The technology of paper! Electronics can be made from cellulose nanopaper?

In recent years, the new material that has attracted the most attention after carbon nanotubes and graphene is Cellulose Nanofiber (CNF). Its Tensile Strength and Specific Modulus far exceed those of aluminum and stainless steel, but its density is only 1/5 that of steel. Furthermore, its Stiffness is close to that of carbon fiber, while its size is comparable to that of carbon nanotubes. It is the ideal material in terms of both lightness and strength.

In addition, CNF has a low thermal expansion coefficient, stable chemical properties, good heat dissipation, a relatively large specific surface area, high biocompatibility, the ability to be naturally degraded and regenerated and many other excellent features. Therefore, it has the potential to fully replace metal, plastic, glass, carbon fibers and other materials in the future to become the key next generation material that will enable the global industry to achieve the goal of environmentally sustainable development.

According to the analysis of CNF industry trends, the two innovative applications with the most market opportunities in the future are lightweight electric vehicles and solid-state, high-efficiency batteries. The goal of saving energy by reducing the weight of vehicle bodies has always been an important trend in the development of the automotive industry. Research has shown that, if a vehicle’s weight is reduced by 10%, fuel consumption will be reduced by about 7%, and every 100 kilograms that a car loses reduces fuel consumption by 0.3 to 0.6 liters per 100 kilometers, thus reducing carbon emissions by about 5 grams per kilometer. This not only greatly improves fuel economy but also meets environmental protection and energy saving goals.

In truth, the Ministry of the Environment of Japan launched an NCV (Nano Cellulose Vehicle) research project as early as 2016. The focus of this project was to adopt new CNF-reinforced resin materials in the development of automotive-related components to reduce the weight of vehicles and study the influence of weight reduction on the improving of energy efficiency. About 20 well-known enterprises and research institutions, including Kyoto University, Tokyo University, Denso, Toyota Boshoku, and DaikyoNishikawa, participated in the project. By 2020, a total of more than 12 billion yen had been invested.

Japan is currently leading the charge in global CNF research and development. Its main competitors are the Nordic and North American countries, which are also major forest countries. Taiwan is also rich in woody biomaterial resources. With its abundance of raw material resources, Taiwan also has considerable advantages for the development of CNF material technology. 

Director of R&D Center & Marketing Division, Chris Chen, 2022/08/20

The technology of paper! Electronics can be made from cellulose nanopaper?

Professor Feng-Cheng Chang

School of Forestry and Resource Conservation, National Taiwan University

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Paper is one of the oldest materials used by mankind for the storage and exchange of information. Traditional paper is highly porous, rough and hygroscopic and has low compatibility with most coatings and printing inks. Therefore, it is usually blended with a variety of non-fibrous additives to improve its strength, smoothness and optical properties as well as to improve its resistance to liquid and gas penetration.

Nanotechnology was initially introduced into the paper manufacturing process simply to optimize the processing technology, reduce energy consumption and improve paper formation. This, however, resulted in the unexpected production of a product with very unique features. Nanopaper can be made from a variety of nanomaterials, such as cellulose nanomaterials, nanocarbon materials, polymer nanofibers, metal oxide nanowires, and more. These materials are made into porous, ultra-thin sheets with at least one dimension in the 1–100nm range. These are fabricated using techniques such as electrospinning, solution processing, self-assembly, and simple suction filtration then filled with additives such as nano fillers, nano coating pigments or nano printing inks to improve dimensional stability and specific properties.

Many nanopaper products with additional functions have already been developed, including protective backing paper, low air permeability paper, transparent paper, super hydrophobic paper, flame retardant paper, photocatalytic paper, antibacterial paper, conductive paper, magnetic paper, sensor paper, printed electronic paper, shape memory paper, and even special paper for energy harvesting and storage, etc. [1].

In recent years, the development of flexible electronics has attracted the attention of the masses and come to be regarded as one of the next great changes in the electronics industry. Unlike traditional electronic components, which use silicon wafer or glass substrates produced via etching processes, flexible electronic devices use solution coating or jet printing processes to fabricate microelectronic components on flexible substrates. This makes the components and devices flexible.

Flexible electronics have lower material costs. Its low temperature process is also suitable for related electronic and optoelectronic components, such as Organic Thin Film Transistor (OTFT), Organic Light Emitting Diode (OLED) and Organic Solar Cell (OS). What’s more, it can also be used in flexible smart electronic and optoelectronic products, such as radio frequency identification tags, flexible displays, flat lighting panels, solar power supply systems, wireless smart sensors and wearable electronic products, etc. [2].

The substrate materials most commonly used for the substrates of traditional electronic components are plastic, metal and glass, etc., with plastic being the main material used in flexible substrates [3], which are commonly used as: polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polycarbonate (PC) etc. [4-5]. However, due to the low thermal expansion coefficient, low operating temperature, and non-renewable source of plastic, it is not conducive to printing, Therefore, it is necessary to find a good, flexible substrate material with which to replace it [3-5].

The cellulose nanopaper substrate is flexible, thin and lightweight, draws upon rich raw material reserves and is cheaper than other substrates in addition to being easier to process. What’s more, cellulose nanopaper can be made from the structural cells extracted from a variety of natural wood fibers and possesses environmentally friendly characteristics such as biodegradability. All these advantages make it very suitable as a substrate for the development of flexible electronic devices in the future.

Typical wood fiber is a composite material comprised of cellulose, hemicellulose, lignin and other extractives. The proportions of these chemical components and their structures vary between different types of raw materials. The cell walls of wood and natural fibers have a multilayered structure. Cellulose microfibers are arranged within the cell wall. The cell wall structure is decomposed to the micro-nano scale through mechanical and chemical treatments to produce single or bundled fiber units for the extraction of cellulose nanomaterials [6].

Figure 1 is a schematic diagram of the multilayered structure of wood fibers and cellulose fibers in which the cellulose microfibers can be further divided into crystalline and non-crystalline regions. This kind of multilayer fiber structure can be separated into fibers of different scales by various physical and chemical methods for further use.

Figure 1. The Multilayered Structure of Wood Fibers and Cellulose Fibers [Dias et al. 2020].

Many different types of cellulose nanomaterials (CNM) have been developed. They can be roughly classified according to their geometric shapes as: cellulose microfibers (CMF), cellulose nanofibers (CNF) or cellulose nanocrystals (CNCs) (also known as cellulose nanowhiskers). These cellulose nanomaterials are hydrophilic, easy to modify and come in a variety of sizes and shapes (Figure 2).

Top-down production of cellulose nanomaterials from plant lignocellulosic fibers requires the material to be mechanically separated via grinding, low-temperature crushing, or high-pressure homogenization, etc. By adjusting the pressure, number of cycles and other conditions, it is possible to produce CMF with diameters of between 100 nm to 1 µm and lengths from hundreds of micro µm to 1 mm. CNF with smaller diameters and lengths can be produced by increasing the pressure and adding more cycles[7]. The main disadvantage of this production method is the high energy consumption.

Cellulose nanocrystals (CNCs) are rod-shaped fibers with high crystallinity and high rigidity and a relatively low aspect ratio. Typically, they are 2–20nm in diameter and 100–600nm in length [8]. CNCs are often made using acid hydrolysis on pulp fibers or micro-crystalline cellulose. Hydrolysis using mineral acids such as sulfuric acid, hydrochloric acid, or phosphoric acid, is combined with ultrasonic treatment to remove amorphous regions of cellulose fibers. There is also bacterial cellulose (BC) (Figure 2-d), a pure and highly crystalline cellulose fibers produced from bacteria, which is also being developed in a variety of ways.

Nanopapers made of these cellulosic materials have many excellent properties including low thermal expansion coefficients, high smoothness, high optical transparency, improved barrier performance, and the potential for surface function modification and adjustment, etc. [9]. Interaction networks with different specific surface areas and pore structures may be produced from cellulose nanomaterials. Due to complex nanofibers/crystalline entanglements, CNF and CNC membranes can reduce the penetration speed of oxygen molecules in cellulose nanopaper, thus improving its oxygen barrier properties [10]. However, the water vapor barrier properties of cellulose nanopaper are reduced by its hydrophilicity and need to be improved through surface modification or the addition of other additives [11].

Adding cellulose nanomaterials to conventional paper increases its surface smoothness, making it a suitable printing substrate for inkjet printing technology. Dissolving or dispersing functional nanoparticles in solutions forms functional ink, which can be printed or coated onto paper using suitable technology. In this way, functional applications or devices can be built right on the paper. Dense accumulations of CNFs and CNCs allow ink to be absorbed through the pores while functional particles are retained on the paper’s surface. This allows for thinner lines and lower resistances, thus making it possible to develop soft nanopaper electronic devices [13].

Functional specialty paper can be produced through various technologies such as: the deposition of functional nanomaterial solutions on paper substrates. The OH groups of the cellulose fibers serve as anchor points. Ion-dipole interactions enable in situ synthesis of metal nanoparticles, which are stabilized via bonding to the surface atoms [14]. Furthermore, multilayer films can be deposited on cellulose fibers using layer-by-layer (LBL) assembly technology. This can involve the immersion of cellulose fibers in polyelectrolyte and nanoparticle colloid solutions or the deposition and growth of metal and metal oxide nanoparticles on cellulose fibers using electrostatic assembly [15-16].

The functional materials added to the paper structure can be either organic or inorganic. Those used in the development of electronic devices usually have conductive, semi-conductive or insulating properties. Materials are mixed and matched to achieve the desired functions, such as combining inorganic nanoparticles with insulating polymers to produce insulation layers with high dielectric constants and good printability [17-18]. Organic nanomaterials are generally less costly than inorganic materials and easier to handle through solutions. In addition, organic material and thin film combinations usually have good elasticity as well as higher flexural and tensile toughness, making them suitable for use in soft substrates.

However, most organic nanomaterials have poor electrical conductivity. Therefore, if high conductivity is required, metal nanoparticles or nano precursors need to be added. The most commonly used metal is silver, but silver is costly. As such, some relatively low cost metals such as copper, nickel and aluminum have also been used in paper substrate research. In addition, carbon nanomaterials such as Graphene and carbon nanotube (CNT) can also be combined with cellulose nanomaterials for the research and development of conductive cellulose nanopaper, the printing of circuits and the fabrication of devices on paper using conductive ink [19-21].

Using an appropriate surfactant to disperse CNT uniformly in water and mix it with CNF produces cellulose nanopaper with high electrical conductivity, good flexural toughness and high tensile strength [22]. Some scholars have combined Graphene coating with cellulose fibers to make nanopaper. The continuous network formed provides high conductivity. Excellent electrical conductivity with a small amount of RGO(5%) can be achieved using reduced graphene oxide in combination with CNF and conductive nanopaper with a sandwich structure (RGO/CNF/RGO) prepared via vacuum filtration. Figure 3 shows a common method for preparing conductive materials via the modification of cellulose nanomaterials [23].

Since the size of cellulose nanomaterials is smaller than the wavelength of visible light, the nanopaper prepared from it often has high light transmittance, making it suitable for use in developing transparent device substrates. The optical properties of cellulose nanopaper depend on the fibers’ diameters and packing density. Light transmittance and optical haze can be changed by adjusting the structural porosity and pore sizes. CNFs and CNCs can be combined to control the transmittance of nanopaper. Increasing the CNC content can reduce the optical haze of the cellulose nanopaper and improve transparency. High transmittance and high haze is the ideal combination for thin film solar cells.

Cellulose nanomaterials can be mixed with ordinary cellulose. For instance, mixing CNF/cellulose in a 60/40 ratio will result in a transmittance similar to that of PE. The haze level, however, will be much higher than that of PET. On the other hand, nanopaper made using TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical) and chemically modified CNF (TEMPO-Oxidized cellulose nanofiber, TOCN) will result in higher light transmittance and lower haze. Chemically modified cellulose nanopaper, however, is usually poor in thermal stability, which can affect the sintering and other post-processing procedures for nanomaterial layers.

Due to its environmental friendliness, the application of cellulose nanopaper in energy storage, especially as flexible electrodes or supporting substrates for energy storage devices such as batteries, super-capacitors, and solar energy and fuel cells, has attracted a great deal of attention. Nanopaper that combines carbon material and cellulose, for instance, is lightweight, strong, flexible, and rechargeable and has high energy density as well as other characteristics [23].

Figure 4. Conductive Nanopaper (a) Dispersion of Cellulose Nanomaterials and Nanocarbon Materials in Water; (b) Use (a) to Make Thin Films; (c) Conductive Nanopaper with a Thickness of About 200μm Made After Freeze-Drying; (d)–(f) The Internal Structure of Nanopaper [25]

Solar cells require highly transparent substrates, but they also need high haze to enhance light scattering in order to increase the absorption of the active materials. At present, most transparent substrates being used are glass or plastic. Solar paper batteries can be made by printing conductive lines of silver nanowire on transparent nanopaper made of CNF. It was discovered that, when nano silver particles are printed on traditional paper, particles tend to flow into the pores. The silver nanowires printed on cellulose nanopaper, on the other hand, can be narrower and have better performance.

Lee et al. (2016) produced foldable, one-time-use organic memory on CNF nanopaper that could be directly printed. Nogi et al. (2013) used cellulose nanopaper as a substrate material to develop antenna devices for transmitting signals that could be used in satellites, computers and other wireless communication equipment (Figure  6). Inui et al. (2014) mixed conductive silver nanowires with CNF to make high-dielectric composite nanopaper that could be used in electronic applications such as transistors and antennae. Fang et al. (2013) fabricated highly transparent OTFT on cellulose nanopaper. Its physical and electrical properties could be improved because of the high binding capacity between the polymer dielectric and cellulose nanopaper. Zhang et al. (2016) fabricated MoS2 phototransistors on flexible, transparent nanopaper. The phototransistor demonstrated high light emissivity, and the device developed had high transparency. Nagashima et al. (2014) assembled silver nanoparticles on cellulose nanopaper to create a resistive non-volatile memory device for information storage. Zhang et al. (2020) prepared highly transparent nanopaper by using TOCN and in situ photopolymerization on paper with a polymerizable deep eutectic solvent (PDES). The resulting film was bent to 150° for 6000 times and still had excellent conductive performance. This film can be used to make soft, electronic light-emitting devices (Figure 7). Therefore, it is expected that cellulose nanopaper will be used in high performance, disposable electronics, such as smart labels, paper displays and biosensor applications, etc. Furthermore, as cellulose nanopaper has good thermal stability and chemical durability, it can be used in the fabrication of OTFT substrates. The process is similar to that of using traditional glass substrates [21].

Figure 6. (a) Coated Photo Paper, (b) Ordinary Paper, (c) Silver Nanowires Folded on Cellulose Nanopaper; (d) Return Loss of Cellulose Nanopaper Antenna Before and After Folding [27]

Nanopaper analytical devices have made major advances in the detection of physiological fluid analytes. They can be used in the analysis of drugs, proteins, viruses, bacteria, hormones and chemical pollutants, etc. [29]. Sadollahkhani et al. (2014) developed a disposable colorimetric paper coated with ZnO@ZnS core-shell nanoparticles that can detect copper ions in water-based solutions.

Zhao et al. (2008) developed a bioassay paper for the detection of DNase I using a gold nanoparticle colorimetric probe. By incorporating gold nanoparticles into cellulose nanopaper, it is possible to enable reactions through electrostatic interactions or coupling between different functional groups. This paper can be used for chemical detection [30]. Yan et al. (2014) used the vacuum filtration method and embedded highly stretchable piezoresistive graphene in cellulose nanopaper to create a mechanical strain sensor. In addition, cellulose nanopaper can, through the grafting of luminescent rare earth particles on cellulose, make a transparent light-emitting device that can be made into an ion probe [31].

The cellulose nanopaper developed by Giese et al. (2014) undergoes a rapid change in color when inflated, and the reaction is reversible. So, it can be used in pressure sensing, bio-sensing, optics, functional thin films and tissue engineering, etc. The hydrophilicity of cellulose nanopaper facilitates the transport of liquid within devices. What’s more, its porous structure allows for a greater active surface area, improving sensing response rate and sensitivity. This makes it suitable for use in biomedical applications, such as low cost, disposable medical diagnostic devices. Nassar et al. (2017) developed a nanopaper hybrid system for medical monitoring (Figure 8). The data collected from the sensor can be wirelessly transmitted to a smart phone application and displayed in real time. This, in turn, has the potential to be developed into an integrated platform for a wearable healthcare monitoring system and multifunctional sensors.

Figure 8. Wearable Integrated Monitoring System Developed with Cellulose Nanopaper Substrates [32]

Norrrahim et al. (2021) proposed the use of cellulose nanomaterials in the construction of wearable electronic devices for military applications. They can be used to monitor personnel health (blood pressure, heartbeat, body temperature) status, personnel positioning, communication and environmental temperature monitoring, etc.

Paper usage has long been associated with packaging, texts and graphic printing, but the range of its applications is greatly expanding since its size and surface characteristics can be adjusted through process design to achieve various desired characteristics. Cellulose nanopaper is a material with a porous network structure. Pore size distribution and porosity differ according to the fiber composition. By adjusting the density of the filling, fiber diameter and functional material additives, it is possible to obtain multiple functions suitable for the subsequent development of various applications. This potential has been proven by numerous studies.

From an environmental point of view, using nanopaper for substrates would have a low impact on the environment. This is essential as the gradual replacement of the energy-consuming and space-consuming hard electronic products of the traditional manufacturing process with energy-saving, thin, light and environmentally friendly flexible electronic products has long been a focus of consideration in the development of new generation electronic devices.

In contrast to commonly used materials like glass and plastic, cellulose nanopaper not only has various properties that meet the needs of flexible electronic substrates but is also environmentally friendly and derived from renewable raw materials. As such, research on various emerging functional cellulose nanopapers is booming. It is anticipated that cellulose nanopaper will eventually help realize various functional products with low costs, low energy consumption and high biocompatibility, thus becoming the mainstream material of future flexible electronics.

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Cellulose comes from plant fibers. The nanonization of cellulose enables the extraction of light, strong and environmentally friendly Cellulose Nanofibers (CNF), which can be widely used in daily life. The density of CNF is only 1/5 that of iron, but it is more than 5 times as strong. Furthermore, it has a large specific surface area (>250m²/g) and good dimensional stability (its thermal deformation is only 1/50 that of glass). Therefore, it is generally recognized by the industry that it has the best chance to replace carbon fiber in the future. It is an emerging nanomaterial with limitless application potential. In 2019, the CNF global market was about 290 million USD, and it is expected to reach 1 billion USD by 2027 with an annual growth rate of about 20%. Its main product applications include composite structural parts, energy storage materials, filter materials, organic electroluminescent (OEL) substrates, textiles, paper, cosmetics and additives, etc.

As for the possible future applications of CNF, the attention and expectations of the market are currently focused on electric vehicles. Using CNF nanomaterials in place of the original steel and glass fibers when making vehicle bodies would allow not only for better heat dissipation but also for the better fuel economy brought about by a lighter weight. This would also significantly reduce carbon dioxide emissions. In addition, according to the latest research findings, CNF also has great power storage efficiency and can potentially be used to produce lightweight, high energy-density solid state batteries without safety concerns. Furthermore, because CNF batteries have both the lightness and thinness of paper materials, they can be folded to store a larger amount of electrical energy. There is also no need to use electrolytes, which will greatly improve both the battery life and safety of electric vehicles.

In addition, CNF materials have another important feature. After they are processed into thin films, they can have good transparency and be folded like paper, so they are very suitable for use as substrates for transparent devices such as large, expandable screens and solar cells, etc. Commonly used glass substrate materials have the disadvantage of being fragile. CNF, on the other hand, is transparent, strong, and flexible. As such, it can be applied to the body or interior of electric vehicles on streamlined surfaces, to form large, external energy-harvesting surfaces (solar energy), or to enhance smart display functions (control panels, car windows and windshields). Furthermore, since the pore size of the CNF structure is smaller than that of viruses and the specific surface area is very large, it can be used to make filters for capturing dust and harmful substances in the environment or as a deodorizing device that absorbs fine, odorous substances. This offers many possibilities for innovation around the maintenance of clean air within the cabins of electric vehicles.

In fact, Japan’s Kyoto University, Toyota Motor, and UPM-Kymmene Oyj, etc. have already successfully launched concept cars with lightweight bodies made of composite CNF materials. More recently, Tohoku University and the Nippon Paper Group in Japan and Brown University and the University of Maryland in the United States, etc. have also made excellent progress in the research of high-density CNF folding batteries. In 2018, Toyota collaborated with the Ministry of the Environment of Japan to create a vehicle based on its Toyota 86 sports car that used new CNF materials. They created a lightweight body where the door trim, seatbacks, and even the hood and tailpipes were made of CNF materials. The resulting weight reduction was significant. The overall weight of the vehicle was reduced by about 48% of the weight of the original steel version. The lid of the trunk, for instance, was only 700 grams. In addition, a Kyoto University Survival Circle Research Institute research team is striving to develop new structural materials that use CNF to replace metal vehicle bodies and frames.

In regards to the reduction of vehicle weights, the current practice is typically to mix CNF with resin, rubber or fibers to form special composite materials, in order to make strong, lightweight auto parts. In terms of material strength at this stage, traditional carbon fiber is still slightly better and therefore more suitable for use in the production of vehicle bodies and frames. CNF is strong while also being easy to bend and process, so it can be used to replace resin and rubber parts such as bumpers, interior trims, and tires, etc. Previously, the extremely complicated manufacturing process for CNF composite materials led to overly high manufacturing costs (about 90 USD per kilogram). This impeded the willingness of major car manufacturers to use these materials. However, in 2017, Furukawa Electric in Japan successfully developed a new process technology with the ability to reduce the production cost of CNF-reinforced resin composites to 1/10 of the current costs, opening up the possibility for application in the automotive industry. The company is scheduled to complete the mass production development of this new material by 2024 and begin applying it to the interior, electronic components and external panels of vehicles. The market size is expected to reach more than 3.5 billion USD in the future. With the rapid development of composite materials, there is a chance that we will see CNF fully replace carbon fibers and glass fibers in the future to become the mainstream choice for lightweight automobiles.

In the research of high energy battery applications, Oji Paper, a leading Japanese paper manufacturer, and Mitsubishi Chemical Group jointly developed a transparent continuous sheet made of CNF in 2013. Due to its light and strong structure and its ability to be folded, it is very suitable for making large displays and solar cells. In order to allow sunlight to irradiate the energy storage elements in the battery without loss, most solar cells must be made with a substrate that has good light transmission properties. CNF sheets look like acrylic sheets and are not only transparent and strong but also flexible. Furthermore, they are lighter than quartz glass and have only 1/50 the thermal expansion coefficient. In addition, in 2021, a joint study conducted by the Nippon Paper Group and Tohoku University found that CNF materials themselves also have a strong power storage effect. Therefore, a large amount of electricity can be stored by using stacking structures. It is also capable of fast charging and discharging. The storage capacity can reach 2.5 times that of current lithium batteries. Since CNF solid state batteries do not need to use electrolytes and do not contain rare metals, they are not only resistant to high temperatures and safe but also expected to have a lower manufacturing cost than lithium batteries. CNF has great potential to be fully applied to the power supply modules of electric vehicles in the future.

The development of CNF mass production technology is also one of the key points of global market competition. There are roughly three methods to extract CNF from plants. These are the mechanical method, enzymatic method and biological method. The mechanical method generally uses a high-pressure homogenizer to apply pressure and high speed motion to the pulp raw material to crush it in order to obtain nanocellulose. The enzymatic hydrolysis method uses cellulase to selectively decompose amorphous cellulose, leaving behind cellulose crystals. Since the reagent enzymes and cellulase used in the enzymatic hydrolysis method are all renewable resources, this is currently a hot research topic in academic circles. The biological method mainly involves preparing synthetic cellulose using microorganisms. The cellulose produced via this method is also called bacterial cellulose. The advantages of using biological methods to prepare CNF are the low energy consumption, no pollution, and the ability to precisely control the structure of the cellulose, crystal form, and particle size distribution, etc., but the disadvantage is that the preparation procedures are complicated and time-consuming, expensive and low yield. At present, the main global suppliers of CNF are Marusumi Paper, the Nippon Paper Group, AsahiKASEI, Daio Paper, CelluComp, CelluForce, Sappi, Borregaard, and Engineered Fibers Technology, etc. Most are Japanese companies.

The CNF fibers in pulp have strong binding properties. Therefore, it usually requires a lot of energy to separate them evenly. This is the greatest technical bottleneck when it comes to industrialization. As such, although Canada, the United States, and Sweden, etc. have all established CNF demo plants, they have not been able to scale up production due to high manufacturing costs. The fact is that the CNF preparation technology employed by most American and European companies use mechanical methods to forcefully grind wood fibers. It is difficult to maintain the uniformity of the width of fibers produced this way. This makes it difficult to take full advantage of the nanomaterial properties that CNF should have. The reason why Japanese companies are leading this field is their mastery of a key technology for the preparation of CNF raw materials. This technology was born from a major breakthrough in the practical applications of CNF made by Professor Akira Isogai of Tokyo University. He developed a production method that uses the “TEMPO” catalyst. In 2013, Professor Akira Isogai and his team found that a special catalyst known as TEMPO has properties that break down CNF fibers. The practice is to add a small amount of TEMPO catalyst to pulp that has been dissolved in water and let it react for 2 hours then stir. The surface of the CNF will be electrically charged. Since there will be negative electrons on the surface of each CNF strand, they will repel each other. When the repulsive force is greater than the binding force, the CNF can be successfully separated. After the reaction caused by the TEMPO catalyst, extremely fine CNF with diameters of 4nm will be dispersed in the water. Once the pulp fiber has all decomposed into CNF, it will form a colloidal transparent liquid called CNF dispersant. This chemical treatment requires only 1/60 to 1/300 of the electricity needed for traditional mechanical grinding methods, and although the price of the TEMPO catalyst can be as high as tens of thousands of yen per kilogram, the amount that needs to be added is only about 1％ of the pulp raw material. Thus, the impact on the total cost is minimal. At present, many companies, including the Nippon Paper Group and DKS, have already adopted this highly efficient, patented method to mass produce CNF. In September of 2015, his development of this important TEMPO catalyst application method made Professor Akira Isogai the first person in Asia to win the Marcus Wallenberg Prize, which is considered to be the “Nobel Prize in Forest Timber Science”. Currently, the global industry and academia are also actively researching and developing catalysts with the same effects as TEMPO but which have a lower cost in the hopes of breaking through the bottlenecks of mass production and materials supply and accelerating the commercialization of CNF.

CNF is an almost perfect wood nanomaterial with infinite possibilities for industrial application. However, to achieve universal application, there are still many challenges that need to be overcome. Among these challenges, the most critical is how to improve its production efficiency and reduce costs. At present, even the most mature CNF mass production technology faces the problem of high manufacturing costs. In terms of market price, CNF costs about 40 to 75 USD per kilogram, whereas steel is only about 1.50 USD, and plastic is less than 4 USD. Clearly, CNF is far from competitive. This remains true even when CNF is used in combination with different materials. According to current market prices, the cost of CNF materials composited with carbon fiber and aramid fiber will exceed 22 USD and 37 USD respectively. In order to be the first to seize the huge business opportunities in the CNF industry, the Japanese government has already clearly stated in the “Revised Japan Revitalization Strategy 2015” that it will promote the international standardization and material utilization of CNF, establish the “Nanocellulose Symposium”, and use policy to guide the development of domestic industries with the goal of reducing the cost of CNF manufacturing to 300 yen (about 2.25 USD) per kilogram by 2030, increasing the annual supply to 50 thousand tons, and cultivating a CNF product application market with a scale of 1 trillion yen. With the continuous progress being made in CNF production technology, the cost too is continuing to decline, and we can foresee the future CNF industry gradually expanding from high-end and high value application fields to low and mid-end markets.

However, the production process of extracting CNF from woody materials also faces problems such as long planting cycles, excessive tree reclamation, and air pollution. In addition, since CNF naturally has strong hydrophilic properties, it often loses its original strength and durability in water or high humidity environments. Therefore, how to improve CNF’s water resistance properties and prolong its service time is also an important issue in regards to promoting its industrialization. In addition, most plastics and rubber, etc. contain a certain amount of oil. If you want to mix them with CNF to make composite materials, its hydrophilic nature can easily lead to poor mixing results.

With the rapid development of material technology, however, many solutions have begun to emerge. So far, the industry has been able to successfully extract CNF from wood and agricultural industrial waste, such as pineapple leaves, banana stems, grapefruit peels and even coffee grounds. According to the latest research, the proportion of cellulose contained in the hop plant used to make beer is almost equal to that of wood. In truth, only the flowers of the hop plant are used during the beer brewing process. The stems and leaves account for about 75% of cultivated hop, but they usually end up in landfills after the harvest. By using hop stems as raw material for the extraction of CNF, it would be possible to replace a great proportion of wood sources and reduce the burden on the forestry environment. In addition, some research teams have developed a process that can uniformly disperse CNF in various resins and successfully produced CNF composite materials like PE, PP, PVC, PS, PMMA, ABS and other general purpose resins and engineering plastics, not only making them biodegradable but also greatly increasing their tensile strength. This research is anticipated to help solve the environmental pollution problem caused by traditional plastics, which do not decompose and rot. It is anticipated that this nanomaterial technology derived from plant fibers will lead the world’s acceleration towards a green and sustainable future.

This article focused on the preparation technology related to cellulose nanomaterials and the development of its applications in flexible electronics. The goal was to provide a comprehensive introduction to help readers quickly learn about this emerging material technology and its great future development potential. The author of this article, Professor Feng-Cheng Chang, is currently working in the Department of Forest Environment and Resources, National Taiwan University, where he also completed his bachelor’s and master’s studies. After receiving his Ph.D. in Wood Science from the Department of Wood Science at the University of British Columbia in 2011, Professor Chang returned to his former department at National Taiwan University to teach and continue his research into cellulose nanomaterials. His team has published more than a hundred related academic journal and conference papers and implemented many research project plans for the Ministry of Science and Technology, carrying out the development of key technologies for the application of nano-micro-crystalline cellulose fibers in reinforced composite materials and filter materials. Professor Chang is one of the few top, domestic scholars with extensive, in-depth experience in researching woody biofibers and composite materials. MA-tek is very honored to be able to work with Professor Zhang this year in industry-university collaborations by providing all the analysis services his team needs for their research of cellulose nanomaterials. MA-tek has a comprehensive set of testing equipment and the professional testing experience to be able to fully meet the various analysis and testing needs of advanced materials research.