The bio-composites derived from renewable resources have been subject of attention since 1942 when Ford applied soy-based plastics in their cars . The abundant and cheap availability of petroleum-based material at that time restricted the earnest efforts for the development of eco-friendly materials. Presently, increasing environmental concerns and regulations have put a deliberate interest in this direction. The very important advantages of natural fibers as filler over traditional carbon and glass fibers are the eco-friendly nature [1, 2]. In most of the cases, unfortunately, the natural fiber composite does not reach the same strength level as glass fiber composites mainly because of incompatibility between generally hydrophobic host polymer matrix and hydrophilic natural fiber, combined with lower thermal resistance of the cellulosic material.
In the last couple of years, it has been observed that highly crystalline cellulose (micro- or nanocrystalline cellulose, sometimes referred to as cellulose whiskers or cellulose crystallites) have some unique and outstanding potential to increase the composite material properties at lower filler concentration, in comparison to unfilled polymer matrix or to their microcomposite counterparts. Cellulose nanofibers have to overcome many obstacles against industrial practices due to time consuming preparation procedure with very low yield, highly hydrophilic surface, commercial unavailability, poor dispersion due to high agglomeration tendency, low thermal stability and most importantly, in general, comparatively higher cost through expensive source. The aim of this review is to concisely present the different angles of the prospects of future applications of cellulose based nanocomposites and to find the answer to whether future product development strategies can compromise with the mechanical properties at the cost of eco-compatibility.
Structure of Crystalline Cellulose
Typical plant cell walls contain a wide range of additional compounds that modify their mechanical properties and permeability. The major polymers that make up the plant structure include cellulose, hemicelluloses, and a complex phenolic polymer called lignin. Lignin permeates the spaces  in the cell wall between cellulose, hemicelluloses and pectin components, strengthening the wall (Fig. 1) . The cellulose is a complex carbohydrate made up of several thousand glucose molecules linked end to end and working like steel wires embedded in rubber matrix of pneumatic tire. Depending on the spatial arrangement of different molecules, the cellulose presents six different polymorphs with the possibility of conversion from one to another. Cellulose chains are biosynthesized, deposited in continuous fashion and aggregated to form microfibrils, ultimately forming long thread-like bundles of molecules stabilized laterally by hydrogen bonds between hydroxyl group and oxygen of adjacent molecules. The extended chain conformation and microfibril morphology result in significant load carrying capability, whereas the question of whether and how cellulose fibrils are organized in aggregates and their interaction with other polymers is still a thrust area of research [3, 5]. Depending on their origin, the fibrils diameter range vary from 2 to 20 nm to several nanometers . Each microfibril is considered as a string of cellulose crystals joined by amorphous domains, exhibiting modulus around 150 GPa and strength of 10 GPa, which are similar to those of aramid fiber . The microfibrils then aggregate further and form cellulose macrofibers , where single cellulose chains are deeply situated inside the matrix of hemicelluloses and other cementing material. Thus, the cellulose microfibrils have no regular surface because they are constituted by crystalline and amorphous regions. The amorphous regions are randomly oriented in a spaghetti-like arrangement, leading to a lower density in these noncrystalline regions [4–7]. Consequently, the amorphous regions are susceptible to acid attack. The hydronium ions can penetrate the cellulose chains in these amorphous domains promoting the hydrolytic cleavage of the glycosidic bonds and finally releasing individual crystallites. These crystallites can grow in size because of the large freedom of motion after hydrolytic cleavage. Therefore, cellulose crystals after acid treatment may be called microcrystalline cellulose. The term “cellulose crystallites” may be used for rod like cellulose crystals of nanometer dimensions (2–20 nm wide) extracted after acid hydrolysis and mechanical treatments [5, 7]. The different treatments of these charged crystallites, such as mechanical dispersion or ultrasonication, permit the dispersion of the aggregates and finally produce colloidal suspensions. Because of their stiffness, thickness, and length, these rod-like particles are commonly called “whiskers”. The terminology varies most of the time in current literature and has been tabulated in Table 1, with description of extraction process and source.
|CNW||Cellulose nanowhsikers||Ramie||H2SO4 hydrolysis|||
|Grass fiber||H2SO4 hydrolysis||[11, 12]|
|CNXL||Cellulose nanocrystals||Cotton Whatman filter paper||H2SO4 hydrolysis|||
|Bacterial cellulose||H2SO4 hydrolysis|||
|Cotton (cotton wool)||H2SO4 hydrolysis|||
|Norway Spruce||H2SO4 hydrolysis|||
|CNW-HCl||Cellulose nanowhiskers||Cotton linters||HCl Hydrolysis|||
|Wh||Whiskers||Cellulose fibers||H2SO4 hydrolysis|||
|NF||nanofibers||Wheat straw||HCl + Mechanical Treatment|||
|NCC||Nanocrystalline cellulose||MCC||H2SO4 hydrolysis|||
|MFC||Microfibrillated cellulose||Pulp Gaulin||Homogenizer|||
|NFC||Nanofibrilated cellulose/ Cellulose nanofibrils||Sulfite pulp||Mechanical|||
|MCC||Microcrystalline cellulose||Alpha cellulose fibers||Hydrolysis|||
|—||Cellulose crystallites||Cotton Whatman filter paper||H2SO4 hydrolysis|||
|—||Nanocellulose||Sisal fibers||H2SO4 hydrolysis|||
|—||Cellulose microcrystal||Cotton Whatman filter paper||HCl hydrolysis|||
|—||Nanofibers||Soybean pods||Chemical Treatment + High pressure defibrilator|||
|—||Nanofibers||Waste newspaper||H2SO4 hydrolysis|||
Extraction of Cellulose Nanocrystals
Cellulose nanofibers have been characterized and extracted from algae (Valonia) , wood , tunicate , sugar beet , brown algae (Oomycota) , bacterial and commercially available microcrystalline cellulose . Some cost effective sources such as wheat straw , flax, hemp, kraft pulp, rutabaga [5, 37], grass [11, 12], and sisal fibers , have also been explored. The extraction can be conducted broadly by two methods after alkali and bleaching treatments, namely acid hydrolysis and mechanical homogenization. Under controlled conditions, the extraction of cellulose whiskers consists of the disruption of amorphous regions by acid treatment while leaving the microcrystalline segments intact. Nanocrystalline cellulose produced after mechanical homogenization is commonly known as Micro-fibrillated Cellulose (MFC), which are long thread like fibers exhibiting web-like structure and having length in the micrometer scale. Amorphous and crystalline regions may be preserved in MFC whereas in nanocrystals from acid hydrolysis, very high amount of crystalline parts exist.
Cellulose nanocrystals have been obtained from hard and soft wood by traditional acid hydrolysis methods ; however, for the black spruce pulp, longer hydrolysis time resulted in shorter cellulose rods with narrow particle length distribution. The optimization of extraction process has been carried out by Bondeson et al.  from Norway Spruce (Picea abies) through monitoring the effect of hydrolysis time, temperature, and the ultrasonication treatment. The authors found that a sulfuric acid concentration of 63.5% (w/w), was optimum to obtain cellulose nanocrystals/whiskers with a length between 200 and 400 nm and a width less than 10 nm, in ∼2 h with a yield of 30% (of initial weight). Sulfite digestion reaction of orange peel has been used to extract high quality cellulose . The peroxide alkaline, peroxide alkaline-hydrochloric acid potassium hydroxide treatments have also been applied for the extraction of cellulose nanofiber . This method removes the middle lamella between the fibers as well as lignin and xylan from the cell wall, so that obtained single fibers can be used for biomechanical analysis. However, the removal of hemicelluloses may significantly change the structure of remaining cellulose . Cellulose crystals with the dimension of ∼5 nm x 150–300 nm have also been obtained from wheat straw . The raw material was kept under vapor pressure (1.95 MPa) at 210°C for 4 min in a reactor after impregnation with sodium sulfite (5% vol.). The steam exploded straw cellulose was washed with boiling 2% sodium hydroxide for 4 h under mechanical stirring. The material was then bleached until it became white, pressed in cheesecloth filters and redispersed in water.
The kraft pulp has been used to extract MFC  by fibrillating it in a 3% water suspension using a refiner, followed by repeated passes through the micro-gap of a high-pressure homogenizer up to 30 times to obtain different degrees of micro-fibrillation. The micro-fibrillated kraft pulp fiber slurry was subjected to centrifugation to isolate nanofibers of cellulose. Kulpinski  suggested that the MFC may be obtained by electro-spinning process by spinning drops containing cellulose dissolved in an N-methylmorpholine-N-oxide/water system. The spinning drops were made with an IKA-VISC laboratory-scale knitter (Heitersheim, Germany), and a small, oil-heated glass electro-spinning device was used. In another attempt, Lyocell was treated by a novel mechanical method to isolate micro and nanofiber, and a mat was made from the well stirred water suspension of the mixture of fiber and polymer .
Processing of Cellulose-Based Nanocomposites
Cellulose nanocrystals are hydrophilic in nature and difficult to employ with hydrophobic polymer matrixes. More commonly they are best suitable with water soluble or dispersible polymeric systems such as polyvinyl alcohol , starch, and natural rubber . Dispersion of cellulose nanocrystals in polar solvent  has accelerated its use as a natural nanofiller. Freeze dried cellulose suspension was dispersed in dimethylsulfoxide (DMSO) and N,N-Dimethylformamide (DMF) by sonication, keeping a small amount of water (0.1%). Although the aggregation in organic media was observed in comparison to water, still the suspension is potentially useful for making organic solvent-based nanocomposites. Further, the traditional melt compounding process of polymer composites is extremely difficult for cellulose nanofiber-based composites. If an aqueous dispersion of cellulose nanowhiskers is allowed to mix with hot polymer, it may generate lots of vapor due to the fast removal of water, which again reaggregates the cellulose whiskers. Oksman et al.  attempted the extrusion process by applying N,N-Dimethylacetamide containing lithium chloride swelling/separation agent and liquid feeding was carried out using separate venting systems to remove vapor. However, degradation was observed, which may had caused the reduction in mechanical properties of the obtained composites. Nanocomposite of 5 wt% cellulose nanowhiskers and cellulose acetate butyrate, plasticized by triethyl citrate has been produced by melt extrusion . The tensile modulus and strength indicated an improvement of 300 and 100%, respectively, at the cost of elongation at break. Results from Dynamic Mechanical Thermal Analysis (DMTA) showed that the tanδ peak temperature was shifted by 31°C, from 117°C to 148°C with addition of cellulose nanowhiskers in the matrix. Hot pressing method has also been applied to prepare the cellulose nanofiber-based composites. All the composites were prepared at 140°C by applying 10–50 MPa pressure at 70% (w/w) fiber loading  where increase in flexural strength and modulus with increasing molding pressure was partially attributable to the high density.
There are possibilities of surface modification of cellulose whiskers to increase the dispersion inside the hydrophobic polymer matrix and hydroxyethylation and hydroxypropylation have been conducted on the available surface hydroxyl groups by Wang et al. . The cellulose nanowhisker suspension was prepared by mixed acid hydrolysis (sulfuric and hydrochloric acid) and reaction was mediated by use of isopropyl alcohol and adequate reagent. During the reinforcing study of a copolymer matrix from latex phase by tunicin whisker, it was observed that processing parameters may have effect in the following ascending order of their reinforcement efficiency (tensile modulus and strength): extrusion < hot pressing < evaporation; and this behavior was attributed to the probable breakage and/or orientation of whiskers during processing . Soykeabkaew et al.  employed a surface selective dissolution method for the preparation of All-cellulose nanocomposites using bacterial cellulose where the effect of the immersion time of cellulose in lithium chloride/N,N-dimethylacetamide on the nanocomposite properties was also measured. Ten-minute immersion time was found best to obtain the optimum mechanical properties of composites. Authors observed that, when cellulose pellicles were mechanically fragmented and sheets prepared, the modulus and strength decreased in comparison of sheets from non disintegrated cellulose pellicles which was attested to the loss of continuity of the original network structure. The effect of cellulose nanofibers on the mechanical performance of resulting hybrid was compared with nanoclays . Nanoclays are layered silicates, commercially available and well known for enhancing the material properties of polymer upon uniform distribution inside the matrix. The results showed a difference in exfoliation and interaction of the two nanoreinforcements (crystalline cellulose and clay) with the polymer matrix, which resulted in a large difference in the mechanical properties between the two nanocomposites. The silicate system showed great improvements in both tensile modulus and yield strength, while the composites of microcrystalline cellulose showed tendencies to improve the yield strength. The two materials also have very detrimental effects on the elongation at break, where the microcrystalline cellulose system showed a reduction.
In the traditional natural fiber-reinforced polymers, the higher the adhesion between filler and polymer, the higher the mechanical performance of the resulting products, whereas opposite trends were found in case of cellulose nanowhisker reinforcements . A decrease in tensile modulus of natural rubber matrix was found when it was filled with chemically modified chitin whiskers . A transcrystallization process takes place when nanowhiskers are mixed to a biodegradable polymer matrix [57, 58] where preferential crystallization of amorphous polymer chains was detected during cooling at the surface of the filler. The exact phenomenon of mechanical properties alteration upon reinforcing with cellulose nano crystals and their role inside the matrix is not very clear in the available literature. For example, surfactant-coated cellulose whiskers acted as very good nucleating agents for isotactic polypropylene, whereas untreated cellulose did not modify the crystallization behavior of isotactic polypropylene .
Application of Cellulose-Based Nanocomposites
Cellulose nanocrystals have been used as geometrically and structurally defined model cellulosic filler with few practically useful products. Commercial forms of microcrystalline cellulose as a colloidal system are available as an aqueous suspension at high solid concentration such as Celish, a trade name from Daicel Corporation (Tokyo, Japan), providing a 10% slurry of cellulose nanofibers. Thixotropic gel system was described by Battista and Smith in 1961 through a patent [60, 61]. Solidified liquid crystals have been applied for optical applications like in security paper . To evaluate the potential application as display substrates, organic light-emitting diode (OLED) materials were manufactured on the wood cellulose nanocomposites by Okahisa et al. . Researchers succeeded in producing optically transparent wood cellulose nanocomposites with the performances of both low thermal expansion and low Young's modulus and deposited an electroluminescent layer on these flexible, low-CTE and transparent wood cellulose nanocomposites (Fig. 2). Cellulose whiskers have also been employed for low thickness polymer electrolytes for lithium batteries at concentrations below 10% to avoid significant decrease in ionic conductivity . Films of high Young's modulus and low density for application as loudspeaker membranes are obtained from microfibrillated cellulose (may have some amount of cellulose nano whiskers in suspension), and melamine formaldehyde . An attempt has been made by Ma et al.  to use electro spun cellulose nanofibers as affinity membranes. Affinity membranes permit the purification of molecules based on physical/chemical properties or biological functions rather than molecular weight/size. Rather than operating purely on the sieving mechanism, affinity membranes separation properties are based on the selectivity of the membrane to “capture” molecules, by immobilizing specific ligands onto the membrane surface. Cellulose acetate nonwoven mesh with fiber diameter ranging from 200 nm to 1 μm was prepared by electrospinning technique. Heat treatment of nanofiber mesh under 208°C for 1 h improved its structural integrity and mechanical strength significantly. The thin cellulose nanofibers tend to collapse by capillary action during the evaporation of water, and the deformed condition is fixed by hydrogen bonds that form between hydroxyl groups of the cellulose, thus producing a high-strength material without the use of binders. Because of this, an optically transparent paper from 0.1 wt% water suspension of well dispersed cellulose nanofibers was prepared by slow filtration with a moisture content of 560 wt%  (Fig. 3). The wet sheet was sandwiched between a combination of wire meshes (#300 inner layer) and filter papers (outer layers), and dried at 55°C for 72 h, while a pressure of ∼15 kPa was applied. Researchers developed a foldable nanofiber material with low thermal expansion (CTE < 8.5 ppm K−1), prepared using 15 nm thick cellulose nanofibers with the same chemical constituents as conventional paper, and a production process also similar to that of conventional paper.
The potential of cellulose nanofibers-based material as ligament or tendon substitute was evaluated by Mathew and Oksman . Initial studies indicated that inherent properties such as low toxicity, biocomptability and biodegradability with excellent mechanical properties of the nanofibers derived from soft wood pulp by mechanical fibrillation, makes cellulose based nanocomposites an excellent candidate for load bearing components in biomedical applications. Cellulose crystals are optically active and can form chiral ordered nematic phase, which could be used to obtain solid film showing cholesteric phase  and the addition of electrolyte to make it possible to fabricate colored films. These colored films have applications like in security papers, bank notes, passports, visa, electoral card and other secure certificates. The conducting composites have been prepared from cellulose whiskers and semiconducting polymers . The approach was based on the fact that noncovalent interactions with sulfate groups and hydroxyl groups could be exploited for the sulfate functionalized whiskers. The incorporation of cellulose nanowhiskers in the polymer matrix may provide a new material with original properties depending on the dispersion and interactions of filler with matrix.
Biodegradation of Cellulose-Based Nanocomposites
The primary objective to fill the polymer matrix with nanocrystalline cellulose is to develop eco-friendly green composites with the potential of degradation in the bio-cycle by the action of different microbes, leaving behind unharmful residue biomass with the emission of carbon dioxide and water. Therefore, the evaluation of the environmental biodegradability of nanocellulose-based composites is a highly important factor in order to expand their applicability. Highly crystalline cellulose is not uniform in structure and there are some imperfections, mostly due to the various chain dislocations and ends. Macro crystalline cellulose is known to degrade by the action of exoglucanases initiated though the action from the end . There is limited research conducted in the area of biodegradation of cellulose nanofiber filler-based composites. In a report of biodegradation study of crystalline cellulose-reinforced rubber, biodegradability of the sample was enhanced with the amount of filler, where the results indicated that crystallinity caused important effects in promoting the biodegradability of rubber . Similarly, the presence of bagasse whiskers resulted in an increase in moisture sorption of rubber films where the highest weight loss in soil was observed at 12.5% whisker content fueling the conclusion that the presence of cellulose whiskers increased the rate of degradation of rubber in soil . Nanocomposites of poly(lactic acid) reinforced with cellulose whiskers highly dispersed with poly(ethylene glycol) were examined for biodegradation in simulated body fluid, where an improvement in the water absorption and biodegradation of the nanocomposites was observed . Cellulose whiskers isolated from bagasse have been filled in polycaprolactone after modification with n-octadecyl isocyanate and nanocomposites were fabricated by a casting/evaporation technique. Bio-disintegration studies of the PCL/cellulose whisker nanocomposites in soil were carried out and an increase in the bio-disintegration was found after addition of 7.5% modified whiskers. At higher loadings of modified cellulose whiskers, the weight loss tended to decrease, but it was still higher than that of neat PCL . The effect of compatibility on the biodegradation of cellulose nanofiber reinforced composites has not been quantified till now with relation to the mechanical performances. However, the reports on the macronatural fiber-filled composites indicated a role of compatibilization on the degradation of resulting composites, and there was a significant effect of the method of preparation on the degradability of the composites. The composites prepared by direct reactive mixing were found more degradable. It has been proposed that compatibility may increases the properties and biodegradation of the host matrix [75–77].