Hydrogels have been applied as intelligent carriers in controlled drug -delivery systems. In the dry state, they are usually hard and glossy and can protect the active ingredients from the influence of oxygen, UV light and other possible degradation. Once swollen in water or bodily fluids, they allow the passage of drug molecules out of the body of the hydrogel. The practical use of hydrogels is mainly limited to applications of high water absorption because gels have low mechanical strength. However, hydrogels with high water content, strength and elasticity can be synthesized easily by repeatedly freezing and thawing the hydrogel.

Hydrogels are made of three dimensional semi-crystals held together by hydrogen bonds. The amount of time the gel is frozen does not affect the strength of the gel. However, the slower the frozen gel is allowed to thaw the stronger the hydrogels become (crystallization of the 3-D semi-crystals is promoted by slow thawing).


Hydrogels can be used to deliver drugs to the body, in dressings to heal wounds as they create and maintain a moist environment and in sensors due to their flexibility and strength. In this activity, acetylsalicyclic acid is held within a hydrogel and then the release of the acetylsalicyclic acid can be followed either by titration or by UV.

  • PVA
  • PAA
  • Acetyl salicylic acid
  • Deionised water
  • Weigh boat(s)
  • Spatula
  • Magnetic stirrer
  • Hot plate and stirrer
  • Fridge
  • Heat 40mL of deionised water to 60°C
  • Weigh out 1.0g of PVA and slowly add to the hot water over 1 hour. The solution should be constantly stirred by a magnetic stirrer.
  • Weigh out 0.2g of PAA and slowly add to the beaker and stir for a further half hour.
  • Add 1 g of acetylsalicylic acid to the mixture and allow to mix for 5-10 minutes.
  • A thick gel will have formed. Pour the gel into a mould (e.g. an empty weigh boat)
  • Freeze the hydrogel for 4 hours at approx -20°C.
  • Allow the gel to thaw slowly in a fridge and repeat the freezing and thawing process once more.
  • Place the hydrogel in 500mls of water and leave to one side.
  • Note any changes in shape to the hydrogel and if the water level has decreased/increased/stayed the same.
  • Test the aqueous solution after 1 hour, 2 hours, 5 hours etc. to determine the release of the acetylsalicylic acid from the hydogel. This can be done either by UV analysis or by titrimetric analysis. The release rate can be determined by plotting the acetylsalicyclic acid concentration released versus time.