November 25, 2020

3D-printed multi-stage microfluidic mixer with swap structures

Scientists at the University of Bremen introduce a novel approach for microfluidic mixing. Within a research project at the Institute for Microsensors, -actuators and -systems (IMSAS) a Nanoscribe 3D printer was used to additively integrate a freeform 3D mixing element into a premanufactured microchannel on a wafer scale. The micromixer can handle high flow rates of up to 100 microliters per minute, fitting applications that range from drug and nanoparticle manufacturing to fast chemical reactions, biological measurements and analytical medicine.

3D-printed three-stage microfluidic mixer
A 3D-printed three-stage microfluidic mixer with screw-like microstructures and a wall thickness of about 2 µm in a premanufactured 2D microchannel. Image: Martin Oellers, Frieder Lucklum and Michael J. Vellekoop, University of Bremen

There is a number of disciplines that apply microfluidics to handle very small quantities of liquids, such as analytical chemistry, drug synthesis, microbiological analysis and lab-on-chip devices for point-of-care applications. A key aspect of microfluidic devices is miniaturization. Due to the miniaturized size of the fluidic systems, turbulent mixing of liquids is not possible. The main mixing process is diffusion mixing, which requires very long microchannels, increasing device footprint, or the implementation of time-consuming micromachining to fabricate complex mixing elements. A team of researchers at IMSAS – University of Bremen apply a convenient method to reduce diffusion length by stacking liquid streams that alternate next to each other. They present a novel concept of microfluidic mixing: a multi-stage swap mixer.

Embedding a filigree microfluidic mixer by 3D printing

The swap microfluidic mixer is additively manufactured using Nanoscribe’s Photonic Professional system. The element is entirely embedded into a predefined and prepatterned 2D microchannel system. In other words, the scientists applied 3D printing to fabricate a freeform 3D fluidic mixer directly into a microfluidic chip. Each micromixer was printed in less than 30 seconds, securing a short processing time to process a whole wafer in typically less than one hour. 3D Microfabrication thus allows for a fast realization of the mixer directly prepared from a computer-aided design (CAD) and through a straightforward printing workflow.

Moreover, adding and removing design elements to test the mixing performance can be achieved without additional prototyping costs. These elements ensure a high mechanical stability to fit within the predefined channel, reducing leaking risks.

Standard photolithography meets Two-Photon Polymerization

The swap mixer is realized by combining standard photolithography with Two-Photon Polymerization (2PP). The 2D microchannel system is fabricated on a silicon wafer by photolithography of SU-8 photoresist. In a second step, the 3D mixing element is printed into the open microchannel using 2PP. After printing, the remaining unpolymerized material is washed away during the development phase, removing all resist residue in the channels to avoid blocking the micromixer. In a final step the microfluidic device is sealed by pressing a polydimethylsiloxane (PDMS) sheet on top of the microchannel.

This fabrication approach underpins the compatibility of standard photolithography and Two-Photon Polymerization for the integration of additively manufactured 3D microstructures into pre-patterned 2D microfluidic channels on a wafer scale. The researchers were able to position a complex shaped 3D microfluidic mixer into planar microfluidic channels with enormous precision using the design freedom of a fast printing process.

Swap microstructures for efficient mixing

The finished micromixer consists of a 3D channel system which swaps incoming fluid streams. This results in laminar alternating streams, which consist of the two liquids to be mixed and are rearranged in 8 alternating streams.

As a result, the diffusion length used to completely mix the two liquids reduces by a factor of 8, shortening mixing times dramatically to 50 milliseconds or less. The 3D structure design allows for a balanced fluidic resistance in each swapper channel, yielding equal distribution of the alternating streams. Moreover, the 3D mixer is strongly anchored in the channel and can handle high flow rates of up to 100 µL per minute, which is comparable to the typical flow rate for detection of biomolecular interaction (1-30 µL per minute), analytical medicine or nanoparticle manufacturing (in the order of 100 µL per minute). The proposed device thus opens the way to mixing elements that operate at a large range of flow rate from 0 to 100 µL per minute for a variety of applications.

Read the open access publication here: On-chip mixing of liquids with swap structures written by two-photon polymerization

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