Q&A: Liquid-Solid Co-Printing of Complete Functional Systems

Professor Robert MacCurdy and his team at the University of Colorado at Boulder have developed and characterized a method to print 3D structures with the simultaneous use of different materials, including liquids and solids, to produce complete functional systems, not just individual components.

Tech Briefs: What got you started on this project?

Professor Robert MacCurdy, University of Colorado at Boulder

Professor Robert MacCurdy: My background is in robotics, and I got really excited at the beginning of my Ph.D work, in trying to think about ways that we could automate the design and fabrication of robots — that is still the central motivation of my work.

My goal is to develop tools that allow human designers to very rapidly respond to new needs and very rapidly design, and then fabricate robots for those needs.

Tech Briefs: Can you tell us more about the process?

MacCurdy: Typically, all inkjet inks are deposited as a liquid and then they are exposed to heat or light and they transition into a solid. But if we also include materials that permanently stay as a liquid, we can use the liquid as a support material.

We have three different scenarios, based on the two different materials we are considering. If one of the materials is more buoyant than the other, they will separate like water and oil. If there are surface tension differentials between the two materials, the upper droplet, which otherwise would fall through the lower, would be maintained on the interface by its surface tension.

And finally, even if the density is wrong, and even if the surface tension is wrong, if you can photopolymerize it rapidly enough so that it transitions from a liquid to a solid while it’s still at the surface and you have other droplets adjacent to it, they’re also photopolymerizing. Then you could potentially create physical structures that would be locked in place and not continue to sink down through the liquid.

That allows us to print structures, for example, spirals, which would be impossible to clean out using existing support materials. The inkjet method requires support material but eventually you have to remove it.

However, existing support materials are quite difficult to remove from channels in the sort of complicated spiral and capillary-like structure that we created in our lab. In our method, because the support material is so low in viscosity, it easily flows out of all the channels with a little bit of fluid flow.

We print the solid and the liquid at the same time. So, it’s all ready to go as soon as you wash the support material off.

Tech Briefs: Is what you’re working on is sort of the basic nuts and bolts that could be used to build things in the future?

MacCurdy: That’s right — we want to add more paints to the painter’s palette. And liquid is an important one because it conveys advantages that just adding a different color of a solid wouldn’t necessarily help.

We are going to continue with the theme of robots but explore and expand on the different kinds of structures that we can make.

An edited version of this interview appeared in the June 2022 issue of Tech Briefs.