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 regulatory 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 communication paths.
To be adopted successfully as an industrial technology, however, DNA self-assembly faces challenges 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 technologies 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 lithography, 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.
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 prosthetic 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 attuned 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 neurons that guide the moth’s direction of motion. In the robot, the silkworm moth’s antennae are connected 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.