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Engineered Molecules for Smarter Medicines

Specially designed polymers can dodge the body’s immune defenses to deliver vital medicine where it is needed most.

Darlene K. Taylor, Uddhav Balami

Molecules That Feel the Heat

A primary focus of the work in our lab is developing smart polymers that respond to changes in temperature. If the chemical structure of a polymer resembles a long string of identical beads, thermo-responsive materials are made of polymers whose beads, suspended in a solution, change their behavior when the temperature of the solution reaches a certain threshold. Above that critical temperature, the individual molecules of the polymer are transformed from a hydrophilic to a hydrophobic, or water-repellant, state—that is, they alter their shape, allowing less of their surface area to come in contact with surrounding water molecules. This change disrupts the hydrogen-bonded network between the polymer chains and the water molecules that are in the solution. The water is rapidly expelled and the polymeric chains collapse into globules, which causes the thermo-responsive material to precipitate out of the solution. If the temperature drops back below the critical point, the polymeric molecules change back from hydrophobic to hydrophilic, absorbing a significant amount of water and transforming the material from a solid back to a solution.

Even though this phase transition has been widely observed to occur in both directions and has been documented in many different polymeric materials, scientists continue to debate exactly why it occurs. According to one theory, disruption of the polymer-to-water interactions is the main factor that controls the phase transition. Another theory holds that the most significant change takes place in the local structure of the water molecules surrounding the hydrophobic groups of the polymer. A third theory offers a compromise of sorts: Perhaps, on reaching a critical temperature, the polymeric chains undergo a combination of changes occurring in both the hydrogen bonding and the hydrophobic interactions within the solution.

The phase transition in thermo-responsive polymers can also be explained from the point of view of thermodynamics. At some critical temperature, it is energetically more favorable for phase separation to occur. The entropy of the water is higher in the less ordered state, without polymer chains suspended in it.

Diving deeper into the thermodynamic interpretation: Initially the water molecules are trapped by hydrogen bonding with the polar groups of the polymer chain. These trapped water molecules form a thin shell of ordered structure around the hydrophilic part of the polymer. Meanwhile, elsewhere in the solution, water molecules interact with one another to form an ordered, ice-like structure around the hydrophobic part of the polymer chain. This hydrogen-bonded network contributes to a larger negative entropy change—in other words, a transformation to a more ordered state. Raising the temperature supplies enough energy to disrupt the hydrogen bonding within the water-polymer interaction; as a result, the hydrated water shells around the polymers break down, and within seconds the water molecules are forced into the bulk of the water.

But the most important fact about thermo-responsive materials is that they work.

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