The Zhenan Bao Research Group at Stanford University, Department of Chemical Engineering has succeeded in designing a self-healing, highly-stretchable material (polymer – a larger molecule made up of several sub-units) that the team hypothesizes to use as the basis for what is in essence an artificially engineered muscle. The team’s research areas consist of not only the synthesis of polymer materials, but of functional organic materials as well, in addition to researching the design, fabrication, production and development of organic electronic devices. Their approach is a self-declared “multidisciplinary” one, combining concepts and methodologies used in the fields of chemistry, chemical and electrical engineering, and physics.

“The team’s research areas consist of not only the synthesis of polymer materials, but of functional organic materials as well, in addition to researching the design, fabrication, production and development of organic electronic devices.”

 

Their findings, published in the April 2016 issue of the monthly journal Nature Chemistry were put together by an eclectic group of researchers, led by Professor of Chemical Engineering at Stanford University, Zhenan Bao, who was selected by Nature’s 10 as one of the top ten people of 2015 whose work has proven to be of great significance, namely her outstanding progress in the development of artificial electronic skin. Bao’s name was also included in 2002 on the list of the top 100 young innovators of the century, a title granted by MIT Technology Review magazine, “the world’s longest-running technology magazine”.

The article entitled “A highly stretchable autonomous self-healing elastomer” highlights the challenge that the current project entailed, which can be easily reduced to a process of synthesizing materials “that possess the properties of biological muscles – strong, elastic and capable of self-healing. However, just like many other great scientific results were stumbled upon in a serendipitous fashion, so did the team discover the properties that would allow the material to stretch to more than 100 times its original length. It was one of Bao’s team members, a visiting scholar from China, Cheng-Hui Li, who began testing the stretching properties of an elastomer (rubber) he had just finished synthesizing. Usually, tests for this type of properties are conducted with the help of a clamping machine that stretches materials in order to find their breaking point. Typically, an elastomer can only stretch up to three times its length.

“However, just like many other great scientific results were stumbled upon in a serendipitous fashion, so did the team discover the properties that would allow the material to stretch to more than 100 times its original length. (…) Typically, an elastomer can only stretch up to three times its length.”

 

This time however, the material kept stretching and stretching, the researcher having to take the polymer quite literally into his own hands, the device being able to measure its elasticity to only 45 inches (provided that the piece that was being tested was 1 inch in length). Li and another member of the team took the elastomer and each held one of its opposing ends, standing further and further apart, in the end the material having reached more than 100 inches through stretching. Bao was stunned by the result. This extreme property of the elastomer was attributed to the improvements that the team had added to a type of chemical bonding called crosslinking.

The process consists of connecting linear chains of linked molecules in such a way to design a fishnet-like appearance of a network of polymer chains. The design offers the polymer a higher elasticity, flexibility and resilience. Custom designed organic molecules were attached to polymer strands (links) creating long structures of chains, called ligands. These ligands functioned similarly to “spring-like coils”. Then, the team added material metal ions to the ligands in order to secure their chemical binding, and thus allow the material to be both flexible and highly stable.

“Due to their specially concocted material, this new type of polymer was not only super-stretchable, but was able to self-heal at room temperature and even at temperatures as low as 4 degrees Fahrenheit (-4 degrees Celsius).”

 

In fact, lack of stability was one of the shortcomings of typical polymers. Their structure was easily damaged if punctured, ruptured or scratched, and thus had hardly any chance to function similarly to a proper bicep, for instance. Nonetheless, due to their specially concocted material, this new type of polymer was not only super-stretchable, but was able to self-heal at room temperature and even at temperatures as low as 4 degrees Fahrenheit (-4 degrees Celsius).

When varying the ions’ ratio or type, the researchers also noticed that the material’s stretchiness and healing rates could be modified. And lastly, the cherry on top of this wonderfully hopeful cake is the fact that the new polymer was also shown to react by twitching when coming into contact with an electric field, due to the material’s metal ions. Although further research has to be carried out in order to gain more control over the expanding and contracting properties of the polymer, in essence, all of the major boxes concerning its suitability to function as replacement for a ‘real’ muscle have already been ticked. Plus, this latest finding perfectly complements the team’s other research endeavors, one of them concerning the development of artificial skin, which again, is a highly difficult task to accomplish.

“And lastly, the cherry on top of this wonderfully hopeful cake is the fact that the new polymer was also shown to react by twitching when coming into contact with an electric field, due to the material’s metal ions.”

 

In any case, it is quite possible that in the nearest future, considerable gratitude will be in order, thanking this team of researchers for their painstaking work in biomechanics, bringing the dream of surmounting our biological shortcomings that much closer. So what better way to put the final touches to this paper, than the simple but inspiring words of one hopeful team member, Franziska Lissel’s broad vision of what is yet to come: “We want to create a very complex system”.

 

O.P.

April 29, 2016

Sources: http://www.nature.com/nchem/journal/vaop/ncurrent/full/nchem.2492.html, https://baogroup.stanford.edu/, https://news.stanford.edu/2016/04/18/stanford-researchers-create-super-stretchy-self-healing-material-lead-artificial-muscle/, https://www.youtube.com/watch?v=nK0KsWHlW2U