The U.S. Navy is attempting to enlist the ability of biomoleculesto induce their constituent atoms to self-assemble. Ifsuccessful, the Navy hopes to save time, money and theoffshore environment; in the process, it also sees a bio-pharmaceutical spinoff.
The sea-going Navy wages a constant battle against submarineenemies -- mainly barnacles, fungi and wild grasses, whichattach in ever-thickening layers to the hulls of ships, causingdrag that reduces their speed and performance.
To combat this marine biofouling, the bottoms of ships arepainted with polymers containing tin compounds that are toxicto the offending marine life, but also to shellfish in general. Oneconsequence: a decline in marine life. San Francisco Bay, forexample, has lost much of its shellfish population, causingCongress to mandate phasing out of toxic-metal hull coatings.So the Navy has a real interest in finding environmentallyacceptable anti-fouling paints.
Physical chemist Joel Schnur, director of the Naval ResearchLaboratory's Center for Bio/Molecular Science & Engineering,told BioWorld that the Navy hopes to protect the environmentas well as cut operational costs. "Every time you bring a ship into scrape and paint, it's like $50 million bucks," he said. "If youcan keep them at sea an extra year or two, you've saved a lotof money."
Schnur's laboratory is constructing microscopic, lipid-basedtubules as carriers for eco-friendlier anti-fouling agents, to bereleased in a time-controlled manner into paint so ships canspend more years between hull jobs. His researchers are alsotesting similar sub-visible hollow cylinders, charged withcytokines, to incorporate in artificial skin for wound healing.
Schnur has a review article in today's issue of Science titled"Lipid Tubules: A Paradigm for Molecularly EngineeredStructures" and a paper in the current Journal of ControlledRelease called "Biologically engineered microstructures:controlled-release applications."
To replace toxic tin and copper as its anti-fouling paint additiveof choice, the Navy is looking into the sea pansy (Renillamultiformis), a plant that grows on coral reefs, where it repelsbarnacles and grasses. "The trouble with Renilla," said Schnur,is that when "you mix it with paint, it lasts about three hours."
Just as liposomes (or their polymer analogs) assemblethemselves in an agitated oil-and-water mixture by theinterplay of hydrophilicity and phobicity, the Navy'sphospholipids pull themselves together into "lipotubes" byphysical chemical processes involving chirality, anisotropy andpolarity, among others. The resulting structures, under thescanning electron microscope, resemble drinking strawssnipped into inch-long pieces.
But instead of an inch, Schnur's hollow cylinders range from 10to 250 microns in length and 0.5 microns in external diameter.Their wall thickness is around 500 angstroms and theirinternal diameter 0.25 to 0.4 nanometers. This makes Schnur'sstructure much more slender than a human hair but largerthan a cell microtubule.
To give the lipid core rigidity, it's coated with a thin layer ofgold, copper, nickel or permalloy. Poured into a container full ofliquid active ingredients, the tubules suck these into theirlumen by capillary action. They are then stirred into marinehull paint.
Permeable liposomes unload their drugs through their poresunder increasing osmotic pressure. The Navy's tubules areimpermeable but open-ended, so they discharge their cargothrough both ends into the paint, then out to the ocean. But tocontrol release time, the Naval researchers first mix the Rinellacompound into a second polymer.
"By varying the permeability of that polymer inside the tubulewe can vary the release rate from 30 days to what looks like10 years," Schnur said. "It's a straightforward modelingproblem, which we've just solved."
Less straightforward is the cost factor. The Navy's lipid, whichit makes in-house, costs about $3,000 a pound, or $600 pergallon of paint. But Schnur's group has devised a method forrecycling the lipid out from under its metallic coating, whichmight cut that cost to $300 or even $200 a gallon.
"At even $500 per gallon per ship, with refurbishing costs attens of millions of dollars, doing that overhaul every eightyears instead of every two, you'd be way ahead of the game,"Schnur said. "The paint is only a $2 or $3 million-dollar item."
Schnur fears that Rinilla, which must be harvested from coralreefs, may turn out to be the costliest component of all.Scientists at Duke University are trying to synthesize moleculesthat will replicate its function.
From Hull Cleaning to Wound Healing
Adapting this hollow-cylinder technology to medical uses raisesother questions that Schnur's group is now addressing. For onething, metal coatings are toxic. For another, without metal, willthe template-like lipid tube still lend itself to controlledrelease? "We may have to use a surfactant on these lipidtubules to make them stronger and still biocompatible," Schnursaid. And the toxicity of the lipid itself -- if any -- must bedetermined.
Findings to date suggest no biocompatibility problem; bacteriagrow in the presence of the material and "release rates areintriguing PP on the order of hours to maybe 30 days." Thecylinders have been implanted in healthy mice and applied tofull-thickness skin wounds in rats, with no toxicity.
"Our initial efforts to develop lipid microcylinders as releasevehicles for wound repair have focused on issues of toxicity ofthe vehicle itself and demonstrating in vitro controlled releaseof transforming growth factor-beta," Schnur said. This beganwith an initial release of five to 10 nanograms over the courseof a day, then slowed to one ng/day. Interleukin-2 gave similarrates.
Macrophages adhered and stretched along the long axis ofmicrocylinders, but made no effort to engulf them.
Schnur finds the nautical, medical and agricultural possibilitiesfor his microcylinders "very attractive commercially PP if theprice is right." He said he has recently held discussions withsome pharmaceutical companies, which expressed interest.
Perhaps biotechnologists, he observed, "could clone materialscontaining the chiral and packing factors that would self-assemble into tubular structures. That would cut the costdown."
Schnur noted that Johnson & Johnson owns the product patentfor the lipid and the Navy holds the process patent for itssynthesis and formulation into tubules, with patents pendingfor the controlled release technique.
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.