Future nano-devices are expected to underpin many of the technologies that
society relies on, ranging from household electronics to medical implants. One
of the great challenges of bringing this promising future into reality lies in
developing practical methods for constructing these highly intricate structures:
How will we assemble electronic circuits that feature many more components than
today's commercial circuits and where each component approaches the atomic
Fractal is a rough or fragmented
geometric shape that can be subdivided in parts, each of which is (at least
approximately) a reduced-size copy of the whole.
'Self-assembly' holds great promise as a technique for building commercial
nano-circuits. Adopting this approach, the nano-engineer allows the circuit to
build itself by exploiting natural growth processes. Self-assembly offers two
striking advantages. Not only is it more efficient at assembling vast numbers of
components compared to traditional fabrication techniques, this fundamentally
'green' technique constructs circuits by the addition of material rather than
the wasteful removal of material that lies at the heart of previous 'top-down'
One of the remarkable consequences of harnessing natural growth processes is
that the resulting circuits exhibit natural patterns rather than the smooth,
straight lines that form the framework of today's commercial circuit designs. In
particular, many self-assembly processes generate fractal patterns. Fractals are
shapes that repeat at many magnifications and are prevalent throughout nature,
appearing in natural environments1, biological
systems and human physiology2.
Computers modeled on the brain's fractal geometry
could possess large circuit connectivity and the associated computing
Nature uses fractals frequently because they possess a number of highly
desirable properties. Topping this list is the fact that the repeating shapes
build objects with huge surface areas. Nature exploits this property for example
in trees, where the large surface area of the tree canopy ensures an
unprecedented ability to absorb sunlight. The same approach could equally be
employed to great effect by designing novel solar cell structures based on
Solar cells modeled on a tree's fractal geometry
could capture vast amounts of sunlight
Another consequence of large surface areas is that two merging patterns
connect together very efficiently. For example, the dendritic structure of the
neurons in the human brain exploits this fractal connectivity to produce
enhanced information processing. The same connectivity could equally be
exploited for future commercial computers by using artificial fractal electrical
Simulation of the self-assembled fractal
This philosophy of learning from nature's successes may well revolutionize
many fields within nanotechnology. Although some electronics applications
already exploit fractal geometry (cell phone antennae being a famous example),
many fields lie at the start of this exciting journey, with many discoveries and
challenges lying ahead.
Taylor's investigations focuses on two families of electronic device in
which millions of metallic nano-particles (each approximately 50 nanometers
across) are self-assembled into fractal circuits. In the first family of device,
the particles merge together to form 'nanoflowers'3
using a growth process called diffusion-limited aggregation. In the second
family, the nano-particles are attached to DNA strands4 which assemble to form a fractal circuit. In both cases,
the self-assembly process generates a tree-like pattern similar to one shown in
These projects are driven by the potential to tune the growth conditions so
that the fractal characteristics of the circuits match those found for example
in the neural structure of the human brain. Imagine a future where computers
operate like our own minds and, ultimately, where fractal circuits may act as
implants to be inserted into specific regions of the brain, restoring or
enhancing a patient's mental functionality. Such goals represent the exceptional
promise of nanotechnology - where researchers from a diverse range of
disciplines work together to improve the basic quality of human life.
1. B.B. Mandelbrot, The Fractal Geometry of Nature, Freeman,
San Francisco (1982).
2. J.B. Bassingthwaite et al, Fractal
Physiology, Oxford University Press (1994).
3. S.A. Scott and
S.A. Brown, Journal of European Physics 39 433 (2006).
Warner, and J.E. Hutchison, Nature Materials 2, 272 (2003).
Copyright AZoNano.com, Professor Richard Taylor (University of
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