Featured on Materials 360
By Tim Palucka
By taking a previously non-compressible conducting polymer and making it into a foam of compressible spheres of various sizes, researchers at Stanford University have made a pressure sensitive sensor that surpasses the capabilities of our own skin.
"This is better than human touch," says Zhenan Bao of the Department of Chemical Engineering at Stanford. "For a gentle touch, the amount of pressure we feel is in the kilopascal range; this sensor is below the ten pascal range of sensitivity."
Indeed, Bao and her colleagues report in a recent issue of Nature Communications that their sensor detected the gentle placement and removal of a flower petal with a mass of only 8 mg, corresponding to a pressure of about 0.8 Pa. Such a sensor could improve the tactile sense in prosthetic devices or robotic hands so that subtle differences in the texture of an object could be differentiated.
Previous attempts to fabricate touch sensors using elastic conducting polymers have had some success, but high thermal expansion coefficients of the polymers used have made it hard to distinguish pressure effects from thermal ones. And non-elastic materials simply require too much pressure to be applied to be of any practical use.
Ironically enough, the solution turned out to be to use a polymer that is normally stiff and brittle. While trying to improve on their previously reported method of preparing conducting polymer hydrogels of polyaniline, the researchers decided to try the more highly conductive polypyrrole. But the pyrrole monomer proved to only moderately soluble in water or alcohol. Micelles formed instead. When polymerized, these micelles became microspheres in a polypyrrole foam, which they researchers called an elastic microstructured conducting polymer (EMCP). These hollow microspheres gave the finished foam a springy, elastic quality.
So Bao's team decided to test this new material as a piezoresistive pressure sensor. They sandwiched an EMCP film between an electrode made of copper foil and one made of poly(ethyleneterephthalate) (PET) coated with indium tin oxide to form a 1 cm 2 pressure sensitive pad. The resistance of this pad varied from > 105 ohms at less than 10 Pa to 103 ohms at >10,000 Pa, yielding a sensitivity range of 100 kPa-1 to 0.01 kPa-1, respectively. The response time of the pad to changes in pressure was less than 50 milliseconds. Furthermore, the piezoresistance of the sensor remained stable over 8,000 cycles, and from temperatures of 0 to 100°C.
Additional investigation revealed that the pressure sensor relies on a contact resistance mechanism; that is, the piezoresistance originates from physical contacts at discrete spots between the asperities of the EMCP film and the electrode surface. So by micropatterning the EMCP surface topology with triangular arrangements of lines 0.5 mm high and 1 mm wide, the researchers were able to further increase the number of contacts, and hence the sensitivity.
"There are two things happening when we press the material," Bao says. Because the spheres have a range of different sizes in the film, when pressure is applied the electrode contact area with the spheres increases, thus increasing the measured current. "At the same time, the spherical particles are pressed closer to each other and the improved contact between the spheres also contributes to the high sensitivity," she says.
The sensitivity of the micropatterned EMCP sensor is, to the best of the researchers' knowledge, the highest reported to date, with a sensitivity of approximately 56.0-133.1 kPa-1 in the low pressure regime of < 30 Pa - enough to detect the presence of a single flower petal. But they are not stopping here. As a next step, Bao's team will try to improve the synthesis of the EMCP to make it more conductive. They also we want to integrate multiple sensors into a more complex functional prototype that can be used for health monitoring, for example.
Read the abstract in Nature Communications here.
For the original article, please click here.