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Biomedical subjects

A Bolz

Publications and source records attributed to A Bolz.

At least 73 records · Page 4Linked to original sources
PubMed1996

Coating of cardiovascular stents with a semiconductor to improve their hemocompatibility.

Thrombogenesis on artificial surfaces can be described as an electron transfer reaction. To inhibit this oxidation process and thereby achieve high hemocompatibility, a surface requires semiconducting properties. However, because all known solid semiconductors are extremely brittle, the requirements for high hemocompatibility and mechanical stability cannot be met by any single material. Therefore, a hybrid design is introduced as a new approach to improve the hemocompatibility of cardiovascular stents.

Angiography↗
PubMed1995

[Measurement and analysis of monophasic action potentials using fractally coated electrodes--I].

The monophasic action potential (MAP) contains a wealth of information about the stat of the myocardium, which makes it very useful for numerous diagnostic and therapeutic applications in patients with heart disease. The silver-silver chloride electrodes which are currently used for the measurement of MAP have poor long-term stability in contact with biological tissue. This study was therefore undertaken with the aim of investigating the electrochemical behaviour of fractally coated leads in terms of their signal-detection performance. Experience gained with these leads in cardiac pacemakers has already demonstrated the long-term stability and biocompatibility of the fractally coated leads. Present results show that, due to their large electrochemically active surface area, fractally coated leads have a very low impedance over a wide frequency range. The negligible polarization artifact of these leads permits the measurement of cardiac potentials immediately after a stimulus. Fractally coated leads are thus highly suitable for the measurement of MAP, and have clear advantages over Ag/AgCl electrodes. The second part of this study reports on the results of MAP measurements using fractally coated leads.

Electrocardiography↗
PubMed1995

[Measurement and analysis of monophasic action potentials using fractally coated electrodes--II].

The monophasic action potential (MAP) represents a summed signal formed by overlapping action potentials of myocardial cells close to the tip of the lead. Analysis of the MAP therefore provides detailed information about the electrophysiological effects of autonomous nervous and pharmacological influences on the myocardium, for example adrenergic or cholinergic stimulation of the heart. All known MAP recordings were obtained with Ag/AgCl electrodes, which, thanks to their low polarization properties, ensure reliable MAP measurement. Owing to their toxicity and inadequate long-term stability, however, Ag/AgCl electrodes cannot be implanted. With the aim of making MAP measurement available for implantable devices, fractally coated leads were therefore developed. The aim of the present study was to evaluate the in vivo measurement of fractally coated leads which are characterized by negligible polarization, low impedance over a wide frequency range, high biocompatibility and good long-term stability. In addition, as a result of their extremely high Helmholtz capacities (up to 50 mF/cm2), fractally coated leads permit stimulation and virtually undisturbed recording of MAP with the same pair of electrodes. For the evaluation of MAP measurements with fractally coated leads, a quadrupolar catheter enabling simultaneous MAP recordings with 2 Ag/AgCl electrodes and 2 fractally coated leads was devised. The stimulation pulses were always applied via the fractally coated leads. With both types of electrode, with spontaneous excitation and stimulation, the well-known MAP morphology, with amplitudes of between 10 and 25 mV in the ventricle, and between 5 and 10 mV in the atrium, was seen.(ABSTRACT TRUNCATED AT 250 WORDS)

Atropine↗
PubMed1993

Haemocompatibility optimisation of implants by hybrid structuring.

State of the art in biomaterial research and implant design is a compromise between functionality and biocompatibility. Consequently, results often have disadvantages with respect to both aspects. With regard to biocompatibility, the activation of the clotting system by alloplastic materials is of great significance, because it necessitates anticoagulant therapy. Further improvements in implant technology require an understanding of the interactions between blood and implants. Therefore a microscopic model of thrombogenesis at alloplastic surfaces is briefly presented, relating thrombogenicity of a material to the electronic structure of its surface. The electronic requirements for high haemocompatibility, which result from this model (especially a low band-gap density of states and a high surface conductivity) are fulfilled by an amorphous alloy of silicon and carbon (a-SiC:H). The advantage of amorphous materials is that they do not obey stoichiometric rules. Thus they allow a continuous adjustment of the electronic parameters without fundamental changes in their mechanical and chemical properties. The theoretical results were checked in vitro by total internal reflection intrinsic fluorescence (TIRIF) spectroscopy as well as thrombelastography experiments (TEG). In comparison with conventional materials such as titanium or LTI carbon, the TEG-clotting time of a-SiC:H-coatings was prolonged by in excess of 200 per cent. As a consequence, a-SiC:H is well suited as a haemocompatible coating material for hybrid structuring of cardiovascular implants.

Biocompatible Materials↗
PubMed1993

Low polarization pacing lead for detecting the ventricular-evoked response.

The cardiac response to a pacing pulse is potentially useful for rate adaptive pacemakers and threshold tracking systems. However, until now capture recognition of the ventricular-evoked response by the use of a single electrode for stimulation as well as detection was limited by the electrode polarization. Electronic measures against the stimulus polarization artifact have not been successful due to the variability of the after potential or the requirement of additional battery power. Following the idea of Lewin, Myers and Parsonnet, who introduced the idea of a non-polarizable porous electrode for physiological stimulation, titanium nitride (TiN) and iridium (Ir) coatings with fractal surface structure have been developed with high electro-chemical active surface areas and Helmholtz double-layer capacities of up to 50,000 microF/cm2, thus reducing the polarization artifact significantly. Two types of endocardial leads (10 with a fractal TiN coating and 5 with a fractal Ir coating) were implanted in the apex of the right ventricle and the polarization artifact, as well as the evoked response, was measured. Both types of pacing leads show a 90% reduction in the polarization artifact in comparison to conventional leads. If an autoshort of approximately 20 to 50 ms is applied after the pacing pulse, the polarization artifact of these leads is negligible, thus enabling reliable detection of at least the repolarization phase of the ventricular-evoked response, which is fully sufficient for capture recognition. Additionally, due to their low polarization losses, TiN- or Ir-coated electrodes with fractal surface structure have a unique stimulation and detection performance.

Aged↗
PubMed1992

[Physical mechanisms of solid-protein interactions in the interface between amorphous silicon carbide and fibrinogen].

State of the art in biomaterial research and implant design is a compromise between functionality and biocompatibility. Consequently the results often have disadvantages with respect to both aspects. In regard to biocompatibility the activation of the clotting system by alloplastic materials is of great significance, because it necessitates anticoagulant therapy. Further improvements of implant technology require an understanding of the interactions between blood and implants. Therefore a microscopic model of thrombogenesis at alloplastic surfaces will shortly be presented, which relates thrombogenicity of a material to the electronic structure of its surface. The requirements for high hemocompatibility, which result from this model--especially in regard to the density of states and the conductivity at the surface--are fulfilled by an amorphous alloy of silicon and carbon (a-SiC:H). The advantage of amorphous materials is that they do not obey stoichiometric rules. Thus they allow a continuous adjustment of the electronic parameters without fundamental changes of their mechanical and chemical properties. The theoretical results where checked by total internal reflection intrinsic fluorescence spectroscopy (TIRIF) as well as thrombelastography experiments (TEG). In comparison to conventional materials like titanium or LTI carbon the TEG-clotting time of a-SiC:H-coatings is prolonged in excess of 200%. As a consequence a-SiC:H is well suited as a hemocompatible coating material for hybrid structuring of cardiovascular implants.

Biocompatible Materials↗
PubMed1990

Artificial heart valves: improved blood compatibility by PECVD a-SiC:H coating.

Implants are steadily increasing in importance as substitutions for body functions. With the present state of the art, the limitations of the application of cardiovascular implants are due to insufficient performance of biomaterials. Present research in this field is being concentrated on efforts to improve the thrombus resistance of conventional materials by coating with semiconducting materials to actively influence the electrochemical interaction between the condensed matter and blood proteins. Based on an electrochemical model of the interaction of fibrinogen with an artificial surface and the resulting requirements for improving hemocompatibility, a coating of amorphous hydrogenated silicon carbide deposited by plasma-enhanced chemical vapor deposition (PECVD) is presently under evaluation as a special coating material for cardiovascular prostheses and is herein described. In particular, first results are published concerning the optimum deposition parameters in the PECVD process and cell culture tests. Experimental results of comparative partial thromboplastin time studies serve the purpose of proving the validity of the electrochemical reaction model referring the hemocompatibility of implantable materials to their semiconducting surface properties. The aim of this article is to demonstrate a feasible method for an antithrombogenic surface modification based on doped amorphous silicon carbide films that is in full conformance to the above mentioned model.

Biocompatible Materials↗
PubMed1990

[Mechanical aspects of the development of artificial heart valves].

The development of an antithrombogenic coating permits a hybrid design for artificial heart valves. A substrate material optimized for its application is coated to meet the electrochemical requirements of improved hemocompatibility. But future progress in artificial heart valves requires an improvement in design as well as of the material. The basis of both aspects is the determination of such fundamental mechanical properties as the elasticity and plasticity of the valve ring and the deformation and fraction behaviour of the occluder. Analytical and numerical calculations of various different models result in different requirements for the substrate of ring and occluder. A combination of high elastic temper and low resistance to flow requires a ring material with a Young's modulus of 40 GPa or more, and a 0.2% proof stress to (Young's modulus)2/3 ratio of 0.3 MPa1/3. The best occluder materials should have a Young's modulus of more than 50 GPa and a flexural strength of at least 800 MPa. On the basis of these criteria, a heart valve consisting of a TiA15Fe2,5 ring and occluders made partially stabilized zirconia is introduced.

Biomechanical Phenomena↗