Armin Dr Bolz

A FRACTALLY COATED, 1.3 MM2 HIGH IMPEDANCE PACING ELECTRODE

Minimizing the geometric surface area of pacing electrodes increases impedance and reduces the current drain during stimulation, provided that voltage (pulse‐width) thresholds remain unchanged. This may be feasible by coating the electrode surface to increase the capacity of the electrode tissue interface and to diminish polarization. Ten unipolar, tined leads with a surface area of 1.3 mm2 and a “fractal” coating of Iridium (Biotronik SD‐V137) were implanted in the ventricle, and electrogram amplitude (unfiltered), slew‐rate, pacing threshold (0.5 ms), and impedance (2.5 V; 0.5 ms) were measured by the 5311 PSA (Medtronic). On days 0, 2, 5, 10, 28, 90, 180, 360 postimplant, sensing threshold (up to 7.0 mV, measuring range 1–14 mV on day 360 only) and the strength duration curve (0.5–4.0 V; 0.03–1.5 ms; steps: 0.5 V; 0.01 ms, respectively) were determined, the minimum charge delivered per pulse (charge threshold), and the impedance were taken from pacemaker telemetry (Intermedics 294–03). Data were compared with those of an earlier series of 20 unipolar, tined TIR‐leads (Biotronik) with a surface area of 10 mm2 and a “fractal” coating of titanium nitride. With the model SD‐V137 versus TIR, intraoperative electrogram amplitudes were 15.1 ± 6.1 versus 14.4 ± 3.9 mV(NS), slew rates 3.45 ± 1.57 versus 1.94 ± 1.06 V/s (P < 0. 05), pacing thresholds 0.16 ± 0.05 versus 0.52 ± 0.15 V (P < 0.01) and impedance measurements 1,136 ±175 versus 441 ± 73 Ω (P < 0.0001), respectively. During follow‐up, sensing thresholds were the same with both leads. Differences in pulse width thresholds lost its significance on day 28 but resumed on day 360 (SD‐V137: 0.08 ± 0.04 ms; TIR: 0.16 ± 0.06 ms at 2.5 V; P < 0.01). With an electrode surface of 1.3 mm2, charge per pulse and impedance consistently differed from control, beingO.15 ± 0.15 versus 0. 66 ± 0. 20 μC (P < 0.001) and 1,344 ± 376 versus 538 ± 79 Ω, respectively, one year after implantation (P < 0.0001). In summary, “fractally” coated small surface electrodes do not compromise sensing; by more than doubling impedance against controls they offer pacing thresholds (mainly in terms of charge) that are significantly lower than with the reference electrode.

Longterm Stability of TiN

Metallic materials which are used in orthopaedics, heart valves and pacemaker electrodes have to be biocompatible; i.e. they have to avoid the reaction with the surrounding tissue. Especially for long-term stability it is essential to inhibit chemical reactions. Titanium and its compounds like oxides and nitrides are well known to behave biocompatible. Therefore, electrodes with a porous surface were produced by sintering titanium alloy powder or PVD of TiN. The electron conductivity can be further increased by Boron doping. To differentiate between threshold changes resulting from surface oxidation or from fibrotic layer, secondary ion mass spectroscopy (SIMS) was performed. Depth profiles of the oxygen concentration allow the evaluation of the electrode interface characteristics with regard to time dependent on the formation of oxide layers. The results indicate that the high long-term efficiency of titanium compound pacing electrodes is a result of the interface structure, the high dielectric constants of the thin surface oxide layer determines the potential and charge distribution resulting in an improved biocompatibility. Therefore the low stimulation threshold is a good indicator of an improved tissue compatibility inhibiting the protein activation.

Surface Coating of PECVD a-SiC:H to Improve Biocompatibility

Hemocompatibility of a material is essentially determined by the electronic properties of its surface, whereas functionality is provided by the bulk properties. As most materials are not satisfying both requirements, a hybrid design is the only way to solve the material related problems in replacement surgery. In the following a microscopic model of thrombogenesis induced by solid surfaces is shortly reviewed and the electronic requirements for high hemocompatibility are deduced. The physical properties of amorphous silicon carbide (a-SiC:H) concerning the band gap, the density of states and the conductivity show, that a-SiC:H coatings meet these requirements. In vitro measurements (TIRIF and TEG) prove the hemocompatibility of a-SiC:H coated parts. Additionally corrosion tests confirm a good long time behaviour, so that a-SiC:H is well suited as antithrombogenic coating material for implants.