Medical device manufacturing adds a number of critical factors to welding operations. Devices like replacement heart valves and cardiac pacemakers are CE Class III products, which means that their quality could eventually be the difference between life and death for a patient. Hermeticity and joint integrity are also paramount, since contamination of an implanted device or leakage of its internal materials could harm a patient.

The medical industry’s unique needs also impose constraints on the choice of a welding technique. In particular, a technique needs to adapt to unique part geometries and metallurgy, and must have the ability to make a single spot weld on a small part without introducing an outside material like solder or braze.

A few welding techniques — primarily laser beam welding (LBW) and resistance welding (RW) — have especially suitable characteristics for medical devices and supplies.

Laser beam welding

Laser beam welding is a non-contact process that uses a focused beam of light to melt two pieces together. It features high-power density, a small heat-affected zone (HAZ) and the ability to make spot welds a fraction of a millimeter in width. The technique is also relatively easy to automate and does not require a vacuum chamber like its cousin electron beam welding (EBW), attributes that simplify the process and increase production volume.

Figure 1: LBW is crucial for providing a clean, hermetic seal between a cardiac pacemaker’s two metal halves. Source: Steven Fruitsmaak/CC BY 3.0

The inherent flexibility of LBW is also useful in producing devices like hypotubes, which are often used in minimally invasive catheterization procedures. These are long, metal tubes that require micromachined features along their entire length. A manufacturer could increase efficiency by using the same integrated LBW system to mark, weld and cut a tube.

LBW is critical for sealing pacemakers and is used to weld stents, drug pumps and implantable valves.

Resistance welding

Resistance welding uses current resistance to generate welding heat. Although its process is quite different, its application shares many similarities with LBW. In the medical industry, both can make small spot welds and are compatible with most metals used in medical devices. Both are suitable in a typical lab or cleanroom environment, and both can create autogenous welds free of solder or braze.

While LBW can only create a fusion weld, resistance welding can create a fusion, solid-state or reflow weld. Resistance welding produces an internal weld not exposed to air, which eliminates the need for a shielding gas to prevent contamination. The technique is especially useful for welding batteries within pacemakers.

Choosing between laser and resistance welding for medical devices involves a few broad selection criteria:

• Resistance welding is most useful for making internal welds or when either or both components are gold-, tin- or solder-plated. It is a contact process that does not require a shielding gas but requires greater part size exposure. Resistance welding is ideal for joining thin foils and fine wires.
• Laser beam welding is ideal for non-contact situations requiring an external fusion weld on a small part. It produces robust, hermetic seam welds, and often requires a shielding gas.

Joining polymer materials in the medical industry

While laser and resistance welding are the most common techniques for joining metals in medical devices, many techniques exist to join polymer materials as well.

While not a typical “welding” technique, hot bar welding fuses two thermoplastic films using a heated metal bar — a technique often used in the packaging industry to seal bags. Similarly, the medical industry uses hot bar welding to close packages of tubing, outer packets for intravenous bags, ostomy bags and others.

Hot bar welding is also useful for bonding thin layers of non-polymer materials. Wearables often require flexible materials, such as flexible batteries or layers of gold to monitor heart rates — all of which can benefit from hot bar welding.

Ultrasonic welding (USW), involving high-frequency vibrations to join two parts under pressure, is useful for joining rigid thermoplastics.

The diversity of metal and plastic welding techniques matches the wide variety of medical devices on the market today. For more on all things welding, visit AWS Welding Digest.