Jan 23, 2018 | By Benedict

Researchers from Pennsylvania State University have successfully 3D printed shapes with complex geometries using a mix of PDMS elastomers, varieties of a common silicon-based organic polymer. The 3D printed PDMS registered a superior tensile strength to molded or cast PDMS.

In a very general sense, one the biggest advantages of using 3D printing over casting and molding is that complex forms can be achieved, forms with intricate internal and external geometries that could not be replicated by pouring a liquid material into a mold. What you don’t always hear is that 3D printing produces stronger and more mechanically robust parts when it does this.

Occasionally, however, by making certain adjustments to the additive manufacturing process, 3D printed parts made from a given material can end up stronger than traditionally made parts made from that same material. A group of Penn State researchers has just achieved something similar in its quest to optimize PDMS (polydimethylsiloxane, or silicone) for 3D printing. It has done so by combining two PDMS elastomers, resulting in improved mechanical characteristics and better biological adhesion.

Sometimes used to make things like lab-on-a-chip devices and 3D culture cell platforms, PDMS is nonetheless most commonly found in more ordinary items: heat-resistant silicone spatulas and other kitchenware. But while simple-shaped objects like spatulas can be made with molding apparatus, sometimes tiny objects like lab-on-a-chip devices demand more subtle methods of fabrication.

There are other disadvantages to molding or casting PDMS as well. According to Ibrahim T. Ozbolat, Hartz Family Associate Professor of Engineering Science and Mechanics and bioengineering at Penn State, casting or micro molding “yields materials with weak mechanical properties and also weak cell adhesion.” This means researchers are often led to use extracellular proteins like fibronectin in order to make cells adhere.

But this doesn’t automatically mean that engineers should turn to 3D printing to process their PDMS, because the material doesn’t always have the right properties for extrusion. Sylgard 184, for example, an elastomer of PDMS, is not viscous enough to use in 3D printing: it flows out of the nozzle like water and forms puddles. So how do you make it 3D printable?

By mixing Sylgard 184 with SE 1700, another PDMS elastomer, the Penn State researchers were able to make the mixture 3D printable, taking advantage of the material’s habit of shear thinning, the process of viscosity decreasing under shear strain. “We optimized the mixture for printability, to control extrusion and fidelity to the original pattern being printed,” says Ozbolat.

Materials that exhibit shear thinning are great for 3D printing, because their viscosity fluctuates in just the right way to suit 3D printing equipment: the material is viscous enough to rest in the nozzle without dripping out like water, but extrudes neatly when pressure is applied through the nozzle. It then becomes more viscous again when out in the open, allowing it to be made into complex shapes without collapsing. Most materials behave in the opposite way, getting more viscous when subject to shear pressure.

The Penn State research wasn’t just about making PDMS printable though. The researchers also wanted to test the biological adhesion of the printed material, to see whether it might be useful for biological applications like cell culture. Generally, this would not be the case, because molded PDMS has a smooth surface and is also hydrophobic, making it a hard material for cells to stick to. But with the 3D printed PDMS structures, the researchers could create non-smooth crevices, perfect for cells to adhere to.

The biological adhesion tests involved the 3D printing of various models of body parts, including a human nose, with the 3D model obtained from the National Institutes of Health 3-D Print Exchange. The nose could be 3D printed without support structures, and included hollow cavities.

By examining their 3D printed nose with an MRI scanner, the researchers found the structure to accurate, with very few deformities. This was thanks to a micrometer-size needle used in the 3D printer which served to remove any bubbles in the viscous material. Excitingly, the 3D printed PDMS nose also exhibited useful mechanical properties. “When we compared the mechanical signatures of molded or cast PDMS with 3D printed PDMS, we found the tensile strength in the printed material was much better,” Ozbolat says.

The conclusion? Printed PDMS can be made stronger than molded PDMS, and could be used in biological applications, functional devices made from conductive materials, and multi-material structures.

The other researchers involved in the project were Veli Ozbolat, Madhuri Dey, Bugra Ayan, Adomas Povilianskas, and Melik C. Demirel.



Posted in 3D Printing Application



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