Seeking An Eternal Solution | November 3, 2008 Issue - Vol. 86 Issue 44 | Chemical & Engineering News
Volume 86 Issue 44 | pp. 25-29
Issue Date: November 3, 2008

Seeking An Eternal Solution

Fluorinated fluid is the protagonist of an ongoing experiment in preserving biological specimens
Department: Science & Technology
News Channels: Biological SCENE
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Exhibit A
The Smithsonian's 24-foot giant female squid is preserved in dilute formaldehyde and stored in a 3M hydrofluoroether fluid.
Credit: 3M
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Exhibit A
The Smithsonian's 24-foot giant female squid is preserved in dilute formaldehyde and stored in a 3M hydrofluoroether fluid.
Credit: 3M

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To learn more about Novec, visit C&EN's blog, "C&ENtral Science," at www.cenblog.org.

AT THE CENTER of the Smithsonian Institution's National Museum of Natural History's gleaming new Sant Ocean Hall lies a preserved giant female squid—the arresting, spineless star among the vibrant exhibition's animal specimens. Tentacles menacingly outstretched and seemingly frozen in time, the 24-foot squid embodies humans' fascination with the briny deep. But this squid also symbolizes something else: an ongoing experiment in the chemistry of preservation, without which the Smithsonian's new exhibition would not have been possible.

The fluids most widely used for long-term museum conservation are solutions of alcohols, such as ethanol, and formalin, a dilute solution of formaldehyde. Museums worldwide have been preparing and displaying soft-bodied animals such as squid in much the same way for centuries, despite the fire and health risks these fluids pose. But the Smithsonian was forced to try a new approach after the Sept. 11, 2001, attacks, when the Washington, D.C., fire marshal drastically limited the amount of flammable preservatives allowed in public buildings.

Museum curators partnered with Minnesota-based 3M to apply a very different type of fluid to the task—3M Novec 7100 engineered fluid, a safe, nonflammable hydrofluoroether originally developed for silicon-chip cleaning and other electronics industry applications. Although Novec comes with its own set of challenges, preservation experts looking for alternatives to the status quo are eagerly watching the Smithsonian's experiment to see what happens.

Alcohol-based preservatives dehydrate specimens while also slowing the rate of decay by killing bacteria. The only widespread alternative to alcohol solutions is formalin, a dilute, buffered solution of formaldehyde that came into use as a preservative near the end of the 19th century. Formaldehyde is a fixative, which means that it permeates tissue, hardening and preventing the specimen from decomposing. Chemically speaking, formalin makes cross-links between certain functional groups, such as basic amino acid residues.

Unfortunately, these established preservatives have disadvantages. For example, museums using flammable fluids have long had to take extra precautions to reduce the risk of a blaze. Even in 1858, the founding donor of Philadelphia's Mütter Museum stipulated that his collection of fluid-preserved human tissues and organs be housed in a fireproof building, says Anna Dhody, the Mütter's curator.

Formalin is less flammable than ethanol and isn't as restricted by the fire code in Washington, D.C., says Brian F. Spatola, collections manager at the Washington-based National Museum of Health & Medicine, part of the Armed Forces Institute of Pathology (AFIP). Most of that museum's collection is stored and displayed in formalin. However, flammability isn't the only concern associated with the fluid.

Besides its distinctively unpleasant odor, formalin is an irritant and has been linked to certain cancers through animal tests, which is why some museums are phasing it out. Unlike AFIP, the Smithsonian does not store specimens in jars of formalin, and the Mütter Museum has nearly finished moving its specimens out of formalin and into ethanol.

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Posterity's Price
A drastic change in color is evident in this small tree frog, shown before (left) and after it was fixed with formalin and stored in 70% ethanol.
Credit: Courtesy of John Simmons
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Posterity's Price
A drastic change in color is evident in this small tree frog, shown before (left) and after it was fixed with formalin and stored in 70% ethanol.
Credit: Courtesy of John Simmons

Safety concerns aside, the aesthetic qualities of traditionally preserved specimens leave something to be desired. Ethanol leaches color from specimens, so "first they turn brown and then a dingy white," while the fluid itself turns yellow, observes Elizabeth Musteen, the Sant Ocean Hall's project manager. Ethanol is less dense than the specimens, which collapse at the bottom of storage jars. "For an exhibition, we put brackets in the jars to raise specimens up," Musteen explains. Formalin also tends to discolor specimens over the long term, and neither preservative faithfully retains a specimen's true texture, Dhody says.

A more recent goal for museums has been the ability to preserve DNA, which comes in handy for classification and other studies. Ethanol preserves DNA, but formalin's cross-linking mechanism may hinder its DNA-preserving ability.

Perhaps the most significant drawback of traditional preservatives is a dearth of scientific records about them, making it tough to rationally develop safer, better alternatives, argues John E. Simmons, an independent museum consultant and former collections manager at the University of Kansas' Natural History Museum. AFIP keeps records on the formulas of its preservatives, and researchers there were engaged in an active research program in preservation during the 1940s and '50s, Spatola says. However, collections donated to AFIP from elsewhere are less stringently cataloged.

"Will we ever abandon alcohol entirely? No, mostly because we don't have 200 years to test new things out."

Simmons contends that research programs like AFIP's have historically been the exception rather than the rule. "In fluid preservation the tradition has always been to just try something out," he says. Many published preservative formulas reflect traditions handed down in a given lab, without much testing of specific variables to look for improvements, Simmons says. "There is a general preservation procedure in place, but the people who prepared specimens" in the past "often had their own way of doing things," Dhody adds.

The Smithsonian's giant squid bucks that tradition—it is a model of meticulous record keeping. Photographs and extensive documentation chronicle how the squid's soft body tissue was injected with formalin by experts in northern Spain, where the squid was caught. They also describe the squid's journey stateside, where it was submerged in Novec and where the record keeping continues apace.

NOVEC 7100 comprises two inseparable isomeric hydrofluoroethers, methyl nonafluorobutyl ether, CF3(CF2)3OCH3, and methyl nonafluoroisobutyl ether, (CF3)2CFCF2OCH3. The fluid differs from both formalin and ethanol in that it's neither a cross-linking agent nor a preservative, says David A. Hesselroth, a 3M chemist. Instead, Novec is a nonflammable, nontoxic, and ozone-friendly storage medium for already-preserved specimens, he says. Novec products have been in use since the mid-1990s, when they were developed to replace ozone-depleting chlorofluorocarbons in applications such as cleaning electronics.

Novec works by forming a chemical envelope around preserved specimens, explains Joseph Koch, marketing manager for 3M's electronics materials division. The fluid has very low surface tension, so "it completely spreads around a specimen's surface, displacing water in all the nooks and crannies," he says. Novec's low water solubility keeps the fluid from getting cloudy over time, and it doesn't leach color from specimens the way alcohol does.

Musteen can vouch for that. "You can still see the squid's brick-red skin as clearly as on the day it was caught, and the fluid itself is crystal clear," she says.

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Pickled
Millions of specimens are kept in alcohol at the Smithsonian's National Museum of Natural History's storage area near Washington, D.C.
Credit: Chip Clark/Smithsonian Institution
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Pickled
Millions of specimens are kept in alcohol at the Smithsonian's National Museum of Natural History's storage area near Washington, D.C.
Credit: Chip Clark/Smithsonian Institution

Smithsonian preservation experts knew exactly how to prep the squid for its Novec-filled resting place because of the tests 3M scientists and museum veterans performed beforehand. For their earliest tests on whether Novec might fit the preservation bill, 3M scientists started small. "We went to a bait shop and picked up a dozen night crawlers," which are earthworms primarily used for fishing, Hesselroth recalls. Worms in Novec degraded significantly after 10 weeks, leading the teams to conclude that Novec itself couldn't be used as a preservative. After follow-up tests on fish, shrimp, and small squid, they found that the best results came from first fixing specimens in formalin and then moving the fixed specimens into Novec—a procedure ultimately carried out for the giant squid.

Unfortunately, Novec isn't a perfect solution. The fluid is denser than water, and unrestrained specimens will float to the tops of tanks, where they could decompose. Restraining a specimen to keep it submerged in the fluid could cause tension damage to it over the long term. To minimize that risk, Smithsonian scientists used a metal screen to reinforce the part of the squid attached to the main restraining bracket and broad transparent straps to better distribute tension on the tentacles, says Michael Vecchione, a zoologist and Sant Ocean Hall curator. "The edges of the straps could still cut into the tissue," he admits. To avoid these challenges, "ideally, you'd want a storage fluid that has the same density as seawater," he comments.

IN ADDITION, because Novec has a high vapor pressure and boils at a lower temperature than ethanol, museum staff had to take extra precautions to minimize evaporation, including using specially designed jars that could keep an extra-tight seal on the specimen and lighting that does not give off a lot of heat, Musteen says.

"Will we ever abandon alcohol entirely?" Musteen asks. "No, mostly because we don't have 200 years to test new things out." For all the flaws of the established technology, museum experts know that formalin and ethanol keep specimens preserved for the long haul, Musteen says. For that reason, the Smithsonian doesn't currently store one-of-a-kind or other highly valuable specimens in Novec. "People know that if they put a specimen into formalin or ethanol it'll still be there" for years to come, Spatola says.

Despite the challenges that come with using Novec, Simmons emphasizes that it offers some improvements over established fluids. For one thing, he notes, it remains clear while preserving a specimen's color. And Novec's nonflammability is a significant plus, he says.

Tests on the giant squid and Novec continue, even while the animal is on display. Every organization that donated specimens for the new exhibition had the same request: "They wanted us to get lots of data on Novec fluid," Musteen says. "They are looking for alternatives to alcohol, just like we are." Specially designed needle ports in the squid's display case permit periodic sampling of the squid's tissue and the surrounding fluid. The sampling is "similar to the sort of biopsy a person might get to test for cancer," Musteen explains. Smithsonian experts will test the Novec fluid to see whether any compounds are leaching from the squid and examine the squid's tissue under the microscope to check for changes to cellular structure.

"I'm excited to see the Smithsonian's staff tackle fundamental issues in fluid preservation," Simmons says, but he cautions that it will take a long time to build a reliable knowledge base about Novec or any new fluid that might come along. "This is very much an experiment," Vecchione says. "We're very interested in seeing how it will all turn out."

 

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