Microfluidics bonding techniques can be indirect or direct. Indirect bonding involves the use of an adhesive layer to seal two substrates and encapsulate microchannels fabricated in one or both of the substrates. Direct bonding methods permanently bond the two substrates without any additional materials. Indirect bonding methods require an intermediate adhesive that results in channel sidewalls with different chemical, optical and mechanical properties than the bulk polymer. This may be a preferable requirement as a more hydrophobic surface may facilitate the fluidics of your sample and reagents.

In general, bonding forces between mating surfaces arise from either molecular entanglement or charge interactions. Entanglement can occur by mechanical interlocking of diffusion between surfaces, while bonding due to charge interactions can result from electrostatic or chemical (covalent) bonding, acid-base interactions, or van der Waals forces.

Thermoplastic bonding methods like thermal fusion bonding, ultrasonic bonding, chemical and solvent bonding, and laser welding are mainly achieved by molecular entanglement.

Your initial inclination may be to focus on the bonding method that offers the most consistent adhesion for your specific assay and substrate but there are many other factors to consider. Our suggestion is that you create a bonding method evaluation rating sheet like the one shown below. This will allow you to objectively rate each method you test. To take this to the next level, weight each specific characteristics that are most important to your assay rather than weighting each one equally.

Bonding two pieces of thermoplastic together is trickier than it may seem which is why you need to spend time understanding those limitations before you choose a specific method. Shown below are some of the most common bonding methods along with pros and cons of each.

Once you have narrowed down a few bonding methods you believe to be a good match with your particular assay, you'll need to conduct some performance tests. These tests can take many forms from simple to complex. Here are some common methods engineers use to test their bonding technology.

Delamination Tests - Just like it sounds this test relies on brute force to peel layers apart. This might be done with a scalpel or knife to wedge in between the two layers and pry them apart. While this is the cheapest and easiest test to conduct, there is no quantitative data output and it's difficult to reproduce consistently. This is the kind of test you might conduct when you're first doing initial “quick and dirty” comparative tests of different bonding techniques.

Peel Resistance Tests - This test requires one or more of pieces of the substrate to be flexible and be able to bend to 90 degree angle without breaking. Using a clamp, one piece of the substrate is bent to a 90 degree angle, slowly peeling it away from the other piece to which it is bonded. The peel force needed to separate the two pieces is then measured.

Tensile Strength Tests - Using a common tensile testing machine, grips or stronger adhesives are used to forcibly separate the two layers and in doing so measure the tensile strength of the bond. This method is commonly chosen because it is highly reproducible.

Shear Strength Tests - Although not as commonly used, this test uses a technique of sliding the substrate laterally along the bonding surface to measure the force needed to separate it from the lower piece which is held stationary. The challenge with this test is that the layers separate gradually and require a lot of monitoring.

Leakage Tests - This is definitely one of the more reliable ways to test the efficacy of the bond. Leakage tests are pretty simple to understand - you inject a liquid into the microchannels and see what happens over a set period of time. Often these tests are done using colored water/dye so leaks can easily be spotted. Other assays may necessitate using specific solutions that test durability and mechanical effects, e.g., mixing. Liquid injected can be static (pressurized or not) and/or have specific flow rates applied to also test component resilience.

Burst Pressure Tests - In this test, a specific PSI of pressure (example: 30-60 PSI) is applied in the form of gas or liquid and can be used to test the bond between substrates or between the blister and the substrate in the case of onboard reagent storage. For higher heat applications (e.g. PCR), it’s important to simulate the maximum reaction temperature with your test liquids since adhesives sometimes fail at higher temperatures.