The overall goal of this technique is to provide a simple and versatile method for fabricating thermoplastic microfluidic devices with high quality bonds. This solvent bonding procedure will allow us to fabricate well bonded and durable thermoplastic microfluidic devices for a variety of applications from microfluidic cell culture systems to prototypes for point of care diagnostic tools. The main advantage of this technique is that it allows us to quickly and easily bond thermoplastic device layers.
The procedure can be easily adapted to bond different thermoplastic materials. Design and fabricate microfluidic device layers from the thermoplastic of choice, using an appropriate fabrication method. Visually inspect the device layers to ensure that edges are clean without burrs or ridges of leftover material from the fabrication process.
For best results, check all machined microfeature edges, in addition to the outside edges of the device, under an optical microscope. 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. Then, clean the device surfaces with laboratory soap and water.
And, dry with compressed air. After submerging the device layers in 2-propanol for two minutes, dry them with compressed air. This video is demonstrated with poly methyl methacrylate, abbreviated PMMA.
Please see the text protocol for slight alterations in the procedure when using cyclic olefin polymer, or COP. Preheat, a heated press to 70 degrees celsius and allow the temperature to stabilize. To prepare the solvent for bonding process, measure 0.5 milliliters of ethanol, per square inch of bonding area for PMMA.
It's important that an appropriate volume of solvent is applied to the device layers because too much solvent can lead to challenges in aligning layers. Dispense 0.1 milliliters of solvent per square inch of bonding area between cleaned plastic layers and bring the layers together. Visually inspect the bonding interface for air bubbles, which are common and should be removed as much as possible.
If bubbles are present, slide the two plastic layers along the bonding interface so that they nearly come apart. And, then, slide them back together. To align the layers of the device by hand, use fingers to align the outer edges of the device.
If more accurate alignment is required, include L shaped corner brackets to hold the device layers together. Next, place the device with solvent into the preheated press and apply 2, 300 kPa of pressure for two minutes. Use tweezers to safely remove the hot device for inspection.
Bonding is now complete. Finally, remove any remaining liquid in the device using compressed air. Shown here are representative results of a pair of device layers before cleaning and after cleaning.
The device layers have been milled to remove burrs. A representative image of a device with solvent added shows bubbles present through the bonding surface that can inhibit bonding. Conversely, well distributed solvent that is free of bubbles leads to strong, high quality bonds.
Rapid solvent evaporation, during heating, can lead to regions of unbonded material near free edges of the device, resulting in poor bond coverage, leaky microfluidic features, and low overall bond strength. These regions are visible as lighter patches with colored interference fringes. An optimized solvent plastic system achieves good bond coverage and strength and minimizes damage to microfeatures, while still properly sealing them.
Once mastered, a new thermoplastic device design can be fabricated in as little as two hours if the bonding technique is done properly. Remember that working with cyclohexane can be hazardous and should be handled in a fume hood and while wearing personal protective equipment. After watching this video, you will have a simple and versatile method for fabricating thermoplastic microfluidic devices with high quality bonds that are leak free and appropriate for use in cell culture, in point of care diagnostic applications.
