Straight micromixer manufacturing combining stereolithography and pulsed laser ablation and simulation - INVITED

. Achieving efficient mixing of fluids is a great challenge in microfluidics that has been addressed using microstructures. In this work, Stereolithography (SLA) and Pulsed Laser Ablation (PLA) were combined to manufacture a straight micromixer for uniform mixing of fluids. Computational Fluid Dynamics (CFD) simulation was performed to test the device. The results suggest that the combination of these optical technologies can be an effective method for fabricating microfluidic devices with great mixing capabilities.


Introduction
Microfluidics is a multidisciplinary growing field that deals with the manipulation and control of small volumes of fluids, typically at microliter scales.The field has gained significant attention in recent years due to its wideranging applications in areas such as biomedicine, engineering, or chemistry.The ability to manipulate fluids at the microscale offers numerous advantages, including reduced agent consumption, improved reaction efficiency, and low waste production [1].
One of the critical challenges in microfluidics is the mixing of fluids, which is essential for various applications such as chemical reactions, biological assays, and analytical measurements.Achieving efficient mixing at small scales can be challenging.However, several strategies have been developed to enhance mixing, such as the use of microstructures [1].
Historically, photolithography has been the main method for micro structuring substrates, but it has some disadvantages such as difficult and time-consuming prototyping protocols and very polluting chemical waste.However, laser technologies such as stereolithography (SLA) and Pulsed Laser Ablation (PLA) have emerged as quick and clean alternatives for substrate fabrication.SLA 3D printers use selective photopolymerization of liquid resins to manufacture objects with great structural detail in 3D, achieving resolutions in the range of hundreds of microns.This method is particularly useful for creating complex structures with high precision, such as microfluidic devices or scaffolds for tissue engineering.PLA, on the other hand, is a technique that uses pulsed laser to micropattern 2D surfaces with outstanding resolutions in the tens of microns range.This method can be used for a wide range of applications, including surface texturing, microfabrication of electronic devices, and the creation of biomimetic surfaces.
In this work, both technologies have been combined to easily manufacture a challenging microfluidics device: a straight micromixer [2].After that, a simulation using computational fluid dynamics (CFD) software was conducted to test the efficiency of the mixer in achieving uniform mixing of fluids at the microscale.

3D Printer
A Formlabs Form 3B SLA printer was used to produce a substrate featuring a 550 μm inward square channel on its surface.Commercial Model resin from Formlabs was selected as printing resin given the precision it offers, its response to laser ablation, and its performance when replicating polymers.

Pulsed Laser Ablation
PLA of the bottom of the 3D printed microchannel was performed using the Rofin PowerLine E diode endpumped Nd:YVO4 laser system.This Q-switched laser has a pulse duration of 20 ns and a fundamental wavelength of 1064 nm, which corresponds to the infrared spectral range.The laser setup was combined with a galvanometer system and a flat field lens (f=160 mm) that focus the laser with a homogeneous energy distribution in a 120x120 mm 2 area.

Achievable micropatterns
By PLA, it is possible to create a series of micropatterns on the resin surface.Several ablation studies were performed on Model resin to find the proper laser parameters and the achievable geometries, using different arrays of straight lines.Then, confocal images were taken to perform surface inspection (Fig. 1) of the micropatterned bottom of the channels.In the first place, a channel was left unpatterned (Fig. 1a) to serve as a control.This piece was printed with no orientation (over the printing platform) to grant a proper square section.In the second place, ridges of 100 μm width, 800 μm length of 200 height, 380 μm of period and 45º of orientation (Fig 1c) and ridges of 100 μm width, 550 μm length of 200 height, 260 μm of period and 90º of orientation (Fig 1e ) were micropatterned on the bottom of the channel using an average power of 6W, a repetition rate of 5 kHz, a velocity scan of 7 mm/s and 1 repetition.As can be seen in the images, the results of ablation depicted in the images exhibit accurate and regular structures with high reproducibility.Successful removal of the resin is even produced without damaging surrounding structures.

Mixing simulation
After the structures were fabricated and analysed through confocal software, they were replicated (Fig. 1b,d,f) using computer-aided design (CAD) software to conduct further simulations (Fig. 2).
In the first image (Fig. 2a), a micro-mixer is shown without any structure at the bottom, resulting in poor mixing performance as the liquids fail to mix effectively in the sides of the channel.The second image shows the first proposed micromixer, with 45º angled structures (Fig. 2b), promoting mixing efficiency by inducing disordered flow.The third image exhibits another micromixer, with 90º angled structures (Fig. 2c), which can also enhance mixing.The technique versatility has been proved since both geometries are useful, and the selection of each of them would depend on the specific application requirements and the desired mixing performance.

Conclusions
In conclusion, the combination of SLA and PLA in the fabrication of a microfluidic micromixer has proven to be a precise and effective method.The results obtained using computational fluid dynamics (CFD) simulations in the manufactured geometries are promising, as they demonstrate the ability of the micromixer to properly mix different fluids at a microscopic scale.In summary, a significant advancement for the fabrication of versatile microfluidic devices by combining two optical technologies in its respective appropriate dimensional range is presented., 09020 (2023)

Fig. 1 .
Fig. 1.Left column shows confocal images of a) unpatterned channel (control), b) channel with 45º ridges and c) channel with 90º ridges.Right column presents the 3D corresponding geometries used in simulations.

Fig. 2 .
Fig. 2. Top view and transversal section of the volume percentage (from 0 to 100%) of two flow samples inside a a) unpatterned microchannel and manufactured micromixer featuring micropatterned b) 45º and c) 90º ridges.