Injectable Shear Thinning Hydrogels

Hydrogels provide a promising new way to deliver drugs or other materials with which to regenerate damage tissue because of their flexibility and biocompatibility. Up until now, though, these could not be fully used because their solid structure tended to degrade under the harsh conditions of the human body, particularly our high body temperatures. To solve this problem, MIT researchers synthesized a special kind of shear thinning hydrogel.

Shear thinning hydrogels are notable because they can switch between a solid and liquid state depending on their surroundings. This makes them ideal as injectable gels because they're liquid when being pushed through a needle but become solid once they're inside the body. However, if the researchers left these gels as is, they would still be subject to stresses of a mechanical nature inside the body and might revert back to liquid.

To fix this, they created a self-healing, reinforcing network inside the gel that only activates when the temperature of its surroundings are body temperature. That way, their new structure would not interfere with the gel's functionality in the needle but would still be able to solve their problem of durability once inside the body.

They opted to create protein hydrogels instead of the more standard polyethylene glycol gels so that they would have better biological properties, and altered the structure so that they would have the desired characteristics. The proteins were loosely held together by links called coils to result in a rope-like structure, and then a second network was built inside that.

At higher temperatures, cross-linking results in a reinforcing network.

At low temperatures, the protein gels are hydrophilic and thus float freely within the gel. As soon as the temperature becomes high enough, though, the gel becomes hydrophobic and groups together, making polymers at the ends of the protein bind to each other. This results in cross-linking so that the gel is significantly more elastic and durable, able to withstand mechanical strength.