Today’s high-performance materials are expected to maintain optimum performance under increasingly severe operating conditions that may involve temperature and pressure extremes (high and low) and are in locations (such as wind turbines) that are difficult to access. But these environments can cause materials to break down through such phenomena as corrosion and wear. 

One approach for minimizing damage that is promising is to have the material self-heal while in operation. Michael Strano, Carbon C. Dubbs professor of chemical engineering at the Massachusetts Institute of Technology in Cambridge, Mass., says, “The idea of self-healing has been around for about 20 years. Ideally, a material that incurs damage such as the formation of a crack undergoes a process triggered possibly by external stimuli to fill the crack.”

Past efforts to develop self-healing materials have been only slightly successful. Strano says, “The problem is that the mass involved in self-healing a damaged material does not change. A hole may be punched in a material leading to a loss of mass that is compensated for by the self-healing material.”

A previous TLT article discussed an example of a past effort that focused on a self-healing coating (1). The researchers incorporated microcapsules of a monomer and a catalyst in a specific coating. These microcapsules only opened when the coating was ruptured, generating a polymer that re-established the integrity of the coating. Evaluation of this self-healing concept showed that it worked well in producing anticorrosive coatings when exposed to salt water. 

Strano believes that an alternative approach derived from Mother Nature could be very promising. He says, “We have focused on studying plants, which need to operate in some extreme environments such as deserts and the arctic tundra. Fragile materials such as organelles exist within plants and can easily rupture. Yet, plants have found ways to thrive in difficult environments through self-healing.”

An appealing aspect of working with plants is that they generate carbon mass through photosynthesis. Carbon dioxide and water produce the sugar, glucose in a plant organelle known as a chloroplast. A process has now been developed to utilize photosynthesis to prepare a carbon-based polymer that can act to self-heal a material. 

Embedded chloroplasts
Strano and his colleagues have developed a self-healing material by embedding chloroplasts into a synthetic material. Under ambient conditions and after exposure to light, glucose produced in the chloroplasts is exported into the material, reacted through an enzymatic process to gluconolactone that is then reacted with aminopropylmethacrylamide under mild conditions to produce a glucose containing polymethacrylamide (GPMAA). 

GMPAA forms a hydrogel that in sufficient concentration can fill a crack created in the material as shown in Figure 1. Strano says, “We are using a natural pathway, photosynthesis, to create new mass through the synthesis of a carbon fixing material. Another benefit of this environmentally friendly process is that we are using a readily available raw material, carbon dioxide.”


Figure 1. The self-healing characteristics are demonstrated by embedding chloroplasts (light green) in a hydrogel (dark green) that was split by a crack (shown on top). Upon exposure to light, the material reacts with carbon dioxide in the air to form new carbon mass, which expands to fill the gap healing the material as shown on the bottom. (Figure courtesy of the Massachusetts Institute of Technology.)

One of the challenges faced by the researchers was stabilizing chloroplasts outside of plant cells. Strano says, “We used nanoparticles based on cerium oxide (known as nanoceria) as antioxidants to scavenge reactive oxygen species generated during photosynthesis that can destabilize chloroplasts. This enabled the chloroplasts to produce higher levels of glucose in the material.”

With the chloroplasts stabilized, the researchers also evaluated techniques for accelerating the formation of GPMAA. Strano says, “We worked with graphene oxide, which exhibits a pretty high Young’s modulus, to both stiffen and strengthen the growing hydrogel and to act as a support for the enzyme catalyst, glucosidase, which converts glucose to gluconolactone.” The rate of hydrogel formation increased three times when graphene oxide was used.

Another method for increasing GPMAA formation is to accelerate movement of glucose (a potential rate-limiting step) outside of the chloroplasts. The researchers found this could take place through the presence of glucosidase outside of the chloroplasts. Strano says, “In the absence of light, glucose and the disaccharide, maltose are the main sugars that leave the chloroplast. Maltose is appealing to work with because it is composed of two glucose molecules. We found that when glucosidase is added to the hydrogel during a dark period, glucose formation increased three times. This confirms that maltose is produced by the chloroplasts and enters the material with glucose.”

The researchers found that within six hours after the chloroplast-containing material was exposed to light, hydrogel formation was observed. The thickness of the hydrogel increased to 20 microns after approximately 18 hours. 

This work demonstrates the potential of producing carbon fixing material to self-heal materials. Strano says, “We feel this approach will most usefully be applied in self-healing coatings but is still limited to only a few applications because of concern with stabilizing chloroplasts outside of their natural plant environment.”

Currently the researchers are developing a new high-value polymeric material that can self-heal using the same process without the need for chloroplasts. Additional information on this concept can be found in a recent paper (2) or by contacting Strano at strano@mit.edu. 

REFERENCES
1. Canter, N. (2009), “Coating: Heal thyself,” TLT, 65 (6), pp. 16-17.
2. Kwak, S., Giraldo, J., Lew, T., Wong, M., Liu, P, Yang, Y., Koman, V., McGee, M., Olsen, B, and Strano, M. (2018), “Polymethacrylamide and Carbon Composites that Grow, Strengthen, and Self-Repair using Ambient Carbon Dioxide Fixation,” Advanced Materials, 30 (46), 1804037.