Ultra-short pulse laser systems for manufacturing nebulizers in medical technology

The coronavirus crisis in particular has highlighted the importance of innovative medical technology.

One medical technology product that is particularly important for respiratory diseases is a device known as a nebulizer. Nebulizers are used to atomize inhaled medications so that the liquid is converted into an aerosol, which can then be distributed throughout the lungs.

Function of a nebulizer in medical technology

The functional principle of a nebulizer is based on a membrane with micrometer-sized holes. This atomizes medications so that the liquid is converted into an aerosol. To ensure that this aerosol is homogeneous, nebulizers require a continuous drive. Specially shaped piezo discs act as ultrasonic transducers and stimulate the perforated membrane to produce ultrasonic vibrations of up to several hundred kilohertz.

Current findings confirm that smaller holes in the filters lead to smaller active ingredient particles, which in turn are distributed deeper in the lungs and can therefore be more effective. Therefore, the goal is to make the micro-holes as small as possible.

Conventional manufacturing processes reach their limits here, as either the required holes cannot be physically produced or tool wear is high. An innovative approach is the use of laser systems with ultrashort pulsed radiation, because the shorter the pulse duration, the smaller the holes that can be drilled. Depending on the focusing lens and wavelength used, drill holes with a diameter of around five micrometers can be produced.

Since the distribution and shape of the drill holes can be adjusted using software settings, it is possible to manufacture different products on a single laser system.

The drilling process with the ultrashort pulse laser – explanation and influencing factors 

Laser drilling is based on the targeted removal of material from a predefined workpiece. In classic, yet highly precise laser drilling, an ultrashort pulse laser source, beam guidance and, if necessary, beam shaping, a galvanometer scanner with two scanning mirrors, and a focusing module are used to generate the laser beam and deflect it onto the workpiece. This setup focuses each individual laser pulse on the workpiece, causing absorption in the workpiece. The absorption and the strong local heating of the material cause the ablation. The drill holes produced in this way are conical. This means that the hole is larger at the laser entry point than at the laser exit point. See also the example in the graphic below.

Various parameters influence the quality and cycle time of the drilling process. Lasers with short pulse durations in the pico- or femtosecond range do not produce melt ejections and have only very small heat-affected zones, which allows for more precise and smaller machining than with other laser systems with longer pulse durations.
However, due to the lower heat influence, the drill hole is usually not created with a single pulse, but by emitting many individual pulses onto the material, and the process is normally slower.

The aspect ratio generated, i.e., the ratio of the exit diameter to the borehole depth, is conventionally max. 1:5.
To ensure efficient processing, the repetition rates for ultrashort pulse lasers are high. In percussion drilling, where the material is removed by emitting several pulses to the same locations, frequencies between 10 and 500,000 kHz can be generated. However, even here, the repetition rates cannot be arbitrarily high, as the absorption of the laser radiation in the material vapor generated and the accumulation of heat mean that less energy is converted into material removal.

Laser drilling of nebulizers

There are various approaches to laser drilling of nebulizers, which depend, among other things, on the customer's requirements. Thus, in addition to the required drill hole diameter, the material thickness, the substrate, and the desired shape of the drill hole are also taken into account in the process development.

As with many other medical technology products, nebulizers are usually made of stainless steel, so the material can be processed with ultrashort pulse lasers with infrared, green, and ultraviolet wavelengths. When selecting the wavelength, various advantages and disadvantages must be taken into account.

While the spot size is larger at longer wavelengths, such as in the infrared range, the Rayleigh length, i.e., the range in which a focus deviation can occur without changing the processing result, is also longer. Even though a smaller laser focus, such as with a green and ultraviolet wavelength, promises even smaller and more precise processing, disadvantages such as the lower removal volume in the same amount of time must also be taken into account. The appropriate wavelength must therefore be selected not only based on the quality of the result, but also on other factors such as the homogeneity of the source material and the desired throughput rates.

The desired drill hole diameter is often less than 3 µm because, as mentioned at the beginning, the effect of the nebulizer increases as the diameter decreases. The film thickness of the starting material is usually several tenths of a micrometer, so that with the technically possible aspect ratio of max. 1:5, the drill hole diameter can only be produced at the hole entrance, but not at the exit. The production of a correspondingly small exit diameter can then be achieved with all three wavelengths mentioned. Because the holes are conical as a result, the borehole entrance diameter is significantly higher. This means that a greater amount of material must be removed and a longer processing time must be used than would be the case with cylindrical boreholes with a diameter of 3 µm, which are not technically feasible.

To counteract this and achieve a further increase in cycle time, it is not enough to simply increase the average power of the laser system and thus act according to the principle of “more is better.” The resulting increase in energy density would then exceed the ideal point of the material and thus lead to overheating and, as a result, material deformation. To counteract this effect, the so-called burst mode can be used. In this process, the high energy density is broken down into individual pulses. However, the average power remains the same. The many individual pulses allow the processing rate to be increased and thus the cycle time to be reduced.

Using the procedure described above, after initial parameter determination and cycle time optimization, 4,000 conical holes with a diameter of approx. 3.5 µm were produced in a total laser time of around 25 seconds at the Pulsar Photonics Laser Application Center using a laser system with an infrared wavelength. When used in a nebulizer, the membrane produced in this way can generate droplet sizes with a diameter between 4 and 10 µm.

   

Precise laser-produced drilling grid in a stainless steel membrane with corresponding microscope image.
© Pulsar Photonics GmbH.

Outlook

Thanks to the many different hardware and software adjustment options, the current results can be optimized through further test loops.

As described above, droplet sizes of around 4 µm can currently be produced. However, this is not sufficient for infants and young children. By using special optics with a very small focus diameter – called Microscan Extension (MSE) – the drill holes and the resulting aerosol particles can be further reduced.

The results presented have shown that fast cycle times can already be achieved with a single laser beam. Thanks to the flexibility of the digital tool “laser” and the software used for this purpose, the laser parameters can be adjusted on a program basis to further reduce the throughput rates with the single beam.

For series processes, the cycle time can also be reduced by using multiple partial beams. In this process, a drilling grid with several holes at a fixed distance can be drilled simultaneously with one laser source and one scanner. If, for example, four partial beams are used, it is possible to implement the process up to four times faster than with the single beam.

For series processes, the cycle time can also be reduced by using multiple partial beams. In this process, a drilling grid with multiple holes at a fixed distance can be drilled simultaneously using a laser source and a scanner. If, for example, four partial beams are used, it is possible to implement the process up to four times faster than with a single beam.

Roll-to-piece automation, in which the raw material can be positioned in the system using coils, also aims to increase throughput. After drilling, each microfilter is separated so that only minimal manual handling is required and the non-productive times of the process can be reduced.

If you would also like to optimize the production of your nebulizers or have other drilling tasks where mechanical processes reach their limits, please contact us and we will carry out initial feasibility tests at our laser application center. We look forward to hearing from you.