298 CATALYZING INQUIRY
Also, a wide variety of potential geometries exists for crossover tiles. There have been experiments with
a so-called 4 × 4 tile, where the sticky ends extend at right angles.
DNA also has the property that its length scale can bridge the gap between molecular systems and
microelectronics components. If the issues of surface attachment chemistry, secondary structure, and
self-assembly can be worked out, hybrid DNA-silicon nanostructures may be feasible, and a DNA-
controlled field effect transistor is one possible choice for a first structure to fabricate. Some other
specific near-term objectives for research in DNA self-assembly include the creation of highly regular
DNA nanoparticles and the creation of programmable DNA self-assembling systems. For the cell regu-
latory systems and enzymatic pathways, some specific near-term objectives include the creation of sets
of coupled protein-DNA interactions or genes, the simulation and emulation of kinase phosphor-relay
systems, and the creation of networks of interconnecting nanostructures with unique enzyme commu-
nication paths.
To be adopted successfully as an industrial technology, however, DNA self-assembly faces chal-
lenges similar to solution-based exhaustive search DNA computing: a high error rate, the need to run
new laboratory procedures for each computation, and the increasing capability of non-DNA technolo-
gies to operate at nanoscales. For example, while it is likely true that current lithography technology has
limits, various improvements already demonstrated in laboratories such as extreme ultraviolet lithogra-
phy, halo implants, and laser-assisted direct imprint techniques can achieve feature sizes of 10 nm,
comparable to a single DNA tile. Some other targets might be the ability to fabricate biopolymers such
as oligonucleotides and polypeptides as long as 10,000 bases for the creation of molecular control
systems and the creation of biochemical and hybrid biomolecular-inorganic systems that can be self-
assembled into larger nanoscale objects in a programmable fashion.
8.4.3.4 Hybrid Systems
A hybrid system is one that is assembled from both biological and nonbiological parts. Hybrid
systems have many applications, including biosensors, measurement devices, mechanisms, and pros-
thetic devices.
Biological sensors, or biosensors, probe the environment for specific molecules or targets through
chemical, biochemical, or biological assays. Such devices consist of a biological detection element at-
tuned to the target and a transduction mechanism to translate a detection event into a quantifiable
electronic or optical signal for analysis. For example, antennae from a living silkworm moth have been
used as an olfactory sensor connected to a robot.^144 Such antennae are much more sensitive than
artificial gas sensors, in this case to moth pheromones. A mobile robot, so equipped, has been shown to
be able to follow a pheromone plume much as a male silkworm moth does. When a silkworm moth’s
antennae are stimulated by the presence of pheromones, the moth’s nervous system activities alternate
between active and inactive states in a pattern consistent with the activity pattern of neck motor neu-
rons that guide the moth’s direction of motion. In the robot, the silkworm moth’s antennae are con-
nected to an electrical interface, and a signal generated by the right (left) antenna results in a “turn
right” (“turn left”) command. This suggests that such signals may play an important role in controlling
the pheromone-oriented zigzag walking of a silkworm moth.
(^144) Y. Kuwana et al., “Synthesis of the Pheromone-oriented Behaviour of Silkworm Moths by a Mobile Robot with Moth
Antennae as Pheromone Sensors,” Biosensors and Bioelectronics 14:195-202, 1999.