In this blog post, we’ll explore the characteristics and potential applications of hydrogels, which are used in a wide range of fields—from molecular gastronomy to artificial joints and tissue engineering.
What are the hydrogels used in molecular gastronomy?
Have you ever heard of blueberry-flavored caviar? This dish is a prime example of molecular gastronomy—applying scientific principles to cooking to create forms and textures unlike those of traditional foods. One of the secrets behind this almost magical dish is hydrogel.
A hydrogel is a material with a three-dimensional polymer network structure that retains a large amount of water. Simply put, it is a gel-like material formed by polymer chains linked together to create a net-like structure, with a large amount of water trapped between them. As a result, it exhibits unique physical properties that make it difficult to classify as either a solid or a liquid. Hydrogels can be engineered to possess a wide range of physical and chemical properties depending on the type of polymers and solutions used, as well as the method by which the polymer chains are linked. Key characteristics of hydrogels include their ability to maintain a stable shape despite containing a large amount of water, as well as the ability to achieve desired properties by adjusting their constituent materials and structure.
The blueberry-flavored caviar introduced earlier is made by filling a syringe with a solution of alginate mixed with blueberry juice and food coloring, then dropping it one drop at a time into a calcium chloride (CaCl₂) solution. During this process, cross-links form between the polymer chains by utilizing the interaction between the negatively charged regions of alginate and calcium ions (Ca²⁺). As a result, small spherical hydrogels are created, with a membrane-like outer layer that solidifies while containing liquid inside. Once you understand the principle, the potential applications are endless. If you mix watermelon juice instead of blueberry juice into the alginate solution, you can make watermelon-flavored caviar; if you use orange juice and adjust the size of the droplets falling into the calcium chloride solution to be smaller, you can create small, spherical orange-flavored treats. As shown here, even when using the same principle, you can create foods in various shapes and flavors by adjusting the ingredients, concentrations, and droplet sizes.
Meanwhile, some readers may be concerned about whether it is safe to eat dishes made using these chemicals. In fact, the alginic acid used in artificial caviar is a substance extracted from seaweed—it is not an unfamiliar chemical found only in laboratories. Food-grade alginic acid is widely used to control the viscosity of foods or to form gels. However, not all hydrogels are edible or safe for the human body. Since the properties of hydrogels vary depending on the polymers, additives, and manufacturing methods used, safety assessments tailored to their specific applications are necessary. Even when making artificial caviar, only ingredients approved for food use and solutions at appropriate concentrations should be used, and the calcium chloride solution employed in the manufacturing process must also be used under conditions suitable for food production.
How are hydrogels utilized in medicine and biotechnology?
Hydrogels designed with biocompatibility in mind are widely used not only in the food industry but also in the fields of medicine and biotechnology. In the field of biotechnology, hydrogels have been used to immobilize cells or enzymes within bioreactors. They have also been utilized in life science research as materials that mimic the environment of the extracellular matrix—which influences cell differentiation, proliferation, and morphology from outside the cell—and their range of applications is expanding in tissue engineering as scaffolds necessary for culturing cells and tissues. Research is also underway to utilize hydrogels as carriers for delivering drugs to specific cells or tissues. Thus, hydrogels are not merely gels with a high water content; they offer the advantage of being utilized as diverse biomaterials—such as by creating environments similar to biological tissues or delivering cells and drugs to desired locations.
However, there were challenges that needed to be addressed for hydrogels to be more widely utilized as biomaterials. While many existing hydrogels have the advantage of being soft and similar to biological tissues due to their high water content, they also suffer from the disadvantage of being mechanically weak and prone to tearing or breaking. In particular, hydrogels derived from alginate had limited elasticity—they could be destroyed even when stretched to only about 1.2 times their original length. Furthermore, typical hydrogels at the time struggled to achieve sufficient strength and toughness for applications requiring them to withstand repeated heavy loads, such as those found in cartilage. Therefore, to use hydrogels as a direct replacement for damaged cartilage, it was necessary to significantly improve their mechanical properties.
One of the key studies aimed at overcoming these limitations was the development of a highly elastic and highly ductile hydrogel announced by a research team at Harvard University in 2012. In this study, the researchers combined two different polymer networks to develop a hydrogel that was significantly stronger and more ductile than existing hydrogels. This material could stretch to more than 21 times its original length and, despite containing approximately 90% water, exhibited a fracture energy of about 9,000 J/m². In particular, the dual-network structure—combining alginate and polyacrylamide—suppressed the rapid propagation of cracks within the hydrogel and allowed for the dissipation of externally applied energy. This mechanism works by the alginate component absorbing energy as its ionic bonds break, while the covalent bond network of the polyacrylamide supports the overall structure. As a result, even when the material is damaged, the rapid propagation of cracks throughout the entire structure is prevented. In actual experiments, even samples with large scratches were found to stretch up to 17 times their original length. Furthermore, the broken ionic bonds can reform over time, allowing the material to partially recover.
This research has garnered attention for its potential to significantly expand the applications of hydrogels. In particular, the research team suggested that these high-toughness hydrogels could be used not only as materials to replace damaged cartilage but also in various fields requiring high elasticity and toughness, such as artificial muscles, soft robots, optical devices, and materials for protecting wounds. However, this does not mean that these research findings will immediately lead to new artificial joint surgeries. This is because, for use as actual medical devices, additional verification is required for various conditions—including biocompatibility, long-term safety, durability, stability under repeated loads, and performance in the in vivo environment. In particular, real joints require complex characteristics such as low friction, load-bearing capacity, and long-term durability—not just high strength.
What possibilities lie ahead for hydrogels?
We have briefly examined how hydrogels have been applied as biomaterials to date. Hydrogels are not only used in molecular gastronomy to create unique shapes and textures but are also being researched as important materials in the fields of life sciences and medicine—for example, to immobilize cells and enzymes, mimic the extracellular matrix, and create scaffolds for tissue engineering and drug delivery systems. In particular, their ability to retain a stable shape through a polymer network while containing a high water content makes them well-suited for creating environments similar to actual biological tissues.
Above all, a key advantage of hydrogels is that they are not a single, fixed material. Their physical and chemical properties can be finely tuned depending on which polymers are used, how they are cross-linked, and what substances are incorporated into them.
Given that their properties—such as softness, elasticity, strength, toughness, degradation rate, and drug-release characteristics—can be tailored to specific purposes, hydrogels are likely to remain a material with significant potential in the field of biomaterials. Of course, before a specific hydrogel can be used as an actual medical device or artificial joint, its safety and performance must be thoroughly verified for each intended application. Nevertheless, given their ability to be tailored with desired physical and chemical properties for use in various fields, hydrogels are likely to continue playing a crucial role in the development of new biomaterials.
