Alexander Kros

Conducting polymers with confined dimensions : Track-etch membranes for amperometric biosensor applications

Organic conducting polymers can be synthesized inside the pores of a track-etch membrane, and the resulting hollow tubules are shown to have enhanced electrical properties compared to their corresponding bulk materials. The polymerization of monomers (e.g., pyrrole, thiophenes) inside the confined space of these pores, combined with electrostatic interaction, ensures the alignment of the organic polymers on the interior, leading to higher conductivity. The application of these conducting tubes in the development of amperometric glucose sensors is discussed. Due to the special properties of conducting polymers inside a track-etch membrane, biosensors with a unique electron-transfer mechanism have been developed.

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.

A printable glucose sensor based on a poly(pyrrole)-latex hybrid material

A printable glucose sensor was obtained by immobilisation of glucose oxidase onto the surface of poly(pyrrole)-coated latex spheres, which were mixed with a conducting ink. The obtained hybrid material was able to amperometrically detect glucose under aerobic as well as anaerobic conditions, without the use of electron mediators. Since all of the steps involved in the preparation of this latex-poly(pyrrole)-based ink are performed in solution, in-expensive mass production will be possible. A possible mechanism for this sensor is proposed based on the direct communication between the enzyme and the conducting polymer under anaerobic conditions.

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.

Silane-based hybrids for biomedical applications *

In this paper, the preparation of different hybrid silane materials is presented and their possible use in biomedical applications is discussed. The first example describes the development of biocompatible coatings based on sol-gel silicates, which can be used as a protective coating for implantable glucose sensors. Blending the silica with different organic polymers modified the properties of the resulting sol-gel materials. Their biocompatibility, both in vivo and in vitro, and their applications on biosensors were investigated. In the second example, an amphiphilic block copolymer comprising hydrophilic poly(ethylene oxide) blocks and hydrophobic poly(methylphenylsilane) segments is presented. In aqueous medium, this polymer forms vesicles in which a fluorescent dye is encapsulated. It was demonstrated that the vesicle aggregates could be broken up using UV irradiation, indicating that these vesicles were potentially interesting as controlled release systems. Monolayer studies confirmed that after photolytic cleavage of the poly(methylphenylsilane) segments, no organized structures were formed from the remaining material.

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.