Nature knows how to fail, and Binghamton University researcher Christopher Maiorana is determined to replicate it. In a recent Scientific Reports article, Maiorana and his co-authors describe a method for guiding fractures in soft materials based on principles observed in skin, bone, and seashells. “If you’re going to fail,” says Maiorana “do it in a controlled way.”

Soft materials are being used to build artificial muscles, swimming robots, and other biomimetic devices. Many of these applications impose intense and cyclical force loading which can cause material fatigue and fracture. Maiorana, a sixth-year PhD student working in Guy German’s laboratory at Binghamton University, set out to explore fracture guidance as a potential solution. If fractures are going to form, he reasoned, perhaps we can design materials such that fractures will propagate away from critical or hard-to-repair components.

Guiding and controlling fracture is observed throughout nature. Mother of pearl, also known as nacre, is an iridescent material that forms both the inner lining of some seashells and the outer coating on pearls. Nacre is remarkably strong due in part to a nanoscale brick-and-mortar structure in which tough mineral crystals are arranged in a staggered pattern with soft proteins layered between them. Molecular bonds in the protein layers are easier to break compared to those in the mineral layer. The protein layer is thus easier to fracture and provides an energetically favorable path for fracture propagation. Fractures in nacre are guided by the protein layers into convoluted pathways that cause dissipation of fracture energy and a halt to the fracture’s progression. 

Maiorana took inspiration from nacre as well as human skin. In studying the mechanics of human skin, Maiorana and his colleagues noticed that skin rarely cracks in a straight line, following instead a jagged path that tracks a network of natural channels etched across the skin’s surface. By providing tracks that are easier to crack, skin and nacre are able to redirect fractures. But could this be replicated in synthetic materials?

Using a three-dimensional (3D) printed mold, Maiorana formed soft membranes made of a widely used and flexible silicon material known as polydimethylsiloxane (PDMS). V-shaped reliefs designed into the 3D molding generated superficial channels in the PDMS membranes, mimicking those observed in skin. Each membrane contained a single channel which deviated its course by an angle of 10-30 degrees. The researchers then applied uniaxial force and studied whether the channel’s angle could influence the direction of a growing fracture.

“We had this working hypothesis this whole time...we spent a year trying to get to this point; then we finally test the thing and it works!” says Maiorana, reflecting on his excitement after seeing the fracture path change course with the embedded channels.

Follow-on tests involved single- and double-layered membranes, the latter being formed by creating a single layer membrane (with a superficial channel embedded) and subsequently pouring a second layer of PDMS on top of the first. This process generated a uniform surface which, Maiorana says, may be required for some soft-material applications. Testing of both single- and double-layered membranes showed that the presence of embedded channels did not significantly affect the material’s toughness and that, as in single-layered membranes, channels built into double-layered membranes also affected the fracture path. However, this influence was shown to be limited in both membrane types, meaning the fracture-path could be temporarily redirected but ultimately returned to a path that was perpendicular to the applied force. The team could increase the amount of influence they had over a fracture’s path by adding a series of successive channels, going so far as to redirect a fracture by 45 degrees.

Kai Guo, a postdoctoral researcher at the Massachusetts Institute of Technology, finds these results to be novel and an important avenue of research, but is left wanting for a deeper analysis of why, and how, the fracture path is altered. “I think it could be a better paper if appropriate modeling or simulations are performed and thus the mechanisms that lead to those crack deviations can be revealed,” says Guo.

To that end, Maiorana also sees room for more investigation. These experiments demonstrate that materials can be designed with embedded channels that may help to guide fractures away from critical components. “We would like to investigate more complicated patterns...and see if we could make the material stronger by replicating the mother of pearl type of structure,” Maiorana says.

Much more research is needed to explore which patterns are most effective and how these principles could be incorporated into soft robotics. Nonetheless, the promise of such an approach is hard to deny.

Read the article in Scientific Reports.