Hydrogels, a class of polymeric materials that can absorb and retain large amounts of liquid (usually water) without disintegrating, are used in a variety of industries, most notably the medical sector, Here, they are primarily used in contact lenses, wound dressing, drug delivery systems, tissue engineering, and hygiene products. The varying chemical character, high compositional freedom (they can consist of one, two, or more kinds of polymers) and nature of crosslinks (physical, chemical) between the polymer chains — the determining feature of hydrogel stability — enable scientists to obtain materials with unique properties.
As a result of continuous innovation, the global hydrogel market, valued in excess of USD $20 billion in 2019, is expected to continue growing and exceed $30 billion by 2027. In particular, novel hydrogel solutions are anticipated to significantly boost the contact lense, hygiene, and drug delivery markets.
Contact lenses:
By far the biggest hydrogel medical application area is in therapeutic (i.e., vision correction) and cosmetic (artificial eye) contact lenses, whose sales totaled more than USD $16 billion in sales in 2019. Naturally, materials for these uses must not only satisfy a broad range of technical criteria such as high transparency, permeability to water and oxygen, and biocompatibility, but also guarantee a comfortable fit and ease of handling for the wearer. The early rigid and poorly permeable poly(methyl methacrylate) (PMMA) contact lenses have been supplanted by more flexible and oxygen-permeable acrylates, including (hydroxyethyl)methacrylate (HEMA) and silicone hydrogel materials, which afford the patient greater comfort and longer wear time.
Other emerging materials include polyvinyl alcohol (PVA), attractive for its low cost and excellent wetting ability, along with polyethylene glycol (PEG) and hyaluronic acid (HA) hydrogels that offer enhanced biocompatibility and properties such as resistance to protein deposition.
More recently, owing to high oxygen transmission and bounded water content, siloxane-based contact lenses have become popular in the market, as exemplified by the success of Alcon’s Dailies Total1 and Johnson & Johnson’s Acuvue products.
As for the latest material trends, much research effort has been put into investigation of double-network hydrogels, a type of gel composite, as well as pH- and temperature-responsive polymers. The use of the former has been shown to improve functionality by, for example, enhancing toughness in the hydrated state and providing low protein adsorption as a result of the intertwined molecular networks of the dissimilar hydrogels. The latter are attractive for ocular drug delivery systems in which the environmental stimulus (e.g., a certain pH or temperature value) activates the release of a drug. The key advantages and disadvantages of the main materials used for contact lens applications are summarized in Table 1.
Table 1. Advantages and disadvantages of the main hydrogel materials used for contact lenses; adapted from Musgrave C. S. A. and Fang F. / Materials (2019), 12, 261.
Thanks to their unique but not yet fully exploited properties, hydrogels have become and are bound to remain the principal materials for contact lenses.
Hygiene products:
Another major application area of hydrogel materials is in hygiene products, whose total market size is about twice as big as that of contact lenses. Here, these materials are primarily used for diapers, adult incontinence products, and feminine hygiene products because of their ability to absorb great amounts of liquid — up 1,000 times their own mass — and create a moisture barrier that reduces the risk of skin irritation and bacterial contamination. Since such products are disposable and have historically been mostly made of synthetic polymers like acrylic acid and acrylamide-based copolymers, their use has led to the accumulation of large amounts of nonbiodegradable waste.
Accordingly, there is a strong push toward the use of biodegradable materials such as cellulose, starch, and natural fibers, instead. Cellulose-based hydrogels, in particular, have been extensively investigated, revealing comparable liquid retention properties to those of synthetic polymers. Here, the carboxymethyl and hydroxyethyl cellulose copolymer cross-linked with divinyl sulfone is of particular interest. However, in general, hydrogels based on natural materials suffer from inadequate properties important to this application. For instance, their liquid absorption capacity varies widely depending on the composition, they are mechanically inferior to synthetic hydrogels, and they lack adequate antimicrobial activity.
An alternative approach has been successfully adopted by the German chemical giant BASF, which offers superabsorbent materials obtained from renewable stock such as organic waste and vegetable oils. See Table 2 for the summary of the main hydrogel materials suited for hygiene products.
Table 2. Advantages and disadvantages of the main hydrogel materials used for hygiene products.
Although highly effective, synthetic hydrogels in disposable hygiene products greatly contribute to environmental pollution, which cannot yet be effectively reduced through the use of natural materials due to their inadequate properties.
Drug delivery systems:
The unique properties of hydrogel materials also render them suitable for drug delivery systems (whose total market size was in excess of USD $15 billion in 2019). Specifically, in addition to low toxicity and biocompatibility, the porous hydrogel structure, along with the possibility of adjusting hydrophilicity/hydrophobicity, can be manipulated during the molecular design process and thus makes an excellent scaffold for controlled and sustained drug release systems. In the right medium, the hydrogel, activated by a certain pH or temperature value or the extent of swelling, begins releasing the active pharmaceutical ingredient (API).
As shown in Table 3, hydrogel materials are used in all five traditional drug delivery routes. Whichever the delivery system, hydrogels usually utilize cellulose-based compounds, various acrylates, natural polymers, and other common hydrogels such as polyvinyl alcohol (PVA). For instance, hydroxypropyl cellulose (HPC) underpins the formulations of various buccal, oral, and ocular drug delivery systems as illustrated by a range of pharmaceutical products, including Zilactin-B Gel for mouth pain relief, Suprax for the treatment of bacterial infections, and Lacrisert for the relief of dry eye symptoms.
Table 3. Drug delivery routes and examples of the hydrogel materials; adapted from Cascone, S. and Lamberti, G. / International Journal of Pharmaceutics (2020), 573: 118803
Despite some commercial success and numerous studies of hydrogel-based drug delivery systems, they remain meagrely marketed as a result of complex design, risk of inadequate or out-of-control drug release, and high production costs.
Summary
Hydrogel materials play a major role in the medical industry, where their highly desirable properties — the freedom of molecular design, compatibility with living systems, and the ability to absorb and retain large amounts of fluids, to name but a few — have been successfully adapted for specific uses in contact lenses, hygiene products, and drug delivery systems. Though generally well established, hydrogel materials are yet to be fully exploited in medical applications, so further progress in the field is expected to bring exciting innovations.
