(Nanowerk News) The mass spectrometer is a highly precise chemical analyzer that has many applications, from evaluating the safety of drinking water to detecting toxins in a patient’s blood. But building an inexpensive, portable mass spectrometer that can be used in remote locations remains a challenge, partly due to the difficulty of shrinking the vacuum pumps required to operate at low cost.
MIT researchers used additive manufacturing to take a major step toward solving this problem. They 3D printed miniature versions of a type of vacuum pump, known as a peristaltic pump, which is about the size of a human fist.
Their pumps can create and maintain a vacuum at an order of magnitude lower pressure than so-called dry and coarse pumps, which do not require liquid to create a vacuum and can operate at atmospheric pressure. The researchers’ unique design, which can be printed in one go on a multimaterial 3D printer, prevents liquid or gas from leaking while minimizing frictional heat during the pumping process. This increases the service life of the device.
These pumps can be incorporated into portable mass spectrometers used to monitor soil contamination in isolated parts of the world, for example. The device is also ideal for use in Mars-bound geological survey equipment, as it would be less expensive to launch a light pump into space.
“We’re talking about very cheap hardware that’s also very capable,” said Luis Fernando Velásquez-García, principal scientist at MIT’s Microsystem Technology Laboratory (MTL) and senior author of a paper describing the new pump (“Compact peristaltic vacuum pump via multi-material extrusion”). “With a mass spectrometer, a 500 pound gorilla indoors is always a pump problem. What we’re showing here is groundbreaking, but it’s only possible because it’s 3D printed. If we wanted to do this the standard way, we wouldn’t approach it
Velásquez-García is joined on paper by lead author Han-Joo Lee, a former MIT postdoc; and Jorge Cañada Pérez-Sala, a graduate student in electrical engineering and computer science. This paper appears today in Additive Manufacturing.
When the sample is pumped through the mass spectrometer, electrons are released to convert the atoms into ions. An electromagnetic field manipulates these ions in a vacuum so that their mass can be determined. This information can be used to precisely identify sample constituents. Maintaining the vacuum is key because, if the ions collide with gas molecules from the air, the dynamics change, reducing the specificity of the analytical process and increasing false positives.
Peristaltic pumps are usually used to move liquids or gases that will contaminate pump components, such as reactive chemicals. They are also used to pump fluids that need to be kept clean, such as blood. The substance being pumped is entirely contained in a flexible tube that is wound around a set of rollers. The rollers press the tube against its housing as it rotates. The squeezed portion of the tube expands after the capstans, creating a vacuum that pulls liquid or gas through the tube.
While these pumps create a vacuum, design problems have limited their use in mass spectrometers. The tube material redistributes when force is applied by the rollers, which creates gaps that lead to leaks. This problem can be solved by operating the pump quickly, forcing fluid through it faster than it can leak. However this causes overheating which damages the pump, and gaps remain. In order to completely seal the tube and create the vacuum necessary for the mass spectrometer, the mechanism must provide additional force to compress the protruding areas, causing more damage, explains Velásquez-García.
He and his team rethought peristaltic pump design from the ground up, looking for ways they could use additive manufacturing to make improvements. First, by using a multimaterial 3D printer, they were able to fabricate flexible tubes from a special type of hyperelastic material that can withstand large amounts of deformation.
Then, through an iterative design process, they determined that adding notches to the tube wall would reduce the stress on the material when squeezed. With notches, the tube material does not need to be redistributed to counteract forces from the rollers.
The manufacturing precision afforded by 3D printing allowed researchers to produce the precise notch sizes needed to eliminate gaps. They can also vary the thickness of the tube so that the wall is stronger in the area where the connector is attached, thereby reducing stress on the material.
Using a multimaterial 3D printer, they printed the entire tube in one go, which is important because post-assembly can introduce defects that can lead to leaks. To do this, they had to find a way to score the thin, flexible tube vertically while preventing it from jiggling during the process. Ultimately, they create a lightweight structure that stabilizes the tube during printing but can be easily peeled off later without damaging the device.
“One of the main advantages of using 3D printing is that it allows us to aggressively prototype. If you do this work in a clean room, where many of these miniature pumps are made, it will take a lot of time and money. If you want to make changes, you have to start the whole process over from scratch. In this case, we can print our pumps in a matter of hours, and each time it can be a new design, ”says Velásquez-García.
Portable, yet performing
When they tested their final design, the researchers found that it was capable of creating a vacuum that has a lower pressure than state-of-the-art diaphragm pumps. Lower pressure produces a higher quality vacuum. To achieve the same vacuum as a standard diaphragm pump, one would need to connect three in a series, says Velásquez-García.
The pump reaches a maximum temperature of 50 degrees Celsius, half that of the state-of-the-art pumps used in other studies, and only takes half the force to completely close the tube.
“Fluid movement is a huge challenge when trying to build small and portable equipment, and this work elegantly leverages the advantages of multimaterial 3D printing to create a highly integrated and functional pump for creating a vacuum for gas control. Not only is the pump smaller than almost anything similar, but it produces a vacuum 100 times lower,” said Michael Breadmore, professor of analytical chemistry at the University of Tasmania, who was not involved in the work. “This design was only possible using a 3D printer and it nicely demonstrates the ability to design and create in 3D.”
In the future, the researchers plan to explore ways to further reduce the maximum temperature, which will allow the tubes to move faster, create a better vacuum, and increase flow rates. They are also working on 3D printing the entire mass spectrometer miniature. As they develop the device, they will continue to refine peristaltic pump specifications.
“Some people think that when you 3D print something there must be some kind of compromise. But here our group has shown that this is not the case. This is really a new paradigm. Additive manufacturing will not solve all the world’s problems, but it is a solution that has real legs, ”says Velásquez-García.