DNA is well-known for its role in biology as the genetic material. In recent years, however, DNA has begun to be used as a material for creating technology. In particular, DNA can be used to make complex nanometer-scale patterns which, in turn, can be used as templates to arrange nanometer-scale devices. For example, DNA patterns might be used to organize nanowires and nanoswitches to create computer circuits much smaller, cheaper, and faster than current semiconductor computer chips.
Recently the investigators invented a method called DNA origami, whereby a long DNA strand is folded into any desired pattern. The method is powerful but has limitations: current DNA origami only contain 200 pixels, which means they can organize at most 200 different devices---not enough to create a complex circuit. In practice it takes 10-20 pixels to align a single carbon nanotube wire on DNA origami, so the most complex device created using DNA origami is a field effect transistor composed of two crossed carbon nanotube wires. Another difficulty is that DNA origami are made in solution, but must be used on surfaces like silicon. Transferring DNA origami to silicon currently results in random placement and orientation, but to build circuits DNA origami must positioned accurately.
The investigators are interested in overcoming these limitations. They are working on: (1) combining DNA origami into larger patterns with larger numbers of pixels by treating DNA origami as puzzle pieces that fit together based on "stacking interactins", (2) precisely placing and orienting DNA origami on lithographically-defined sticky patches on silicon, and (3) using DNA origami to organize multiple carbon nanotubes to create more complex circuits, such as NAND logic gates.
The modern computer and communications revolutions have been made possible by the continued miniaturization of electronics. The silicon chips in modern computers and cell phones typically have on the order of 1 billion switches in them, connected by a complicated network offine wires. The technology which makes this possible is a printing process called photolithography, and it us allows us to print with very fine details---the smallest lines that can be printed areabout 50 nanometers wide. This resolution sets the size of the smallest wires and switches (transistors) that can be made. To get an idea of how small this is, it is about 1/100th the size of a red blood cell, and about 1/1000th the width of a human hair. Photolithography for modern computer chips is typically performed in a high-technology manufacturing plant, called a "fab" which typically costs several billion dollars. To make smaller, faster, cheaper and more energy efficient computers,we will need to make smaller transistors, or other equivalent devices. Unfortunately, photolithography cannot make transistors much smaller and many researchers are looking to other technologies. In the last 10 years, the field of DNA nanotechnology has learned how to make complex DNA structures, called DNA origami, which are about 100 nanometers in size. These DNA origami, made from sythetic DNA in a test tube, can be thought of as little templates or stencils which have a resolution that is much higher than photolithography. The individual pixels of a DNA origami are just 5 nanometers in size, and so it may be possible to use DNA origami to organize wires and switches of even smaller computer chips. Another advantage of DNA nanotechnology is that it doesn't require a billion-dollar fab---the relevant chemistry could all be done cheaply in a home kitchen. However, there are many different problems which must be solved before DNA origami can be used as a template for the manufacture of electronic circuits. The goal of this NSF Emerging Technology Grant was to start to solve a few of these problems. One of the major problems is that DNA itself is not an electronically interesting material---it does not conduct electricity well or have other characteristics necessary for a transistor. Thus it is important to figure out how to use DNA origami to organize materials that make better wires and switches. To this end we used DNA origami to make crosses of carbon nanotubes---wire-like structures just 1 nanometer across and thousands of nanometers long. When crossed, carbon nanotubes form transistors and so we were sucessful in using DNA origami to self-assemble individual nano-sized transistors in a test tube. One problem with these carbon nanotube transistors is that they were made on the DNA origami while the DNA origami were in solution---that is floating around in water. In order to test the carbon nanotube transistors we had to wash them over a silicon surface where some of them stuck, use special high-power microscopes to find the transistors, and then use a difficult process called e-beam lithography to wire them up. The whole process was relatively slow and inefficient. The general problem is that DNA origami are made in solution, and so any devices or circuits that they carry will be spread willy-nilly on a surface when they are spread out and dried. Itis a little like taking a deck of cards, and throwing them on the floor. To solve this problem we have been working with the semiconductor industry, especially IBM research, to find a way to put origami where we want on a surface. Our solution is to create sticky patches, the size and shape of the origami so that when the origami are washed over the surface, they bind exactly where we want them. In this way we have been able to organize origami into well-defined grids rather than random patterns---the next step is to build devices on top of them. Finally, DNA origami, while very small, are not very complex. Each one has only about 200 pixels, and so each one cannot carry too many different wires an switches. Think of the icons on an old computer or cell phone---they were only coarse images of what they were meant to represent. Recently, we have found a simple way to combine multiple DNA origami into larger structures based on their shape---the origami stick together like puzzle pieces. Now we can make structures that have more than 1000 pixels which can be used to position the devices of the circuits we would like to make. (Figure shows four origami being combined into a long chain). So far, enabled by grants from the NSF, the power and complexity of DNA nanotechnology has been increasing by leaps and bounds---our goal is that one day soon DNA nanostructures will play a vital role in the manufacture of computers.
