Minimally invasives: an inside story

Percutaneous transluminal coronary angioplasty (PTCA) is the medical term for the expansion of a constricted coronary blood vessel from the inside without having to undertake open-heart surgery. PTCA is a standard procedure to improve the symptoms of chronic coronary disease, as well as a life-saving emergency measure after an acute heart attack. A balloon catheter is pushed into position along a special guide catheter inserted via the groin artery or the under arm artery. The balloon catheter has a balloon at the end which is expanded at approximately 8 – 12 bar and positioned inside the constricted blood vessel. This reopens the blood vessel to allow the blood to flow freely again. This technique primarily makes use of the elasticity of blood vessels.

The key component of the PTCA catheter is the balloon, which has to be welded so that it is completely tight. Welding these components is highly complex because of the very delicate structure of the parts and because, in addition to achieving the specified degree of tightness, another critical aspect is complying with the stringent component tolerances of a few hundredths of a millimetre. Potential joining methods include gluing, hot air or hot element welding and laser welding.

Various processes are available for welding plastics using lasers. The most frequently used industrial method is transmission laser welding. This method involves combining one material transparent to the wavelength of the laser beam – usually between 808 nm and 1064 nm – with another material which absorbs the specific wavelength. The laser beam is focused through the laser-transparent moulded component on to the absorbing joining component. This melts its surface. The transparent component is also plastified by heat transfer, and the accompanying pressure from the application of a specific amount of force creates the material bond joining the two materials. One of the preconditions for the success of this method is that one of the two components being joined can be made to absorb the laser light by mixing it with additives. Because this was not possible in the case presented here, a different solution had to be found.

It is possible to use laser light with longer wavelengths to join together two colourless polymers. Use is made here of a property of thermoplastic polymers: The absorption in colourless polymers rises successively starting from a wavelength of approximately 1.3µm, and is almost completely absorbed by the polymer above wavelengths of approximately 2.7µm. A standard method for welding two transparent polymers uses a laser with a wavelength of 1.5µm. When very thin materials are used, such as in a PTCA catheter, it is also possible to weld using inexpensive CO2 laser systems (wavelength 10.6µm). Laser light of this wavelength is directly absorbed by the upper film. The melting of the lower film and welding exclusively depend on heat transfer.

After a detailed evaluation of all the alternative methods, laser welding with a CO₂ laser was selected as the optimum solution. Laser-based joining was chosen instead of heating element or hot air welding for a large number of reasons: in addition to much shorter cycle times, the main benefits include higher controllability and the high process security and stability. Laser welding can demonstrably boost the quality of the finished product. And the reject risk associated with the use of excess energy can be reduced to a minimum because the energy footprint can be restricted to a very small area. Another favourable aspect is the absence of any contamination of the component.

Since the advent of laser welding technology, gluing is only used as an alternative in the medical technology sector under very exceptional circumstances: in addition to the much lower process reliability, the amount of space required for a glued bond would be completely unacceptable in the assembly presented here.

A suitable system for producing the component was then elaborated in co-operation between eucatech AG and LPKF Laser & Electronics AG. The system uses a customised laser plastic-welding plant based on standard modules from the laser plastic-welding manufacturer’s product line. The machine is very compact even though all of the components such as the laser, cooler and controls are integrated within the equipment housing. Because of the production logistics, manual assembly of the welding plant was selected as the best solution.