A new metamaterial mimics the connection between stiff, porous bone and flexible, fibrous tendons, offering a novel solution to a longstanding challenge in biomedical engineering. To create this metamaterial, scientists at the Delft University of Technology in the Netherlands took two distinct classes of materials and merged them via a transition zone that resembles the natural interface between bones and tendons. They say their method could lead to better, more durable implants as well as improvements to other devices that rely on interfaces between materials with contrasting properties.
Junctions between bones and tendons can weaken with age, leading to musculoskeletal pain and, in extreme cases, disability. Researchers have explored ways of using biomedical scaffolds to help repair this damage, but this has proved challenging because bone is relatively stiff and isotropic, whereas tendon is much softer and highly anisotropic. “Reproducing the characteristics of both within a single architecture usually requires concessions that limit the performance of one or the other,” says Mohammad J Mirzaali, who led the TU Delft study. “The main challenge was therefore not simply to design two different types of lattices but find a reliable way to connect them.”
Best of both worlds
The researchers began with two distinct classes of architected materials: strut-based lattices and sheet-based ones derived from structures known as triply periodic minimal surfaces. Mirzaali explains that the latter provide relatively high stiffness and load-bearing capacity, while the former offer greater geometric flexibility and can be designed to exhibit more compliant and anisotropic mechanical responses.
Because it is so difficult to create effective connections between such disparate materials, Mirzaali says this approach previously received little attention. “Directly connecting the two often creates weak interfacial zones in the composite that concentrate mechanical stresses, making it more likely to fail,” he explains.
To avoid this, the TU Delft team developed a computational strategy that identifies compatible geometries between the two structures and introduces a transition region that bridges their architectures over a relatively short distance. This bridging is accomplished via so-called twinned unit cells that share some characteristics of both structures and allow a gradual change from one to the other. The result is a transition that more closely resembles the natural bone-tendon interface, rather than an abrupt boundary.
The researchers turned several of these computer-generated designs into real objects using 3D printing. After performing mechanical tests to evaluate failure mechanisms, the fatigue behaviour of the hybrid lattices, and the effects of different connection strategies on stiffness, they confirmed that their transitional interfaces are robust under different loads. Micro-computed tomography (µCT) images also showed the printed structures closely resembled the computational versions, with no unintended voids visible in the examined regions.
Work could advance implants and tissue engineering strategies
“Our work shows that we can design multi-zonal structures in which each region is optimized for a different function without being restricted to a single family of lattice architectures,” Mirzaali tells Physics World. Hybrid biomedical scaffolds are one possible application, he says, “but the concept is more general and could be extended to a host of other areas where different regions in a component need contrasting properties.”
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For now, the TU Delft researchers are concentrating on biomedical applications and investigating how different cell types respond to the architectural transitions. Initially, they are doing this via in vitro monoculture and co-culture studies, with promising preliminary results. “Musculoskeletal disorders are among the leading causes of disability worldwide,” Mirzaali says, “and improved methods for connecting hard and soft tissues could help advance future implants and tissue engineering strategies.”
The research is described in Nature Communications.
