The use of a new type of dual polymer material capable of reacting dynamically to its environment, Brown University researchers have developed a set of modular hydrogel components that may be useful in a variety of "soft robot" and biomedical applications. applications.
The components patterned by a 3-D 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 showed a soft grip that could activate the demand for collecting small objects. They also designed the LEGO-like hydrogel building blocks that can be gently mounted and then tightly sealed to form custom microfluidic devices ̵
The key to the new material's functionality is its dual polymer composition, the researchers say.
"Essentially, one polymer provides structural integrity, while the other allows these dynamic behaviors to bend or self-adhere," said Thomas Valentin, a recent Ph.D. students of Brown's Engineering School and the main author of the paper. "Then the two put together a material larger than the sum of its parts."
Hydrogels solidify when polymer strands in them become bonded to each other, a process called crosslinking. There are two types of bonds that contain crosslinked polymers together: covalently and ionically. Covalent bonds are quite strong but irreversible. When two wires are covalently connected, it is easier to break the string than it is to break the tape. Ionic bindings 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. Removal of ions will cause the bonds to fall apart.
For this new material, the researchers combined a polymer which is covalently cross-linked, called PEGDA, and one which is ionically cross-linked, called PAA. PEGDA's strong covalent bonds hold the material together while PAA's ionic bonds make it sound fast. Placing the material in an ionic environment gets the PAA interconnected, which means it becomes more rigid and contracts. Remove these ions and the material softens and swells as the ionic bonds break. The same process also allows the material to be self-adhesive when desired. Put two separate pieces together, add some ions, and the pieces sit tightly together.
This combination of strength and dynamic behavior allowed the researchers to make their soft grips. They each patterned the "fingers" of the gripper to have pure PEGDA on one side and a PEGDA-PAA blend on the other. Adding ions caused the PEGDA-PAA side to shrink and strengthen, pulling the two gripper fingers together. The researchers showed that the setup was strong enough to lift small objects weighing about one gram and holding them against gravity.
"There is great interest in materials that can change their shapes and adapt automatically to different environments," said Ian Y. Wong, a technical assistant and the equivalent author of the paper. "So here we show a material that can be bent and reconfigured in response to an external stimulus."
But potentially more immediate use is in microfluidics, researchers say.
Hydrogels are an attractive material for microfluidic devices, especially those used in biomedical testing. They are soft and flexible as human tissues, and generally non-toxic. The problem is that hydrogels are often difficult to pattern with the complex channels and chambers required in microfluidics.
But this new material – and the LEGO block concept it allows – provides a potential solution. The 3-D printing process allows to incorporate complex microfluidic architectures into each block. These blocks can then be assembled using a pedestal configuration resembling real LEGO blocks. Adding ions to the assembled blocks makes a waterproof seal.
"The modular LEGO blocks are interesting because we could create a prefabricated toolbox for microfluidic devices," Valentin said. "You keep a number of preset parts with different microfluidic architectures available, and then you just take the ones you need to make your custom microfluidic circuits. Then you heal them together and it's ready to go."
And storing the blocks for long periods before use does not seem to be a problem, the researchers say.
"Some of the tests we tested for this study were three or four months old," said Eric DuBois, a brown bachelor and co-author on paper. "So we think these could stay usable for a long time."
The researchers say they will continue to work with the material, possibly adapting the properties of the polymers to achieve even more durability and functionality.
Scientists create new "smart" material with potential biomedical, environmental applications
Thomas M. Valentin et al., 3D-printed self-adhesive PEGDA-PAA hydrogels as modular components for soft actuators and microfluidics, Polymer Chemistry (2019). DOI: 10,1039 / C9PY00211A