The permeability of silicone rubber to metal compounds: relevance to implanted devices.
Most implanted electrical devices use encapsulant as insulation. The encapsulant may remain functional for many years, bonded to the metallic surfaces, but eventually become partly detached allowing corrosion to occur. To understand whether the corrosion products will cause toxic effects, we need to know how quickly they will permeate through the encapsulant. In these experiments, silicone capsules (the encapsulant) containing metal compounds were left in jars of initially pure water for 6 months, and the concentration of the metal in the water was measured. The amount of metal depended on the type of compound; for the organometallic compounds tested, permeation was very rapid. However, for most of the other compounds, whether oxides or salts, the amount of metal was below the control level and therefore could have been the result of contamination. These compounds were tin sulfate and oxide (<10²), lead nitrate and oxide (<10²), copper sulfate (<10³), and nitrates of bismuth (<10¹), chrome (<10²), nickel (<10³) and zinc (<10²). The numbers in brackets are the maximum mass (ng) of permeated metal after 6 months. Three silver compounds were tested but without proper controls; however, the amount of permeated silver appeared to be low: silver oxide (1.3 × 10²), silver nitrate (6.3 × 10¹), and silver chloride (6 × 10⁰). The resolution of this method is limited by contamination that is detected by control capsules. The conclusion is that compounds that are likely corrosion products permeate through silicone encapsulant at a low rate and seem unlikely to cause toxic effects.
Department of Medical Physics & Bioengineering, University College London, London WC1E 6BT, United Kingdom. nick@medphys.ucl.ac.uk
Nick
N
Donaldson
Preeti
P
Baviskar
Jim
J
Cunningham
Darren
D
Wilson
D020099:Coated Materials, Biocompatible; D006801:Humans; D057230:Limit of Detection; D008422:Materials Testing; D008670:Metals / Q000737:chemistry*; D010539:Permeability; D019736:Prostheses and Implants; D011475:Prosthesis Failure; D012826:Silicone Elastomers / Q000737:chemistry*; D012996:Solutions / Q000737:chemistry
D020099:Coated Materials, Biocompatible; D008670:Metals; D012826:Silicone Elastomers; D012996:Solutions