Highly Sensitive Pressure/Strain Sensor with Nanowire Composite for Skin-attachable Multifunctional Electronics

Abstract

In this dissertation, we describe the highly sensitive pressure/strain sensor with nanowire composite for skin-attachable and wearable electronics. Multiscale structured nanowire composite composed of silver nanowires (AgNWs) and polydimethylsiloxane (PDMS) was designed and fabricated for the high performance multifunctional sensor. Based on this multiscale structured nanowire composite, high performance pressure/strain sensors were fabricated and characterized. Nowadays, flexible and stretchable physical sensors such as pressure, strain, and temperature sensors have been widely investigated for the application to the wearable electronics. Especially, high performance pressure/strain sensors which show high sensitivity, fast response time, and high cycle stability are required. Due to these requirements, we developed the nanowire composite, the flexible and stretchable electrode, and introduced the multiscale structure with nanometer-sized rough surface and micrometer-sized wavy structure to the nanowire composite. By integrating this nanowire composite and polymeric dielectric layer/printed Ag electrode, we fabricated capacitive-type pressure sensors and arrays. High pressure sensitivity (S >3.8 kPa-1), fast response and relaxation time (t <0.15s), high cycle stability (1500-cycle of repeated loading/unloading of pressure and 5000-cycle of repeated bending with bending radius of 3mm), and multiple sensing such as pressure and bending were obtained. Nanowire composite with multiscale structure can be easily scaled up for a large area sensor array and sensor arrays with 3 × 3 and 5 × 5 pixels were fabricated. The sensor arrays can detect the spatial distribution of the applied pressure with the sensitivity as high as that of the single sensor. Wearable fingertip pressure sensor was fabricated to demonstrate the fingertip pressure sensing prototype device. We attached each pressure sensor on the four fingers except a little finger and measured the capacitance change by grabbing plastic beaker. We developed simple method to control the pressure sensitivity of the sensor. Simply, by controlling the mixing ratio of the matrix PDMS of the nanowire composite, we can tune the pressure sensitivity of the sensor. Three types of PDMS with different mixing ratio, a 5:1, 10:1, and 15:1 mixture of liquid PDMS and curing agent, were used to fabricate nanowire composites and sensors. Owing to the difference of Youngs modulus and the shape of the crest area of nanowire composites with different mixing ratio, pressure sensors showed different pressure sensitivity according to different mixing ratio. We investigated the highly bendable pressure sensor with high bending stability and pressure sensing ability in the bending state. By introducing bending sensing part beside the pressure sensing part, the bendable sensor can detect both pressure and bending and distinguish the pressure and bending. We used the surface functionalization method and the PDMS spacer to demonstrate the bendable sensor. Strong siloxane bonding between the surface functionalized bottom plane, PDMS spacers and patterned nanowire composite was obtained and based on this bonding, our sensor showed high bending stability. Pressure sensitivity of the bendable sensor was increased up to 9 kPa-1 owing to the air gap from the PDMS spacer. The bendable sensor can detect ultra-low pressure of 0.7 Pa and show fast response and relaxation time below 0.075s. Even in the bending state, the bendable sensor can detect the normal pressure. By using this bendable sensor, we fabricated the wearable sensor to measure the wrist pulse and vibration of smart phone. Sensor array was also fabricated and we estimated the pressure in the bending state by calculating the capacitance change. We demonstrated the pressure sensitive transistor (PST) by integrating the bendable sensor and prined single-walled carbon nanotube thin film transistor (SWCNT TFT). PST can be operated in low voltage below 5V and ultra-low power consumption of PST below 15 nW can be achieved (I_DS< 15nA, V_DS= -1V). By using the PST and commercially available electronic devices such as LED chip, resistor, OP amp and battery, we fabricated the user inter-active pressure sensing device and pulse monitoring device. Finally, we developed the stretchable multifunctional sensor which can detect the 3-axis force, the normal force and shear force, and strain by using nanowire composite for the application to the electronic skin. By introducing various sensing component to the sensing system, we can obtain multifunctional sensor to mimic human tactile receptors and skin. For the 3-axis sensing, four individual capacitive sensors composed single sensing cell. Our sensor can sense and distinguish the pressure and shear force by analyzing the capacitance change of four individual capacitive sensors. For the strain sensing, flat nanowire composite was used. During the stretching process, the flat nanowire composite was deformed and this deformation resulted in the resistance change of the flat nanowire composite. Our sensor can detect the strain by analyzing the resistance change of the flat nanowire composite.