Solvent Bonding for Fabrication of PMMA and COP Microfluidic Devices

Summary

Solvent bonding is a simple and versatile method for fabricating thermoplastic microfluidic devices with high quality bonds. We describe a protocol to achieve strong, optically clear bonds in PMMA and COP microfluidic devices that preserve microfeature details, by a judicious combination of pressure, temperature, an appropriate solvent, and device geometry.

Abstract

Thermoplastic microfluidic devices offer many advantages over those made from silicone elastomers, but bonding procedures must be developed for each thermoplastic of interest. Solvent bonding is a simple and versatile method that can be used to fabricate devices from a variety of plastics. An appropriate solvent is added between two device layers to be bonded, and heat and pressure are applied to the device to facilitate the bonding. By using an appropriate combination of solvent, plastic, heat, and pressure, the device can be sealed with a high quality bond, characterized as having high bond coverage, bond strength, optical clarity, durability over time, and low deformation or damage to microfeature geometry. We describe the procedure for bonding devices made from two popular thermoplastics, poly(methyl-methacrylate) (PMMA), and cyclo-olefin polymer (COP), as well as a variety of methods to characterize the quality of the resulting bonds, and strategies to troubleshoot low quality bonds. These methods can be used to develop new solvent bonding protocols for other plastic-solvent systems.

Introduction

Microfluidics has emerged over the past twenty years as a technology well suited for studying chemistry and physics at the microscale¹, and with growing promise to significantly contribute to biology research^2-4. The majority of microfluidic devices have historically been made from poly(dimethylsiloxane) (PDMS), a silicone elastomer that is easy to use, inexpensive, and offers high quality feature replication⁵. However, PDMS has well-documented shortcomings and is incompatible with high-volume fabrication processes^6,7, and as such, there has been a growing trend toward fabricating microfluidic devices from thermoplastic materials, because of their potential for mass manufacturing and thus commercialization.

One of the major barriers to wider adoption of plastic microfabrication has been achieving easy, high quality bonding of plastic devices. Current strategies employ thermal, adhesive, and solvent bonding techniques, but many suffer from significant challenges. Thermal bonding increases autofluorescence⁸ and often deforms microchannel geometries^9-11, while adhesive techniques require stencils, careful alignment, and ultimately leave the thickness of the adhesive exposed to the microchannel¹⁰. Solvent bonding is attractive due to its simplicity, tunability, and low cost^10,12-14. In particular, its tunability enables optimization for a variety of plastics, which can yield consistent, high quality bonding that minimizes deformation of microfeatures¹⁴.

During solvent bonding, solvent exposure increases the mobility of polymer chains near the surface of the plastic, which enables inter-diffusion of chains across the bonding interface. This causes entanglement via mechanical interlocking of the diffusing chains, and results in a physical bond¹⁰. Thermal bonding works in a similar manner, but relies on elevated temperature alone to increase chain mobility. Thus, thermal methods require temperatures near or above the glass transition of the polymer, whereas the use of solvents can significantly reduce the temperature needed for bonding, and thus reduce unwanted deformation.

We provide a specific protocol for bonding both PMMA and COP devices. However, this protocol and method describes a simple, generic approach for solvent bonding of thermoplastic microfluidic devices that can be tailored for other plastic materials, solvents, and available equipment. We describe numerous methods for assessing the quality of bonds (e.g., bond coverage, bond strength, bond durability, and deformation of microfeature geometries), and provide troubleshooting approaches to address these common challenges.

Protocol

Note that all of the steps described below have been developed and performed in a non-cleanroom environment. The solvent bonding steps can certainly be performed in a cleanroom, if available, but this is not required.

1. Preparation of Thermoplastic Microfluidic Device Layers

1. Design and fabricate microfluidic device layers from the thermoplastic of choice, using an appropriate fabrication method (e.g., micromilling¹⁵, embossing^16-18, injection molding).
2. Visually inspect device layers to ensure that edges are "clean" (i.e., no burrs or ridges of leftover material from the fabrication process). For best results, check all machined micro-feature edges in addition to the outside edges of the device under an optical microscope.
3. If leftover material is found during visual inspection, use a razor blade, or scalpel to carefully remove any material that prevents the device layers from lying flat against one another so that the interfaces of the layers come into conformal contact.
4. Clean device surfaces with laboratory soap and water and dry with compressed air. Submerge device layers in 2-propanol for 2 min and dry with compressed air.

2. Solvent Bonding

1. Prepare heated press (for PMMA) or hotplate (for COP).
   1. For PMMA (cast acrylic, glass transition temperature of ~100-110 °C)¹⁸ preheat press to 70 °C, and allow temperature to stabilize.
   2. For COP (glass transition temperature of 102 °C, from manufacturer), preheat hotplate to 25 °C, and allow temperature to stabilize.
2. Prepare solvent for bonding process.
   1. For PMMA, measure 0.5 ml of ethanol per square inch of bonding area.
   2. For COP, prepare a 65:35 mixture of 2-propanol and cyclohexane, with a total volume of 0.5 ml of the mixture per square inch of bonding area.
      NOTE: For COP, use glass pipettes and containers, as cyclohexane will dissolve common polypropylene labware. Perform all mixing and bonding in a fume hood, as cyclohexane is toxic.
3. Dispense 0.1 ml of solvent per square inch of bonding area between cleaned plastic layers and bring the layers together. Visually inspect for air bubbles at the bonding interface, which are common, and should be removed as much as possible.
   NOTE: It is beneficial to work quickly once the solvent has been dispensed, as volatile solvents will begin to evaporate (and hence, solvent mixtures will change in composition).
   1. If bubbles are present, slide the two plastic layers along the bonding interface so that they nearly come apart (but remain in contact), and then slide them back together.
4. Align the layers of the device with alignment pins, a custom jig, or simply by hand (see Discussion section for further details).
   1. If using alignment pins, align the holes for the pins, and insert the pins into the device stack.
   2. If using a custom jig, insert the device stack into the jig and tighten around the device.
   3. If aligning by hand, use fingers to align the outer edges of the device.
5. Place the device with solvent into the pre-heated press (for PMMA) or onto the pre-heated hotplate (for COP).
   1. For PMMA, apply 2,300 kPa of pressure for 2 min.
   2. For COP, apply 350 kPa of pressure. Increase the temperature from 25 °C to 70 °C at a rate of 5 °C/min. After reaching 70 °C (after 9 min), bond for an additional 15 min.
6. Use tweezers to safely remove the hot device for inspection. Bonding is now complete.
7. Remove any remaining liquid in the device (in microchannels or other features).
   1. For PMMA, remove any remaining liquid with compressed air. For COP, place bonded device on hotplate and bake at 45 °C for 24 hr to remove any remaining cyclohexane.

Results

A schematic of the general solvent bonding procedure is shown in Figure 1. The easiest way to assess bond quality is to visually inspect bond coverage, since poor bond coverage is easily visible as regions of unbonded plastic, and is indicative of weak bonding. Such regions are typically near free edges (e.g., periphery of device, or near open ports or microchannels), and can also often appear around any particles of dirt or dust at the bonding interface. Poor bo...

Discussion

The feasibility of potential bonding strategies depends on available equipment. While hotplates are relatively common and free weights can be purchased inexpensively, high pressure strategies will require the use of a heated press. For example, our optimal PMMA bonding recipe requires high pressure to bond with ethanol (see Table 1), and the required pressure is not attainable for typical device sizes using free weights. Thus, if only a hotplate and weights are available, PMMA can instead be bonded with ...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
COP	Zeonor	604Z1020R080	20 kg COP Pellets - 1020R. Multiple suppliers can be used, but may affect bonding characteristics.
PMMA	McMaster Carr	8560K173	1.5 mm sheet thickness for our typical applications. Multiple suppliers can be used, but may affect bonding characteristics.
Cyclohexane	Sigma-Aldrich	227048	Cyclohexane, anhydrous, 99.5%. Multiple suppliers can be used. Toxic, requires fumehood.
Ethanol	Sigma-Aldrich	24102	Ethanol, absolute, ≥99.8% (GC). Multiple suppliers can be used.
Acetone	Sigma-Aldrich	179124	Acetone, ACS reagent, ≥99.5%. Multiple suppliers can be used.
2-Propanol	Sigma-Aldrich	278475	2-Propanol, anhydrous, 99.5%. Multiple suppliers can be used.
Hot plate(s)	Torrey Pines Scientific	HP60	Fully programmable digital hotplate. Multiple suppliers can be used.
Free weights	Cap Barbell	RPG#2	Standard cast iron plate. Multiple suppliers and different weights can be used.
Heated press	Carver	Auto CH	Auto series heated hydraulic press. Multiple suppliers can be used. A press that fits in a fumehood would allow the most flexibility (this model does not).
CNC Milling Machine	Tormach	PCNC 770	3 Axis CNC mill. Multiple suppliers can be used.
Endmills	Various	Various	Required sizes depend on designs. Multiple suppliers can be used.