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Flourishing smart flexible membranes beyond paper Bingbing Gao, Hong Liu, and Zhongze Gu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b00743 • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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Analytical Chemistry

Flourishing Smart Flexible Membranes Beyond Paper Bingbing Gao†, Hong Liu‡* and Zhongze Gu‡* †School

of Pharmaceutical Sciences and School of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing

211816, China. ‡State

Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering, Southeast University, Nanjing

210096, China.

Recently, smart flexible membranes (SFMs) especially paper has been flourished, SFM-based devices are characterized by passive response to external stimuli and manipulate liquids and electronics, making SFMs suitable for (bio)chemical sensing, display, wearable sensing, and energy harvesting. In this feature, we summarize both the historical development and recent advances in SFM-based devices built with both traditional papers and other flexible membranes, including passive responses to external stimuli, manipulation of microfluidics and electronics and multiple applications based on such manipulation. ■ Introduction Since ancient times, paper has been a ubiquitous material in daily life with various functions.1-4 The application of paper in the analytical field (litmus paper) has been dated back to the early 1800s in scientific reports;5 since then, the development of paper as an analytical substrate has been pursued continuously.6 With its thin, porous, flexible, inflammable, water-absorbent and affordable properties, paper has been widely used as the most popular carrier for information transmission,7, 8 for both writing and printing substrates,9 for analytical devices,6 and for energy harvesting and storage devices.10 Paper-based devices are easy to manufacture, rich in source materials, low in cost, renewable, biocompatible, disposable, recyclable, and easy to chemically modify.11, 12 The word smart for smart paper represents two aspects of the material. One aspect is a passive response to external stimuli: for example, a paper device can respond photoelectrically to external objects to be measured, making the device suitable for (bio)chemical sensing.13, 14 Second, paper can be driven by the outside world and can manipulate liquids15 and electronics,16 making paper suitable for display,17 wearable sensing,18 and energy harvesting.19 A piece of paper is typically made of cellulose fibers: the fibers are pressed into a membrane while moist and dried to form paper. This procedure gives paper some valuable properties: 1) liquid can be automatically transported in paper; and 2) paper is stable in many (bio)chemicals, as paper is composed of a hollow hydrophilic network structure that directs liquid flow, is inert and is unaffected by both inorganic and organic agents. Among various POCT (point-of-care testing)20 detection methods,11, 21 22 paper-based analytical devices are widely used as a major development direction of POCT testing23 with their merits of simple operation, low cost, portability, disposability, and the lack of a need for complicated external instrumentation.24 The first report of a microfluidic paper-based analytical device (μPAD) for chemical analysis was by Martinez et al. in 2007.25 The authors used photoresist as a patterning reagent to make hydrophobic and hydrophilic patterns, and the hydrophilic channels enabled liquid transfer (carrying samples) from the sampling area to the sensing area. Later, Whitesides and his coworkers developed a paper microfluidic chip manufacturing method based on commercial wax printers26, 27, which greatly reduced the production cost and improved the production speed of the paper chips, thus promoting the development of paper microfluidics. In 2011, Liu and his coworkers developed a three-dimensional (3D) origami microfluidic chip (oPAD) based on two-dimensional (2D) paper 1 ACS Paragon Plus Environment

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microfluidic chips using the principle of origami14, 28. The oPAD also advanced the electrochemical signal conversion and amplification technology of paper microfluidic chips, which solved the need for complex instrumentation to address quantitative detection problems related to microfluidic chips. The use of paper as an analytical platform has several advantages: 1) microchannels are self-driven without additional electricity and pumps; 2) the large specific surface area can store test substances; 3) paper is the least expensive material for microfluidics, and the printing method is relatively cheap; 4) paper can easily be stacked together to form a multilayer structure, thereby achieving 3D cell cultures; and 5) paper can filter particulate matter in samples, such as in filtering out blood cells. However, paper chips also have many obvious shortcomings: the sensitivity of detection is not satisfactory because the channel fiber mechanism interferes with the signal and adsorbs the sample, and traditional microfluidic methods such as capillary electrophoresis and droplet formation are rarely applied to paper. Furthermore, high-precision integrated chips are difficult to achieve, the smallest channel is approximately 200 μm, and solutions are volatile in open channels. In addition to the great achievements that paper has made as an analytical platform that can respond to external stimuli, paper can also be driven by the environment. Paper can manipulate both liquids and electronics, thus enabling the potential application of paper in electronic fields. Recently, flexible porous membranes have been widely used in flexible electronic and energy storage devices. Paper exhibits piezoelectric behavior and is porous, biodegradable, affordable, and easy to use. Due to these excellent features, paper is a promising low-cost substrate material for the construction of various electronic carriers and attractive in the fabrication of flexible electronics, display applications, energy storage, soft robotics, and energy harvesting and storage devices. For decades, researchers in various fields, including both science and engineering, have reported a wide range of (pseudo-)paper-based devices that focus on the fabrication and development of new cost-effective microfluidics and electronics. With the fusion of various disciplines, the combination of microfluidics and electronics on paper has exhibited great potential in promoting the rapid development of flexible smart devices in fields such as artificial skin, soft robotics and flexible electronics. Many interesting breakthroughs regarding new types of integrated paper devices have been achieved. With the development of paper, the rigid identification of paper has become blurred: pseudo-paper and membranes with appropriately flexible or porous structures can be defined as paper. For example, photonic crystal papers have recently been developed as attractive platforms for display, microfluidics, and analysis due to their unique optical properties and high-order structures. To date, a comprehensive review on paper devices is still lacking, especially considering the advancements in recent years. We believe that such an overall review of paper devices would have a great influence on this flourishing field and will inspire scientists in various research fields. Therefore, we present here an overall review that focuses on an overview of recent developments in SFM-based devices, especially paper devices, and considers aspects of SFM devices built with both traditional papers and pseudo-papers (paper-like membranes). These aspects include passive response to external stimuli ((bio)chemical sensing), driven manipulation (microfluidics, electronics), multiple applications based on such manipulation, and the fabrication methods of SFM devices, from historical developments to recent advances and future prospects. First, we present SFM analytical devices for passive response to external stimuli ((bio)chemical sensing). Then, interesting SFM devices are summarized according to the driven manipulation of microfluidics, various driving forces and pattern dimensions (1D, 2D and 3D), and SFM electronic devices based on paper electronics, followed by a detailed discussion of multiple functional SFM devices based on these features. Then, we provide an overview of SFM materials and device fabrication methods (in the Supporting Information). Finally, we present a summary and several perspectives on many development directions in this fascinating field. The purpose of this review is to provide an overall view of functional SFMs beyond paper devices in both microfluidic areas and electronic areas, promoting communications among analytical chemistry, materials science and electrical science. ■ Passive

response to external stimuli 2 ACS Paragon Plus Environment

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SFM devices can respond passively to outside stimulation and can respond photoelectrically to external objects to be measured, thus making such devices suitable for (bio)chemical sensing. Paper and cellulose have been attractive platforms for sensing applications in (bio)chemistry research for decades. Paper-based cellulose ester microfiltration membranes/devices are widely used for fast, low-cost, and sensitive sample pretreatment because of their advantageous disposable, low-volume, portable, and easy-to-use properties.29 Typically, cellulose membranes act like a diaphragm and separate liquid into two areas; with liquid pumping, particles or elements can be separated. Luo et al.30 showed (Figure 1(A)) a 3D origami paper-based electrophoretic device (oPAD-Ep). This 3D paper chip enabled the fast separation of fluorescent molecules and proteins in average of 5 min under a very low driving voltage (~10 V only). The driving voltage was much lower than that in conventional electrophoresis, which usually calls for a driving voltage of ~100 V. Optical detection is simple and intuitive and is the most common detection method. The basic principle of optical inspection is to use a chemical reaction or an enzyme-linked immunoreactivity reaction to change the color of a dye or to develop a color based on the agglomeration of nanoparticles or quantum dots. Commonly used examples include pH test paper, urine test strips, and lateral-flow immunochromatographic test strips.31 Xu et al. reported the design and fabrication of conjugate probes using aptamer-labeled AuNPs replaced with antibodies (Figure 1(B)).32 With the help of inkjet printing, the rapid preparation of analytical regions can be performed. For example, to achieve the most effective multiple enzyme-based reactions, inkjet printing enables the printing of enzyme combinations with various proportions. Despite the wide use of dye-based colorimetric analysis methods, structural color (for example, colloidal crystals or photonic crystals) has also been used in sensing methods, especially for label-free-based detection,33, 34 as the color yield is more stable and vivid. Figure 1(C) shows a photonic crystal pattern with multiplestructural color shifting capability under vapor. 35 Electrochemistry is the science that studies the relationship between electrical and chemical reactions. The instruments required for electrochemical detection devices are simple and portable, and the cost is low. Electrochemical detection is renowned for its high sensitivity, good selectivity, easy integration and miniaturization. The integration of electrochemical means and paper chips has been widely reported.36-39 Figure 2(A) shows an oPAD; as reported by Liu et al.,14 analytes in a sample were introduced from the sampling area, and aptamers captured the analytes. A glucose oxidase (GOx) tag was used to modify the relative concentrations of an electroactive redox couple, and the self-powered oPAD generated an electrical signal automatically, thus enabling a digital multimeter (DMM) readout. Fluorescence refers to a phenomenon of photoluminescence known as cold luminescence. The use of fluorescence for detection has many advantages, such as good selectivity and a low detection limit. However, fluorescence detection often requires instrumentation, and the paper material in paper chips has a certain fluorescent background. It has been shown to be possible to detect proteins, bacteria and DNA on paper chips. In a typical approach, Figure 2(B) shows a paper device for automatic sample preparation and LAMP, as reported by Connelly et al.40 Biomarkers in human body fluids are important for day-to-day personal healthcare sensing and monitoring. However, the small number of appropriate markers in fluid restricts such sensing. Recently, SERS has emerged as an attractive platform for small-molecule sensing with ultrahigh sensitivity and label-free properties.41, 42 Park et al. reported a plasmonic Schirmer strip for the quantitative SERS detection of UA in human tears. The testing strip was rapid, simple, inexpensive, and label-free. Using cellulose paper as a substrate to grow nano Au islands, the film thickness and the deposition rate during thermal evaporation could be controlled (Figure 2(C)).43 Current detection methods using PADs as sensing platforms always call for specific analyte requirements or require analyte labeling for spectroscopic and electrochemical sensing.44 Mass spectrometry (MS) analysis is more precise and requires no extra prelabeling of samples;45-47 however, a large-scale instrument is always required for MS analysis. The Cooks team used paper as a carrier for the spray48 for high-throughput detection and combined the paper spray platform with a portable mass spectrometer for testing, greatly enriching the application of MS (Figure 2(D)).48 3 ACS Paragon Plus Environment


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Since more than 95% of all new drugs have poor pharmacokinetic properties, such as low water solubility and low bioavailability, the controlled release of drugs plays an important role in efficiently utilizing drugs. The layer form of paper offers an ideal platform for 3D cell culture, or paper-based cell culture, and can serve as a disease model to imitate in vitro normal and diseased tissues, thus making paper-based cell culture an attractive method for researching the physiological and pathological behaviors of cells.49-51 To date, many studies have shown that cell culture on paper can simulate various natural pathophysiological microenvironments.52, 53 The microenvironment (for example, oxygen supply, metabolic conditions and pH) in a paper-based culture platform can be regulated to meet the needs of the application.54Mosadegh et al.55 reported cells cultured on a paper-based platform and assembled together to form 3D culture models. A stretched IO paper-like

membrane was used for cell orientation regulation: tendon fibroblasts were arranged in a certain direction on stretched IO paper (Figure 3(A)), as reported by Lu et al.56, 57 ■

Driven manipulation Liquid manipulation

Microfluidic chip analysis systems are systems for controlling fluid in microchannels.58-60 Such systems can perform many complicated operations,61-66 including sampling, dilution, injection, reaction, separation, analysis and detection. Microfluidic chip analysis systems are a scientific technology that uses tens or even hundreds of micrometer-scale microchannels to control small amounts of liquid. The channels in the chip are at the micron scale, ensuring a small amount of sample consumption during analysis, so lab-on-a-chip are widely used. For bioanalysis, the key technology in lab-on-a-chip POCT drug research is the driving of liquids. For example, in coin-sized microdevices, researchers can set up experiments of protein crystallization, requiring only 10 nL of protein sample per experiment. In structural biology, the reduction in sample consumption is significant, saving time and cost with a large number of samples, and miniaturization has far-reaching implications in developing next-generation biological tissue engineering. Microfluidic driving and control technology is complex and diverse. However, the driving of liquid is crucial. There have been two main developments in the development of liquid-driving techniques in biomedical microanalysis in the past decade: electroosmotic flow and pressure-driven flow; pressure-driven flow can be achieved in a variety of ways, including volumetric pumps, reciprocating pumps, gravity, surface tension, and centrifugal force. Water can spontaneously penetrate through paper; there is no need for an additional pump. In addition, paper can be functionally modified to change its hydrophilicity. In recent years, paper has commonly been used in the manufacture of sensors because of its versatility, abundant availability, and low cost. Furthermore, paper-made devices are generally flexible, portable, disposable, and easy to operate. Liquid flow at the bulk scale is different from flow at the micro-/nanoscale, and microfluidic devices can deliver and control small amounts of samples and are widely used in chemical reactions and biological assays.67-71 In contrast to traditional liquid flow in microfluidic channels with pumps, which limits wide application,72, 73 the use of capillary forces for liquid self-pumping is promising.15 In self-driven microfluidic devices, liquid flow is controlled by directional surface structures or surface chemistry. Conventional microfluidic devices usually require extra 3D wall fabrication to form channels (glass, silicon and polymers), Figure 3(B) shows the fabrication of a ryegrass leaf structure, as reported by Guo et al.74 PDMS was polymerized on ryegrass leaf, followed by poly(vinylidene fluoride) (PVDF) replication of the template to obtain a replica of the positive microstructure of the leaf. The artificial surface was (super)hydrophobic and used for liquid regulation. Centrifugation is highly important in medical diagnostic facilities.75-77 Centrifugation helps extract plasma from whole blood and pathogens and parasites from blood or urine for analysis and is always the first step in diagnostics.78, 79 An ultralow-cost (20 cents), lightweight (2 g), human-powered high-speed (125000 r.p.m) paper centrifuge (paperfuge) inspired by an ancient whirligig (or buzzer toy; 3300 BC) was introduced. Driven by centrifugal forces, the paperfuge was used to separate pure plasma from whole blood in less than 1.5 min and 4 ACS Paragon Plus Environment

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isolate malaria parasites in 15 min, according to Bhamla et al.80 As shown in Figure 3(C), an optical reader that can measure the transmittance in a very sensitive and rapid manner on a hybrid paper-polymer centrifugal disc platform was reported. This device enables real-time monitoring of multiple samples by measuring the absorbance of the light transmitted through the paper integrated on the disc between the light emitting diode (LED) and the photodiode (PD) regardless of the ambient light condition.81 Transpiration is a natural phenomenon in plants.82, 83 This phenomenon is the process by which water is lost from the surface of living plants (mainly leaves) to the atmosphere in the form of water vapor.84, 85 Jing-min Li et al. created a microfluidic pump based on the pull of transpiration.86 Liu et al.87 presented a ring-shaped oven integrated on a paper-based immunodevice for self-pumping and self-washing.

Multidimensionality Paper and other film materials are used in a wide range of applications, especially in rapid testing, such as in commercial test strips and test cards. Conventional test strips are usually based on ELISA to produce color changes and are often only used for qualitative analysis, not quantitative analysis. However, traditional analytical test strips have the disadvantages of single function, low sensitivity and selectivity, large sample consumption, and an inability to perform quantitative detection and are increasingly unable to meet the increasing demand for POCT. mPADs, i.e., paper chips, can solve this problem and become more miniaturized, integrated and multifunctional. Paper microfluidic analytical technology has been combined with analysis via test paper technology and microfluidic chip technology. With the development of paper microfluidic devices, multiple functional and complicated chips have been developed. The basic unit of paper microfluidic devices is the channel; devices can be simply divided into 1D,88, 89 2D90 and 3D91-93 and distinguished by the channels, and the channels can be considered as lines. The “dip-and-read” assay format eliminates the capillary flow-based analyte depletion step in microfluidic paper channels (Figure 4(A)), as described by Yamada et al..94 Carrilho et al. fabricated (Figure 4(B)) paper-based plates by patterning sheets of paper into hydrophilic zones surrounded by hydrophobic polymeric barriers using photolithography.95 3D paper chips offer a convenient platform for integrating microfluidic and electronic components (Figure 4(C)). Ainla et al.96 reported that with the emergence of 3D chips, complicated chips could be fabricated for the purpose of automatic analysis. A 3D POC paper-based four-valve lateral flow immunosensor was developed by Gerbers et al. A channel network formed with several layers of paper and tape directs three different fluids to flow sequentially over the detection area, which enables automatic ELISA for rabbit IgG sensing.97 ■ Electronic manipulation Based on the excellent properties of paper substrates, such as low cost, porosity, stability and flexibility, paper electronics have attracted much attention and are considered promising technologies and exciting materials for future flexible electronic device fabrication.98, 99 Paper patterning methods for the fabrication of microfluidic devices are also suitable in patterning conducting materials to fabricate e-papers. Many e-papers, such as circuits, electrodes, resistors, antennas, and radio frequency (RF) devices, have been developed.100 The performances of these devices have been determined by researchers, and the conductive properties (both density and connection) of the materials fabricated on paper highly influence the stability of the e-paper devices. In this section, we will summarize the research of e-paper devices, starting with the basic units of electronics: paper circuits, electrode-to-paper electronic devices, PCBs, and energy harvesting and storage devices. Circuits and electrodes are the most basic units of electronic devices. To realize electrochemical analysis on paper microfluidic devices, paper electrodes have been widely fabricated, and the fabrication methods of paper microfluidic devices are also suitable for paper electronics. The fabrication of paper-based electrochemical devices (PEDs) using a commercial digitally controlled desktop plotter/cutter, for example, substrates with pillar arrays were used to automatically guide nanoparticle assembly into curved shapes. With this method, curved NP arrays were printed on flexible PDMS to form paper-like electronic membranes, and the papers were used as wearable sensors of small strains on human skin to identify motions on the face (Figure 4(D)), as Su et al. reported.101 Since paper channels can work as wires, electrodes, and microfluidic channels at the same time, 5 ACS Paragon Plus Environment


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the cofabrication of electronic and microfluidic channels on paper was realized in an electrofluidic device (Figure 4(E)), as reported by Hamedi et al.102 With its unique porous bulk structure and rough and absorptive surface properties, paper is promising in energy-based applications for both energy harvesting and storage with comprehensive performance. For energy harvesting on paper, paper-based triboelectric nanogenerators (TENGs) have attracted the most attention due to their low-cost, environmentally friendly merits. Due to the foldability of paper, an X-shaped paper TENG (XPTENG) was proposed by Xia et al. to increase the output performance and harvest the mechanical energy generated by motion of the human body (Figure 4(F)).103 ■

Multifunctional applications Display

Paper-based flexible electronics 99, 104, 105 have many unique advantages over traditional electronics and are widely used in smart pixels, storage devices, electrowetting, printing circuits, batteries, photovoltaic devices, and touch sensors.106-112 Flexible displays have outstanding mechanical flexibility and are lightweight and impact-resistant. According to recent research, flexible displays using flexible substrates are more durable and lightweight than rigid glass, and various types of flexible displays based on plastic substrates, liquid crystals, organic light-emitting diodes and electronic paper have emerged as cost-effective and eco-friendly materials. Paper made from cellulose, which is a natural wood resource and abundant in nature, is a promising material for the fabrication of display substrates. Printing, spraying or transferring patterns on flexible substrates are the most direct ways of patterning materials for display. Figure 5(A) shows the fabrication of a solid noniridescent structural color by spraying a PDA@SiO2 amorphous colloidal array. 113 Figure 5(B) shows a simple practical method for fabricating diffraction gratings by stress gradient treatment of elastic photonic crystal films, as reported by Ding et al.114

Motion sensing and wearable sensors As the aging population continues to increase, the growth of the world population has forced a rapid rise in medical expenses. The healthcare system is undergoing a transformation in which continuous monitoring of inhabitants is possible even without hospitalization.115 The advancement of sensing technologies, embedded systems, wireless communication technologies, nanotechnologies, and miniaturization has made it possible to develop smart systems to continuously monitor human activities. Wearable sensors detect abnormal and/or unpredictable conditions by monitoring physiological parameters and other symptoms.116, 117 Thus, necessary help can be provided in cases of urgent need. Wearable sensors have attracted considerable interest in recent years due to their great promise in widespread applications.117-119 However, the lack of reliable noninvasive chemical sensors has greatly hindered progress in the field of human sensing. Because of their high performance, inherent miniaturization and low cost, electrochemical sensors provide considerable prospects as wearable chemical sensors for various applications.120-122 There already exist various kinds of wearable sensors (usually produced by introducing functional nanomaterials into flexible support materials)123, 124 that enable real-time and noninvasive monitoring of electrolytes and metabolites in human body fluids for personal health monitoring.125, 126 Due to their high integration with the human body and their ability to monitor individual health by methods including biochemical analysis and physiological monitoring, wearable sensors (including flexible microfluids and wearable electronics) help to diagnose early diseases and evaluate sports injuries.122 127, 128

Figure 5(C) shows a paper-based humidity sensor used to measure breathing rates by converting humidity changes into electricity. The sensor is a good example of a combination of paper sensors and traditional electronics, as described by Güder et al.129

Actuators and soft robotics 6 ACS Paragon Plus Environment

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In contrast to hard robotics, the latest advances in soft robotics utilize the flexibility of materials to create highly adaptive robots for soft interactions with the environment.130 Flexible tendril manipulators are based on different driving technologies, such as pneumatic actuators, cable drive systems and shape memory alloy (SMA) actuators.131, 132 Actuator materials can transform chemical or physical energy into mechanical energy.133 Soft actuator materials have attracted increasing attention due to their potential applications in soft robotics, artificial muscles and bionic devices.133 Among the various actuator driving methods, electrical, chemical and photoactivated methods are the most widely used and researched. Recently, to save the energy used for actuating, great efforts have been made to reduce the driving voltage and temperature of thermal response actuators, thus making the development of chemical actuators very attractive., Fu et al.134 reported artificially synthesized paper-like IO hydrogel membranes that had structural color with self-regulating ability. As shown in Figure 5(D) they also reported a cellular mechanical visualizable biosensor formed by assembling engineered cardiac tissues on the Morpho butterfly wings. The assembled cardiomyocytes benefit from the periodic parallel nanoridges of the wings and can recover their autonomic beating ability with guided cellular orientation and good contraction performance.135

Energy harvesting and storage The efficient production and storage of energy in an eco-friendly and sustainable way has become a global goal.136 In particular, new energy storage methods are needed to utilize energy from renewable sources, such as wind and solar energy, where irregular power generation (volatile and inefficient) poses a considerable challenge to traditional energy storage systems. Thus, various energy storage technologies, such as mechanical, physical, thermal, chemical and electrochemical energy storage systems, have been developed.137 138, 139 Flexible electronic devices with the characteristics of simplicity, low cost, light weight, recyclability, easy operation and one-time use are especially needed for building energy storage devices.140 Papers have many unique properties and are widely used in μPADs.141 10, 142 With the rapid development of paper devices, the use of paper materials to generate energy has attracted wide attention.143, 144 110

Other devices We have already discussed paper devices, including paper microfluidics, for analysis and paper electronics for circuits and energy storage. However, even though we limit our discussion range to chemistry, the robust range of paper devices are beyond our ability to show to readers. Nevertheless, we also introduce here some other interesting paper devices for the purpose of inspiring scientists in different fields. Zhang et al. reported a graphene film heater prepared by laser reduction of GO.145 A stainable high-frequency RF GaN device was made by epitaxial growth on 2D boron nitride for chemical-free transfer to a soft, flexible paper-like substrate, as reported by Glavin et al.146 Fan et al147 showed (Figure 5(E)) a microphone for sound recording based on the wave energy harvesting of a 125 μm thick rollable TENG. Tao et al.148 showed the use of ionic liquids as ultratemperature-sensitive fluids to fabricate an ultrafast-response and temperature-stable paper thermometer. Zhang et al. reported a photonic crystal viscometer for determining the viscosity of a liquid sample from the infiltration time to soak the PC film, as shown in Figure 5(F).149 ■

Conclusion and perspectives

In this overall review, the historical status and recent developments in SFM-based devices were presented, including SFM analytical devices (passive response to external stimuli), SFM microfluidics, SFM electronics (driven manipulation), multifunctional applications, materials and fabrication methods. We first summarized SFM analytical devices for (bio)chemical sensing. Then, the driven manipulation of microfluidics and electronics, including 3D SFM-based analytical devices and SFM electrodes, circuits and energy storage devices was discussed, followed by a detailed discussion of multifunctional SFM devices and the materials and fabrication methods for SFM device construction. Although numerous SFM devices have been developed in recent years, there are several directions for further research and development in this attractive and flourishing research field. 7 ACS Paragon Plus Environment


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First, with the rapid development of flexible membranes, the rigorous barriers between paper and membrane have become blurred. Paper should be considered as a series of SFMs (e.g., photonic crystal films) instead of cellulose papers only, and additional novel materials, especially materials with functional structures (e.g., ordered structures or metastructures), can be used as paper substrates for the fabrication of specific paper devices. To date, research has focused on GO, liquid metals, shape memory materials, 4D printing materials, and healable substrates. The combination of the porous structure of paper and these materials may reveal special properties of paper and endow paper with new functions. Second, with the rapid development of SFM devices, single-performance SFM chips are far from meeting the demand. Therefore, the manufacture of multifunctional paper chips is imminent. To achieve this goal, not only innovations in SFM materials are needed but also the integration of various technologies, such as microfluidics and microelectronics. Integration in SFM substrates is a good example of how in the future, with the help of microfabrication, various emerging industrial technologies such as micromotors and artificial intelligence will be integrated into SFMs. Third, future manufacturing methods require higher pattern resolution and a lower cost. Large-scale production of SFM products is an inevitable topic outside the laboratory. Fortunately, there has been some research on expanding paper microfluidic platforms. Despite these promising explorations, there are still many challenges for commercial-scale manufacturing. In summary, SFM devices have developed into a powerful research direction and have valuable applications across many fields and disciplines. We hope that this review will inspire researchers from different backgrounds to address these issues and contribute to the development of paper devices as a broadly reliable, truly industrialized technology. We also believe that SFM-making equipment will accomplish even more exciting achievements.

Author Information Corresponding Author *E-mail: liuh@ seu.edu.cn, [email protected].

Notes The authors declare no competing financial interest.

Biographies Bingbing Gao is currently work in Nanjing Tech University as an associate professor. In 2013 he joined Prof. Zhongze Gu’s group and received his Ph.D. degree in biomedical engineering at Southeast University in 2017. Then he continued his research work as post-doctor in Southeast University for two years. His current scientific interests are focused on paper microfluidics, flexible electronics and wearable devices. Hong Liu received his bachelor and master’s degree from Nanjing University with Dr. Huangxian Ju, and he received his PhD from the University of Texas at Austin in the USA with Dr. Richard M. Crooks. In 2013, he joined Southeast University, and he is now a professor and an associate director of State Key Laboratory of Bioelectronics. His research interests include electrochemistry and bioelectronic devices for diagnostics and treatments. In his spare time, he likes cycling, travelling and spending time with his family. Zhongze Gu received his Ph.D. degree in 1998 from the University of Tokyo under the direction of Professor Akira Fujishima. He started an academic career in 1998 at the Photochemical Conversion Materials Project in KAST as a researcher. From 2003, he has been a professor at the State Key Laboratory of Bioelectronics, Southeast University. His research interests include bio-inspired nanomaterials, tunable photonic crystal materials, biosensors, electrospinning and the biomedical applications of nanofibers.

Acknowledgments We gratefully acknowledge financial support from the China Postdoctoral Science Foundation project (2018T110428, 2017M621597), the Fundamental Research Funds for the Central Universities (2242018R20011), the Innovative and Entrepreneurial Talent Recruitment Program of Jiangsu Province, the 8 ACS Paragon Plus Environment

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National Natural Science Foundation of China (21635001), Key Research and Development Plan of Jiangsu Province BE2016002, the Project of Special Funds of Jiangsu Province for the Transformation of Scientific and Technological Achievements (BA2015067), the 111 Project (B17011, Ministry of Education of China), Key Research and Development Plan of Jiangsu Province BE2016002, the State Key Project of Research and Development (2016YFF0100802) and the Fundamental Research Funds for the Central Universities (2242018K41023). Supporting Information Available: Fabrication of SFMs: Materials and Fabrication methods

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9 ACS Paragon Plus Environment


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Figure ToC



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Figure 1

Figure 1. (A) An 11-layer origami paper-based electrophoretic device used to separate calf serum under a low voltage of 10.0 V. (B) Schematic diagram of an aptamer-functionalized AuNP conjugate probe-based LFA for human thrombin sensing. (C) Photonic crystal-based label-free sensing, structural color changes in a mesoporous silica nanoparticle (MSN) pattern under N2 vapor. Reproduced from ref 30 and Copyright 2014, American Chemical Society for (A). Reproduced from ref 32 and Copyright 2009, American Chemical Society for (B). Reproduced from ref 35 and Copyright 2014, American Chemical Society for (C). Figure 2


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Figure 2. (A) Electrochemical sensing on SFMs, operating principle of a self-powered oPAD. (B) SFM-based fluorescence sensing, schematic of a sliding-strip device. (C) Plasmonic SERS Schirmer strip for tear screening. (D) Paper spray ionization for analysis of a dried blood spot on paper. Reproduced from ref 14 and Copyright 2012, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim for (A). Reproduced from ref 40 and Copyright 2015, American Chemical Society for (B). Reproduced from ref 43 and Copyright 2016, American Chemical Society for (C). Reproduced from ref 48 and Copyright 2010, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim for (D). Figure 3

Figure 3. (A) Tissue engineering on SFMs, optical image and SEM images of a gradient-stretched IO substrate. (B) Surface-based liquid self-pumping. (i) Illustration of the artificial structured surface fabrication process. (ii) SEM image of the obtained PDMS surface with structures on the surface. (C) Hybrid paper-polymer centrifugal optical devices (i) The manufactured acrylic case (ii) Hybrid paper- polymer discs with18 inlet channels and 19 ACS Paragon Plus Environment


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detection zones. Reproduced from ref 56 and Copyright 2015, American Chemical Society for (A). Reproduced from ref 74 and Copyright 2012, The Royal Society of Chemistry for (B). Reproduced from ref 81 and Copyright 2018, 2018 Elsevier B.V. for (C). Figure 4

Figure 4. (A) Photograph of an assembled 3D-printed single-channel PAD for Ni detection. (B) Photolithographic fabrication of multichannel paper plates. (C) Photograph of device fabrication where microfluidic channels were printed on a single sheet of paper, the paper was folded around an electrotextile and the device was laminated with polyethylene films. (D) Curved nanoparticle arrays printed on flexible electronic devices for skin motion sensing. (E) PCB on paper as a microcontroller-based heater. (F) Image of the stacked XP-TENG shape with arm motion. Reproduced from ref 94 and Copyright 2018, The Royal Society of Chemistry for (A). Reproduced from ref 95 and Copyright 2015, Elsevier B.V. for (B). Reproduced from ref 96 and Copyright 2017, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim for (C). Reproduced from ref 101 and Copyright 2015, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim for (D). Reproduced from ref 102 and Copyright 2016, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim for (E). Reproduced from ref 103 and Copyright 2018, Elsevier B.V. for (F). Figure 5


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Figure 5. (A) Multicolored patterns of PDA@SiO2-AM arrays fabricated on multiple substrates, such as glass, PMMA, Al foil and paper. (B) Optical image and reflectance spectra of a photonic crystal paper film. (C) (i) Paper-based sensor in a facemask. (ii) Image of the data acquisition device. (D) The construction of the visualized detection platform by assembling engineered cardiomyocytes on the modified Morpho wings actuators (E) Image of a paper-thin TENG for sound recording. (F) PC viscosity test paper for detecting the viscosity of silicone oil. Reproduced from ref 113 and Copyright 2018, The Royal Society of Chemistry for (A). Reproduced from ref 114 and Copyright 2014, American Chemical Society for (B). Reproduced from ref 129 and Copyright 2016, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim for (C). Reproduced from ref 135 and Copyright 2018, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim for (D). Reproduced from ref 147 and Copyright 2015, American Chemical Society for (E). Reproduced from ref 149 and Copyright 2017, WILEYVCH Verlag GmbH & Co. KGaA, Weinheim for (F).


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