Design and evaluation of "Lab-on-paper" sensing platforms based on conductive polymers

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DESIGN AND EVALUATION OF “

LAB-ON-PAPER”

SENSING PLATFORMS

BASED ON CONDUCTIVE POLYMERS

By

Walter Julián Rosas Arbelaez, B.S.

Presented to the Chemical Engineering Department of the University of Los Andes In partial fulfillment

of the requirements for the Degree of

Master in Chemical Engineering

University of Los Andes

Bogota D.C.

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Dedication

To my beloved mother, for her infinite patience, support and love. To my father (R.I.P) who encouraged me to be a better scientist and person

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Acknowledgments

During the work on my research I have received support, help and guidance from a lot of people whom I am very grateful with. I would like to express my special thanks from the bottom of my heart to my supervisor Dr. Watson L. Vargas for his tremendous amount of support, advice, patience, and encouragement and for providing me the opportunity to work in his group with sufficient freedom to complete my work. I also would like to acknowledge the members of my dissertation committee, Dr. Rocio Sierra, and Dr. Fredy Segura for their valuable feedback and corrections.

I would like to express my special appreciation to Eng. Sara Segura, for his advice, support, assistance and guidance and for sharing her knowledge in electronic engineering and mainly her friendship. I wish to express my gratitude to M.S. Alvaro Achury for his guidance, support during the measurements, and sharing his knowledge in electronics. I also wish to thank my colleagues and friends in Dr. Watson Vargas´s group for all their support, understanding and valuable friendship.

Finally, I deeply appreciate the encouragement and devotion from my mother Luz Marina Arbelaez, she has provided love and emotional support throughout my life.

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Figure 31. Log Impedance of different compounds on a screen printed silver sensor (Thickness: 40 µm). Figure 32. Phase Plot of different compounds on a screen printed silver sensor (Thickness: 40 µm). Figure 33. Log Impedance of different compounds on a screen printed PANI sensor (Thickness: 85 µm). Figure 34. Phase Plot of different compounds on a screen printed PANI sensor (thickness: 85 µm)

Figure 35. Log Impedance of different compounds on a screen printed PANI sensor. (Thickness: 125 µm). Figure 36. Phase Plot of different compounds on a screen printed PANI sensor.(Thickness: 125 µm).

Figure 37. Experimental imaginary part of impedance and calculated according to Eq. 7 for 50% ethanol-water solution on printed PANI sensor.

Figure 38. Experimental imaginary part of impedance and calculated according to Eq. 7 for 50% ethanol-water solution on printed PANI sensor.

Figure 39. Experimental setup with the VNA and the coupling antenna with the sensor. Figure 40. Results of wireless measurement on a silver sensor by cover mask deposition. Figure 41. Results of wireless measurements on printed PANI sensor by cover mask deposition. Figure 42. Results of wireless measurements on printed PEDOT sensor by cover mask deposition. Figure 43. Results of CO detection on the printed silver sensor and the printed PANI sensor

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Figure 44. Setup of the preliminary pressure sensor.

Figure 45. Electrical resistance registered after the thin film was rolled once. Figure 46. Electrical resistance registered after the thin film was rolled twice. Figure 47. Results of Strain Gauge Test on printed PANI 5.cm stripes. Figure 48. Results of deformation by angle.

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List of Tables

Table 1. Volume ratio for modified inkjet printing ink, using a total volume 0f 10ml as base. Table 2: Results of EDS analysis of commercial conductive ink.

Table 3. Conductivities of the synthetized polymeric solutions.

Table 4. Roughness and thickness of the thin layer. Measurements performed on a PANI -PSS modified ink, printed on paper.

Table 5. Computed and experimental capacitances of PANI sensor by cover mask deposition Table 6. Computed and experimental capacitances of PANI sensor by screen printing

Table 7. Correction factor of substances using PANI sensor screen printing

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Chapter 1: Introduction

Currently, there are a variety of techniques to detect different types of compounds or substances in different areas of interest, such as environmental monitoring and pollution control, health and medicine, industry, social issues, among others. These techniques have significant sensitivity for such applications, but the biggest drawback of these techniques is that they are not feasible to implement in the field, given its significant cost and its size which might not be practical for transport and use in remote areas [1,2].

In addition, sensors are related to electronic features; therefore it is important to take the electronic characteristics into account. Hence, the growing demand of electronic products for functionality, high performance, adequate reliability and rapid product delivery is making the electronic industry find alternatively producing methods to satisfy the demands. Future electronic devices are required to connect “everything to everyone” in an attractive form especially in the high technology electronics field [3]. Besides, low-end products are becoming more cost-sensitive along with the emerging issue of manufacturing systems and materials that would lower overall product costs [4]

Printing technologies are taking advantage of cost-sensitive, using a wide range of materials. In some applications, each type of printing technology offers specific and interesting advantages. Currently, electronic manufacturing can satisfy the low cost, high performance demands by combining several printing methods on cheaper substrates.

This thesis evaluates the implementation of different screen printing technologies and low-cost material such as paper for different sensing applications and as an alternative material for other high-end electronic fields. In this work, a conventional architecture of a sensor is proposed which is implemented with conducting polymers inks and compared with a conventional metal ink and substrate combination. Such comparison serves as the purpose of establishing the differences on selectivity for different sensing applications.

Moreover, this thesis also discusses the different challenges of the printing techniques, its limitations and potentials in order to satisfy several requirements in the electronic and sensing fields.

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Chapter 2: Background

2.1 Chemical Sensors

In recent years, sensors specifically chemical sensors and biosensors have become of significant importance in science, chemistry, medicine, electronics and biotechnology. Modern sensors are based on the same principles as the human senses, including, also, human intelligence. Sensors can be defined as devices which provide an output in response to a specified measurand [5]. The output is usually an electrical quantity since data processing is still based on electronic circuits. On the other hand, the measurand is a physical, chemical or biochemical quantity, property or condition [6]

Chemical sensing is part of a process in which a response can be obtained about the chemical composition of a given system; in such process, an amplified electrical signal results from the interaction between a chemical compound and the sensing structure. The interaction includes two main steps: recognition and amplification.

Figure 1. Sensing principle of chemical sensors [7]

Usually, there exist two types of interactions: a surface interaction in which the species are adsorbed on the surface, and a bulk interaction in which the compound of interest is partitioned into the sample and the sensor, and is absorbed.

Reversibility in the context of chemical sensors means that the output can change as concentration changes, both up and down [7]. Thus, sensors can be either thermodynamically reversible or irreversible. If they respond to a step up or a step down in the concentration of the substrate, they are said to be “reversible” in sensing.

Selectivity is the most important issue in chemical sensors. It determines what type of application the sensor can be used in. Selectivity can be defined as the ability to respond under

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the presence of specific species of interest in presence of other species. Therefore, one of the most important aims is to design a selective layer in such a way that the sensor has a wide dynamic range and low detection limit; it also means that the layer rejects others interfering species and has a unique response to the analyte of interest.

A specific chemical compound can be detected by more than one type of chemical sensor, thus there is a wide range of chemical sensors. Chemical sensors can be classified according to the working principle into the following basic groups [7, 8]:

- Electrochemical: devices that transform the effect of the electrochemical interaction analyte-electrode into a useful signal.Such effects may be stimulated electrically or may result in a spontaneous interaction at the zero-current condition. (Potentiometric, voltamperometric, coulombometric, conductimetric).

- Optical: devices that transform changes of optical phenomena, which are the result of an interaction of the analyte with the receptor part. (Spectrophotometric, luminescence, opto thermal, refractive index, light scattering).

- Electrical: devices based on measurements, where no electrochemical processes take place, but the signal arises from the change of electrical properties caused by the interaction of the analyte (ion selective, field effect transistors, metal oxide semiconductors, organic semiconductors, electrolytic conductivity sensors).

- Sensors sensitive to a change of mass: devices that transform the mass change at a specially modified surface into a change of a property of the support material. The mass change is caused by accumulation of the analyte (piezoelectric, acoustosurface, body-wave sensors)

Besides, it is important to understand that optimal sensory systems should display all the features mentioned before such as selectivity and reversible mechanisms. While some elements can be addressed by instrumentation, the design of robust, sensitive and selective sensory materials remains as an emerging issue.

2.2 Conducting Polymers

Conducting polymers have been investigated in recent years for their chemical and electrochemical properties. The wide range of conducting polymers, their many derivatives, moderate range of doping options and many routes of synthesis result in a large range of tunable material properties such as conductivity, sensitivity, selectivity and redox potential. Thus, this allows them to interact with many different chemical species. A key advantage of

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these materials over other devices using small molecule elements is the potential of polymers to exhibit properties that are sensitive to minor perturbations [9]

Among the most widely conductive polymer used as sensors are polypyrrole (PPy), polyaniline (PANI) and poletilenodioxitiofeno (PEDOT) (See Figure. 2). These polymers have shown a good response to different compounds both gaseous and liquid

Figure 2. Examples ofconductive polymers [10].

Chemical sensors are analytical devices that convert the potential energy of a target analyte in a measurable signal, either electrically or optically. The versatility of active electro conductive polymers lies in the ability of such materials to allow a wide range of molecular interactions, which are subjected to modification during their synthesis [2]

Features may also be monitored after synthesis using electrical stimulation, magnetic or both. This ability, coupled with the fact that these interactions are intimately involved in determining the electrical properties of these conductive polymers, makes them strong candidates for implementing sensors.

Among the different substances detected by these polymers are ammonia, ethanol, methanol, among other organic solvents and contaminants. These polymers can be easily processed using electroplating techniques, self-assembly or "spin-coating"[11]

In recent years these materials have been coupled with nanotechnology, since it improves the properties of the materials at the nano-scale and may be combined with different other materials, thereby improving many properties of both materials [10].

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15 2.2.1 Nanostructured conductive polymers

Today, nanotechnology allows conducting materials to be studied and applied in chemical sensing in novel ways. Controlling the structure of the conducting polymer at nanoscale allows enhancing the properties of the polymer, processes that are not possible with traditional bulk materials.

Nanostructured conductive polymers have several advantages over their bulk counterparts. First, the synthesis can be controlled at the nano-scale; therefore the level of order and structure reproducibility could be improved. Secondly, nanostructured materials have more enhanced surface-area-to-volume characteristics which increase the chemical interactions and electron transport processes. Thirdly, nanostructure materials allow having larger range of composited materials with new physical, chemical, magnetic and electrical properties. Lastly, many fabrication techniques allow more control of both synthesis of the material and deposition of the nanomaterial, resulting in improvements in the fabrication of the sensor [9].

The nanostructured conducting polymers which find use in chemical-sensing applications should achieve two main objectives. They should lead to an improvement in the performance of the sensor itself, in terms of the selectivity, sensitivity, limit of detection which is the lowest value of concentration that can be detected by the sensor, stability and interference rejection which is how the sensor can avoid other signals present during the measurements. On the other hand, they should lead to improvements in materials processability and feasibility to produce working sensor devices.

2.3 Lab-on-a-chip and Lab-on-paper

Lab-on-a-chip (LOC) is a nanotechnology tendency which has emerged in the last decade; it allows using different platforms for different application areas such as biotechnology, diagnostics, medical or pharmaceutical industries. While its importance and utility is widely acknowledged and extensive research has been conducted in the laboratory on device manipulation and demonstrations, there are few commercialized LOC products that are fabricated using clean-rooms [12].

This is partly due to the fact that LOC devices, although much more simplified than conventional analytical instruments, are still not readily accessible to those in developing countries [12].

On the one hand this is due to a lack of lithography-based clean-room infrastructure for the construction of LOC devices where channels, pumps and valves are created on a plastic (or glass, silicon) substrate, processes that can be both complex and expensive [12].

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The other related issue deals with the lengthy regulatory approval of LOC devices. Recent developments suggest that bioassays on paper substrates may be another interesting alternative for both existing porous substrate devices and solid-support “classical” labs on a chip [13].

Paper, normally made of cellulose fibers, is abundant, inexpensive, sustainable, disposable, easy to use, store, and transport, easy to modify physically and chemically and familiar to the public, but perhaps its most important advantage is that an enormous variety of printing techniques is available nowadays for its functionalization.

Paper-based devices have many advantages:

 They are easy to manipulate (particularly suited to nontechnical personnel), inexpensive, low volume, easily adaptable, and are capable of rapid on-site detection.

 Paper is made of naturally abundant materials (i.e., cellulose), and is biodegradable. Moreover, both simple paper strip tests and more complex paper-based microfluidic devices require no or little external power sources.

 Paper products can be easily manufactured on a large scale by the well-established coating and printing techniques which can further lower the cost of the final products

 From the business and marketing perspectives, paper-based kits require a relatively small investment and can get into the market easily, which can facilitate its wide use in practical applications [14, 15].

Despite the mentioned advantages, the stability of the sensor over time becomes an important issue for study, because these devices are designed to be stored and transported over long periods of time. Additionally, the science behind the fluid flow through the matrix of the paper, and bio-sensing, bio-mobilization on paper is not completely understood [16].

Paper-based sensors have a potentially large number of applications. Among these applications are potential areas such as environmental monitoring, food safety, filtration systems, defense and military security [17].

Given the ability to print a variety of existing materials including polymers, metals, nanoparticles, and biological species, among others, it is possible to conceive a nearly infinite number of possibilities for this type of platform. Additionally, new paper-based sensors have more functions than the conventional sensor; an example of this is the possibility of incorporating processes separation, purification, biochemical reactions, detection and communication signals [18].

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2.4 Printing Techniques

Offset lithography, screen, gravure and inkjet printing are the dominant printing techniques, though almost all the previous techniques have been used to satisfy performance, resolution, ultra-low cost efficiency, and high volume requirements for sensors manufacturing. The processes described below and additive printing process have their individual operations conditions, the rheology and drying time of ink and the conditions of curing might vary. These parameters affect the formulation of the ink. An important property is the wetting capability of the substrate, which is closely related to the adhesion strength on the substrate. Printed and cured structures should be electrically functional and mechanically flexible for optimal performance.

2.4.1 Inkjet printing technology

The electronics industry relies on PCB fabrication methods, such as lithography, mechanical milling, or screen printing techniques. The majority of circuits fabricated on PCB use a glass fiber FR-4, while screen printing is used on PET (polyethylene terephthalate) circuits fabrication. Both materials are non-biodegradable substances contributing to the enormous bulk of the electronic waste generated [19].

However, in recent years inkjet printing has been studied for the fabrication of sensors, using paper as substrate. Paper is highly biodegradable requiring short times to break down in landfills, allowing it to become in a promissory “green” material for the elaboration of circuits [20].

Inkjet printing is a non-contact method, which uses inks jetted repeatedly from the printing head to form small drops that can be directed onto the substrate [21].

Inkjet printing methods can be classified based on print-head types, that is, in continuous mode and drop-on-demand (DoD), Figure 3 provides a general classification of inkjet printing technologies. The main difference between these two alternatives is that in continuous mode, only a part of the generated flow of ink drops is directed onto the substrate according to the illustration in Figure 4. On the other hand, in DoD processes drops of ink are only generated if the image to be printed required them [21].

This project focuses mainly on the drop-on-demand technique. The DoD techniques can be divided into thermal, piezoelectric, electrostatic, and acoustic (see Figure 3).

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Figure 3. Map of inkjet printing techniques. Adapted from Ref.[21]

2.4.1.1 Drop-on-demand inkjet technology

DoD inkjet technique uses a CAD image file and provides high speed and scalability, allowing the use of high frequency multiple nozzles. On the other hand, different types of inks can be printed which is similar to the color inkjet printer [22]. DoD processes can be classified according to how the individual drop is generated. In this study, drop-on-demand thermal inkjet is used; the thermal inkjet printer has an electric heater inside the nozzle that quickly raises the temperature of the ink facilitating the flow. The heated ink generates a bubble, which due to the expanding process, ejects a drop of ink through the nozzle. Water-based inks are preferable, because they produce more explosive bubbles than other solvents [22]. A fundamental reason as to why in this study all the used and synthesized inks are water-based.

INKJET TECJHNOLOGY

Continuous

Microdot Hertz

Binary Deflection Multiple Deflection

Drop-on-Demand (DoD)

Super-Fine

Piezoelectric

Bend Mode

Push Mode

Shear Mode Squeeze Tube

Electrostatic

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Figure 4. Schematic of the DoD inkjet printing system. Piezoelectric system [21].

2.4.1.2 Inkjet printing: versatility

In addition to forming less material waste than other techniques through its selective ink spraying ability, inkjet printing also offers a unique versatility related to the material deposition. Unlike bombarding substrates with plasma or ion vapor, inkjet printing uses electrostatic potential to deposited charged ink in inkjet cartridges on a substrate. The print-head sprays out controlled ink through multiple microscopic nozzles 254 microns apart each other about 21,5 microns in diameter. The series of nozzles also have a ﬁlter to retain large particles from the deposited ink size around 0.2 microns [19].

The versatility of inkjet printing with regards to material deposition make it ideal for sensing elements as well as functionalized nanostructures onto circuit platforms for wireless or near- ﬁled sensor telemetry constituting an extremely low-cost alternative to conventional material deposition technologies.

Inkjet printing technology, as required for sensor technology development, can be quite expensive and therefore unreachable in the current circumstances to many developing nations. Therefore, a much more suitable and available low-tech alternative is the use of screen printing.

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20 2.4.2 Silk Screen or Serigraphy

The silk screen process is one of the very few handicraft processes that are not only surviving the machine era but even displacing it in many instances [23]. The silk screen process was first used in USA about the beginning of the twentieth century. It was first limited to rough and simple “show cards”. Today, “show cards” are still being done by silk screen, but the other uses of the process shadow this one. [23].

The screen is made of a piece of mesh stretched onto a square or rectangular frame. The screen fabric may be silk, nylon, wire, cotton organdy, but usually polyester. A stencil is formed by blocking off parts of the screen on the negative image of the design to be printed. When ink is deposited onto the screen and frame assembly, pressure is required to spread and push the ink through those areas where the screen is not blocked by the stencil. This is accomplished by the use of a squeegee, a flexible plastic or rubber blade supported by a holder. When the ink passes through to the surface below, that surface is printed with the image defined by the stencil [24] (see, Figure 5).

Figure 5. Schematic of screen printing process.

Although, silk screen cannot approach the level of detail of inkjet printing or lithography, it makes up for this by the versatility in inks that can be used as well as its simplicity. Besides, silk screen process has two advantages; first, its initial cost is much less, for runs under several thousands, it will successfully compete with any other process. The second advantage is that since silk screen is a basic stencil process, it can print on substrates that a hard metal plate cannot. It is one method that can successfully print on glass or wood[23], as well as other less usual substrates.

The screen printing has widely developed over the last twenty years, mainly for three aspects:

 Width of the deposition, thanks to the substitution of traditional masks into new ones made of steel and obtained by means of a laser, leading to films characterized by a minimum width of 20 µm.

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 Serigraphic pastes, whose improvement relies of a great quantity of shapes in different applications, based on functional materials.

 Miniaturization and compactness of the planar structures of thick films and multilayer configurations were recognized to be appealing characteristics in the electronic field.[25] Nowadays, this technique is also used by the semiconductor industry to produce thick-film hybrid circuits. Also, many others fields of application for this technology, e.g. Television sets, computers, mobiles, and electronics for automotive industry. All these applications converge into one phrase “printed electronics”. Hence, screen printing is already in use for the production of printed electronics components. For printed electronics the ability of a screen press to apply a wide range of ink thickness is critical, because complex products require densities that are rarely uniform [26].

This thick film technology has been recently applied to the sensing field, both for the passive electronic components and for the sensors [25]. However, the combination of the mass production capability to a relatively low cost and the reliability of devices in difficult operating conditions make planar thick film technology one of the most suitable for sensor printing with the following requirements [25]:

 Sensitivity to low pollutant concentrations

 Reliability

 Repeatability

 Low energetic consumption

 Moderate sizes

 Low ratio cost/performance

 Automation possibility

Biosensors have been widely produced using this technique, since the need of biosensors has increased over the last few years [27, 28]. Recent research for instance, has focused on the fabrication of the glucose sensor [27]. An alternative method has been proposed in which a paste, prepared by mixing the enzyme and ferrocene in paraffin is pressed into an electrode pit [20]. Other researchers have been working on enzyme-sensors such as a lactate sensor [28], and other electrochemical sensors.

However, these sensors have been produced on “hard” substrates such as polymers, silicon, metals and glass, making these sensors to be higher in costs and less environmentally friendly. Hence, paper could open a new research area, since it supports capillary flow enabling sample transfer onto desired spots or zones in a controlled manner without the need of pumping. Besides, paper is biodegradable making itself a promissory material to fabricate sensors. Recently, researchers have reported the first SERS sensor on paper [29].

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Additionally, chemical-sensor arrays have been fabricated in a batch process using screen printing as the main technique, for reasons of cost and reproducibility, the sensor-specific structures should be mass fabricated, as are the microelectronics. The challenge lies in merging the standard semiconductor process sequence with the non-standard steps used to form the transducers. It has been demonstrated that screen printing can be used to partition the fabrication into two distinct sequences, semiconductor processing and sensor-specific steps [30].

The interdigitated capacitive chemical sensors have been produced by inkjet printing, screen printing among others. Due to their feasibility and low-cost to fabricate in different type of substrates, and also good responses in the presence a wide range of chemical compounds, they have become a special interest for some researchers. In this study we will focus our attention on this specific type of sensors and use the screen printing technique as an alternative to ink-jet printing, given the current high cost of ink-jet printers for sensing applications.

2.5 Interdigitated Capacitor-based (IDC) Sensor

Interdigitated capacitor electrodes are among the most widely used periodic electrode structures for the chemical sensing applications due to several advantages such as one-side access, control of signal strength, and simplification of modeling. The basic principle of an IDC sensor relies on applying a spatially periodic fringing electric field into the material under test. The combination of this periodic fringing field with the excitation frequency provides the information about the dielectric spectroscopy of the material under test [31].

These electrode arrays have become more prominent as a sensor device due to the ongoing miniaturization of electrodes and the low cost of these systems [32]. An important advantage of these sensors is its simple and inexpensive mass-production process and the ability to use them over a wide range of applications without significant changes in the sensor design [33]. Another benefit is the ability to integrate electrodes with instrumentation to develop autonomous chips. Currently they have been used for the detection of capacitance, dielectric constant and bulk conductivity in chemical and biological systems [34].

A useful method for the calculation of the capacitance of an IDC is the conformal mapping technique. This method provides closed form expressions for the computation of the capacitance of the interdigitated electrodes, mainly, based on the geometry and properties of the sensor [35].

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23 2.5.1 Conformal mapping techniques

A conformal mapping technique can be used to convert a coplanar geometry into parallel plate capacitor, for which the capacitance is a linear function of dielectric constant. Closed-form expressions for the coplanar line impedance characteristics and its dielectric constant are obtained in terms of finite dimensions [36].

Figure 6. Cross-section view of coplanar strip line with fringing electric field [36].

Figure 6 shows the cross section view of coplanar strip line. This figure allows computing the geometric parameters used to predict the theoretical capacitances for this type of mapping. Some authors have assumed that the total capacity of the signal strip and the adjacent ground strip is the sum of the line capacity C1 and C2, where C1 is the line capacitance of coplanar strips with, usually, air layer above and below and C2 is the capacitance of coplanar strips with the dielectric substrate layer present. Because of this reason, C2 is calculated with a dielectric constant of εsub -1, because this region is included in the computation of C1. This technique is so called partial capacitance technique.

The capacitance of the coplanar strip lines with air layer only is estimated by:

(1)

Where, K (k1')/ K (k1) is a ratio of complete elliptical integral of the first kind and ε0 is the free space permittivity (ε0=8.85x10-14 F/cm). The ratio of the integral can be estimated using Hilberg’s approximation, which is given in general form as:

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( √

)

( √ ₎

Where

( ) √

( )( ) (2)

The result for the second capacitance is given as:

(3)

Where

( ) *₍₎₊

√ * ( )+ * ( )+ *₍₎₊ * ₊

And

√ ₍₅₎

Therefore, the total line capacity between the signal strip and the adjacent ground strips is:

(6)

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This conformal mapping technique lead us to have an approach of the capacitance of the sensor, taking into account only the variables regarding the geometry of the sensor used for this project. Some theoretical assumptions are taken such as fully conducting material and homogeneous dispersion of the layer onto the substrate. Nevertheless, with this information we determine some properties which can contribute with the final response of the sensor.

Chapter 3: Methods and Measurements Techniques

3.1 Synthesis

The following sections describe the different synthesis approaches used in the formulation of conductive inks both based on metals and conductive polymers.

3.1.1 Synthesis of water-dispersable Polyaniline-Polystyrenesulfate (PANI-PSS) solution.

PANI-PSS solution was synthetized according to the procedure described by Jyongsik Jang et al [37]. Aniline monomer (0,6 mmol) was introduced drop wise in 0.5 M HCl solution (40 ml) and was stirred for 1 h. An aqueous PSS solution was prepared by using 0.3 g NaPSS in 100 ml of distilled water, then it was poured to the mixed solution and was stirred for 1h. The polymerization was conducted by using ammonium peroxydisulfate (APS) (0,7 mmol) as an oxidizing agent for 12h at room temperature. After polymerization a dark green solution was obtained. The final solution was filtered in order to remove particles which were not dissolved. 3.1.2 Synthesis of water-dispersable polyethylendioxythiophene-polyestyrensulfonate (PEDOT-PSS) solution.

PEDOT-PSS solution was synthetized by the same method mentioned previously. EDOT monomer (0,6 mmol) was introduced drop wise in 0,5 M HCl solution (40 ml) and was stirred for 1h. An aqueous PSS solution was prepared by using 0,4 g NaPSS in 100 ml of distilled water, then it was poured to the mixed solution and was stirred for 1h. The polymerization was conducted by using ammonium peroxydisulfate (APS) (0,9 mmol) as an oxidizing agent for 12h at room temperature Finally, the solution was black color and then filtered.

3.1.3 Synthesis of water-dispersable PANI-PSS/Pd-NPs solution and PEDOT-PSS/Pd-NPs solution.

PANI-PSS/Pd-NPs and PEDOT-PSS/Pd-NPs solutions were prepared according to Anjali A. Athawale et al [38]. Pd nanoparticles were synthesized by a thermal reﬂux method. The reaction mixture (100 ml) containing 1 × 10−3 M PdCl2·5H2O and 0.1 M aniline (stabilizer) with methanol–

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water (1:1.5) as a solvent was reﬂuxed for 3 h at a temperature of 50◦C. PSS was added to the solution and stirred for 1 h. Ammonium persulphate solution (0.1 M) was used as an oxidizing agent for the preparation of the nano-composite.

For PEDOT-PSS/Pd-NPs solution the time of the reflux was modified, taking only 4 hour because of the slow oxidation of EDOT.

3.2 Design of the IDC Sensor

A simple IDC sensor was fabricated using 3 different techniques: 1. Inkjet Printing

2. Screen Printing

3. Cover mask deposition

All modified inks were printed using a household HP 845 C inkjet printer (see Figure 7). The cartridges (see Figure 8) used for printing had 300 nozzles (25 microns diameter), each dispensing a droplet volume of 33 µl. This enabled deposition of lines (3 cm x 1 cm) with a resolution of 600 x 600 dpi. Commercial 40 weight paper was used as substrate for all printings. Also, different layers were printed in order to obtain the number of layers that favors the lowest resistance.

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Figure 8. Commercial HP 15 ink cartridge used in the study.

The cartridges used in the study were acquired and refilled with the modified ink according to the following procedure.

- The cartridges were bought both as new ones and used ones.

- The cartridges were rinsed with isopropyl alcohol using ultrasonic device for about one hour.

- They were totally dried under fume cabin overnight.

- The cartridges were refilled with the different modified inks as shown in Table 1.

- Each cartridge was able to be reused from two to four times, after these refilling cycles, it was not able to work in the printer anymore and it was discarded.

3.2.1.2 Inks preparation for printer

Commercial inkjet-printing ink was purchased for the preparation of in-house formulations of ink. Different volume ratios of commercial ink and polymer solution (see Table 1) were mixed and evaluated based on the lowest electric resistance as criteria for comparison.

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Table 1. Volume ratio for modified inkjet printing ink, using a total volume of 10ml as base.

Sample Volume of commercial ink

(ml)

Volume of polymer solution

(ml)

1 9 1

2 8 2

3 7 3

4 6 4

5 5 5

6 4 6

7 3 7

8 2 8

9 1 9

For inkjet printing, we used the design of the sensor that can be observed in Figure 9. Such IDC design includes an interdigitated sensor array in the center and it has a coupled antenna around for wireless reception of the signal. The lines in between of the interdigitated array are also called “fingers” as whether the hands fingers are clasped. Regarding the use of this sort of design, it has the advantages such as: occupy reduced space, low energy consumption, lower electrical loss, wide bandwidth for wireless applications, good connection with electronic circuits, lower prices. All these features are compared with larger conventional sensors. [31]

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Figure 9. Design of the IDC sensor used.

In addition to the previous sensor design, 5-cm stripes were printed to study a piezo-resistive sensor for basic deformation studies.

In order to stencil the image (shown in Figure 9) on the mesh, we followed the next steps:

 The image was printed in black-and-white positive onto an acetate film.

 The frame with a 90-mesh was coated with a photosensitive emulsion and put to dry in a dark room.

 Once dry, the image is placed over the screen and then exposed to a UV light source.

 The screen is washed off with a large amount of water.

 The areas of emulsion that were not exposed to light dissolve and wash away, leaving a negative stencil of the image on the mesh.

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Figure 10. a) Design of the sensor on the mesh b) Frame and design of the sensor stenciled on it.

When the frame is already prepared, it can be used for printing the image using the PANI or PEDOT based inks. For screen printing, ink is placed on top of the screen, and a squeegee (rubber blade) is used to push the ink through the holes in the mesh. The user begins with the fill bar at the rear of the screen and behind a reservoir of ink. The user lifts the screen to prevent contact with the paper and then using a slight amount of downward force pulls the fill bar to the front of the screen. This effectively fills the mesh openings with ink and moves the ink reservoir to the front of the screen. The user then uses the squeegee to move the mesh down to the substrate and pushes the squeegee to the screen. As the squeegee moves toward the rear of the screen the tension of the mesh pulls the mesh up away from the paper leaving the ink upon the paper surface. Finally the paper is allowed to dry.

This is a technique that has not been previously reported for the design of sensors on paper. This technique includes the following steps:

 Design of the prototype in a program such as Corel Draw or Illustrator

 The prototype must be in negative-image (See Figure 11a.) to facilitate cutting process.

 The prototype is cut using a laser cutter, where the white spaces are cut off and a mask remains (See Figure 11b.). The prototype should be cut in adhesive paper in order to be placed easily on the substrate or in this case on paper.

 The adhesive-paper mask is removed from its base paper and then placed on the substrate. (See Figure 11c.)

 The conductive ink is then added upon the surface where the mask is located, then is either allowed to dry with a fun or let to dry overnight.

 Finally the, mask is removed from the paper and the sensor-shape is transferred to the substrate.

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Figure 11. Cover Mask Deposition Process. (a) Original image of the sensor to be fabricated. (b) laser-cut mask. (c) Transferred sensor shape on paper.

3.2.3.1 Ink for cover mask deposition

The ink used for this technique is prepared using two different conductive inks: one is polyaniline or PEDOT based and the other is a commercial conductive ink made of a carbon compound. The weight ratio of the two inks is 2 to 1 Polyaniline/PEDOT: Commercial conductive ink.

In order to establish what the commercial conductive ink is made of, an Energy-Dispersive Spectroscopy (EDS) analysis was made in a scanning electronic microscope (SEM). The results of such analysis are shown in Table 2.

Table 2: Results of EDS analysis of commercial conductive ink. Elem

ent

App Inten sity Weig ht% Weig ht% Atom ic% Con c. Corrn . Sigma

C K 371.

05

1.429 6

81.38 1.49 85.74

O K 18.8

4

0.336 4

17.56 1.50 13.89

Cl K 2.83 0.836 9

1.06 0.15 0.38

Total s 100.0 0 b a c

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Table 2 indicates that the main component is carbon. Therefore, graphite (or Carbon black) may be a possible compound that is present in the ink because of its colour and its low cost.

Additionally, a FTIR spectrum was taken in order to gather more information on the main components present in the commercial conductive ink.

The Infrared spectrum of commercial conductive ink can be observed in Figure 12.

Figure 12. Infrared Spectrum of commercial conductive ink.

In the spectrum, it can be seen the characteristic peaks of vibration C=O about 1730 cm-1 and 1626 cm-1 . A very strong band centred at 3425 cm-1 is assigned to a phenolic/ alcoholic groups. This is strong evidence that there is graphite present in the sample. Besides, the peaks about 2924 cm-1 and 2850 cm -1 are assigned to CH2- vibrations. Two bands around 1384 and 1320

cm-1 are characteristic of the stretching of CH groups. The small bands between 790 and 450 cm-1 correspond to vibrations C-Cl. It evidences the existence of PVC, but in a smaller amount compared with the graphite, probably as an adhesion enhancer.

This commercial ink was mainly used to enhance the electrical properties of interdigitated array to obtain a better capacitive response.

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3.3 Characterization

3.3.1 Characterization of PANI-PSS/Pd and PEDOT-PSS

The synthetized polyaniline and PEDOT were characterized by FT-IR. The FT-IR spectra were recorded using Thermo Nicolet V-200 FTIR spectrometer in the region 400 to 4000 cm-1. Morphological studies were carried out using SEM (scanning electronic microscope). Electrical conductivity of the polymer solutions was measured using a pH-conductivity meter.

3.3.2 Characterization of sensors printed on paper using inkjet printing

Resistivity measurements of printed 5cm-stripes were carried out using a Fluke Digital Multimeter. Additional resistance measurements were made for studying the number of layers versus resistance for PANI Ink and commercial printing ink using the same multimeter.

Also, the electric properties of the thin films were analyzed by using tapping-mode electric force microscopy (EFM). EFM is a technique which maps electric properties on a sample surface by measuring the electrostatic force between the surface and an AFM (Atomic Force Microscope) cantilever. EFM images contain information on electric properties such as the surface potential of conducting materials, charge distributions of insulating materials, and electric domains of ferroelectric materials. For this technique two thin-film-stripes were chosen, one without conducting ink and the other one with ten layers of ink since it is expected that the surface is more conductive as the number of printed layers is increased.

Optical microscope images were taken to see how the inks were deposited upon the paper surface and to observe the shape of the stripes as the number of layers increases.

Additionally, the thickness and the width of the printed sensors were measured using contact profilometry. The profilometry tool is based on contact measurement of the sample. A stylus is moved across the feature to measure and the vertical displacement of the stylus is converted to a height value in Z equivalent to the step height in the feature studied. This process involves some mechanical and electronic devices in order to perform the conversion. The principle can be described as the one used in early phonograph players [39].

3.3.3 Characterization of sensors printed by screen printing and cover mask deposition Optical microscope images were taken to see how the inks were deposited upon the paper surface and to observe the shape of the interdigitated sensor array.

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The thickness and the width of the printed sensors were measured using contact profilometry. 3.3.4 Sensor Performance Characterization.

3.3.4.1 Impedance Measurements

The impedance of the IDC sensors is measured with VNA Master MS2028C with frequencies in the range (5 kHz to 20 GHz).

The actual measurement of the input impedance is complicated by many factors, mainly due to the connection of the VNA cable to the IDC sensor. An impedance measurement includes the impedance of everything on the measurement side of the sensing circuit. This includes connectors, circuitry in the VNA itself, cables, and many other items necessary to interface the device being measured with the part of the VNA performing the measurement. It is therefore necessary to remove the contributions of all items, excluding the device being measured.

Calibration in this context is therefore defined as mathematically moving the reference plane of the measurement to some other location. As mentioned above, the default reference plane is located at the sensing circuit. Standard known impedances are used to make a series of measurements from which the calibration coefficients are computed. The coefficients are used with all subsequent measurements to mathematically move the reference plane to a new location in the measuring apparatus and to attach units to all measurements [35].

Compensation is similar to calibration because it also removes measurement errors using known standards. Compensation differs from calibration because, in compensation, the reference plane is not moved and units are not assigned. Parasitic signals in the measurement apparatus are compensated or removed.

After the proper calibration and compensation, the tests can be performed using different substances such as methanol, ethanol, water and mixed solution of water and ethanol.

3.3.4.2 Wireless Measurements using VNA

Passive wireless sensors, such as LC resonators, have been developed, where an external magnetic field coming from an antenna couples with the sensor inductor and the effects of the measured variable changes the sensor’s capacitance, resulting on a shift of the resonant frequency ω0 that can be sensed by the coupling antenna. These types of sensors have a

longer life span, and reduce maintenance costs and complexities [40].

An antenna was coupled to the VNA after a proper calibration as it was mentioned previously following standard procedures [40, 42].

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Finally, the sensors were place into containers without direct contact to the antenna. In each container a 12 ml of the substance was poured and the sensor introduced to measure the impedance.

3.3.4.3 Deformation Sensor

Most semiconductors have a piezoresistive effect when subjected to mechanical stress or deformation [43].

Hence, it was decided to study the effect of a mechanical load or deformation of a stripe of paper that has conductive ink on it, in order to evaluate its piezoresistive response. For this purposes, three tests were carried out:

 Preliminary test: a conductive paper stripe was held up with two clips of a multimeter and the resistance was measured, then the paper was rolled in spiral and the resistance changed.

 Strain Gauge test: a conductive paper stripe was placed on an edge of a bar with two connections to a multimeter, and then a mechanical load is also placed on the opposite edge (see Figure 13). With a greater imposed load, the paper is more stretched, thus changing its electrical resistance.

Deformation by angle test: a conductive paper is placed in plastic rings with different radii, thus both extremes of the paper are held up by multimeter clips. Thus, with decreasing radius, the angle increases allowing deformation to be greater on the paper and therefore the resistance changes.

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Chapter 4: Results and Discussion

4.1 Characterization of conducting polymers

All the polymers solutions were correctly prepared obtaining similar conductivities (see Table 3)

Table 3. Experimental conductivities of the synthetized polymeric solutions.

Solution Conductivity (mS/cm) Conductivity (mS/cm) Data from the literature

PANI-PSS 100 13537

PEDOT-PSS 45 6037

PANI-PSS/Pd-NPs 110 -

PEDOT-PSS/Pd-NPs 80 10038

All PANI-PSS solutions showed a color change (see Figure 14). They revealed that nanoparticles are present in the solutions modifying the characteristics of the polymer once the reaction is carried out. On the other hand, PEDOT-PSS solutions showed black color in both cases with nanoparticles and without them.

Figure 14. PANI-based inksa) PANI-PSS/Pd-NPs b) PANI-PSS c) PEDOT-PSS

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In Figure 15, the spectrum of PANI-PSS can be observed. The spectrum, exhibits the characteristic peaks of polyaniline, such as those located near 1560 cm-1 corresponding to C = C vibration quinoid ring. Furthermore, there are peaks around 1492 cm-1 representing the repeated units of the benzene present in the polyaniline, as the peak near 1401 cm-1.

The CN stretching characteristic band is observed near 1298 cm-1, although the band characteristic of PSS is overlapping the CN band, and therefore it cannot be clearly distinguished.

Figure 15. FT-IR spectrum of PANI-PSS.

In Figure 16, the SEM micrographs show the morphology of PANI-PSS, it evidences that the characteristic morphology is made of nano fibers which is in agreement with the literature [41]

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Figure 17 ilustrates the FT-IR spectrum of PEDOTR-PSS. It shows the peaks at 1450 and 1221 cm−1 (C–C and/or C=C stretchings of the thiophene ring), 1085 cm---1 (originated from the C-O-C bond), 1032, 835 and 655 cm−1 (ν(C-S) and δ(C-H)) [6]. Thus one may conclude that the product of the emulsion polymerization process is PEDOT according to the similar results reported in literature.

Figure 17. FT-IR spectrum of PEDOT.

Figure 18 evidences that the morphology of PEDOT is also fibers as it was shown for PANI previously. However, these fibers have greater diameter in comparison to the PANI fibers.

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Figure 18. SEM micrograph of PEDOT.

4.2 Sensor Design

Among the prepared inks for inkjet printing, it was found that only the modified inks PANI-PSS (5:5) and PPY-PSS/Pd-NPs could successfully be prepared and printed.

Although, the rest of modified inks were prepared at the same conditions, they could not be printed as normal as PANI-PSS (5:5). Because the ink prepared was destabilized by the presence of acid in the solution of PANI and then aggregates were formed that prevented the ink being ejected correctly by the cartridge nozzles. Other prepared inks could not be printed at all due to the formation of unstable colloidal suspensions.

We tried to address these issues and therefore different tests were performed in order to overcome these drawbacks:

 PANI solution was centrifuged and dried to obtain powder particles, which were redispersed in commercial ink, but the obstruction issue was still remained, and it was

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even higher, since aggregates were not easily broken and therefore were not well redispersed.

 The acidic solution of the PANI was neutralized to minimize the effect of aggregate formation, but the counterion (PSS) came out from the polymer matrix, causing it to reduce the electrical properties since the polymer counterion is favorable in the polymer at acidic conditions. The electrical properties strongly decreased as if PANI has not been added to the commercial ink.

 Finally, an attempt was made to change the type of commercial ink, but the result was rather the same, the printer could not be changed for another one, because the one available has nozzles with the greatest ejection holes. In contrast, the other printers have smaller nozzles diameter, and therefore, it would have been more difficult try to pass through the nozzles the aggregates formed in the ink.

These difficulties indicated to us that it is not feasible to perform inkjet printing with a normal home-printer in a reproducible manner using the implemented inks, due to limitations in the cartridges. However, in order to test the potential of this technique, the 5-cm-stripes to be used for deformation sensing were printed using the commercial home printer.

The sensors fabricated by the screen method were achieved successfully. They can be seen in Figure 19.

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Figure 19. Paper-based sensors fabricated by screen printing. a) Printed silver sensor with surrounding antenna, b) printed PANI sensor with surrounding antenna c) printed PANI interdigitated (left) printed silver interdigitated

(right).

As it can be seen in Figure 19 there are two sensors fabricated with silver paint (Top-left and Bottom-right). These two types of devices were fabricated in order to compare the response of different materials, since silver is present as a metal micro/nanoparticle, and therefore its behavior is different from that of a polymer.

For the PANI sensors, it was necessary to print 5 layers to obtain a conductive layer for the interdigitated array. In contrast, silver based ink was printed only once.

The sensors in Figure 19 (a) and (b) were implemented for wireless measurement and therefore they have an antenna around the interdigitated array. The sensors in Figure 19 (c) were used for impedance measurements.

a b

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The sensors fabricated by cover mask deposition were also successfully implemented. They are shown in Figure 20.

Figure 20. Paper-based sensors fabricated by cover mask deposition.a) printed PANI sensor with surrounding antenna, b) printed silver sensor with surrounding antenna c) printed PANI interdigitated.

The sensors in Figure 20 were fabricated using PANI and silver based inks. Additionally, there are two more sensors fabricated using PEDOT-Pd and PANI-Pd using this same technique.

4.3 Characterization of sensors printed by inkjet printing

The results for inkjet printing presented in this report are based on the most promising prepared ink that could be successfully used (PANi-PSS,5:5). The printed stripes are shown in Figure 21.

c

a a

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Figure 21. Printed stripes on paper using inkjet printing.

As it can be observed in Figure 22, the electric resistance is reduced significantly compared with commercial ink, particularly at low number of printed layers.

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The results in Figures 22 reveal that the greater the number of printed ink layers the smaller is the electrical resistance for both commercial and modified ink, this is due to a more well-defined layer is formed with every print, increasing the electrical properties on the substrate.

The lowest resistance that could be achieved with ten modified ink layers, was about 21 kΩ in comparison with the lowest resistance obtained with commercial ink (nearly 5 MΩ).

In addition, the profilometry results for the printed stripes by inkjet printing show that the roughness is higher as the number of layers increases, because the layer is well defined during the printing process. Additionally, the thickness of the layers also increases as function of the number of layers (see Table 4).

Table 4. Roughness and thickness of the thin layer. Measurements by profilometry performed on a PANI -PSS modified ink, printed on paper.

Number of Layers

Roughness (µm)

Thickness (µm)

1 3,17 0,279

3 3,26 0,277

5 3,47 0,258

10 3,69 0,285

The results for EFM are shown in Figure 23 and Figure 24. In this case, surface potential was measured for two different films. The first one was the one which was printed with commercial ink (see Figure 23) and the other one was printed with ten layers of modified conducting ink (see Figure 24).

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Figure 24. EFM image of ten printed using modified conducting ink.

As it is shown in Figures 23 and 24, there is a clear difference between the two images. Figure 23 shows that the surface potential is not distributed uniformly on the surface because there is only one layer printed on the paper, therefore there not exists any defined pattern on it. However, Figure 24 shows a uniform distribution on the surface indicating that with a larger number of layers printed on the substrate a more defined morphology is obtained, so that the surface potential is well distributed all over the surface. The electrical properties are better defined as the number of layers increases, a result that is in agreement with the conductivity measurements as function of the number of printed layers.

The optical microscope images for the stripes printed by inkjet printing can be observed in Figure 25

Figure 25. Microscope Images of different printed layers by inkjet printing. (From left to right) 1 printed layer, 3 printed layers, 7 printed layers.

As it is observed, the clearness of the paper is decreased at higher number of printed layers, showing the formation of defined layer upon the paper.

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4.4 Characterization of sensors printed by screen printing and cover mask

deposition

The profilometry for sensors fabricated using screen printing with silver based ink is shown in Figure 26.

Figure 26. Profilometry of sensor fabricated by screen printing.

In Figure 26 the profilometry indicates that the approximate height of the interdigitated fingers is around 20 microns, and the width thereof is about 700 microns. The space between the fingers is 600 microns.

Additionally, the peaks in each finger represent the mesh holes of the screen, through which the ink passes. This indicates that the silver ink dries very fast and cannot be spread in the entire finger as it is supposed to be, such a result suggests that for obtaining a more flat surface a different low-evaporating solvent might be necessary.

The profilometry for sensors fabricated using cover mask deposition with PANI, is shown in Figure 27.

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Figure 27. Profilometry of sensor fabricated by cover mask deposition.

In Figure 27, the profilometry shows a horizontal straight line in most of the measures, indicating saturation in the measurement of the height. This represents the height of each finger is above the limit of the measurement equipment that is 50 microns.

Thus, it was necessary to use a micrometer to measure an approximate height. The height that could be measured was about 120 microns. This indicates that this technique allows the fabrication of structures with layers of greater height and better conductive properties.

In addition, the profilometry evidences that the width of each finger is about 1000 microns or 1 mm. Therefore, the laser cut was successful and in accordance with the preset design.

4.4.1 Microscope Images

The microscopic images of the sensors fabricated by screen printing can be seen in Figure 28.

Figure 28. Optical Microscope Images of sensors (50X) by screen printing. a) interdigitated of silver sensor b) interdigitated of PANI sensor.

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Figure 28a, shows well-defined patterns of interdigitated arrays of silver paint. On the other hand, the patterns for PANI (Figure 28b) are not as good as silver, this might be due to the ink is water-base, thus when the squeegee is passed, it drags along some ink previously deposited upon the surface of the paper.

Also, circular patterns can be observed due to the presence of the mesh when the screen printing process is performed. The microscope images of sensors fabricated by cover mask deposition are shown in Figure 29.

Figure 29. Optical Microscope images of sensors (50X) by cover mask deposition. a) interdigitated of silver sensor b) interdigitated of PANI sensor.

In the images of Figure 29, the interdigitated fingers can be observed, but the spacing between them is not well defined, due possibly to the fact that some ink seeps underneath the adhesive paper. Furthermore, when removing the adhesive paper, edges are affected as can be seen in the images.

However, the thickness of the layer remaining on the paper is quite considerable. This can be evidenced noticing the shadow that is formed when it is illuminated at some angle.

4.5 Impedance Measurements

In order to perform impedance measurements, additional connections were introduced on the sensors, and then connected to the VNA cable. In this way, the measurements were carried out, as these connections allow tracking changes in the capacitance of the sensor. Figure 30 shows the respective sensors with their additional connections.

Design and evaluation of "Lab-on-paper" sensing platforms based on conductive polymers