By Mark Dwortzan
Photonics engineers use light to excite the bonds that connect atoms within molecules, causing them to vibrate at a specific resonant frequency. Using spectroscopy techniques to examine what frequencies of light are absorbed by a material, they can determine what kind of bonds it contains, and thus identify the material. In recent years engineers have designed artificially structured materials, or metamaterials, that produce strong resonance frequency responses in the terahertz ( 1012 Hz) range—distinct responses that can “fingerprint” many biological and chemical agents.
Now a research team from Boston University and Tufts University has found a novel way to turn these electromagnetic metamaterials into implantable sensors and detectors made of tough but biodegradable silk. Incurring no harm on the human body and optically transparent, such devices may ultimately be used to identify toxins in the bloodstream or monitor drug delivery rates or tumor growth in real time.
Drawing on the expertise of three co-principal investigators—Professors Xin Zhang (ME), Richard D. Averitt (Physics) and Fiorenzo G. Omenetto (Tufts Department of Biomedical Engineering), the team has devised a new, simple method to spray electromagnetic metamaterials onto silk substrates, thus enabling them to be implanted in the body. The researchers describe their achievement in the August 24 edition of Advanced Materials.
Featured on the inside front cover of the August 24 edition of Advanced Materials, this image shows a metamaterial structure patterned directly onto a 2 cm × 2 cm freestanding silk film placed on a bed of natural silk threads.
In collaboration with Averitt, an expert in terahertz frequency phenomena, and Omenetto, who introduced silk materials to photonics-based sensor detectors, Zhang and her former graduate student Hu “Tiger” Tao (ME, PhD’10) engineered a unique micro and nanoscale fabrication technology to spray 100-nanometer-thick, split-ring-shaped, metamaterial elements onto freestanding, biocompatible silk substrates. When the researchers implanted the metamaterial silk composite into pig muscle tissue in a series of experiments, they easily detected the transmission of terahertz radiation through the material.
“The resonant response, which emerges from oscillating electrons in highly conductive metals, such as gold in the metamaterial we used, shows that these implantable devices could be used to detect the presence of biological and chemical agents in the body,” said Zhang.
Potential next steps include pre-treating the silk with chemicals that are transformed in the presence of a disease, and then using the metamaterial as a biosensor for the disease; and expanding the new method to other frequency domains in the electromagnetic spectrum to enable additional sensing applications. The research was partially funded by the National Science Foundation, Air Force Office of Scientific Research, Army Research Laboaratory, and the Defense Advanced Research Projects Agency.