Mother-of-pearl might best be known for its gleaming, ethereal appearance—much prized in the jewelry world—but the natural material is also actually one of the toughest on Earth. If you’re not familiar with it, mother-of-pearl is the incandescent interior of mollusk shells, also known as nacre. The material’s resilience has made it of particular interest to a research group at the Viterbi School of Engineering at the University of Southern California, which has created a 3D printed replica of the natural material.
The research breakthrough, led by Yong Chen, Professor in the Daniel J. Epstein Department of Industrial and Systems Engineering and the Center for Advanced Manufacturing, could lead to advancements in the development of new responsive smart materials and safety devices, including helmets and armor for both sporting and military use. Another area of interest for the 3D printed material is in the biomedical field, where it could be used for smart wearable devices.
The 3D printed material, whose development was detailed in a recent study in Science Advances, is inspired by the natural microstructures of nacre, which provide it with its strength. In addition to this, the researchers also relied on electrical fields in the material fabrication process, which means that the final product has strong electrical conductivity—ideal for smart devices.
“Nacre is strong because it stacks microscale and nanoscale components together in a brick-like structure and uses soft material to bind them together,” Chen explained, adding that the material has superior pressure resistance that other rigid materials like glass or ceramic. “Even very strong glass can be easy to crack when you drop it. Microcracks on the surface of these materials can quickly propagate all the way through it, whereas nacre combines soft and hard material in an intelligent way.”
That is, when microcracks form in nacre, the soft material that binds it together deflects the impact and actually stops the cracks from spreading. The 3D printed material mimicked the microstructure of mother-of-pearl as well as the multi-material composition to achieve a similar result.
Chen added: “The main motivation for this research was to see whether we could 3D print any shape at a microscale, using the architecture of nacre combining both hard and soft materials, to achieve a much tougher structure.”
The 3D printed material is made up primarily of graphene powder, which acts as the material’s building block. To align the graphene particles, the team introduced an electrical charge of around 1,000 volts. The material can then be cured, and then another layer applied.
“Originally we had this randomly distributed graphene,” Chen said. “When you add it to the electrical field, these random grains of graphene are aligned parallel to each other. Then we cure the material and finalize the layer. We then stack layer after layer on top so that it is similar in microstructure to nacre.”
The soft features of the material were achieved by adding a polymer—effectively making the mother-of-pearl-inspired material a polymer-graphene composite. As mentioned, the electrical charging process imparts conductive qualities into the material: a feature which sets it apart from its natural counterpart.
The material’s conductivity could be useful in the production of protective devices, such as helmets or armor, in that it could sense and alert the wearer if it is compromised or if it has any structural weaknesses.
To test this particular application, the team 3D printed a tiny smart helmet for a lego man with a sensor connected to an LED light. When pressure was put on the helmet, the LED light was activated by the sensor, letting the user know it was under stress. You can see the adorable results of the test in the photos above.
Going forward, the research team will further explore the material’s thermal conductivity properties as well as its mechanical strength and electrical conductivity.