Polydimethylsiloxane (PDMS), also known as silicone rubber, has several useful properties, and there has been a lot of interest lately in 3D printing the material. However, it’s a difficult process, as you generally need to heat the material rapidly or use special chemistry to cure it. But according to a team of researchers from Penn State, 3D printing technology can be used to improve the strength and cell adhesion of PDMS polymer.
While the material is most commonly used in geometrically simple flexible baking pans and heat-resistant silicone spatulas, which can be easily molded, it can also help make biological machines, two- and three-dimensional cell culture platforms, organ-on-a-chip devices, and lab-on-a-chip devices, which require smaller, more complex geometries.
Ibrahim T. Ozbolat, Hartz Family Associate Professor of Engineering Science and Mechanics and bioengineering at Penn State. “Researchers often use extracellular proteins like fibronectin to make cells adhere.”“So far, PDMS (polydimethylsiloxane, or silicone) has limitations in formability and manufacturing of devices. Most research is done using casting or micro molding, but this fabrication yields materials with weak mechanical properties and also weak cell adhesion,” explained
Any material that’s used as 3D printer “ink” has to be able to go through the print nozzle and maintain its shape after it’s been deposited onto the print bed – design integrity is gone if the material flattens, seeps, or spreads out. But the researchers discovered that by combining two different polymer forms – PDMS elastomers Sylgard 184 and SE 1700 – they could 3D print silicone parts with complex geometries that had improved biological adhesion and mechanical characteristics without having to resort to molding, casting, and spin coating to make only simple forms.
The team reported the results of their experiment in a paper, titled “3D Printing of PDMS Improves Its Mechanical and Cell Adhesion Properties,” in ACS Biomaterials Science & Engineering; supported by the Scientific and Technological Research Council of Turkey and the Turkish Ministry of National Education, the paper’s co-authors include postdoctoral fellow Veli Ozbolat, doctoral students Madhuri Dey and Bugra Ayan, bachelor’s/master’s student Adomas Povilianskas, engineering science and mechanics professor Melik C. Demirel, and Ibrahim Ozbolat.
The abstract reads, “This research presents a detailed investigation on printability of PDMS elastomers over three concentrations for mechanical and cell adhesion studies. The results demonstrate that 3D printing of PDMS improved the mechanical properties of fabricated samples up to three fold compared to that of cast ones because of the decreased porosity of bubble entrapment. Most importantly, 3D printing facilitates the adhesion of breast cancer cells, whereas cast samples do not allow cellular adhesion without the use of additional coatings such as extracellular matrix proteins.”
Sylgard 184 is not a viscous enough material for 3D printing, as the material puddles once it’s extruded out of the nozzle. But when mixed in the proper ratio with SE 1700, it works.
The team optimized the mixture of the two elastomers in order to take advantage of shear thinning, a property that has been combined with 3D printing before, in which a material behaves like a solid when still, but like a liquid when force is applied.
Ozbolat said, “We optimized the mixture for printability, to control extrusion and fidelity to the original pattern being printed.”
When under pressure, most materials become more viscous, but others have the complete opposite response and become less viscous. A fluid that is viscous enough to sit in a 3D printer nozzle but becomes less viscous when put under the pressure of being pushed out as an ink is perfect for 3D printing – once it’s out of the nozzle, it regains its viscosity and the fine threads placed on the object retain their shape.
When PDMS material is molded, it has a smooth surface, and the material is also hydrophobic (doesn’t like water). When these properties combine, the molded surface of the material doesn’t offer tissue cells a great place to adhere. Cell adherence can be increased by using coatings, and 3D printed surfaces made up of thousands of tiny PDMS strands have lots of minute crevices that are also perfect for cell adherence.
The team worked with the National Institutes of Health (NIH) 3D Print Exchange to get patterns for biological features, like blood vessels, ears, femoral heads, hands, and noses, to test the fidelity of 3D printing with PDMS, and printed out a nose – one of the human organs that includes complex geometries and hollow cavities without requiring support materials.
“We coated the PDMS nose with water and imaged it in an MRI machine. We compared the 3-D reconstructed nose image to the original pattern and found that we had pretty decent shape fidelity,” Ozbolat said.”When we compared the mechanical signatures of molded or cast PDMS with 3-D printed PDMS, we found the tensile strength in the printed material was much better.”
When a material is passed through a micrometer-size needle, the process removes most of the bubbles locked in the material. So since the PDMS was forced through a 3D printing nozzle, there were far less bubbles in the final material than there would have been with casting or molding processes. In addition, because the PDMS materials are being 3D printed, they could be incorporated with other materials to manufacture multi-material, one-piece devices. Functionalized devices would also be possible, if the PDMS incorporated conductive materials.
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