The EconomistDecember 14th 2019 Science & technology 692 harnessed to store data commercially. Ar-
chival storage is one idea, for it minimises
dna’s disadvantages—which are that,
compared with hard drives, reading and
writing it is fiddly and slow.
Now, though, a team led by Yaniv Erlich
of Erlich Lab, an Israeli company, and Rob-
ert Grass, a chemist at the Swiss Federal In-
stitute of Technology, in Zurich, have had
another idea. As they describe in a paper in
Nature Biotechnology, they want to use dna
data storage to give all manner of ordinary
objects a memory of their own.
The researchers describe a test run in
which they encoded the Stanford bunny—a
standard test image in computer graph-
ics—into chunks of dna. Those chunks
were then given a protective sheath of sili-
ca nanoparticles. That served to protect
them for the next stage, in which they were
mixed with plastic and used as feedstock in
a 3dprinter, which printed a model of the
bunny. The result was an object that con-
tained, encoded throughout its structure,
the blueprints necessary to produce more
copies of itself. By clipping a tiny fragment
of plastic from the finished bunny’s ear and
running the dna within through a se-
quencer, the researchers were able to re-
cover those blueprints and use them to
make further generations of dna-infused
bunnies.
Satisfied with their proof of concept,
they then repeated the trick by encoding a
short video in dnaand fusing it in plexi-
glass, a transparent plastic. They used the
plexiglass to make a lens for a pair of spec-
tacles. Once again, clipping a tiny sliver
from the lens and dissolving the plastic
away was able to liberate the dna, which
could be used to recover the video.
The cost of both producing and reading
dnais falling precipitously. The price of
reading a million letters of the genetic al-
phabet has fallen roughly a million-fold
since the start of the millennium. For that
reason, Drs Erlich and Grass hope their idea
might one day have all sorts of uses. One,
they think, could be to embed relevant in-
formation into manufactured goods. They
give the example of custom-fitted medical
implants that contain a patient’s medical
records and the precise measurements
needed to make another implant.
A second use, for the privacy-minded,
could be steganography—the art of con-
cealing information within something ap-
parently innocuous (this was the idea be-
hind the dna-infused spectacles). Their
most futuristic idea is an entire world full
of objects which, like biological life, con-
tain all the information needed to make
copies of themselves in every part of their
structure. Drs Erlich and Grass have
dubbed their technology the “dna of
things”, and it is certainly a clever idea. But
the next job might be to come up with a
snappier name. 7
O
ver thepast few decades, photovolta-
ic cells have gone from being exotic and
expensive power-packs for satellites and
similar high-end applications to quotidian
generating equipment for grid-scale power
stations. One area where they have not yet
fulfilled their potential, though, is as local
sources of electricity to keep office build-
ings and the like supplied with energy. The
main reason is that no one has a good an-
swer to the question: where do you put
them? Roof-top cells can power a one- or
two-storey house. They will not power an
office block. You could array them on the
walls. But office blocks tend to have high
window-to-wall ratios and to be governed,
for fire-safety reasons, by strict rules on
wall cladding.
What is left is to replace the windows
themselves with solar cells. Unfortunately,
commercially available solar cells are
opaque to the point of blackness. But Seo
Kwanyong of the Ulsan National Institute
of Science and Technology, in South Korea,
plans to do something about that. As he
and his colleagues report this week in Joule,
they have created solar cells that are as
transparent as tinted glass.
Dr Seo’s approach is, in retrospect,
blindingly obvious. It is to punch—or, rath-
er, etch—holes in the material of which a
cell is made, in order to let light through.
Getting the size and layout of the holes
right, though, proved tricky.
Commercial solar cells are made from
wafers of silicon. Dr Seo and his colleaguesworked with sheets of the stuff that were
200 microns thick—the sort of thickness
employed commercially. The holes they
etched were 90-100 microns across, a di-
ameter calculated to be the minimum
needed to permit the passage of visible
light without creating awkward diffraction
effects that would distort what was seen
through the wafer.
Despite this precaution, their first ef-
forts still suffered from strange colours
and opaque regions caused by diffraction
and consequent interference patterns. But
these turned out to be a result of the ran-
dom spacing and arrangement of the holes,
rather than their size. Tweaking the etch-
ing process so that it produced holes which
were regularly rather than randomly ar-
rayed abolished these distortions and re-
sulted in a material that was evenly trans-
parent and which generated no chromatic
aberration. And, crucially, when wired up
as a photovoltaic cell it did indeed produce
electric current.
Clearly, there is a trade-off between the
transparency of a wafer and the amount of
light that can be harvested for electricity
generation. By adjusting the spacing of the
holes, the team were able to make wafers
with transmittances of between 20% and
50% of incident light. Commercial tinted
and coated glass generally has a transmit-
tance of between 30% and 70%.
Wiring up a wafer with 20% transmit-
tance created a device with an efficiency of
12.2%. That compares with 20% for the best
commercial cells, but is not negligible. So,
though 20% transmittance is a bit on the
dark side for office-window glass, what Dr
Seo and his colleagues have created is a
prototype that is within shouting distance
of numbers that might make it commer-
cially viable. Clearly, it would cost more
than standard window glass. But, unlike
window glass, it would pay back its cost in
free electric current. 7Transparent solar cells could be used to
glaze buildingsSolar powerWindows of
opportunity
The hole story