Silk has long been studied for its incredible toughness and strength. It is one of the best-characterized materials. However, that long history does not mean that silk does not have a few surprises left. In a study published in a recent issue of PNAS, researchers report the development of a new, easily-adapted method to create programmable silk proteins that can have multiple mechanical functionalities. These new forms can be especially useful in biomedical applications, such as drug delivery or implants that respond to stress, heat, or other functionalities that can be tailor-made to fit particular medical needs.

At the Silklab at Tufts University, the researchers know a lot about silk. “We know the nuances of silk assembly more than other biopolymers,” says Fiorenzo G. Omenetto, professor of optics and nonlinear engineering at Tufts’ department of biomedical engineering. Omenetto and his team thought that they could push silk to be more dynamically functional. “We wanted to make materials that were more interesting from a functional standpoint.”

Silk is biocompatible, and it can be reconstituted using water rather than harsher solvents. However, in the changing landscape of healthcare, materials need to be more than just biocompatible, Omenetto says. They also need to be responsive to the environment they are put into.

Omenetto and the group at Tufts set out to make a silk product that could be programmed with a host of different functionalities, from responding to heat to mechanical stress or strain. The method hinges on hybridization of other water-soluble molecules that can alter the way the silk protein works.

To begin, they dissolved raw silk fibers in lithium bromide (LiBr) to obtain a suspension of the constituent protein, fibroin. This is the first order of the material, Omenetto says, and is similar to what a silkworm has in its glands. In the animal world, the natural fibroin suspension turns into silk fibers and draws its strength through dynamic water evaporation regulation. In the laboratory, the LiBr was removed with dialysis, leaving behind fibroin suspended in only water. Controlling the water removal in the laboratory makes it possible to generate forms of silk not found in nature.

The team sought to control assembly by using several different methods. If left alone, fibroin will eventually spontaneously form globular or crystalline gels. The team was able to direct the assembly of these gels into a silk monolith by controlling water evaporation. These monoliths are blocks of silk made up mostly of crystalline beta-sheet structures. These monoliths can then be cut and polished into specific shapes, like pins, plates, or screws.

By adding specific molecules during the assembly phase, the silk becomes more than just a formable material. For example, they added polydiacetylene vesicles to the silk fibroin suspensions, creating hybrids that reacted visibly to strain by changing color. When exposed to varying strain levels, a silk pin turned from blue to red, revealing where the material had reached its yield point. Another example was the addition of gold nanorods that could be heated to 160°C by simple infrared light given off by a light-emitting diode. A possible application of this molecular build is to use light and heat to stave off bacterial infection around an implant. 

Regardless of the molecule added, the silk monolith formed and acted as expected, making it easy to reshape from the macroscale into different forms.

“There are these rare cases in reading a scientific paper [such as this study] where you know that the authors present ground-breaking work that is going to change the way we do things today,” says Oded Shoseyov, professor of protein engineering and nano-biotechnology at the Hebrew University of Jerusalem, who is not associated with the study. He adds that this is a case of top-down meeting bottom-up materials research—the perfect blend of biomimicry and human ingenuity.

Omenetto says that this is a promising outcome. “You don’t have to sacrifice functionality” when including different molecules, he says. “You get to add more functionality instead.”

There are still steps to take to really characterize the material, he says. The researchers hope to more closely study how it withstands multiple cycles of deformation. They also want to look at what happens when two different functionalities are paired, such as heat activated mechanisms paired with those of drug release or mechanical responses. Omenetto also wants to study surface and interface characteristics as the monoliths are exceptionally smooth, which could yield interesting interactions between materials.

 Read the abstract in PNAS.