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LEGO-type hydrogel building blocks with tiny fluid channels can be assembled into complex microfluidic devices and then sealed together. Credit: Wong Lab / Brown University
Using a new type of dual-polymer material capable of dynamically reacting to its environment, researchers at Brown University have developed a set of modular hydrogel components that could be useful in various biomedical and "soft robotic" applications. ".
The components, which are configured by a 3D printer, are capable of bending, twisting or sticking together in response to treatment with certain chemicals. For an article published in the journal Polymer chemistry, the researchers presented a flexible gripper capable of acting on demand to pick up small objects. They also designed LEGO-type hydrogel building blocks that can be carefully assembled and sealed to form custom microfluidic devices – "lab-on-a-chip" systems used for drug screening, cell culture and cell culture. other applications.
Researchers say the key to the functionality of the new material is its dual-polymer composition.
"Essentially, one of the polymers provides structural integrity, while the other promotes these dynamic behaviors such as flexion or self-adhesion," said Thomas Valentin, Ph.D. recently. diploma. student at the Brown & s 39; School of Engineering and senior author of the newspaper. "So, assembling the two makes a material bigger than the sum of its parts."
Hydrogels solidify when the strands of polymer they contain attach to each other, a process called crosslinking. There are two types of bonds that keep the crosslinked polymers together: covalent bonds and ionic bonds. The covalent bonds are quite strong but irreversible. Once two strands are covalently linked, it is easier to break the strand than to break the bond. Ionic bonds, on the other hand, are not as strong, but they can be reversed. The addition of ions (atoms or molecules with a net positive or negative charge) will result in the formation of bonds. By eliminating the ions, the bonds separate.
A new hydrogel material is able to respond dynamically to its environment. In the presence of iron ions, the material bends itself to close a clamp that can pick up small objects. Credit: Wong Lab / Brown University
For this new material, researchers have combined a covalently crosslinked polymer, called PEGDA, and an ionically crosslinked polymer called PAA. The strong covalent bonds of PEGDA keep the material together, while the ionic bonds of PAA make it reactive. Placing the material in an ion-rich environment causes the PAA to cross-link, which means that it becomes stiffer and shrinks. Remove these ions and the material will soften and swell when the ion bonds break. The same process also allows the material to be self-adhesive when desired. Gather two separate pieces, add ions and the pieces bind closely.
This combination of strength and dynamic behavior allowed researchers to build their flexible gripper. They modeled each of the "fingers" of the gripper to have pure PEGDA on one side and a PEGDA-PAA mix on the other. The addition of ions caused shrinkage and strengthening of the PEGDA-PAA side, which brought the two fingers closer to the forceps. Researchers have shown that the configuration is strong enough to lift small objects weighing about one gram and protect them from gravity.
"Materials that can change shape and automatically adapt to different environments are of great interest," said Ian Y. Wong, assistant professor of engineering and corresponding author of the newspaper. "Here we present a material that can flex and reconfigure in response to an external stimulus."
But applications could be more immediate in microfluidics, explain the researchers.
Hydrogels are an attractive material for microfluidic devices, particularly those used in biomedical tests. They are soft and flexible like human tissues and generally non-toxic. The problem is that hydrogels are often difficult to model with the complex channels and chambers needed for microfluidics.
A new type of hydrogel material developed at Brown has the ability to dynamically react to its environment – bending, wringing and self-adhering to demand. Above, the researchers demonstrated a self-adhesive behavior on the tail of a 3D printed hydrogel salamander. Self-adhesive behavior has also been used to fabricate hydrogel building blocks that interlock as LEGO blocks. Credit: Wong Lab / Brown University
But this new material – and the LEGO block concept it allows – offers a potential solution. The 3D printing process makes it possible to integrate complex microfluidic architectures into each block. These blocks can then be assembled using a socket configuration very similar to real LEGO blocks. The addition of ions to the assembled blocks makes it possible to obtain a tight seal.
"The LEGO modular blocks are interesting in that we could create a prefabricated toolbox for microfluidic devices," said Valentin. "You keep a variety of predefined parts with different microfluidic architectures, then you only type the ones you need to create your custom microfluidic circuit, then you cure them together and you're ready to start."
And storing the blocks for long periods of time before use does not seem to be a problem, say the researchers.
"Some of the samples we tested for this study are three to four months old," said Eric DuBois, a Brown undergraduate and co-author of the paper. "We therefore think that these could remain usable for an extended period."
The researchers said they would continue to work with the material, potentially altering the properties of the polymers for even greater durability and functionality.
Explore further:
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More information:
Thomas M. Valentin et al, Self Adhesive PEGDA-PAA Hydrogels Printed in 3D as Modular Components for Lightweight Actuators and Microfluidics, Polymer chemistry (2019). DOI: 10.1039 / C9PY00211A
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