Fabrication and testing of a 3D microfluidic micromixer: On-chip particle manipulation

This short review is based on the research paper “Innovative 3D Microfluidic Tools for On-Chip Fluids and Particles Manipulation: From Design to Experimental Validation”. It was authored by Sofia Zoupanou, Maria Serena Chiriacò, Iolena Tarantini and Francesco Ferrara. The study was published in the journal Micromachines on 21st January, 2021. The researchers in this paper have made use of a pressure-driven microfluidic pump to fabricate and test a 3D microfluidic device made of PMMA. The tests performed on the device using a flow controller is to confirm its applicability to multiple fields of research in chemistry and cell biology.

Abstract

Micromixers are considered to be important for lab on chip devices, and their low efficiency can prove to be a roadblock for many bio-application studies. Efficient mixing with the ability to automate is still vital. This research paper presents a novel method to fabricate a 3D poly(methylmethacrylate) (PMMA) fluidic mixer. This is done by putting together computer-aided design (CAD), micromilling technology, along with experimental applications through the manipulation of fluids and nanoparticles. The platform consists of 3 microfabricated layers with a reservoir-shaped microchannel at the bottom, a central serpentine channel, a through-hole for interconnections and an upper layer containing inlets and outlets. Factors such as the sealing process, the high precision and customizable fabrication techniques contribute to the monolithic 3D architecture. This creates buried running channels that are able to facilitate passive chaotic mixing and dilution functions, owing to the portion of the pathway in common between the reservoir and the serpentine layers.

The plug and play micropumping possibility makes it possible for researchers to demonstrate the working principle of the device by making use of colored fluids, and fluorescent nanoparticles to illustrate various application such as particle manipulation.

Introduction

In the last few decades, microfluidic structures have been established as the cornerstone for general lab-on-chip devices. Existing fabrication methods include the use of basic soft lithography, which is followed by PDMS/PDMS, PDMS/Glass, or PDMS/SU8 bonding protocols to finally obtain an assembled microfluidic device. [1-4]. Even though these conventional molded devices performed well, their compatibility with biological studies, ease of setting them up were some limitations that did not meet the requirements for many applications [5,6]. Also, PDMS is sensitive to exposure to some certain chemicals and might even adsorb proteins on its surface [7-9].

Thermoplastic polymers (plastics) like poly (methyl methacrylate) (PMMA), polystyrene (PS), cyclic olefin copolymers (COC), or polycarbonate (PC) are all gaining widespread interest in the last decade. This is because they allow easy surface treatment/modification, are transparent, biocompatible [5], and most importantly they are suited for various industrial applications in the lab on chip (LOC) market [10].

Also, by making the switch from 2D to 3D microfluidics, it is being considered as revolutionary technology due to the unique capabilities of miniature fluidic systems within the field of cell biology, on-chip chemistry and droplet microfluidics [11].

Micromixers are an important element, needed to shift standard assays to on-chip reactions. They have a range of bio-applications – studies on living cells for medical diagnostics, synthesizing nanoparticles, and bio-tech analysis like polymerase chain reaction (PCR) [12-14]. To bring together robustness and reproducibility into a single device, a plug and play device made of PMMA was designed and developed into three different levels. The device has the potential to be used for mixing in chemistry and cell biology.

Aim & objectives

The researchers set the following objectives for this study:

• Design, simulation and fabrication of 3D microfluidic device
• Bonding and making sure the device is functional
• Plugging the microfluidic setup and in-flow passivation of microchannels
• Mixing and gradient particle generation and manipulation

Materials & methods

Fluidics and particle tracing of interconnected h-junction microchannels in 3D were simulated with the help of a Computational Fluid Dynamics CFD module. For the entire simulation, the particles’ diameters were defined at 200 nm and 1µm.

An Elveflow microfluidic system, which is perfectly suited for these kinds of studies, was used to pump the solution into the device. The Elveflow micro pumping system was equipped with an OB1 base module, two MK3+ channels for a pressure controller, and two microfluidic flow sensors.

This microfluidic system enables the flow of the medium and can be controlled temporally. During the experiments, the two inlets were connected with a vial containing the particle samples. Finally, the microchip was positioned under the microscope for observation, and the result was noted down with real-time image acquisition.

Figure 1: Microfluidic experimental setup with Elveflow flow sensors and capillaries connected to the 3D microfluidic device, observed under a microscope. Courtesy of Maria Serena Chiriacò

Key findings

Conceptual design of the 3D microfluidic tool

Figure 2:  FEM simulation of values of flow velocity (a) and pressure (b) in a two-level interconnected microchannels system. Courtesy of Maria Serena Chiriacò

The design of the 3D microfluidic device was the first part of this study. The fluidic parameters were investigated to plan the features required for the final architecture. The interconnected channel design and the simulation results for the mixing index are given in Fig 2.  Figure 2a shows the velocity distribution of the proposed design while Figure 2b shows the pressure distribution across the entire network.

To discover more about how to better plan your microfluidic setup, please find below a selection of application note and our microfluidic calculator!

• General Overview of microfluidics
• Microfluidics and microfluidic devices: A review
• Microfluidic calculator

Mixing and gradient particle generation & manipulation

Efficient mixing of different elements is considered to be the ultimate goal, and thus the micromixer was tested by performing a flow test.

In table 1, a summary of tests undertaken on the holes (A, B and C) were used as inlets or outlets. Flow tests were performed using the Elveflow microfluidic setup (Flow controller & flow sensors)

Table 1. Schematic of experiments based on the alternative use of holes as inlet/outlet combined with injection of colored fluids or fluorescent particles suspensions. Courtesy of Maria Serena Chiriacò.

Figure 3 (a) picture of the experimental setup under the optical microscope. The violet drop emerging from the outlet of the assembled and connected device is clearly visible as a result of the mixed pink and blue fluids. (b) Picture of the separate and common pathway taken through the microscope framing. Interconnection hole is highlighted by a red arrow. Scale bar: 500 µm. Courtesy of Maria Serena Chiriacò.

Figure 3a shows a photograph from the experimental setup. The microfluidic chip is visible during the in-flow test, where the resulting violet mixed color at the outlet is observed at the output.

Figure 5b shows a more zoomed in and clearer image obtained by a microscope.

The final phase of the experiment involves the mixing and particle manipulation. A green fluorescent polystyrene suspension is injected through hole A.

The path of these particles was followed under the microscope, and a few images were captured before and immediately after the mixing point. (Figure 4).

Conclusion

Lab on chip devices allow to incorporate multiple laboratory features into a few square centimeters.

Microfluidic devices have the breakthrough potential for making radical changes to the traditional methods in the fields of chemistry, environment and life sciences.

Prototyping methods used to design and fabricate polymeric lab on chip devices are on the rise, due to the high flexibility and precision they offer.

The fabrication of a device requires steps of optimization in order to fine-tune the architecture and geometry based on the application and observation. In this study, optimization of the microfluidic device from the design to the particle generation and manipulation is accomplished.

Figure 4: (a – b) image of microchannels with green and red particles while running separately before reaching the interconnection point and (d – e) immediately after. (c) and (f) merged image of green and red fluorescence acquisition. Courtesy of Maria Serena Chiriacò.

The obtained device has demonstrated its ability to work with two different functions – mixing and obtaining gradients into microchannels, which would take advantage of all properties that 3D PMMA is capable of. Future developments in applications such as cell labeling, mixing two types of cells in a particular ratio, and even dilution of cells for counting.

These exciting results were achieved with the help of pressure-driven flow controlled microfluidics to perform tests on the fabricated 3D microfluidic device. For an in-depth analysis on this study, the complete paper is available here.

References