Hydrogel is a network of polymer chains that are hydrophilic, sometimes found as colloidal gel in which water is the dispersion medium. Hydrogels are highly absorbent (they can contain over 90% water) natural or synthetic polymeric networks. Hydrogels also possess a degree of flexibility very similar to natural tissue, due to their significant water content. As per Wikipedia, common uses for hydrogels include:

• Scaffolds in tissue engineering. When used as scaffolds, hydrogels may contain human cells to repair tissue.
• As sustained-release drug delivery systems.
• Provide absorption, desloughing and debriding of necrotic and fibrotic tissue.
• Used as biosensors - those hydrogels that are responsive to specific molecules, such as glucose or antigens.
• used in disposable diapers where they absorb urine, in female hygiene products.
• Contact lenses
• Rectal drug delivery and diagnosis

Hydrogels can reversibly change their size and shape under different conditions. This property makes them attractive for a wide variety of applications, including artificial muscles, drug delivery and sensors. But even though stimuli-sensitive hydrogels have been studied for a few decades, they have not yet been commercialized for applications. One of the biggest problems is that they are usually weak and brittle, causing them to easily break when stretched, as per phys.org.  As per a study published in Nature Communications, researchers at Nagoya University and The University of Tokyo have designed hydrogels with temperature and pH sensitivities that are extremely stretchable as well as mechanically strong. The improvements mark an important step toward enabling hydrogels to reach their full commercial potential.
Many previous attempts have been made to improve the strength of hydrogels by modifying the polymer structure, but doing so often alters the stimuli sensitivities, as well.
The new hydrogel structure was inspired by recent research on a 'slide-ring gel' in which molecules can slide through the holes in a figure-8-shaped junction of cross-linked polymers. This sliding is called the 'pulley effect'. By minimizing the stress on the polymer network, the pulley effect greatly strengthens the hydrogel. Here, the researchers prepared a hydrogel with materials chosen specifically to exploit the pulley effect. They also introduced ionic sites in the polymer network, which increases stretchability and can be spatially distributed to regulate the pH dependence of the hydrogel's temperature response. The resulting hydrogels exhibit many remarkable properties. They can be stressed, compressed, coiled, and knotted without breaking. The strong hydrogels also cannot be easily cut with a sharp knife. In addition, they can absorb large amounts of water, becoming 620 times heavier and gaining a much larger volume when placed in water.
The results demonstrate that making relatively minor modifications to the hydrogel polymer network can result in dramatic changes in its chemical properties. The preparation method is simple and general, allowing it to be applied to a wide variety of applications. 'Now we are studying the elastomers that do not contain any solvents, using the Polyrotaxane cross-linkers', Takeoka said. 'I believe that we can obtain more stretchable elastomers using our cross-linkers.'

Hydrogels could replace damaged cartilage in people or serve as artificial muscles and nerves for soft, deformable robots. But it is difficult to make the materials both stiff and tough at the same time; as stiffer hydrogels are brittle, while those less apt to break are too squishy. A team at Harvard University has used a substance derived from seaweed to make a hydrogel that is both stiff as well as tough (ACS Macro Lett 2014), as per Chemical & Engineering News.
Joost J Vlassak, Zhigang Suo and their colleagues created the hydrogel by mixing distilled water and acrylamide, along with two types of naturally occurring sodium alginate, which comes from the cell walls of brown algae. They added a combination of calcium sulfate and calcium chloride to act as ionic cross-linkers in the polymer network. The two alginates had different lengths, one up to nine times as long as the other. The short-chain alginates produce a low viscosity mixture, making it easier to pack high concentrations of the polymer into the hydrogel, which increases the material's stiffness. The long-chain versions contribute to the gel's toughness. They form long stretches of cross-links between the other polymers, which can bridge cracks in the network. Also, these ionic links can unzip to dissipate stress. With the right proportions of components, the new material can have an elastic modulus, a measure of stiffness, of approximately 1 megapascal, and a fracture energy, a measure of toughness, of about 4,000 J/m2. Vlassak says the material is as stiff as cartilage, but is tougher than the tissue, which has a fracture energy of 1,000 J/m2.
The properties of the hydrogel might be tuned further by using different lengths of alginates, Vlassak says. The range of lengths of naturally occurring alginates is limited, but hitting them with gamma radiation can produce different chain lengths.