TWI rescue operation

The Medical Research Council (MRC) approached TWI to help with a one-chance, delicate operation to attach a coil to the pin of a feedthrough. These are parts of an experimental femoral prosthesis (artificial bone implant) from which stresses and strains are being measured. The strain gauges and electronics are inside the hollow prosthesis which is made from titanium. Power is supplied by radio-frequency induction to the coil and the signals are returned by 'passive' telemetry.

The coil, made of 0.3mm gold wire, had to be joined to 0.5mm diameter niobium pins. One end of the coil had already been welded using pulsed TIG spot welding. However, welding at the feedthrough end had not run so smoothly and the small arc welding electrode could no longer access the areas to join the components.

TWI chose to use a low power pulsed Nd:YAG laser to carry out the welding operation and trials were carried out to join samples of the gold wire to the niobium pin. However, because of oxide formation on the pin, direct welding of the gold wire to the niobium could not be achieved on the real component. To overcome this a shim of nickel was placed between the gold and niobium and the combination was welded with one pulse of laser energy. After successful welding the coil was encapsulated in silicone rubber. The prosthesis has since been implanted into a patient.

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