Recent Advancements in Functionalized Paper-Based Electronics

Review www.acsami.org

Recent Advancements in Functionalized Paper-Based Electronics Yang Lin,† Dmitry Gritsenko,† Qian Liu,‡,# Xiaonan Lu,⊥ and Jie Xu*,† †

Mechanical and Industrial Engineering, University of Illinois at Chicago, Chicago, Illinois 60607, United States Department of Chemistry, The University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada # Department of Microbiology and Immunology, The University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada ⊥ Food, Nutrition and Health Program, Faculty of Land and Food Systems, The University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada ‡

ABSTRACT: Building electronic devices on ubiquitous paper substrates has recently drawn extensive attention due to its light weight, low cost, environmental friendliness, and ease of fabrication. Recently, a myriad of advancements have been made to improve the performance of paper electronics for various applications, such as basic electronic components, energy storage devices, generators, antennas, and electronic circuits. This review aims to summarize this progress and discuss different perspectives of paper electronics as well as the remaining challenges yet to be overcome in this field. Other aspects included in this review are the fundamental characteristics of paper, modification of paper with functional materials, and various methods for device fabrication. KEYWORDS: paper-based electronics, paper characteristics, functional materials, supercapacitors, generators, antennas, electronic circuits

1. INTRODUCTION Along with rapid development of the electronic industry, the consumption of electronic products worldwide has grown significantly, and the lifetime of electronics has become much shorter than it was before.1 Therefore, a considerable amount of electronic waste generated per year has resulted in great demand for landfill space and caused severe environmental issues such as wastewater discharge, ozone depletion, and climate change.2,3 To reduce the consumption of related toxic materials such as gallium arsenide and to minimize environmental pollution, various eco-friendly, biodegradable, and costeffective electronics have emerged and attracted great attention recently. For instance, transient electronics has drawn extensive attention recently. Devices that belong to this category possess the ability to dissolve totally or partially after finishing their functions.4 Thus, they are widely used for implantable biomedical components, zero-waste consumer electronics, and other applications.5,6 As one of the most popular materials involved in conventional electronics, silicon has also been applied in transient electronics.5 Besides, zinc oxide, which has superior electron mobility and transparency, is another © 2016 American Chemical Society

promising material for transient electronics. For example, Dagdeviren and co-workers have successfully built an energy harvester based on ZnO.7 In addition to inorganic methods, organic electronics has recently shown great promise as an alternative method for reducing environmental pollution and lowering the cost of electronic devices. This technology capitalizes on organic molecules or polymers instead of silicon or gallium arsenide, and it has the capability to control functionality down to the molecular level.8,9 As a flexible, cheap, ubiquitous, and environmentally friendly material, cellulose-rich paper has attracted growing interest in the research community, and various applications such as immunoassays,10 food safety,11 urinalysis,12 environmental monitoring,13 and electronics14 have been realized on paper substrates. For instance, paper was introduced by the Whitesides group in 2007 to build biosensors, which are then termed microfluidic paper-based analytical devices (μPAD).15 Received: April 23, 2016 Accepted: July 27, 2016 Published: July 27, 2016 20501

DOI: 10.1021/acsami.6b04854 ACS Appl. Mater. Interfaces 2016, 8, 20501−20515

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ACS Applied Materials & Interfaces

thus making it suitable for different purposes. For instance, sizing (e.g., starches, gums) is usually used for reducing the tendency of paper to absorb liquid; thus, the strength is enhanced.25 Calendering is another pivotal step that has been widely applied to make paper smoother and more lustrous.30 Besides the influence from processing steps, the composition of the original materials is also of great importance to the characteristics of papers. For example, papers made from chemical pulp are referred to as wood-free papers, whereas those made from mechanical pulp are considered to be woodcontaining papers. Lignin in mechanical pulp is often considered to cause paper to become brittle and appear yellow over time,31 whereas chemical pulp that is free of lignin can be more porous and stronger. Moreover, because paper is composed of fibers with a high aspect ratio, properties of paper are often anisotropic. For instance, paper elongates more in cross machine direction (CD) than in machine direction (MD).28 In contrast to the opacity characteristics of regular papers, optically transparent paper that is made of nanocellulose recently has been demonstrated.32,33 Nanocellulose nowadays can be classified into various types, including microfibril, cellulose nanofibers (CNF), nanocrystalline cellulose (NCC), and bacterial nanocellulose that is produced by bacteria. CNF contains nanoscale cellulose fibrils that have a very high aspect ratio (∼150).34 CNF is also considered to be a thixotropic material that flows after shaking for a certain time while keeping high viscosity under regular conditions. Owing to these special properties, nanocellulose has already been widely applied in numerous applications such as paper industries. It enhances the fiber−fiber bonding strength within papers, thus increasing the dry and wet strengths of paper products, and renders transparency to its final products.35 For instance, Hu and collaborators developed a transparent and conductive paper by coating conductive materials such as carbon nanotubes (CNTs) on nanocellulose and applied the as-prepared papers in solar cells because its large light scattering is preferred.36 Moreover, Malti and collaborators have successfully developed nanopaper with high electronic and ionic conductivities very recently.37 Besides novel nanocellulose materials, transparency can be obtained by filling pores with transparent materials (e.g., wax) as well. When it comes to paper electronics, the electrical properties of paper become significant because they affect the performance directly. However, original papers usually cannot meet all of the requirements. Therefore, different processes are often applied to convert paper into an appropriate substrate for various electronics applications. For instance, in one of the previous studies, cotton paper was coated by a single-wall carbon nanotubes (SWCNs) suspension to fabricate a solid-state supercapacitor.38,39 The authors demonstrated that SWCNs provided excellent electrical properties and large surface areas exposed to the ions in electrolyte. Nevertheless, original paper still has its own characteristics, which might be preferred in some applications. The key characteristics of paper substrates and examples of modification methods, which are used for enhancing performance, are summarized in Table 1. The performance of typical paper electronic devices depends not only on their electric characteristics but also on other ones such as surface roughness, bending stiffness, and ink absorbency. Surface roughness denotes the deviations of a surface in its normal direction. Paper with higher surface roughness has a higher ratio of surface area to volume and, thus,

Since then, a myriad of new fabrication methods (e.g., wax printing), sensing mechanisms (e.g., colorimetric method), and applications (e.g., heavy metal detection) have come out.16 With the aid of paper, external components (e.g., tube and pumps) are not indispensable because flow is controlled by capillarity forces. Moreover, even the simplest fabrication methods such as drawing and printing can convert normal paper into simple devices.17 Furthermore, various paper-based devices have been developed as complements to more complex sensing systems. To name a few, electrochemical detection has been built on paper to determine glucose, lactate, and uric acid simultaneously.18 The results showed it has competitive performance with traditional tests but in a cheaper and simpler way. Detection of heavy metal ions such as Cu2+ using paperbased devices recently has also been explored and shown high specificity and sensitivity.19 Owing to the rapid development of μPADs, paper electronics has also witnessed significant improvements by adopting stateof-the-art techniques such as new fabrication methods.20 In general, the use of papers as a platform for building electronics has advantages in the following aspects: (1) Paper is one of the most abundant materials worldwide with extremely low cost.21 (2) Unlike other precious materials such as silicon, paper is an annually renewable material. (3) Simple fabrication methods such as printing and writing are available for making paper electronics. (4) Paper is biocompatible, biodegradable, and environmentally friendly. (5) Paper is porous with a high surface-volume-ratio; thus, functional materials can be absorbed inside easily. (6) Paper is flexible and can be easily folded or bent to form 3D structures without causing structural damage.22 (7) Paper is lightweight. Finally, (8) paper devices can be disposed of by simply putting them on fire. It is worth nothing that, besides cellulose-rich paper, other thin materials such as flexible glass, plastic films, and metal foils have also been used as the substrates for creating flexible electronics.23 They will be referred to as “paper-like electronic devices” hereafter. Despite several previous reviews covering transient electronics,24 organic electronics,8 flexible electronics,23 μPADs,16 paper electronics,25 and others, myriads of improvements and new applications regarding paper electronics have been proposed and implemented during the past five years, which will be the main focus of this review. Specifically, we will categorize key characteristics of paper substrates and emerging functional materials developed for paper or paper-like electronics. Moreover, various emerging applications of paper electronics, such as capacitors, diodes, transistors, and antennas, will be introduced and discussed. Finally, conclusions will be drawn and future research works will be proposed. 1.1. Characteristics of Paper Substrates. Generally speaking, the term paper is defined as a thin sheet that is produced from cellulose pulp, and it is widely applied in many uses such as writing, drawing, printing, and cleaning. It is noted that paper has been developed for more than 2000 years,26 but its consumption has still grown rapidly over the past decades.27 Despite its long history, the process of manufacturing papers remained unchanged until 200 years ago, when people started to consider economic and ecological requirements.28 In principle, the main process consists of the preparation of an original raw materials suspension (e.g., pulps, fillers, pigments, chemical additives, and coating colors), dewatering of the suspension into an endless web, further dewatering by pressing and drying, sizing, coating, and calendering.29 Among these steps, the last three endow the paper external characteristics, 20502


thermal

thermal

printing

printing

electrical

electrical

thermal stability

thermal conductivity

ink absorbency

hydrophobicity

resistivity

conductivity

optical

transparency

mechanical

optical

opacity

wet strength

physical

porosity

mechanical

physical

pore size

specific bending stiffness

physical

smoothness

mechanical

physical

surface roughness

strain to rupture

physical

density

type

physical

thickness

characteristic

related impacts

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conductive paper can be used for various electronic applications such as smart paper technology, and add conductive additives before the paper it also depends upon temperature is formed

conformal coating

surface sizing or chemical additives used before paper is formed

high hydrophobicity of paper provides ease of removing water droplets from the surface;48 however, it results in difficulties of printing

general paper has good insulating performance. however, it depends extensively upon the relative humidity and temperature

coating or sizing

change material composition

catalytic oxidation


surface sizing

use nanocellulose or add fillers/polymers to the pulp

add additives to the pulp



calendering

calendering

modification method

ink absorbency determines the availability of using inkjet printing to fabricate paper electronics devices

thermal conductivity characterizes the heat transfer in paper

high thermal stability allows adaptation under extreme conditions

wet strength determines the resistance to rupture when paper is saturated with water

specific bending stiffness determines the bending resistance

strain to rupture indicates the resistance of rupture

transparent paper is suitable for those experiments that require microscopic observations

opacity of paper determines how much light can pass through the substrate

porosity of paper determines the amount of absorbed medium (e.g., conductive ink)

average pore size determines the size of the external particles that can be retained in the paper

appropriate smoothness is required for printing

rough surface increases the actual surface area; however, it increases printing difficulty

heavier paper provides more mechanical strength than lighter ones

thicker paper provides more stiffness and strength than thinner ones

Table 1. Characteristics of Papers example value

36

∼90% in forward direction

1−10 S cm−1

7 cm2 (V s)−1 and an on−off current ratio >105. The results are comparable with the ones obtained from conventional paper, making the design a promising candidate for highperformance disposable electronics applications. Wang and colleagues demonstrated the stable operation of cellulose 20505


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Figure 1. (A) Schematic illustration of paper supercapacitors and (B) related ion movement. Adapted with permission from ref 56. Copyright 2015, RSC.

nanocrystal-glycerol-based top-gate OFETs at low voltages.86 The latter feature is of high importance as it allows reduced device energy consumption. Another issue that arises for topgate OFETs with a solution-processed semiconductor layer is stability maintenance. As shown by the authors, it can be achieved by isolation of the semiconductor layer from chemical species in the substrate. To realize this, a thin aluminum oxide layer was deposited between the substrate and the organic semiconductor layer. Finally, a low voltage threshold and a mobility of 0.11 cm2 (V s)−1 were achieved. To achieve the low price of the components on paper substrates, it is advantageous for all parts to be printed. Grau and others presented a working prototype of all-printed OTFTs on paper.87 The authors proposed innovative techniques to overcome difficulties associated with paper roughness and ink absorption. More specifically, they employed gravure printing to create locally smoothing pads. These pads played a role in absorption barriers. A saturation mobility of 0.1 cm2 (V s)−1 and an on−off ratio of 3.2 × 104 were achieved. To sum up, there are several crucial parameters that should be optimized for the testing of various prototypes of transistors with paper substrates, namely, high stability, low operation voltages, and high carrier mobility and on−off current ratio. Nanopaper can be considered as a promising choice for further development of paper-based transistors that may provide an opportunity for optimization of almost all of the above-mentioned parameters. 2.1.3. Passive Components. Recent advancements in simple passive electronic components are mainly related to the development of pencil-on-paper technologies for the creation of precise sensing platforms. Kurra and Kulkarni provided an overview of electronic components that can simply be drawn on paper.41 For instance, a trace amount of graphite on a paper substrate is actually a resistor, and its electrical properties can be simply modified by varying the length and thickness of the graphite. Depending upon the type of the clay used (inorganic or organic), different trace patterns can be obtained. For instance, regular pencils can use inorganic clay materials, whereas grease pencils can include organic or polymer binders. The latter are more suitable for drawing on hard and smooth surfaces. Lin and colleagues created a bilayered chemiresistor onto the paper substrate.88 The first layer was drawn with a flexible pencil and served as an active sensing element. The second layer was created on top of the first one with an HB pencil and used to improve electrical contacts. Kang reported tunable piezoresistive sensors.89 The author showed that an increase in gauge factor along with graphite resistance was

observed when the mechanical strain was applied. The effect was associated with the tunneling effect in graphite during the process of drawing. Dinh and co-workers successfully developed a paper-based thermoresistive sensor with high negative coefficient of resistance. The authors demonstrated that this resistance change was due to the rise in thermionic emission with increasing temperature.90 Summarizing recent advancements in applications of nanopaper for basic electronic components, it can be stated that this novel unique material may open avenues for breakthrough development of paper-based electronic devices that possess a wide spectrum of unique properties. 2.2. Energy Storage Devices. Energy storage devices are also considered as important applications for paper electronics. Conventional energy storage devices, such as button batteries, are usually hard and rigid; thus, they are not suitable for the future electronics that are expected to be twistable and deformable. Additionally, energy storage devices on paper provide the possibilities for integration of all essential components without external ones. For example, Liu and coworkers integrated battery and electrochromic readout on paper to detect glucose and H2O2.91 Nowadays, energy storage devices, such as supercapacitors, electrochemical batteries, and biofuel cells, are widely used. Lithium-ion batteries and supercapacitors usually have larger power compared to electrochemical batteries and biofuel cells; thus, they are more suitable for high-power applications.92,93 However, other devices, such as paper-based MFCs, could provide cleaner power, thus reducing the pollution from energy consumption.52,94 2.2.1. Supercapacitors. Supercapacitors are also termed electrochemical capacitors because they employ Helmholtz double layer or Faradaic electron charge transfer.95 The device based on Helmholtz double layer is called an electric double layer capacitor (EDLC), whereas the other one is called a pseudocapacitor. These supercapacitors have various advantages (e.g., long cycle life, large power density, and high energy density); thus, researchers have been striving to improve their performance during the past decade.96,97 Energy stored in EDLCs was achieved by ion adsorbing and dislodging.98 Besides, conductive electrodes (e.g., carbon electrodes) with high electrostatic double-layer capacitance were applied in EDLCs for achieving separation of charges at the interface.99 As a rough material, modified conductive papers are referred to as good electrodes because they possess highly specific surface area and thus have higher capacitance.100 On the other hand, 20506


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Figure 2. (A) Fabrication procedure for coating SWCNs on cotton paper as supercapacitor. Adapted with permission from ref 38. Copyright 2012, American Institute of Physics. (B) Performance of supercapacitor fabricated by pencil-writing and electrodeposition. Adapted with permission from ref 69. Copyright 2013, Elsevier. (C) Photographs of foldable supercapacitor. Adapted with permission from ref 111. Copyright 2015, Elsevier. (D) Novel fabrication method for PEDOT paper: (a) schematic illustration of paper preparation; (b) image of PEDOT-paper; (c) image of surface of Scotch tape peeling from PEDOT-paper; (d) image of flexible thin capacitor; (e) image of capacitor that lit LED. Adapted with permission from ref 112. Copyright 2015, RSC.

gravimetric and volumetric capacitances.105 Moreover, pseudocapacitive materials with higher capacitance and energy density are the “state-of-the-art” for paper supercapacitors. For example, polyaniline has attracted growing interest recently. Shen and coauthors capitalized on polyaniline and multiwall carbon nanotubes (MWCNs) hybrid electrodes associated with poly(vinyl alcohol)/H2SO4 electrolyte, resulting in supercapacitors with superb high specific capacitance (656 F g−1).106 Because pristine polyaniline electrodes lack a stable cycle life,107 self-supported conducting polymer composites have been used to enhance their capacitive performance.108 Various experiments have been conducted to enhance the performance of supercapacitors in terms of energy density, power density, volume/area capacitance, and cycle life. For instance, SWCN-coated cotton paper and poly(vinyl alcohol)/ phosphoric acid were applied as electrode and electrolyte, respectively, to fabricate thin and lightweight supercapacitors.38 In this study, supercapacitor was fabricated via simple coating (Figure 2A). Finally, cyclic voltammetry (CV) and charging/ discharging by constant current were applied to test the performance of as-fabricated supercapacitor. The results indicated that it had highly specific capacitance (i.e., 115 F g−1) and large energy density (i.e., 48.9 Wh kg−1). Recently, simple fabrication (e.g., pencil-writing) has emerged to modify paper into flexible supercapacitors.69,109

pseudocapacitors capitalize on Faradaic current through redox reaction,101 intercalation,102 and electrosorption.103 They commonly use metal oxide, metal nitrides, metal sulfides, or conducting polymers as electrodes. For instance, MnO2/carbon nanotube hybrid electrodes were applied for fast redox reaction and stable electrochemical performance in pseudocapacitors.101 In fact, EDLCs usually have higher stable cyclic performance and power density than pseudocapacitors, but have low capacitance and energy density.56 Like conventional supercapacitors, paper devices also consist of electrodes, electrolyte, and current collectors, as shown in Figure 1. Among these components, the key ones for paper supercapacitors are the electrodes, which are usually made by modifying paper and can take a role as current collector. To obtain better performance, electrodes require low electrical resistance, good stabilities in electrochemical and mechanical performance, highly specific surface area, and favorable pore size distribution. Additionally, electrolytes demand electric insulation, ionic conductivity, and electrochemical stability. Among commonly used electrolytes, solid-state electrolytes (e.g., polymers) have been increasingly applied to paper devices because they require simpler fabrication methods than others.104 Recently, EDLCs have been explored extensively for paper electronics applications. Carbonized waste paper followed by surface modification was used, resulting in large and specific 20507


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another example of the storage and transfer of huge energy amounts that is moreover environmentally friendly.123 To make the impossible possible, numerous devices, especially generators, have been hitherto proposed including thermoelectric, triboelectric, and others. 2.3.1. Thermoelectric Generators. Owing to the capabilities of converting waste heat into electric energy, diverse thermoelectric materials have been explored. The figure of merit (ZT) of thermoelectric materials is given by

Expensive materials, such as SWCNs and noble metals, were replaced by cheaper ones for mass production. Furthermore, these cheaper materials have higher electrochemical stability in acid or base electrolytes.53,110 Yao and co-workers developed solid-state supercapacitors via pencil-writing and electrodeposition.69 In this study, conventional printing paper was converted into conductive material using pencil-writing and then modified by electrodeposition of polyaniline to enhance its conductivity and electrochemical activity. The as-fabricated supercapacitor showed outstanding flexibility and good performance in lighting a LED (Figure 2B). Other fabrication methods, such as chemical and physical deposition,113 surface coating,62 printing,114,115 and papermaking technique,116 were also applied. Similar to pencil writing, surface coating is commonly used due to the sufficiency of converting a paper surface into a conductive one while leaving the bulk paper as a mechanical support. Yuan and collaborators developed polypyrrole-coated paper supercapacitors by “soak and polymerization”, and the as-fabricated paper showed high conductivity (i.e., 15 S cm−1) with low cost.62 Moreover, Li and co-workers demonstrated asymmetric paper supercapacitors by coating cellulose paper with different materials.117 The as-fabricated positive and negative electrodes showed excellent areal capacitance and cycling performance compared to their counterparts with poor cycle stability. The best achieved supercapacitor showed extremely high energy density (80 Wh kg−1), power density (25.6 kW kg−1,), and striking cycle life (20000). Furthermore, foldable supercapacitors were proposed by combining cellulose paper, SWCNs, and polyaniline nanoribbons.111 The final device showed excellent capacitance retention after hundreds of times of folding (Figure 2C). Recently, a novel synthetic technique has been developed using polyethylenedioxythiophene (PEDOT), which is highly pseudocapacitive.112 In this study, interfacial polymerization without any surfactant at the interface of immiscible liquids was used. The related process demonstrated a highly scalable method for the production of large PEDOT paper. A schematic illustration of the fabrication process and photographs of the supercapacitors are shown in Figure 2D. 2.2.2. Other Energy Storage Devices. Other energy storage devices on paper, such as lithium-ion batteries,118 biofuel cells,52 and solar cells,36 have also been explored in recent years. Among these, lithium-ion batteries provide relatively high power.92,119 For instance, Hu and co-workers have developed thin and flexible lithium-ion paper batteries by using paper as mechanical support and separator.119 However, lithium-ion batteries usually contain other heavy metals, such as cobalt (Co), which casts a threat to the environment and public health. Therefore, paper-based MFCs and solar cells are much more promising candidates due to their environmentally friendly feature. Recently, the performance of paper-based MFCs has been significantly improved through an origami method,120 multianode structure,52 and array structure.121 However, the power densities remain very low. 2.3. Generators. With the increasing demand for energy and the ever-reducing fossil fuels on Earth, new sources of energy are required in the near future. Therefore, energyharvesting technologies have drawn significant interest from both academia and industry over the past decade. For instance, waste heat from vehicle exhaust provides approximately a 350 °C temperature difference, which can be recovered through thermoelectric devices.122 The water wave is

ZT = (σS2)/κT

where σ, S, κ, and T denote electrical conductivity, Seebeck coefficient, thermal conductivity, and absolute temperature, respectively. The expression σS2 is usually referred to as the power factor and determines the performance of thermoelectric materials. To date, various materials including inorganic and organic ones have shown favorable thermoelectric performance in terms of high electrical and low thermal conductivities.124,125 However, the applications of these materials are still very limited. The power factor can rarely exceed 100 μW m−1 K−2. Thereby, intensive treatments have been done to improve the performance of thermoelectric materials. Luo and colleagues recently capitalized on the post-treatment to enhance the thermoelectric properties of poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS),126 which is considered to be a promising organic substitution for inorganic hazardous materials (e.g., Bi2Te3, Sb2Te3). In this study, after treatment by the mixture of different concentrations of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) and dimethyl sulfoxide (DMSO), PEDOT:PSS film reached a high ZT of 0.068. CNTs are another commonly used material in thermoelectric devices, and they can be applied directly as a p-type material with a Seebeck coefficient of 49 μV/K. Moreover, this pristine CNT can be easily turned into n-type material by doping with phosphine-containing aromatic dopants, which was previously challenging.125 As paper exhibits higher thermal stability than polymer substrates, it has been employed as a substrate for thermoelectric generators. Jiang and collaborators demonstrated that paper can be used for the performance of PEDOT:PSS in terms of Seebeck coefficient, electrical conductivity, and thermal conductivity.127 As a simple method, writing PEDOT:PSS directly on paper was applied to fabricate a composite of these two components. The authors also showed that paper and PEDOT:PSS have a nice affinity by which no peeling off and dispersing were observed after this composite had been immersed in water for 48 h. In contrast, PEDOT:PSS-coated glass substrate possesses low affinity as it detached after 1 min when soaking in the water. Moreover, Wei and co-workers developed a screen-printed paper device that took advantage of PEDOT:PSS and successfully lit an LED for 100 h.128 In this study, PEDOT:PSS ink was prepared by adding ethylene glycol to increase its electrical conductivity.129 In contrast to conventional devices with π-type structure with both p- and n-type materials, plain structure of only p-type material was used to adapt thin property of paper substrate. After connecting the as-printed lines of PEDOT:PSS with silver paste, the authors achieved an output power of 4 μW. Furthermore, Crispin and co-workers have recently combined nanofibrillated cellulose and polymers to develop thermoelectric devices.130 2.3.2. Other Generators. Interaction between humans and devices has recently been investigated to harness energy using paper electronics. The use of paper devices usually involves 20508


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Figure 3. (A) Paper touch pad based on piezoelectric property of ZnO nanowires. Adapted with permission from ref 136. Copyright 2014, ACS. (B) Schematic illustration of paper nanogenerators based on electrification and piezoelectric effect. Adapted with permission from ref 138. Copyright 2015, IEEE. (C) Improvement of electric output through folding: (a) schematic and (b) image of multiple-fold paper generator; (c) output currents and (d) total charge transfer of paper generator. Adapted with permission from ref 139. Copyright 2015, Elsevier. (D) Principle of paper generator based on electrostatic effect: (a) original, (b) pressing, and (c) releasing state of paper generator with external load R; (d) relationship between ratio of charge density and thickness of PTFE. Adapted with permission from ref 140. Copyright 2013, RSC.

Figure 4. (A) Schematic illustration of microstripe antennas. (B) Antennas with no slot, one U-slot, one modified U-slot, and two U-slots. Adapted with permission from ref 145. Copyright 2014, IEEE. (C) Relationship between return loss and frequency with different structures. Adapted with permission from ref 146. Copyright 2013, IEEE. (D) Antenna with hybrid structure. Adapted with permission from ref 147. Copyright 2015, IEEE.

been extensively used for various applications, such as biosensors,133 solar cells,134 and electrical generators.135 Hitherto, researchers have extended its applications to paper generators. For instance, Liu and coauthors have grown ZnO nanowires onto chromatography paper for the fabrication of a piezoelectric touch pad.136 When touched by fingers, the

rubbing, sliding, and touching. Thus, generators were developed on paper to collect related energy and convert it to electric energy for digital displays or actuating other components.131 Owing to its extraordinary performance in semiconductive, photoluminescent, and piezoelectric properties,132 ZnO has 20509


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Figure 5. (A) Schematic illustration of fabricating electronic circuits on paper by Gerber format-based etching. Adapted with permission from ref 14. Copyright 2015, IEEE. (B) Fabrication of capacitor on paper: (a) dimensions of the patterns; (b) preparation process for capacitors; (c) exploded view of the array of capacitors. Reproduced under the terms of the Creative Commons Attribution License.155 (C) Process of building graphenebased electronic circuits on paper: (a) filter the graphene dispersion through membrane filter; (b) turn the filter over and place on paper; (c) draw the pattern by a pen and then remove the membrane filter. Adapted with permission from ref 158. Copyright 2013, Wiley.

mark and an antitheft sensor based on transparent paper generators.94 2.4. Antennas. Conventional antennas for transmitting and receiving radio waves are ubiquitous in our daily life. However, these antennas (e.g., monopole, dipole, aperture, and loop antennas) are bulky and costly. Therefore, building antennas on paper has caught growing interest.142 Microstripe antennas built upon flat surfaces (e.g., papers) have paved the way for inexpensive, lightweight, conformal configurative, and environmentally friendly electronic products.143 Microstripe antennas consist of three components: a radiating patch, a dielectric substrate, and a ground plane (Figure 4A). The shape of a radiating patch usually adopts as a rectangle to simplify the analysis of performance, although other geometries are used as well. To obtain a better radiation, the dielectric substrate should be thick and have a low relative dielectric constant.144 Device fabrication using inkjet printing is considered to be the Holy Grail for paper electronics due to its convenience. Recently, a U-slot monopole antenna has been fabricated through inkjet printing.148 In this study, the U-slot was cut from a rectangular radiator to excite a narrow band. Inkjet printing was applied to print conductive silver ink on paper for radiator, microstripe feed line, and ground plane. The fabricated antenna showed a triband operation in 1.57, 3.2, and 5 GHz, which is suitable for GPS, WLAN, etc. Similarly, Chen and co-workers developed a triband monopole antenna for wireless applications using two U-slots (Figure 4B).145 Moreover, Rizk and others also capitalized on silver nanoparticles ink associated with inkjet printing to fabricate Z-shaped antenna on a glossy paper.149 Conductive ink was also printed onto cardboard, which was pretreated by a dielectric board to reduce conductive losses.150 However, these paper antennas may exhibit limited ability in receiving and transmitting signals due to the losses of dielectric

deformation of the paper substrate could lead to the generation of an electric charge output due to the piezoelectric property of ZnO nanowires on paper fibers (Figure 3A). The authors also demonstrated that the average magnitude of the generated current peak was proportional to the growth percentage of ZnO nanowires and the pressure applied onto the touch pad. Finally, the preprogrammed password was used to investigate the feasibility of using this paper generator. Another example of using ZnO was realized by integrating ZnO nanowires modified with carbon fibers and Au-coated ZnO modified paper.137 The fabricated piezoelectric nanogenerators exhibited an improved power output, and its performance can be adjusted by controlling the fiber number, strain rate, and connection modes. Furthermore, the piezoelectric effect of ZnO nanostructures and its electrification in contact with poly(methyl methacrylate) were integrated for paper nanogenerators (Figure 3B).138 Zhou and his collaborators have developed a metal-free foldable generator.139 In this study, CNT and polypropylene were used to fabricate an electric generator on the basis of electrostatic effect. The authors demonstrated the improvement of power output by folding (Figure 3C) and good performance in driving a liquid crystal display (LCD) by human stimulation. Moreover, electrostatic charges generated by corona technique were also applied.140 The paper generator was integrated by silver-coated paper and polytetrafluoroethylene (PTFE)/silvercoated paper. Because the surface charge density of silver-paper electrode was inversely proportional to the distance between the two electrodes,141 the current started to flow when this distance was changed (Figure 3D). Recently, the triboelectric effect was validated to be applicable for generators on paper. For example, Wang and others successfully developed a page 20510


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First, a PTFE filter was used for graphene dispersion; this filter was then turned over, and the circuit was drawn onto the paper using a pen through the filter. After the filter was removed, electronic circuits were completely built. Although the fabrication methods aforementioned are novel and easy to operate, they are still in their infancy, and further research is required to focus on improvement of efficiency and resolution during fabrication. Owing to the outstanding flexibility of paper, foldable circuit boards have been built on paper and shown exciting performance even though being folded or creased.53 The as-fabricated paper has shown excellent abilities in a wide range of applications such as smart envelope, skin bandage, paper-based microelectromechanical system, and disposable devices. Nevertheless, the study of paper-based circuit boards is still in its early stage. Drawbacks such as low resolution, unstable performance, and strict range of use still inhibit their mass adoption and commercialization.

materials and conductive nanoparticle ink.151 Therefore, electromagnetic band gap (EBG) structure was applied to paper antennas to compensate for the losses and improve the gain.152 Moreover, fractal design for antennas also indicated its ability to enlarge bandwidth and improve its performance.153 Twisting and bending performance of paper antennas showed great importance because of the mechanical properties of paper in the applications of curved surfaces.143 Nordin and coauthors demonstrated that resonant frequencies shifted right or left, respectively, by bending antennas with circular or rectangular EGB.151 However, these two prototypes indicated a better performance in antenna matching. Wang and Li also investigated the twisting effect of inkjet-printed antennas, and the as-fabricated antenna showed good robustness toward twisting.143 Additionally, Misran and collaborators have developed another design to reduce the return losses and broaden the bandwidth of antennas on the paper.146 On the basis of upper and lower slots, they demonstrated that return losses could be as low as −34.19 dB and the bandwidth can be 400 MHz at 6.61 GHz (Figure 4C). Radio-frequency identification (RFID) has become another main application for paper antennas in recent years. For instance, Turkish researchers recently have developed a hybrid structure on paper as antenna and integrated it with passive RFID chips (Figure 4D); thus the total price could be lowered to U.S. $1.147 2.5. Electronic Circuits on Paper. Building electronic circuits onto the surface of paper has witnessed burgeoning growth in recent years because of its inexpensive and environmentally friendly merits.154 Various methods have been developed for the fabrication of paper-based printed circuit boards (PCBs), such as direct writing,155 screen printing,14 and printing of liquid metal.156 Due to the intrinsic porosity of the paper substrate, the conductive materials (e.g., silver ink) tend to penetrate into these pores, resulting in discontinuous electronic circuits.154 Moreover, the conductive materials are easily oxidized. Although these limitations restrict the performance of paper devices to some extent, a variety of applications have been demonstrated. For instance, simple and cheap paper devices were achieved by off-the-shelf materials, such as paper and foil tape.157 However, the resolution of the related electronic circuits was very low and, thus, not suitable for complicated applications. Recently, Choi and colleagues took advantage of Gerber format-based etching to deposit metal pattern onto paper, followed by soldering.14 Specifically, a stencil was fabricated by patterning AutoCAD pattern into an OHP film and then cut by an electronic cutting machine. Afterward, a metal layer (∼160 μm) was deposited through the stencil onto paper. After applying conductive epoxy to corresponding positions, the authors obtained final electronic circuits by soldering (Figure 5A). Furthermore, because liquid metal inks have remarkable adhesion and electrical properties, they have been demonstrated to build electronic circuits onto paper.155 GaIn10-based liquid metal ink was synthesized from gallium and indium metals with superhigh purity. The synthesized ink was then applied to fabricate electrical components, such as resistors, inductors, and capacitors, onto the paper by direct writing. As a paradigm, capacitor was fabricated onto the paper, as shown in Figure 5B. In addition, Chin and others utilized graphene as a conductor to build electronic circuits onto paper.158 In that study, another simple fabrication technique was applied to deposit graphene pattern onto a paper substrate (Figure 5C).

3. CONCLUSION The development of paper electronic devices has recently demonstrated impressive progress despite being in its infancy. Various indispensable applications ranging from basic electronic components to complex circuit boards have been proposed, investigated, and successfully implemented. In competition with traditional electronic counterparts, paper-based electronic devices are likely to gain more popularity in the foreseeable future due to numerous advantages such as low cost, flexibility, ease of fabrication and modification, disposability, and userfriendliness. For instance, high conductivity of paper can be achieved via specific functionalization. In contrast with conventional techniques, simple fabrication methods such as printing or drawing can be employed for paper substrates. Moreover, as a recyclable material, paper also allows the consumption of precious resources to be reduced, and the final devices can be easily disposed of by simple incineration. Despite all of the above-mentioned unbeatable advantages, paper electronics still suffer unavoidable drawbacks at the moment. For example, the properties of paper are highly dependent on humidity and temperature; thus, strict condition control is required. Moreover, the lifetime of paper devices is currently not comparable with that of traditional ones. Furthermore, to date, the resolution of electronic circuits on paper substrates still requires significant improvement, which creates a delay in replacing current PCBs. Nevertheless, because future mass production demands fast and mature techniques to generate real commercial products, paper substrates are among the most promising candidates for flexible, inexpensive, and environmentally friendly electronic devices.

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AUTHOR INFORMATION

Corresponding Author

*(J.X.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support to X.L. in the form of a Discovery Grant from the National Sciences and Engineering Research Council of Canada (NSERC RGPIN-2014-05487) is gratefully acknowledged. Y.L. acknowledges support from the Sigma Xi Grants-inAid of Research program (G201510151654964). 20511


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Recent Advancements in Functionalized Paper-Based Electronics

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