A group of bioinspired polymers are being studied by researchers at the Department of Energy’s Ames Laboratory to understand how they are able to form and react to stimuli similar to the way proteins, lipids and DNA react in nature. Unlocking how these soluble block polymers are able to self-assemble could potentially lead to a variety of uses such as controlled release systems for sustained and modulated delivery of drugs or gene therapies.
Ames Laboratory materials chemist Surya Mallapragada and her research team are focusing on pentablock polymers - polymers that form in strings of five chains. Each string is comprised of two cationic (positively charged) blocks, two hydrophilic (water loving) blocks, and one hydrophobic block. Because the hydrophobic block tries to avoid water, it forms the centre of the string, with the hydrophilic next and the cationic blocks on the outside. In solution, these strings form in small clusters called micelles, again with the hydrophobic blocks at the centre.
“The interesting thing about these polymers is that they respond to changes in temperature and pH,” Mallapragada says. “As the temperature goes up, the micelles cluster together more, forming a polymer gel. A similar reaction takes place as pH rises - the hydrophobicity of the cationic blocks increases which also helps in gel formation.”
As temperature and/or pH drops, the process reverses itself and the gels dissolve back into micelles and polymer strands. Using cryotransmission electron microscopy, Mallapragada’s group is working to understand just how these micelles look and how fast the polymers respond to changes in temperature and pH.
“ Samples are plunged into liquid ethane which freezes them so quickly that ice doesn't form and disrupt the crystal structure,” she says. “We’re able to then view the gel formation at various stages (temperature and pH) under very controlled conditions.” She adds that this work will be complemented by conducting x-ray scattering studies at the Advanced Photon Source facility at the DOE’s Argonne National Laboratory.
The structure appears to be the key in how the polymers react to stimuli similar to the way biomolecules react in nature. These substances carry out a wide variety of tasks, responding to subtle changes in body chemistry regulating those changes. The problem in working with proteins and similar biomolecules, according to Mallapragada, is that it is difficult to isolate the materials without damaging them.
“ Biomolecules often exist in extremely small quantities,” she says, “and are not very robust. In separating them from a source, they become denatured or damaged. The polymers we are studying are much more stable, readily available and therefore easier to study.”
Because they are easier to work with, the polymers could potentially be modified and used as a way to deliver drugs or gene therapies. For example, incorporating the glucose oxidase enzyme in the polymer would make it sensitive to changes in glucose levels in the body. Soluble at room temperature, the polymer could be injected under the skin where it would form in a gel due to the higher temperature of the body. When the gluconic acid level falls, the resulting drop in pH would cause the polymer to swell and release insulin.
The injectable gels would be much less invasive than surgically implanting automatic insulin delivery systems and the gels would dissolve on their own after about a week.
For potential gene therapies, the positively charged (cationic blocks) polymers can complex with DNA (negatively charged). The polymers could be used to deliver so-called suicide genes and chemotherapy drugs directly and selectively to tumours, since normal cells would be less likely to react with the polymer and express the incorporated gene. A preliminary in vivo study in rats is now underway in conjunction with the John Stoddard Cancer Center at Iowa Methodist Medical Center in Des Moines. The basic research on polymer synthesis and characterisation is funded by the DOE’s Office of Basic Energy Sciences. The gene therapy and bioapplication work is funded by a Bailey Career Development Grant.