The Institute - June 2020 - TI7
engineers with ingenuity
the instrument. Each cartridge can have a different design of
its microfluidics and hold different chemicals, creating a PCR
[polymerase chain reaction] specific to any DNA sequence you
want, and so sensitive that you can detect just a few segments
of that DNA in a milliliter of liquid." Petersen received the 2019
IEEE Medal of Honor for contributions to the field of MEMS.
The GeneXpert test, like most COVID-19 tests to date, starts
with a nasal sample taken with a swab. The person collecting
the sample drops the swab into a specimen transfer tube. To
start the test, liquid containing the sample is pipetted into a
disposable test cartridge, and the cartridge is inserted into
the test machine. The procedure takes no special training,
and after those steps, the process is automatic.
Petersen explains the technology behind the test [see illustration below]:
"The test cartridge contains microfluidic channels made
of plastic using high-precision injection molding. All the
chemicals needed for the process are stored in chambers
within the system. In the center of the cartridge, a rotary
valve turns to open different pathways, while a tiny plunger-
like a syringe-moves fluids in and out as needed.
The plunger pulls the sample into the center, the valve
rotates, and the plunger pushes it into another region of
the cartridge to do an operation on it. The system can do
that multiple times, moving the sample to different regions
with different chemicals, extracting RNA, mixing it with
the reverse transcriptase that synthesizes complementary
DNA that matches the RNA, and eventually pushing it into
a PCR tube, where rapid heating and cooling speeds up the
process of copying the DNA.
Each new copy of the DNA gets a fluorescent molecule
attached-which allows an optical system to determine
whether the targeted gene sequence is in the sample."
Petersen, now an angel investor, says he is gratified that
a technology he worked to develop is
being used to help address the pandemic.
"The instrument hasn't changed that
much," he says. "It's pretty much what
we designed 20 years ago."
Cepheid's test for COVID-19 was the
first approved in the United States. Since
then, Abbott Laboratories has received
FDA approval for a five-minute test that
runs on its ID Now analyzers. Roughly
18,000 of them have been installed in
U.S. health care facilities. According to
a recent study on flu virus identification
that compared the Cepheid and Abbott
systems along with a similar technology
from Roche, "the Cepheid test showed
the best performance" and was generally
more sensitive. Such comparisons with
respect to identifying the new coronavirus are not yet available.
This article originally appeared online as "A
Rapid COVID-19 Test Arrives via a 20-yearold Technology Already in Many Hospitals."
Using a swab, a nasal sample is taken from
the patient. The swab is then transferred
to a specimen tube and placed in a disposable cartridge [bottom, left], which is
prefilled with required chemicals. The cartridge is then placed into the GeneXpert
rapid molecular diagnostic machine
[bottom, right] to test for the virus.
The Institute - June 2020
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