Microencapsulation is widely used in the manufacturing of pharmaceuticals, and it is a process where a small particle of one material is enclosed within a shell produced from another. This process can also be used in other areas such as solar power and self-repairing materials.
However, most applications of microencapsulation need particles of the same size, and the existing fabrication techniques fail to reliably provide this. In pharmaceuticals and other high profit margin products it can be cost-effective to mechanically divide particles of the appropriate size from those particles that are too small or too large, but this is not possible in niche or small-margin products.
The recent issues of the journal Lab on a Chip features a report presented by researchers from MIT’s Microsystems Technology Laboratories. In this report, the researchers talk about a new microencapsulation method that helps to obtain particles of extremely reliable sizes, while at the same time increasing the rate of production.
An affordable commercial 3D printer was used to manufacture the devices that helped produce the spheres. The potential to 3D print fabrication systems will not only maintain low manufacturing costs but will also enable researchers to rapidly develop systems to create microencapsulated particles for specific applications.
When you print your microsystems, you can iterate them very fast. In one year, we were able to make three different generations that are significantly different from one another and that in terms of performance also improve significantly. Something like that would be too expensive and too time consuming with other methods.
Luis Fernando Velásquez-García, Principal Research Scientist, Microsystems Technology Laboratories
Velásquez-García and Olvera-Trejo, a postdoc at Mexico’s Tecnológico de Monterrey, contributed to this paper. Olvera-Trejo is a visiting researcher at MIT under the auspices of a new nanoscience research partnership between the two universities.
The new system developed by the researchers uses the same key technology that was earlier explored by Velásquez-García’s group in order to deposit material on chip surfaces, generate X-rays, etch chips, propel nanosatellites, and spin out nanofibers for use in a variety of applications.
All of these applications rely on dense arrays of emitters that eject streams of ions, electrons, or fluids. The emitters could be rectangular, cylindrical, or conical; 3D printed or etched microscopically; solid, or hollow, like nozzles. In all instances Velásquez-García’s team used electric fields to monitor their emissions instead of using microfluidic pumps.
The new emitters are an alternative on the hollow 3D-printed design. A concentric ring and a hole are the two openings that each emitter comprises of, instead of just having one opening at its tip. These openings are fed by individual microfluidic channels.
If the electrical conductivity and viscosity of the fluids fed through the channels, the diameter and length of the channels, and the strength of the electric field that draws them up are accurately calibrated, the emitters will develop extremely small spheres where the material drawn through the outer ring covers the material drawn through the center hole.
Velásquez-García states that the physics explaining the relationship of forces that creates the microcapsules is just around a decade old. Other researchers have developed separate emitters that can create microcapsules, but Velásquez-García’s team is the first to position the emitters in a monolithic array - 25 emitters packed on a chip that is less than an inch square - while maintaining both uniformity and efficiency. The modular design of the arrays allow them to be attached together in order to create larger arrays.
Microencapsulation is used by pharmaceuticals manufacturers to protect drugs from degradation before they could attain their targets. Researchers have also explored microencapsulation as a process to develop self-healing materials: The same stress that leads to cracking of a material also breaks the capsules, discharging an epoxy capable of patching the crack.
There, consistency of capsule size is significant to ensure that distributing the capsules throughout the material does not compromise its structural integrity.
Yet another possible application of microencapsulation, dye-sensitized solar cells are a cost-effective alternative to silicon solar cells. They use extremely small particles of dye-coated metal suspended in another material, mostly a fluid.
The dye changes light to electricity, which is then transmitted by the metal to electrodes. The efficiency of the cell is maximized by preserving an accurate ratio of dye-covered surface area to volume of metal.
In their primary experiments, Velásquez-García and Olvera-Trejo used sesame oil and water as their fluids, and the emitters were produced from plastic. The obtained microspheres were almost 25 μm in diameter. However, there are 3D printers that use ceramics or metal, capable of developing emitters that have the potential to bear harsher or hotter fluids.
The research team used helical fluid channels to pack the emitter arrays into the smallest possible volume. These channels spiral around the interiors of the emitters, minimizing their height.
The channels taper from 0.7 mm at their bases to 0.4 mm at their tips in order to monitor the emission rate. Velásquez-García states that it would be virtually impossible to develop these complex and small devices by using conventional microfabrication processes.
“These devices can only be made if you print them,” Velásquez-García says. “We’re not doing printing because we can. We’re doing printing because it enables something that didn’t exist before that brings very exciting possibilities.”
The full implications of this are so large that it’s not easy to fully appreciate what this could do. It has the possibility of revolutionizing the making of very sophisticated large-area devices. This would be the kind of technology that would allow you to do the Internet of things, to build functionality into structures at much, much lower cost than you could by gluing a silicon chip on. And you could actually have higher performance because the sensing is built into the physical structure. My group would be users of this, and many of the faculty using the [Stanford nanofabrication] facility would be very excited to get their hands on this.
Roger Howe, Professor, Stanford University