Underwater current leakage between encapsulated NiChrome tracks: Implications for strain-gauges and other implantable devices

Abstract

We present the results of experiments aimed at identifying a suitable polymer for the encapsulation of thin-film strain gauges for underwater applications (with a view of using it in an instrumented bone fusion nail). The leakage currents across grooves cut (using a laser) in thin films of NiChrome over borosilicate glass were studied for encapsulated samples, immersed in water at 37°C. The selected encapsulants were five silicone rubbers (of both medical and engineering grades), produced by Nusil (MED-6015, MED4-4220, MED3-4013, CV14-2500 and EPM-2420) and Elast-Eon™2A, a co-polymer developed by Aortech. The effect of a primer, as well as that of a black dye mixed with the rubber, was also investigated.13% of samples exhibited slow current increases, peaking at 1–4nA, and 9% exhibited brief peaks up to 30nA (only one sample exhibited both). These were due to corrosion of the NiCr following ionic contamination. For the remaining 80%, the leakage current remained remarkably low (<1nA).Silicone rubber encapsulation appears as a realistic low-cost alternative to the hermetic packaging of thin-film strain-gauges. However, this is conditional to achieving a suitable degree of cleanliness of all surfaces prior to encapsulation. Cleaning and rinsing procedures should therefore be evaluated before opting for this method.