In the very early stages, many designers start fabricating with silicone or 3D printing using commercially available pumps and valves – a typical and reasonable approach for demonstrating proof of concept and fundraising. The next step is to get to next level of reproducibility and accuracy with a more developed design that might involve producing small lots (10-100 units).

By creating small lots of prototypes with different features (e.g., geometries, channel designs and surface chemistries) the fluidic behavior of the microfluidic channel “circuits” can be tested and further refined.

There is no standardized approach providing the right combination of materials and processes to use for your prototyping project. There are so many different combinations of substrate materials and methods that when the complexities of reagent properties are added, you really need an expert on your side to make sense of the options and the development path. The table below provides a summary of the basic pros and cons of different microfluidic prototyping methods. Each has its respective trade-offs, and the best choice depends on the application and stage of development. This is not a comprehensive list, but it does include the most commonly applied techniques.

The method you choose to make your prototypes is important but your chosen materials will dictate compatibility and scalability. This is an often-misunderstood area vital to the performance of your device. Several different areas need to be considered when choosing materials.

• Chemical compatibility and surface chemistries -  Need to make sure  your cartridge substrate is compatible with your chosen reagents and   targets (e.g. contact angle, biocompatibility)
• Heat resistance and thermal properties
• Optical properties - transparency, auto fluorescence
• Mechanical strength and/or flexibility
• Bonding or assembly options
• Cost to scale/speed of production

Polydimethylsiloxane (PDMS) is one of the most commonly used thermoplastic substrates for making microfluidic cartridge prototypes and for limited scale production. While PDMS is affordable, easy to fabricate, able to hold tight tolerances and is compatible with many liquids, it can be somewhat limiting the technique does not provide the required draft angle in the microchannels needed separation in any scale injection molding process. PDMS is an acceptable “bridge” material but it is not suitable for high volume manufacturing – the goal is to get to the final material as quickly as possible.

Therefore, other polymers representing final commercial product material can be considered for prototype injection molding and micromachining as well. The following polymers are commonly used for injection molding, micro machining prototypes and commercial products. They tend to have good mechanical and optical properties and are easier to fabricate than materials such as silicon or glass:

• Polymethyl Methacrylate (PMMA)
• Cyclic Olefin / Copolymers (COC, COP)
• Polycarbonate (PC) 

Your device design is solid. You have tested the microfluidic circuit and individual components. You have considered all material options. And you know which methods of prototyping makes the most sense for your disposable cartridge. Have you considered how it all the pieces literally come together?

Bonding is an often-underestimated discipline of microfluidic prototyping but it is vital to the performance of your device. There are several possible bonding
methods used in microfluidic cartridges including pressure-sensitive and resin adhesives, thermal fusion, ultrasonic, laser welding and solvents. We won’t go into the specifics of which ones are best for specific applications.

Prototypes also offer and excellent way to test valves, switches, micromixers, reaction chambers and other features as parts of a fluidic “circuit”, using a methodology known as “couponing.” If you initially test fully integrated designs and assemblies it can be very difficult to isolate the root cause of problems identified, i.e., substrate material, bonding techniques or method used to fabricate your assay. By testing prototypes of the valves, mixers and chambers separately you can isolate the functionality of flow mixing and switching first, and then add other features such as chambers. This is less risky than assembling all components and testing them together at one time before you commit to a more advanced design. You don't want to end up creating a final prototype mold and then make small changes thereafter as this is far more costly than testing components separately and can slow your time to market.

It is very tempting to take shortcuts and move into production quickly to satisfy overly ambitious and optimistic timelines. However, based on 20+ years of microfluidic design and manufacturing experience, we have seen those shortcuts turn into production fiascos that significantly hamper the commercialization efforts of the company. Having a partner that understands microfluidic thermoplastic material options, surface chemistry, bonding techniques and fluid dynamics is critical. Understanding how all of those things affect your final design and transition to full scale manufacturing is even more vital.

Choosing experienced partner may seem more expensive initially but is less costly overall when you consider that fewer iterations will be needed and your time to market may be shorter.