October 31, 2012
Researchers Describe Process In Which Ingesting Silver Can Turn Your Skin Blue
April Flowers for redOrbit.com - Your Universe Online
When is your skin like a black and white photograph? Sounds like the start to a bad riddle, right? Instead, it's the essential question behind a new study from Brown University. The study shows, for the first time, how ingesting too much silver can cause argyria, which is a rare condition that turns a patient's skin a striking shade of grayish blue.
"It's the first conceptual model giving the whole picture of how one develops this condition," said Robert Hurt, professor of engineering at Brown. "What's interesting here is that the particles someone ingests aren't the particles that ultimately cause the disorder."
It's been known for years that silver had something to do with argyria. Argyria has been documented in patients who have undergone alternative treatments involving silver and people who — ill advisedly — drink antimicrobial health tonics containing silver nanoparticles. Researchers have found silver particles lodged deep in the skin of the patients, but haven't been able to understand how they got there.
Hurt and his colleagues say that argyria is the result of a set of complex chemical reactions.
The research team revealed that nanosilver is broken down in the stomach and then absorbed into the bloodstream as a salt. This salt is deposited in the skin where exposure to light converts the salt back into silver metal. This creates the telltale bluish hue. This final stage in the process is exactly the same photochemical reaction used to develop black-and-white photographs.
Hurt's research team has been investigating the environmental impact of silver nanoparticles for years. So far, they have found that nanosilver tends to corrode in acidic environments. As it corrodes, it gives off charged ions in the form of silver salts, which can be toxic in large amounts. Jingyu Liu, Brown graduate student and postdoctoral researcher at the National Institute of Standards and Technology (NIST), suggested that those same toxic ions might be produced when silver enters the body, playing a role in the development of argyria.
To test this theory, the researchers mixed up a series of chemical treatments that could simulate what might happen to silver inside the body. The experiment had three steps. One treatment simulated the acidic environment in the gastrointestinal tract, the second mimicked the protein content of the bloodstream, and the final step was a collagen gel which replicated the base membranes of the skin.
Nanosilver corrodes in stomach acid much as it does in other acidic environments. When the nanosilver corrodes, it strips the silver atoms of electrons, forming the positively charged silver salt ions. Like other salts, silver salts are easily absorbed into the bloodstream, which is a crucial step according to Hurt. Silver metal particles aren't very likely to make it from the GI tract to the bloodstream, but in salt form they are easily absorbed.
The team then found that silver ions bind easily with sulfur that is already present in the blood proteins, giving them a free ride through the bloodstream where some of them end up in the skin. This is where they are exposed to light.
In order to re-create this end stage, the team shined ultraviolet light on the collagen gel containing silver ions. Once exposed to the light, the electrons from the surrounding materials jump onto the unstable ions. This returned them to their original state — silver metal. This reaction is what ultimately turns the skin blue. The way silver is used in black and white photography is similar to the photoreaction in the skin. Silver salts on photographic paper, when exposed to light, reduce to silver metal and darken to create the image.
Silver has been valued for centuries for its ability to kill germs despite its potential toxicity, which is why silver nanoparticles are used today in everything from food packaging to bandages. There are regulatory limits for occupational exposure to silver, but this research raises questions as to whether there should be special limits on the nanoparticle form.
Bioavailable silver — the form that is absorbed into the bloodstream — is the silver salt made in the stomach. Any silver metal ingested is just the raw material to create that bioavailable salt, so ingesting silver in any form, nano or not, would have basically the same effect.
"The concern in this case is the total dose of silver, not what form it's in," Hurt said. "This study implies that silver nanoparticles will be less toxic than an equivalent amount of silver salt, at least in this exposure scenario."
Their findings were published online in the journal ACS Nano.