Original source: Materials Today
By combining two different types of silicone, a team of Penn State researchers has been able to produce silicone parts with complex geometries by 3D printing. These printed parts also have better mechanical characteristics and biological adhesion than parts produced by conventional silicone production processes such as molding, casting and spin coating.
“So far, PDMS (polydimethylsiloxane, or silicone) has limitations in formability and manufacturing of devices,” said Ibrahim Ozbolat, associate professor of engineering science and mechanics, and bioengineering at Penn State. “Most research is done using casting or micro molding, but this fabrication yields materials with weak mechanical properties and also weak cell adhesion. Researchers often use extracellular proteins like fibronectin to make cells adhere.”
PDMS can be used to make lab-on-a-chip devices, organ-on-a-chip devices, two- and three-dimensional cell culture platforms, and biological machines. The material is also commonly used to produce heat-resistant silicone spatulas and flexible baking pans, but these are geometrically simple devices and can easily be molded. If the material is instead used for growing tissue cultures or testing, the geometries become much smaller and more complex.
For any material to serve as ‘ink’ in a 3D printer, it must be able to go through the printing nozzle and maintain its shape once deposited. The material cannot spread, seep or flatten, otherwise the integrity of the design is lost. Sylgard 184, an elastomer of PDMS, is not viscose enough to use in 3D printing – the material simply flows out of the nozzle and puddles. However, when it is mixed with SE 1700, another PDMS elastomer, in the proper ratio, the mixture becomes printable.
“We optimized the mixture for printability, to control extrusion and fidelity to the original pattern being printed,” said Ozbolat. The researchers optimized the mixture to take advantage of a material property called ‘shear thinning’. They report their results in a paper in ACS Biomaterials Science & Engineering.
While most materials become more viscose under pressure, some materials have the opposite, non-Newtonian response, becoming less viscose. This is perfect for 3D printing because a fluid that is viscose enough to sit in the nozzle then becomes less viscose when pressure is applied to push the ‘ink’ out of the nozzle. As soon as the material leaves the nozzle, it regains its viscosity and the fine threads placed on the object retain their shape.
PDMS, when molded, has a smooth surface; it is also hydrophobic, meaning it does not like water. These two properties ensure that the molded surface of PDMS is not an easy place for tissue cells to adhere. Because of this, researchers frequently use coatings to increase cell adherence. By contrast, the 3D-printed surfaces, because they are made up of thousands of tiny strands of PDMS, possess minute crevices that offer cells a place to stick.
To test the fidelity of 3D printing with PDMS, the researchers obtained designs for biological features – hands, noses, blood vessels, ears and the head of a femur – from the National Institutes of Health 3D Print Exchange. Using these designs, they 3D printed a nose with their PDMS mixture, which showed that organs like this can be printed without support materials and include hollow cavities and complex geometries.
“We coated the PDMS nose with water and imaged it in an MRI machine,” said Ozbolat. “We compared the 3D reconstructed nose image to the original pattern and found that we had pretty decent shape fidelity.”
Because PDMS is forced through a nozzle for printing, the number of bubbles in the final material is far less than with molding or casting. Passing the mixture through a micrometer size needle removes most of the bubbles.
“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,” said Ozbolat.
Because the PDMS materials are being printed, they could be incorporated with other materials to make one-piece devices composed of multiple materials. They could also incorporate conductive materials to produce functionalized devices.