High adhesion strength and hybrid irreversible/reversible full-PDMS microfluidic chips

Highlights

• SWB offers simple, fast, and low cost operation requiring only a laboratory oven.

• Microdevices herein described showed burst pressures of up to 4.5 MPa.

• Reversible behavior is crucial in stages of research and development.

• Chips obtained after successive detachment/bonding steps withstood high pressure.

• Emulsifications at high flow rates produced monodisperse dispersions.

Abstract

To the best of our knowledge, this paper outlines for the first time high adhesion and hybrid irreversible/reversible microfluidic devices fully composed of polydimethylsiloxane (PDMS). These chips were fabricated by the sandwich bonding (SWB), a method that was recently deployed by our group. SWB offers simple, fast, and low cost operation requiring only a laboratory oven. The devices showed burst pressures of up to 4.5 MPa. This value is more than tenfold the pressures withstood by the full-PDMS chips described in literature. In terms of the reversible behavior, the ability for disassembling the chip slides is crucial in research and development stages, especially when the device integrates high-cost components or harsh cleaning steps are needed. Following successive steps of detachment and bonding, the channels still withstood high pressures of approximately 1.8 MPa. Finally, the emulsification of corn oil 4.0% w/w to polyglycerol polyricinoleate with 10.0 μmol L⁻¹ rhodamine B aqueous solution was realized to show the relevance in enhancing the flow rate in microfluidics. Such experiment was conducted at total flow rates of 0.8–160.0 μL min⁻¹. The decrease in size and polydispersity of the droplets was observed at increasing flow rates. Monodisperse emulsions were achieved only at 160.0 μL min⁻¹.

Introduction

Polydimethylsiloxane (PDMS) is the polymer most used in microfluidic applications. This elastomer has greatly streamlined the use of microscale platforms in academia and industry by providing a simple and fast fabrication. PDMS is low-cost, optically transparent, flexible, biocompatible, and capable of integrating multifunctional units with the intent to deploy lab-on-a-chip devices. In addition, such polymer ensures a conformal covering over large areas even for nonuniform surfaces. This property bypasses the use of cleanroom facilities in some cases and contributes for the bonding step once the van der Waals interactions between the slides are increased [1], [2], [3], [4], [5]. For instance, reversibly bonded PDMS/glass channels show a burst pressure of 0.04 MPa [6].

With regard to its drawbacks for applications in microfluidics, the PDMS exhibits nonspecific absorption and adsorption, poor efficiency separation of hydrophobic compounds in microchip capillary electrophoresis (MCE), and swelling in different organic media. Meanwhile, methods to solve these disadvantages were already described in the literature. Some alternatives include bulk modification [7] or coating of the PDMS surface [8] with SiO₂ and removal of unreacted oligomers followed by oxidation in air plasma [9], [10]. Conversely, a little attention has been focused on other substantial disadvantage of PDMS-based microfluidic chips, namely: the low adhesion strength of such devices.

The techniques commonly used for irreversibly bonding PDMS devices rely on surface oxidation [11], [12], [13], adhesive layer [14], [15], [16], [17], [18], [19], [20], or chemical modification with different functional groups [21], [22], [23], [24], [25], [26]. In all of these cases, the adhesion strength is somewhat poor with burst pressures ranging from 0.2 to approximately 1.0 MPa considering full-PDMS and PDMS-based hybrid channels. For instance, the glass microdevices (require a more complex fabrication) withstand pressures of up to 5.3 (sacrificial adhesive bonding) [27], 7.0 (thermal bonding) [28], and 13.9 MPa (bonding assisted by surface modification) [29].

The capacity of the device to endure high flow rates is important by increasing the mass transfer rate. This phenomenon assists the processes of mixing. In harsh conditions of flow rate, e.g., two liquids into Y-shaped channels can be homogeneously mixed even without the presence of intricate structures (passive mixers) or external perturbations (active mixers) [30]. The generation of fast mixings presents significant implications in diverse areas such as inertial particle focusing, chemical synthesis, emulsification, protein folding, flash chemistry, and high pressure liquid chromatography [29], [30]. Indeed, the development of approaches for enhancing the mixing efficiency is one of the current challenges in microfluidics. Herein, a laminar flow is observed limiting the homogenization of the reactants by diffusion. The mixers incorporated into the microfluidic chips are based on the coupling of the diffusion with chaotic advection [5].

This manuscript describes the fabrication of high adhesion strength PDMS/PDMS microfluidic platforms, presenting burst pressures of up to 4.5 MPa. Such value is more than tenfold the pressures withstood by the full-PDMS chips described in the literature [12], [21], [22]. The chips herein addressed were fabricated using the sandwich bonding (SWB). This method was recently proposed by our group for hybrid PDMS microchannels (with glass and metals) [31], presenting simple operation, low cost, and short time consumption. Furthermore, the SWB eliminates the use of surface oxidation, half-cured PDMS as adhesive layer, organic solvents, or surface chemical modifications. Therefore, this technique adds flexibility in surface modifications (before the bonding), alignment, enclosing of submicron structures, and fabrication of hybrid platforms. The SWB relies on the reversible bonding of a flat coverslip over the substrate slide that contains the channels. The coverslip is smaller than the substrate, leaving a border around this latter exposed. Then, a liquid composed of PDMS monomers and curing agent is poured onto the previous structure, covering the exposed sides of the substrate and the entire surface of the coverslip. After the cure of the cover, the bonding is completed. In this paper, all the three pieces of the device (substrate, coverslip, and cover) were composed of PDMS. Otherwise, holes were engraved on the coverslip to improve the adhesion strength of the device by increasing the contact area between substrate and cover as discussed next.

Apart from the high adhesion strength, the SWB PDMS devices exhibited the remarkable property of irreversible/reversible hybrid behavior as observed for the SWB PDMS/glass channels [31]. The reversible nature was achieved peeling off the cover in acetone. Therefore, the substrate and coverslip could be detached for reuse. The devices obtained after the cover removal remained withstanding high flow rates. This hybrid behavior is observed only on the SWB method. The ability to disassemble is crucial in stages of research and development, especially when the microdevice incorporates high-cost functional units, functionalizations need to be repeated, or a harsh cleaning is needed [32], [33]. The main advantage of the SWB over the reversible bonding techniques addressed in the literature is the high adhesion strength. These approaches usually rely on aspiration [34], [35], magnetism [36], [37], or packing [38], [39], [40]. Aside from problems such as additional apparatuses and restrictions for microdevices with high density of microfluidic structures, such reversible bonding techniques produced a poor adhesion strength, ranging from 0.05 to 0.16 MPa only.

We expect this paper can assist the researchers to create new PDMS microfluidic chips to high flow rate-based applications. The following tests are shown here: adhesion strength for different experimental conditions, reversibility, and run-to-run repeatability in a microchip electrophoresis. In addition, processes of mixing and emulsification were realized at increasing flow rates to show the relevance in achieving high flow rates in microfluidics. In these applications, we monitored the mixing homogeneity and the dimension and polydispersity of the emulsion droplets.