There were two 4.2 4.2 mm2 square blocks at two ends of the
CNT-based sensors for the electrical contacts for further characterization
experiments. For the control of the sensor thickness, weight
of the CNT powder used in the vacuum filtration process can determine
the thickness of the CNT-based sensors. More CNT powder
used can fabricate thicker sensor. In our experiment, 10.0 mg
CNT powder was first ultrasonically dispersed in 150 ml DI water
with 1% SDS surfactant as the master solution. During the vacuum
filtration process, a certain volume of CNTs suspended solution
was used to fabricate the CNT-based sensors on paper. The volume
was calculated by the proportional weight of the CNTs in the master
solution. For example, 7.5 ml master solution contained 0.5 mg
CNTs. Therefore, the length, width, and thickness of the CNT-based
sensors on paper can be well-controlled using this fabrication
method. The paper was still flexible and the CNT-based sensors
did not show cracks on the surface under microscopic observation.