In recent years, considerable research has been put into the development of printed electronics (PE) as well as the best ways to effectively and efficiently print electronic devices on flexible substrates. This thesis focuses majorly on the design and fabrication of novel flexible sensors using conventional printing processes. Initially, a flexible printed strain gauge was fabricated successfully on a flexible paper substrate using flexography printing process. Silver (Ag) ink was printed on the paper substrate as metallization layer. The performance of the printed device was investigated by subjecting the strain gauge to a 3-point bend test, with a displacement of 1 mm, 2 mm and 3 mm at 3 Hz operating frequency.
The electro-mechanical response of the fabricated strain gauge, as a function of electrical resistance revealed that the sensor has a longer functional life for smaller displacements which is especially advantageous for structural asset monitoring applications. Then, a fully flexible carbon nanotube (CNT) capacitive pressure sensor was developed for the detection of varying applied pressures. The sensor was successfully fabricated using the screen printing technique. Polydimethylsiloxane (PDMS) was used as a dielectric layer and it was prepared using a PDMS pre-polymer and a curing agent mixed in a 10:1 ratio. The electrode design was directly screen printed using conductive CNT ink onto the PDMS dielectric layer. The capacitive response of the printed sensor for varying applied pressures demonstrated the feasibility of employing CNT based electrodes and PDMS dielectric layer for the development of an efficient, flexible and cost effective pressure sensors in sports, military, automotive and biomedical applications.
Finally, a novel printed impedance based electrochemical sensor has been successfully developed on a paper substrate for the detection of various bio/chemicals. This flexible analytical device was fabricated by screen printing a two electrode sensor configuration on a wax-printed chromatography paper substrate. Ag ink was used for screen printing the working and circular electrodes. The electrical impedance spectroscopy (EIS) based response of the fabricated electrochemical sensor revealed the capability of the sensor to distinguish among varying levels (pico, nano, micro and milli) of potassium chloride (KCl) and glucose (C6H12O6) concentrations as well as the potential of employing paper substrate based electrochemical sensors for bio/chemical sensing applications.
INTRODUCTION TO SENSORS
The amplitude and/or the velocity of the propagated wave changes with respect to the characteristics of its surrounding medium (propagation path of the wave) and these changes can be measured using phase and frequency characteristics of the acoustic sensor, which will be in correlation with the input quantity (such as pressure, humidity, magnetic fields, viscosity, temperature, biological matter, chemical vapors, mass) being measured. The concept of the acoustic sensor can be explained using a basic acoustic wave gas sensor (Fig. 2.4).
Optical sensors mainly work on techniques such as light scattering, spectral transmission changes, radioactive loses or micro bending. Optical sensors have many advantages such as electrical passiveness, multiplexing capabilities, wide dynamic range, and high sensitivity. The concept of optical sensors can be explained using the through-beam photoelectric sensor (Fig. 2.6). This sensing system has two components i.e., emitter and receiver in separate housings.
Every sensing system has its own operating boundaries or limits in which the output results will have a linear relationship with the input measure and (Fig. 2.9). Sensors become non-responsive beyond its operating limits for whatever the amplitude of input signal is applied (above some threshold input value). This is called as saturation region and there will be no output of the sensor in this region.
INTRODUCTION TO PRINTING TECHNIQUES
Screen printing is a technique in which a paste-like material (ink) is transferred onto the substrate. Screen printing is typically done either by hand, or semi or fully automated. The screen printer mainly consists of the squeegee, stencil and a fabric screen with the design on it. Typically, screen fibers are made of plastic, natural silk or metal fibers and the squeeze is made of rubber. A section in the screen will be imposed by the design of the desired print and the ink is allowed to pass through it and the desired design is created on the substrate being used. The ink is transferred either by pushing or forcing the ink by squeeze through the fabric screen and thus, this printing process is also referred as push through the process (Fig. 3.2).
Flexographic printing is a well-established R2R high throughput rotational printing method. It is an indirect impact based printing technique that can provide a wide range of ink thickness with the same resolution. The main parts of the flexographic printer are anilox roller, an impression cylinder, doctor blade, and the ink reservoir. Anilox roller (a steel cylinder), that has finely engraved cells on its surface made of chromium or ceramics, collects the specific amount of ink from the ink reservoir. Then, the collected ink is transferred onto the elevated structures of the printing plate. With the help of the plate cylinder, the ink on the printing plate is printed on the substrate as shown in Figure 3.4. It is capable of addressing problems such as contact finger geometry as well as production line output.
A NOVEL FLEXOGRAPHIC PRINTED STRAIN SENSOR ON A PAPER PLATFORM
The strain sensors were placed on the supports of a 3-point bend test fixture (Mark-10 ESM 301 motorized test stand with a vertically movable platform) (Fig. 4.5). The movable platform, capable of moving upwards and downwards was used to apply different displacements (1 mm, 2 mm, and 3 mm ) on the meander trace of the strain sensors in the Y-direction. A Ag conductive epoxy paste (Circuit works CW2400) was used to bond the connecting wires to the contact pads of printed sensors.
A total of 50 strain sensors have been fabricated with different meander trace lengths and widths. For experimental convenience, six sensors (Sensor #1, Sensor #3, Sensor #18 Sensor #42, Sensor #44 and Sensor #45) were chosen and the photographs of the six sensors with different meander trace lengths and widths (Table 1) are shown in Figure 4.1. No analysis was done on the sensor #18 since it had a break in the grid traces.
DEVELOPMENT OF A NOVEL CARBON NANOTUBE BASED PRINTED AND FLEXIBLE PRESSURE SENSOR
The screen printing press and screen were then thoroughly cleaned using ethylene glycol di-acetate. Special care was taken when cleaning the squeegee and screen as to prevent damage and prolong their use. The schematic of the sensor design and picture of the printed sensor with top, bottom, and dielectric layers are shown in Figure 5.1 (a) and 1 (b), respectively.
The experiment setup is shown in Fig. 5 3. A sensor was placed between a force gauge (Mark-10 model M5-200) and vertically moveable platform (Mark-10 ESM 301 motorized test stand) connected to the LCR meter. The sensor can be subjected to various pressures by varying the compressive forces that can be applied due to vertical motion of the movable platform.
DEVELOPMENT OF A PRINTED IMPEDANCE BASED ELECTROCHEMICAL SENSOR ON PAPER SUBSTRATE
The sensor was designed with a working electrode of 1700 µm radius and a circular electrode with inner and outer radius of 2900 µm and 3900 µm, respectively as shown in Fig. 6.1(a). The screen printed sample was sintered using a photonic curing system (NovaCentrix Pulseforge® 1200) at 400 V for a duration of 500 ms for two passes. Figure 6.1(b) shows the fully fabricated electrochemical sensor on wax printed paper substrate.
The experimental setup is shown in Fig. 6.3. EIS measurements were performed on the printed sensor using an Agilent E4980A precision LCR meter, at room temperature. Small outline integrated chip (SOIC) test clips from Pomona® Electronics were used to connect the LCR meter to the printed sensor. Initially, to establish a reference signal, 120 µl of DI water was drop-casted onto the sensor.
It was observed that the reactance part of the impedance is capacitive. A model equivalent circuit for the EIS measurement is shown in Fig. 6.6. The equivalent circuit consists a resistor (R) and capacitor (C) in parallel. Theoretically, at low frequencies, capacitor behaves as an open circuit and real part dominates the reactance part. At higher frequencies, the capacitive reactance gradually decreases and dominates the real part.
CONCLUSION AND FUTURE WORK
In the course of this thesis work, the author successfully demonstrated the fabrication of a flexible strain gauge on the paper platform using traditional printing process i.e., flexography. A carbon nanotube (CNT) based capacitive pressure sensor was fabricated using screen printing process and its capability to monitor pressure has been demonstrated successfully. In addition, an electrochemical sensor was fabricated on wax printed chromatography paper substrate using screen printing technique has also been demonstrated. The performance results obtained through the research work of all the three projects are listed below: In the first research project, the author developed a flexographically printed flexible strain sensor on paper based substrate, using silver (Ag) as a metallization layer. The capability of the printed strain sensor to detect changes in its resistance due to the displacement of the sensor was investigated by using 3-point bend test on a Mark-10 ESM301 test stand.
Based on the experience gained during the course of the thesis, the author believes that the current three projects can be improved by implementing some of the following suggestions.
A Novel Flexographic Printed Strain Sensor on Paper Platform
1. Use stretchable inks such as silver flexo ink from Liquid X as metal layer to avoid the breakage of the conductive lines to withstand even higher displacements.
2. Use DuPont Tyvek paper as a substrate which offers high flexibility and resistive to deformation and can withstand higher displacements.
Development of a Novel Carbon Nanotube based Printed and Flexible Pressure Sensor
1. Improve the adhesion between the CNT ink and PDMS by modifying the surface properties of the PDMS.
2. Apply passivation layer onto the both sides of electrodes before testing to avoid leaching of the CNT ink due to poor adhesion.
Source: Western Michigan University
Author: Dinesh Maddipatla