A Fully Inkjet-Printed Strain Sensor Based on Carbon Nanotubes

Abstract

A fully inkjet-printed strain sensor based on carbon nanotubes (CNTs) was fabricated in this study for microstrain and microcrack detection. Carbon nanotubes and silver films were used as the sensing layer and conductive layer, respectively. Inkjet-printed CNTs easily undergo agglomeration due to van der Waals forces between CNTs, resulting in uneven films. The uniformity of CNT film affects the electrical and mechanical properties. Multi-pass printing and pattern rotation provided precise quantities of sensing materials, enabling the realization of uniform CNT films and stable resistance. Three strain sensors printed eight-layer CNT film by unidirectional printing, rotated by 180° and 90° were compared. The low density on one side of eight-layer CNT film by unidirectional printing results in more disconnection and poor connectivity with the silver film, thereby, significantly increasing the resistance. For 180° rotation eight-layer strain sensors, lower sensitivity and smaller measured range were found because strain was applied to the uneven CNT film resulting in non-uniform strain distribution. Lower resistance and better strain sensitivity was obtained for eight-layer strain sensor with 90° rotation because of uniform film. Given the uniform surface morphology and saturated sheet resistance of the 20-layer CNT film, the strain performance of the 20-layer CNT strain sensor was also examined. Excluding the permanent destruction of the first strain, 0.76% and 1.05% responses were obtained for the 8- and 20-layer strain sensors under strain between 0% and 3128 µε, respectively, which demonstrates the high reproducibility and recoverability of the sensor. The gauge factor (GF) of 20-layer strain sensor was found to be 2.77 under strain from 71 to 3128 µε, which is higher than eight-layer strain sensor (GF = 1.93) due to the uniform surface morphology and stable resistance. The strain sensors exhibited a highly linear and reversible behavior under strain of 71 to 3128 µε, so that the microstrain level could be clearly distinguished. The technology of the fully inkjet-printed CNT-based microstrain sensor provides high reproducibility, stability, and rapid hardness detection.

Fully Inkjet-Printed Carbon Nanotube-PDMS-Based Strain Sensor: Temperature Response, Compressive and Tensile Bending Properties, and Fatigue Investigations

“…10 6 µΩ cm for a 10 wt% composition of MWCNTs. Since our reported value is still at least one order of magnitude smaller, we can conclude that the further increase of MWCNT content in our inkjet-printed sensors (25 wt% compared to 10 wt%) results in a lower resistivity and agrees well with the detailed investigation by Yan et al Another reference value is found in the work of Kao et al [26] where they use a commercially available CNT ink (CNT-22, lab311, Korea) for a fully inkjet-printed strain sensor. Unfortunately, the exact composition of the ink is not disclosed and the thickness for a series of samples with different number of passes printed is not given.…”

“…This behaviour should not affect any properties of the device as it occurs only in the first few drops. Similarly, Kao et al [26] report an agglomeration of CNTs at the printing origin due to edge-enhanced evaporation and an overall decrease of CNT density along the printed lines. We do not observe such an extended trend for our printed structures.…”

“…It is demonstrated that the sheet resistance is decreasing with the number of printed layers. Based on this observation, the authors claim that the "increase in the CNTs density [shows that] the sheet resistance decreased with the number of passes" [26]. How-ever, by using the given sheet resistance and extrapolating the layer thickness from values given before in the paper, an almost constant resistivity of ρ = (1.60 ± 0.20) • 10 5 µΩ cm can be calculated.…”

Wearable Sensors for Healthcare: Fabrication to Application

“…Similar to polymers, certain types of nanomaterials, such as carbon-based allotropes [ 42 , 43 , 44 ] and metallic nanomaterials [ 45 , 46 , 47 ], have been used as processed materials to form sensors. The carbon allotropes include Carbon Nanotubes (CNTs) [ 48 , 49 , 50 ], graphene [ 51 , 52 , 53 ], and graphite [ 54 , 55 , 56 ], while some of the types of metallic nanomaterials are nanoparticles [ 57 , 58 ], nanoribbons [ 59 , 60 ], and nano-beads [ 61 , 62 ].…”

Integration of Different Graphene Nanostructures with PDMS to Form Wearable Sensors

“…This caused the researchers to further develop sensors that included flexible materials. The sensors have been devised using extensive printing techniques [ 18 , 19 ] such as screen printing [ 20 , 21 ], inkjet printing [ 22 , 23 ], 3D printing [ 24 , 25 ], offset lithography [ 26 , 27 ], flexography [ 28 , 29 ], and gravure printing [ 30 , 31 ].…”