Your laptop or phone charger that breaks where the flexible cable connects to the hard adapter, we’ve all experienced it. This is just one example of how difficult it is to effectively connect hard and soft materials. With a unique 3D printing process researchers from TU Delft hybrid joints made of multiple materials that are remarkably close to the natural design of bone-tendon joints. Their research results recently appeared in Nature Communications.
- TU Delft researchers use human bone-tendon connections as inspiration to effectively connect hard and soft materials;
- For this they use a 3D printing process and different configurations;
- They managed to increase the toughness of the connections by 50%, with universally applicable design guidelines to improve mechanical performance.
- The developed technology makes the production of entire products possible in one go.
Despite the large difference in hardness between bones and tendons, this connection never breaks in the human body. It is this bone-tendon connection that inspired the team of researchers from the Faculty of Mechanical Engineering, Maritime Technology and Applied Materials Sciences (3mE) to optimize the hard-soft transition of man-made materials.
Design inspiration
Amir Zadpoor. Image: TU Delft
When there is a mismatch between two connected materials, this results in a stress concentration, explains Amir Zadpoor, professor of Biomaterials and Tissue Biomechanics. That means the mechanical stress goes to the connection point and usually results in the softer material breaking. One of the things you see in nature is a gradual change in properties. “A hard material does not suddenly become a soft material,” says Zadpoor. “It changes gradually and that levels off the tension concentration.” With that in mind, the researchers used different configurations and a multi-material 3D printing technique to increase the contact surface between hard and soft materials, mimicking the design found in nature.
Another consideration is that the force a soft material can withstand before failure is lower than that of a hard material. “It is only relevant to make the transition as strong as the soft material, because if it is stronger, the soft material will fail,” says Mauricio Cruz Saldivar, the first author of the paper. The researchers were able to increase the toughness values of compounds by fifty percent compared to a control group. Approaching the limits of what is theoretically possible is one of the most important contributions of this research, according to the team. But the research also led to design guidelines for improving the mechanical performance of soft-hard connections, and these principles are universally applicable.
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An entire product in one go
The technology developed by the team also makes it possible to make an entire product in one go. This is important because products consisting of multiple materials are usually attached to each other with glue. For example, parts are assembled or mechanically connected, such as in the automotive or aviation industry. “But what we are trying to do is eliminate the extra steps involved and make everything in one go,” says assistant professor Zhenya Doubrovski. “That makes it possible for us to combine even more exotic and different materials, for example materials that have more damping resistance versus materials that are very powerful.” These combinations lead to even more practical applications.
Future applications
This technology can be widely used. Potential applications include medical devices, soft robotics and flexible devices. But the team also wants to investigate how they can make connections with living cells, for example to connect implants to the surrounding soft tissue. “Ultimately, we want to regenerate bone and the connection between bone and muscle,” says Assistant Professor Mohammad J. Mirzaali. “That would mean that we would have to integrate living cells into this connection, which increases the complexity of the construction.”
2023-12-13 07:04:04
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