High-Performance Materials from Bio-based Feedstocks. Группа авторов
Читать онлайн книгу.
coated fertilizers, the hydrophilicity of native biopolymers results in the buildup of osmotic pressure inside the fertilizer beads. This causes the coatings to break prematurely, resulting in burst release of the nutrients. Naturally obtained biopolymers are thus not ideal fertilizer‐coating materials owing to the lack of hydrophobicity and modifications are necessary to help control the nutrient release. By replacing hydroxy groups with ester groups, cellulose acetate (CA), which is a common cellulose derivative, can be prepared. The presence of ester introduces its pH sensitivity property to the materials and can be beneficial for applications not only in slow‐release fertilizers (Chapter 16) but also in drug delivery (Chapter 8).
In other applications, the hydrophilicity and swelling behavior of biopolymers are used as an advantage. A different class of fertilizers is that of hydrogel‐based fertilizers, which absorb and retain high amounts of water. Hydrogel composites with plant nutrients embedded in their network structure using alginate or chitosan have been developed to reduce the frequency of irrigation as well as controlling the rate of nutrient release (Chapter 16). Hydrogels also find applications in targeted drug delivery. The biocompatibility, biodegradability, and non‐toxicity as well as high affinity for water make biopolymers like alginate, hyaluronic acid, pectin, and carrageenan attractive in controlled release and targeted drug delivery systems for HIV prophylaxis (Chapter 8). Alginate microparticles and films have also been used for anti‐HIV drug delivery due to their excellent biocompatibility and biodegradability (Chapter 8), and alginate has been blended with other polymers to adjust the hydrophobicity of polyelectrolyte films for food‐packaging applications (Chapter 15).
An interesting group of oligosaccharides with both hydrophilic and hydrophobic properties is that of cyclodextrins. The truncated cone structure of these circular oligosaccharides exhibits a hydrophobic inner cavity, while the upper and lower rims are hydrophilic. These unique properties enable cyclodextrins to contain hydrophobic molecules, but the high costs limit the applicability in foods (Chapter 14). Cyclodextrins can also be used to form metal–organic frameworks (MOFs) that have been investigated for a variety of potential applications including molecular separations, drug delivery, and biomedicine. Organic ligands are the main factors that determine if an MOF is bio‐based, but the sustainability and safety of the metal ions should also be considered (Chapter 12).
Biodegradability is preferred in some applications such as packaging and fertilizers. The biodegradability needs to be tuned, however, to meet the desired product specifications. For example, in fertilizers, polysaccharide‐based coatings based on, e.g. starch or cellulose, are too biodegradable, leading to premature nutrient release into the soil. To reduce the rate of degradation in soil and extend its service time, the biopolymer can be grafted with rubber or a different polymer with a lower biodegradability and higher hydrophobicity (Chapter 16). In food packaging, many packaging materials are based on blends of biodegradable polysaccharides such as starch, cellulose, and chitosan. In addition, cellulose nanocrystals, nanofibers, and bacterial cellulose have been used as biodegradable reinforcing fillers in various packaging films (Chapter 15).
Due to their biological nature, the biocompatibility of polysaccharides can be taken advantage of in applications where the polymer requires intimate contact with cells. For example, mucoadhesive films based on derivatives of cellulose, alginate, or chitosan can provide sustained release of several antiretroviral agents (Chapter 8). Moreover, biomaterials are critical to success in tissue engineering, act as a scaffold for tissue and cells to grow on. Such scaffolds can be derived from protein or carbohydrate biopolymers, including silk, collagen, fibrin, chitosan, alginate, and agarose. Biocompatibility and the ability to contribute to biological functions with the cells are necessary properties. Chapter 11 highlights bio‐based feedstock that can be used in scaffold manufacturing for tissue engineering.
Carbohydrate‐based materials play an essential role in cellular recognition processes. Carbohydrate‐protein interactions can be probed by synthetic glycomaterials to diagnose viral and bacterial infections. In Chapter 10, bio‐based glycomaterials and carbohydrate‐functionalized materials are discussed including their application in drug/gene delivery, wound healing, biorecognition, and sensing.
Chitosan and chitin/glucan complexes have been used in a wide range of applications such as anticancer, antibacterial, antioxidant as well as gene delivery due to their unique biochemical properties (Chapter 9). The major limitation of these biopolymers is their insolubility in water and, as such, steps have been taken to improve their solubility by various modification methods. Other applications of chitosan include fertilizers (Chapter 16) and antibacterial food packaging (Chapter 15). For food‐packaging applications, chitosan is limited by its low mechanical strength, rigid structure, and low thermal stability, which can be overcome by blending chitosan with other (bio)polymers (Chapter 15).
Due to their safety and biocompatibility, polysaccharide‐based materials find applications in the food industry as emulsion stabilizers. Starch modified with octenyl succinic anhydride (OSA) generates an amphiphilic character of the starch, making it an effective stabilizer of oil‐in‐water emulsions. Gums such as gum acacia, gum tragacanth, xanthan gum, and guar gum are used as surfactants to stabilize emulsions and liposomes in the food industry, as well as the formation of coacervates. Additionally, cellulose nanocrystals find applications in food applications in the stabilization of Pickering emulsions (Chapter 14).
Polysaccharides have a high abundance of functional groups, e.g. hydroxyl groups of cellulose, amine of chitosan, and carboxylic acid of alginate. These functional groups have a good affinity for complexation of metal ions through mechanisms such as reduction, chelation, and complexation. The polysaccharides alone are poor sorbents due to their low stability and need to be improved by chemical (e.g. introduction of other functional groups) as well as physical modification (e.g. forming composites with other polymers as well as inorganic materials). The use of bio‐based composites for the recovery of precious and heavy metals is discussed in detail in Chapter 7.
In electrochemical storage devices, synthetic binders such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) are traditionally used, which suffer from environmental and safety issues. As a green and renewable replacement, various biopolymer‐based binders have been used in either pristine or modified forms. For example, cellulose and its modified forms such as CMC, EC, and CA, but also chitosan, alginate, and gums have shown great potential as environmentally friendly binders (Chapter 5). Biopolymers such as chitosan, cellulose, and carrageenan can also be used as components of proton exchange membranes in fuel cells. Especially, chitosan is a highly promising biopolymer for fuel cell applications due to its low cost, environmental friendliness, hydrophilicity, ease of modification, and low methanol permeability (Chapter 5).
As mentioned in Section