4 Key Materials When Building Microfluidic Devices

Although small in size, microfluidic devices carry an enormous burden. Patients and caregivers rely on them for accurate test results and crucial information at the point-of-care. They screen for cancer, monitor blood glucose, detect pathogens. As they transport and protect samples of blood or other fluids, consistent and reliable performance is imperative. What could be more important?

Since their invention in the 1950s, microfluidic devices have come a long way, evolving to meet modern healthcare industry needs with accuracy, portability, and modularity. As they’ve evolved, so have the materials used to create them. In this article, we’ll explore the key materials needed when manufacturing microfluidic device components.

Here are what we’ve identified as the 4 key materials when it comes to building microfluidic devices:
1. Silicon and Glass
2. Paper
3. Polymers
4. Hydrogels

Please remember, as important as it is to use trusted materials, it’s equally important to partner with people who have converting expertise in medical and healthcare manufacturing.

1. Silicon and Glass for Microfluidics

Silicon and glass are grouped together because they were the original materials used for microfluidic applications. For the most part, time has left these two behind, with technological advancements taking over, including new materials such as polymer substrates, composites, and paper.

Silicon is highly resistant to organic solvents, has high thermo-conductivity, and has stable electroosmotic mobility. However, its hardness makes it difficult to handle, so converting can be challenging. Plus, dangerous chemicals are used during fabrication, and it’s expensive.

Unlike silicon, glass is optically transparent, yet it does have high thermo-conductivity and has stable electroosmotic mobility. An electrically insulating material, glass has similar drawbacks as silicon (hardness and high fabrication cost). Typical applications: on-chip reactions, droplet formation/specimen testing, and solvent extraction.

Another inorganic material that deserves mention here is ceramic. Low-temperature cofired ceramic (LTCC), an aluminum oxide-based material, comes in laminate sheets, allowing for the integration of multiple circuits (sensors, heaters, electronics) in one module.

2. Paper-Based Chips in Microfluidics

One of the most recent materials used in microfluidics, flexible, biocompatible, cellulose-based paper is an intriguing microfluidic substrate for several reasons:

• Cost — A cheap substrate, paper can easily be chemically modified through changes in composition/formulation or by using surface chemistry
• Availability — Paper is easily found around the world
• Disposability — Disposing of paper is as easy as burning or by natural degradation
• Porousness — Paper’s structure allows for a combination of flow, filtering, and separation; its capillary action pulls solutions through a device
• Visibility — Paper’s white background provides contrast for color-based detection

Paper’s downside? It can only be used for a limited number of applications because of its weak mechanical properties. Plus, it’s difficult to get intricate detail onto a chip.

3. Polymers for Microfluidics

Not long after silicon/glass chips, polymer-based chips were introduced, providing tremendous flexibility in choosing the right material with specific properties. Because of their accessibility and low cost (compared to inorganic materials), polymers are the most-commonly used microchip material today.

There are three common classifications of polymers: elastomers, thermosets, and thermoplastics.

Elastomers

PDMS (polydimethylsiloxane) is the most popular material for microfabrication because it’s easy to fabricate, it bonds strongly to glass (and other substrates), it’s optically transparent, it naturally repels aqueous solutions, and it has a reasonable cost.

This rubber-like, medical-grade silicone is also known for its flow properties and its flexibility to work with new applications. In general, PDMS is inert, non-toxic, and non-flammable.

Thermosets

This polymer’s chains are irreversibly bound together when cross-linked, creating high mechanical and physical strength. Thermosets are easy to craft quickly, they’re optically transparent, and they don’t melt or swell with certain solvents. Because of their high cost, thermosets’ applications in microfluidics are limited.

However, as far as thermosets used in microfluidics, thermoset polyester (TPE) is one of the most widely used. A hydrophobic material, TPE requires surface modification via the use of buffer additives or chemical reactions. This allows water to flow through microfluidic channels easily.

Thermoplastics

Thermoplastics (PMMA, polycarbonate, polystyrene) are likely the second-most used materials in microfabrication. Similar to thermosets, thermoplastics are highly cross-linked polymers, so they retain their shape well after cooling (soft when heated, hard when cooled). Able to be remolded multiple times, optically clear thermoplastics are resistant to permeation of small molecules, plus they’re stiffer than elastomers.

Excellent for commercial production (high rate, low cost), thermoplastics’ production process uses thermomolding (templates in metal or silicon at high temperatures). Unlike PDMS, thermoplastics cannot create conformal contact with another surface. Depending on the application, the surface may be modified by coating or surface grafting.

Biaxially oriented polypropylene (BOPP): offers good clarity, low autofluorescence with
considerable strength and is easy to process in terms of coating, printing, and slitting. Special consideration should be given to avoid surface scratching during processing.

Polyethylene terephthalate (PET): naturally clear with good strength. Low moisture transmission. Easy to add surface treatment. Very easy to process, coat, slit etc. Good rigidity with lower susceptibility to plastic deformation compared to BOPP

4. Hydrogel for Microfluidics

More and more, microfluidics are involved in biological research and biomimicking. Hydrogels are used to study micro-cultures and serve as cell support for various applications because they resemble the extracellular matrix. Unlike PDMS devices that study tissue-level cell culture, hydrogel devices are mostly cell related.

Microchannels are able to be built into hydrogels, which deliver solutions, cells, etc. Hydrogels are highly porous, and the pore sizes can be controlled to allow small molecules to diffuse through. Non-toxic to cells, hydrogel easily allows for feature designs and sizes to be molded onto it. However, because of their low strength, hydrogel microfabrication can only support micrometer scale compared to other polymers in the nanometer scale.

Advantages of hydrogel include its cost and commercial accessibility. Also, hydrogels can be molded from masters that consist of almost any material that’s insoluble in water, however, bonding is challenging because hydrogels usually don’t stick with contact alone.

The Right Partnership Complements the Right Materials

Material selection should be based on an analysis of the device’s intended use, the durability of the material and, of course, its cost. You need to use trusted materials in the construction of your microfluidics devices. Yet, to achieve consistent and reliable performance, lot after lot, it’s important to have a reliable partnership on which to lean. The top of your “must have” list should be converting expertise in medical and healthcare manufacturing.

As a 3M Preferred Converter of 3M Medical Materials and Technologies, Strouse passes these benefits along to our customers: competitive pricing, custom quantities, rapid turnaround, and access to 3M specialists, support, and technology.

Strouse works with advanced materials for microfluidic devices including:

• Bioassay compatible tapes (PCR, qPCR, and ELISA) to minimize the potential for chemical and optical interference
• Hydrophilic films for fluid transport, including specialty coatings and materials that enable efficient flow of fluids through capillary channels
• Films and specialty materials for solutions such as membranes, polyester films, coatings, engineered fluids, and more
• Spacer tapes with low build-up of residue to ensure the thickness of the material meets the specifications and requirements of your device
• Porous membranes used for filtration
• And more!

Ready to discuss how 3M microfluidic tapes and Strouse expertise can elevate your microfluidic device? Ask an engineer about adhesive solutions today!