Using a new type of dual polymer material capable of responding dynamically to its environment, Brown University researchers have developed a set of modular hydrogel components that could be useful in a variety of “soft robotic” and biomedical applications. The components, which are patterned by a 3D printer, are capable of bending, twisting or sticking together in response to treatment with certain chemicals. For a paper published in the journal Polymer Chemistry, the researchers demonstrated a soft gripper capable of actuating on demand to pick up small objects. They also designed LEGO-like hydrogel building blocks that can be carefully assembled then tightly sealed together to form customized microfluidic devices — “lab-on-a-chip” systems used for drug screening, cell cultures and other applications.
Hydrogels solidify when the polymer strands within them become tethered to each other, a process called crosslinking. There are two types of bonds that hold crosslinked polymers together: covalent and ionic. Covalent bonds are quite strong, but irreversible. Once two strands are linked covalently, it’s easier to break the strand than it is to break the bond. Ionic bonds on the other hand are not quite as strong, but they can be reversed. Adding ions (atoms or molecules with a net positive or negative charge) will cause the bonds to form. Removing ions will cause the bonds to fall apart.
For this new material, the researchers combined one polymer that’s covalently crosslinked, called PEGDA, and one that’s ionically crosslinked, called PAA. The PEGDA’s strong covalent bonds hold the material together, while the PAA’s ionic bonds make it responsive. Putting the material in an ion-rich environment causes the PAA to crosslink, meaning it becomes more rigid and contracts. Take those ions away, and the material softens and swells as the ionic bonds break. The same process also enables the material to be self-adhesive when desired. Put two separate pieces together, add some ions, and the pieces attach tightly together.
That combination of strength and dynamic behavior enabled the researchers to make their soft gripper. They patterned each of the gripper’s “fingers” to have pure PEGDA on one side and a PEGDA-PAA mixture on the other. Adding ions caused the PEGDA-PAA side to shrink and strengthen, which pulled the two gripper fingers together. The researchers showed that the setup was strong enough to lift small objects weighing about a gram, and hold them against gravity.