Effect of the metallic seal of a hermetic enclosure on the induction of power to an implant.

Most neuroprostheses which use integrated circuits protect these chip components in a hermetic package. No satisfactory method of sealing such an enclosure has been found which does not use metals. Therefore, in general, the seal is an electrically conducting ring. If induction is used to supply power to this device, this ring will be a 'short-circuited turn' which will affect the performance of the inductive link. In the paper, theory is presented for a model in which the metallic seal is a tertiary inductance, coupled to the primary and secondary. The tertiary has finite Q. Equations for this model are given from which formulae for the gain and efficiency are derived for the particular condition of tuning that the carrier frequency equals the resonant frequency of both the primary and secondary circuits. From the formulae, gain curves are plotted which show how the seal affects the link. However, it is clear that general solutions to the problem are needed if the theory is to be of practical use.