"It looks a bit like a heap of spaghetti," said Dr Jenny Melia trying to describe "smart gel", a substance that could transform the way medicines are administered.
Smart gels are actually polymers, chains of molecules that are cross-linked to each other. The polymers under study by Dr Melia and colleagues at the Materials Ireland Polymer Research Centre, in Trinity College, are from the acrylamide family. These have a particular fondness for hydrogen bonds and so take up and can hold on to large amounts of water, trapping it like a sponge.
Unlike a sponge, however, Dr Melia's smart gel can be programmed to discharge its cargo of water on cue, responding to a stimulus such as raised temperatures or changed acidity. Smart gel polymers can also be constructed to react to light and electric fields.
Their particular value, Dr Melia said, is their ability to respond in so predictable a way, for example, releasing their trapped water when acidity reaches a given level or when a temperature is reached. The acrylamide polymers "show a very precise phase transition", she said, responding both quickly and within a very tight band of a degree or two.
This would give them great potential in intelligent drug delivery. The water could be made to carry dissolved drugs which would remain locked up and unavailable until the trigger acidity or temperature was reached. They would then release drugs but only for as long as the trigger stimulus persisted, switching on and off as the situation demanded.
Such a drug delivery system would respond in a dynamic way to the recipient and would be highly tuned to the illness it was treating, Dr Melia said. "The drugs would tailor themselves to the body's condition," so less drug would be needed. Existing drug delivery systems release their drug load over a period of time irrespective of changes in body signals or medication levels.
Trinity's Materials Ireland lab, one of the Programmes for Advanced Technologies, has already carried out research in conjunction with a US firm on an intelligent drug delivery system based on smart gels. It is involved in research with the college's Pharmaceutics Department.
The group is also looking at smart gels that absorb metals. These could be useful for trapping heavy metal wastes in industrial effluent and chemical residues.
The key to their precise response to a stimulus relates to the way they are initially constructed. By adding side chains, adding water-repelling or -loving molecules you can adjust the temperature at which the polymer switches. "You have to be very accurate in your synthesis," Dr Melia said.
They work because of their "bonding interactions". At room temperature their hydrogen bonding is very strong, allowing them to become "hydrophilic" and able to trap large amounts of water. Once they reach a trigger temperature, however, they become "polymerphilic", losing their affinity for hydrogen and bonding instead with other polymer strands.
The group has not yet begun to look at biodegradable polymer types which could be implanted or swallowed. The emphasis, Dr Melia said, was on intelligent skin patches which released drugs when they were needed.
There are other possible uses such as an additive in superglues that would allow tackiness to be activated or deactivated by heating or cooling. It would combine a thermo-switchable polymer with the adhesive polymer, and would reduce the tissue damage caused by skin-contact adhesives.
Dr Melia described another possible use in intelligent packaging. At warmer storage conditions, food products require more breathability in the packaging. If thermo-switchable polymers were integrated into the packaging, pores in the packaging could form, allowing for greater oxygen release. When the correct cooler storage temperature was reached the pores would close.
