The medical industry continues to develop new devices that are smaller in size and more sophisticated in functionality. From in vitro diagnostics and treatment of chronic disease to completely taking over bodily functions, today’s medical devices are performing tasks that were hardly even imagined just a decade ago. In every case, their design requires perfection in form and function, and new assembly technologies are needed to meet these challenging requirements.

Fig. 1 – Microfluidic device welded with laser technology.
Laser welding of plastics is a proven, innovative welding technique based on a principle where two plastic components are brought together under a controlled clamp force, while laser energy is passed through one plastic component (transmissive part) and absorbed by the second component (the absorptive part). This absorption results in heating and melting of the parts at their interface, creating a strong bond that welds the parts together when the laser is shut off and the weld surface cools—all in a matter of seconds.

Because laser welding does not require mechanical forces to heat and melt the component parts, it ensures hygienically clean, particulate-free surfaces, hermetic-sealing capabilities, and the ability to join plastic components that can be just a few millimeters in size.

Such unique capabilities have resulted in laser welding becoming the technology of choice for joining plastics for today’s increasingly advanced, technologies: complex cardiac devices, wearables, microfluidics, devices for in vitro diagnostic or drug delivery, sterile kits, implants, tissue-engineering technologies, disposables, implantables, stent assemblies, and lab-on-a-chip technologies.

All such devices have minimal tolerance for contaminants or particulates that potentially damage delicate structures and compromise device functionality. Laser welding is proven as a technology that makes such purity possible.

Laser welding is also considered more capable of joining dissimilar materials than other forms of plastic joining, which often require the materials being joined to have similar chemistries so that they melt at the same time. Elastomers with different properties, such as thermoplastic ethylenes, propylenes, and soft-touch plastics are being over-molded onto hard-plastic substrates to enhance the comfort and ergonomic appeal of medical instruments and other devices. Unlike over-molding, which is often chosen for this application, lasers can shoot energy through the overmold material to the substrate, melting the interface of both materials to create an actual bond that ensures better performance and longer life.

In addition, device manufacturers are embedding advanced functionalities, such as sensors that read bodily conditions and communicate with pumps, or GPS components that monitor the movement of patients and alert caregivers to respond. Because laser welding does not use the mechanical friction energy to bond component parts, it will not compromise the performance of such sensors, sensitive electronics, or delicate structures.

There are currently two predominant laser welding techniques in use today: trace laser welding and simultaneous through-transmission infrared welding.

Trace Laser Welding

Fig. 2 – Simultaneous Through-Transmission Infrared (STTIr) welding fiber optic array that delivers laser energy to the wave guide.
Trace laser welding employs a single, columnated beam of laser light that “traces” along the length of a weld surface to melt and join the plastic components. This is proven as an effective joining method for parts like microfluidics, which have simple geometries, fine diameters, and a flat weld plane. It produces the clean, particulate-free welds required by medical devices, but it also requires a clear line of sight from the laser emitter to the weld surface. That becomes a disadvantage if the geometry of the part needs complex angles and turns. (See Figure 1)

In cases of complex, three-dimensional geometries, the single beam of laser light in trace welding cannot always reach the entire weld surface, because it cannot “shoot” around a corner. To compensate for this, trace welding requires a time-consuming process that involves rotating the parts around the laser beam in order for it to reach all the weld surfaces. Because of that, trace welding is not an efficient solution for high-volume applications that require millions of parts annually.

Simultaneous Through-Transmission Infrared Welding (STTIr)

STTIr welding is a more advanced laser welding technology that has been introduced. It is not only an effective welding technique for simpler part designs, but it is also proven to be very effective for joining devices with complex geometries, while also meeting the industry’s growing need for high-volume production. (See Figure 2)

STTIr welding employs multiple laser beams located in a wave guide positioned along the entire length of the weld surface being joined. During each weld cycle, all the lasers in the wave guide fire simultaneously. The entire length of the weld surface melts and bonds all at once in a controlled melt that can be accomplished in fractions of a second.

Depending on the size of the part, materials used, the number of laser banks, and the weld parameters, STTIr welding can produce as many as 750 parts per minute—a valuable capability for high-volume production.

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