Materials Scientists from the University of California-Los Angeles (UCLA) together with their collaborators have designed a new technique to create synthetic biomaterials that replicate the internal structure, strength, durability and stretchiness of tendons and other biological tissues.
The team has designed a two-pronged process to improve the strength of present hydrogels that could be utilized to make ligaments, artificial tendons and cartilage that are 10 times harder when compared to the natural tissues.
Despite containing mostly water with very less solid content (around 10% polymer), the hydrogels are more sturdy compared to Kevlar and rubber, which are both 100% polymer.
This discovery has never been realized in water-laden polymers until this study was recently published in the Nature journal. The new hydrogels could offer coating for wearable or implanted medical devices to enhance their comfort, fit and long term performance.
This work shows a very promising pathway toward artificial biomaterials that are on par with, if not stronger than, natural biological tissues.”
Ximin He, Study Leader and Assistant Professor of Materials Science and Engineering, Samueli School of Engineering, University of California-Los Angeles
Hydrogels are an extensive class of materials consisting of interior structures that constitute crisscrossing gels or polymers.
They exhibit the potential for being used as alternative tissues, either to close wounds for a short period or as a long-term or even permanent solution. The gels might be used in wearable electronics and soft robots.
But existing hydrogels are not durable or robust enough to replicate or replace tissues that require shifting and flexing repeatedly while bearing weight.
To overcome these problems, the UCLA-led team used a combination of structural and molecular engineering methods that were not used together earlier to make hydrogels.
Initially, the team employed a technique known as 'freeze-casting'—a solidifying process that leads to the formation of concentrated and porous polymers, similar to a sponge. Then, they performed a 'salting-out' treatment to aggregate and crystallize polymer chains into strong fibrils.
The new hydrogels that were obtained include a range of connecting structures throughout different scales—from molecular levels up to a few millimeters. The hierarchy of such multiple structures, quite similar to that of their biological equivalents, allows the material to be more expandable and powerful.
The researchers demonstrated that this universal technique is highly adaptable and could mimic different soft tissues in the human body.
The team created their hydrogel prototype by using polyvinyl alcohol, a material approved already by the U.S. Food and Drug Administration. Its durability was tested and there were no indications of deterioration following 30,000 cycles of stretch testing.
The new hydrogel generated a vivid shimmer under light, similar to real tendons, which confirms the micro or nano structures that developed in the gel.
Besides biomedical applications, the breakthrough could be promising for bioelectronics or surgical machines that run countless cycles, and 3D printing of a configuration that was unachievable earlier, as a result of the flexibility of the hydrogel.
The researchers showed that such 3D-printed hydrogel architectures could change into other shapes pending variations in humidity, acidity, or temperature. When used as artificial muscles, they are much more resilient and could exert huge force.
The co-lead authors of the study, both from UCLA, are materials science doctoral student Mutian Hua and postdoctoral scholar Shuwang Wu. Other authors of the study are UCLA’s Yanfei Ma, Yusen Zhao, Zilin Chen, and Imri Frenkel; Joseph Strzalka and Hua Zhou from Argonne National Laboratory in Illinois; and Xinyuan Zhu from Shanghai Jiao Tong University in China.
The study was financially supported by the National Science Foundation, Air Force Office of Scientific Research, and the Office of Naval Research. A part of the study was performed at Argonne National Laboratory, which is run by the U.S. Department of Energy.
Hua, M., et al. (2021) Strong tough hydrogels via the synergy of freeze-casting and salting out. Nature. doi.org/10.1038/s41586-021-03212-z.