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HOME > PAST ISSUE > March-April 2014 > Article Detail


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

Sidebar: Tripping the Light Fantastic

Alongside materials that respond to changes in temperature is another category of intriguing smart polymers whose changes are triggered instead by light. Because light is a clean stimulus that allows remote control without physical contact, these photo-responsive polymers are useful for a wide variety of applications, including smart optical systems, micro-electrochemical systems, sensors, textiles, coatings, and solar cells.

2014-03TaylorFp113.jpgClick to Enlarge ImageOne area receiving attention now is photo-responsive smart materials that change their color in response to light. In our laboratory we are currently investigating a particular family of polymers that incorporate an organic compound called spiropyran. This molecule has ring-shaped structures that open and close when exposed to certain wavelengths of light—or, in some cases, when exposed to changes in acidity. Cleavage of the ring at the site of a specific carbon-oxygen bond creates a chromophore (color-producing region) that strongly absorbs visible light. Like other smart materials discussed in the article, this polymer holds promise for biomedical application in the encapsulation and targeted release of drugs. In a completely different application, the textile industry uses photo-responsive polymers woven into apparel to produce garments that change their color or reveal a print pattern in the sunlight, or fibers that act as an indicator of over-exposure to the sun—a feature that might be appreciated, for example, in children’s clothing.

Smart materials can be designed to conduct electrons in response to the absorption of electromagnetic radiation. Instantaneously, an electron can be raised to a higher energy level. The electron then relaxes from the high energy state to the ground state as it loses a photon.

Smart materials are also being used to harvest sunlight for photovoltaic systems. At present, however, these polymers do not perform as efficiently as the semiconducting materials used in the computing industry. However, polymers are considered the next-generation material for photovoltaic devices because they are so inexpensive to manufacture. Research is under way to improve the performance of polymer-based solar cells. Ideally, the polymer models would respond to a broad range of light (infrared to visible wavelengths), increasing their efficiency to (for example) power electronic devices. Working toward this goal, our group has an ongoing interest in developing materials known as paraphenylene oligomers. Experimental and theoretical studies carried out in our group show that tagging those molecules with additional molecular groups causes the photo-responsive polymers to absorb longer, redder wavelengths of light. These materials could be tuned to respond to an even broader range of light, we find, by adding a strong electron acceptor group.

Polymers containing transition metals such as ruthenium are particularly well suited as antennae for attracting light, a feature that facilitates its absorption and electron transfer (and/or energy transfer) within the material. Our group, in collaboration with colleagues at the University of North Carolina–Chapel Hill, has been using ruthenium in combination with a polymer to form a hybrid system (such as ruthenium (II) polypyridyl derivatized polystyrene) that looks particularly promising. This line of investigation should yield a better understanding of how electron and photon energy are transported within the photo-responsive polymers. Our latest studies suggest that the relative spatial arrangement between the ruthenium ions in the polymer is a key parameter influencing the way it transports electrons through the system. Such discoveries are bringing us closer to developing polymer-based solar cells that can compete with coal- and natural gas–fired power plants. Understanding the internal physics of light-absorbing polymers is essential to making further advances.

Although conventional, silicon-based photovoltaic arrays are already in widespread use, polymer-based solar devices would offer significant advantages. Polymers are inexpensive to develop, easily scaled to large manufacturing quantities, lightweight, and adaptable to a wide variety of design criteria. Refining and commercializing this technology would benefit not only the solar panel industry but also the textile industry, as it becomes possible to weave electron-conducting smart materials into fabrics. In fact, the two industries may someday come to overlap. Incorporating light-responsive smart materials into textiles could transform the way we interact with our electronics, perhaps leading ultimately to devices that can be charged by contact with our clothing.


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