Researchers from Brown University have developed a new type of dual polymer material that is capable of responding to its environment in dynamic ways. The innovative hydrogel material, demonstrated with 3D printing, could be used in a range of soft robotic and biomedical applications to simplify the assembly of complex devices.
In their research, the Brown researchers used a 3D printer to produce components from the hydrogel which could be programmed to bend, twist or stick together when exposed to certain chemicals. In one particular demonstration, the team 3D printed a soft gripper which was capable of actuating on demand to pick up small objects.
In another example, LEGO-inspired building blocks were printed from the hydrogel and subsequently assembled and sealed to form customized microfluidic devices. This last use could be used in drug screening, cell culture and other similar applications.
PEGDA and PAA
The hydrogel material is distinguished by its dual polymer composition, which enables its special functionalities. As Thomas Valentin, a recent PhD graduate from Brown’s School of Engineering and the lead author of the study, explained: “Essentially, the one polymer provides structural integrity, while the other enables these dynamic behaviors like bending or self-adhesion. So putting the two together makes a material that’s greater than the sum of its parts.”
More specifically, the hydrogel combines a covalently crosslinked polymer called PEGDA with an ionically crosslinked one, PAA. Polymers that are covalently crosslinked have strong, irreversible bonds, meaning that once two strands are linked, it is easier to break the strand than the bond itself. Ionic crosslinked polymers, however, have weaker bonds that can be reversed. In the case of ionic bonds, the addition of ions causes the bonds to form while their removal causes the bonds to break.
By combining PEDGA and PAA, the hydrogel has both types of bonds. The PEGDA’s covalent bonds effectively hold the material together, while the PAA’s ionic bonds make it responsive. By exposing the printed hydrogel to an ion-rich environment, the PAA in the material crosslinks, causing it to become more rigid. When the ions are removed, the material softens and swells. This composition also enables separate printed parts to be attached to each other with the addition of ions (as seen in the lizard print).
Soft robotic applications
As mentioned, the researchers utilized the innovative material to produce a soft gripper capable of picking objects up. This component integrated fingers made from pure PEGDA on one side and a PEGDA-PAA mix on the other. When exposed to ions, the mixed side of the gripper fingers shrinks and hardens, bringing the grippers together. In the experiment, the gripper successfully lifted and held small objects (weighing about a gram).
“There’s a lot of interest in materials that can change their shapes and automatically adapt to different environments,” explained Ian Y. Wong, an assistant professor of engineering and the paper’s corresponding author. “So here we demonstrate a material the can flex and reconfigure itself in response to an external stimulus.”
Another important application area for the hydrogel is in the production of microfluidic devices, which must be non-toxic and flexible for biomedical use. Traditionally, producing microfluidic devices with complex channels has been challenging because of the scale, precision and sealing required.
The new material, however, can be printed into LEGO-like blocks embedded with microfluidic architectures. The blocks can then be assembled using a socket configuration and tightly sealed by adding ions.
“The modular LEGO blocks are interesting in that we could create a prefabricated toolbox for microfluidic devices,” Valentin added. “You keep a variety of preset parts with different microfluidic architectures on hand, and then you just grab the ones you need to make your custom microfluidic circuit. Then you heal them together and it’s ready to go.”
Notably, the hydrogel components have demonstrated a relatively long shelf life. The researchers say they tested samples that were three or four months old with good results.
Going forward, the research team from Brown University will continue to develop the dual polymer to achieve higher durability and better functionality. A study detailing the research was recently published in the journal Polymer Chemistry.