Through a new process, it is now possible to laser-weld two optically clear plastic parts made of a wide range of polymer materials for medical applications. This innovation combines patented Branson Simultaneous Through-Transmission Infrared® (STTIr®) laser welding technology from Emerson with precision ultrasonic spray deposition technology from Sono-Tek to open up new design possibilities for the makers of medical devices, drug-delivery systems, and in-vitro testing products.

This breakthrough eliminates a design constraint associated with laser-welded medical products, namely, the requirement that they be made from one clear or transmissive plastic part and a second dark or absorptive plastic part. However, the new process overcomes this limitation by precision-treating one of the clear mating parts with a biocompatible laser-absorber prior to laser welding. The absorber is comprised of microparticles of pigment dye or carbon black that are suspended in a carrier fluid such as isopropyl alcohol or acetone.

There are several methods for applying laser absorbers: master batching within the resin prior to molding; pad printing; or for the most precision, the absorber can be applied using Sono-Tek ultrasonic spray-deposition equipment. A specially designed precision ultrasonic atomizer deposits laser-absorbing dye on the surface of one of the mating parts, creating a spray pattern width as small as 0.5 mm. When deposited using an optimal ultrasonic frequency and spray pattern, the carrier fluid essentially flashes off, leaving a precise pattern of laser-absorbing particles, 1 μm or smaller in size, on the surface of the thermoplastic. During the welding process, laser energy is absorbed by these particles and consumes them, releasing heat energy that conducts through the weld zone of the mating parts which are then bonded together under compressive force.

According to Anita Ennest, regional sales manager for Sono-Tek, the ultrasonic spray-deposition systems “are designed to deposit minute quantities of pigment dye, measured in microliters, with coating thicknesses that are evaluated in the hundreds of nanometers on the surface of the coated parts.” She adds that the equipment used to apply the laser-absorbing suspension is modeled on earlier equipment developed to deposit ultra-thin, ultra-precise coatings on medical stents.

Ennest adds that the laser-absorbing suspension essential to the success of the new plastic welding process found its roots in light-absorbing coatings originally developed for use on photovoltaic cells but is formulated with a biocompatible solvent and pigment dye combination. Developing the right combinations involved an iterative process that required the technology partners to transition the coating, deposition, and weld processes from absorbent to transmissive materials, then transmissive to fully transparent materials.

Fig. 1 - Tongue-and-groove laser joint example showing the biocompatible laser absorber and flash containment zone. (Credit: Emerson)

Another challenge facing the process was the need to dispense and deposit the laser-absorbing dye into precisely the right location on the joint of an injection molded part. Typical targets include the interior surface of a tongue-and-groove joint, or the interior surface of a “double V” joint on one of the mating parts (see Figure 1). Precisely depositing the absorber within enclosed joints like these not only focus the laser-generated heat needed to produce the part-to-part melt, but also create a flash-trap that contains the treated area of the melt, isolating it from any nearby microfluidic flow paths in the welded part.

As Sono-Tek perfected the means of applying the laser-absorber, Hugh McNair, manager, laser applications and systems for Emerson Automation, led the effort to perfect the spray-enabled laser welding process. He explains, “This new clear-on-clear plastic laser welding technology can reliably produce complex fluid paths while maintaining superior aspect ratios in the submillimeter flow path pitch.” Such a degree of precision is essential for microfluidic parts, which must be reliably bonded, yet maintain consistent dimensions along extremely small flow paths.

Two clear mating parts can now be laser welded using an innovative new welding process that can reliably produce complex fluid paths while maintaining superior aspect ratios. (Credit: Emerson)

McNair adds that due to the precision of the spray deposition process, there is a reduced need to precisely image the laser energy, since clear-on-clear welding only occurs where the laser-absorbing dye has been deposited. Additionally, when a tongue-and-grove joint, or a similar joint design, is employed within the mating parts, the joint can be used to properly align the entire assembly prior to welding. And, as noted, the area containing the finished weld can be completely isolated from the functional microfluidic path in the completed assembly — a must for medical device applications.

New Technology Combines with Many Existing Laser-Weld Processes

Because this new “clear-on-clear” laser welding capability relies on equipment outside the standard STTIr laser-welding process — the biocompatible laser absorber and ultrasonic spray deposition equipment — it can be “bolted on” to many existing laser-welding applications. Then, once implemented, it delivers cycle times measured in seconds, making industrial-scale laser welding of medical-quality clear-on-clear microfluidic parts economically viable for the first time. And, since the spray deposition process can be adapted to process multiple parts at once, both the spray-deposition and welding processes can be further scaled up to accelerate production throughputs.

The Branson GLX Micro laser welder from Emerson is the smallest of five units in the GLX Series. The welding unit (left) features a color touchscreen HMI, a lift table measuring 150 × 150 mm, and clamp forces up to 0.05 kN, making it an excellent choice for welding microfluidic parts used in medical applications. The unit at right contains the welder’s laser source and power supply. (Credit: Emerson)

The new process also allows for a far greater range of clear thermoplastics to be combined, regardless of chemistry. Because the laser-absorbing pigment dye so efficiently generates and conducts heat and melt, its use can compensate for even larger differences in the glass transition temperature (Tg) of different polymers used in mating parts.

A typical example of the process involves the use of two clear plastic coupons — injection-molded parts that, together, mate to form a serpentine flow path like those found in in vitro diagnostic systems, human drug-delivery systems, or implantable medical devices. The coupons that will become the absorptive half of each assembly are placed into the Sono-Tek spray deposition equipment to deposit the laser absorber.

From there, the parts are transferred to a laser welder, where the result is a clear-on-clear, laser-welded part. For medical or IVD devices that demand optically clear flow paths, the technology is invaluable, simplifying everything from automated blood cell counts in capillarysized fluid paths, to providing visual validation to a technician that a micro-dose of a powerful therapy is being properly administered to a patient.

This article was written by Tom Hoover, Sr. Medical Business Development Manager – Americas, Assembly Technologies at Emerson, St. Louis, MO. For more information, visit here .