Intelligent Gels

Another State of Matter

Most people are familiar with the three states of matter: solid, liquid and gas. Actually, there are many more exotic states, and one task of material scientists is to try to understand their structure and properties, and derive useful applications from such understanding.

One exotic state of matter is gels. They are not quite solid; neither are they liquid. Examples include jello, hair-styling gels, sea cucumbers and soft contact lenses. Dr. Wu Chi of the Department of Chemistry has been working on polyelectrolyte gels.

Most common molecules are fairly small, consisting of only a few atoms. For example, the oxygen molecule consists of only two oxygen atoms; the water molecule consists of two hydrogen atoms and one oxygen atom. Macromolecules, or huge molecules, also exist. They consist of tens of thousands, even millions of atoms in one single molecule. The typical structure is a long chain of carbon atoms, on which various side groups are attached. If only hydrogen is added, one can get polyetheylene, a familiar soft plastic; with other side groups, one can get, for example, nylon.

Polyelectrolytes are a special kind of macromolecules; in these, many of the side groups can easily become charged by gaining or losing electrons. By developing branches and intermolecular connections (called cross-linking) these macromolecules can form a three-dimensional network (Fig. 1). The networks can collapse into rather tight structures (like jello powder before water is added); but if water is added to fill up the space between the network, it can swell up and form gels. The interaction between water and the polymer networks determines the swelling and shrinking of a given polyelectrolyte gel. In fact, familiar gels such as jello and sea cucumbers are all of this type.

Some types of polyelectrolyte gels undergo `phase transitions', and are extremely useful materials in medicine and chemical technology.

Phase transitions are sudden changes in the properties of a material when the external environment is slightly altered. The classic example is the boiling of water: by just changing the temperature from 99.9...9°C to 100.0°C, water suddenly becomes steam, expanding 1700-fold. This somewhat trivial observation is the basis of the steam engine, and indeed the industrial revolution.

Those polyelectrolyte gels that undergo phase transitions can have sudden changes in volume, ranging from 100 to 500 times of their original size. These transitions are caused by tiny changes in temperature, solvent composition, salinity, acidity (pH), ionic strength and electric field. For example, the gel called poly(N-isopropylacrylamide) can suddenly swell 100-fold when the temperature is changed by as little as one degree, from 32°C to 31°C. It is as if these gels would perform tasks on demand; they are thus sometimes called intelligent gels.

Dr. Wu Chi's work focusses on these intelligent polyeletrolytic gels. This important project won competitive funding of $546,000 from the Research Grants Council in 1993.

Some medication is destroyed in acidic conditions. They would therefore be rendered useless in the acidic environment of the stomach. Scientists therefore develop a gel which shrinks in acidic conditions and expands in alkaline conditions. If the medication is encapsulated in such a gel, it will be tightly protected when passing through the stomach; however, in the relative alkaline conditions of the intestines, the gel will expand and release the medication, which is then absorbed by the intestinal walls.

At the University of Trondheim in Norway, researchers are working on an even more interesting scheme. They use an intelligent gel that expands upon the application of an electric field. Insulin is encapsulated in such a gel, and implanted into diabetic patients. Then controlled release is achieved by applying an electric field external to the body; needle-pricks are thus spared.

Currently little is known about the sudden volume change, and its relationship with the gel structure. The development of gels with specific properties is still based on trial-and-error, which is expensive, time-consuming, and usually unreliable. Therefore the focus of Dr.

Wu's research is to obtain a deeper understanding of the transition, which will enable the industrial sector to develop polyelectrolyte gels in a more cost-effective way.

Specifically, the research will try to determine the kinetics of the swelling and shrinkage, in other words, the course of time development when external conditions are changed. Upon shrinkage, water has to come out from the inside of the entangled cross-linked web; upon swelling, water has to burrow deep into the inside. This occurs through a relatively slow process of diffusion, and may be a crucial step that determines the course of time development; so the diffusion process will be investigated. The gel structure will also be examined, both for the gel network as a whole, and for the individual chains in the gel. It is hoped that the relationship between the structure and the dynamics can be revealed.

The method is simple. One simply `looks' at the sample. More precisely, one probes the sample by shining light on it. The scattered light, monitored by modern optical tech-niques such as miniature interferometry, laser light scattering, differential refractometry, holographic relaxation spectroscopy and time-resolved small-angle x-ray scattering, will reveal detailed information about the sample. This is then analysed to obtain information about the gel, both statically and also as it undergoes volume changes.

Two model systems have been studied in detail. The first is simply gelatin, which has been used extensively in the food/feed and pharmaceutical industries as an important stabilizer and gelling ingredient. The second is poly(N-isopropylacrylamide). This is a model gel system with a transition temperature of 31°C.

Macromolecules exist in a range of molecular weights — just as people may be heavier or thinner. The first step was to synthesize a set of samples with narrow distributions of molecular weights — just as focussing on groups of people in different weight ranges. The picture always becomes clearer and sharper when one looks at such segregated samples.

With these samples, the research group has found that the swelling of a thin gel film can be better described by first-order kinetics. The group has also accomplished the study of the swelling and shrinking of single polymer chains. With a better understanding of single chain properties they will move onto the gel networks. They also intend to study both micro gel particles and thin gel films. Their ultimate goal is to find out why some polyelectrolyte gels undergo intelligent phase transitions.