The resultant ink contained about 5 wt% CNTs and formed a smoothly uniform suspension. For screen printing, the ink much showed a proper fluidic property to maintain good printing resolution. If the viscosity was too low, the ink can be too loose and cause leaking during screen printing; if the ink was too thick, it may cause unprintable patterns or uneven film thickness. Moreover, the surfactant SDS can disperse more CNTs to avoid agglomerations. Our ink achieved a balanced surface tension at about 33 dyn/cm and viscosity at around 6.7 Pa s, which was suitable for screen printing. The screen printing was conducted by the screen mask to define the sensor channel and electrode dimensions on paper, as shown in Fig. . After the printing process, CNTs were deposited on the void areas and formed a uniform film after thermal annealing. As the CNTs networked into the paper substrates, they formed a porous 3D structure within paper. By varying different W of the channel, the sensors were fabricated with a range of L/W ratio. As shown in Fig. , the two ends of the CNT channel had a squared silver electrode contact to output the resistance signals. To ensure the even thickness of the sensor, all devices were screen printed one time. The thickness of the CNT film was measured by the cross-sectional optical imaging, which showed a half in-paper thickness of 50 μm and on-paper thickness of just 3.5 μm.