Bone and tendon could hardly be more different. One is hard and rigid, the other soft and flexible. Yet in the human body they are connected by a remarkably strong interface that rarely fails. For decades, scientists have struggled to recreate this natural transition. Now, a TU Delft team has taken a major step forward. By providing new strategies to combine two types of metamaterials, they have engineered structures that closely mimic the bone-tendon interface. The work was published in Nature Communications .
Nature has spent millions of years optimizing the connection between bone and tendon. Over a very short distance, the body transitions from stiff bone to flexible tendon while maintaining strength and durability. Reproducing such a gradual yet robust transition has proven challenging for engineers.
Yet, being able to mimic this transition could have a lot of impact. "Musculoskeletal disorders are among the leading causes of disability worldwide," explains associate professor Mohammad J. Mirzaali, "and improved methods for connecting hard and soft tissues could help advance future implants and tissue engineering strategies."
The challenge of connection
Scientists have turned to metamaterials, engineered materials whose properties are determined by their internal architecture rather than their chemical composition. Two major classes exist. Sheet-based metamaterials consist of continuous surfaces and are well suited for stiff structures and bone cell integration, while strut-based metamaterials are made of interconnected beams and can better mimic softer tissues.
"Combining these two types of metamaterials has been a true challenge for engineers. Directly connecting the two often creates weak zones where failure can occur. The first engineering instinct is to gradually transition from one architecture to the other; but that causes the transition region to become far too long and too far away from the compact bone-tendon interface found in nature," explains first author dr. Jianxing Yang.
Building a seamless transition
To overcome this challenge, the TU Delft team developed an algorithm that identifies compatible sheet-based and strut-based lattices and determines how they can best be joined. The method aligns the geometries of the two architectures and introduces a transition cell that shares characteristics of both. "Rather than creating an abrupt boundary, the transition cell enables a short, gradual change from one structure to the other, much like the natural bone-tendon interface", explains dr. Vahid Moosabeiki.
The researchers validated their approach by 3D printing several of the designs and subjecting them to mechanical tests. The experiments confirmed that the computationally designed transitions can create robust connections between the two metamaterial families.
Implants that direct cell response
In follow-up research, the team is now investigating how living cells respond to these hybrid structures. While scientists have previously studied cell behavior on sheet-based and strut-based metamaterials separately, little is known about how cells react when both architectures are combined within a single construct. "By culturing cells on the newly developed structures, we hope to understand how geometry influences cell attachment, growth and differentiation" say Prof. Amir A. Zadpoor. Ultimately, they aim to determine whether architected transitions can guide the formation of different tissue types, just as the natural bone-tendon interface does.
The findings provide researchers with a new toolbox for designing transitions between hard and soft materials. In the future, such strategies could contribute to implants and tissue-engineered structures that not only match the mechanical properties of the body, but also help direct its biological response.
