Matthew Nipper, PhD, Director of Engineering
As medical catheters become smaller and more complex, with added functionality, they become more challenging to laser-machine. OEMs increasingly require laser technologies that can drill micron-sized features in a range of materials, including ceramics, glass, metals and alloys, polymers, and semiconductors with tight, single-micron tolerances. Polymers are especially popular for medical catheters because they are highly stable and good electrical insulators.
The type of laser selected, and the pulse and wavelength used, depends on the chemistry and thickness of the polymer being processed, its intended use, and the precision/tolerance that is required.
Achieving optimal laser processing results is dependent on matching the proper laser pulse width and wavelength with the material to be processed. The material chemistry, thickness, precision, and price considerations all factor into this decision process. This blog will explore the analysis methods used for deciding these parameters.
The first consideration for matching the right laser is understanding the wavelength requirement. The three wavelength categories for lasers are ultraviolet, visible light, and infrared. Different materials absorb and reflect different amounts of light, according to the chemical properties of the material and the wavelength of the light. The greater the amount of energy absorbed by the material from a particular laser wavelength, the easier and more accurate the laser processing will be. If a high proportion of light is reflected away from the surface, less energy will be absorbed for cutting, making it a less-efficient process, with lower quality.
A common polymer used in catheters is polytetrafluoroethylene (PTFE). The equation for photon energy shown in the graphic below is the one you would follow if you wanted to determine the minimum wavelength need to break the chemical bonds for PTFE for cutting, where E is photon energy, h is the Planck constant, C is the speed of light in vacuum, lambda is the photon’s wavelength, and NA is the Avogadro constant.
Following the formula, to photochemically break down the polymer backbone through a direct photon excitation mechanism, a minimum laser wavelength of 264 nm is required. This is an intuitive answer given the well-documented UV resistance properties of PTFE. Machining PTFE is challenging, given the poor absorption efficiency of UV light. So to increase the machining efficacy, we must consider the effects of pulse width.
Pulse width (PW) is the elapsed time between the beginning and the end of a single pulse of energy, measured in seconds. The shorter the pulse width, the greater the effectiveness of the cut, with fewer burrs or defects, such as heat-affected zones (HAZ). Ultrafast lasers, those defined as having pulse widths equal to or less than 10E-12s, interact with materials in a fundamentally different way compared to nanosecond lasers. The ultrafast photons strip electrons of the atoms in the material, ionizing the atoms and forcing them to explode out of the bulk material, with virtually no heat transfer. For our example of PTFE, this means we can machine the material with a femtosecond laser wavelength in the IR or green visible light regions, where we couldn’t do the same with a nanosecond laser. Ultrafast lasers have an average pulse width of 150 femtoseconds which is 150 quadrillionths of 1 second. To understand how fast 150 femtoseconds is, think of a matchbox car moving 1 inch in one second. Now imagine that same matchbox car circling the earth approximately 10,000 times in that same second!
Pulse width defines the three most popular industrial lasers for micromachining:
- Femtosecond–with a pulse width that lasts 10E-15s, femtosecond lasers produce the least amount of heat damage and are critical for the high-precision manufacturing of implantable medical devices.
- Picosecond—with a pulse width of 10E-12s, picosecond lasers have higher average power and are better for thicker dielectrics like ceramics and glass. They provide a nice bridge between the femtosecond lasers and nanosecond lasers.
- Nanosecond—at 10E-9s , nanosecond lasers have the highest average power and are ideal for high-speed processing of polymers and thin metals that have a wider tolerance range.
Medical catheter designers are always trying to balance quality and price. If extra high quality is needed, femtosecond or picosecond lasers are always a good choice. If economics are a significant constraint, a nanosecond laser may be suitable.
Laser processing procedures will continue to advance as medical device manufacturers incorporate more complex designs, materials, and added functionality. For example, we can drill through one layer of the catheter structure at a time to create 1-2 micron-wide holes or other shapes to the desired depth within the catheter, or completely through the material, with the highest precision. Often the best solutions result from working directly with the customer’s engineering teams to create proprietary laser systems that push the technical limits of laser micromachining.
Matthew Nipper, Ph.D., is Director of Engineering for Laser Light Technologies in Hermann, Missouri. He received his Ph.D. from the University of Georgia and completed a postdoc appointment in Biomedical Engineering at Georgia Tech. Dr. Nipper has over 13 years of experience in photonics and leads an interdisciplinary engineering team that continuously challenges what is possible for laser processing.