Wiring electrical circuits by self-assembly
5 Nov 2001
A study initiated by a North Carolina State University chemical engineer has discovered a new method of growing microscopic wires that can conduct electricity in a liquid environment.
The research by Dr. Orlin Velev and four colleagues shows that colloidal nanoparticles - dispersed particles ranging in size from 15 to 30 nanometres - can spontaneously self-assemble into wires when placed under the force of an alternating (AC) electric field, a process known as dielectrophoresis. The microwires are about a micrometre, or one-millionth of a metre, in diameter, and up to a few millimetres long. The formation of these microwires can be controlled and used in rudimentary electrical circuits.
Making electrical circuits in wet environments may lead to a host of bioelectrical uses, such as providing electrical connections to living cells and tissues. There's no need for organic liquids or special solutions as the process can be accomplished in small amounts of water. Moreover, microwires will spontaneously connect to a conductive element - some carbon paint, for instance - that is placed in the wet environment. Eventually, it is expected that microwires will be used to connect living cells to electrodes.
Velev and his colleagues have already shown that spontaneously self-assembling microwires can be used as sensors to detect certain chemicals, like cyanide. Velev said the process will advance the field of electrical detection, which could supplant optical detectors.
The discovery caught Velev and his colleagues by surprise. 'Nothing was expected to happen with waterborne metallic nanoparticles in the AC electric field because the force between these tiny particles is so small,' he said. In fact, using just two particles would not lead to assembly. Many particles are needed for wire growth to take place in a collective effort, unlike the simple chaining in organic liquids observed previously.
'When you miniaturise things, self-assembly is important,' Velev said. 'With dielectrophoresis, there is no chemical reaction, no need for soldering, and the electrical connections are made spontaneously.'
The research showed that microwires that burn out due to higher currents or other factors spontaneously repair to form a new connection. Also, microwires stay intact when current is switched off, preserving the self-assembled circuits.
Velev and his colleagues used gold nanoparticles which are essentially inert and benign. He said silver, platinum, a variety of other metals and possibly carbon could also be used successfully in dielectrophoretic assembly.
The research will be continued at NC State as part of Velev's scientific program.
