Image: Christine Daniloff
properties of hydrogels—highly absorbent, gelatinous polymers that shrink and
expand depending on environmental conditions such as humidity, pH, and
temperature—have made them ideal for applications from contact lenses to baby
diapers and adhesives.
In recent years,
researchers have investigated hydrogels’ potential in drug delivery,
engineering them into drug-carrying vehicles that rupture when exposed to
certain environmental stimuli. Such vesicles may slowly release their contents
in a controlled fashion; they may even contain more than one type of drug,
released at different times or under various conditions.
difficult to predict just how hydrogels will rupture, and up until now it’s
been difficult to control the shape into which a hydrogel morphs. Nick Fang, an
associate professor of mechanical engineering at Massachusetts Institute of
Technology (MIT), says predicting how hydrogels transform could help in the
design of more complex and effective drug-delivery systems.
“What kind of shape is
more efficient for flowing through the bloodstream and attaching to a cell
membrane?” Fang says. “With proper knowledge of how gels swell, we can start to
generate patterns at our wish.”
Fang and postdoctoral
researcher Howon Lee, along with colleagues at Arizona State
University, are studying
the mechanics of shape-shifting hydrogels: Looking for relationships between a
hydrogel structure’s initial shape, and the medium in which it transforms, in
order to predict its final shape. In a paper to appear in Physical Review Letters, the researchers report that they can now
create and predict complex shapes—including star-shaped wrinkles and waves—from
The findings may
provide an analytical foundation for designing intricate shapes and patterns
From PowerPoint to 3D
To create various hydrogel structures, Fang and his collaborators used an
experimental setup that Fang helped invent in 2000. In this setup, researchers
project PowerPoint slides depicting various shapes onto a beaker of
photosensitive hydrogel, causing it to assume the shapes depicted in the
slides. Once a hydrogel layer forms, the researchers repeat the process,
creating another hydrogel layer atop the first and eventually building up a 3D
structure in a process akin to 3D printing.
Using this technique,
the team created cylindrical shapes of various dimensions, suspending the
structures in liquid to observe how they transformed. All cylinders morphed
into wavy, star-shaped structures, but with characteristic differences: Short,
wide cylinders evolved into structures with more wrinkles, whereas tall,
slender cylinders transformed into less wrinkly shapes.
Fang concluded that as
a hydrogel expands in liquid, various forces act to determine its final shape.
“This kind of tubular
structure has two ways of deforming,” Fang says. “One is that it can bend, and
the other is that it can buckle, or squeeze. So these two modes actually
compete with each other, and the height tells how stiff it is to bending, while
the diameter tells how easy it is to stretch.”
observations, the team drew up an analytical model representing the
relationship between a structure’s initial height, diameter and thickness and
its ultimate shape. Fang says the model may help scientists design specific
shapes for more efficient drug-delivery systems.
Fang says the group’s results may also help explain how complex patterns are
created in nature. He points to peppers—whose cross-sections can vary widely in
shape—as a case in point: Small, spicy peppers tend to be triangular in
cross-section, whereas larger bell peppers are more star-shaped and wavy. Fang
speculates that what determines a pepper’s shape, and its number of waves or
wrinkles, is its height and diameter.
Fang says the same
principle may explain other intricate shapes in nature—from the creases in the
brain’s cortex to wrinkles in fingerprints and other biological tissues that “leverage mechanical instability to create a wealth of complex patterns.”
The team plans to
study and predict more hydrogel shapes in the future to help scientists design
drug vesicles that transform predictably.