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The Best Of Both Worlds: Creating A Strong, Stretchable Fiber For A Range Of Materials | Science Trends

The Best Of Both Worlds: Creating A Strong, Stretchable Fiber For A Range Of Materials

Throughout history, humans have sought to find or design materials with desirable properties for different applications. Since each material has its advantages and disadvantages, oftentimes we want to try and combine them to get the “best of both worlds.” However, the way in which the materials are combined has a significant impact on the resulting material properties.

A great example of this can be seen by comparing the properties of metals and rubbers. Metals (e.g. steel) require a lot of energy to deform and break, however, they cannot stretch or elongate to long lengths and instead fracture brittlely. Rubbers (e.g. a rubber band) can stretch to many times their original length before breaking, however, they lack the high strength of the metal. By selecting the appropriate geometry (in this case, a core of metal surrounded by an elastic shell), we have been able to develop fibers that combine the strength of metal with the elasticity of rubber!

The fibers can elongate many times their original length but are also very tough (i.e. harder to deform). This toughness arises from the fact that the metal core breaks over and over while the polymer shell maintains the overall integrity of the fiber.

A) A schematic of how the fiber dissipates energy during stretching. B) The fibers combine the best attributes of metal (black line) and polymer (red line) to show significantly improved mechanical properties (blue line). C) Images of the fiber as it is stretched and the metal cores breaks repeatedly. D) Fiber stretched to eight times its original length. Figure republished with permission from Science Advances from doi.org/10.1126/sciadv.aat4600

Why would we want to create tough, stretchable fibers? The most promising application is for more effective energy dissipation in devices. In general, effective energy dissipation by one material protects other more fragile materials. Consider, for example, the human body, which is composed of an endoskeleton of bone surrounded by a soft matrix of tissue. Many of us have suffered an injury such as a fall in which we broke a bone. Luckily for us, the bone served to dissipate most of the energy from the impact of the fall and thus protected more fragile body parts such as organs from harm. The same energy-dissipating design is being applied in the metal-polymer core-shell fibers. By sacrificially breaking the metal core, the fiber dissipates energy from large loads. Thus, the fibers are useful for a range of potential applications, especially those in which a fiber geometry is particularly desirable. Think wearable electronics, next-generation textiles, soft robotics, and advanced packaging.

 The fibers are able to hold much heavier loads without elongating rapidly (due to their toughness), allowing them to gently lower the weight down. Figure republished with permission from Science Advances from doi.org/10.1126/sciadv.aat4600

The fiber’s metal core is also electrically conductive until the first break. Additionally, increasing the temperature to just above room temperature (when the metal core will melt to a liquid) allows the fiber to quickly and reversibly alternate between soft and rigid mechanical properties and also allows the fiber to repair itself after damage. The fibers could be used in soft robotics by mimicking other examples of effective energy dissipation in the human body, which include collagen fibers that dissipate energy and prevent cuts from spreading in human skin, and titin that does the same for human muscle. The fibers are over twice as tough as titin!

And while we specifically used a metal core and a polymer shell, the fibers could in principle be extended to a range of core-shell materials, thus allowing the mechanism to be highly tunable depending on the optimal strength or operational conditions.

These findings are described in the article entitled Toughening stretchable fibers via serial fracturing of a metallic core, recently published in the journal Science Advances.

About The Author

Chris Cooper

Christopher B. Cooper obtained his B.S. in chemical engineering from North Carolina State University in 2017 as a Park Scholar, completing his honor’s thesis on liquid metal electronics under the supervision of Dr. Michael D. Dickey. He then completed an MPhil in chemical engineering at the University of Cambridge as a Churchill Scholar working with Dr. Jacqueline M. Cole on a design-to-device approach for materials discovery of new co-sensitized dye-sensitized solar cells. Currently, he is pursuing his Ph.D. in chemical engineering at Stanford University working with Dr. Zhenan Bao on the design of stretchable and self-healable electronic devices.

Michael Dickey

Michael Dickey is a research scientist and head of the Dickey Group at North Carolina State University.

The Dickey group is studying new ways to pattern, actuate, and control soft materials (gels, polymers, liquid metals). A common theme of our projects is the importance of thin films, interfacial phenomena, and microfabrication. Our approach is to (i) elucidate the fundamental properties of materials such that they can be harnessed in a useful manner, and (ii) develop new, unconventional approaches to fabricate, actuate, and assemble these materials. Applications of the research include patterning, 3D printing, stretchable / soft electronics, self-folding, actuation of soft robotics, wearable electronics, energy harvesting devices, reconfigurable circuits, and microfluidics..

We are also interested in photo-curable polymeric materials, particularly those used in both photo- and imprint lithography. The properties (curing speed, sensitivity, elemental composition, dielectric constant, mechanical properties, viscosity, etc.) of these materials must be tailored and optimized depending on the application.