RESUMO
Silicone rubber is not, at first sight, a promising material with which to encapsulate a piece of implantable microelectronics expected to last ten or twenty years. It is well known to be very permeable to water. In fact, if correctly applied, it performs very well for ten years, possibly longer. The paper considers the part played by osmosis in achieving satisfactory performance combined with simple technology.
Assuntos
Engenharia Biomédica/métodos , Próteses e Implantes , Elastômeros de Silicone , Eletrônica Médica , Desenho de Equipamento , OsmoseRESUMO
The durability of the adhesion, in the presence of water, of three two-part silicone rubbers to five adherends of practical interest is examined. The results are contrasted with those obtained, in previous work, from single-part material. It is concluded that two-part rubbers promise outstanding performance in particular circumstances.
Assuntos
Desfibriladores Implantáveis , Elastômeros de Silicone , Adesividade , Desenho de PróteseRESUMO
The paper presents some measurements of the hydrothermal stability of experimental adhesive joints in water at 100 degrees C. The joints are between one adhesive silicone rubber and ten metal or metal-oxide adherends, all combinations of interest to the neurological prosthesis maker. The probable adhesion mechanism is then considered, in the search for some parameter by which the experimental results could have been predicted. Evidence is produced that physical adsorption plays little or no part in the adhesion, but that hydrothermal stability seems to be a function of the adherend ionic charge. In pursuit of this idea, the valency, the Slater potential and the iso-electric point for the surface (IEPS) of the adherend are examined as possible prediction parameters. It is concluded that, at least in neutral and acidified water, the IEPS is a promising predictor, with the benefit that it is experimentally determinable for adherends of unknown composition. The most stable joints seem to occur when the charge density on the adherend has an optimum negative value.
Assuntos
Metais , Óxidos , Próteses e Implantes , Elastômeros de Silicone , Adesividade , Materiais Biocompatíveis , Humanos , NeurologiaRESUMO
The Note describes a procedure for encapsulating the implantable microelectronics in an air-curing silicone rubber, by which the shape of the casting is fully defined, and cure is achieved within an acceptable time.
Assuntos
Eletrônica Médica/instrumentação , Desenho de Prótese , Elastômeros de Silicone , Engenharia Biomédica , Humanos , Procedimentos NeurocirúrgicosRESUMO
The simplest kind of radio frequency (RF) coupled neurological prosthesis, in which a burst of RF, generated by the external equipment, provides the stimulus energy via a passive implanted receiver, is the most troublesome in respect of RF interference (RFI). Because the low ratio of pulse duration to interpulse interval is usually the case, the peak power transmitted is necessarily high, of the order of watts. This paper outlines a series of measurements by which the mode of RFI generation by a typical external equipment was precisely identified, enabling rational and appropriate countermeasures to be taken.
Assuntos
Próteses e Implantes , Ondas de Rádio , Auxiliares Sensoriais , Estimulação Elétrica , Fenômenos Eletromagnéticos , Desenho de PróteseRESUMO
In a programme to understand, and learn to control, radiofrequency interference from neurological prostheses, it was discovered that a harmless-looking Hartley oscillator (one type of harmonic oscillator) generated a forest of harmonics extending to 1 GHz. These harmonics were not difficult to eliminate. The point of this paper is to emphasize that, given the selection of instruments used by most development engineers, the harmonics would probably have been missed.
Assuntos
Próteses e Implantes , Ondas de Rádio , Auxiliares Sensoriais , Desenho de PróteseRESUMO
The electrodes of an implanted stimulator must be operated in such a way that the task of converting an electron current in metal to an ion current in tissue is achieved without release of noxious chemical species into the tissue, and without significant corrosion of the electrode metal. Both these effects depend on the charge density at which the electrodes are operated. In bringing a newly designed multichannel stimulator into service, the charge densities can only be known when the operating stimulus strengths have been determined. Even then, unless the stimulator is arranged to telemeter out what it is doing, any charge-density figures obtained have the character of estimates, rather than measurements. The first London Mk. V stimulator is used to enable a paraplegic woman to stand up. This note provides estimates of the average charge density and corrosion rate for the Pt--Ir electrodes used. These fall within limits which we believe to be generally accepted as safe.