Hemicellulose-Based Polymers Processing and Application

Hemicellulose is the second most abundant biomass in the world. Because of its unique chemical structure, hemicellulose has attracted more and more research and industry application under the background of fossil resource shortage and drawbacks. Hemicellulose is tended to be modified with esterification, etherification, cross-linkage and so on. These reactions change hemicellulose properties, so that it could be applied in many fields such as medical field, films, hydrogels, conductive polymers. Herein, we reviewed structure, isolation, reaction and applications of hemicellulose. Especially the useful pathways to change the hydrophilic character of hemicelluloses to hydrophobic are reviewed and several applications of these materials are discussed.


Introduction
Since fossil resources are nonrenewable and lead to environmental issues [1], much research and considerable work have been done to alternate energy resources such as biomass, wind, geothermal and solar energy in order to meet the developmental and environmental requirements [2] [3] [4]. Among these resources, utilization of biomass has attracted a lot of attention over the past several decades. Biomass is considered to be the only sustainable and carbon-neutral source for liquid fuels, which has potential to replace or partly replace fossil resources.
Biomass is organic materials that are plant or animal based, such as crops, trees, fiber residues, aquatic plants, forestry and wood residues, agricultural wastes, and other non-fossil organic materials. Chemically, biomass is a form of lignocellulose and essentially composite plant structural material. Biomass is composite in nature, being composed in varying proportions of three readily

Lignin
Lignin is complex polymers with large molecular weight, which has the structure of phenylpropane units and coupling with the generic term for a large group of aromatic polymers resulting from the oxidative combinatorial coupling 4-hydroxyphenylpropanoids. The main precursors of lignin are trans-coniferyl alcohol (1) trans-sinapyl alcohol (2) and trans-p-coumaryl alcohol (3). These precursors biosynthesize lignin and their chemical structure of precursors is illustrated in Figure 1. The phenylpropane units are through C-O-C and C-C linkages with random order. In the cell walls, lignin is like a binder to combine hemicellulose and cellulose and makes cells more rigid and impervious.

Cellulose
Cellulose, the main polysaccharide of wood, is the most abundant resource on the earth. Cellulose exists widely in plants, animals and some bacteria. Cellulose, a linear polymer, is established as the repeat unit of anhydro-D-glucopyranose for cellulose chains (Figure 2) connected by β-1,4-glycosidic bonds. Because of its   linear structure and intramolecular hydrogen bonding, this polymer forms a very organized structure in the wood fibers. Cellulose fibrils contain alternating crystalline and amorphous regions. The fibrils are highly ordered in the crystalline region, whereas they are less ordered in the amorphous regions. This is an important property since cellulose is much more susceptible to react in its amorphous regions rather than the crystalline regions due to the accessibility of the reagents. Molecular weight measurements have shown that the degree of polymerization (DP) of cellulose is in the range of 10,000 to 15,000 depending on the source. The hydroxy groups formed glycosidic bond and some hydroxy groups on one chain interaction with another chain by Van Der Waals force. These chains are hydrophobic, so that, cellulose is insoluble in wanter and some high polarized solvents. Nowadays, cellulose is applied to nanocontainers, hydrogels, flame retardant etc.

General
Hemicellulose is the second most abundant biomass in the world, which is about 20% -30% weight of lignocellulosic materials (LCMs). Over past decades, Much research have been done to conversion of biomass especially lignocellulosic materials to replace the fossil based product, e.g. biomass fuels, biomass ethanol and organic materials. Hemicelluloses widely existed in nature are a class of hetero-polysaccharides present in the primary and secondary cell wall of wood and annual plants together with cellulose and lignin. Many agricultural crops or their residues, such as sorghum, sugarcane, corn stalks and cobs, cereal straws and husks, as well as forest and pulping side-streams from hardwoods, are good sources for xylans. For example, the xylan content in oat spelts is over 30% with an estimated production of 60 billion tons per year. Unlike cellulose, hemicelluloses consist of different monosaccharide units such as glucose, xylose, mannose, galactose, arabinose, fucose, glucuronic acid, and galacturonic acid in various amounts or traces dependent on the natural source. The units of hemicellulose are illustrated in Figure 3.

Extraction and Purified Hemicellulose
Several methods have been used to extract hemicellulose from woody tissues.
Among these methods, acid hydrolysis, generally speaking, which often used 0.5% -1% sulfuric acid and hydrochloric acid is an effective method that could make the hemicellulose break down to low molecular weight product [23].

Chemical Reactions
Due to its hydroxyl groups, which can serve as a reaction site, hemicellulose study, based on different reactions, the DS can vary at range from 0.095 to 0.75 [33]. hemicellulose acetylation a kind of esterification has been reported frequently [12] [34] [35]. Acetylation has been attained by several pathways. However, the most effective way is that hemicellulose reacts with acetic anhydride with catalysts [36].
Oleolation of hemicellulose is also an esterification. Generally, hemicellulose esterification is facile with fatty acid chloride, the reaction formula is illustrated in Figure 8. The oleolation of hemicellulose has shown the good water resistant properties, due to long chain of fatty acids that can be applied as coat on paper [37] [38] [39].
Fluorination of hemicellulose can provide modified hemicellulose with unique chemical property especially applied in marine as coatings [40]. The fluorination  Hemicellulose cross-linking is mostly used to make hydrogels which have restricted water solubility and swell in water [42]. Salam  was one-third of that for a commercial biopolymer coating. In addition, it has been shown that the surface strength of butylated and alkylated xylan coating was close to that of the reference latex as a binder in pigment coatings [46].

Applications
Hemicellulose as prospect biomass resource attracts much attention, because it is sustainable, renewable and environment friendly.

Hydrogels
Polymeric hydrogels have defined a remarkable research area, mainly due to their wide range of applications, such as matrix chemistry and biology, the removal of endocrine disruptors, media for the delivery of substances in biomedicine, and scaffolds. Hydrogels have potential in biomedical applications due to their good biocompatibility and ability to selectively mimic human tissues. Natural polymer-based hydrogels, such as those made from proteins and polysaccharides, are widely used in injectable engineering, controlled drug release, biosensors, and articular cartilage tissue engineering because they are commonly biocompatible, degradable, capable of promoting cell adhesion and proliferation, and immunologically inert. Hemicellulose-based hydrogels are currently being acknowledged for their non-toxic and renewable properties in addition to the advantages of natural polymers mentioned above [59]. Pen and his coworkers prepared novel ionic hydrogels via free radical graft copolymerization of acrylic acid and xylan-rich hemicelluloses using N,N-methylene-bis-(acrylamide) as cross-linker and ammonium salts as redox initiator. The results showed that the ionic hydrogels had high water adsorption capacity and rapid and multiple responses to pH, ions, and organic solvents, which may allow their use in several areas such as adsorption, separation, and drug release systems [60]. Another study, full interpenetrating polymer networks (IPNs) was developed by performed with O-acetyl-galactoglucomannan via thiolene click reaction. The AcGGM IPNs fabricated had shear storage modulus (G) values than that of the corresponding precursor single networks of AcGGM [43].

Conductive Polymers
Conducting polymers are the polymers which could conduct electricity, that has been widely applied to fabricate electrochemical biosensors, the reason is that this kind of polymers present of conjugated double bond or triple bond, in addition, conductive polymers have ability of conduct both electronically and ionically [61]. Other kinds of conducting materials are composites that blending some conducting materials particle to non-conducting polymer, so that the composites own conductivity. Conducting polymers and composites have attracted a lot of attention due to their wide application on soft materials, electrochemical biosensors, neural interface [62], drug delivery, bio erodible, tissues and other areas. In a research, electrically conductive hemicellulose hydrogels (ECHHs) is performed via cross-linking O-acetyl-galactoglucomannan (AcGGM) with epichlorohydrin in water at ambient temperature in the presence of conductive aniline pentamer (AP). The polymers have more pores and controllable conductivity, swelling behavior and middle strength mechanical properties, which could be applied in biomedical field [63]. In another of Zhao and

Medical Application
Biomedical applications of modified hemicellulose have attracted much attention on hydrophobization and/or cross-linking to produce water-resistant materials. Certain types of hemicelluloses have been used successfully to inhibit the growth of different tumors by inducing the activity of the host immune system Hemicellulose based polymer with typical type could have the effect of antitumor and antimicrobial activities. The mechanism of antitumor is that galactan could stimulate NK cell cytotoxic activity, and thus, activate the immune system against tumor cells [65]. One research showed that mannose has bioactivity to conjugate to vaccine; this could be applied as a target for cancer immunotherapy [66].
Hemicellulose has also been used as a wound dressing. An ideal wound dressing should keep moisture and good ventilation to environment and release the wound pain. In market, there is similar product called Veloderm, which is a biofilm with a polymeric structure based on hemicellulose microfibers. It is produced from sugarcane by a biotechnological process. It has been distributed into the market as a competitor to other wound dressings. This company describes Veloderm as a wound dressing that is able to allow the selective permeability of gases and vapor and prevent the permeability of water and microorganisms.
They describe its ability to adhere to the wound and generate a sealing process that protects the wound externally from dirt and germs [25].

Outlook
Hemicellulose as a second abundant resource on the earth has a large amount of hydroxyl group that leads to drawbacks of applications. Through reacting with hydroxyl group of hemicelluloses, properties of hemicellulose are changed from hydrophilic into hydrophobic. Modified chemical methods mainly include esterification, etherification, crosslinking etc. This reaction can be used to alter hemicellulose hydrophobicity and improve hemicellulose miscibility, thermoplastic stability and mechanical properties. In addition, hemicellulose cross-linking has been developed to create hydrogels used as drug-delivery systems or as water-absorption hydrogels. Recent research into hemicellulose-based conductive hydrogels has combined hemicellulose with chitosan to create a matrix capable of overcoming acidic conditions. Moreover, despite the fact that pharmaceutical applications of hemicelluloses are relatively new, we can anticipate that hemicelluloses can incorporate diverse biomolecules; thus, providing a promising structure for the preparation of new drug-delivery systems. A focus on green pathways that use green solvents and reactants for hemicellulose derivatization must be considered in future applications.