Nanocellulose-Enabled Electronics , Energy Harvesting Devices , Smart Materials and Sensors : A Review

Cellulose nanomaterials have a number of interesting and unique properties that make them well-suited for use in electronics applications such as energy harvesting devices, actuators and sensors. Cellulose nanofibrils and nanocrystals have good mechanical properties, high transparency, and low coefficient of thermal expansion, among other properties that facilitate both active and inactive roles in electronics and related devices. For example, these nanomaterials have been demonstrated to operate as substrates for flexible electronics and displays, to improve the efficiency of photovoltaics, to work as a component of magnetostrictive composites and to act as a suitable lithium ion battery separator membrane. A discussion and overview of additional potential applications and of previously published research using cellulose nanomaterials for these advanced applications is provided in this article. The concept of using cellulose nanofibrils in stimuli-responsive materials is illustrated with highlights of preliminary results from magnetostrictive nanocellulose membranes actuated using magnetic fields.

Cellulose nanomaterials are a renewable resource typically created from plants, and although their inclusion in electronics, energy storage devices and sensors is certainly not a panacea for the negative environmental impact of a growing electronic waste stream, they may provide the opportunity to mitigate the problem.According to the U.S. Environmental Protection Agency, 384,000,000 electronic devices were disposed of in 2010, and 152,000,000 of those were mobile devices [37].Few of these devices contain biodegradable or compostable components, and recovering individual constituents is costly.A number of efforts have been made to incorporate bio-derived and biodegradable materials, including cellulose nanomaterials and biopolymers, into electronic applications.Some of those efforts are included here, and one can envision a future in which sustainable materials compose a large portion of electronic devices and in which the devices can be readily recycled or possibly composted to leave recoverable metal constituents.In many cases, though, cellulose nanomaterials offer additional attributes and performance features over conventional materials that drive the research into their inclusion in electronics.
As cellulose is broken down into nanometer-scaled particles and fibers, the materials begin to exhibit some interesting and unique properties depending on their size, morphology, charge and other characteristics.Since these properties are important to their application, a brief primer into their categorization and properties is warranted here.Typically, cellulose nanomaterials, especially from plant sources, can be categorized as either cellulose nanocrystals (CNCs) or cellulose nanofibrils (CNFs).CNCs are discrete, rodshaped cellulose particles (Figure 1) typically with high crystallinity and are primarily produced through acid hydrolysis of native cellulose.Typically, CNCs have nanometer-scaled diameters with lengths of hundreds to thousands of nanometers, and they often contain surface charge groups and can thus be colloidally stable.Conversely, CNFs are typically network structured nanoscaled fibers (Figure 2) that can be produced from countless methods usually involving some kind of chemical treatment followed by mechanical refining.The morphology of CNFs widely varies from process to process, and they can often have a wide distribution of diameters and lengths.Another type of cellulose nanomaterial is often called bacterial cellulose as it is produced by a species of bacteria generally called Acetobacter xylinum [38].The fibers produced by these bacteria have cellulose I crystallinity with diameters of tens to hundreds of nanometers [2,39].A number of reports have been made on using bacterial cellulose as a substrate in electronic devices [40,41].Most of these findings are likely transferrable to CNFs from plantbased materials.In addition, nanofibers of regenerated cellulose can be created using the gel spinning process, and such cellulose nanofibers have also been employed in electronic applications [42].CNCs and CNFs each bring unique properties and applications, and both have been explored for use in electronics and functional devices.
Cellulose nanomaterials are often touted as potential polymer reinforcements because of their high strength and stiffness, and these good mechanical properties also lend these materials for use in functional devices such as electronics.The cellulose crystal has been measured to have extremely high strength and stiffness with values of about 10 GPa and up to about 200 GPa, respectively, but bulk materials, such as films, made from nanocellulose have considerably lower mechanical properties [43][44][45][46][47]. Nonetheless, cellulose nanomaterial films and composites have mechanical properties that rival or exceed many polymers.The tensile strength and modulus of randomly oriented CNF films, for example, are commonly as high as 200 MPa and 20 GPa, respectively [47][48][49].As a comparison, polyethylene terephthalate (PET), which has been used as a substrate for flexible electronics, has tensile strength and modulus of approximately 55 MPa and 3 GPa, respectively [50][51][52].Typically, only the tensile properties of CNF or CNC films are measured or reported, but their tear and bending behaviors are also important.The incorporation of polymers and other modifiers can aid in improving flexibility and improving durability, but little data is available, especially pertaining to their application in electronics and functional materials.Nonetheless, the mechanical properties of cellulose nanomaterials lend them to be employed as components in a variety of devices.
The optical properties of cellulose nanomaterials are also distinctive from bulk cellulose and can be favorable for electronic applications.Films and composites made from cellulose nanomaterials, including CNCs, CNFs and bacterial cellulose, can be highly transparent (Figure 3), depending on how they are produced [41,49,[53][54][55][56][57][58].Fukuzumi et al. measured the light transmittance through a CNF film made from softwood pulp to be up to 90% but only 78% when made from hardwoods [53], and Nakagaito et al. compared the transparency of films made from a variety of materials, including bacterial cellulose [54].The CNF films have also been measured to have high optical haze and strong scattering, which can potentially be exploited for photovoltaic applications [33,34,59].Interestingly, CNC suspensions are also birefringent and optically active and can be assembled in films such that they exhibit tailorable and even switchable optical and color characteristics [18,20,46,60].These optical properties of cellulose nanomaterials offer enormous potential for use as electronic substrates, sensors, liquid crystal devices and more.
Cellulosic materials have low thermal expansion, which is desirable in electronic components that generate heat, and the coefficient of thermal expansion (CTE) for cellulose nanomaterials and their composites has been measured and reported for various CNF films.The thermal expansion of cellulose has been measured to be as low 0.1 ppm/K [61], and the CTE for films and composites containing cellulose nanomaterials has been reported to be as low as 1 ppm/K [62].Bacterial cellulose typically has a lower CTE than plant-derived CNFs [54], and modification of bacterial cellulose nanofibers was shown to be able to reduce the thermal expansion down to 1 ppm/K [62].Nogi and Yano demonstrated that by adding only 5% of bacterial cellulose nanofibers to an acrylic resin, thermal expansion of the film from 20 to 150 °C could be reduced by more than a factor of 60 resulting in an in-plane thermal expansion of 4 ppm/K [41].Nogi et al. measured the thermal expansion of "optically transparent nanofiber paper" from Douglas fir to be 8.5 ppm/K over the range of 20 to 150 °C [55], and Yagyu et al. measured the CTE of CNF films from wood to be 7-13 ppm/K [63].These values are favorable compared to existing polymeric materials, which can have CTE values up to hundreds of ppm/K [43,56].Because low thermal expansion is important for electronics printed or deposited onto flexible substrates, cellulose nanomaterials may offer favorable properties and potentially improved performance over other polymer films.
Because of all these properties, including sustainability, high strength, flexibility, transparency and low thermal expansion, cellulose nanomaterials have been explored for their use in electronics and functional devices.In some cases, cellulose nanomaterials can provide advantages over traditional materials.For example, cellulose-based nanocomposites can overcome issues with piezoceramics such as brittleness [64], high toxicity in lead zirconate titanate (PZT) [64,65], low energy conversion efficiency [66,67], inability to operate under long life cycles without the issues of depolarization [68][69][70], and the lack of flexibility to operate under strong ambient vibrations [71,72].An overview of many of these applications is provided here, but this account is not intended to be comprehensive.Instead, this work aims to provide a broad perspective of how cellulose nanomaterials may play a role in electronics such as sensors, actuators, displays and energy harvesting devices.

SUBSTRATE MODIFICATION
Substrate quality is critical for the fabrication and performance of electronics on cellulose substrates, and methods for improving CNF substrates electronics have been explored and continue to be the focus of both research and commercialization efforts [30,[73][74][75].Low surface roughness, strong water resistance and robust mechanical properties are all desirable properties for electronic substrates, and in many cases, cellulose nanomaterials may require modification or improvements to meet those needs.Torvinen et al. evaluated the effects of composition and processing on the properties of composite films containing CNF inorganic filler [75].They found that calendering produced mechanically stable and flexible sheets with smooth surfaces on which silver nanoparticle ink could be printed.The films had similar surface roughness and sheet resistance as those from reference materials, including polyethylene terephthalate (PET), and they concluded that these CNF composite substrates would be suitable for use in electronic applications, such as RFID sensors and tags.For the case of transferring silicon nanomembrane (Si NM) devices to cellulose substrates, both the substrate coating and adhesive layers were found to affect the transferability to the substrate [30].Thin, smooth epoxy layers resulted in better transfer to the substrate and also provided enough protection to limit swelling during subsequent wet cleaning steps.Couderc et al. coated CNF sheets with thin polyimide layer, which improved the dielectric properties, reduced the moisture uptake and decreased surface roughness [76].Chinga-Carrasco et al. created CNF films modified with hexamethyldisilazane to improve water resistance and printability [73], and Shimaoka et al. reported a proprietary modification process producing films with substantially reduced moisture uptake and with low brittle ness, such that they could be bent and rolled tightly without cracking [74].Therefore, despite some of the drawbacks of pure CNF films, such as large moisture absorption, modifications and processing techniques have been demonstrated to yield CNF films suitable for electronic applications.

FLEXIBLE ELECTRONICS
Because cellulose nanofibril films can be transparent, have low CTE, are flexible and have good mechanical properties, they are excellent candidates for substrates for flexible electronics.A number of methods for creating electronics on cellulose nanomaterial substrates have been demonstrated and include printing circuits directly onto the substrates, using coating and thermal  deposition techniques and transferring prefabricated devices onto CNF substrates.The results from some of these electronic devices on nanocellulose substrates are described below.
Researchers at the University of Wisconsin and the USDA Forest Service Forest Products Laboratory recently demonstrated that high-speed transistors on silicon nanomembranes can be fabricated onto CNF films, as shown in Figures 4 and 5 [28,30].These flexible electronics were built by first fabricating the device on a special silicon wafer that allows thin silicon nanomembranes (Si NMs) to be released and then by transferring the Si NM to a CNF substrate.In the first step (Figure 5i), the n+ wells for the source and drain regions were formed using a spin-on-dopant diffusion process carried out in a nitrogen and oxygen ambient furnace at 850 °C for 20 minutes.Next, the thin silicon layer was released by wet etching with hydrofluoric acid (Figure 5ii), then the electrodes were created (Figure 5iii).Next the Si NM was transferred to a CNF substrate with an adhesive layer (Figure 5iv-v), and finally the device was completed by photoresist patterning and dry etching steps (Figure 5vi).This thin-film transistor (TFT) built on the CNF substrates was demonstrated to operate in the microwave range with a transition frequency of 4.9 GHz while flat and 3.8 GHz while bent [28].The substrate was found to have low gate leakage and field effect mobility values consistent with other Si NM transistors.The substrate was also shown to exhibit biodegradability, which could potentially offer opportunities to recover rare metals used in electronics.Such high speed electronics have numerous potential applications for communication and computation devices, and preliminary investigations show that cellulose nanomaterials perform satisfactorily.
In a more recent report from this group, various working electronic devices were fabricated on CNF substrates [31].They reported an environmentally friendly method for creating gallium arsenide microwave devices on CNF substrates with performances comparable to those made on rigid devices.They also created Si-based complementary metal-oxidesemiconductor (CMOS) devices, including logic gates and a full adder, which worked at frequencies up to 5 kHz, as shown in Figure 6.The working characteristics of these devices were clearly shown, indicating that full circuits and working electronics can be created on CNF substrates.
In another example, flexible organic field-effect transistors (OFETs) were fabricated on flexible, transparent cellulose "nanopaper," which was created by oxidation and mechanical refining of bleached softwood Kraft pulp [29].They created CNF films with surface roughness of about 5 nm, transparency of 90% and coefficient of thermal expansion as low as 12 ppm/K.A transparent, conductive carbon nanotube layer was then coated onto the CNF substrate, followed by spin-coating of a poly(methyl methacrylate) (PMMA) dielectric layer.Next, they vacuum deposited an n-type organic semiconductor NTCDI-F15, a naphthalenetetracarboxylic diimide derivative.Finally, they evaporated silver electrodes through shadow masks onto the semiconductor.The device was reported to "exhibit good n-type transistor characteristics" and had effective carrier mobility of 4.3 × 10 -3 cm 2 /(Vs) and I on /I off ratio up to 200.The mobility only decreased by about 10% when the device was bent either parallel or perpendicular to the conduction channel direction.They also noted that, unlike other organic semiconductor devices, the good bonding between CNF and carbon nanotubes (CNTs) diminished the problem of protruding CNTs, which can cause an electrical short circuit.The highly transparent, flexible CNF substrate exhibited excellent properties as a field-effect transistor substrate.
Printed electronics are another application in which cellulose nanomaterials have been demonstrated to be suitable as substrates [32,63,77,78].Printed electronics often employ inks consisting of nanometer-scale silver particles in suspension, but carbon nanotube (CNT)-CNF mixtures have also been suggested as potential conductive inks with the CNFs acting as the dispersing agent [78].Nge et al. demonstrated that silver nanoparticle tracks printed on CNF substrates show lower electrical resistance than those printed on commonly used plastics such as polyimide and poly(ethylene naphthalate) (PEN) [77].Those conductive tracks on CNF substrates showed better electrical performance during and after folding than those on plastic substrates.In another example, flexible silver antennas were printed on crosslinked CNF films using gravure printing techniques [32].The devices were demonstrated to have good radio frequency performance for application in radio frequency identification (RFID) antenna tags.Although printed electronics do not typically match the performance of typical silicon-based electronics, there are a wide range of applications, including lightemitting diodes, sensors, medical devices, and radio frequency identification (RFID) antennas, for which CNF substrates can potentially be employed [79].

DISPLAYS AND LIGHT-EMITTING DIODES
Using CNFs as substrates for light-emitting diodes and displays has received a great deal of attention, including from electronics manufacturers, over the last several years.Okahisa et al. reported successful fabrication of an organic light-emitting diode (OLED) device on wood-based cellulose nanofibril composites [56] (Figure 7).They acetylated the CNFs, impregnated them with different acrylic resins and found the composite fabrication and formulation critical to the ability to fabricate a working device.The fabrication of the device involved multiple steps adopted from a device created on polymer substrates [80].First, a smoothing layer was spin-coated on a CNF composite film, then a silicone oxynitride (SiON) barrier film was sputter deposited.The anodes were created by sputter deposition and patterning; the organic layers were vacuum deposited, and the cathode was fabricated by vacuum deposition.Finally, a passivation film layer was created by chemical vapor deposition.
The device was able to illuminate lettering expressing the researcher's affiliations.Legnani  sputtering at room temperature [40].Although their membranes had less than ideal optical transmittance, they achieved promising results with luminance levels up to 1200 cd/m 2 .In another example, Yagyu et al. illuminated light-emitting diodes on CNF substrates by printing silver nanoparticles on the substrate followed by heat curing at up to 160 °C [63].They found that the type and treatment of CNF used was important in reducing discoloration of the device and that heat treatment at temperatures above 150 °C was necessary to achieve good electrical conduction.The advancement of light-emitting diodes on cellulose nanofibril substrates could lead to devices with curved or bendable displays made on transparent, renewable substrates.

OPTO-ELECTRONICS
Another exciting application for cellulose nanomaterials is liquid crystal displays (LCDs) and other electro-optical devices.Cellulose nanocrystal suspensions display liquid crystal behavior, and the CNCs can be aligned under an electric field, making them potentially suitable for optical devices or displays [81][82][83].By altering the electrical signal through the CNC suspensions, the orientation of the CNCs and thus transmittance of light can be controlled [82,83].
Limited research for the use of CNCs in this manner is available, so it's difficult to know whether CNCs offer advantages over other materials for LCDs, but the opto-electric effect is a property that could potentially have a number of interesting applications.

ENERGY HARVESTING AND STORAGE
A number of energy harvesting and storage applications have been successfully fabricated using cellulose nanocrystals and nanofibrils.Cellulose nanomaterials have been used for energy harvesting in solar cells [33,34,36] and for energy storage in battery electrodes, as solid ion-conducting electrolytes and as separator membranes for Li-ion batteries, and a few examples of these applications are provided here [84][85][86][87][88][89][90][91][92].In some cases, the cellulose nanomaterials were shown to offer unique improvements or perform as well or better than conventional materials.
Cellulose nanomaterials have been successfully incorporated as components of photovoltaics as evidenced by multiple reports [33,34,36].Hu et al. demonstrated a working organic photovoltaic on cellulose nanofibril substrate [34].In this example, the cellulose nanofibril substrates were made conducting by deposition of a tin-doped indium oxide layer.The organic layers were spin-coated and heat treated in multiple steps onto the substrate, and then metal electrodes were thermally evaporated onto the film.This organic solar cell on cellulose nanofibrils substrate was measured to have a power conversion efficiency lower than the control but which could likely be improved through processing improvements.Using a similar approach for constructing solar cells, Fang et al. demonstrated that the high transparency and optical haze of CNF films can be advantageously exploited to improve the power conversion efficiency of an organic solar cell by 10% simply by laminating it with a CNF film [33].They measured the transparency and optical haze of the CNF film to be 96% and 60%, respectively, and the device had a power conversion efficiency of 5.88%.Zhou et al. produced a recyclable organic solar cell on a cellulose nanocrystal substrate, as shown in Figure 8, and they demonstrated the device could be readily deconstructed into its constituent components using a low energy process [93].The device was fabricated by first depositing a semitransparent silver layer on the CNC film, which had nanometer-level roughness that eliminated the need for planarization.Organic photoactive layers were spin-coated onto the silver layer, and metal contacts were evaporated onto the photoactive layers.The device reached a power conversion efficiency of 2.7% and demonstrated a potential approach for using sustainable cellulose nanomaterials as a component in recyclable electronic components.
Cellulose-based materials have been widely explored for use in batteries, and using cellulose nanomaterials is a natural extension for battery applications [85,94].Flexible electrodes for batteries and other applications have been created using cellulose nanomaterials as binders, templates, etc., in conjunction with conductive materials [84,87,88,90].For example, Jabbour et al. used graphite and cellulose nanofibrils, which they referred to as microfibrillated cellulose, to create flexible electrodes for batteries [84].They found the electrodes had a conductivity of about 0.3 S/cm even as they were rolled around hoses with radii ranging from 3 to 32 mm, but this conductivity was significantly lower than reference electrodes.The electrode had specific discharge capacity close to the theoretical specific capacity of graphite and good cycling performances, and they expressed high optimism for cellulose nanomaterials to play a role as electrodes in electronic devices.In another example, flexible siliconconductive cellulose nanopaper was demonstrated as a functional Li-ion battery electrode [87].This electrode was fabricated by first creating a CNF-carbon nanotube (CNT) aerogel, followed by plasma-enhanced chemical vapor deposition (PECVD) of a thin silicon layer in the porous aerogel channels.The electrode performance was evaluated in coin-type Li-ion battery for up to 100 charge/discharge cycles, and they concluded that the electrode performed quite well with the discharge capacity remaining above 1200 mA h/g, which is more than three times the theoretical capacity of conventional graphite anodes.Such examples demonstrate the role that cellulose nanomaterials can play in electrodes for batteries and other applications and how they can be incorporated with other nanotechnology for advanced applications.
Cellulose nanofibrils also have the potential to replace thermoplastic polymer films as membrane separators for Li-ion batteries [85,88,91].For example, Chun et al. compared the performance of separators made from CNFs created by high-pressure homogenization to a commercially available tri-layer polypropylene/polyethylene/polypropylene (PP/PE/PP) separator and found that the CNF separator showed comparable electrochemical performance to the commercial one.Furthermore, they concluded that the CNF separator offered some potential advantages over the PP/PE/PP separator, including improved ionic conductivity, electrolyte wettability, and thermal shrinkage.The thermal shrinkage of the CNF separator was judged to be negligible as compared to 36% for the PP/PE/PP one, which could lead to improved electrode isolation and safety of Li-ion batteries.Leijonmarck et al. reported a process in which the electrodes and separator were created from cellulose nanofibrils in a sequential filtration process as shown in Figure 9 [88].The result of the sequential filtration process was a flexible, single-paper Li-ion battery cell, as shown in Figure 10.The anode consisted of CNFs, graphite and Super-P carbon; the cathode consisted of CNFs, LiFePO 4 , and Super-P carbon; and the separator contained CNFs and SiO 2 .The paper batter was soaked in electrolyte, then copper and aluminum foil charge collector were placed on the negative and positive sides of the battery, respectively, and the battery cells were sealed in a pouch.They tested the electrochemical performance of the battery cells and found that they could be used for hundreds of charge cycles while maintaining a large part of their capacity.They also found that the battery could work while being bent, indicating their applicability in flexible devices.Therefore, in addition to potentially improving the sustainability and environmental impact of battery components, cellulose nanomaterials could offer improvements over conventional Li-ion battery separator properties.Cellulose nanomaterials have also been demonstrated to play a role as a component in supercapacitor devices [42,[95][96][97][98][99].Electrochemical capacitors are energy storage devices that operate by rapid charging/ discharging at the interface between an electrode and electrolyte, and various studies have employed CNFs or CNCs as components of the electrodes [95][96][97][98][99].In some cases, cellulose nanofibers were used as sacrificial precursors to produce carbonized fibers through high-temperature treatments and activation [42,96].In the case of carbonized nanofibers, working electrodes were essentially prepared by mixing the carbonized fibers with binders (polyvinylidene fluoride or polytetrafluoroethylene) and carbon or acetylene black and pressed into a flat electrode [42,96].In other cases, the cellulose nanomaterials were used as a component of composite electrodes [97][98][99].These composite electrodes were fabricated by using cellulose nanomaterials and conducting polymers such as polyaniline [97,98] or polypyrrole [99].For example, Zhang et al. started with a CNF aerogel, coated silver onto the aerogel, then electrodeposited polyaniline (PANI) onto the aerogel [98].The supercapacitor was fabricated by soaking aerogels in polyvinyl alcohol/ phosphoric acid electrolyte gel, sandwiching a glass filter paper separator between two of these electrodes, then allowing the assembly to dry.They found the specific capacitance of these supercapacitors to be as high as 176 mF cm -2 and that there was almost no change under bending.In another example, Liew et al. used cellulose nanocrystals coated with a thin film of polypyrrole to create a highly porous electrode structure [99].They found the cellulose nanocomposite to have a high capacitance (256 F/g) and rapid charge/ discharge, which they attributed to the porous 3-dimensional structure of the composite.They compared the cellulose composite supercapacitor to one made with carbon nanotubes (CNTs) and found that the cellulose nanocomposite was more durable than the CNT one with comparable performance.Cellulose nanomaterials have successfully been demonstrated to work in capacitor devices, which can complement or replace batteries in devices needing rapid charge/ discharge properties and may play a role in both rigid and flexible electronic devices.

Water dispersion of NFC + Graphite
Cellulose nanocomposites containing CNCs and polydimethylsiloxane (PDMS) were shown to be capable of harvesting energy by exploiting the triboelectric effect, which converts mechanical energy to electricity [100,101].The initial CNC suspension was wet ball-milled, freeze-dried and ground to produce CNC flakes.The CNC flakes were mixed with PDMS and spin-coated into films.Aluminum electrodes were used, and the separation distance was adjusted by a programmed solenoid.Period compressions were applied to the film assemblies, and the output voltages were measured using an oscilloscope.The cellulose nanocomposites showed three times the voltage output and over ten times the maximum power value compared to pure PDMS films.Cellulose nanocomposites have the potential to be incorporated in energy harvesting devices, exploiting energy from ocean waves, human movement, air flow and more.

SENSORS
Cellulose nanomaterials have also been exploited as sensors and stimuli-sensitive functional materials, including for use in biomedical applications [102][103][104][105][106]. For example, Nielsen et al. modified CNCs with dual fluorescent labeling to create ratiometric pH-sensing nanoparticles [103].The emission spectra of the CNC suspension varied with pH, and the ratio of intensity at multiple wavelengths showed clear transitions as a function of pH.Edwards et al. demonstrated that peptide modified CNCs in tandem with a cellulose membrane could be used as a biosensor wound dressing to detect a destructive protease [104].In another example, pyrene-modified CNCs were demonstrated to be highly selective to sensing Fe 3+ with good discrimination between Fe 2+ and Fe 3+ , demonstrating that cellulose nanomaterials have potential application as chemical sensors [105].Modified cellulose nanomaterials have been shown to be responsive to the presence of specific chemicals and conditions, and one can imagine combining this sensing capability with cellulose-based electronic devices to create fully functional electronic sensing devices for use in biomedical or chemical analysis applications.

PIEZOELECTRIC AND ELECTROACTIVE EFFECTS IN CELLULOSE NANOMATERIALS
Cellulose nanomaterials have also been demonstrated to display piezoelectric and electroactive effects, which potentially can be exploited for use as actuators and sensors in a wide range of devices, including microelectromechanical systems, robotics, speakers, micropumps and more [107][108][109][110].For example, Csoka et al. demonstrated that thin CNC films demonstrated a piezoelectric effect with an effective shear piezoelectric constant of 2.1 Å/V, which is comparable to a reference ZnO film [109].Jeon et al. demonstrated that bacterial cellulose films exhibited electrically driven bending deformation, which could be enhanced by chemical modification [110].Nanocellulose-based actuators and electroactive devices have enormous potential, but there exists a fairly wide gap in fundamental understanding of these effects as well as a lack of effective demonstration of these materials in devices compared to existing materials.

MAGNETIC CELLULOSE NANOCOMPOSITES IN AEROGELS AND NANOPAPER
Composite materials offer a unique opportunity of combining two or more materials that retain their individual properties but when combined can produce new features.An example of this composite approach is the efforts to integrate magnetic particles within cellulose materials.A hydrophobic cellulose aerogel composite was previously prepared with magnetic nanoparticles for oil absorption applications [111].The principle of this composite is that the cellulose would absorb the oil from a spill and then be magnetically retrieved.The aerogel produced was able to absorb oil up to about 28 times of its own weight within 10 minutes and could be easily removed and recovered from the water surface by an external magnet.Olsson et al.
were able to make magnetic aerogels and nanopaper using cellulose nanofibrils as templates by facilitating the growth of ferromagnetic cobalt ferrite nanoparticles [112].The nanocellulose is then compacted to make stiff magnetic nanopaper.The nanofibrils act as templates for the ferromagnetic nanoparticles.Their flexibility, high porosity and surface area make them attractive for use in microfluidics or electronic actuators.Researchers have also shown the ability to incorporate magnetic particles with cellulose materials by coprecipitation methods [113][114][115].Cobalt ferrite nanotubes with an average diameter of 217 nm were synthesized by coprecipitation using freeze-dried bacterial nanocellulose as template [116].The metal precursors (Fe 3+ and Co 2+ ions) bind to cellulose nanofibrils via electrostatic interactions between the hydroxyl groups ( electron-rich oxygen atoms) of the nanocellulose and electropositive transition metal cations.The authors were able to show magnetic behavior at room temperature that they attributed to the fraction of nanoparticles in superparamagnetic state.

MAGNETOSTRICTIVE CELLULOSE NANOCOMPOSITES FOR ACTUATION AND SENSING
In ferromagnetic materials, magnetostriction leads to the variation of their physical geometries according to an external magnetic field.The magnetostrictive effect is usually negligible for regular materials like nickel.The advent of rare earth iron alloys, such as Terfenol-D, that exhibit giant magnetostrictive effect has led to the promising development of sensors and energy harvesting devices requiring higher energy density, faster response and better precision.Additionally, the magnetostrictive effect has been used in transducers for applications in sonars [117][118][119], energy harvesting [120][121][122][123], active vibration control [124][125][126] and position control systems [127,128].However, the monolithic Terfenol-D materials are heavy and not easily moldable into desirable shapes for sensor and actuator fabrication; they are still relatively brittle and can only withstand small strains to failure [129].A potential remedy is to take advantage of the superb mechanical properties of the nanocellulose matrix to form a magnetostrictive composite with magnetostrictive particles.The structure of the cellulose nanofibers provides a layered substrate for magnetostrictive materials.Since the resulting composite is in the form of film and inherit magnetic properties from magnetostrictive particles, it is known as magnetostrictive nanocellulose membrane (MNM).For magnetostriction, the Joule effect describes the change in length due to a change in the magnetization state of the material.Consequently, the magnetic field can be used to actuate or morph the flexible MNM.Whereas through the inverse magnetostrictive effect (also known as the Villari effect), the mechanical stress will change the magnetic susceptibility of a material.Since the film is expected to respond to changes in the external mechanical stresses by changing its magnetic state, these changes in the magnetic state can induce voltage in coil(s) in accordance with Faraday's law, which can then be readily used to deliver power to an electrical load.Moreover, a magnetic field can be used to optimize magnetostrictive properties of the composite by favorably aligning particles [130][131][132][133] during the processing stage (Figure 11). Figure 12 shows a unimorph nanocellulose composite that is actuated by a magnetic field and the Joule effect.The magnetic field is cycled from 0 to 2 kOe and back down in 200 Oe increments.
The composite material fabrication procedure generally followed the CNF preparation procedure discussed in earlier literature [134], with additional steps for introducing Terfenol-D particles.Filtration was carried out until approximately 80% of the water by volume was removed from the composite.An identical volume of CNF slurry was then added atop the partially filtered layer, this time without any Terfenol-D particles.This was done to create a unimorph structure which would allow the material to bend instead of linearly deforming under an applied magnetic field.Filtration was again carried out until approximately 80% water was removed from the material.At this point, the sample was removed from the filtration vessel, and sandwiched between two sheets of wax paper with absorptive pads on either side.The stack was then pressed at 100 N for 24 hours while sitting atop a layer of silica gel desiccant which drew out remaining water.
Testing of the composite was performed via a video extensometry method, using DC magnetic fields inside a laboratory electromagnet to actuate the sample.The magnet was placed atop a damped optics table to eliminate random vibrations from the environment.An optical microscope (Stemi 2000-C, Carl Zeiss AG, Germany) equipped with a digital camera (AxioCam ICc 1, Carl Zeiss AG, Germany) was mounted to the electromagnet for the purpose of non-contact displacement measurement.A black and white grating pattern was painted at the tip of both edges of the particlecontaining sample.The sample was then cantilevered within the center of the electromagnet air gap, with length direction along the central axis of the pole pieces.Ten cycles of a low frequency AC magnetic field were applied to the sample in order to relax any residual stresses introduced during the mounting procedure.DC current was supplied to the electromagnet such that the magnetic field strength incremented by 200 Oe, completing a full loop between −2 kOe and 2 kOe.An optical microscope image of the sample was captured at each increment.The image sequence obtained was then converted to a video file, and the angular displacement of the sample as a function of magnetic field strength was extracted by tracking the painted-on grating with open source video analysis software (Tracker).The process was repeated with the sample flipped over in its holder, and its other edge in view of the microscope.A total of five trials were performed for each sample at each orientation.Images of a sample with grating being analyzed in Tracker can be seen in Figure 12.
Figure 13 shows the angular deflection data for a 30% volume fraction Terfenol-D/CNF unimorph composite film compared with a control sample containing an equal volume of non-magnetostrictive grade 68 ferrite particles.In addition to lacking appreciable magnetostrictive properties, the 68 ferrite has comparable initial DC magnetic permeability to that of Terfenol-D.As such, the control sample experiences  an approximately equivalent attractive force in a static magnetic field.The data shows both positive and negative angular deflections for the Terfenol-D-containing sample, due to the fact that the sample was flipped over as mentioned previously.This reversal, one indicator of magnetostriction, is in contrast to the unidirectional deflection caused by magnetic attraction.Peak angular deflections at 2 kOe magnetic field of 0.26° and 0.21° were observed depending on sample orientation.The difference is likely attributable to imperfect sample alignment with the magnetic field direction, caused by some deformations of the composite film during the drying stage of the manufacturing procedure.In addition, it has been previously reported that localization and boundary effects relative to the distribution of the Terfenol-D particles in a composite may contribute to the variability in the response [135].Regardless, the virtually nonexistent response of the control sample and the deflection reversal when the Terfenol-D sample is flipped strongly indicate that a magnetostrictive response is being observed in the fabricated unimorph specimens.

CONCLUSIONS
Cellulose nanomaterials are a relatively new class of materials that are receiving increased attention for a wide variety of applications because of their sustainability and unique properties.Cellulose nanofibrils and cellulose nanocrystals have been used in electronics, display devices, including organic light-emitting diodes, energy harvesting devices, such as photovoltaics, magnetic and magnetostrictive composites, as battery electrodes and separators, and many other functional or advanced applications.In addition to potentially improving the environmental impact and sustainability of function materials, cellulose nanomaterials often provide improved functionality and/ or durability for these applications.Films from cellulose nanomaterials have high transparency, high optical haze, low thermal expansion coefficient and good mechanical properties, thus offering some advantages over traditional materials, such as thermoplastics.As the economics and availability improve, these nanomaterials are expected to find a variety of commercial applications in functional devices.

Figure 1 Figure 2
Figure 1 Transmission electron micrograph of cellulose nanocrystals from wood; scale bar is 200 nm.

Figure 3
Figure 3 Cellulose nanofibril film overlaid on Forest Service logo.

Figure 4
Figure 4 Flexible electronics on cellulose nanofibril substrate.

Figure 5
Figure 5 Schematic of production process for flexible electronics on CNF substrate.(Reprinted with permission from [28]; Copyright 2015 AIP Publishing LLC)

Figure 6
Figure 6 Digital electronics on CNF.(Reprinted under Creative Commons license from [31])

4 Figure 9
Figure9 An illustration of the sequential filtration steps during the production of a paper battery cell.(Reproduced with permission from[88]; The Royal Society of Chemistry)

Figure 10
Figure 10 (a) Photograph illustrating the flexibility of a paper battery.(b) SEM image of a paper battery crosssection.(Reproduced with permission from [88]; The Royal Society of Chemistry)

Figure 12
Figure 12 Angular deflection data being extracted from microscope images of samples during testing.

Figure 13
Figure 13 Angular deflection data comparing unimorph CNF composite films containing magnetostrictive and nonmagnetostrictive particles.