Laser plastic welding is helping to pave the way for a new era of medical devices. As a technique for bonding two or more thermoplastic components together, it has advantages to other methods, including cleanliness, precision, hermetic sealing, and quality controls. Moreover, laser plastic welding brings economic efficiencies, design flexibility, aesthetic welding, and new material options to the medical manufacturing industry.

Medical devices are getting smaller because the demand for smaller medical devices is becoming larger. Trends such as minimally invasive surgery and the rise of microfluidic devices, coupled with the advent of manufacturing technologies that allow for production of such ultra-precise designs, are driving innovation and challenging the status quo in the medical industry.

Laser plastic welding is one such manufacturing technology. Adding a little drama to the typically unexciting world of manufacturing, devices that were no more than working designs a few years ago currently are in use and saving lives thanks to laser welding.

Laser Plastic Welding
Laser plastic welding is a method of bonding two or more thermoplastic components together. Although there are many methods for joining thermoplastics, laser plastic welding has a few clear advantages for the medical devices industry: cleanliness, precision, hermetic sealing, and quality controls.

Figure 1: Laser plastic welding process

 Laser plastic welding relies on passing laser energy through an upper, laser-transmissive layer down to the surface of the lower, laser-absorbing layer where the energy is absorbed (Figure 1). The resulting heat from absorption melts the plastics, and creates a weld seam.

Changing the Game
Joining and assembly are critical steps in the manufacture of plastics devices. In all cases, devices are designed based on how they are to be assembled. Each joining method will come attached with its own list of design demands and requirements.

Limited by these requirements, device engineers often find themselves trying to put a square peg in a round hole, and innovations are held back based on manufacturing abilities, or lack thereof.

Laser plastic welding has only been a commercially viable joining method for approximately a decade. Gaining most of its traction in the automobile industry, it has finally laid roots in the medical industry, and there is no turning back.

Before lasers, plastic devices were joined by a multitude of other techniques including adhesives, hot plate welding, ultrasonic welding, and friction welding. In most cases, these preceding methods will remain extremely viable, and laser welding may never actually top them. Each of these methods has its own set of advantages and drawbacks, just as laser welding does. However, lasers have brought a new set of capabilities to the table that were previously unknown.

Advantages of Laser
Cleanliness, precision, hermetic sealing, and quality controls are the most significant advantages of laser welding for the medical industry.

1. Cleanliness
Cleanliness is a major factor in the manufacturing of medical devices. Whether it is an intravenous application or a microfluidic device, the smallest contaminates could easily lead to negative results.

Bonding completed by ultrasonic or friction welding processes leave the joint with a scaling effect. When two parts are rubbed together at high speeds, the scales from the joint break away and become loose, dust-like particulates that can contaminate the device.

Glues and adhesives also have potential for contamination. Introducing adhesives, which often are toxic, into a device meant to be completely contaminate-free is complicated, if not impossible.

Laser welding, on the other hand, produces tight, clean joints that are particulate-free. There is no relative motion between the joining parts during the process to cause particulates. Moreover, no additional materials, including glues or extra plastic, are required to complete the weld.

Laser systems are clean room certified, and currently in operation in cleanness class ISO 5 clean rooms.

2. Precision
Small devices require precise welding capabilities. Laser welding systems are capable of producing beam spot sizes as small as 0.07 mm. Therefore, weld seams of relative widths can be realized.

Microfluidic devices are designed to move very small amounts of fluids through highly controlled channels as a means of testing and measuring.

Figure 2: Microfluidic device

Figure 2 is one such application. At a total size of roughly 2 ½ x 1 ½", the device has 2 m or roughly 6 ½ ft of weld seams within it. The ultra-small channels are created by correctly placing a weld seam on both sides of the channel without harming the integrity of the channel. Since the laser beam is only heating the plastic where the beam strikes, the heat affected zone is minimal—there is no chance of damaging or influencing features outside of the weld seam.

Complex patterns are realized by a galvanometric scanning system. Using two high-precision, angled mirrors to guide the laser beam, the most intricate patterns can be traced with superb accuracy. In addition, changing the pattern of the weld seam is as simple as loading new data into the system software. The process is ideal when manufacturing different products on the same system or for prototyping purposes where changes frequently take place.

3. Hermetic seals
Whether fluids are to be kept in or out, hermetic sealing of a joint is required for medical devices in almost all cases. Laser plastic welding is known for producing very strong joints, and the nature of the heating process will leave weld seams that are often as strong as or stronger than the parent materials.

The excellent fusion of the two plastics also ensures that the weld seam is entirely sealed. This balloon catheter not only requires a perfect hermetic seal, but also a high strength weld. The balloon would need to withstand inflation pressures of 8-12 bar to counter the pressure within the artery it is to be inserted, all the while remaining leak-tight.

Due to the ultra-small size of the balloon, part tolerance for the application was specified to the hundredths of a millimeter. Moreover, laser welding produces only localized heating of the plastic, meaning that hermetically sealed and strong joints are possible for any size application.

4. Elaborate quality controls
When devices are designed to save lives there is no room for error. Laser plastic welding has unsurpassed quality controls to ensure that any part coming out of manufacturing is held to the highest standards.

Laser welding systems have been proven suitable for the highest risk category, Risk Class III, proven in the manufacture of intracardiac catheters.

Process Monitoring Techniques
Four elaborate process monitoring techniques make up the repertoire for laser welding system quality controls: melt collapse monitoring, pyrometer readings, burn detection, and reflection diagnosis. These process monitoring controls ensure that accurate quality assurance data can be realized for any type or complexity of weld.

1. Melt collapse monitoring
Melt collapse monitoring is the most robust process monitoring technique. As the two parts are heated during welding and force is applied, the molten parts will compress into one another. Measuring the melt collapse is a simple, yet sophisticated way of measuring the fusion of the two materials.

Parameters for adequate melt collapse are determined in testing. If a part fails to fall within the defined collapse limits during production, it will be rejected, and the data will be stored for later evaluation.

2. Pyrometer readings
Depending on the type of laser process in use, melt-collapse monitoring cannot always be used; as is the case with radial welds, such as balloon catheters.

Pyrometers provide a means of testing quality when no melt-collapse is to take place. A pyrometer measures the amount of heat reflected from the interface of the two parts. In a pyrometer reading, the heat map can help determine weld quality by measuring the consistency of heat distribution at the weld interface. If there was a lack of heat produced at a section of the weld, a poor weld will likely result.

3. Burn detection
A pyrometer measures infrared heat wavelengths from the part. Burn detection is similar in nature, but instead, it measures heat reflected outside of the infrared range.

Heat outside of the infrared spectrum during laser welding typically is in the form of light. The light results from the laser striking a contaminate, or if the plastic itself is heated to the point of vaporization.

4. Reflection diagnosis
Designed to ensure that there is no gapping between the joining parts, reflection diagnosis measures the scattering of light reflected from the surface of the part. Since parts that fit together only have a single surface, the light is reflected in a tight uniform pattern. If there is gapping between the two parts, a second surface at the weld interface will reflect light at alternate angles. The extra scattering is recognized by the system, and flagged for review.

More Advantages
Besides the four advantages outlined above, laser welding boasts many more including economic efficiencies, design flexibility, aesthetic welding, and new material options.

Although laser welding is still new, it is gaining increased attention in the medical device sector. With so much to offer, the applications that will come from laser assembly will only be limited by the creativity of engineers.

Josh Brown is Marketing Development Representative at LPKF Laser & Electronics. He can be reached at 503-454-4231 or jbrown@lpkfusa.com.