NC State Researchers Look to the Structural Stability of Sand Castles to Develop New 3D Printing Method for Silicone Paste

The other day I was walking through a local outdoor mall, and noticed that several wooden platforms had been set up. The platforms weren’t empty: each one contained a rather large pile of sand. I then realized that I was walking through what would soon be a giant sand art competition; I’m hoping to go back this weekend and check out the giant sand sculptures once they’re completed. It made me think of the sand castles we build as children on the beach with buckets and plastic sand tools. Thanks to 3D printed molds, the art of sand castle building has become a lot more high tech, and it’s even possible to 3D print using sand.

According to a recent Facebook post, North Carolina State University (NC State), which recently received a Concept Laser metal 3D printer courtesy of the GE Additive Education Program, was ranked #3 in North America in increasing research publications. Last month, NC State researchers announced their development of a 3D printed cube that’s used to manipulate virtual objects, and now, a different research team has worked out a way to 3D print silicone paste. The technique was inspired by and relies on the principles which allow sand castles to hold together.

Orlin Velev, professor of chemical and biomolecular engineering at NC State, said, “There is great interest in 3D printing of silicone rubber, or PDMS (polydimethylsiloxane), which has a number of useful properties. The challenge is that you generally need to rapidly heat the material or use special chemistry to cure it, which can be technically complex.”

The team recently published a paper on their work, titled, “3D Printing by Multiphase Silicone/Water Capillary Inks,” in the journal Advanced Materials; co-authors include Velev, Simeon D. Stoyanov, Sangchul Roh, Dishit P. Parekh, and Bhuvnesh Bharti.

A sand castle’s structural stability comes from the formation of capillary bridges between the tiny granules of wet sand. According to the introduction in the paper, mixing sand and water to form sand castles could be considered as “the most ancient method of making 3D architectures.”

The abstract reads, “3D printing of polymers is accomplished easily with thermoplastics as the extruded hot melt solidifies rapidly during the printing process. Printing with liquid polymer precursors is more challenging due to their longer curing times. One curable liquid polymer of specific interest is polydimethylsiloxane (PDMS). This study demonstrates a new efficient technique for 3D printing with PDMS by using a capillary suspension ink containing PDMS in the form of both precured microbeads and uncured liquid precursor, dispersed in water as continuous medium. The PDMS microbeads are held together in thixotropic granular paste by capillary attraction induced by the liquid precursor. These capillary suspensions possess high storage moduli and yield stresses that are needed for direct ink writing. They could be 3D printed and cured both in air and under water. The resulting PDMS structures are remarkably elastic, flexible, and extensible. As the ink is made of porous, biocompatible silicone that can be printed directly inside aqueous medium, it can be used in 3D printed biomedical products, or in applications such as direct printing of bioscaffolds on live tissue. This study demonstrates a number of examples using the high softness, elasticity, and resilience of these 3D printed structures.”

The team combined water with both solid and liquid forms of silicone into a pasty ink, and used this material to 3D print silicone rubber structures. The researchers discovered that this liquid silicone rubber acted just like a bridge, helping the small beads of silicon rubber link up, and by pulling the adjacent particles together, 3D particle networks were formed.

The process is very similar to how water helps bind sand particles together to build sand castles. In addition, the paste-like suspensions offer static yield stress, and considerable elastic modulus.

Orlin Velev

“Our method uses an extremely simple extrudable material that can be placed in a 3D printer to directly prototype porous, flexible structures – even under water. And it is all accomplished with a multiphasic system of just two materials – no special chemistry or expensive machinery is necessary,” explained Velev. “The ‘trick’ is that both the beads and the liquid that binds them are silicone, and thus make a very cohesive, stretchable and bendable material after shaping and curing.”

Since the UC State research team’s method works in both wet and dry environments, the flexible, porous structures could have biomedical applications, and could possibly be used in live tissue, like for flexible meshes or soft bandages that can be printed right on a person’s body. The method could have uses in soft robotics as well. Discuss in the Sand Castles forum at 3DPB.com.

[Source: The Engineer]