Nanocellulose Composites Properties and Applications

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PBM·Nanocellulose Composites

Nanocellulose Composites—Propertiesand Applications

Chang Ma1, MingGuo Ma1,2,*, ZhiWen Li1, Bin Wang1

1. Engineering Research Center of Forestry Biomass Materials and Bioenergy, Beijing Key

Laboratory of Lignocellulosic Chemistry, College of Materials Science and Technology, Beijing

Forestry University, Beijing, 100083, China;

2. Key Laboratory of Pulp and Paper Science & Technology of Ministry of Education/Shandong

Province, Qilu University of Technology, Ji’nan, Shandong Province, 250353, China

Received: 2 February 2018; accepted: 1 March 2018.

*Corresponding author:MingGuo Ma, PhD, professor;research interests: green synthe-sis and applications of biomass functional materials; E-mail: [email protected]

Chang Ma, master candidate;E-mail: [email protected]

Abstract: Nanocellulose composites combine the advantages of nanocellulose and composites. Recently, nanocellulose composites have been received more attentions due to their improved properties and promising broad applications. In the past, rapid progress has been made in the synthesis, properties, and mechanism of nanocellulose composites and potential applications were reported. There are a few reports on the increasing applications of nanocellulose composites with focus on the biomedical field, environmental field, electrode and sensor applications. In this article, the recent development of nanocellulose composites was reviewed via some typical examples. In addition to the synthesis methods, improved properties and potential applications were discussed. The problems and future applications of nanocellulose composites were also suggested.Keywords: nanocellulose; composite; property; application; sensor; biomedical field

1 Introduction

Nanocellulose includes cellulose nanocrystals, cellulose nanofibrils and bacterial cellulose. Cellulose nanocrystals could be obtained from native cellulose via acid hydrolysis removing the amorphous regions and preserving the highly-crystalline structure. Cellulose nanofibrils consist of crystalline and amorphous regions from wood pulp through mechanical pressure before and/or after chemical or enzymatic treatment. Fig.1 showed the schematic illustrations of the hierarchical structures of plant cell walls, the four methods of individualization of cellulose nanofibrils (high-intensity ultrasonication method, strong hydrochloric acid hydrolysis method, 2,2,6,6-Tetramethyl-1-piperidinyloxy-mediated oxidation method and strong sulfuric acid hydrolysis method), and the corresponding as-prepared cellulose

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nanofibrils[1]. Nanocellulose has the characteristics of high crystallinity, stiffness, elasticity, flexibility and plasticity[2] and has application potential in the smart screens, papermaking, biomedical and environmental fields. Nanocellulose was reported to have the value of Yong’s modulus of 206 GPa which was similar to the value of steel of 200～220 GPa[3].

Recently, most attention has been paid to the research of the nanocellulose composites due to their special properties and promising applications. Nanocellulose composites combine the advantages of nanocellulose and composites. Nanocellulose has been reported as an efficient strategy to improve the properties of polymer composite. Several reviews on the applications of nanocellulose composites have been published including electroconductive field[4], food packaging[5-6] and biomedical applications[7]. Khan et al reviewed bioactive nanocellulose-based composites in the preparation of cheap, lightweight and very strong nanocomposites for food packaging application[5]. In the review, authors indicated that the application of nanocellulose could extend the food shelf life and improve the food quality as carriers of some active substances, such as antioxidants and antimicrobials. In general, nanocellulose is applied as reinforcing phase for the fabrication of composites owing to its intrinsic

chemical natures, aspect ratio, and crystallinity degree[8]. More recently, Mao et al presented the preparation of nanocellulose by various methods and discussed its applications in electronic field, adsorption field and dispersing & stabilizing fields[9].

This article provided an overview of properties and applications of nanocellulose composites. Various appl ica t ions of nanoce l lu lose composites including biomedical field, environmental field, electrode and sensor applications were reviewed. At last, the problems and future appl ica t ions of nanoce l lu lose

composites were suggested.

2　�Properties and applications of nanocellulose composites

2.1　Biomedical applications

Nanocellulose and its composites are cost-effective advanced material for biomedical applications such as medical implants, tissue engineering, drug delivery, wound healing, diagnostics, 3D printing and magnetically responsive materials because of their biocompatibility, biodegradability and low cytotoxicity[7]. For example, nanocellulose was extracted from pineapple leaf fibers by steam explosion method[10]. The as-obtained nanocellulose is a promising material for biomedical applications including tissue engineering, drug delivery, wound dressings and medical implants. Ma et al reviewed the surface chemical modification of cellulose nanocrystals via various strategies for biomedical applications[11]. Nanocellulose has great biocompatibil i ty and mechanical strength and promising applications in biomedical fields.

Cellulose/polyurethane nanocomposites with nanofibers structure and biodegradation properties were isolated from pineapple leaf fibers for various

Reprinted from Ref. [1] with permission from Wiley.Fig.1　Schematic illustrations of (a) the hierarchical structures of plant cell walls,

(b)～(e) the four methods of individualization of cellulose nanofibrils, and (f)～(i) the corresponding as-prepared cellulose nanofibrils

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versatile medical implants[12]. It observed the increase of strength nearly 300% and the stiffness by 2600% with addition of 5 wt% cellulose nanofibers into polyurethane. Natural rubber/nanocellulose composites were investigated by vermicomposting method with reference to the crosslinking of the matrix[13]. The as-reinforced nanocellulose was proved to highly influence the biodegradation rate of the composite via interaction between rubber and nanocellulose.

A carbodiimide cross-linker was used to fabricate allicin-conjugated nanocellulose and lysozyme-conjugated nanocellulose with good antifungal and antibacterial effects against standard strains of Candida albicans, Aspergillus niger, S. aureus and E. coli[14]. Authors suggested that these materials could be used as an antimicrobial agent in food packaging, inside foodstuffs and textile materials. Berndt et al also prepared porous bacterial cellulose-silver nanocomposites with a strong antimicrobial activity against E. coli as antimicrobial wound dressings[15]. It should be pointed out that this activity was restricted to the as-modified dressing itself, avoiding a release of silver nanoparticles into the wound.

Hierarchically ordered cellulose nanofibers/indomethacin composites were fabricated for sustained release applications through self-assembly and recrystallization of indomethacin on the surfaces of cel lulose nanofibers[16]. The composite fibers possessed an encapsulation efficiency of up to 97%, and a sustained drug-release period of over 30 days. Nanocellulose filter paper with a tailored pore size distribution was researched for virus removal[17]. Natural unmodified nanofibrous polymer-based membrane was capable of removing virus particles solely based on the s ize-exclus ion principle.

Dehnad et al prepared chitosan-

nanocellulose nanocomposites with superior mechanical properties[18]. They obtained approximate values of 47% elongation-at-break, tensile strength of 245 MPa and Young’s modulus of 4430 MPa. Authors found the as-obtained chitosan–nanocellulose nanocomposites with high Tg range of 115～124℃ [19]. Chitosan–nanocellulose nanocomposites also displayed inhibitory effects against both gram-positive (S. aureus) and gram-negative (E. coli and S. enteritidis) bacteria through agar disc diffusion method. It is expected that these chitosan–nanocellulose nanocomposites could be used for antimicrobial agent in biomedical fields. TEMPO-oxidized bacterial cellulose/sodium alginate hydrogel composites were synthesized for cell encapsulation[20]. The composites showed the increase compression strength and chemical stability. Cells were successfully encapsulated in the composites, indicating that it could be a potential candidate for cell encapsulation engineering.

In situ gelling poly(oligoethylene glycol methacrylate)/cellulose nanocrystal nanocomposite hydrogels with enhanced mechanical properties were synthesized through control of physical and chemical cross-linking[21]. Fig.2 displayed schematic representation of hydrogel precursors and injectable poly(oligoethylene glycol methacrylate)/cellulose nanocrystal nanocomposite

Reprinted from Ref.[21] with permission from ACS.Fig.2　Schematic representation of hydrogel precursors and injectable poly(oligoethylene

glycol methacrylate)/cellulose nanocrystal nanocomposite hydrogels

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hydrogels via co-extrusion of the reactive precursor polymer solutions from a double-barrel syringe. The hydrogels displayed the readily tailored macroscopic properties including gelation rate, swelling kinetics, mechanical properties and hydrogel stability. These nanocomposite hydrogels have potential interest for various biomedical applications including tissue engineering scaffolds for stiffer tissues or platforms for cell growth. Tummala et al reported nanocellulose reinforced polyvinyl alcohol hydrogels with exceptionally high water content (90 wt%～93 wt%) and collagen-like mechanical behavior typical for soft tissues, especially in ophthalmology[22]. The hydrogels possess remarkable mechanical strength for their high water content, transparency of 90%～95% at 550 nm, and hyperelastic properties of 250%～350% strain.

Highly biocompatible bacterial cellulose/zein composite nanofiber with a controlled hydrophobic surface was developed through a solution impregnation method, followed by evaporation-induced self-assembly of adsorbed zein protein[23]. It observed cells spread out on the membrane surface and obtained the high cell density after 48 h of culture. The composite membranes showed a significantly increased adhesion and proliferation of fibroblast cells probably due to the rough surface structure of nanofibers as well as the high

biocompatibility of natural zein protein (Fig.3). Authors suggested that the novel composite nanofibers could be of promising applications for the design of biomaterials and bio-devices that require specific surface properties and adhesion.

Dong et al synthesized Ag@Fe3O4@cellulose nanocrystals nanocomposites by a green microwave-assisted hydrothermal method[24]. Ag@Fe3O4@cellulose nanocrystals nanocomposites exhibited good antibacterial activities toward both S. aureus and E. coli, and had good adsorption of dye solution. The nanocomposites have the inhibition zones of 2.8 mm for S. aureus and 2.4 mm for E. coli (Fig.4), showing good antimicrobial activities. Ag ions were released slowly from nanocomposites and went through bacteria membrane and reacted with—SH in protease, resulting in the inactivation of protease and eventually the death of bacteria.

More recently, Wu et al reported biocompatible TEMPO-oxidized bacterial cel lulose pell icle/silver nanoparticles with antibacterial activity by in situ synthetic method for wound dressing[25]. The composites exhibited high biocompatibility of cell viability >95% after 48 h of incubation according to the result of in vitro cytotoxicity test and showed significant antibacterial activities of 100% and 99.2% against E. coli and S. aureus, respectively.

All membranes were subjected to Hoechst staining. Reprinted from Ref.[23] with permission from ACS.Fig.3　Fluorescent images (scale bar 30 mm) of L929 fibroblast cells on bacterial cellulose, bacterial cellulose-zein 0.5%, bacterial

cellulose-zein 1%, and bacterial cellulose-zein 2% membranes after 24 and 48 h of culture

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2.2　Electrode materials applications

Nanocellulose composites combined the advantages of nanocellulose and conductive polymers such as high surface area, high conductivity and high charge capacities, which could be applied as electrode materials.

Microfibrillated cellulose/polypyrrole composite with electronically conductive high surface area was obtained by direct chemical polymerization of pyrrole on wood-derived nanofibers[26]. The as-prepared composite exhibited a surface area of 90 m2/g, a conductivity of 1.5 S/cm and an ion-exchange capacity for chloride ions of 289 C/g corresponding to a specific capacity of 80 mA·h/g. These nanocellulose composites have promising applications in ion-exchange and paper-based energy storage devices fields. Polypyrrole/cladophora nanocellulose composites were used as paper-based energy-storage devices with unprecedented performance at high charge and discharge rates[27]. The as-obtained composites have charge capacities of more

than 200 C/g, energy of 1.75 Wh/kg and power densities of 2.7 kW/kg for paper-based e lect rodes a t potential scan rates as high as 500 mV/s. The nonelectroactive carbon filaments decreased the contact resistances and the resistance o f t h e r e d u c e d p o l y p y r r o l e composite. Nanocellulose coupled po lypyr ro le@graphene ox ide paper was prepared via in situ polymerization for use in high-performance paper-based charge storage devices, exhibiting stable cycling over 16000 cycles at 5 A/g as well as the largest specific volumetric capacitance of 198 F/cm3

f o r f l e x i b l e p o l y m e r - b a s e d electrodes[28]. Wang et al prepared surface modified nanocellulose f ibers/polypyrrole composites flexible supercapacitors electrodes

with enhanced capacitances[29]. The supercapacitor electrodes have the high full electrode-normalized gravimetric of 127 F/g and volumetric capacitances of 122 F/cm3 at high current densities of 300 mA/cm2 approximate to 33 A/g for conducting polymer-based electrodes with active mass loadings as high as 9 mg/cm2. Modified nanocellulose fibers/polypyrrole composites were applied to connect three symmetric supercapacitors (Fig.5). It observed the triangular shape for the charge/discharge profiles at ±20 mA upon bending the super-capacitors at a bending angle of ~340°.

Recently, Zheng et al reported cellulose nanofibers-mediated hybrid polyaniline electrodes with good capacity rate and energy/power density balance for high-performance flexible supercapacitors[30]. The composites achieved a maximum specific capacitance of 421.5 F/g for hybrid electrodes at a current density of 1 A/g. All-solid-state supercapacitors from the hybrid electrodes showed excellent electrochemical

Reprinted from Ref.[24] with permission from Springer.Fig.4　Antimicrobial activities of Ag@Fe3O4@CNC nanocomposites synthesized by

microwave-assisted hydrothermal method for 30 min: (a) S. aureus; (b) E. coli; (c) The schematic illustration of possible antimicrobial mechanism of Ag@Fe3O4@CNC

nanocomposites

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performance and capacitance retention under repeated bending over 1000 cycles.

Cellulose nanofibrils/carbon nanotubes aerogels hybrids were synthesized for mechano-responsive conductivity and pressure sensing by the freeze-drying method[31]. The hybrid aerogels has a relatively high electrical conductivity of 1×10-2 S/cm. Highly-ordered biodegradable cellulose nanofibers/carbon nanotubes flexible dielectric papers with mechanically flexible and improved mechanical strength were fabricated for dielectric energy storage using a vacuum-assisted self-assembly technique[32]. The as-obtained dielectric paper possessed a high dielectric constant of 3198 at 1.0 kHz and enhanced dielectric energy storage capability of (0.81±0.1) J/cm3. Jiang et al developed in situ grown two-dimensional bacterial nanocellulose/graphene oxide composite for flexible supercapacitors in flexible electronics such as bendable mobile phones, flexible displays and wearable devices[33]. It found graphene oxide flakes interlocked within the nanocellulose network during bacterial nanocellulose growth, preventing their restacking and loss of active area and leading to excellent energy storage performance as well as mechanical flexibility.

2.3　Sorption applications

Nanocellulose composites have application in sorption fields for various pollutants due to their large specific surface area, high carbon content and the rich surface functional group, which results in strong adsorbability and great adsorption capacity.

Highly porous TiO2-coated nanocellulose aerogel was obtained via chemical vapor deposition, showing photo swi tching between water-superabsorbent and water-repellent states[34]. Nanocellulose aerogel displayed highly hyd rophob i c w i th contact angle of 140°. Authors

suggested that both structures and pores at several length scales from microscale to nanoscale possibly promoted high water absorption upon UV illumination due to increased capillary effects. The nanocellulose aerogels show photo-oxidative decomposition and wetting phenomena, and have interest for applications in water purification. Bacterial cellulose/TiO2 composites were prepared by in situ modification route[35]. These composites have applications in porous filtering media for drinking water purification and air cleaning.

Calcium hydroxyapatite/microfibrillated cellulose composites were synthesized as a potential adsorbent for the removal of Cr(VI) ions from aqueous solution[36]. It obtained the very high adsorption rate in the beginning and achieved over 94% of Cr(VI) removal within the first 5 min. The adsorption equilibrium data well fitted by the Langmuir isotherm model with maximum adsorption capacity of 2.208 mmol/g of Cr(VI) ions on composites. More recently, Anirudhan et al developed zinc oxide incorporated graphene oxide/nanocellulose composite with self-cleaning and reusability property for the adsorption and photo catalytic degradation of ciprofloxacin hydrochloride from aqueous solutions[37]. The composites have the enhanced band gap of 2.8 eV

Reprinted from Ref. [29] with permission from ACS.Fig.5　(a) Photo of a red LED powered by three flexible polypyrrole@nanocellulose fibers-based supercapacitors coupled in series for a bending angle of ~340°; (b) Charge/discharge

profiles for the in-series supercapacitors under flat and bending conditions for an applied current of 20 mA as well as (c) the corresponding current density responses due to potential steps to 2.4 V

and 0 V, respectively

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in the visible region, the values of surface area of 12.68 m2/g, pore volume of 0.026 mL/g and pore radius of 12.5 nm, respectively. It achieved a maximum degradation efficiency of 98.0%, followed first-order kinetics. The composite is a potential candidate for the removal and degradation of ciprofloxacin from aquatic environment.

Poly(acrylic acid)-modified poly(glycidyl methacry late)-grafted nanocellulose was synthesized by graft copolymerization reaction of glycidyl methacrylate onto nanocellulose in the presence of ethylene glycol dimethacrylate as cross-linker followed by immobili-zation of poly(acrylic acid)[38]. This hydrogel displayed the maximum adsorption capacity of 148.42 mg/g for white lysozyme of chicken egg from aqueous solutions at 30oC and pH value of 6.0 based on Langmuir isotherm model. Natural rubber/nanocellulose nanocomposites were reported to reinforce the membrane transport, rheological and thermal degradation properties[39]. The nanocomposites showed an increase Vrf value from 0.74 to 0.80 at 10% nanocomposite compared with nature rubber. With the addition of nanocellulose, it observed the remarkable decrease in the diffusion coefficient and the equilibrium solvent absorption. pH responsive nanocellulose/amphoteric polyvinylamine nanocomposite microgel with high density of free amine groups was fabricated via a two-step method[40]. The microgel was effective in anionic dye removal under acidic conditions, and displayed the maximum adsorption capacities of 869.1 mg/g for congo red 4BS, 1469.7 mg/g for acid red GR and 1250.9 mg/g for reactive light yellow K-4G, following the pseudo second order kinetics and adsorption isotherms fit well with the Sips model.

Activable carboxylic acid functionalized crystalline nanocel lulose/poly(vinyl alcohol-co-ethylene nanofibrous membrane with enhanced adsorption functionalized with 1, 2, 3, 4-butanetetracarboxylic acid was applied to adsorb heavy metal ions of Pb (II), and the metal ion mixture of Pb (II), Cr (VI), Mn (II) and Cu (II)[41]. The as-modified nanofibrous membrane displayed excellent adsorption capacity of

471.55 mg/g at 15℃ to the single metal ions and metal ion mixture after NaHCO3 activation. Nanocellulose/poly(2-(dimethylamino) ethyl methacrylate) hydrogels with both pH-sensitive and temperature-sensitive properties were prepared via crosslinking free radical polymerization for removal of Pb(II) and Cu(II) ions[42]. The hydrogels possess uniform pore structure, high porosity (>97%), specific surface area of 82 m2/g and compressive strength of 1.26 kPa. It achieved the maximum adsorption capacities of 217.39 mg/g for Cu(II) and 81.96 mg/g for Pb(II) according to Langmuir model, attributing to the chelation of tertiary amine groups and the ion-exchange of carboxyl groups.

More recently, ultralight super-hydrophobic porous three-dimensional carbon aerogels based on cellulose nanofibers/poly(vinyl alcohol)/graphene oxide were synthesized through an freeze-drying process for highly effective oil-water separation and environmental protection[43]. The as-resulting aerogels have low density of 18.41 mg/cm3, high porosity of 98.98%, a water contact angle of 156°(super-hydrophobic) and high oil absorption capacity (97 times its own weight).

2.4　Hydrogels applications

Nanocellulose is an important reinforcing agent for creating reinforced or structured hydrogel composites with desirable mechanical properties and a wide range of biomedical, energy storage, construction, separations, cosmetic and food applications[44].

Cellulose nanocrystals/poly(acrylic acid) nano-composite hydrogels with inter-connected network structures were synthesized by in situ free radical polymerizat ion [45]. The as-prepared cel lu lose nanocrystals/poly(acrylic acid) nanocomposite hydrogels exhibit excellent composition-dependent mechanical properties such as a large elongation ratio (>1100%) and high tensile strength (>350 kPa). It found some new chain entanglements under concentrated conditions after drying treatment above the glass transition temperature (Tg). The cellulose nanocrystals/polyacrylamide core-shell elastomeric hydrogels were synthesized via free radical polymerization

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in the absence of chemical cross-links[46]. The as-obtained hydrogels possess great tensile strength and elongation ratio, compared with chemically cross-linked counterparts, due to both chemical and physical interactions between cellulose nanocrystals and polyacrylamide matrix. Authors suggested that the high extensibilities and fracture stresses were related to the well-defined network structures with low cross-linking density and lack of non-covalent interactions among polymer chains. Cellulose nanocrystals/poly(ethylene glycol) elastomeric nanocomposite hydrogels with high strengths and flexibilities were obtained through a one-stage photocross-linking process[47]. The as-prepared nanocomposite hydrogels were efficient at energy dissipation due to the reversible interactions between cellulose nanocrystals and poly(ethylene glycol) polymer chains. They proposed that the strong gel viscoelastic behavior and the mechanical reinforcement were related to “filler network”. Stretchable and hysteretic isotropic cellulose nanocrystals/poly(N,N-dimethylacrylamide) nanocomposite hydrogels with high mechanical properties and a more efficient energy dissipation mechanism were obtained[48]. It observed a 4.8-fold increase in Young’s modulus, 9.2-fold increase in tensile strength and 5.8-fold increase in fracture strain with only 0.8 wt% of cellulose nanocrystals loading. It found physical interactions within networks as reversible sacrificial bonds, exhibiting large hysteresis as an energy dissipation mechanism via cluster mobility.

Naser i e t a l prepared double cross- l inked interpenetrating sodium alginate and gelatin reinforced cellulose nanocrystals polymer network hydrogels with improved tensile strength and strain via the freeze-drying process[49]. The hydrogels showed a three-dimensional network of interconnected pores and hierarchical pores with a nanostructured pore wall roughness, the high porosity of the scaffolds (>93%), high phosphate buffered saline uptake and cytocompatibility toward mesenchymal stem cells. Semi-interpenetrating chitosan/cellulose nanocrystals hydrogels with improved mechanical properties

and remarkable pH sensitivity were prepared[50]. It achieved the crosslinking degree of 83.6% and the maximum compression of (50.8±3) kPa for the hydrogel. The hydrogels exhibited excellent pH sensitivity and the maximum swelling ratio under acidic condition (pH value of 4.01). These hydrogels have the applications in various fields, such as tissue engineering, pharmaceuticals and drug delivery. Mittal et al combined silk proteins with cellulose nanofibrils to obtain bioactive composites with anisotropic hierarchical structures[51]. Fig.6 showed the formation mechanism and synthetic procedure of composite isotropic films composed of cellulose nanofibrils (90%) and silk (10%) by the solvent casting method. It observed a dense network of fibrils with an apparently isotropic orientation based on the surface of the cellulose nanofibrils film. The composites achieved highly desirable mechanical performance with a stiffness of ~55 GPa, strength at break of ~1015 MPa and toughness of ~55 MJ/m3. Authors suggested that bio-based materials provide abundant opportunities to design composites with high strength and functionalities.

Shao et al synthesized the self-healing cellulose nanocrystal-poly(ethylene glycol) nanocomposite hydrogel via diels-alder click reaction[52]. These nanocomposite hydrogel displayed outstanding mechanical properties with a high fracture elongation up to 690%, a fracture strength up to 0.3 MPa at a strain of 90%, the self-healing capability as high as 78% and excellent self-recovery and antifatigue properties (Fig.7). The hydrogel displayed the increase ultimate elongation and tensile strength with increased healing time. It observed the fracture stress of 52 kPa after 1 h and the recovered stress of 116 kPa after 24 h, compared with the original hydrogel.

2.5　Sensors applications

Nanocellulose is a promising candidate in (bio) sensing technology, related to various fields such as clinical/medical diagnostics, environmental monitoring, food safety, physical/mechanical sensing, labeling and

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bioimaging applications[53]. High-strain sensors were obtained based on

crumpled graphene and nanocellulose with 3D macroporous structure via the vacuum filtration method[54]. The high-strain all-directional stretchable nanopaper sensors exhibited a gauge factor of 7.1 at 100% strain, which was >10 times higher than stretchable carbon nanotubes and Ag nanoparticles sensors. Authors suggested that these graphene high-strain sensors have promising in emerging human-interactive applications.

Highly porous a -Fe2O3 (hematite) nanostructures with a well-defined anisotropic porosity were fabricated via sol-gel transformations of molecular precursors using nanocellulose as template[55]. The as-established nanocellulose templating technique enables the fabrication of a variety of mesoporous crystalline iron oxide scaffolds.

Nanocellulose/poly(vinyl alcohol)/graphene nanocomposite freestanding films were fabricated via the casting method for humidity sensing effect in aqueous system[56]. It obtained a homogenous dispersion of nanocomposite in water with good stability due to the hydrophobic interactions as well as electrostatic repulsion. Composite films exhibited high conductivity, sensitivity as well as faster response (lower hysteresis), and good repeatability of relative humidity.

Burrs et al reported a paper-based graphene-nanocauliflower hybrid composite for use in electroche-mical biosensing of small molecules (glucose) or detection of pathogenic bacteria[57]. Graphene oxide-coated nanocellulose produced a conductive paper with an extremely high electroactive surface area of (0.29±0.13) cm2. The surface of conductive paper was functionalized with biomaterials (oxidase or RNA) for demonstration as a point of care biosensor. It achieved

Reprinted from Ref.[51] with permission from ACS.Fig.6　Formulation of composite isotropic films composed of cellulose nanofibrils (90%) and silk (10%). (a) Schematics showing how the functionalized silk interact with and decorate the surface of cellulose nanofibrils; (b) Fabrication of anisotropic cellulose nanofibrils

films; (c) AFM images of the film surfaces

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the detection limit of (0.08±0.02)mmol/L for glucose and approximate to 4 CFU/mL for E. coli O157:H7, and the response time of 6 s for glucose and 12 min for E. coli, respectively. T h e n a n o c e l l u l o s e / g r a p h e n e /n a n o p l a t i n u m c o m p o s i t e s a r e excellent platform for electrochemical biosensors targeting small molecules or whole cells for use in point of care biosensing.

L iu e t a l f ab r i ca t ed robus t , s t retchable and s train-sensi t ive functional network hydrogels for ultrasensitive wearable soft strain sensors by the interconnection between a “sof t” homogeneous polymer network and a “hard” dynamic ferric cross-linked cellulose nanocrystals network[58]. These hydrogels exhibited unusual mechanical properties, such as excellent mechanical strength, robust toughness and stretchability, as well as good self-recovery property, and displayed tunable electromechanical behavior with sensitive, stable and repeatable variations in resistance upon mechanical deformations. Fig.8 displayed the demonstration of the F-hydrogels as wearable sensors with strain-responsive conductivity. The curve of the relative resistance with pulse-beat exhibited excellent reproducibility. Authors indicated that hydrogels could act as a wearable strain sensor to monitor finger joint motions, breathing and even the slight blood pulse.

3　Conclusions

In summary, a considerable amount o f papers a re re la t ive ly to the

Reprinted from Ref.[58] with permission from ACS.

Fig.8　(a) Photographs of the changes for LED brightness vs. elongation of the F-hydrogels connected in the electric circuit; (a-1) The relative resistance changes vs. time when a loading-unloading cycle of the F-hydrogels at different strains; (b) Photographs of bending the index finger at different bending angles; and (b-1) the relative resistance changes vs. time when bending-unbending the index finger at

different angles

Reprinted from Ref.[52] with permission from ACS.Fig.7　(a) Self-healing performance of hydrogel by direct visual inspection; (b) Stress-strain curves of the original and self-healed hydrogel at various healing times; (c), (d)

Self-healing efficiency of the hydrogel measured from tensile tests at room temperature

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applications of nanocellulose composites in the last years. As described above, the main applications of nanocellulose composites are in the areas of biomedical field, environmental field, electrode and sensor applications. These applications take advantage of high crystallinity, high stiffness and high mechanical properties of nanocellulose as well as the synergy effects between nanocellulose and composites, and result in the improved performance of nanocellulose composites. In many examples, nanocellulose was applied as reinforcement material for the fabrication of composites and greatly improved properties and applications of cellulose composites. Especially, the structured and functional hydrogel composites based on nanocellulose exhibited desirable properties and very charming applications. Obviously, nanocellulose composites are fascinating future materials.

A large variety of synthesis approaches including microwave-assisted hydrothermal method, freeze-drying method, self-assembly technique, in situ modification route, in situ free radical polymerization, photo cross-linking process, vacuum filtration method, etc, were reported for the synthesis of nanocellulose composites. Obviously, the synthetic methods do not seem to be difficult for the synthesis of nanocellulose composites. However, the interaction between nanocellulose and composites, and mechanisms of nanocellulose improving their properties are still poorly understood, there is still much room to improve the applications of nanocellulose composites as well as their properties. As a matter of fact, it is not easy to synthesize nanocellulose from biomass in large-scale industrial scale, because the traditional acid hydrolysis method suffers the drawback of high energy consumption, long time and low yield during the synthesis of nanocellulose.

This paper reviewed the main applications of nanocellulose composites in the areas of biomedical field, environmental field, electrode and sensor applications. Is this all nanocellulose composites could spare? Obviously not. More and more applications should be explored, which could accelerate the research

of nanocellulose composites. We believe that will come true in the near future.

No mat t e r how good the pe r fo rmance i s , nanocellulose composites are still nanocellulose composites in itself. As the structured and functional materials, devices are the foundation to build the building. Obviously, the nanocellulose composites are not devices. But devices could be consisted of nanocellulose composites. We expect that more and more progress would be made in the design of devices using nanocellulose composites.

Acknowledgments

Financial supported from the Fundamental Research Funds for the Central Universities (No. 2017ZY49) and the Foundation (No. KF201607) of Key Laboratory of Pulp and Paper Science and Technology of Ministry of Education/Shandong Province of China is gratefully acknowledged.

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Nanocellulose Composites Properties and Applications

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