A Flexible and Highly Sensitive Piezoresistive Pressure Sensor Based on Micropatterned Films Coated with Carbon Nanotubes

1MEMS Laboratory, Department of Precision Instruments, Tsinghua University, Beijing 100084, China 2State Key Laboratory of Precision Measurement Technology and Instrumentation, Tsinghua University, Beijing 100084, China 3School of Aeronautical Manufacturing Engineering, Nanchang Hangkong University, Nanchang 330063, China 4School of Information Engineering, Nanchang Hangkong University, Nanchang 330063, China 5i-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215125, China

The micropatterned plastic film method is effective in enhancing sensitivity and ductility and reducing response time of microsensors [13].For example, Bao's group demonstrated an ultrasensitive resistive pressure sensor with rapid response and a low-pressure regime of 0.8 Pa by using an elastic microstructured conductive polymer [14].Wang's group presented a kind of self-powered pressure sensor based on micropatterned plastic films, which have a detection limit of 13 mPa [15].Cheng's group reported on a new bioinspired strategy to fabricate flexible pressure sensors with touch sensitivity of 50.17 kPa −1 [16].Because of the superior mechanical flexibility and great conductivity, carbon nanotube had also been applied as a conductive layer of micropatterned film to further improve the performance of the sensor [13,[17][18][19][20].For instance, Zhang's group presented silk-molded, flexible, micropatterned electronic skin coated with single-walled carbon nanotubes, which has ultrahigh sensitivity (1.8 kPa −1 ) for the detection of minute forces with fast response time [17].Meanwhile, because of its great conductivity, carbon nanotube applied as a conductive layer may result in low consumption of the flexible sensors [21].
In this paper, a flexible pressure sensor that uses micropatterned films coated with carbon nanotubes is fabricated and is applied to measure the fluid pressure in the curved microtube.Compared with commonly used MEMS pressure sensors, the proposed flexible pressure sensor is thinner and more flexible and has lower consumption.So, the thin and flexible pressure sensor is easier to be installed on the surface of the curved microtube and has less influence on the measured flow field.

Fundamentals and Methods
The pyramidal-shaped micropatterned PDMS layer is applied in our flexible pressure sensor, whose thickness is 40 ± 10 m, and it can be economically fabricated [22].The schematic of measuring the flexible microtube's pressure using the flexible pressure sensor is shown in Figure 1, which consisted of two PDMS films with micropatterned structure on their contact surfaces for enhancing sensitivity and reducing hysteresis [14,17,23].The PDMS contact surfaces are coated with singlewall carbon nanotube layer to improve conductivity.Given that the resistance of the micropatterned film is very sensitive to the applied vertical force, the flexible pressure sensor is adhered upon the surface of the perforated flexible microtube to sense the vertical force which is generated by the fluid pressure in the microtube.Thus, the wall pressure of the microtube could be measured in real time by monitoring the resistance of the micropatterned film.Two examples for measuring gas and liquid pressure were provided as follows.
According to the fundamentals and analysis mentioned above, the gas pressure measuring experimental system is illustrated in Figure 2(a), which comprises a flexible pressure sensor, a perforated flexible microtube, a pump, a valve, a standard pressure gauge, a picoammeter, and a computer.Herein, the flexible pressure sensor is adhered upon the hole on the perforated flexible microtube's surface.The gas pressure in the flexible microtube is monitored by the standard pressure gauge and is adjusted by the pump as well as the valve.The electric output signal of the flexible pressure sensor is recorded in real time by the picoammeter.The pressure in the tube will be measured when the micropatterned film is subjected to pressure through the hole on the tube, as shown in Figure 2(b).The computer receives the measurement data and then processes and displays the results.Figure 3 shows another method that utilizes the flexible sensor to measure the liquid pressure.In this case, the flexible pressure sensor is directly attached to the curved surface of the sealed liquid pressure chamber.The gas pressure transfers to the liquid, and then the liquid pressure is transmitted to the flexible pressure sensor attached to the wall of the liquid chamber.The liquid pressure in the chamber is also measured by the standard pressure gauge and is adjusted by the pump as well as the valve.Other parts of the experimental system are similar to those in the gas experimental system: the electric output signal of the flexible pressure sensor is recorded in real time by the picoammeter.The computer the measurement data and then processes and displays the results.

Results and Discussion
Figure 4 shows the SEM images of the pyramidal-shaped micropatterned film, which is an array of pyramid microstructures made by PDMS coated with conductive single-wall carbon nanotube layer (inset in the top view SEM image).The experimental results of measuring the gas pressure are shown in Figure 5.There is a positive correlation relationship between gas pressure and current.The sensor's sensitivity  can be defined as where  is the pressure sensor's current when pressure is applied on the device;  0 is the pressure sensor's current in the condition of the initial pressure; p is the applied pressure [17].
As it is shown in Figure 5, the curves of the current versus the pressure could be divided into two appreciated linear parts.In the low-pressure region (0-30 mmHg), the sensor has higher sensitivity (4.7 × 10 −2 kPa −1 ) calculated by (1), while, in the range from 4 to 13.3 kPa (30-100 mmHg), the sensitivity of the sensor is 1.1 × 10 −3 kPa −1 .And the curves of liquid pressure versus current of the sensor are shown in Figure 6, which are also positively related.The calculations show that the proposed sensor also has high sensitivity (5.6 × 10 −3 kPa −1 ) in the low-pressure region (0-140 mmHg).The sensitivity of the sensor is 1.8 × 10 −3 kPa −1 when the liquid pressure changes from 140 to 300 mmHg.The flexible sensors show lower sensitivity in the high-pressure region, which is mainly determined by the characteristics of pyramidal-shaped microstructures.When the pressure comes to the high region, the deformation of the peak of pyramidal-shaped microstructures is harder than the lowpressure region, which results in lower sensitivity.In both cases, the proposed sensors all have low power consumption (<55 W and <180 W, resp.).The red curve is the output current with the increase of pressure; the blue curve is the output current with the decrease of pressure.
The experimental results show that, compared with the commonly used MEMS pressure sensors, the proposed flexible pressure sensors have higher sensitivity, lower consumption, and ultrathin size.For instance, NovaSensor's NPC-100, a type of commercial MEMS pressure sensor, has pressure sensitivity of 3.75 × 10 −5 kPa −1 and consumption of 20 mW with a thickness of 4.32 mm [24].Moreover, the advantages of the sensor also include that the sensor's measurement range could be adjusted by selecting different parameters of the micropatterned film.Furthermore, due to its high and ductility, the sensor could be easily attached to the curve surface of the microtube with minute impact on the flow field.

Conclusion
In this paper, we demonstrate a flexible, highly sensitive, low consumption, and ultrathin sensor based on pyramidalshaped micropatterned films coated with carbon nanotubes for measuring the fluid pressure in the curved microtube.The experimental results show that the flexible sensors can conveniently measure the pressure of the liquid and the gas in the curved microtube.Moreover, the flexible sensor has high pressure sensitivity of 4.7 × 10 −2 kPa −1 in gas sensing, 5.6 × 10 −3 kPa −1 in liquid sensing, low consumption (<180 W), and ultrathin thickness of 40 ± 10 m.We envision that the flexible pressure sensors could be used in implantable medical pressure sensing, industrial pipeline pressure sensing, microfluidic chip pressure sensing, and smart skin pressure sensing.

Figure 1 :Figure 2 :
Figure 1: Schematic of measuring the flexible microtube's pressure using a flexible pressure sensor.

Figure 3 :
Figure 3: Diagram of the liquid pressure experimental system.

Figure 4 :
Figure 4: SEM images of the micropatterned film.

Figure 5 :
Figure5: Gas pressure in the curved tube versus current at constant voltage = 1 V.The red curve is the output current with the increase of pressure; the blue curve is the output current with the decrease of pressure.

Figure 6 :
Figure 6: Liquid pressure versus current at constant voltage = 1 V.The red curve is the output current with the increase of pressure; the blue curve is the output current with the decrease of pressure.