Penn State researchers use 3D printing to enhance silicone

STATE COLLEGE, Pa.—A team of Penn State researchers recently discovered how to improve the cell adhesion and strength of a silicone polymer with 3D printing.

Polydimethylsiloxane (PDMS) has limitations in formability and manufacturing of devices. To obtain better mechanical characteristics and biological adhesion, researchers opted not to use the traditional molding and casting manufacturing techniques, and instead use 3D printing.

PDMS has been popular in lab-on-a-chip devices and organ-on-a-chip devices, and used for drug testing devices, disease screening and disease modeling purposes, said Ibrahaim Tarik Ozbolat, Hartz Family associate professor of engineering science and mechanics and bioengineering.

"PDMS is commonly used by researchers and they mostly use casting or micro molding techniques," said Veli Ozbolat, a postdoctoral visiting scholar in the Engineering Science and Mechanics Department.

Yet, when they 3D-printed PDMS, they found better mechanical and cell adhesion properties, he added.

"PDMS is extensively used in medical applications, but the manufacturability of PDMS devices is limited to soft-lithography and its derivatives," Veli Ozbolat said.

"We decided to 3D print PDMS, which enables the fabrication of geometrically complex shapes."

During their research, the team created a 3D-reconstructed nose and image.

"We first did a printability study for different mixing ratios, tip sizes, pressures and speeds to determine best parameters," Veli Ozbolat said.

After determining the suitable parameters to print 3D geometries, he said, print paths were generated. The print path of the human nose was taken from Cellink, and 3D printing was performed with the determined parameters.

"Cells usually do not adhere to PDMS substrates due to the hydrophobic nature of PDMS," said Madhuri Dey, a doctoral candidate in the Chemistry Department.

"However, when PDMS is coated with proteins like fibronectin, it makes the environment congenial for cell attachment by introducing certain amino acid sequences which the cells can readily adhere to," Dey said. "Thus fibronectin is a commonly used protein for growing cells on PDMS substrates.

Although the team uses diverse types of material in their research, they did not consider other materials for this project.

"We cannot say it is the best material, but this PDMS blend is very useful for 3D printing of complex shapes and we showed that 3D printing improves its mechanical and cell adhesion properties," Veli Ozbolat said.

The "ink" material for the 3D printer must be able to go through the printing nozzle and maintain shape throughout the process. It cannot spread or flatten. When searching for an optimized mixture for printability, researchers combined two different PDMS elastomers.

Sylgard 184 is not viscous enough to use in 3D printing on its own, but combined with SE 1700, a second PDMS elastomer, it creates a printable mixture in the proper ratio, Ibrahaim Tarik Ozbolat said.

"We were looking (for a) way to print Sylgard 184 and we thought mixing with more viscous material makes it printable," added Veli Ozbolat. "We found SE 1700 from literature."

Researchers utilized shear thinning to get the most from the mixture.

"Shear thinning is the characteristics of some non-Newtonian fluids whose viscosity decreases with increasing shear stress," Veli Ozbolat said.

"Shear thinning is desirable for 3D printing to improve the printability because increasing shear stress makes material more injectable and removing shear stress after extrusion makes printed structure stronger."

Ultimately, the research showed that highly intricate geometrics can be 3D printed using this PDMS with sufficient resolution, Veli Ozbolat said.

3D structures have higher mechanical properties compared to cast counterparts.

"In addition, we showed for the first time that the 3D printed surface of PDMS inks promoted better cell adhesion and spreading compared to the cast surface," Veli Ozbolat said.

Looking to the future, he said the researchers will incorporate nanoclay instead of SE 1700 to enhance the 3D printability of highly indicated free-standing objects.