Piezoresistive Pressure Sensor for application in e-skin devices

Nuno Miguel Gonçalves Pinela Licenciado em Ciências da Engenharia de Micro e Nanotecnologias Piezoresistive pressure sensor for application in e-skin devices Dissertação para obtenção do Grau de Mestre em Engenharia de Micro e Nanotecnologias Orientador: Doutor Rui Alberto Garção Barreira do Nascimento Igreja, Professor auxiliar, Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa Co-orientador: Doutor Hugo Manuel Brito Águas, Professor auxiliar, Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa Júri Presidente: Doutor Rodrigo Ferrão de Paiva Martins Arguente: Doutor Carlos Jorge Mariano Miranda Dias Vogal: Doutor Rui Alberto Garção Barreira do Nascimento Igreja Setembro, 2017 Piezoresistive pressure sensor for application in e-skin devices Copyright © Nuno Miguel Gonçalves Pinela, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa. A Faculdade de Ciências e Tecnologia e a Universidade NOVA de Lisboa têm o direito, perpétuo e sem limites geográficos, de arquivar e publicar esta dissertação através de exemplares impressos reproduzidos em papel ou de forma digital, ou por qualquer outro meio conhecido ou que venha a ser inventado, e de a divulgar através de repositórios científicos e de admitir a sua cópia e distribuição com objetivos educacionais ou de investigação, não comerciais, desde que seja dado crédito ao autor e editor. Este documento foi gerado utilizando o processador (pdf)L A TEX, com base no template “novathesis” [1] desenvolvido no Dep. Informática da FCT-NOVA [2]. [1] https://github.com/joaomlourenco/novathesis [2] http://www.di.fct.unl.pt "Try not to become a man of success, but rather try to become a man of value." Albert Einstein Acknowledgements I would like to acknowledge everyone who contributed for this master thesis to happen and to be concluded, starting from Professor Rodrigo Martins and Professor Elvira Fortunato, who have developed these two multidisciplinary research centers, which are CENIMAT|i3N and CEMOP, where I have spent my last nine months and where I found all the tools to surpass barriers and acquire knowledge. I would also like to show my acknowledgement to this great institution, FCT-UNL, where I find myself growing and evolving everyday, since the first day I joined it. Secondly, I would like to express my gratitude to Professor Rui Igreja, for accepting and introducing me to this wide world of sensors. Furthermore, I would like to thank for the guidance and orientation in every single meeting. I am also grateful to Andreia, who has supported me and contributed significatively for this thesis. To Professor Hugo Águas, I also want to thank for the guidance. Moreover, I want to thank CENIMAT|i3N members Alexandra Gonçalves, Sónia Pereira, Ana Carolina Marques, Ana Samouco, Beatriz Coelho, Ricardo Ferreira, Tiago Mateus, Rodrigo Santos, Pedro Alves, Daniela Gomes and Tomás Calmeiro, who have always provided me the help I needed, in terms of suggestions, logistics and technical support. To all my friends from Open-Space, I want to give special thanks because in spite of our duty to complete this hard task of writing a master thesis, we managed to have fun and to help each others in one or another situation. To my course friends who have been these last five years with me, João Afonso, Marco and Shiv. To my friend Tiago Gameiro, I would really like to express my gratitude, for the good times we have spent during these years. To my long-time friends Alexandre Ferrão, Gonçalo Félix and Duarte Pais, who have been an important support. Also, I want to thank to my girlfriend’s family for the support. I want to specially acknowledge all my family. To my grandparents for all the pride they put on me. To my parents Anabela and António, for conceding me this opportunity of education. I will be forever grateful, and will always praise on follow the principles I have been taught. To my brother Pedro, who has been a major pillar throughout the years. I also want to thank Noa for the unconditional love. To Mariana, who has made this walk alongside me, making it much easier. A few lines here would not be enough to express how thankful I am for all the love, support, motivation and for everything she is. vii Abstract In recent years, the advancement of science and technology tends to evolve towards the exploitation of electronic skin (e-skin) and functional prosthetic devices, enabling innovating applications in various fields such as biomedical systems, sports health-monitoring and healthcare. Owing to their significant role in health monitoring, pressure sensors come as essential components in the development of artificial systems that can mimic the impressive human skin. The development of such sensors comprises the search for flexible and stretchable materials suitable for implementation in robust devices that enable the integration of multiple sensing-functionalities. To quantitatively monitor pressure, these sensors use transduction methods based on piezoresistivity, capacity, piezoelectricity, and triboelectricity. In this work, piezoresistive devices were chosen over others due to their ease in structure design and readout mechanism. The mechanism of such piezoresistive pressure sensor relies on the transduction of a pressure change into a change in resistance that, in this case derives from variations in the contact area. In the approach presented in this work, a semi-sphere microstructuring patterning made by laser engraving on hard-poly(dimethylsiloxane) (h-PDMS) was introduced. h-PDMS works as a mold from which standard-poly(dimethylsiloxane) (s-PDMS) microstructured membranes with approximately 200 µ m thickness are peeled off. Carbonink, working as active material, was deposited on top of the microstructured s-PDMS membranes. The fabrication of such pressure sensors based on organic membranes combines advantages such as the production in a low-cost and fast way, device flexibility, and tunability of the sensor’s design. Moreover, sensitivities of 2.4 ×10−1kPa−1 were reached for the sensors developed. Keywords: Electronic skin, piezoresistivity, microstructures, semi-spheres, PDMS. ix List of Tables 1.1 State of the Art of piezoresistive pressure sensors ................ 4 3.1 Membranes peeled offfrom molds fabricated in s-PDMS using three different designs ........................................ 11 3.2 Membranes peeled offfrom molds fabricated in h-PDMS using laser engraving in raster mode, power of 50 W, and a speed of 0.762 m/s. ........... 13 3.3 Summary of height measurements of semi-spheres using different combinations of designed base diameters and laser powers. ............... 14 3.4 SEM images acquired of microstructured PDMS films. ............. 15 3.5 SEM images of microstructures produced on PDMS with carbon or PMMA + carbon coatings. ................................... 21 A.1 Semi-spheres real diameter and real pitch measured on horizontal and vertical directions ....................................... 37 C.1 Measurements of dimensions of the fabricated microstructures with carbon coating and with carbon coating with PMMA. .................. 41 G.1 Estimated sensor price regarding materials costs ................ 51 xvii Acronyms CNTs carbon nanotubes. h-PDMS hard-poly(dimethylsiloxane). LCD liquid crystal display. LOD limit of detection. PDMS poly(dimethylsiloxane). PEDOT:PSS poly(3,4-ethylenedioxythiophene) polystyrene sulfonate. PEN polyethylene. PI polyimide. PMMA poly(methyl methacrylate). PVDF poly(vinylidene) difluoride. PZT lead zirconate titanate. SEM scanning electron microscope. s-PDMS standard-poly(dimethylsiloxane). xix Motivation and Objectives Nowadays, the pursuit for information-sensing inspired by human skin has been motivated by the possibility of application on functional health monitoring systems and on robotic systems. Therefore, in order to achieve this, thin film pressure sensors are being widely exploited. Furthermore, efforts have been made towards the fabrication of thin film sensors in a non-clean room environment, which would considerably lower the device cost. Thus, the main goal of this work is to design, fabricate and develop low-cost, flexible pressure sensors based on the piezoresistive effect, which could be easily adapted and conformed to different surfaces, using microstructured substrates. Large scale production greatly benefits from low manufacturing costs in flexible pressure sensors, and the simplification of the device’s structure and manufacture is also desirable. These piezoresistivity-based devices allow an easy readout mechanism, as well as a simple structure design, which in turn will allow an easy device optimization. Moreover, through the use of the perfect combination of materials one expects to produce each sensor for approximately 0.351 €(materials costs). Finally, the fabricated materials will be extensively characterized, both morphologically and electrically, as to compare different designs, with the primary goal of developing a sensor with the best sensitivy possible. xxi Chapter 1 Introduction 1.1 Electronic Skin Human skin is an outstanding organ, being our interface with the surrounding world and allowing us to perceive mechanical stimuli such as pressure, shapes and textures [1]. This sense of information is only achievable because human skin comprises mechanoreceptors that receive a mechanical stimulus which is than transduced into a biological response [2]. Nowadays, inspired by this illusory simplicity of nature, efforts are being made to develop skin-inspired electronic devices. The pursuit for these e-skin devices is motivated by the possibility of application on functional prosthetic devices [3], humanoid robotics [4] and human health monitoring, where it could play a key role [5,6]. Therefore, accurate quantitative monitoring requires an effective transduction where transduction mechanisms such as piezoresistivity [7–9], capacitance [10–12], piezoelectricity [13–15], and triboelectricity [16–18] are being widely exploited to develop different types of pressure sensors. 1.2 Pressure Sensor Fundamentals Firstly, for a better comprehension of pressure distributions, pressures such as human touch, object manipulation, and human body circulation are considered to be in the low pressure (<10 kPa) and medium-pressure (10-100 kPa) regimes [12]. Secondly, and given that a pressure sensor transduces a mechanical pressure into an electrical signal an outline of some noteworthy key parameters that include sensitivity, limit of detection (LOD), linearity, response time and stability is presented. Among these, sensitivity, which is defined by the ratio between the variation of the quantitative output signal and variation of the applied pressure, is one of the most important parameters because it defines the accuracy and effectiveness of the measurement [19]. Sensitivity is defined as S= ∆R R0! ∆P(1.1) Where Pdenotes the applied pressure and ∆R/R0 is the relative resistance change of the sensor. LOD represents the lowest quantity of pressure that can be distinguishable. Lowering the LOD of a pressure sensor represents an improvement on lower pressure detection which is a requirement in lower pressure regimes [20]. Another relevant parameter is linearity, which is a classification expressed as percentage of the deviation of the sensor’s output curve from a specified straight line over a 1 CHAPTER 1. INTRODUCTION certain pressure range [20]. Hence, one desires to have pressure sensors with broad linear ranges, which simplifies the conversion of mechanical pressure into electrical output. Response time can be defined as the time required for a pressure sensor output to go from its previous state to a final stable value [20]. This parameter is especially important in dynamic real-time pressure sensing devices, when producing real-time monitoring systems or instant-response displays. Furthermore, to accurately measure the magnitude of a pressure stimulus, the most relevant transduction methods are piezoelectricity, capacitance, and piezoresistivity . Piezoelectricity, which is the ability (quantified by the piezoelectric strain constant d33 ) of a material to generate electrical charges in response to the occurrence of electrical dipole moments due to applied mechanical stresses. This property of crystals and certain ceramics is good for development of low-power-consumption or self-powered sensing devices [21]. Moreover, the piezoelectric element of the device can be combined with transistors to improve sensitivity [13]. The development of piezoelectric pressure sensors has been receiving lots of attention due to their fast response speed and self-powered operation. The piezoelectric materials most used for this application include poly(vinylidene) difluoride (PVDF) and its copolymers, lead zirconate titanate (PZT) [13], and Zinc Oxide (ZnO) [22]. Capacitive sensor transduction consists of a change in capacitance caused by the deflection of the plate when an external stimulus is applied. As dielectric constant (permittivity) of the medium between the plates is a constant, the external stimulus, either an applied pressure or shear force, usually represents a change in area or in the distance between the plates, respectively [1]. Capacitive sensors offer advantages such as high sensitivity, but the main advantage of this types of sensors is the simplicity of their governing equation, which allows a direct analysis and a simple device design [23]. These sensors have also demonstrated the characteristic of high strain sensitivity for detection of a static force with low-power consumption [24]. However, once capacitance is proportional to the area, a reduction in the size of these devices for miniaturization means a reduction of the capacitance and the signal-to-noise ratio [25]. Additionally, capacitive sensors are vulnerable to external interferences [26]. The other transduction method commonly used is based on piezoresistive effect. Piezoresistive sensors have been widely investigated due to their simple structure and readout mechanism [27]. These sensors transduce a force variation into changes in resistance of a device that is detected by an electrical measuring system. The resistance variation derives most commonly from changes in: the geometry of the sensing element; the contact resistance ( RC ) and the resistivity of a composite due to changes in separation between particles. Usually, for these type of sensors, when the resistivity of the material is constant, a change in resistance derives from changes in the geometry of the sensing element. For conductive materials, the mechanism relies on changes in the RC between both materials. A RC change caused by change in contact area between two conductors is 2 1.3. PIEZORESISTIVE PRESSURE SENSOR DEVICES proportional to the square root of the force, which is an advantage as it provides high sensitivity at lower pressures as well as it expands the usable range [1]. For conductive elastic composites, piezoresistance depends on the morphology, composition, and strain range of the system, and the mechanism relies on changes in conductive path. Furthermore, piezoresistive sensors exhibit a fast response speed [1]. However, piezoresistivity-based sensors usually show undesirable drift and hysteresis [28]. As seen, the presented transduction methods provide different sensing capabilities which allows systems to integrate multifunctional sensors. For example, piezoresistive are usually used to reliably measure large strains [29], capacitive devices to sense normal forces and piezoelectric devices to measure vibrations [1]. 1.3 Piezoresistive pressure sensor devices Being human skin considered as a performance benchmark for the development of e-skin, some considerations need to be followed to grant this electrical material the mechanical properties of human skin, such as stretchability, flexibility, and low Young’s modulus [1]. To fulfil these considerations, the design and fabrication of the device are critical. An important parameter is regarding device’s stretchability and there are two main strategies used to improve it [30]. The first method (Figure 1.1a) uses a thin conductive material bonded to an elastic substrate, such as poly(dimethylsiloxane) (PDMS) [10]. The second method (Figure 1.1b) is based on the fabrication of devices by mixing conductive materials into an elastomeric matrix [31]. F F (a) F F (b) Figure 1.1: (a) Thin conductive material bounded to a micro-structured elastomer. (b) Mixture of a conductive material on an elastomeric matrix. Another important parameter in pressure sensors design is the contact area, which can be increased by constructing various device geometries, for example through the materials microstructuration. Creating the desired geometry requires techniques such as lithography [32], coating [33], and micro-channel molding and filling [34]. This concept has shown to be an ideal candidate for e-skin applications [8,12]. For this reason, and to be able to meet the needs stated above, the materials choice is crucial for the development of flexible pressure sensors and herein, an outline of substrate and active materials for piezoresistive pressure sensors is presented. Table 1.1 summarizes piezoresistive pressure sensors developed over the recent years as well as their performance parameters, to 3 CHAPTER 2. MATERIALS AND METHODS coating layers was measured in a profilometer (Ambios XP-Plus 200 Stylus) for one to five stacked layer depositions, using a tracking force of 0.5 mg and a scanning speed of 0.20 mm/sec. I-V curves were acquired using Keithley 2000 Multimeter connected to each silver-ink electrode of the membrane and a voltage sweep from -2 V to 2 V, in steps of 0.5 V, was applied. The output signal corresponds to current flowing through the 2 cm x 2 cm microstructured domain. 2.6 Electrical characterization of devices For quantitative analyses, output signals from a mechanical stimulus were acquired by recording changes in electrical resistance as a function of applied pressure using a homemade system developed on the scope of this work (as described in Section 3.3), capable of applying different pressure values. Additionally, I-V curves of the devices were acquired using the same method as for membranes, to test for ohmic-like behaviour. 10 Chapter 3 Results and Discussion 3.1 Patterning The microstrutured domain shape optimization to reach semi-sphere-like structures went through several steps such as exploitation in PDMS molds fabrication, laser engraving parameters, and shape design in software. Table 3.1: Membranes peeled offfrom molds fabricated in standard-PDMS using a speed of 0.254 m/s and laser power of 12.5 W for images (a)(c)(e) or laser power of 25 W for images (b)(d)(f), all with laser engraving in vector mode and for the three different designs previously mentioned. (a) and (b) Microstructures resultant from aligned circles with with both a diameter and a circles distance of 200 µ m. (c) and (d) Microstructures resultant from aligned squares of 200 µ m x 200 µ m with a distance between squares of 200 µ m. (e) and (f) Microstructures resultant from aliegned squares of 200 µ m x 200 µ m with two diagonals and a distance between squares of 200 µ m. Abbreviations used in this table: Power (P), soft-Polydimethylsiloxane (s-PDMS), Speed (S). Vector Mode s-PDMS P = 12.5 W, S = 0.254 m/s P = 25 W, S = 0.254 m/s (a) (b) 200 µm 200 µm 200 µm (c) (d) 200 µm 200 µm 200 µm (e) (f) 200 µm 200 µm 200 µm 11 CHAPTER 3. RESULTS AND DISCUSSION Firstly, tests over shape design in software were performed to choose the perfect design to achieve the desired form on laser engraving. To do so, three different shapes were studied: circles, squares, and squares with two diagonals. The circles had a diameter of 200 µ m, the squares were 200 µ m x 200 µ m, and the pitch between each feature was fixed at 200 µ m. These designs were engraved in vector mode in s-PDMS films in two different batches – the first with a laser power of 12.5 W and a speed of 0.254 m/s and the second with a laser power and speed of 25 W and 0.254 m/s, respectively. Table 3.1 shows tilted images (45 º ) acquired from the membranes peeled offfrom molds. Molds engraved with circles clearly show a much more semi-sphere like structure, whereas the others stay more faithful to their squared designs. All six examples show some irregularities in the microstructures due to the difficult in peeling offthe membranes from the mold made of the same material. Additionally, given that molds engraving is done in a material that melts very easily, the engraved cavities do not melt in a homogeneous way, and so the PDMS membranes that are peeled offfrom these molds get the negative pattern of those irregularities. Membranes peeled offfrom the mold engraved with higher power present features that are more irregular, possibly due to the over-melting of PDMS during laser engraving. Therefore, molds engraved with a power of 12.5 W and a speed of 0.254 m/s appear to be preferred candidates for ideal semi-sphere molds. 200 µm (a) 200 µm (b) Figure 3.1: SEM images acquired from a top view of peeled membranes from molds made in s-PDMS in vector mode using a laser speed of 0.254 m/s and a laser power of (a) 12.5 W and (b) 25 W. A top view of the semi-sphere like features from the achieved membranes was also captured, as shown on Figure 3.1. Such figure confirms a good fidelity to the design in terms of shape, but highlights differences in terms of pitch over the horizontal and vertical engraving directions. This discrepancy results from the fact that the laser has a better resolution on the vertical direction than on the horizontal direction as further examined in this section. Figure 3.1 also shows that the pitch (200 µ m) is close to the laser beam resolution (127 µ m) as semi-spheres in horizontal direction are almost touching their horizontal neighbour semi-spheres. This represents a limit in design’s pitch (for these ranges of laser power), as a pitch lower than the one tested (200 um) would lead to 12 3.1. PATTERNING an over-melting of cavities on PDMS mold, resulting in distinct features than the ones designed. Meanwhile, tests on h-PDMS were also performed to evaluate the combination of raster mode engraving on molds made of h-PDMS. The designs tested for this engraving mode had to be changed because raster mode engraves the material in a different way when compared to vector mode. Also, one expected that a design based on circles would give rise to cavities with the shape of inverted cones instead of inverted semi-spheres. Therefore, the patterns designed for this study were squares of 200 µ m x 200 µ m with a pitch of 300 µ m or 1000 µ m. Laser engraving parameters were fixed at a speed of 0.762 m/s and power of 50 W. From patterns shown on Table 3.2 one can observe the formation of 3D structures far from being semi-spheres. For instance, in the image for the aligned pattern with the pitch = 300 µ m, straight lines engraved by the laser beam are perfectly distinguishable, which confirms that raster mode is not suitable for regular semi-sphere microstructuring. Table 3.2: Membranes peeled offfrom molds fabricated in h-PDMS using laser engraving in raster mode, power of 50 W, and a speed of 0.762 m/s. (a) Microstructures resultant from aliegned squares of 200 µ m x 200 µ m with a distance between squares of 300 µ m. (b) Microstructures resultant from aliegned squares of 200 µ m x 200 µ m with a distance between squares of 1000 µ m. (c) Microstructures resultant from misaligned squares of 200 µ m x 200 µ m with a distance between squares of 300 µ m. (d) Microstructures resultant from misaligned squares of 200 µ m x 200 µ m with a distance between squares of 1000 µm. Raster Mode h-PDMS Pattern Pitch = 300 µm Pitch = 1000 µm (a) (b) Aligned 200 µm 200 µm 400 µm 400 µm (c) (d) Misaligned 200 µm 200 µm 400 µm 400 µm Once the design and the laser engraving mode for microstructuring semi-spheres were chosen, the optimization process demanded a choice of the mold material. As seen before, 13 CHAPTER 3. RESULTS AND DISCUSSION membranes peeled offfrom s-PDMS molds were usually difficult to peel and showed irregularities on their structure. Therefore, experiments in vector mode on h-PDMS were also performed to appraise this combination comprising circle patterns. Herein, to investigate the effect of laser power on engraving microcavities, molds were engraved using laser power of 25 W, 12.5 W, 7.5 W, and 2.5 W whereas the speed was fixed at 0.254 m/s for all patterns. The reason to maintain laser speed at a high value is due to the fact that the higher the laser speed, the shorter is the working time of laser on PDMS and, consequently, the less high and sharp the structures will be. Table 3.3 and Figure 3.2 present results from molds fabricated using laser vetor mode on h-PDMS with circles, where one observes that the higher the laser power, the higher the microstructures are. In this specific study, a 45 % increase in laser power gives rise to structures with a height five times bigger. Table 3.3: Summary of height measurements of semi-spheres using different combinations of designed base diameters and laser powers. 200 µm Height (µm) 60 80 112 151 180 206 232 320 Diameter (µm) 100 200 100 200 100 200 100 200 Laser Power (W) 2.5 2.5 7.5 7.5 12.5 12.5 25 25 0 5 1 0 1 5 2 0 2 5 0 5 0 100 150 200 250 300 350 400 D i a m e t e r = 1 0 0 µm D i a m e t e r = 2 0 0 µm H e i g h t (mm ) L a s e r P o w e r ( W ) Figure 3.2: Semi-spheres height measured from SEM images versus laser power used to engrave the molds from where the resultant structures were peeled off. Values presented correspond to average values ± standard deviation of a minimum of 15 measurements. A relation between diameter and height is also observable, once the higher the designed diameter, the higher the semi-sphere height is. This results from the fact that engraving larger areas comprises deeper engraving on molds. Regarding these results, laser power of 2.5 W and 7.5 W showed up as being the most likely to produce semisphere like structures. 14 3.1. PATTERNING Furthermore, microstuctures resultant from two similar designs were used to compare the theoretical diameter and pitch with the resultant ones, as well as to compare the resolution over vertical and horizontal directions. The first design is based on circles with a theoretical diameter of 200 µ m and a pitch of 150 µ m, while the second design is based on circles with a theoretical diameter of 200 µ m and a pitch of 200 µ m. To do so, molds were made using a laser power of 2.5 W and 7.5 W, maintaining speed at 0.254 m/s for both. Table 3.4 shows the general view of the resultant PDMS structures exhibiting nice homogeneity for each pattern, while highlighting the differences between pitches and diameters over horizontal and vertical directions. From the images one notices that each feature can easily be discerned even for the lower pitch, meaning that theoretical pitch of about 150 µm is achievable with this technique. Table 3.4: SEM images acquired of microstructured PDMS films. Images (a)-(h) result from molds engraved with a laser power of 2.5 W and 0.254 m/s laser speed for different pitches and diameters. Images (i)-(p) result from molds engraved with a laser power of 7.5 W and 0.254 m/s laser speed for diferent pitches and diameters. Horizontal Pitch Vertical Pitch 150 µm 200 µm 150 µm 200 µm P = 2.5 W, S = 0.254 m/s Diameter (a) (b) (c) (d) 100 µm 200 µm 200 µm 200 µm 200 µm (e) (f) (g) (h) 200 µm 200 µm 200 µm 200 µm 200 µm P = 7.5 W, S = 0.254 m/s Diameter (i) (j) (k) (l) 100 µm 200 µm 200 µm 200 µm 200 µm (m) (n) (o) (p) 200 µm 200 µm 200 µm 200 µm 200 µm As previously seen, measurements over both horizontal and vertical directions should be performed in order to thoroughly analyse features. Hence, all semi-spheres were measured over both directions of the engraving process. Table A.1 summarizes the measured 15 CHAPTER 3. RESULTS AND DISCUSSION diameter and pitch of the fabricated features, showing that although diameter and pitch values were usually far from what was designed, the sum of diameter and pitch tends to be very close to the expected sum of the designed ones. This can be explained by the fact that laser beam melts more PDMS than it was designed, performing engravings much larger than expected, which is corroborated by the discrepancy, sometimes larger than 100 µ m, on measured diameters. Therefore, pitch for its turn, shows to be much smaller 100 150 200 250 300 350 100 150 200 250 300 350 R e a l D i a m e t e r (mm ) D e s i g n e d D i a m e t e r ( mm ) P i t c h = 1 5 0 mm H o r i z o n t a l V e r t i c a l P i t c h = 2 0 0 mm H o r i z o n t a l V e r t i c a l y = x 2 . 5 W L a s e r P o w e r (a) 100 150 200 250 300 350 400 100 150 200 250 300 350 400 R e a l D i a m e t e r (mm ) D e s i g n e d D i a m e t e r ( mm ) P i t c h = 1 5 0 mm H o r i z o n t a l V e r t i c a l P i t c h = 2 0 0 mm H o r i z o n t a l V e r t i c a l y = x 7 . 5 W L a s e r P o w e r (b) Figure 3.3: Real diameter of PDMS semi-spheres, measured over horizontal and vertical directions of laser engraving versus designed diameter in Adobe Illustrator, produced with a laser power of (a) 2.5 W or (b) 7.5 W. The lines y = x illustrate a real diameter equal to the one designed. Values presented correspond to average values ± standard deviation of a minimum of 15 measurements. 0 50 100 150 200 250 0 5 0 100 150 200 250 R e a l P i t c h (mm ) D e s i g n e d P i t c h ( mm ) D i a m e t e r = 1 0 0 mm H o r i z o n t a l V e r t i c a l D i a m e t e r = 2 0 0 mm H o r i z o n t a l V e r t i c a l y = x 2 . 5 W L a s e r P o w e r (a) 0 50 100 150 200 250 0 5 0 100 150 200 250 D i a m e t e r = 1 0 0 mm H o r i z o n t a l V e r t i c a l D i a m e t e r = 2 0 0 mm H o r i z o n t a l V e r t i c a l y = x R e a l P i t c h (mm ) D e s i g n e d P i t c h ( mm ) 7 . 5 W L a s e r P o w e r (b) Figure 3.4: Real pitch between PDMS semi-spheres, measured over horizontal and vertical directions of laser engraving versus designed pitch in Adobe Illustrator, produced with a laser power of (a) 2.5 W or (b) 7.5 W. The lines y = x illustrate a real pitch equal to the one designed. Values presented correspond to average values ± standard deviation of a minimum of 15 measurements. 16 3.1. PATTERNING than expected to maintain the design. Laser engraving resolution in vertical direction is notably better than in horizontal direction once features measured in vertical direction of the laser engraving show values of diameter and pitch closer to the designed ones, as it is shown on Figure 3.3 and Figure 3.4, respectively. As a result of all the explained before, the molds chosen for fabrication of PDMS membranes come as a combination of using designs with circles to be engraved on hPDMS with a laser engraving machine on vector mode. The parameters chosen were 7.5 W of laser power and speed of 0.254 m/s, as with this combination more reproducible semi-sphere structures are achivable. Two molds using this combination of parameters were produced, both with theoretical diameters of 200 µ m – the first with a pitch of 150 µ m and the second with a pitch of 200 µ m. Figure 3.5 shows the two molds produced and Figure 3.6 shows a microscope image of each PDMS mold that was used to fabricate membranes present in the studies further explained in this work, which were measured to have a thickness of 215 µ m ± 19 µ m (average values ± standard deviation) corresponding to a minimum of 15 measurements. Figure 3.5: Molds produced for fabrication of membranes. Mold on the left designed with circles with 200 µ m of diameter and pitch = 150 µ m. Mold on the right designed with circles with 200 µm of diameter and pitch = 200 µm 200 µm (a) 200 µm (b) Figure 3.6: Microscope acquired images of the fabricated molds. (a) Mold designed with diameter = 200 µ m and pitch = 150 µ m. (b) Mold designed with diameter = 200 µ m and pitch = 200 µm. 17 CHAPTER 3. RESULTS AND DISCUSSION 3.2 Ink Study 3.2.1 Carbon-ink dilution study Studies over carbon-ink dilutions and number of layers were performed to further explore the best conductive film to fabricate piezoresistive sensors. Herein, PMMA was investigated as a coating layer between carbon-ink and PDMS to check if it could both improve the adhesion of carbon ink to PDMS and the final stability of the device. Firstly, to investigate the possibility of using less carbon-ink without compromising the good functionality of the piezoresistive sensors, sheet resistance of smooth carbon-coated PDMS and smooth carbon-coated PDMS with PMMA was measured by varying the dilution of the coating in water (100 wt %, 67 wt %, 50 wt %, 40 wt %). Figure 3.7 shows a comparison of sheet resistance between carbon-ink deposited on PDMS with and without PMMA. Both curves tend to decrease in sheet resistance as carbon-ink concentration in water increases, as expected. From this study it was found that sheet resistance for 50 wt % and 40 wt % dilutions, for both PDMS with and without PMMA, is excessively high for a conductive film, with average resistance values above 30 k Ω/ . For films with approximately 67 wt 40 50 60 70 80 90 100 0 5 0 100 P D M S w i t h P M M A P D M S w i t h o u t P M M A S h e e t R e s i s t a n c e ( k W / ÿ ) C a r b o n - i n k i n w a t e r ( w t % ) Figure 3.7: Sheet resistance of smooth carbon-coated PMDS and PMMA-coated PDMS for four different carbon-ink dilutions in water. Measurements presented here correspond to average values ± average absolute deviation of a minimum of 3 measurements. % dilution, the sheet resistance is (6.7 ± 1.2) k Ω/ and (6.4 ± 0.8) k Ω/ , respectively for smooth carbon-coated PDMS and smooth carbon-coated PDMS with PMMA. Using a concentration of 100 wt % of carbon-ink ensures a sheet resistance of about (1.3 ± 0.1) k Ω/ and (1.6 ± 0.3) k Ω/ , respectively for PDMS with and without PMMA, which is suitable for the fabrication of piezoresistive pressure sensors. Figure 3.7 illustrates that 67 wt% and 100 wt% dilutions allow an easy replication of the membranes produced as the sheet resistance average values are deviated by a small error. 18 3.2. INK STUDY 3.2.2 Ink layers study To further understanding the relation between carbon coating layers and their behaviour on the device, a study comparing sheet resistance of different deposited layers using the dilutions of 67 wt% and 100 wt% was accomplished. Figure 3.8a shows a comparison of sheet resistance as a function of the number of layers for PDMS and PMMA-coated PDMS. Regarding the 67 wt% dilution of carbon-ink on PDMS with PMMA, sheet resistance of one layer is approximately (6.4 ± 0.8) k Ω/ . Sheet resistance, as expected, decreases with the number of carbon layers deposited by spin-coating as the amount of conductive coating increases. The deviation error also decreases which means that the conduction mechanism is becoming homogenous for the whole membrane. Both dilutions tend to a limit of sheet resistance with the increasing layers. Nevertheless, as expected, the 67 wt% dilution requires more stacked layers to reach the same values of sheet resistance of 100 wt%. Therefore, for 100 wt% dilution, there was no need to perform studies over more layers, once the values were already low enough. The sheet resistance on PDMS values are close to the ones with the same dilution on PMMA for both dilutions studied here, however, it appears to have higher deviation error, which means PMMA on PDMS seems to contribute for a greater stability of carbon coating adhesion. The first carbon coating layer for all four curves shows a larger deviation error of sheet resistance than for all other layer depositions due to the heterogeneity of deposited carbon. This observation is corroborated by the deviation errors for thickness, which were also measured in this study. 12345 0 2 4 6 8 N u m b e r o f c a r b o n c o a t i n g l a y e r s 6 7 w t % w / P M M A 6 7 w t % 1 0 0 w t % w / P M M A 1 0 0 w t % S h e e t R e s i s t a n c e ( k W/ÿ) (a) 12345 0 4 8 1 2 1 6 2 0 2 4 2 8 T h i c k n e s s ( mm ) N u m b e r o f c a r b o n c o a t i n g l a y e r s 6 7 w t % w / P M M A 6 7 w t % 1 0 0 w t % w / P M M A 1 0 0 w t % (b) Figure 3.8: (a) Sheet resistance of 67 wt% and 100 wt% carbon coating dilutions in water on smooth PDMS films and PMMA-coated PDMS films with 1 to 5 layers of coating. (b) Thickness of 67 wt% and 100 wt% carbon coating on smooth PDMS films and PMMAcoated PDMS films with 1 to 5 layers of coating. Measurements presented here correspond to average values ± average absolute deviation of a minimum of 3 measurements. Thicknesses presented on Figure 3.8b were measured between the top of the coating 19 CHAPTER 3. RESULTS AND DISCUSSION - 2 - 1 0 1 2 - 2 - 1 0 1 2 V o l t a g e ( V ) C u r r e n t ( m A ) P M M A - c o a t e d P D M S m i c r o s t r u c t u r e d m e m b r a n e s A B (a) - 2 - 1 0 1 2 - 0 . 4 - 0 . 2 0 . 0 0 . 2 0 . 4 V o l t a g e ( V ) C u r r e n t ( m A ) P D M S m i c r o s t r u c t u r e d m e m b r a n e s A B (b) - 2 - 1 0 1 2 - 2 - 1 0 1 2 V o l t a g e ( V ) C u r r e n t ( m A ) P M M A - c o a t e d P D M S m i c r o s t r u c t u r e d m e m b r a n e s A B (c) - 2 - 1 0 1 2 - 0 . 2 0 . 0 0 . 2 C u r r e n t ( m A ) P D M S m i c r o s t r u c t u r e d m e m b r a n e s A B V o l t a g e ( V ) (d) Figure 3.13: Voltage sweep from -2 to 2 V proving the ohmic behaviour of two devices produced with (a) PMMA-coated PDMS membrane with semi-spheres with a pitch of 150 µ m. (b) PDMS membrane with semi-spheres with a pitch of 150 µ m. (c) PMMA-coated PDMS membrane with semi-spheres with a pitch of 200 µ m. (d) PDMS membrane with semi-spheres with a pitch of 200 µm. Furthermore, to better understand the signal over time in response to applied pressure, output signal was acquired as function of a pressure range, at the frequency of 1 Hz. Figure 3.14 shows the output signal for each type of fabricated sensor. Herein, one observes the good output signal reproducibility over cycles, confirming the good stability of the device, and proving that the design of the device is suitable for this type of applications. As seen in Figure 3.14c, sensors fabricated with PMMA coating on PDMS with a pitch of 200 µ m present a higher output signal for the same pressure than other fabricated sensors. Both curves on each graph present a phase difference of about 0.01° to 0.8° which may be introduced by the assembled system composed by the electronic components and the motor, even so, this does not seem to cause interference to the device’s 26 3.4. ELECTRICAL CHARACTERIZATION OF DEVICES performance. 01234 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 DR/R0( % ) T i m e ( s ) P i e z o r e s i s t i v e s e n s o r s i g n a l 0 5 0 100 150 200 250 300 350 P r e s s u r e ( P a ) P i e z o e l e c t r ic s e n s o r s i g n a l (a) 01234 0 . 0 0 . 5 1 . 0 1 . 5 2 . 0 T i m e ( s ) DR/R0( % ) P i e z o r e s i s t i v e s e n s o r s i g n a l 0 5 0 100 150 200 250 300 P r e s s u r e ( P a ) P i e z o e l e c t r ic s e n s o r s i g n a l (b) 01234 0 . 0 0 . 5 1 . 0 1 . 5 2 . 0 2 . 5 DR/R0( % ) T i m e ( s ) P i e z o r e s i s t i v e s e n s o r s i g n a l 0 5 0 100 150 200 250 300 350 P i e z o e l e c t r ic s e n s o r s i g n a l P r e s s u r e ( P a ) (c) 01234 0 . 0 0 . 5 1 . 0 1 . 5 DR/R0( % ) T i m e ( s ) P i e z o r e s i s t i v e s e n s o r s i g n a l 0 5 0 100 150 200 250 300 P r e s s u r e ( P a ) P i e z o e l e c t r ic s e n s o r s i g n a l (d) Figure 3.14: Resistance changes in response to applied pressure over time (a) PMMAcoated PDMS membrane with semi-spheres with a pitch of 150 µ m. (b) PDMS membrane with semi-spheres with a pitch of 150 µ m. (c) PMMA-coated PDMS membrane with semispheres with a pitch of 200 µ m. (d) PDMS membrane with semi-spheres with a pitch of 200 µm. Moreover, Figure 3.15 shows the approximately linear relation between ∆R/R0 and the applied pressure (regarding the principle illustrated in Figure E.1, Section E), for the two designs on membranes with and without PMMA, from where one may calculate a sensitivity. Sensitivities of S = 4.9 ×10−2kPa−1 for external pressures ranging from 9 Pa to 330 Pa for PMMA-coated PDMS containing semi-spheres with a pitch of 150 µ m, S = 1.3 ×10−1kPa−1 for external pressures ranging from 5 Pa to 360 Pa for PDMS containing semi-spheres with a pitch of 150 µ m, S = 2.4 ×10−1kPa−1 for external pressures ranging from 4 Pa to 150 Pa for PMMA-coated PDMS containing semi-spheres with a pitch of 200 µ m, and S = 9.2 ×10−2kPa−1 for external pressures ranging from 10 Pa to 340 Pa for PDMS containing semi-spheres with a pitch of 200 µ m, were acquired. In Section Fa signal response to the approximately same applied pressure, at both a frequency of 1 Hz and 15 Hz are shown, proving that the fabricated sensors maintain their performance for this range of frequencies as the relative resistance change stays identical. 27 CHAPTER 3. RESULTS AND DISCUSSION 0 . 0 E + 0 0 1 . 0 E - 0 1 2 . 0 E - 0 1 3 . 0 E - 0 1 0 % 1 % 2 % DR/R0 y = 4 . 9 E - 2 x P r e s s u r e ( k P a ) P M M A - c o a t e d P D M S , P i t c h = 1 5 0 mm (a) 0 . 0 E + 0 0 1 . 0 E - 0 1 2 . 0 E - 0 1 3 . 0 E - 0 1 4 . 0 E - 0 1 0 % 1 % 2 % 3 % 4 % 5 % P r e s s u r e ( k P a ) P D M S , P i t c h = 1 5 0 mm y = 1 . 3 E - 1 x DR/R0 (b) 0 . 0 E + 0 0 5 . 0 E - 0 2 1 . 0 E - 0 1 1 . 5 E - 0 1 0 % 1 % 2 % 3 % 4 % 5 % y = 2 . 4 E - 1 x DR/R0 P r e s s u r e ( k P a ) P M M A - c o a t e d P D M S , P i t c h = 2 0 0 mm (c) 0 . 0 E + 0 0 1 . 0 E - 0 1 2 . 0 E - 0 1 3 . 0 E - 0 1 0 % 1 % 2 % 3 % DR/R0 P D M S , P i t c h = 2 0 0 mm y = 9 . 2 E - 2 x P r e s s u r e ( k P a ) (d) Figure 3.15: Resistance response to different pressures. Dot line is a linear regression from which the sensitivity of each device is extracted. (a) PMMA-coated PDMS membrane with semi-spheres with a pitch of 150 µ m. (b) PDMS membrane with semi-spheres with a pitch of 150 µ m. (c) PMMA-coated PDMS membrane with semi-spheres with a pitch of 200 µm. (d) PDMS membrane with semi-spheres with a pitch of 200 µm. 28 Chapter 4 Conclusions and future perspectives In the present work, the major objective was to develop a simple and low-cost method for fabrication of carbon-coated PDMS with uniformly microstructured patterns. To accomplish so, a thin PDMS film was microstructured with semi-spheres, and posteriorly covered by a flexible carbon coating. As previously seen, the combination of laser engraving parameters, the design of microstructures, and the type of mold showed to be of significant importance in order to obtain the desired microstructures on the fabricated PDMS membranes. One major conclusion of this work is that the ideal combination to obtain semi-spheres is to use designs with circles engraved on h-PDMS molds with laser engraving machine on vector mode. Although the real values of pitch and diameter do not maintain fidelity to the designed values, this method allows a fabrication of controlled and reproducible semi-sphere-like structures. Semi-sphere dimensions such as pitch size, height and diameter have a serious effect on the performance of the pressure sensors as the microstructures play the most important role on the variation of contact area upon pressure, affecting the pressure sensors sensitivity. Other conclusion of this work is that the laser engraving machine has better resolution over vertical direction of laser engraving than over horizontal direction, as real values of semi-sphere’ pitch and diameter tend to be much closer to the values designed in software, when measured on the vertical direction. Regarding studies with the active layer material, carbon coating, the results showed that using one single coating layer of 100 wt % carbon coating dilution would be preferred over using various layers of lower dilutions, as it requires less work and time consumption and ensures lower sheet resistances and thickness. A homemade pressure applying system was developed in order to electrically characterize the fabricated devices. This system can provide pressures in frequencies from 0.5 Hz to 20 Hz, and the range of the applied pressure can be easily adjusted depending on what is the final object applying the force. For example, an object with larger area will apply lower pressure. Further improvements on this system could be made by changing the 3D printed components to carbon fiber ones and changing the rubber tip to a softer material with larger area. In what concerns the pressure sensors, devices comprising a PMMA layer between PDMS and carbon coating were fabricated in order to investigate PMMA adhesion properties. In terms of sensitivity, the sensor with higher sensitivity was the one fabricated with a larger pitch and with a PMMA layer but at the same time, the lower sensitivity was found in the sensor with a PMMA layer but with lower pitch. Such observation can be derived from the fact that for the lower pitch, the presence of an extra layer (PMMA) and, therefore, additional thickness, would mean a spoilage of the sensing properties as 29 CHAPTER 4. CONCLUSIONS AND FUTURE PERSPECTIVES the interlocked domain may act as a smooth film, instead of increasing the contact area between both microstructured films. The higher sensitivity reached was 2.4 ×10−1kPa−1 for external pressures ranging from 4 Pa to 150 Pa. In fact, one expects that the use of thinner PDMS films enhances the sensitivity of the full device. These sensors were estimated with a price of 0.351 € of materials costs, as seen in Section G. In conclusion, the approaches presented here contribute to a new direction of flexible, low-cost, and easily assembled for future health monitoring devices because despite the fast progress in e-skin devices, there is room for investigation in the field of low-cost and low power consumption sensors, where the integration of multiple other sensing properties, namely temperature, shear, and vibration, is also appealing. Other appealing functionalities such as biocompatibility, self-healing also need further attention for the implementation in e-skin to fully mimic its organic parallel. Self-powering, for example, also constitutes a demand once it would allow an autonomy degree which is not possible with an immovable power supply. This could be achieved by adding, for example, solar cells, batteries and wireless antennas. 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All the assigned variables are also shown. 1// Motor Pins 2const int stepPin = 7; 3const int dirPin = 8; 4 5// Lower steps 6#define MS1 3 7#define MS2 5 8#define MS3 6 9 10 int steps = 1; // Steps chosen by the operator during program. Steps can go from 1 to 10 meaning 70 um to 700 um 11 int DT = 1000; 12 float TT = 0.05; 13 int cycles = 9999; // Cicles 14 float frequency = 0.5 ; // Frequency chosen by the operator by controling the potentiometer 15 16 void funcao1(){ 17 18 digitalWrite(MS1,HIGH); // Programing 1/16 step 19 digitalWrite(MS2,HIGH); 20 digitalWrite(MS3,HIGH); 21 22 int cycle1 = 0; 23 24 TT=1.0/frequency;// Period 25 DT=int(TT/(4.0*steps)*1000.0); //Delay Time DD=(Period/4*steps)*1000 26 27 while (cycle1 <cycles){ // Program keeps running until 9999 cycles 28 29 digitalWrite(dirPin ,HIGH); // Steps in one direction 30 for(int x=0;x<steps ;x++) { 31 32 digitalWrite(stepPin ,HIGH); 33 delay(DT); 34 digitalWrite(stepPin ,LOW); 35 delay(DT); 36 } 37 38 39 digitalWrite(dirPin ,LOW); // Steps in the other direction 43 APPENDIX D. PRESSURE APPLYING SYSTEM 40 for(int x=0;x<steps ;x++) { 41 42 digitalWrite(stepPin ,HIGH); 43 delay(DT); 44 digitalWrite(stepPin ,LOW); 45 delay(DT); 46 } 47 48 49 50 cycle1 =cycle1 + 1; 51 } 52 } (1) (2) (3) (4) (5) (6) (7) (8) Figure D.1: Schematic representation of the developed system. (1) 3D printed stepper motor support (2) and (3) 3D printed components responsible for converting radial movement in vertical movement (4) 3D printed compontent that slides over two parallel bars and holds the rubber (5) Rubber that acts as the final object applying the force (6) Piezoelectric sensor capable of detecting force (7) Arduino board controlling the system (8) LCD for an easier user interface. 44 Figure D.2: Sketch in SketchUp of component 1 of the homemade system to posterior 3D printing. Figure D.3: Sketch in SketchUp of components 2 and 3 of the homemade system to posterior 3D printing. Figure D.4: Sketch in SketchUp of component 4 of the homemade system to posterior 3D printing. 45 APPENDIX D. PRESSURE APPLYING SYSTEM Figure D.5: RC Low-Pass Filter with Op Amp Buffer with 3 poles with a cutting frequency of 10 Hz. 46 Appendix E Sensor operation principle Time (s) Resistance (Ω) ΔR ROFF RON Unloading Unloading Loading Figure E.1: Sensor operation principle based on resistance changes in responses to loading and unloading (ROFF: unloading, RON: loading). 47 Appendix F Resistance changes for different frequencies 01234567 0.00% 0.10% 0.20% 0.30% 0.40% 0.50% 0.60% 0.70% 1 H z DR/R0 T i m e ( s ) (a) 0 . 0 0 0 . 0 5 0 . 1 0 0 . 1 5 0 . 2 0 0 . 2 5 0.00% 0.10% 0.20% 0.30% 0.40% 0.50% 0.60% 0.70% 1 5 H z T i m e ( s ) DR/R0 (b) Figure F.1: Resistance changes in response to an applied pressure of PMMA-coated PDMS membranes with semi-spheres with a pitch of 200 µ m with a frequency of (a) 1 Hz (b) 15 Hz. 49 Appendix G Estimated sensor price Table G.1: Estimated sensor price regarding materials costs Price (€) Quantity (kg) Amount used (g) Cost (€) PDMS 250 1.1 1.187 0.326 Carbon-ink 70.54 0.1 0.322 0.002 Silver-ink 500.80 0.1 0.456 0.023 Total 0.351 51