A technique for joining rigid and elastic materials together, inspired by nature’s way of connecting muscle to bone in the human body, has been developed by a team of Brazilian, European and American researchers at the Complex Materials group of the Federal Institute of Technology (ETH) in Zurich, Switzerland. Using the new technique, the researchers produce bio-inspired composites that show great potential for application in biomedical implants, not to mention components for the automobile and aerospace industries. The method for joining polymers to ceramics was described in a paper published in December 2012 in the journal Nature Communications.
The group’s leader, André Studart, a Brazilian professor and engineer, says that attachments between rigid and flexible materials are very common in living beings. “In our own bodies, for example, highly elastic parts like tendons are connected to extremely rigid ones, like bones,” he explains. “Unlike what we see in artificial products, our bodies are able to sustain high mechanical loads at the point where these two materials are joined, without that connection point failing.” Rafael Libanori, a Brazilian chemist, has also applied nature’s principles to produce high-performance artificial materials. The other members of the group include two Swiss researchers, one French, one Austrian, and one American.
Transforming these natural characteristics into technology, creating an artificial mechanism that makes it possible to connect elastic materials to rigid ones, is not as easy as nature would make it seem. On the contrary, joining two products with different mechanical properties is currently a major challenge for many fields of engineering. This is why the work being done by Studart’s group is so important. “We developed a method for producing artificial heterogeneous materials that can be used to connect rigid structures to elastic ones efficiently, like in nature,” he says.
The group observed that nature solved the problem by gradually changing the mechanical properties of the coupling structure, known as a tendon-bone insertion. “Near the tendon, the insertion is relatively elastic and is composed mainly of collagen fibers,” Libanori explains. “But as it gets closer to the bone, the concentration of reinforcing mineral elements increases gradually, resulting in a heterogeneous composite that can distribute mechanical strain uniformly along the length of the insertion.” This gradual transition of mechanical properties takes place both lengthwise and crosswise, minimizing the development of intense mechanical strain at the tendon-bone junction.
Transition in teeth
The mechanical properties of collagen are typical of elastic materials, whereas reinforcing mineral elements like hydroxyapatite exhibit the usual characteristics of rigid ceramics. Hydroxyapatite is made of calcium phosphate, the main component in bones. Teeth are another example of a biological material that gradually transitions between different types of mechanical properties. “The inner part of our teeth is made of dentin, which is more elastic, whereas the outer layer, the tooth enamel, is much more rigid and hard,” Libanori explains. “This gradual transition occurs perpendicularly, from the inside of the tooth outward toward the enamel.”
The method created by the group, called “hierarchical reinforcement of polyurethane elastomers”, was developed during Libanori’s doctoral studies at ETH, where Studart was his advisor. “In this case, the word ‘hierarchical’ is used because the polymer matrix is reinforced with increasingly rigid components at different size scales: molecular, nanometric, and micrometric,” says Libanori. “This way, we can combine layers of materials with differing degrees of rigidity, using a procedure called solvent welding.” In their Nature Communications paper, the researchers describe a matrix of polyurethane – a polymer used in the manufacture of foams, shoe soles, textiles, and adhesives, among other things – reinforced with nanometric and micrometric ceramic particles (respectively, a synthetic clay called laponite and aluminum oxide). Nanometric sizes are equivalent to 1 millimeter divided by 1 million, and micrometric particles are thousandths of a millimeter in diameter.
According to Studart, this method enables the creation of polymeric composites that were unimaginable until now. “For instance, we created a material whose rigidity at its upper surface is equivalent to that of our teeth and bones, whereas the elasticity of its bottom surface resembles that of our skin,” the professor reveals. The researchers have also shown that rigid electronic devices integrated with a flexible substrate, as in the case of LED circuits, can be effectively protected from mechanical failure. This significantly increases the equipment’s useful lifetime.
Flexible devices produced via this method can be stretched to as much as 4.5 times their initial size without compromising the response of their rigid electronic components. According to Libanori, the project is still in the academic research stage and the group is looking for companies interested in licensing the technology. “At this time, we are discussing the possibilities for collaboration with a major company in the electronics industry,” he says.
Professor Edson Roberto Leite, from the Chemistry Department at the Federal University of São Carlos (UFSCar), has been keeping track of Libanori and Studart’s work for several years. “I was Rafael Libanori’s advisor during his undergraduate research and masters program, and I referred him to Studart,” he says. “Their work is very important because they create composite material processing methods that make it possible to emulate nature’s hierarchical way of organizing materials. This is the group’s major breakthrough. Going beyond simply studying how nature works, they are artificially reproducing the way it builds materials, without using biochemistry or genetics.” According to Leite, research in that field is still incipient in Brazil. “Some groups are working on artificial photosynthesis, like ours here at UFSCar, and only a handful of others are working on bio-inspired composites,” he says. “Elsewhere in the world, this subject is gaining prominence and large groups are performing cutting-edge research.”
LIBANORI, R. et al. Stretchable heterogeneous composites with extreme mechanical gradients. Nature Communications, v.3, article 1.65, 11 December 2012 (online).