Beyond Jell-O: New ideas gel in the lab

The gelatin in your dessert bowl and the absorbant in your baby’s diapers are known technically as hydrogels. Now, researchers have created a new variety of hydrogels that they hope to use for more complex pursuits, including drug delivery, bone repair, and organ replacement.

HYDROGEL, CLOSE UP. The microscale structure of a hydrogel’s polymer network. The bar (lower left) represents 20 micrometers. B. Ozbas and Pochan

The nanoscale structure of a hydrogel’s polymer network. The bar (lower right) represents 0.2 micrometer. L. Pakstis and Pochan

Hydrogels are networks of polymers that don’t dissolve in water. Instead, they swell up, or gel. Many hydrogels, like the colorful edible gelatin, are made from natural proteins, while others are artificial.

To make the new hydrogels, Timothy Deming of the University of California, Santa Barbara and his colleagues made polymers of amino acids by using synthetic-chemistry techniques. For each polymer, the researchers used two types of amino acids. Each molecule contained a chain of one amino acid that’s water soluble linked to a chain of another that isn’t.

This created polymers with a hydrophilic, or water-loving, part and a hydrophobic, or water-averse, part. Generally, chemical organizations of this kind fold into spherical structures called micelles in water. In these structures, the water-averse part becomes the center of a sphere and the water-loving part becomes the outside. However, the new polymers surprised the researchers by forming hydrogels instead, says team member Andrew Nowak of UC-Santa Barbara.

The researchers discovered that their new hydrogels have traits that could make them useful. They don’t liquefy even at temperatures as high as 90C–much higher than the melting point of many hydrogels. Also, they quickly regain their gel-like quality, or stiffness, after being physically broken up. Moreover, compared with most hydrogels, the new materials gel in water containing much less polymer.

The research provides important insight into what makes good hydrogels, says team member Darrin Pochan of the University of Delaware in Newark. The strongest hydrogels formed from polymers that included hydrophobic amino acids that take on helical shapes, such as strings of leucine. Stiff gels also formed when the researchers used a string of valine, a zigzagging structure, as the hydrophobic amino acid. In contrast, polymers made with blocks of amino acids that assume more random conformations formed much weaker hydrogels. Such knowledge is useful for designing hydrogels with particular properties, says Nowak.

Since the new hydrogels are made of amino acids found in natural proteins, they could be used in materials intended to break down in the body, says Pochan. In addition, he says, the new hydrogels have well-ordered networks of pores on both the nanometer and micrometer scales. Typically, hydrogel structures are less ordered, akin to cooked spaghetti. The new structures could be designed to carry drugs or serve as scaffolds for growing tissue or depositing minerals.

The porous networks are certainly the appropriate size for such applications, especially for growing cells into tissue, comments Shuguang Zhang of the Massachusetts Institute of Technology.

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