12/3/2014 Meredith Staub
Written by Meredith Staub
Assistant professor Shelby Hutchens is bringing a physical chemistry perspective to MechSE. She received all of her degrees in chemical engineering: a BS from Oklahoma State University, and an MS and PhD from the California Institute of Technology.
At Caltech, Hutchens began work with polymer physicist Zhen-Gang Wang studying nucleation theory. However, she finished up her degree in Julia Greer’s group exploring small-scale mechanics of materials. In this group, she analyzed the deformation behavior of vertically-aligned carbon nanotube foams.
“What I got out of my graduate work was that I enjoyed working with soft materials,” Hutchens said, “which was different from the rest of my group; they were all mostly working with metals. I wanted to do something more with soft materials because of all the nonlinear deformations and unique, complex behaviors that can be observed. I wanted to do something soft, and polymers are the most ubiquitous soft material out there.” This led Hutchens to pursue a postdoc with Alfred Crosby in the Polymer Science and Engineering Department at the University of Massachusetts Amherst.
Her main project as a postdoc was trying to further the development of a technique that could measure the mechanical properties of extremely soft materials known as “cavitation rheology.” Ultrasoft materials often sag under their own weight, so they can’t be tested using the same geometries as hard materials. Additionally, if a material is inhomogeneous on a variety of length scales, traditional testing techniques will tend to coarse-grain any measurements of mechanical properties, losing information that could be key to that material’s performance.
“One reason why these measurement techniques are important is that your body is made up of a lot of soft materials, and people are finding more and more that tissues and cells respond to the mechanical environments around them,” Hutchens said. “People don’t necessarily know how to characterize that in a fast, cheap, and non-invasive way, particularly in the non-linear regime.”
The technique was simple: after pushing a needle into something soft, a bubble would be blown into the material. The pressure of this bubble is measured, and as it increases, at some point a mechanical instability occurs and the pressure rapidly drops. The peak pressure value ended up being a very sensitive property for measuring the differences between materials, and one of Hutchens’ projects was to find out which mechanical properties these differences corresponded to.
Here at Illinois, Hutchens plans to continue characterizing soft materials at small scales.
“Soft materials fracture is really cool—it does amazingly different things depending on the constitutive behavior,” Hutchens said. “I want to work on characterization techniques in general, because I’m also interested in developing a new class of materials that uses plants as inspiration. In plants, you have a closed-cell cellular solid that you can load with salt to establish osmotic pressure gradients across the material. Then if you put this material into water, it will deploy in some way that you have decided in advance based on how you’ve architected the structure of that particular solid. By tuning the material properties and the geometry, you can create a set of materials that can provide you with an inhomogeneous stress state, or an inhomogeneous strain at the surface.”
Hutchens thinks these properties could be useful in the future as implantable “soft casts.” If you break a bone, you put a hard cast on it. If you “break” a soft tissue, you may sew it shut, but there is less in terms of therapy to help heal soft tissue that’s been traumatized, and many types of tissue prefer a certain stress state to heal. One example of this is heart tissue. As heart tissue heals, performance recovers best when all the collagen fibers are aligned, but that is very difficult to achieve in unloaded tissue. If soft materials modeled after plant tissue could be designed in a knowledgeable way, they may be able to help collagen fibers align during healing, and lead to better recovery of the tissue’s original function.
As for most new faculty, the choice to come to Illinois was easy for Hutchens. She observed, “One of the things in particular that was very attractive to me here was the range of shared equipment and facilities that are available. It seems to me that almost everything I would need to try something experimentally is here. It really lowers that barrier to trying new things. The possibilities are endless!”