Colloidal inks feel the squeeze

Researchers at the University of Illinois at Urbana-Champaign have found a new way to assemble complex, three-dimensional structures from specially formulated colloidal inks.

Researchers at the University of Illinois at Urbana-Champaign have found a new way to assemble complex, three-dimensional structures from specially formulated colloidal inks that could find use in advanced ceramics, sensors, composites, catalyst supports, tissue engineering scaffolds and photonic materials.

The colloidal, gel-based inks form self-supporting features through a robotic deposition process called robocasting. A computer-controlled robot squeezes the ink out of a syringe, building the desired structure layer by layer.

‘Our goal is to make designer materials that can’t be made by conventional forming techniques,’ said Jennifer Lewis, a professor of materials science and engineering and of chemical engineering at the University of Illinois at Urbana-Champaign.

‘The directed assembly of fine-scale, three-dimensional structures containing spanning elements required the development of concentrated colloidal, gel-based inks,’ Lewis said. ‘These inks must first flow through a very fine deposition nozzle and then quickly ‘set’ to maintain their shape while simultaneously bonding to the underlying layer.’

The researchers are said to have created structures with features as small as 100 microns and have spanned gaps as large as 2 millimetres.

The elastic properties and the viscous response of the ink can be ‘tuned’ by tailoring the strength of the interparticle attractions, Lewis said. Because of the dynamic nature of the resulting gel, the particle network forms very quickly after the ink is pushed through the syringe, providing the desired shape retention.

Through careful control of colloidal forces, the researchers can not only produce complex shapes that can’t be made by conventional moulding or extrusion processes; they can also build in complexity with respect to chemical composition.

‘The robotic deposition equipment has the capability of handling multiple inks and dispensing them simultaneously,’ Lewis said. ‘As the relative rates of deposited ink are regulated, structures can be built that have compositional variations in them.’

Inks are housed in separate syringes mounted on the robotic deposition apparatus and can be mixed or deposited independently.

The ink exits the nozzle as a continuous, rod-like filament that is deposited onto a moving platform, yielding a two-dimensional pattern. After a layer is generated, the stage is raised and another layer is deposited.

This process is repeated until the desired structure is produced. A computer program called RoboCAD, developed by Illinois graduate student James Smay controls the machine’s motion. The software allows users to rapidly design and build complex, three-dimensional structures by simply designing layers as two-dimensional drawings.

‘Ink can be made from nearly any particulate material that can be suspended in solution, as long as the interparticle forces can be tuned to yield the desired viscoelastic response,’ Lewis said.