Johns Hopkins Magazine -- February 1997
Johns Hopkins Magazine


S C I E N C E    &    T E C H N O L O G Y

An all-plastic battery... acid test for a new contraceptive... the hot secret behind a fiery new foil... testing dirt's strength... butter your eggs

Just one word: plastics
Perhaps Benjamin Braddock, the confused young man in The Graduate, ought to reconsider the now-famous pithy advice. Johns Hopkins engineers have built a battery made of just one thing: plastics.

Or, to be precise: polymers called poly-fluorophenyl-thiophenes.

Unlike conventional batteries that contain toxic heavy metals such as lead or nickel-cadmium, or lithium, which is highly explosive, the polymer battery is non-toxic and safe. What's more, it's only about as thick as a business card, and malleable enough to be tucked into existing crevices of a device, thus eliminating the need for separate battery compartments.

Popular Science recently listed the all-plastic battery as one of the 100 top scientific developments and inventions for 1996.

"It's lightweight. It's flexible. You can cut it to any shape. It's made from carbon-based materials, so there are essentially no environmental concerns," says Peter Searson, professor of materials science and engineering. And it costs about the same amount of money to manufacture as a lithium battery, he adds.

The plastic battery, for which a patent is pending, is the brainstorm of Searson and Theodore Poehler, vice provost for research, along with post-doctoral students Yosef Gofer and Hari Sarker, and graduate student Jeffrey Killian, with funding from the U.S. Air Force. An Air Force-sponsored test and demonstration of the new device will be conducted aboard a satellite in 1998, says Poehler.

Searson's group has so far used the batteries to power an alarm clock and a calculator. They can also be used on other portable devices such as cellular phones, wristwatches, or hearing aids, says Searson. Not surprisingly, battery manufacturers have shown interest in the invention.

The plastic battery's one drawback is its capacity. While it produces 2.5 volts (enough to be competitive with the 3-volt lithium batteries now on the market), its capacity is five times lower. In other words, it needs to be recharged about five times as often as a comparable lithium battery.

So how can plastic, supposedly an insulator, be used to conduct electricity? Some plastics, it turns out, are conductors.

A battery consists of two electrodes--an electron producer called an anode and an electron acceptor called a cathode--and a conductive material, the electrolyte. When the Air Force first asked Hopkins researchers to develop the battery, researchers had already built batteries containing a plastic cathode, but no one had been able to find a stable polymer for use as the anode, explains Searson.

The Hopkins team knew it was in luck last year when it synthesized a stable polymer suitable for an anode from a class of plastics called poly-fluorophenylthiophenes. Combined with a related poly-fluorophenylthiophene serving as cathode, and a polymer gel electrolyte, the all-plastic battery was born. "To our knowledge, this is the first all- polymer battery anyone has made," says Searson. --MH

Contraceptive takes a leap forward
A new contraceptive gel that appears to prevent sexually transmitted diseases, as well as pregnancy, is now being tested in women. The clinical trials, which are sponsored by the National Institutes of Health, will test whether the gel is safe and effective.

Hopkins researchers who developed the gel have shown that it prevents pregnancy in animals, and protects mice against herpes type II. Their research also suggests that it may prevent transmission of gonorrhea, HIV, and chlamydia. Biophysics professor Richard Cone was recently awarded a $2 million grant from NIH to support further research on the gel.

The contraceptive works by maintaining the vagina's normal acidity, explains Kevin Whaley, a research scientist in biophysics and an adjunct faculty member at the School of Hygiene and Public Health. Normally, the vagina has a ph of 4. All other things being equal, such acidity would destroy sperm, bacteria, and viruses. However, semen is alkaline, and raises the vagina's ph to 7. The "key ingredient" in the contraceptive gel, says Whaley, is an acidic polymer, which acts like a buffering agent and keeps the ph at 4.

Sexually transmitted diseases account for five of the 10 most common diseases reported last year to the Centers for Disease Control and Prevention, according to a recent report by the Institute of Medicine (IOM). The IOM reports that in 1994 there were an estimated 4 million cases of chlamydia, 800,000 cases of gonorrhea, 101,000 cases of syphilis, between 200,000 and 500,000 cases of genital herpes, more than 1 million cases of pelvic inflammatory disease, and 79,897 cases of AIDS. --MH

Hot flashes
A new foil invented by Hopkins engineer Tim Weihs looks like something you'd wrap around a baking potato. But light a match to it, and within a second a square-foot section of the foil will heat up to 2,900 degrees, as a heat-generating reaction occurs. The fiery exothermic reaction subsides within seconds.

This hot flash ability suggests several engineering applications, including use as a tough new welding material, says Weihs. A strip of the new foil can be placed between two objects, and ignited, forming a tight seal between the two pieces. Because of its brevity, the blast of heat will not damage the materials.

The foil contains layers of aluminum and another metal such as nickel, explains Weihs, an assistant professor of materials science and engineering at Hopkins's Whiting School of Engineering. Each layer is only 5 to 5,000 atoms thick (1 to 1,000 nanometers), at most.

The foils are made through magnetron sputtering, a technique in which ions are fired at a target material, knocking off single atoms that then land on a substrate. By alternating aluminum and nickel targets, Weihs creates a torte of layered aluminum and nickel. With co-inventor Troy W. Barbee, a senior research scientist at Lawrence Livermore National Laboratory in California, Weihs was recently awarded a U.S. patent for the foils.

The foil's hot secret is the intimacy between its two types of atoms, says Weihs. "Certain atoms would rather be bonded to other types of atoms than to their own kind. Aluminum would rather bond to nickel than to aluminum, and vice versa." So all it takes is a "very tiny pulse of heat or friction" to get the bonding started. Because the atoms are so close to one another, bonding occurs rapidly, and once the first nickel and aluminum atoms bond, heat is released, which triggers bonding among neighboring atoms. A self-propagating reaction ensues.

An added attribute is that the reaction does not require oxygen, meaning that, technically, the foil does not burn. The new material could, therefore, be used underwater or in the vacuum of outerspace, says Weihs. The foil's current high cost (about $60 per six-inch square, a cost that could be considerably reduced, notes Weihs) may limit its applications to such special uses. Weihs has also pondered the possibility of using the foils as a means of burning off cancer cells without harming healthy cells. --MH

What lurks beneath the dirt?
Imagine this scenario. The United States is called in to help fight a war in an isolated region of a foreign country. An airfield is rapidly constructed by mixing polypropylene fibers into tilled soil, then packing down the earth. The plastic reinforces the soil, making it strong enough to support the weight of helicopters and planes.

This approach has advantages over existing methods for building temporary airfields, such as laying down interlocking structures covered by aluminum sheets, says Radoslaw Michalowski, an associate professor of civil engineering. Aluminum, for example, which has been used by the U.S. Air Force in the past, is far heavier and bulkier than plastic fibers.

With funding from the Air Force and the National Science Foundation, Michalowski developed a theoretical model for predicting the strength of such soil-fiber composites. His model has relevance for determining, for example, whether an airfield composed of the composite will be able to withstand the weight of a C-130 aircraft.

It all starts in a basement laboratory at the Whiting School. Open the lab's refrigerator. What's to eat? Yum! A sample of frozen plastic and dirt--"We like to call it soil," says Michalowski. Samples such as this one are placed in a sealed cylinder and subjected to increasing amounts of pressure until-- at some critical point--the mixture collapses. Michalowski's model predicts that failure point. After testing more than 80 specimens of fiber-laced soil under a variety of conditions, he says, "The model seems to be very accurate at predicting the strength or failure of soil."

Plastic grids layered within hillsides prevent dirt from sliding away.
The fibers belong to a class of materials called "geosynthetics"- -synthetic materials used to reinforce earth.

In related research, Michalowski is analyzing geosynthetic grids and fabrics commonly used to reinforce slopes, embankments, and the soil behind retaining walls.

The engineering of such structures has undergone major changes in recent years. Retaining walls, which are designed to hold back dirt where builders have cut into hillsides, have traditionally been made of concrete that is two- to three-feet thick, says Michalowski. Floods and earthquakes can total these walls, turning them into hazardous chunks of concrete. So engineers now generally reinforce hillside soil with layers of geosynthetic mesh or fabric. If the synthetics do their job, they prevent collapse of the hillside and generally do the job of a retaining wall.

But just the right amount of geosynthetic material must be used, and it must be placed just so in order to work. If the material is weak, the soil will collapse. If a length of geosynthetic is too short, the overlaying soil may slide or roll off. On the other hand, using more material than is required incurs unnecessary expense.

Michalowski has created algorithms to determine precisely how strong a geosynthetic should be, how many layers of the material should be used, how long each piece should be, and how the material should be positioned to give the maximum support. So, for example, given a 50-foot-high slope, with a 40-degree angle of inclination, Michalowski's formula might predict that 20 layers of a plastic grid should be used, each 50 feet long.

Although geosynthetic materials are extensively used, the public is not aware of the science that goes into their design--or, for that matter, that they even exist down there, under the dirt, says Michalowski.

It's that way with much of civil engineering, he says. Though feats of civil engineering may not be as scintillating as a super nova, he says, they have more relevance to everyday life.

So the next time you drive past a retaining wall, think of these unsung heroes. --MH

Butter your eggs
Despite its sadly tarnished public image, cholesterol has fascinated biologists and the like for more than 200 years-- mostly because, as it turns out, organisms tend to be more dead without it. Cell membranes are supple mostly because 10 percent or so of their structure is cholesterol. It is a major ingredient in bile, which digests fats. And testosterone, estrogen, and other steroid hormones necessary to reproduce begin as cholesterol molecules.

All long established. But now it appears that without cholesterol we might have never been alive in the first place.

Last fall, Philip Beachy, Jeff Porter, Chin Chiang, and their colleagues at Hopkins School of Medicine reported in the October 11 Science that cholesterol anchors in place a protein that orchestrates development in embryos. This new function suggests a reason why organisms might have evolved this waxy substance in the first place. Indeed, the more adventurous speculate that cholesterol could have helped boost primitive, single-celled life over the evolutionary hump to the wonders of multi-cellular existence.

Without Hedgehog, the mouse embryo at right develops a long proboscis and Cyclops-like eye.
Very early in an embryo's life, its notochord (the embryonic governor) produces a protein called Hedgehog--so named because without it, fruit flies assume a rounded, bristly shape. In vertebrates, this dexterous protein signals developing tissue to form skin, ribs, backbone, and motor neurons, as well as support cells for the embryonic spinal column. It also rearranges fingers and toes in their proper sequences.

Hedgehog has two remarkable traits, neither of which, Beachy discovered, it can accomplish without cholesterol. First, it activates itself by cleaving in two--but only when a cholesterol molecule binds to a particular part of the protein and coordinates the snip.

The cholesterol remains attached to the now-activated half, which carries the signal to the surface of the cell that made it. What an unspecialized cell turns into depends upon how much Hedgehog protein it is exposed to--the second remarkable trait. Cells adjacent to the notochord get tickled by a lot of Hedgehog and become support cells, called floor plate. Farther away, cells receive little protein and so turn into the nerve cells that control movement. On limbs, a strong signal makes pinky fingers and a weak one thumbs.

Thus cholesterol's role: Without its cholesterol anchor, the Hedgehog signal extends farther than it should--presumably because the protein wanders off. Distant cells are fooled into forming floor plate instead of motor neurons. Pinkies proliferate, and so on.

The result then, as Beachy puts it, is "a collection of screw-ups so severe that fruit flies"--the geneticist's laboratory rat-- "never hatch into larvae." Mice that lack the Hedgehog gene have a Cyclops-like eye and long proboscis. Human beings may suffer in a similar way. Children born with Smith-Lemli-Opitz syndrome (SLOS) cannot make cholesterol: they are missing the final enzyme in its synthesis. Although their mothers likely contribute cholesterol in the womb, some of their head and brain abnormalities resemble those of the Hedgehog-less mice.

How did such fundamental orchestration come about? It's logical that early, single-celled life used some sort of signals to recognize each other and mate, for example. Over time, organisms could have evolved these primitive nametags into signals to customize cells for specific tasks. And a key step in customizing, or cell differentiation, would be getting the proper signal to its proper place. Enter cholesterol.

That scenario may end up less fantastic than one might think, says Beachy. Eugene Koonin of the National Library of Medicine recently called his attention to a coincidence. Koonin, a geneticist, had noticed that the part of Hedgehog where cholesterol forms its bridge bears a remarkable resemblance to part of something recognized less than 10 years ago: self-splicing proteins.

"Self-splicing proteins may be nature's tidiest parasites," says Beachy. These proteins get a host cell to replicate them by quietly inserting their DNA into specific places along genes. The cell's own machinery manufactures the parasite along with the real product. Then the self-splicing protein just as quietly cuts itself out, neatly reties the ends of its host protein, and goes on its way, the host unharmed.

The sequence that Koonin saw in common was where one end of splicer meets host, and where the cleaving part of Hedgehog joins the signaling part. Is this an evolutionary coincidence, or did an enzyme in cholesterol metabolism have some ancient entanglement with the machinery of a self-splicing protein?

Another coincidence shows up with Hedgehog's three-dimensional, structure. Nestled within its protein chain are a zinc atom and a water molecule. They are arranged in a tetrahedral shape that Hedgehog turns out to share with an enzyme in soil bacteria called Streptomyces (from which come several antibiotics). This enzyme uses the zinc-water complex to prevent formation of cross-supports between the sugar chains of cell walls. Could Hedgehog, closely associated with cell membranes, have inherited the complex from this enzyme? Perhaps the protein sometimes needs to free itself from its cholesterol anchor, or to move in some other fashion through tissues. Speculation to be sure, says Beachy: "There may well be aspects of Hedgehog signaling that we haven't discovered yet."

There are signs that cholesterol may help process more proteins than Hedgehog. Porter and Keith Young checked their work in Beachy's lab by feeding a kidney-cell culture with radio-labeled cholesterol. Hedgehog lit up, as expected--but so did several other proteins. They are as yet unidentified.

"This is very preliminary," warns Porter, "but we may be dealing with a fundamental process in development that no one has ever seen before."

Not bad for a molecule of such, well, greasy repute. --EM

Written by Melissa Hendricks and Elizabeth Manning (MA '90).