A new kind of hydrogel material has the ability to react dynamically to its environment – bending, twisting, and self-adhering on demand. The self-adhering behavior is shown on the tail of a 3D-printed hydrogel salamander. The self-adhering behavior was also used to make hydrogel building blocks that fit together like LEGO blocks. (Wong Lab/Brown University)

Using a new type of dual-polymer material capable of responding dynamically to its environment, 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. The researchers created a soft gripper capable of actuating on demand to pick up small objects, as well as 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.

The key to the new material's functionality is its dual-polymer composition; one polymer provides structural integrity while the other enables the dynamic behaviors like bending or self-adhesion.

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. Ionic bonds are not quite as strong but can be reversed. Adding ions will cause the bonds to form and removing ions will cause the bonds to fall apart.

For the new material, the researchers combined one polymer that's covalently crosslinked (called PEGDA) and one that's ionically crosslinked (PAA). 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 a soft gripper. Each of the gripper's “fingers” was patterned 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 setup was strong enough to lift small objects weighing about a gram and hold them against gravity.

The new material — and the LEGO block concept it enables — allows complex microfluidic architectures to be incorporated into each block. Those blocks can then be assembled using a socket configuration much like that of real LEGO blocks. Adding ions to the assembled blocks makes a water-tight seal.

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