Princeton engineers have developed a sensor that may
revolutionize how drugs and medical devices are tested for
contamination, and in the process also help ensure the survival of
two species of threatened animals.
To be fair, some of the credit goes to an African frog.
In the wild, the African clawed frog produces antibacterial
peptides — small chains of amino acids — on its skin to protect
it from infection. Princeton researchers have found a way to attach
these peptides, which can be synthesized in the laboratory, to a
small electronic chip that emits an electrical signal when exposed
to harmful bacteria, including pathogenic E. coli and
salmonella.
“It’s a robust, simple platform,” said Michael McAlpine, an
assistant professor of mechanical and aerospace engineering and the
lead researcher on the project. “We think these chips could replace
the current method of testing medical devices and drugs.”
A paper outlining their development of the sensor was published
online October 18 in the Proceedings of the National Academy of
Science. The research was funded by the American Asthma
Foundation and by the Air Force Office of Scientific Research.
The current testing method has a major drawback: It relies on
the blood of the horseshoe crab, a species that is roughly 450
million years old. The horseshoe crab population has declined in
recent years, and as a result, so too has the population of a bird
that feasts on the crab.
![]() |
||||
The crab became desirable for testing because its immune system
has evolved to cope with the constant threat of invasion from its
bacteria-rich environment. Its blood contains antimicrobial cells,
known as amebocytes, that defend the crab against bacteria —
similar to the way the peptides protect the African frog’s
skin.
For almost 40 years, an aqueous extract made from horseshoe crab
blood cells, called Limulus amebocyte lysate (LAL), has been used
for testing drugs and medical devices for contamination.
In the era before the use of these animal extracts for testing,
although drugs and medical devices were sterilized, they would
sometimes cause patients to develop fevers due to an immune
reaction to endotoxins, which are remnants of bacteria destroyed by
the sterilization process. When a sample from a drug or device is
added to LAL and the solution hardens into a gel, it indicates the
sample is contaminated and not safe for human use.
New approach could help save animal populations
To produce LAL, the crabs are captured and roughly 30 percent of
their blood drained before they are returned to the ocean. There is
disagreement on how many crabs die as a result of the procedure,
but their estimated mortality rate can be as high as 30 percent,
according to the United States Geological Survey.
A conservative estimate puts the number of horseshoe crabs on
the Atlantic Coast between New Jersey and Virginia at between 2.3
to 4.5 million, according to the Ecological Research &
Development Group. In recent years, the populations of the
horseshoe crab and shore birds that rely on them for food both have
been in decline, with the red knot, a rust-colored species of shore
bird, of particular concern.
Each spring the bird migrates 20,000 miles from the islands of
Tierra del Fuego, off the southern tip of South America, to the
Delaware Bay on the east coast of the United States. From April to
May, the bird feasts on horseshoe crab eggs found on beaches,
nearly doubling its body weight to sustain its health for the long
flight south.
Studies have discovered a precipitous decline in the red knot
population. One study by researchers at the University of Toronto
found that the Tierra del Fuego population of red knots declined
from 53,000 birds to 27,000 birds between 2000 and 2002. The
decline has been linked to the reduction in the number of horseshoe
crabs, as a result of harvesting their blood for medical testing
and their use as fishing bait for eel and conch.
In response, Delaware, Maryland and New York have limited the
number of crabs that can be harvested each year to less than
150,000, and New Jersey has implemented a moratorium on harvesting
the crabs.
In 2009, since implementing the measures, the number of red
knots visiting Delaware Bay was estimated at 24,000, up from 18,000
the year before, but still far lower than the population of 100,000
to 150,000 of two decades ago.
McAlpine and Manu Mannoor, a Princeton graduate student who
worked on the project, hope that technology based on their
electronic chip will eventually replace LAL as the standard for
contamination testing, obviating the need for horseshoe crab blood
and helping both the crabs and the red knots rebound.
At the same time, producing this new sensory device would not
put pressure on the frog species. “No frogs were harmed in the
making of this sensor,” he said.