Researchers from the Swiss Federal Institute of Technology Lausanne (EPFL) have developed a novel 3D printing method to fabricate strong and supple composite polymers that possess similar properties to those of naturally occurring materials, such as cartilage and skin.
Many soft natural tissues display mechanical properties which so far have not been replicable in manmade alternatives. To address this, EPFL scientists introduced a new specifically-engineered ink that can be 3D printed into strong and tough double network granular hydrogels (DNGHs).
According to the team, this development opens the door to new possibilities in the design of adaptive, strong, and tough hydrogels with potential applications in soft robotics, membranes for wastewater treatment, and even cartilage prosthetic implants.
“We’re still a long way from being able to control the structure of synthetic materials at so many different scales,” said Esther Amstad, an assistant professor at EPFL’s Soft Materials Laboratory and the paper’s lead author. “In nature, basic building blocks are encapsulated in compartments, which are then released in a highly localized way. This process provides greater control over a material’s fine structure and local composition.
“We took a similar approach, arranging our own building blocks into compartments then assembling them into a superstructure.”
3D printing hydrogels
The field of 3D printed hydrogels seems to be of great interest to EPFL as of late, after another research team from the institute recently developed a mechanical microdevice capable of biopsy and drug delivery when implanted in human skin. The implant is powered by a 3D nanoprinted pump made of hydrogel, which possesses the same stiffness as human skin and can be remotely actuated via ultrasound.
Elsewhere, researchers from China’s Sichuan University and Xiamen University have developed 3D printed self-adhesive bandages capable of delivering nerve-healing drugs, which are comprised of two click-activated hydrogel layers, while a Spanish-led research team 3D printed a hydrogel capable of accelerating the production of T-cells in cancer patients.
EPFL isn’t the first to investigate the 3D printing of life-like materials such as skin and cartilage, as researchers from Duke University previously explored 3D printable hydrogel materials to create life-like knee implants for use in meniscus knee injuries.
Replicating natural materials
Despite significant improvements in mechanics, man-made hydrogels are typically incapable of adapting their properties in response to external stimuli, in contrast to many natural counterparts. Generally, synthetic hydrogels fall into two contrasting categories, the first of which includes materials that are hard and loading-bearing but are poor at absorbing energy, such as window glass. Materials in the second group are more able to resist cracking but are extremely soft and there cannot bear heavy loads.
Yet, some natural composites which are made from a combination of biological materials and proteins are both strong and crack-resistant, a combination of properties that the EPFL researchers hoped to achieve with their 3D printed DNGHs.
To produce the ink, the researchers swelled polyelectrolyte-based microgels in a monomer-loaded solution, within which the monomers bound together to form a network of polymers. These microparticles were then soaked in another type of monomer (acrylamide) to create a paste. This two-step process separates the fabrication of the microgels and their annealing in order to combine the injectibility and printability advantages of jammed granular solutions with the desirable mechanical properties of DN hydrogels.
After jamming the hydrogels together using vacuum filtration, the scientists 3D printed the paste through a 410 µm diameter nozzle and exposed it to UV radiation, causing the monomers added in the second stage to polymerize and intertwine with those formed earlier in the process. This process hardened the paste to produce a strong, hard-wearing material.
Putting it to the test
To assess the robustness of their material, the researchers carried out fracture strength tests and compression measurements. A 3mm tube was able to withstand a tensile load of up to 10kg, and a compressive load of up to 80kg, without damaging the material’s structural integrity. The material’s elasticity was also explored through a series of tensile tests, which revealed the optimum crosslink density of the microgels should remain at 3.5 mol%.
The scientists further claim their ink overcomes limitations surrounding weak adhesion between sequentially deposited layers in 3D printed materials, as the second, percolating network forms after the ink is 3D printed. In theory, this means the interfaces between the layers should be as strong as the grain boundaries within the printing plane. To test this hypothesis, the researchers printed two solid DNGH rectangular strips, one with the printing direction along its length, and the other where the printing direction is perpendicular to it. The printing direction appeared to have no significant influence on the mechanical properties of the stripes, in contrast to polymers which are 3D printed using conventional, homogeneous inks.
Potential applications of DNGHs
The results of the study suggest that the researchers’ specially-engineered ink is well-suited to the 3D printing of strong and tough 3D hydrogels, which has been difficult to achieve in the past. The shape fidelity and mechanical stability of the constructs fabricated through this method indicates the potential for the microgel-based ink to design mechanically robust granular materials with complex geometries.
The scientists also demonstrated the potential for imparting shape-morphing properties on the fabricated DNGHs through to co-printing of microgels with different crosslink densities. To do this, they 3D printed a flower, of which the first layer was composed of microgels with a lower crosslink density than those contained in the second layer. During the drying and soaking stage, the flower folds in opposite directions. This suggests the researchers’ method could have the potential to produce responsive, smart, and soft materials that are sufficiently strong and stiff to bear significant loads.
Relating this to tangible applications in the real world, the DNGHs 3D printed with the researchers’ specially-engineered ink could open up new possibilities in designing a new generation of strong and tough soft robotics and implants, such as for cartilage and skin, which can adapt their properties in response to external stimuli.
Further details of the study can be found in the article titled “3D printing of strong and tough double network granular hydrogels”, published in the Advanced Functional Materials journal. The study was co-authored by E. Amstad, A. Charlet, and M. Hirsch.
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Featured image shows 3D printing and testing of the DNGHs. Image via Advanced Functional Materials.