Fabrication of a leak resistant microfluidic chip – surface plasmon resonance applications

This short review article is based on a research paper titled “Leak-free integrated microfluidic channels fabrication for surface Plasmon Resonance applications” authored by M-T. Bakouche, S. Ganesan, D. Guérin, D. Hourlier, M. Bouazaoui, J-P. Vilcot, S. Maricot. This research paper was published in the Journal of Micromechanics and Microengineering. It explores the fabrication of leak resistant microfluidic SPR sensors using (3-mercaptopropyl) trimethyloxane (MPTMS) self-assembly process. The ideal bonding is achieved by treating the PDMS chip to UV-O3 and MPTMS before attempting attachment to the gold surface. A study is conducted to compare the bonding of corona-discharge and chemically bonded PDMS samples using tensile strength. Most importantly, the 4 channel PDMS-based microfluidic chip is also tested under pressurized fluid injection using the high accuracy flow controller for multiple liquid leak resistance checks.

Abstract

The research paper which has inspired this short review article describes a new method of fabricating a microfluidic integrated SPR (Surface Plasmon Resonance) gold chip based on a (3-mercaptopropyl) trimethoxy silane (MPTMS) self-assembled monolayer. This monolayer was formed at the surface of the microfluidic chip made of polydimethylsiloxane. Its existence was established using a contact angle and FTIR measurements on the treated PDMS surface. An accurate 4-channel microfluidic system was developed on PDMS and reported on a gold SPR sensor. The sealing also had to be inspected through tests that involve injection of continuous flow of solutions under a gradient pressure of up to 1.8 bar. The bonding strength was measured through tensile tests for the chemical as well as corona bonding, and studied in detail. Finally, the integrated microfluidic SPR sensor was tested on an SPR bench which validates its usability, and confirms that no leakages were found between the different microfluidic channels.

Introduction

Surface Plasmon Resonance (SPR) can be defined as a collective oscillation of conduction electrons at the surface of metal nanoparticles, caused by excitation by the corresponding resonant wavelength of light. At present, sensors based on surface plasmon resonance (SPR) physical phenomena are among the sensors exhibiting the highest sensitivity in material permittivity measurements. One of its main adaptations concerns molecular interaction analysis [1-3]. When these sensors are usually converted to biosensors using surface chemistry, the SPR detection scheme is considered to be label-free, real-time and extreme kinetics qualifiers. These properties can be considered advantages compared to other molecular interaction detection techniques. At present, SPR measurements are made by introducing an SPR sensor into the system that is handling the pressure-contact microfluidic system. Preceding use, biologists functionalize the SPR sensor on the measurement equipment after the request of their analyses. This makes the assembly of a completely ready-to-use SPR sensor by integrating the appropriate microfluidic system, a challenging feat.

Polymer-based microfluidic systems, for example polymethylsiloxane (PDMS) have been widely used for the design and fabrication of lab-on-chips, point of care devices in the last 2 decades [6]. However, the technology is still not well understood [7]. Hence, PDMS can be utilized as the reference material owing to its properties of biocompatibility and inertness towards biological materials. Its permeability, flexibility, non-toxicity, affordable price and ease of use also make it the ideal solution. Furthermore, its optical transparency in the visible spectrum makes it an asset specifically for experimental monitoring [8-10]. The intrinsic properties of PDMS allows it to easily stick to silicon-based surfaces such as silicon or glass.

Aim and objectives

The specific steps that the researchers followed throughout the fabrication and testing procedure are listed below:

• The fabrication of the SPR (Surface Plasmon Resonance) sensor and the PDMS based microfluidic chip.
• Processing the microfluidic chip for integration
• PDMS chip surface characterization to validate surface modifications (contact angle goniometry) for assembly preparation
• Fabrication of the integrated fluidics SPR sensor
• Tensile strength testing for classical corona-discharge treated & chemically bonded samples
• Two step leak resistance testing: visual observation & an SPR test bench

For a detailed understanding of the processes involved in the surface treatment and characterization of the PDMS-based microfluidic chip, please refer to the original research paper by M-T. Bakouche et al. Images of the experimental setup used for this study are displayed below in Figure 1 & 2.

Key findings

Assembly of the microfluidic SPR sensor

Firstly, for fabrication of the integrated microfluidics SPR sensor, the MPTMS-treated PDMS chip is attached onto the SPR surface. It is done by hand-pressing the chip onto the gold surface and a vacuum-annealing procedure is used for 60 minutes at a temperature of 60°C. This results in a homogeneous & irreversible bond with the help of a Au-S bond (Figure 3)

The final integrated fluidic SPR chip is demonstrated in Figure 4. The adhesion was checked by hand, and no disbonding was observed. The important finding from this section of the study is that a non-treated PDMS chip would have easily detached in comparison to the chemically treated PDMS chip.

Microfluidic SPR sensor test

This test involved the fabricated microfluidic device being tested, first under visual observation using an ink test, and secondly on the SPR test bench.

The leak resistance of the microfluidic system was tested by injecting a black ink solution through the four microfluidic channels for 5 minutes.

A high accuracy pressure-driven flow control flow controller (Elveflow OB1) was used to generate a pressure gradient of up to 1.8 bar. The chip was then observed under an optical microscope. The observations from the ink injection are shown in Figure 5.

Finally, the SPR test is performed as a functional test, as well as to test the leak resistance once again. Because of its intrinsic low penetration depth, SPR measurements are also accurate in detecting any leaks from the chip surface. The SPR experimental setup is based on an angular interrogation scheme [14]. The test fluid in this section was an ethylene glycol-based solution, and a similar flow controller was used to regulate the flow rate at a similar level as compared to the ink test. The stability of the bond is observed for an extended experiment duration of up to 20 minutes without any observable leak. Figure 6 shows the plasmon profile on both corona-discharge and chemically treated microfluidic chips. The chemically-bonded sample is observed to perform better than the corona-bonded sample.

Mechanical Test

A comprehensive tensile strength test was performed on both the classical-discharge treated and chemically bonded samples. (Figure 7) The corona bonding was finalized by treating both clean glass and PDMS surfaces for 2 minutes using a handheld corona treater device, after which both of these were aligned and attached together. The assembled device was left overnight at a temperature of 80°C to obtain an ideal bond. The dimensions of the samples demonstrated in Figure 8A were 25×75 mm with bonded PDMS at the ends. During the tensile strength test, they were secured by jaws to ensure uniform pressure distribution.

The resulting graph was obtained, for force vs elongation. Both chemically and corona-discharge bonded samples are seen to exhibit a linear behaviour under elongation. The key observation from this mechanical test was the average failure load. The failure load or break point for the chemically-bonded sample was found to be 30% higher, concluding that the sample could withstand more pressure when compared to the corona-discharge bonded one.

The authors managed to develop a versatile way to test two different kinds of bondings with the help of high accuracy pressure-driven flow-controlled technology. It provides the flexibility & accuracy required to test the leak resistance of the fabricated microfluidic system under various elaborate test conditions.

If you’re interested in what the authors have achieved, feel free to contact our team of experts. For a detailed insight into the study, please refer to the original paper by M-T. Bakouche et al.

References