Taking Cues From The Sea

Sarina Tracy, sarina.tracy@photonics.com

When man copies nature, it tends to be a smart move. Using nature’s design and strategy to solve human problems can allow us to bypass a lot of experimentation. One of the earliest instances of biomimicry may be Leonardo da Vinci’s sketches for a “flying machine” that mimics the wings of a bat. And architects have used the design of an egg – nature’s strongest, most stable shape – to create domed ceilings for centuries.

Now, a rather surprising stimulus has inspired a new lifesaving technology: an imaging system that detects cancerous and dysplastic tissues, based on the eyes of the mantis shrimp.

The mantis shrimp can see differences in polarized light through its compound eye, which comprises groups of photocells called ommatidia and a cornea that focuses external light. The focused light is filtered through a pigment cell for color sensitivity, passing through a series of photosensitive retinular cells containing microvilli, which act as polarization filters. The retinular cells signal the brain through the optic nerve, and the brain extracts visual information based on the input from the ommatidia.

Researchers at Washington University in St. Louis achieved a similar effect by using aluminum nanowires as linear polarization filters. Photosensitivity is achieved through a CMOS imaging paradigm. The signal from the diode is converted into a current inside the pixel, forming an image.

“Sometimes we follow nature as close as possible, and sometimes we try to mimic the functionality in biological counterparts with the best tools we have,” said Dr. Viktor Gruev of WUSTL. “Our sensors have to be more generic in recording light properties than what the mantis shrimp visual system does. After all, these creatures don’t search to find cancer – they use polarization to eat, mate or camouflage.”

The team attached the polarization sensor to an endoscope to look for colorectal tumors in mice. A near-infrared fluorescent dye was applied to suspect regions inside the mice. The cancer cells were easy to see, as their disorganized and invasive structures scattered light differently than healthy cells did. This compact polarization imager is said to be the only device of its kind that can be integrated into the tip of a flexible endoscope. And, surprisingly enough, creativity and ingenuity are the traits that make this application possible, as the imaging sensor’s applications never propelled its development.

“My idea was that if we want to utilize nanotechnology, we have to find a way to merge it with CMOS. Along the way, we want to create extraordinary devices,” Gruev said. “This effort was purely driven by technology and not by application. Once we solved some of the engineering and material science questions, my colleagues kept asking me what [I was] going to use this technology for. This made me think even harder and outside my comfort zone.”

The polarization sensor has potential in many fields, including real-time imaging of soft biological tissue and neural recording in freely moving animals through an implantable device. The sensor is already established as part of a technique for understanding swordtail fish mating patterns. As for Gruev, however, his eyes remain locked on the clinic.

“These sensors have the potential to transform how we diagnose diseases, how we understand the functionality of the brain,” he said. “I hope that this technology will be used in many places that do not have access to sophisticated medical technology and improve their living standards.”

The research was published in Proceedings of the IEEE (doi: 10.1109/JPROC.2014.2342537).