4D Printing of Bioinspired hydrogel structures

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Johns Hopkins University
Biological structures in nature are made out of components which respond to external cues present in their environments. For example, sea creatures and plants swell and deswell to adapt and grow in their environment. Stimuli-responsive hydrogels are synthetic polymers that exhibit similar behavior and undergo large volumetric or phase transitions in response to external stimuli. The integration of these hydrogels with other material systems can create engineered self-sufficient structures that can function in complex environments like the human body. This study provides insights into the possible routes to program shape changing structures using stimuli-responsive hydrogels and 3D/4D printing for applications in soft robotics, microfluidics, and biomedical engineering. 4-D printable inks were developed using a thermoresponsive active hydrogel Poly(N-isopropylacrylamide) and a passive hydrogel Poly(acrylamide). By printing tubes with alternating walls of active and passive hydrogel arranged either vertically, horizontally or at angles, structures with different functionalities which were reversible with temperature. A gripper was printed based on a coral polyp which could extend and then grip a structure. All these structures can be extended towards applications in soft robotic endoscopy, in which endoscopes have to extend, bend and grip. Enzymatic degradation is another stimulus to trigger transitions in hydrogels. It takes different times to digest various kinds of food by enzymes in our body depending on its macromolecular composition. We developed 4D printable edible inks that degraded due to the hydrolysis of the glycosidic bonds of the polysaccharides in an apple by the enzyme amylase. By controlling their concentration in the hydrogel, the degradation of the structures could be tuned. Functional structures were printed using these inks which could actuate over time as the materials degrade. These actuating structures could be used as drug or nutrient delivery systems as a nontoxic and cheap alternative for people with dietary restrictions. In summary, this study outlined the development of smart hydrogels and their application in designing stimuli responsive structures for biomedical applications.
biomedical engineering, stimuli-responsive polymers ,4-D printing, soft robotics implants, edible hydrogels.