Ion exchange membranes are used across industries for water desalination, energy storage, and pharmaceutical applications. Because these membranes work through ion exchange, designing membranes with low ionic resistance is a major materials engineering goal. In recent years, researchers have designed patterned ion exchange membranes with areas of varied thickness. The thinner regions help lower overall ionic resistance, allowing target ions to pass more freely through the membrane.

Patterned membranes are typically fabricated by etching a pattern on silicon or stainless steel molds from which the membranes are cast. This fabrication process can be time-consuming, costly, and difficult to alter. Now, a method of three-dimensional (3D) printing of micro-patterned ion exchange polymeric membranes, as reported in a recent study, may allow for rapid design, fabrication, and easy modifications, leading to a new way to manufacture these membranes for different applications.

“Molds take a really long time [to make],” says Michael A. Hickner, professor of materials science at The Pennsylvania State University and co-author on the study published in Applied Materials & Interfaces, referring to the traditional mode of membrane fabrication. After a design is modeled in a computer, the pattern is etched out on a silicon or stainless steel mold. The membrane is then cast in that mold, a process that typically calls for a host of organic solvents like N-Methyl-2-pyrrolidone (NMP).

“Solvents can be dangerous and environmentally toxic,” Hickner says. Add solvent cost and amount of time needed to create molds, and the whole process of making a membrane can be lengthy and expensive. Furthermore, once a mold is designed, it cannot be altered without making an entirely new mold. “We need a new way of making membranes quickly at large scale.”

Hickner and his team saw a possible solution in the new technology of 3D printing. This would allow for quick design and fabrication, and it would also be easier to make tweaks to the membrane design. Nothing would have to be etched from scratch; it would just be a matter of changing the design in the computer and doing another print.

Hickner and his team designed a micro-patterned anion exchange membrane. They began with selecting a material that could not only be patterned, but had good membrane properties and could be cured without use of solvents. They chose a mixture consisting of diurethane dimethacrylate (DUDA), poly(ethylene glycol) diacrylate (PEGDA), dipentaerythritol penta-/hexa- acrylate, and 4-vinylbenzyl chloride. The combination of materials made for a membrane that would have a variety of useful mechanical properties, such as flexibility, moderate water uptake, and high ion exchange capacity.

These specific materials were chosen for their reactive end groups that could be UV photopolymerized during 3D printing. When a photoinitiator is added to the mix, light generates free radicals from the initiator. Those free radicals turn the material from liquid to solid. The materials also contain precursors to quaternary ammonium groups, which give the material its ion exchange functionality.

Photolithography is not a new technique, but when employed in a 3D printer, precise and complex structures can be created. A pattern of light is projected into the liquid, causing the liquid polymer to solidify into a pattern. But the resolution of the technique allows for the hardening process to occur layer by layer. Being able to use photolithography to finely control how the material builds up along the z-axis allows for varying-thickness membranes to be made rapidly—and without solvents.

“This study is a great example of unique product solution enabled by the combination of clever materials science and 3D printing,” says Jason Rolland, vice president of materials development at Carbon, a tech firm specializing in 3D printing polymeric materials with light curing, unaffiliated with the current study. “Patterning membranes in this fashion allows for rapid investigation of a variety of geometries that would otherwise prove impractical or not possible by conventional fabrication methods.”

The demonstrated patterned membrane did indeed have a lower overall ionic resistance than smooth membranes of the same surface area size and material volume. However, this first test was just the beginning of what Hickner hopes to produce with 3D-printed membranes.

“We want to look at fluid dynamics at the interface,” he says. “We want to know how resistance changes as the patterns change. We want to be able to do both engineering and materials science and manufacturing [at the same time].” And, without the use of solvents or time-consuming etching of molds, the research team will be able to test more membranes more quickly, making for a more robust and fast-paced research program in this area.

Read the abstract in Applied Materials & Interfaces.