Polymers, both natural and synthesized, are found everywhere in our daily lives and have applications in countless industries. Due to growing concern for the environment, the use of natural polymers, or biopolymers, has risen as they have potential applications for a variety of fields without requiring the use of fossil fuels.1 The biochemical industry is familiar with their form and functions, and some exiting ones, including fossil versions, have been easily replaced with biopolymers of the same properties.2-3 Chitin in particular is abundantly found in nature in invertebrates from crustacean shells or insect cuticles.4 It can be retrieved from food production waste, and is non-toxic. Its deacetylated form, chitosan, has been applied in relation to the cosmetic and biomedical industries.2
Chitosan’s prominence within both industries corresponds to its status as a “green polymer that is “nontoxic, biodegradable…with antitumor, antioxidant and antimicrobial activities.”3,5 Its suitability is related to its structure, as shown in Figure 1. Chitosan is positively charged and its solubility is pH-dependent. Its amino groups are protonated with a low pKa-value of 6.5, allowing it to be soluble in acidic solutions.6 As a polycation, it is able to form ionic bonds with natural and synthetic anionic species.4 This makes it highly applicable to the personal care industry, especially for hair care products.
Figure 1. Structure of chitosan. Image “Chitosan Synthese,” by user NEUROtiker, is in the Public Domain (Reprinted from Wikimedia Commons).
To focus on one particular product, the chemistry behind shampoo makes it a feasible application for chitosan. Shampoos serve as scalp cleaners, and one of their main components are surfactants, molecules that are both hydrophobic and hydrophilic. The cleaning agent surfactants are typically anionic in nature. A commonly known one is sodium laurel sulfate. Its hydrophobic tails attach to the nonpolar residues and the hydrophilic heads are attracted to water’s hydrogen atoms. Water molecules have strong hydrogen bonding which creates high surface tension. Upon immersion of the amphiphilic surfactants, their hydrophobic components disrupt water’s intermolecular forces. The water molecules exclude the surfactants from the solution and pus them to the surface, allowing them to be washed away.7-8 This structure of this process is shown in Figure 2.
Figure 2. Structure of a surfactant.
Image “Micelles,” by user Jwleung, is licensed under the Creative Commons BY-SA 3.0 License (Reprinted from Wikimedia Commons).
Surfactants are highly effective as cleaning agents. However, due to their anionic state, they can cause an increase in electrical negative charge on the hair shaft by reducing the presence of nonpolar molecules. This creates frizz and friction, causing hair texture to be damaged and coarse. Companies thus often add cationic surfactants to shampoos and conditioners to neutralize this.8
Chitosan’s cationic nature makes it highly suitable for this application. Hair has an overall negative charge in addition to the effects of anionic surfactants, and this becomes more prevalent if cuticles are damaged.8 Chitosan can serve as a cationic surfactant as it has an electrostatic attraction to the negative net charge of hair. The film-forming properties of chitosan and silk blends are currently being studied on human hair to assess their potential applicability to hair conditioner formulations that promote regeneration.2 The blend would create a film around damaged hair cuticles and smooth them, creating a protective cationic-anionic hydrophobic complex.8
Another field in which chitosan has advantageous applications in is the biomedical field. One example is its ability to stop blood flow. Due to its positive charge, chitosan can interact with the negatively charged membranes of red blood cells and cease bleeding, making it beneficial for wound dressing. After physical or chemical modification, films, sponges, and scaffolds can be created from chitosan. Chitosan 2D scaffolds and sponges can be used for wound-dressing. A chitosan sponge absorbs the secretions from a wound and helps it recover with its antimicrobial, non-toxic and hemostatic properties. Several chitosan-based medical technologies such as wound dressings and wound dressings containing additional antimicrobial drugs have already been cleared by the FDA.
Chitosan can also be used in 3D scaffolds for tissue engineering, specifically in the form of hydrogels. Chitosan is a polysaccharide with breakable glycosidic bonds6, and its cationic nature allows it to be broken down by human enzymes through hydrolysis. In such a reaction, a molecule of water is added, splitting the bonds and creating monosaccharide units that are biocompatible, biodegradable and non-toxic.9 As such, it is suitable to be replaced by healing tissue. Its amphiphilic properties would also allow for cell proliferation and attachment and for it to capture large amounts of water. High water content allows hydrogels to resemble natural skin tissue.10
One way in which chitosan hydrogels can be formed is through physical cross-linking which involves non-covalent reversible interactions, such as electrostatic interactions, hydrophobic interactions, or hydrogen bonding.4 One method is through combination with -glycerol phosphate with corresponding temperature dependence. The driving force for this reaction is a transfer of protons from chitosan to -glycerol phosphate, which reduces electrostatic repulsion, leading to the proliferation of chitosan. Another method is a mechanism called polyelectrolyte complexation. An entropy-favored reaction occurs between chitosan and a negatively charged polysaccharide, creating the complexed hydrogel.6,9
Additionally, chitosan hydrogels can also be prepared through chemical cross-linking. The amino and alcohol groups in chitosan’s structure allows it to have stable covalent bonds4, and the covalent linking (intramolecular or intermolecular) in this process is irreversible and more stable than physical cross-linking. Typically, chitosan cross-links with another polymeric chain of its own. Glutaraldehyde is extensively used as a cross-linking agent.9 The products formed from the reaction between its aldehyde group and chitosan’s amine group can be stabilized as suitable reducing agents for cross-linking. However, while the gel formed from this combination was relatively stable over time, glutaraldehyde and other cross-linkers may leave toxic agents within it and can cause cell-growth inhibition.4
Alternative non-toxic cross-linkers are being tested, one of which being genipin. According to an article in the journal Research in Pharmaceutical Sciences, genipin is “a natural, water soluble and bifunctional cross-linking agent…[and can] react with amines, proteins and amino groups of chitosan.” It is less toxic and more biocompatible than glutaraldehyde, and genipin-cross-linked chitosan hydrogels degrade more slowly as well.9
Chitosan is a versatile polymer with its cationic nature (it is the only naturally-occurring polysaccharide that is positively charged4) and nontoxic, biodegradable and antimicrobial properties. This makes it favorable in various applications within a variety of industries, including as a cationic surfactant in personal care products or as a wound-dressing or hydrogel component in the biomedical industry. Also, there is current research on improving or modifying chitosan properties (i.e. antifungal properties, solubility in a wider range of pH, mucoadhesiveness, enhanced biodegradability9) to make it more usable for other biomedical applications such as drug delivery. Other research is being conducted about creating blends between natural polymers, including chitosan, collagen, silk, and keratin with respect to the cosmetic and biomedical industries.2 This could open the doors for better formulations, functionality and stability than existing products. It would be interesting to explore other applications for chitosan, such as food or pharmaceutical applications. Chitosan is one example as to how a natural polymer can be widely and favorably used while having less of a negative impact on the environment. The next step to take would be to explore the applications of other natural polymers as well.