Composites containing long carbon nanotubes show promise for producing stretchable sheets with high electrical conductivity.
Stretchable electronic components would be ideal for applications where circuits need to be repeatedly bent and moved, such as joints on robots or for biological purposes. However, deterioration and sudden decreases in conductivity is a problem with conventional materials, where wires and connections can fracture and snap.
Now,Takao Someya and colleagues1 have succeeded in producing stretchable electronic circuits using a nanowires-polymer composite, which preserve their conductive properties even when stretched.
The material was made by mixing ultralong carbon nanotubes with an ionic liquid to create a ‘bucky gel’ in which the tubes were well dispersed. By adding a fluorinated copolymer to the gel, a composite material was produced which could be drop-cast on a surface to make a thin film. The film was flexible as soon as it had air-dried, but to improve its elasticity, the researchers perforated it with a hole puncher to make a net structure (Fig.1) and coated it with silicon rubber.
Traditional conducting rubber containing carbon particles has very low conductivity― only 0.1 S/cm at all extensions―despite its flexibility, which exceeds 150%. When the resistance of a uniform sheet of stretchable conductor was measured, it exhibited a conductivity of 57 S/cm, which was maintained even when stretched up to 38%. When the sheet was mechanically processed to form a net-shaped structures, the conductivity diminished slightly, but even up to 134% extension, a conductivity of 6 S/cm was maintained.
The researchers also found that traditional connections were one of the most delicate parts of test circuits, so they created a paste that could be used for interconnects instead. To do this, they took the polymer/nanotube gel and chemically created crosslinks between some of the polymer chains. As a result, the partially crosslinked gel was far more robust than wire interconnects.
To test their conductive sheet and paste, the authors created a 20cm by 20cm active matrix with printed organic transistors. They showed that the sheet could be stretched by up to 70% without causing any mechanical damage or changing the characteristics of the transistors.
The group envisage many applications that will involve human-electronic interfacing. “Objects that come into contact with humans are often not square or flat. We believe interfaces between humans and electronics should be soft,” says co-author Sekitani.
References
Sekitani, T. et al. A Rubberlike Stretchable Active Matrix Using Elastic Conductors. Science 321, 1468–1472 (2008).
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Flexible electronics: Fantastic elastic. NPG Asia Mater (2008). https://doi.org/10.1038/asiamat.2008.141
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DOI: https://doi.org/10.1038/asiamat.2008.141