New welding process improves microfluidic chip fabrication for life sciences
Researchers have developed a highly precise plastic welding process using a thulium fibre laser that solves the challenges faced when fabricating transparent microfluidic chips for life science applications.
The process could also be used to increase flexibility and efficiency in the industrial production of other medical technologies.
Microfluidic chips have proven their worth in life sciences as they are able to transport, mix and filter even the smallest amounts of liquid efficiently. Fabricating them is still a challenge, however, particularly due to the media-tight encapsulation required of the microchannels integrated in the chips.
Conventional joining technology reaches its limits in the micrometre range, and therefore is unsuitable for encapsulating the channels. Laser transmission welding with beam sources in the near-infrared (NIR) range is a promising alternative, as it enables high precision and flexibility.
However, laser transmission welding also has a problem: volume absorption creates a heat-affected zone (HAZ) that extends vertically over the entire cross-section of the component. The thermal expansion during the melting process promotes the formation of blowholes and cracks, which cause leaks in the seam structure. In addition, there is a risk that the material will warp, especially in flat components.
German research institute Fraunhofer ILT therefore launched the ‘SeQuLas’ (segmental quasi-simultaneous laser irradiation) project in 2017, together with Amtron from Aachen, Ortmann Digitaltechnik from Attendorn and Bartels Mikrotechnik from Dortmund.
With the developed joining process, in which a thulium fibre laser is used, high-precision welding of microfluidic components can be achieved. (Image: Fraunhofer ILT)
Within the project, the partners developed a welding process using a thulium fibre laser – with an emission wavelength of 1,940nm, which plastics absorb naturally – which reduces the heat-affected zone from expanding vertically throughout the chip.
In the developed process, the laser beam is guided several times along the weld contour at high speed with the aid of a scanner system. This heats the entire seam contour simultaneously, which would otherwise melt sequentially when using contour welding.
In tests with polycarbonate components, Fraunhofer ILT demonstrated that during the welding process the heat is dissipated at the outer surfaces while heat accumulates inside the material. The increasing number of passes and the high scanning speed even reduce the vertical expansion of the heat-affected zone by up to 30 per cent compared to contour welding.
Early detection of thermal damage
In a second step, the partners developed a process control for the laser welding process. A pyrometer integrated in the beam path measures the temperature in the component during welding. By coupling the measurement signal with the position of the scanner mirrors, it was made possible to record the heat distribution in the component in a spatially resolved manner. In this way, thermal damage can be recorded and precisely localised during the welding process.
The newly developed process can, therefore, react quickly to temperature deviations and control the laser power accordingly. In this way, homogeneous seam properties along the seam contour can be ensured.
The ‘SeQuLas’ project was completed in February 2020 and ran for three years. It was funded by the European Regional Development Fund (ERDF) and the state of North Rhine-Westphalia.