A mold insert from MiniFAB being milled. With its milling and metrology capabilities, the company can control the depth, profile and texture of the microfluidic device. (Image Credit: MiniFAB)

When making the tiny channels, V-grooves, holes and other features that control fluids in microfluidic devices, there are a number of options to choose from, including chipmaking techniques and that headline-grabbing phenomenon known as 3-D printing.

Still, the preferred choice is often good, old-fashioned milling.

Why? Milling microfluidic devices can be relatively fast, easy and inexpensive. What’s more, the results often exceed the capabilities of competitive manufacturing techniques. On the other hand, milling can’t meet the requirements of every application, so makers of microfluidic devices must make sure in advance that milling can “cut it” in the role it will be asked to play.

Device or mold?

Milling can be used to either directly machine microfluidic devices or to machine the molds used to make these devices. One way to decide which of these options is best for a particular situation is to consider manufacturing volume. Direct machining is probably more economical for small numbers of microfluidic devices, while milling a mold would make sense for higher-volume device production, according to Bob Brown, project engineer at microfluidics molder PEP microPEP, East Providence, R.I.

Another factor in deciding whether to mill directly or mill a mold is the material from which the device will be made.

“If the material is not suitable for machining, like some type of soft polymer, you would need to machine a mold,” said Onik Bhattacharyya, vice president of sales and marketing for Chicago-based Microlution Inc., a builder of CNC micromachining centers. Microlution’s machines can mill molds made from brass, aluminum and stainless steel.

A number of milling advantages—including easy access to the technology, fast turnaround, low cost and the ability to cut many materials—make it a great method for rapid prototyping of microfluidic devices, according to Karel Domansky, a staff engineer at Harvard University’s Wyss Institute for Biologically Inspired Engineering, which mills both microfluidic devices and the molds used to make them.

Milling vs. the competition

In addition, fabrication of multi-height features is much easier with milling than with photo-lithography, which usually produces features of the same height, noted Domansky, who has 15 years’ experience making microfluidic devices at Harvard and the Massachusetts Institute of Technology.

Another disadvantage of photo-lithography is that it’s “more what they call a 2½-D process,” Bhattacharyya said. “So, if you want to make a nice rolling hill, you can’t do that with lithography.” Milling, on the other hand, “is a good option when you need to create a real 3-D geometry.”

A brass mold for a microfluidic device made from PDMS (polydimethylsiloxane). The polymer is widely used in the manufacture of microfluidic chips. (Image Credit: Microlution)

As for laser cutting, it may be faster and easier than milling, but it won’t yield consistent results, noted Jack Heald, president of Minitech Machinery Corp., a Norcross, Ga.-based builder of desktop CNC machines used to mill microfluidic devices. For example, Heald said, using a laser to cut a 100µm-thick micro-fluidic channel may result in a good bit of deviation along the channel because of inconsistencies in the burning process.

Then there’s the option of using a 3-D printer to make microfluidic devices. Though it may be all the rage these days, 3-D printing is slower than milling and won’t produce as smooth a surface due to the nature of the material-deposition process, according to Heald. In addition, he said, a 3-D printer may not give you the option of using metals, “so you end up with a plastic or composite part that may or may not be as good as metal” for a microfluidic application.

Milled materials

According to Heald, work materials commonly milled in microfluidic applications include aluminum, brass and PMMA (polymethylmethacrylate), an acrylic. All are easy to machine, he said, and PMMA isn’t affected by fluids flowing in microfluidic channels.

Because different materials have different properties—and interact with fluids in different ways—material choice for a microfluidic device depends in part on the type of fluid that will be flowing through it, Bhattacharyya said. Microlution machines have milled the devices from harder plastics, such as PEEK and polycarbonate, as well as from metals, such as steel, titanium, brass and aluminum.

Acrylic and polycarbonate are also common choices for microfluidic devices, according to Wyss Institute’s Domansky. One reason is because they respond well to thermal bonding processes that are used to produce closed microfluidic channels. But care must be taken to avoid excessive stress-cracking when milling.

The pillars shown are 10µm in diameter and 50µm high. They were machined to mold PDMS membranes with calibrated holes. (Image courtesy Minitech Machinery.)

Accuracy and tolerances

Both work materials and feature geometries impact accuracy and tolerances when milling microfluidic devices. For example, Domansky said, milling a more “challenging” material, such as polypropylene, will probably result in looser tolerances, while milling a more brittle and easier-to-machine material, such as acrylic, will allow tighter tolerances. In addition, he explained that tight tolerances will probably be more difficult to achieve when machining deep, narrow channels due to the tendency of the micromill to bend.

Another key factor is the construction of the milling machine. Heald noted that Minitech’s latest machine has a solid-black-granite base and a Z-axis column that increases stability and significantly reduces vibration. This, he said, translates into higher milling accuracy and tighter tolerances, as well as smoother finishes.

As for feature sizes that can be achieved when milling microfluidic devices, that depends on the cutting tools. According to Heald, some of today’s smallest tools are 10µm-dia., side-cutting endmills. He also believes 5µm-dia. tools may now be in use. But diminutive tools like these are difficult to work with and break very easily, he said.

On the downside

A limitation of machining microfluidic devices is that endmills and drills may not be good options for creating small features with high aspect ratios. With a small tool like a 25µm-dia. endmill, the highest aspect ratio that can be achieved is probably about 3:1, according to Microlution’s Bhattacharyya. As endmill diameters increase, he added, aspect ratios can go higher because the tools become more rigid. So, a shop using a 100µm-dia. endmill, for example, could potentially cut a microfluidic feature having a depth of 900µm or more, he said.

Still, when it comes to fabricating high-aspect-ratio features such as deep and narrow trenches, milling is limited compared to photolithography-based methods, according to Domansky.

At the Wyss Institute, researchers perform in-house micromilling to directly manufacture an extracorporeal blood cleansing device for sepsis therapy. The polysulfone or aluminum devices contain networks of surface channels milled to opposing surfaces of a thin sheet that are connected by 340µm-wide and 340µm-deep slits. (Image Credit: Wyss Institute)

In addition, he noted, the surface roughness of milled microfluidic devices is usually much greater than that produced by photolithography. This can be acceptable, however, when producing larger microfluidic features. For this reason, as well as the possible breakage and deflection of milling tools with dimensions in the tens of microns, Domansky thinks milling is best suited for making microfluidic structures with feature sizes measuring hundreds of microns or larger.

However, the smoothest possible surface isn’t always desirable in microfluidic applications, according to Oliver Rapp, engineering and business development manager for PEP microPEP. Rapp has seen cases where less-than-smooth surfaces were actually advantageous for customers’ microfluidic devices.

Even when a smooth surface is important, the story doesn’t necessarily end with the milling process. After milling, microfluidic device manufacturers that want finer finishes for their devices and molds can turn to secondary operations. When the microfluidic features are big enough, for example, mold polishing can be employed, Rapp noted. “But if you have a channel that’s so small that you can’t get a stick or ultrasonic probe in there for polishing, all bets are off,” he said. “In a case like that, you might have to look at something like very fine EDMing.”

A secondary operation, however, may not even be needed to improve upon a milled surface, Rapp maintained. “We’ve seen some pretty good surface finishes, especially from high-speed cutting tools,” he said.

“Small-diameter cutting tools require higher spindle speeds to achieve a good finish,” noted Sebastian Garst, manufacturing manager for Melbourne, Australia-based MiniFAB, which designs and manufactures microfluidic devices and molds. According to Garst, milling can produce metal surfaces with a roughness of less than 100nm. But high-quality surface finishes are expensive to produce, he added, requiring cutters made of materials such as PCD (polycrystalline diamond), which is used to mill nonferrous materials.

In addition, Garst pointed out that fine surface finishes require longer machining processes, which conflict with the goal of rapid turnaround. So, in order to get a microfluidic device with a topnotch surface, “there’s often a tradeoff,” he said.

About the author: 

William Leventon is a New Jersey-based freelance writer. He has a M.S. in Engineering from the University of Pennsylvania and a B.S. in Engineering from Temple University. Telephone: (609) 926-6447. E-mail: wleventon@verizon.net. Telephone: (609) 926-6447. E-mail:  wleventon@verizon.net.

Source and Copyright: Micro Manufacturing