be dependent on the additive. Glucose measurements using glucose oxidase-based sensors coated with the different hybrid films were performed both in buffered solutions containing bovine serum albumin and in serum. Stable glucose responses were obtained for the coated sensors in both media. The SG-DS containing coating appeared to be most promising for future in vivo glucose measurements.

Biocompatibility evaluation of sol}gel coatings for subcutaneously implantable glucose sensors

The objective of the current investigation is to determine the soft-tissue biocompatibility of sol-gel matrices which can be used to optimize the properties of implantable glucose sensors. The biocompatibility of sol-gel matrices with heparin, dextran sulphate, Nafion, polyethylene glycol, and polystyrene sulphonate was examined in vitro in simulated body fluid and with cell culture experiments using human dermal fibroblasts. Finally, an in vivo study was performed. Therefore, sol-gel coated polystyrene discs were inserted subcutaneously in the back of rabbits. After 4 and 12 weeks, the implants with surrounding tissue were retrieved and processed histologically. In simulated body fluid, the formation of a granular calcium phosphate precipitate was observed. Cell proliferation on polyethylene glycol, Nafion, and heparin coated substrates was comparable to control samples and significantly higher than on dextran sulphate and polystyrene sulphate coated substrates. Light microscopic evaluation of the retrieved in vivo samples showed a fair tissue reaction to all materials. Histomorphometric analysis demonstrated that there were no differences in tissue response to the different sol-gel coatings. In conclusion, sol-gel matrices exhibit a fair biocompatibility both in vitro and in vivo. These results will form the basis for further research into the real merits of sol-gel coatings in optimizing the properties of subcutaneously implantable glucose sensors.

Performance of Subcutaneously Implanted Glucose Sensors: A Review

Despite a considerable amount of research attributed to the development of an implantable glucose sensor, to date there is no clinically applicable concept for continuous glucose monitoring. Investigations to validate the subcutaneous tissue for continuous glucose sensing mostly comprise short-term implantations of glucose sensors. Most implanted glucose sensors showed a significant decay in sensitivity over the implantation period. This bioinstability was not to be expected from the in vitro performance of the sensors. In this article, the influence of possible failure mechanisms on the poor in vivo performance of subcutaneously implanted glucose sensors is reviewed.

Wound healing around bone-anchored percutaneous devices in experimental diabetes mellitus

The objective of this investigation was to study the influence of impaired wound healing on the tissue response to bone-anchored percutaneous devices. For this reason, diabetes mellitus was induced in rabbits with alloxan. Untreated rabbits were used as controls. Skin-penetrating titanium implants were inserted in the tibial bone of diabetic and healthy animals. The enossal part of half of the implants was provided with a thin magnetron-sputtered calcium-phosphate coating. The soft-tissue and bone response was evaluated clinically, histologically, and histomorphometrically. We did not observe more infectious complications in diabetic animals. Furthermore, histological analysis revealed no differences in soft-tissue response between diabetic and healthy animals. A close bone-implant contact was observed for all implants. Nevertheless, the density of cortical bone around the implants was clearly lower in diabetic animals compared with control animals. In control rabbits, but not diabetic animals, coated implants showed more downgrowth of bone into the marrow cavity than uncoated ones. In general, diabetes mellitus was shown to have no adverse effect on the clinical performance of the percutaneous devices. We think that this is due to the good fixation of the implants in diabetic as well as control animals. Therefore, we conclude that the presence of impaired healing in chronic health disorders like diabetes is no contra-indication for the anchorage of percutaneous implants in cortical bone.

The influence of impaired wound healing on the tissue reaction to percutaneous devices using titanium fiber mesh anchorage

The objective of this investigation was to study the influence of impaired wound healing on the soft-tissue response to percutaneous devices using titanium fiber mesh anchorage. For this reason, diabetes mellitus was induced in rabbits with alloxan. Untreated rabbits were used as controls. Two implant types were inserted subcutaneously: two-stage percutaneous devices as well as separate titanium fiber mesh sheets. The soft-tissue response to both implants was assessed by clinical, histologic, and histomorphometric evaluation. Clinically, we observed a higher number of infectious complications around percutaneous implants in diabetic animals. Histologic and histomorphometric analyses revealed that severe diabetes effected matrix maturation and delayed neovascularization (p<0.05). We also observed higher numbers of inflammatory cells in the mesh porosity of percutaneous implants in severely diabetic animals (p = 0.09). Our results indicate that severe, uncontrolled diabetes negatively influences the maturity and neovascularization of connective tissue inside the fiber mesh porosity. The higher number of infectious complications in diabetic animals suggests that the presence of impaired healing conditions facilitates infection around skin penetrating devices.

A percutaneous device as model to study the in vivo performance of implantable amperometric glucose sensors

Glucose kinetics were investigated in subcutaneous tissue of rabbits, in which a percutaneous device was implanted. The device was used for collection of tissue fluid and as carrier of an amperometric glucose sensor. Changes in glycaemia were reflected in subcutaneous tissue fluid. However, a limited number of responses of the implanted sensors were observed. Histologic evaluation showed thin fibrous capsules surrounding the implants. Accumulations of inflammatory cells were observed inside the subcutaneous chamber. The experiments again showed that changes in blood glucose concentration can be measured in subcutaneous tissue fluid collected with a percutaneous device. Nevertheless, implanted glucose sensors could not reliably monitor these changes. Supported by our histological observations and sufficient in vitro performance, we suppose that the cellular reaction to the sensor plays an important role in this poor in vivo performance. In combination with adsorption of tissue fluid proteins, this results in a reversible deactivation of implanted sensors. The exact mechanisms involved in this process are currently unknown and need further investigation.

Biocompatible polystyrenes containing pendant tetra(ethylene glycol) and phosphorylcholine groups

The synthesis and characterization of styrene-based polymers and copolymers containing pendant tetra(ethylene glycol) and phosphorylcholine groups is reported. These polymers are obtained via radical polymerization reactions using α,α′-azobis(isobutyronitrile) as the initiator, and are developed as protective biocompatible coatings for implantable biosensors. Cell morphology studies show that none of the synthesized polymers and copolymers are toxic, and that the rate of cell growth can be tuned by changing the monomer composition. The presence of tetra(ethylene glycol) groups in the coatings lowers the protein adsorption, thereby influencing the rate of cell growth. An equally profound effect is observed when a low percentage of phosphorylcholine groups is present in the polymers.

Evaluation of the tissue reaction to a percutaneous access device using titanium fibre mesh anchorage in goats.

The tissue reaction to a percutaneous access device, applicable as a carrier for an implantable glucose sensor, was evaluated in goats. Titanium fibre mesh structures were used for anchorage of the device in superficial as well as deeper soft-tissue locations. The percutaneous part was subcutaneously anchored with a fibre mesh sheet. The distal part was placed intraperitoneally and anchored in deeper soft-tissue layers using a fibre mesh cuff. All implants showed good healing with the surrounding tissue. Histological evaluation showed that the subcutaneous fibre mesh sheets and peritoneal fibre mesh cuffs were filled with immature connective tissue, generally free of inflammation. Problems concerning disconnection of the silicone catheter from the titanium holding element and filling of part of the peritoneal fibre mesh cuff with silicone glue have to be overcome by more appropriate preclinical testing and improved implant design. Our results demonstrate that titanium fibre mesh structures can be used effectively for soft-tissue anchorage of percutaneous access devices. A sufficient ingrowth of connective tissue was obtained in superficial as well as in deeper soft-tissue layers. The access device could have application as a carrier for an implantable glucose sensor for glucose monitoring in different tissue compartments.