Hydrogels | smart materials

Hydrogels are found in many commonly available products, including disposable nappies, cheap hair gel and plant water storage crystals. The practical work (Experiments with a smart material – hydrogels) – is fun to do and the results are clear and easy to see (in two cases, they are both sudden and dramatic).

This is followed by written work (Hydrogels and how they work). Information is provided on the structure of hydrogels and students consider how this structure relates to the properties they have observed.

Finally, a possible future use for very small particles of hydrogel (microgels) in drug delivery systems is introduced (Drug delivery and smart materials). This material could be used to enhance teaching of ionic and covalent bonding or equilibrium. Alternatively, hydrogels could be considered as interesting polymers and as an example of smart materials and nanotechnology.

Prior knowledge required

Students will need to have some knowledge and understanding of:

• Ionic and covalent bonding
• Reversible reactions
• Acids and bases
• Rates of reaction and particle size – for Drug Delivery and smart materials only.

Background information

Hydrogels are polymers that can retain many times their own weight in water. They are often polymers of carboxylic acids. The acid groups ionise in water, leaving the polymer with several negative charges along its length. This has two effects.

Firstly, the negative charges repel each other and the polymer is forced to expand.

Secondly, polar water molecules are attracted to the negative charges.

This increases the viscosity of the resulting mixture because the polymer chain now takes up more space and resists the flow of solvent molecules around it.

The polymer is in equilibrium with the water around it, but the equilibrium can be disturbed in a number of ways. If the ionic concentration of the solution is increased, for example by adding salt, the positive ions attach themselves to the negative sites on the polymer, effectively neutralising the charges.

This causes the polymer to collapse in on itself again. Adding alkali removes the acid ions and causes the position of equilibrium to move to the right; adding acid has the opposite effect. There are a large number of hydrogels and they expand and contract at different pH values, temperatures and ionic concentrations.

By using a mixture of monomers to create the polymer these characteristics can be fine-tuned.

The commonly available hydrogels that are suggested for use in this practical activity are sensitive to salt concentration but do not show much change across the pH range which can be investigated readily in the classroom. They lend themselves very well to a range of investigative practical work.

For example, their volume in different amounts of water or in different salt concentrations could be measured.

Equipment required

Do not be put off by the long list of requirements given below; many items are needed for all the experiments.

The equipment is listed separately for each experiment so that just one or two parts of the activity can be prepared if preferred.

Plant water storage crystals This experiment is referred to in the worksheet Drug Delivery and smart materials. If this worksheet is to be used, then tea must be included in the experiment. If you do not intend to use the worksheet, the water crystals can be coloured with a few drops of food colouring (for wonderful, lurid colours) or not at all (but they look great when coloured).

For each pair or group:

Part 1

• 1 teaspoon water crystals – available from garden centres and sold under various names, eg Phostrogen Swellgel
• Large (at least 1 dm³) beaker or plastic tub – ice cream or similar tubs are fine
• 500 cm³ strong tea – use 1 tea bag per 500 cm³, pour on boiling water and leave to brew overnight (this tea will stain some containers).

Part 2

• Sieve (plastic ones are fine) or large funnel lined either with paper towels or with filter paper – groups will be able to share these
• Beakers, 2 x 250 cm³
• Very concentrated or saturated sodium chloride (table salt) solution, 200 cm³ 
• Distilled water, 200 cm³ 
• Dessert spoon or similar – plastic disposable spoons are fine and could be re-used
• White paper (to place under beakers to make it easier to see the results)
• Stirring rods x 2
• Sieve or tea strainer – if a funnel was used earlier, tea strainers are needed now; the same sieves could be used throughout
• Petri dishes – lids not required x 2

Hair gel

For each pair or group:

• Approx 1 large teaspoon hair gel – the cheaper and nastier the better
• Salt
• Petri dish or lid
• Teaspoon or similar – an ordinary spatula is a bit small.

Disposable nappies

For each pair or group:

• A disposable nappy – the ultra-absorbent type
• Scissors
• A large ice cream tub or similar container for collecting the inside of the nappy – this is safer than using newspaper or similar; if tubs are in short supply, large ziplock bags could be used (students put the nappy in the bag, zip it up and manipulate it until all the hydrogel has been extracted)
• Approx 500 cm³ distilled water – tap water can be used but the results are not as spectacular.
• Salt
• Dessert spoon or similar measure
• Stirring rod
• Large beaker or plastic tub – at least 600 cm³
• Eye protection
• Plastic gloves for those with sensitive skin.

Note: As an alternative to using nappies and extracting the hydrogel, sodium polyacrylate can be ordered from Sigma-Aldrich. Hydrogel and sugar Students are asked to predict the outcome of this experiment towards the end of the worksheet Hydrogels and how they work, and then to test their prediction.

The experiment should be carried out in the same way as part 2 of Plant water storage crystals but using sugar instead of salt solution.

• Remaining hydrated plant water storage crystals
• Sugar or sugar solution in distilled water
• Distilled water – it is important that distilled water is used, both in the sugar solution and as the ‘plain’ water in this experiment
• Beakers, 250 cm³ x 2
• Tea strainer or sieve.

Timing

It will take over an hour to do all the practical work at once. If lessons are shorter than that then part 1 of Plant water storage crystals can be done in a separate lesson before the rest of the experimental work.

The crystals will keep for a few days in the tea solution, although the tea may stain some types of container. The remaining practical work should fit into an hour if students are organised. They should set up the parts of the experiments that need to be left then do the rest of the work while they wait.

The time required for the written work will depend on the ability of the group. The Hydrogels and sugar experiment is best done part-way through the written work. The hydrated crystals can be kept for a few days if covered with water.