Hydrogels are widely used materials for various applications in areas ranging from medicine, cosmetics, and tissue engineering to food and agriculture.
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Naturally sourced polymer-based hydrogels offer advantages over synthetic forms owing to their biocompatibility, physicomechanical and environmentally friendly properties.
Their safety, biodegradability, and comparatively low cost are other desirable features that capture the interest of scientists.
In a review recently published in The Journal of the Science of Food and Agriculture, Miri Klein and Elena Poverenov from the Agricultural Research Organization in Rishon LeZion, Israel have provided a comprehensive overview of the various nature-sourced polymer-based hydrogels that are applied in various industries, with a focus on food and agriculture.
They begin by introducing the different biopolymers incorporated into the materials, namely proteins, polysaccharides, as well as a combination of the two, before moving onto the polymer cross-linking techniques used, including covalent and non-covalent bonding methods.
Finally, they provide examples of how these hydrogels may be applied in food and agriculture.
Proteins, polysaccharides or a combination of the two
As nutrients found in food and a source of amino acids with functional groups that enhance encapsulation, proteins are regarded as safe and biocompatible polymers for making hydrogels that can deliver health-benefiting substances such as vitamins, antioxidants, and probiotics.
With their functional groups enabling physicochemical interactions, along with their biocompatibility and biodegradability, polysaccharides such as starch, chitin, and pectin are considered widely available polymers for the formation of safe and nontoxic hydrogel materials.
Using a combination of proteins and polysaccharides, scientists can also make composite materials featuring the desirable characteristics of both polymer types.
In the case of protein-based hydrogels, the material is usually formed through non-covalent bonding between proteins via hydrogen bonds, for example, although covalent disulfide bonding sometimes occurs.
For polymer chains that do need linking, synthetic crosslinkers such as the dialdehydes glyoxal and glutaraldehyde can be used to covalently bind the chains, but this can change the chemical structure and properties of the polymers and influence whether crystallinity forms.
The reviewers say that preferred alternatives to synthetic crosslinkers are the naturally occurring crosslinkers found in fruits and vegetables, which are thought to be more eco-friendly and to have a superior safety profile. Examples of these natural crosslinkers are described below.
Genipin, which is an aglycone widely used as a natural crosslinker in herbal medicine is found in the Gardenia Fruit. Scientists have used it in place of glyoxal and glutaraldehyde to covalently link polymer chains.
Poverenov and Klein refer to a study where researchers who used genipin to make chitosan-based hydrogels found that a higher concentration of genipin resulted in more hydrophilicity.
Another study used genipin to make chitosan nanogels that could be delivered to difficult-to-access parts of the body. The nanogels also responded quickly to environmental stimuli.
One study used the phenol tannic acid to enhance the performance of gelatin and another used the phenol caffeic acid to make a gelatin hydrogel with properties that were not reversed on exposure to heat.
One more study the reviewers refer to used the polyphenol procyanidin to modify collagen and induce microfibril aggregation, which increased the hydrophobicity of collagen films and reduced their permeability to water vapor.
Applications in food and agriculture
The use of these natural polymer-based hydrogels is increasingly capturing the interest of scientists working on applications in food and agriculture.
In agriculture, they can be used for the controlled delivery of fertilizer to make soil and water use more efficient and enhance the properties of produce without posing an environmental threat.
Minimizing water loss from soil ensures more is available to feed plants that need it to grow, even in the case of plants that are grown in dry soils.
Hydrogels designed to be highly absorbent can address problems that arise in agriculture such as irrigation, erosion and water run-off. Their super absorbency can also increase soil aeration and the activity of microbes.
In the food industry, these hydrogels can be applied to ensure packaging has antimicrobial activity, absorbs moisture released from foods and extends storage time.
These biopolymer-based hydrogels can be used in the food and agriculture as part of advanced packaging, as carriers of bioactive components, for the delivery of nutrients or fertilizers, as controlled-release systems and as superabsorbent,”
Poverenov and Klein
The team says that although extensive progress has been made in this area, biopolymer-based hydrogels are currently mainly used in biomedical applications.
“For their implementation in food and agriculture, there is a need for more intensive research, including optimization and cost-effective considerations,” they conclude.
Klein M and Poverenov E. Natural biopolymer‐based hydrogels for use in food and agriculture. The Journal of the Science of Food and Agriculture 2020. https://doi.org/10.1002/jsfa.10274
Bahram M et al. An Introduction to Hydrogels and Some Recent Applications. Emerging Concepts in Analysis and Applications of Hydrogels. Intechopen 2016. Available at: https://www.intechopen.com/books/emerging-concepts-in-analysis-and-applications-of-hydrogels/an-introduction-to-hydrogels-and-some-recent-applications