A new twist in free-form fabrication

In a new twist, researchers have developed a method to produce 3-D microgeometries in a free-form fashion using controlled extrusion. The process could be used to create complex components for micro-electro-mechanical systems (MEMS), such as gears and springs. It could also create flexible electrical connections, freestanding electromagnetic shielding meshes and inductance components in organic micro-electronics. Potential biomedical applications include microprosthetic devices and tissue-engineering scaffolds.

“We are working on developing load sensors with this method,” said Daniel Therriault, associate professor of mechanical engineering, l’École Polytechnique de Montréal (Montreal Polytechnic). “The fabrication of a microcoil was used to demonstrate the potential of the process, but the coil could be inserted in a MEMS device, used as a spring or an actuator or even as a load sensor.”

The fabrication of a microcoil demonstrates the UV-assisted method to produce 3-D micro geometries in a free-form fashion. The size of a full microcoil structure is a few millimeters. All images: École Polytechnique de Montréal.

With this fabrication process, a robot dispenses a polymer enhanced with carbon nanotubes that hardens as soon as it leaves the micronozzle. An ultraviolet light quickly sets the structure in place.

“The process is inspired by the direct-write assembly method,” said Therriault. “It is basically the combination of the motion of a micro-positioning platform and the extrusion of a paste-like material using a dispensing system.”

Other direct-write fabrication methods, such as robotized layer-by-layer deposition, are used to fabricate 3-D components. These methods are limited mainly to fabrication of supported structures. The advantage of free-form fabrication over layering is enhanced process flexibility. “The layering method forms a specific geometry,” he said. “To produce a coil without any support would not be possible with the layer-by-layer deposition approach.”

Therriault, together with Louis Laberge Lebel, a doctoral student at Montreal Polytechnic, and collaborators at the Institut National de la Recherche Scientifique—Énergie, Matériaux et Télécommunications developed the process.

The fabbing process

The direct-write fabrication apparatus consists of a syringe barrel containing the uncured material fitted with an extrusion nozzle having a defined cross-section, such as circular or square. The nanocomposite is extruded through a capillary nozzle by applied pressure and forms a filament that is exposed to UV light by optical fibers attached to a ring above the filament. The robot dispenses the material at an approximate speed of 1mm every 3 seconds. The height of the ring, and thus the UV zone, is adjusted so the filament is exposed to the UV radiation slightly after extrusion. This allows curing to occur away from the extrusion point. However, the UV radiation must remain as close as possible to the extrusion point to reproduce the specific path of the moving extrusion nozzle.

Drawing of the direct-write microcoil fabrication process. A polymer is extruded through a nozzle and then cured by UV light.

The researchers developed the UV-curable nanocomposite, consisting of a blend of single-wall carbon nanotubes in a polyurethane matrix, to manufacture the coils. The carbon nanotubes provide enhanced mechanical and electrical properties.

“We create the material by dispersing the carbon nanotubes in a liquid thermosetting polymer,” Therriault said. “The carbon nanotubes make the polymer significantly stronger than a plain polymer and also make it electrically conductive. We thought it would be a great approach to combine this novel, direct-write fabrication method with these carbon nanotubes to have a composite with better properties.”

Upon curing, material rigidity increases and provides structural support for the material being deposited in a continuous manner and in different directions. This allows for free-form direct writing in open air.

The research team plans to improve the nanocomposite material—specifically the dispersion of the carbon nanotubes and the quality and orientation of the carbon nanotubes to enhance electrical and mechanical properties.

Only getting smaller

“The smallest filament we have been able to produce is slightly smaller than 100µm in diameter,” Therriault said. “We are working toward 50µm, 30µm and 10µm. That is something we feel can be reached in the next few months.”

To go smaller, the team needs to reduce vibration and develop a more precise micropositioning system. Currently, there is a physical limitation with the robotic dispensing system. “When it goes down to displacement of a few tens of microns of material, it vibrates too much.”

The biggest filament diameter the team was able to produce was a few hundred microns. “But as you go bigger, the process becomes less attractive because other technologies, such as rapid prototyping, can produce larger components [at a more] competitive price,” Therriault noted.

The researchers want to improve the entire process. “We are working in parallel,” he said. “We have a team focused on the polymer and, specifically, incorporating the carbon nanotubes to create a better nanocomposite. But we are also pushing the envelope with the fabrication process to develop a new apparatus with a higher intensity UV light for faster fabrication (several millimeters per second) and more flexibility with the material selection.” µ

About the author: Susan Woods is a regular contributor to MICROmanufacturing and Cutting Tool Engineering magazines. E-mail: susanw@jwr.com.