Recently, I've noticed an increase in the use of polymers for stents and scaffolds in medical device manufacturing, largely because it offers a range of interesting features and applications. The only way to manufacture stents and scaffolds made of these materials, however, is by using a femtosecond (fs) laser, which provides both the necessary cutting capability and cut quality.
Regular readers of this blog already know that fs lasers process materials utilizing very short pulse durations – in the 10-15 s range. For metals, this enables cold processing with almost no heat signature on the part. However, the situation is a bit different for plastics and polymers, because they don't tend to absorb laser radiation between 355 and 1070nm wavelengths particularly well.
As we reduce the pulse duration from nanosecond to picoseconds and then down to femtoseconds, the regular absorption rules are essentially voided by the photon energy and density of the fs lasers, and we enter into the world of non linear absorption. Simply put, think of the route to ablation like a multi-chair ski lift to a mountain summit. Each “chair” is the femtosecond laser exciting the material at an electronic level with many small hits. When the summit is reached vaporization occurs.
This very short pulse effect – known as multi photon absorption – was once thought to trump wavelength. However, with the different kinds of multiple wavelength fs lasers now on the market, that is no longer the case: green, and, in some cases, ultraviolet wavelengths, do offer an absorption advantage.
The cutting of polymer stents is a great example of an application that really benefits from the shorter wavelengths. For one medical device customer, Amada Miyachi application engineers looked at cutting polylactide (PLA) material with both 1030 nm and 515 nm wavelengths. We found that even after giving up 50 percent of the laser power to convert from infrared to green, the 515nm wavelength could cut the material with better quality, at around the same speed, with a significantly larger processing window which is great for the production environment to maintain yield.
Unfortunately, due to non-disclosure agreements, I cannot show a picture of the comparative cuts . But, if you’d like to see for yourself why the green fs laser is the way to go for cutting polymer stents, send us some parts. We'll process free of charge! Just use the link below...