Designed to mimic bone, wood and other natural materials, the novel porous material is lighter than traditional products and can be strategically inserted into structures to provide higher stiffness in areas with high demand. Image: Paulino et al.A material developed in a research lab at Princeton University is full of holes – but that's a good thing. Designed to mimic bone, wood and other natural materials, the porous material is lighter than traditional products and can be strategically inserted into structures to provide higher stiffness in areas with high demand.
This porous material, created by researchers at Princeton University and Georgia Tech, features spinodal microstructures – networks of specially designed holes that can be tuned to achieve optimized behavior at the macroscale. In a paper in Advanced Materials, the team reported combining different realizations of these spinodal microstructures to design and prototype facial implants for reconstructive surgery and stiff, lightweight parts for aircraft.
Davide Bigoni, a professor of solid and structural mechanics at the University of Trento in Italy, who was not involved with the research, called the results a "breakthrough".
"The authors have found a clever way to allow a continuous transition between zones with different architectures,” he said. “This is the ultimate concept of biomimicking, as all natural structures form continuous systems. This is a fact known since ancient times – 'natura non facit saltus', nature does not make jumps."
Many natural materials, including bone, animal horns, wood and the skeletons of sea urchins known as sand dollars, are full of holes. The empty spaces make the materials light, and in some cases allow body fluids to move through the pores. In bones, these spaces allow for a remodeling process that makes the bone more or less dense in response to physical demands. Creating synthetic materials with similar properties has been a challenge for engineers.
In the new study, the researchers mimicked these natural materials by designing microstructures with holes of different sizes, shapes and orientations. The new objects are known as architected materials, as they have a customizable performance based on the relation between material and geometry.
The holes can be shaped like spheres (like the ones in sand dollar skeletons), diamonds (bone), columns (wood) or lentils (horn). By varying this shape, the researchers could imbue the material with stiffness in different directions. They could also control the material’s density by changing the holes’ size, while altering the holes’ orientation increased stiffness in regions under strain.
"You have the actual structure and the microstructure working together to get superior performance," said Fernando Vasconcelos da Senhora, a graduate student at Georgia Tech and first author of the paper.
To demonstrate potential uses, the researchers designed and 3D-printed a facial implant, such as used to repair a major facial injury after a car accident. Currently, surgeons use plastic or titanium to create porous implants that allow bone to regrow through the holes, but these implants do not have the same tunability as can be achieved with spinodal architectures. The researchers combined sections with column- and lentil-shaped holes to create an implant that was stiff enough to withstand the forces of chewing, and had the right size holes to promote bone growth and healing. The prototype implant was made of a photopolymer resin, but it could be 3D printed using biocompatible materials for future use in patients.
According to the researchers, this technique opens the door to creating implants with many different types of material, as the combination of geometry and material allows designers to fine-tune performance.
“It’s not the base material that is better. It is the microscale features that are better,” said Emily Sanders, a co-author of the paper and an assistant professor of mechanical engineering at Georgia Tech. “In theory, we could make the scaffolds out of any material – most appropriate would be to explore biocompatible materials.”
To demonstrate an entirely different use, the researchers combined three different types of microstructure to construct a jet engine bracket – a critical part of an aircraft, which holds the engine in place and must be both strong and lightweight.
"We have a technique that is quite powerful in the sense that it combines material architectures with optimization at different scales and its integration with additive manufacturing," said Glaucio Paulino, professor of engineering at Princeton University and principal investigator on the project. "It can have a broad range of applications in the sense that it scales, so it can be applied in nano- and microtechnology, as well as at meso- and macroscales."
A key aspect of the materials' success is the seamless transitions from one type of microstructure to another within the same object. Abruptly switching between microstructures without connecting the network of pores would cause the material to split along the seams. Materials made with spinodal microstructures are also less likely to have weak spots, because the holes occur randomly instead of in regular patterns.
"A major part was figuring out how to take advantage of the manufacturing platform and [work out] mathematically the structure of these architected materials and then link the two together so that we could actually fabricate something," said Sanders.
The researchers are already exploring additional uses for the microstructures. Currently, the technology is at the prototype stage, but they are eager to test the properties of the materials more fully. "I'm interested in understanding the fundamental questions about how these architected materials behave," Sanders said.
This story is adapted from material from Princeton University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.