Volume 7, Number 1, 1997

On Research

Polymers promise earthquake-proof buildings
and new drug-delivery methods

Since her days as an undergraduate at Washington University in St. Louis, UD chemical engineer Annette D. Shine has been fascinated by polymers-the long, chain-like molecules found in a vast array of products, from the shiny polyester suits of yesteryear to such flexible, insulating plastics as polyethylene.

These days, Shine is training polymers to perform new tricks.

Making liquid polymers "morph," or turn solid and then liquid again in response to an electrical field, is one of her current objectives. Because they stiffen and relax on cue, these morphing liquids may someday serve as high-IQ lubricants in cars, earthquake-proof buildings and joint rehabilitation devices, Shine says.

In the meantime, she's also collaborating with Jack Gelb Jr., UD professor of food and animal sciences, to fine-tune a technique for capturing medicines within a thin polymer shell. Already, the researchers have produced a prototype poultry vaccine based on the new technology, and Gelb says the work may lead to better drug-delivery systems for humans, too.

"Polymers are incredibly versatile molecules, and scientists are constantly coming up with new uses for them," says Shine, an associate professor. "I'm particularly interested in polymer rheology, which means that I study how these molecules behave as they flow and change, and how their molecular structure affects that behavior-say, during an industrial process. The more we learn about polymer rheology, the more we will be able to do with these molecules."

Chocolate shock absorbers?

How could a liquid turn solid, then liquid, then solid again in the time it takes to snap your fingers? Certain fluids are "electrorheological," or capable of solidifying in response to an electric field, Shine explains. These morphing liquids demonstrate the "Winslow effect," a behavior first discovered in 1947 by researcher Willis M. Winslow.

A wide range of fluids exhibit the Winslow effect. In fact, scientists recently coined the term "chocolate effect" to describe electrorheological (ER) fluids because high-voltage electricity will transform a molten Hershey bar into a solid chunk of sweet stuff. Though carmakers aren't scrambling to develop new lubricants based on chocolate, they do hope to cushion bumps and vibrations by using materials with similar properties, Shine says.

"Cars of the future may ride much more smoothly, thanks to the so-called chocolate shock absorbers now under development," Shine says. Sensors inside the car would detect a bump and then feed that information to a computer, which would instantly send a jolt of electricity through an electrorheological fluid, causing it to stiffen or relax in response to road conditions, she explains.

Shine predicts a variety of applications for "smart fluids." Physical therapists might be able to use ER fluids, for example, in harnesses designed for joint-replacement patients. Smart lubricants could be built into a suspension system attached to a patient's walking harness. "If they began to fall," she says, "you would want the harness to catch them, without jerking them too abruptly. Electrorheological fluids could control the rate of capture, pulling them gently back to a standing position." The same principle may improve buildings designed to withstand earthquakes, too, she adds.

But first, Shine notes, researchers must make smart fluids smarter.

Like melted chocolate, most ER fluids consist of many droplets in an oily fluid, much like cornstarch suspended in corn oil, she explains. Over the years, she says, most morphing liquids have been made the same way, by suspending "polar" particles, which acquire positive and negative charges on opposite ends, in a non-polar or neutral liquid. When an electric field is applied to these substances, she says, the negatively-charged ends of particles quickly latch onto the positively-charged ends of other particles, thereby forming long, rigid chains. Unfortunately, small changes in temperature or moisture level can dramatically change the rigidity of these particle chains, so the fluid response is not reliable. And, over time, the suspended particles either sink to the bottom or float on the surface of the liquid. This settling robs ER fluids of their chameleon-like properties.

To correct these problems and create less finicky ER fluids, Shine's research team has been experimenting with two long chain molecules: a polyamino acid derivative and PHIC, also known as poly(n-hexyl isocyanate). She dissolves these polymers in a liquid hydrocarbon solvent such as xylene, and with their attached carbon side-chains, the rod-like molecules resemble a "fuzzy hydrocarbon sheath around a polar core," Shine says. An electric field forces the pole-shaped molecules to line up, which thickens the solvent. Their sheath-core structure immunizes the molecules against the ill effects of moisture and temperature, Shine says, while their molecular size makes them more resistant to settling.

Shine says she expects that the polymers can be produced readily and inexpensively, by such organisms as bacteria. Her morphing liquids are still being refined because they remain relatively thick after being removed from an electric field, but, she says, "that would not be a big obstacle for use in shock absorbers and a whole host of other key applications."

Moreover, she says, UD researchers have developed a theory for modeling the behavior of smart lubricants, given such different parameters as the molecular weight and concentration of polymers. "This model should allow us to predict the behavior of different ER fluids, to achieve increasingly desirable characteristics. With this knowledge, we can better design devices to use the fluids."

A spoonful of polymer makes the medicine go down

With Gelb, Shine also studies drug-delivery systems based on polymers. A special process, initially developed to encapsulate a poultry vaccine, may someday mean better drug-delivery systems, Shine says.

"Our first test of this new technology involved encapsulating a vaccine for infectious bursal disease, an immunosuppressive viral illness of chickens," Gelb says. "We simply wanted to use this virus as a model, to demonstrate the feasibility of our encapsulation method, which could prove beneficial in many other species."

The UD encapsulation technique, dubbed PLUSS (for Polymer Liquefaction Using Supercritical Solvation), lets researchers capture the desired protein components within a polymer shell. In theory, Gelb says, the vaccine could be injected into a chicken embryo and then remain safely encapsulated for a period of weeks before emerging to trick the young chick's immune system into fighting against IBVD.

Such encapsulated vaccines may prove crucial in various young animals or even children, according to Gelb. "Natural antibodies, present at birth, help the young fight diseases before their own immunity has been established," he explains. "Although these maternal antibodies are beneficial, their presence prevents effective vaccination at birth, because they destroy the vaccine." Polymer-coated, controlled-release vaccines may protect the vaccine from being destroyed by maternal antibodies, while also protecting the vaccine recipient from disease.

The key to the prototype vaccine, Shine says, is a biodegradable polymer called PLGA (polylactic-coglycolic acid), a material already used in surgical sutures. Before PLGA could be manipulated to contain the poultry virus, the researchers first had to liquefy the polymer so that they could easily shape it around the vaccine. They also needed to encapsulate the virus without killing it-no small feat, since existing processes require too much heat as well as potentially dangerous organic solvents.

Shine's simple solution involves placing a freeze-dried packet of powderized vaccine together with carbon dioxide and polymer inside a pressurized cell. The pressurized carbon dioxide causes the polymer to melt at a much lower temperature than it would under normal circumstances. Under pressure, the liquid polymer and vaccine can be mixed together at room temperature or lower. When the pressure is released, however, the polymer hardens again, leaving behind tiny, irregularly shaped capsules containing live viral fragments-and no hazardous byproducts.

Gelb and Shine plan to patent the PLUSS technology. "I can imagine a variety of pharmaceutical applications for this system," she says, "from the continuous delivery of insulin to diabetic patients to the controlled release of viral hosts for gene delivery."

-Ginger Pinholste