Winfree and New York University’s Ned
Seeman, which is focused on DNA computing and nanotech.
While these DNA projects have received a great deal of attention in scientific journals and even in the mainstream press, other, less well-known
approaches to molecular computing
might lead to entirely new computing
paradigms, say researchers. One example is the work of Andrew Adamatzky,
professor of unconventional computing in the department of computer science at the University of the West of
England. Adamatzky’s research focuses
on reaction-diffusion computing, in
particular on a chemical reaction called
the Belousov-Zhabotinsky (BZ) reaction,
which causes waves of ions to propagate
through an environment. By controlling
the BZ propagation pattern, Adamatzky
has shown it is possible to produce biological logic gates.
Given the proper environment, the
BZ reaction can operate much like a
parallel processor in which each point
on the wave front, mapped to a particular grid, can serve as a point of calculation. To create a Boolean logic gate, for
example, Adamatzky represents True
and False by the presence or absence
of a wave fragment. When two or more
wave fragments collide, they fuse, dissipate, generate new wave fragments,
or change their trajectory or velocity,
representing the Boolean variables and
implementing the computation. The
trajectories of the traveling wave fragments can be changed dynamically, and
adjusted and tuned by colliding other
wave fragments against them, making
for complex interactions. “The medium can implement such sophisticated
tasks as computation of the shortest
collision-free path, approximation of
Voronoi diagrams of arbitrary geometrical objects, and development of a skeleton of a planar shape,” he says. “We
proved the computational universality
of the BZ medium by constructing a set
of functionally complete logical gates
in laboratory experiments. Our results
indicate that the BZ system is a general-purpose parallel computer.”
But as with other work in molecular
computing, several problems must be
addressed in reaction-diffusion computing. Sensitivity of the BZ reaction is
one of the major issues facing Adamatzky. “We design logical circuits according
future applications
include environment-
ally friendly systems
that can automatically
decompose and drug
delivery systems that
can be embedded in
human bodies.
to principles of collision-based computing, where information is represented by
traveling wave fragments,” he says. “
Unfortunately, these localized excitations
are unstable; they collapse or expand after some period of time.” But even with
unsolved problems and unanswered
questions, the experiments are proving
to be useful for other fields. The core
ideas of reaction-diffusion computing
already have spread to those working in
massively parallel computing; and even
some conventional silicon processors
execute wave-based algorithms.
These projects represent a small
cross section of the ongoing developments in molecular computing. Unlike quantum-computing projects that
require sizeable machinery and clean-room-style environments, the promise
of molecular computing is that it can
operate in natural environments, without electrical power. Whether scientists
are able to achieve Feynman’s vision of
nanoscale computers consisting of circuits and switches made of a handful of
atoms remains to be seen, but researchers today are hopeful that paradigms
developed through work in molecular
computing will lead to entirely new
applications, such as environmentally
friendly systems that automatically
decompose, energy-efficient machines
that generate minimal amounts of
heat, and biocompatible communication systems that can be embedded in
human bodies.
based in Los angeles, Kirk L. Kroeker is a freelance
editor and writer specializing in science and technology.
Tatsuya suda, National science foundation, contributed
to the development of this article.
Medicine
Detecting
Breast
Cancer
a large-scale british study has
found that one radiologist aided
by a computer is as accurate as
two radiologists when it comes
to detecting breast cancer in a
mammogram, according to an
article published online in the
New England Journal of Medicine.
Mammograms are used to
screen women for early signs
of breast cancer, but the tests
are not perfect. in the u.s.
most mammograms are read
by a single radiologist, while
in britain, they are read by two
radiologists or technicians.
The british researchers, led
by radiologist Fiona J. gilbert
of the university of aberdeen,
analyzed the results from a
randomized study of 31,000
british women. it found that a
radiologist aided by a computer
detected 198 cancers out of
227, while a pair of radiologists
detected 199 cancers.
computer-aided detection
(caD) systems use computer
algorithms to analyze digital
mammogram images and to
pick out and mark suspicious
areas. caD systems are used in
approximately 25% of mammogram readings in the u.s., and
this percentage should increase
as more medical centers switch
from film x-rays to digital images.
“in the united states, it’s just
not practical in most practices
to do double readings by
physicians,” said carol h. lee, a
radiologist at Memorial sloan-Kettering cancer center in New
york and head of the american
college of radiology’s breast
imaging commission. “These
results are reassuring to me that
a single reading with caD can
achieve that same sensitivity.”
Double reading is the
standard practice in at least 12
european countries. “up to now
double reading has been the gold
standard of mammography,”
said Dr. Marco rosselli del Turco,
president of the european society
of breast cancer specialists.
“We have been waiting for a
well-designed, prospective,
randomized study to establish the
role of caD. This study provides a
definitive answer about the value
of adding caD to single reading,
and is likely to lead to a change in
european guidelines.”