December 17, 2007
Research yields new class of shap-retaining rubber
jonathan.sherwood@rochester.edu
Rochester researchers have developed a shape-memory
rubber that may enable applications as diverse as biomedical implants,
conformal face-masks, self-sealing sutures, and “smart” labels.
The material, described in the journal Advanced Materials, forms a
new class of shape-memory polymers, which are materials that can be
stretched to a new shape and will stay in that form until heated, at which
time they revert to their initial shape.
Unlike conventional shape-memory polymers, however,
the new material is transparent, rubbery, and most importantly, engineers
will be able to control the speed at which it returns to its original
shape. Other such polymers use crystallization to hold a temporary
shape, which often makes them opaque, hard, and brittle in their frozen
states, and this can limit their use.
“At higher temperatures the material stretches
like a rubber band, but, at lower temperatures, it stiffens up,” says
Mitchell Anthamatten, assistant professor of chemical engineering and
inventor of the material. “This property can be used to temporarily
hold the material in a deformed shape; and its original shape can be
recalled by heating. Imagine an optical lens that can be triggered to
change shape, a face-mask that can fit any user, or a biomedical implant
that changes shape slow enough for a surgical procedure.”
The new rubber functions differently than conventional
shape-memory materials by using “sticker groups”—hydrogen
bonding groups that form temporary bonds. These sticker groups break and
reform constantly. It’s akin to tearing a net apart only to find that
new knots have formed between different strands. When the material is
stretched, new bonds form that hold the material, temporarily, in its
deformed shape. Creating the rubber with different amounts of sticker
groups controls the rate at which the rubber returns to its original shape.
With this control, Anthamatten envisions applications that today’s
shape-memory polymers simply can’t fulfill.
“The pressure at which you hold together a
sutured wound determines a lot about how it will heal,” says
Anthamatten. “This polymer could be made into a thread that responds
precisely to body temperature, tightening the sutures to the perfect
pressure.”
Anthamatten is currently investigating how dyes
diffuse through his networks. “We expect the rate of dye diffusion to
increase with temperatures,” says Anthamatten. This property may
enable “smart” labels that account for time and temperature and
can inform customers when products are about to expire. “We may not
always have to rely on the expiration date. What if our milk was not
refrigerated properly? What if the air conditioner failed for some time at
the pharmacy? People want to know that their products are fresh.”
One aspect of the clear rubber that surprised
Anthamatten was how easy it is to make. “It’s ridiculously
simple,” he says, “and we’re fascinated by how small
modifications lead to major changes in how the material
behaves.”
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