Apr 16, 2018 | By David

A team of researchers at Cleveland’s Cape Western Reserve University have recently completed some interesting work on a new group of 3D printed materials, which could have a variety of useful applications. The project was carried out by Professor Rigoberto Advincula, alongside Qiyi Chen and Peng‐Fei Cao, at the Department of Macromolecular Science and Engineering. Their work was detailed in a paper entitled ''Mechanically Robust, Ultraelastic Hierarchical Foam with Tunable Properties via 3D Printing'', published in the Advanced Functional Materials journal. The team fabricated superelastic foams, using the viscous solution printing (VSP) 3D printing technique, also known as direct ink writing (DIW).

The use of direct printing from a digital 3D model enables complex structures to be produced with a high level of accuracy, giving engineers a lot of control over porosity both at the micro- and macro- scales. The research was carried out using polyurethane, a common plastic material. 3D printing enables the structure of the material to be controlled at various levels, giving it a porosity that leads to significantly improved, desirable properties. 3D printing offers a much higher level of complexity in terms of the final structures of the foam, when compared to molding or casting methods.

The VSP 3D printing technique makes use of a syringe, which extrudes a viscous ink material on to a build plate, setting in place in order to create a 3D structure layer-by-layer. This 3D printing technique has advantages over regular FDM extrusion methods, as it can print with a much wider variety of materials. These include metals, hydrogels, and aerogels, as well as ceramics and thermoplastics.

The ink used in the 3D printing technique was a thixotropic material, which means that it doesn’t flow smoothly and is deformable under external stress. These inks are formulated by a simple one‐pot process, in which duo-nanoparticles (nanoclay and silica nanoparticles) are dispersed in a polyurethane suspension.

Precise control of the viscosity of the ink, as well as the design of the syringe, the print parameters, and the 3D design itself, allowed for a high level of control over the final 3D printed structure. The thermoplastic polyurethane (TPU) was fabricated with a hierarchically porous structure. At the macro-scale, pores of a larger size were fabricated in the structure by putting them in the initial 3D design. At the next level down, large micro-pores were generated through a post-printing phase separation process, when the object was immersed in water. The smallest micro-pores were generated through chemical etching.

The resulting TPU foam structures were lightweight, and they exhibited good mechanical strength. They also boasted unprecedented elasticity, standing over 1000 compression cycles, as well as remarkable robustness, rapidly and fully recovering after a load more than 20,000 times their own weight.

The mechanical properties of the superelastic foams can be tuned, according to the application they are being used for. This is also true of their conductive performance. As a demonstration, a small sponge made from the foam was dipped in a solution of carbon nanotubes (CNTs) in water. The CNTs tightly grip the surface of the TPU foam, due to a strong van de Waal’s force. After drying, the foam was attatched to a circuit board and used as a highly sensitive pieszoresistivity sensor. This is effectively an elastic power switch, which can be compressed in order to turn a circuit on or off.

(all images, credit: Rigoberto Advincula)

As well as flexible electronis, this 3D printable TPU foam can be used to improve many other existing applications of polyurethane, including footwear (like New Balance's 3D printed trainers) car seating, packaging, and tissue engineering scaffolds. The ink mixture could also be changed to get similar effects with plastics other than polyurethane.



Posted in 3D Printing Application



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