Coil transforms human implants

Coil transforms human implantsSteve Bush
Increasingly, electrical devices are being implanted into people. Some of the more ambitious in-body systems, including artificial hearts, are too power-hungary to be powered from batteries and have to be supplied from outside the body.  
  Body talk… Research team’s transcutaneous transformer can transmit 20W of power at 90 per cent efficiency.
One option for getting the power in is to run a cable right through the skin. This is possible, but infection is a constant threat.
Transformers, with one coil outside the body and one coil inside, avoid the infection problem. Unfortunately, coupling efficiency is compromised because the skin is thick, between 5 and 15mm.
In a paper submitted to the IEE’s Electronic Letters (volume 35, number 2), a team from the Nanyang Technological University in Singapore has described a transcutaneous transformer which can transmit 20W of power at 90 per cent efficiency.
The transformer employs an unusual geometry (see diagram). The primary core is ferrite around which a 21 turn coil is wound. The secondary core is made from amorphous metal ribbon and has 27 turns.
Litz wire, made from many thin insulated strands, is used for both windings to reduce losses due to the skin effect.
An external 100kHz oscillator provides power which is rectified inside the body. The coupling coefficient between the coils is 0.65 with a 5mm skin thickness.

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