Inspired by the human heart, Penn Engineers design tear-resistant soft material

Engineers have designed a soft material for robotics, medical devices, and wearable technologies that are both tear-resistant and able to resist deformation.

The human heart beats more than two billion times in an average lifespan. The aorta’s tissue is strong enough to withstand this stress, yet flexible enough to maintain a steady flow of blood. But how does this soft tissue not tear over time? This is a question that engineers are trying to answer as they examine the physical properties of soft materials, such as silicones or hydrogels, which are commonly used in robotics, medical devices and wearable technologies.

Glass fibers embedded in stretchy silicone.
Using a 3D printer, Penn Engineering researchers are able to precisely control the alignment of glass fibers embedded within this stretchy silicone. The stripes represent regions with different fiber alignment patterns, and thus different levels of resistance to the tear making its way across the sample. (Image: Penn Engineering Today)

In the engineering world, traditional soft materials are either tough, meaning difficult to tear, or stiff, meaning able to resist deformation. The tradeoffs that come with each of these properties make it challenging to design a material that is both. Previous approaches involve combining two types of materials, alternating the placement of rigid and flexible materials throughout a final composite material to help prevent and slow tears. However, manufacturing a product made from multiple material types increases the cost and slows production time, as it requires investment in creating chemical compatibility between the two materials to allow them to stick together.

To tackle the issue of making soft materials tough, Penn Engineers have examined the problem from a different angle—literally, the angle at which glass fibers embedded in silicone come out of a 3D-printing nozzle.

Taking solutions from nature, Jordan Raney, assistant professor in mechanical engineering and applied mechanics in Penn’s School of Engineering and Applied Science, along with Chengyang Mo and Haiyi Long, graduate students in his lab, looked at the variations of fiber alignment in biological tissues to find an answer in their new study published in PNAS.

“The answer to many problems in nature is spatial variation,” says Raney. “Tissues are made up of features with varying patterns and degrees of order, and this heterogeneity of the structure allows it to perform multiple functions simultaneously. In engineering soft materials, you can either create heterogeneity by employing multiple material types or by changing a few aspects of one type of material, such as fiber alignment. Our approach is the latter.”

The researchers mimicked this variation by manipulating one type of silicone, polydimethylsiloxane. They embedded glass fibers into this soft silicone to provide it with structural fibers akin to collagen in skin.

“Here we created a simple tear-resistant material by just manipulating the fiber alignment,” says Mo, the study’s lead author. “We were able to slow the tear and we can easily manipulate the number of stripes to attune the material to different applications where it might need flexibility over toughness or vice versa. But we didn’t stop there. We wanted to improve these properties further, and that’s when we looked at the aorta.”

This story is by Melissa Pappas. Read more at Penn Engineering Today.