New 3D Printing Technique Shows Promise for Developing Smart Materials and Self-Adaptive Infrastructures

Thanks to a new study detailed in the journal Nature Materials published online on January 21^st, 2019, the piezoelectric materials inhabiting everything from musical greeting cards to cellular phones may be receiving an upgraded.

A research team, which included Xiaoyu ‘Rayne’ Zheng, a member of the Macromolecules Innovation Institute and assistant professor of mechanical engineering in the College of Engineering, and his group, has devised techniques to 3D print piezoelectric materials that can be customized to transform stress, movement, and impact from all kinds of directions to electrical energy.

“Piezoelectric materials convert strain and stress into electric charges,” explained Zheng.

Featuring just a few defined shapes, the piezoelectric materials are made of brittle ceramic and crystal, which can be manufactured only through a clean room. To 3D print these piezoelectric materials, Zheng’s group has created a new method so that they are no longer limited by size or shape. It is also possible to activate the material—offering the next generation of smart materials and intelligent infrastructures for energy harvesting, impact and vibration monitoring, tactile sensing, and many other applications.

Unleash the freedom to design piezoelectrics

Since the original discovery of the piezoelectric materials in the 19^th century, the developments in manufacturing technology has given way to clean-rooms and a complicated procedure that creates blocks and films, which post machining, are joined to electronics. The costly process combined with the innate material brittleness, has reduced the ability to increase the potential of the material.

The researchers created a unique model that enables them to exploit and design random piezoelectric constants, causing the material to create electric charge movement in response to incoming vibrations and forces from any direction, through a series of 3D printable topologies. The novel technique is different from traditional piezoelectrics, in which the intrinsic crystals prescribe the electric charge movement, and enables users to propose and program voltage responses to be reversed, suppressed, or magnified in any direction.

We have developed a design method and printing platform to freely design the sensitivity and operational modes of piezoelectric materials. By programming the 3D active topology, you can achieve pretty much any combination of piezoelectric coefficients within a material, and use them as transducers and sensors that are not only flexible and strong, but also respond to pressure, vibrations and impacts via electric signals that tell the location, magnitude and direction of the impacts within any location of these materials.

Xiaoyu ‘Rayne’ Zheng, Assistant Professor, Department of Mechanical Engineering, Virginia Tech College of Engineering

3D printing of piezoelectrics, sensors, and transducers

A factor in the present fabrication of piezoelectric materials is the use of natural crystals. The atoms’ orientation is fixed at the atomic level. A substitute developed by Zheng’s team imitates the crystal but enables the orientation of the lattice to be modified by design.

“We have synthesized a class of highly sensitive piezoelectric inks that can be sculpted into complex three-dimensional features with ultraviolet light. The inks contain highly concentrated piezoelectric nanocrystals bonded with UV-sensitive gels, which form a solution—a milky mixture like melted crystal—that we print with a high-resolution digital light 3D printer,” stated Zheng.

The investigators showed the 3D printed materials at a scale measuring just fractions of the diameter of a single strand of human hair. “We can tailor the architecture to make them more flexible and use them, for instance, as energy harvesting devices, wrapping them around any arbitrary curvature,” stated Zhengd. “We can make them thick, and light, stiff or energy-absorbing.”

Compared to flexible piezoelectric polymers, the material has 5-fold higher sensitivities. The material’s shape and stiffness can be adjusted and developed as a thin sheet resembling as a stiff block, or a strip of gauze. “We have a team making them into wearable devices, like rings, insoles, and fitting them into a boxing glove where we will be able to record impact forces and monitor the health of the user,” Zheng said.

The ability to achieve the desired mechanical, electrical and thermal properties will significantly reduce the time and effort needed to develop practical materials.

Shashank Priya, Associate Vice President for Research, Department of Materials Science and Engineering, Penn State

Priya was a former professor of mechanical engineering at Virginia Tech

New applications

The researchers were able to print and demonstrate smart materials swathed around curved surfaces, and worn on fingers and hands to harvest the mechanical energy and convert motion; however, the applications go much beyond consumer electronics and wearables.

Zheng perceives the technology as an advancement in tactile sensing, energy harvesting, robotics, and intelligent infrastructure, in which a structure is fully made with a piezoelectric material—detecting impacts, motions, and vibrations—and enabling those to be tracked and located.

The researchers have also printed a tiny smart bridge to show how it can be utilized for detecting the locations of dropping impacts, and also its magnitude, and, at the same time, it is strong enough to take in the impact energy. The investigators also showed how a smart transducer is capable of changing underwater vibration signals into electric voltages.

“Traditionally, if you wanted to monitor the internal strength of a structure, you would need to have a lot of individual sensors placed all over the structure, each with a number of leads and connectors,” stated Huachen Cui, first author of the Nature Materials paper and a doctoral student with Zhengr. “Here, the structure itself is the sensor – it can monitor itself.”

The researchers’ work is partly supported by the National Science Foundation, Air Force Office of Scientific Research, the Office of Naval Research, and the Virginia Tech Institute of Critical Technology Junior Faculty Award.

The Nature Materials paper features the following authors: Huachen Cui (Mechanical Engineering), Ryan Hensleigh (Virginia Tech Macromolecules Innovation Institute), Desheng Yao (ME), Deepam Maurya (ME), Prashant Kumar (ME), Min Gyu Kang (ME), Shashank Priya, (ME & Penn State’s Materials Research Institute), and Zheng.

Zheng is also an affiliate faculty member of the Department of Materials Science and Engineering.