Hydrogel Synthesis

Hydrogels are a class of cross-linked polymers capable of absorbing hundreds of times their weight in water. Water enters the network and solubilizes hydrophilic and/or ionic species on the polymer backbone. The water molecules are larger than the solubilized groups and their presence inside the network causes the hydrogel to swell (Figure 1). The cross-links connecting the polymer backbone prevent the hydrogel from dissolving or breaking.

Figure 1: Hydration of a hydrogel.

In this example, the hydrogel is synthesized via free radical polymerization. A free radical is an unpaired, highly reactive electron created from a free radical initiator, such as 2,2-Dimethoxy-2-phenylacetophenone (DMPAP). UV light cleaves the carbon-carbon bond in DMPAP to form a free radical on each carbon atom (Figure 2).

Figure 2: 2,2-Dimethoxy-2-phenylacetophenone fragmenting into two free radical-carrying molecules.

The radical species reacts with double and/or triple bonds found in the polymer backbone and cross-linker. For free radical polymerization, the polymer backbone contains one double bond that propagates the chain. The free radicals react with the carbon-carbon double bond in 2-hydroxyethyl methacrylate (Figure 3) to form a propagating chain with a free radical on the end (propagation step in Figure 4). The hydroxyl group coming off the backbone is soluble in water, causing the crosslinked-network to swell.

Figure 3: 2-hydroxyethyl methacrylate.

Figure 4: UV initiated free radical polymerization steps.

The radicals also react with the two carbon-carbon double bonds in tetraethylene glycol dimethacrylate (TEGDMA) (Figure 5), the chemical cross-linker, to link the backbone chains together. Hydrogel synthesis is complete when the free radicals have been consumed or have completely reacted.

Figure 5: Tetraethylene glycol dimethacrylate.

The final hydrogel monomer is shown in Figure 7, and the synthesized hydrogels are shown in Figure 8. The degree of swelling was found to be approximately 136% for the 1 min sample, 387% for the 1.5 min sample and 81% for the 5 min sample. These results demonstrate the relationship between degree of crosslinking, or the extent to which the network is connected, and swelling ability. More links between the polymer molecules mean more elastic restraining forces on those polymer chains, which inhibit them from expanding to the same degree as a less-crosslinked hydrogel.

Figure 7: Monomer created from photoinitiator DMPAP, HEMA backbone, TEGDMA crosslinker, EG solvent and photochromic pigment after free radical polymerization. 

Figure 8: Hydrogels after polymerization. From left to right: 1 minute under UV light during polymerization, 1.5 minutes under UV light during polymerization, 5 minutes under UV light during polymerization. The 1-minute sample appears more transparent and gel-like then the 1.5 minute and 5 minutes samples, which had increasing degrees of polymerization

Hydrogel synthesis is a technique for producing crosslinked polymeric materials which can swell in response to liquid, UV light, pH, or a range of other stimulants. Synthesis by combination of liquid solutions is advantageous for the simplicity of mixing and forming hydrogels, though the final product is generally impure and tends to contain polymers with low molecular weights. This specific procedure, while simple, involves chemicals that are both toxic and flammable, and therefore requires extreme care and preventative measures. The hydrogels produced by this method are useful in applications ranging from drug delivery to sensors to absorbent hygiene products.

Hydrogels are used in a variety of consumer products, medical devices, and sensors. Consumer products such as hospital pads, feminine hygiene pads, and diapers contain sodium polyacrylate, one of the most common superabsorbent polymers. The hydrogel swells in the presence of the fluid between 300-800 times its weight. This allows manufacturers to use less material and create products that are slim and comfortable for the user to wear.

Additionally, soft contact lenses are made of silicone hydrogels, which allow oxygen to easily pass to the cornea and are more comfortable than hard contact lenses. Hydrogels are also commonly used in drug delivery because the cross-linked network allows for drugs to be stored in the three-dimensional network and slowly released into the body.

Hydrogels can also be tuned to swell as a function of salinity, pH, or other signals, making them suitable in sensor applications. The hydrogel synthesized in this video is used as a sensor in a sprinkler lawn sensor. The hydrogel is in contact with the soil and while the lawn is being watered, it swells until it triggers the sprinkler shut-off.