The system works
The puzzle pieces in the TriggerINK project in Aachen are slowly but surely coming together, bringing the future of cartilage regeneration closer than ever. Last year, the participating research groups made rapid progress—with initial tests already demonstrating cartilage growth.
Worn or damaged cartilage tissue can’t regrow on its own, which is why researchers in the TriggerINK project at DWI – Leibniz Institute for Interactive Materials in Aachen are lending nature a helping hand. They’re envisioning an innovative cartilage regeneration strategy in which a 3D printing robot injects a gelatinous substance—a customised bio-ink—directly into damaged cartilage. There, the bio-ink serves as a supportive scaffold and orientation for the new tissue and is simultaneously enriched with novel substances that trigger regeneration.
Last year, the various research groups in the project made several important advances. Project leader Laura De Laporte’s group continued experiments on the bio-ink: working with samples in test tubes, they investigated how the microscopic, rod-shaped elements in the ink, aligned using a magnetic field, affect cell growth. Previous studies had already shown that stem cells in scaffolds with a clear spatial orientation are more likely to develop into cartilage cells.
Cartilage cells in the matrix
In order to be functional, however, cartilage tissue requires more than just cartilage cells. The cells must also produce components such as collagen or proteoglycans—the components in the extracellular matrix (where the cartilage cells are anchored) that lend cartilage its essential elastic and tear-resistant properties. The team recently demonstrated enhanced production levels of these matrix components in the aligned hydrogel scaffolds. “For the first time, we’ve been able to show that the spatial alignment of our scaffold leads to functional improvements,” Laura De Laporte says with satisfaction.
In collaboration with Gerjo van Osch from Erasmus University Rotterdam, the researchers also conducted the first animal experiments in the project: they removed a sample containing cartilage and bone tissue from the knee of a dead cow, then drilled a hole in this graft, filled the hole with the bio-ink mixture and inserted the graft under the skin of a laboratory mouse.
After adding growth factors, the researchers observed that stem cells did indeed grow from the cow bone into the bio-ink—and that cartilage formed. These initial findings suggest that the aligned microgel rods promote cartilage generation. Or, as De Laporte says: “Our system seems to work.” Positive results notwithstanding, a whole series of further tests are still needed. To this end, the researchers have set up a lab for studying their treatment in cartilage-bone samples from cows ex vivo in an incubator under physiological-like mechanical loading.
Targeted release
That growth factors are essential for cartilage regeneration comes as no great surprise to the TriggerINK team. After all, their overall plan is centred around the targeted release of these factors to control tissue regeneration: first comes a growth factor to attract stem cells from the surrounding bone tissue to the treatment site, and then a differentiation factor to trigger the stem cells into forming cartilage cells.
The controlled release of these factors is the focus of the work conducted by Andreas Herrmann and his group of researchers, who have developed a promising technology for integrating the substances: ultrasound-responsive nanoflowers. These petal-shaped nanostructures, made from DNA strands, contain a protease (an enzyme) and are integrated into the bio-ink. A highly specific amino acid sequence known as an intein (an intervening protein) inactivates the growth factors, which are introduced into the nanoflowers—in such a way that they can be reactivated using ultrasound.
Excise and bond
The ensuing activation process works as follows: first, an ultrasound signal switches on the protease, causing the intein to excise itself from the protein so that two halves of the protein remain. Afterwards, the intein can link the two halves of the desired protein in such a way that it makes the protein—in this case, the growth factor—functional. “We’ve taken a significant step forward with this technology,” Andreas Herrmann says.
Matthias Wessling’s group can also report progress. The researchers have developed a prototype of the two-armed 3D printing robot that will inject the bio-ink into damaged cartilage tissue. The printhead and a light source are mounted on one of the arms. The printhead is designed to print bio-inks containing different microgel particles, layer by layer, while the light source emits a specific wavelength with the aim of connecting certain molecules in the bio-ink, enabling them to form a grid-like scaffold. The robot’s other arm contains a magnetic ring used to spatially align the magnetic microgel rods via an external magnetic field before the bio-ink is crosslinked and stabilised by light irradiation.
Although it will take time to fit all these puzzle pieces together, the future of cartilage regeneration is now in sight.
