First Practical Smart Material Could Propel Use of Tissue-Penetrating Light in Medical Diagnostics

Scientists at the University of California, San Diego (La Jolla) are reporting that they have developed and completed successful initial tests of the first practical smart material suitable for medical imaging applications. The new material, they say, may supply the missing link in efforts to use a form of light in medical device applications that can penetrate four inches into the human body. Their report on the new polymer material, which can potentially be employed for diagnosing diseases and engineering new human tissues in the lab, appears in the American Chemical Society journal Macromolecules.

Adah Almutairi and colleagues explain that near-infrared (NIR) light, which is just beyond what humans can see, can penetrate into the body. Because this type of light can disintegrate plastics, it could be used to disassemble drug-filled polymers in drug-delivery applications. However, current NIR-responsive smart materials require high-power NIR light, which could damage cells and tissues, and materials capable of responding effectively to low-power NIR have not been available until now. In response, Almutairi and her team have have begun research on developing a new smart polymer that can respond to low-power NIR light.

When it penetrates deep into tissues, NIR can be remotely applied with high spatial and temporal precision. The UC San Diego researchers' polymeric material can disassemble in response to biologically benign levels of NIR irradiation upon two-photon absorption. The design relies on the photolysis of the multiple pendant 4-bromo7-hydroxycoumarin protecting groups to trigger a cascade of cyclization and rearrangement reactions, leading to the degradation of the polymer backbone. The new material undergoes a 50% M_w loss after 25 seconds of ultraviolet irradiation by single-photon absorption and 21 minutes of NIR irradiation via two-photon absorption. Most importantly, the scientists note, is that even NIR irradiation using a biologically benign laser power is sufficient to cause significant polymer disassembly. Furthermore, this material is well tolerated by cells both before and after degradation. These results demonstrate for the first time that an NIR-sensitive material can potentially be used in in vivo applications.

The researchers envision that this polymer could be used in an implantable hydrogel capable of releasing medications or imaging agents upon contact with NIR light. "To the best of our knowledge, this is the first example of a polymeric material capable of disassembly into small molecules in response to harmless levels of irradiation," the researchers comment.