Researchers have shown that a MEMS-based gas sensor manufactured with a desktop device performs at least as well as commercial sensors built at conventional production facilities.
In the other paper, they show that the central component of the desktop fabrication device can itself be built with a 3-D printer. Together, the papers suggest that a widely used type of MEMS gas sensor could be produced at one-hundredth the cost with no loss of quality.
The researchers’ fabrication device sidesteps many of the requirements that make conventional MEMS manufacture expensive. “The additive manufacturing we’re doing is based on low temperature and no vacuum,” says Luis Fernando Velásquez-García, a principal research scientist in MIT’s Microsystems Technology Laboratories and senior author on both papers. “The highest temperature we’ve used is probably 60 degrees Celsius. In a chip, you probably need to grow oxide, which grows at around 1,000 degrees Celsius. And in many cases the reactors require these high vacuums to prevent contamination. We also make the devices very quickly. The devices we reported are made in a matter of hours from beginning to end.”
Researchers used electrospray emitters that had been built using conventional processes. However, in the December issue of the Journal of Microelectromechanical Systems, Velásquez-García reported using an affordable, high-quality 3-D printer to produce plastic electrospray emitters whose size and performance match those of the emitters that yielded the gas sensors.
In addition to making electrospray devices more cost-effective, Velásquez-García says, 3-D printing also makes it easier to customize them for particular applications. “When we started designing them, we didn’t know anything,” Velásquez-García says. “But at the end of the week, we had maybe 15 generations of devices, where each design worked better than the previous versions.”
The advantages of electrospray are not so much in enabling existing MEMS devices to be made more cheaply as in enabling wholly new devices. Besides making small-market MEMS products cost-effective, electrospray could enable products incompatible with existing manufacturing techniques.
There are plenty of very promising materials for new sensors, utilizing nanostructured materials, which have been published in high-level scientific papers, haven’t resulted in reliable products.
Abstract – Electrospray-printed nanostructured graphene oxide gas sensors
We report low-cost conductometric gas sensors that use an ultrathin film made of graphene oxide (GO) nanoflakes as transducing element. The devices were fabricated by lift-off metallization and near-room temperature, atmospheric pressure electrospray printing using a shadow mask. The sensors are sensitive to reactive gases at room temperature without requiring any post heat treatment, harsh chemical reduction, or doping with metal nanoparticles. The sensors’ response to humidity at atmospheric pressure tracks that of a commercial sensor, and is linear with changes in humidity in the 10%–60% relative humidity range while consuming less than 6 μW. Devices with GO layers printed by different deposition recipes yielded nearly identical response characteristics, suggesting that intrinsic properties of the film control the sensing mechanism. The gas sensors successfully detected ammonia at concentrations down to 500 ppm (absolute partial pressure of ~5 × 10^−4 T) at ~1 T pressure, room temperature conditions. The sensor technology can be used in a great variety of applications including air conditioning and sensing of reactive gas species in vacuum lines and abatement systems.
Abstract – SLA 3-D Printed Arrays of Miniaturized, Internally Fed, Polymer Electrospray Emitters
A paper reports the design, fabrication, and characterization of arrays of miniaturized, internally fed, polymer electrospray emitters fabricated with stereolithography. The freeform additive manufacturing process used to make the devices has associated two orders of magnitude reduction in the fabrication cost per device and fabrication time (from thousands of dollars to tens of dollars, and from months to hours, respectively) and a two orders of magnitude reduction in the cost of the manufacturing infrastructure (from millions of dollars to tens of thousands of dollars) compared with a silicon MEMS multiplexed electrospray source. The 3-D printed devices include features not easily attainable with other microfabrication methods, e.g., tapered channels and threaded holes. Through the optimization of the fabrication process 10-mm tall, isolated, straight, solid columns with diameter as small as 300 μm, and 12-mm long, straight tubes with inner diameter as small as 400 μm and wall thickness as small as 150 μm were demonstrated. Arrays with as many as 236 internally fed electrospray emitters (236 emitters in 1 cm2) were made, i.e., a twofold increase in emitter density and a sixfold increase in array size compared with the best reported values from multiplexed, internally fed, electrospray sources made of polymer. The characterization of devices with a different array size suggests a uniform emitter operation.