Eensy Weensy Spider Silk Takes the Temperature of a Single Cell
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(Inside Science) -- Even though he works with spiders regularly, Yao Zhang, a physicist at Jinan University in China, admits he's afraid of them. In fact, most of the people in his lab are, except for graduate student Zhiyong Gong, who keeps spiders as pets in his dorm. Naturally, Gong was the one who volunteered to harvest spider silk in the lab as part of the group's efforts to study how the silk can be used to benefit the human world.
"He was the real 'Spider-man'," said Zhang.
Whether you're afraid of spiders or not, it's hard not to marvel at the wonders of spider silk. By weight, it's stronger than steel and tougher than Kevlar. It's also exceptionally elastic and can stretch a further one-half its length before it breaks -- that's twice as long as nylon can stretch. Also, spider silk doesn't harm living tissue and breaks down safely inside the body, which has led scientists to investigate the biomedical applications of this extraordinary material.
Spider silk's transparency and high refractive index means light moves more slowly through it than in air. This property allows spider silk to trap and channel light over long distances. With a diameter of no more than several microns -- a hundred times thinner than a human hair -- spider silk can be used as a Lilliputian optical fiber to direct light to and from extremely small and hard-to-reach places, such as fluorescent nanoparticles.
Zhang and his colleagues showed that if the nanoparticles fluoresce colors differently depending on temperature, then stringing spider silk with the particles turns the thread into a thermometer. In a paper published earlier this year in the journal Nano Letters, the researchers report that their tiny spider-silk thermometer can measure the temperature across a single cell.
The idea for this unconventional thermometer struck when the Jinan researchers noticed that their nanoparticles stuck strongly to spider silk. Spider silk is negatively charged, so it attracts positively charged objects such as the researchers' nanoparticles. This means the nanoparticles won't fall off the spider silk when jiggled in a fluid environment, such as inside a human body.
Because lone nanoparticles fluoresce diffusely, the researchers slather many nanoparticles onto the surface of spider silk. Then they can excite all the nanoparticles at once by shining an infrared laser into one end of the thread. The thread can also collect and concentrate the fluorescent light from the nanoparticles for a stronger signal.
When the researchers draped the nanoparticle-embellished silk thread across a cell, the thread could map the cell's hot spots. A cell's temperature reveals what stage of life it's in, including whether it is cancerous. According to Zhang and his team, their thermometer could be used to diagnose cancer or study cancer development. Someday this could perhaps be done inside the human body, but "how to deliver the modified silks to the cells through the body is a big challenge," said Zhang.
"Temperature is such a fundamental quantity in biochemical and chemical processes, because reaction rates are temperature-dependent," said Peter Maurer, a physicist at the University of Chicago who didn't participate in the research. Many alternative methods to measure cell temperature are challenging to deploy and read out, he noted. Conversely, the spider silk method seems much simpler. "I'm sure [the Jinan researchers] could use this for detection of other things besides temperature as well," he added.
For all the diverse things people have made with spider silk (among the wackiest are a violin, bulletproof armor and flamboyant fashion), this light-based thermometry for cells makes a lot of sense, said Kenny Hey Tow, a senior scientist at the Research Institute of Sweden who wasn't involved in the study. He said the Jinan researchers' demonstration is "one of the ways that we should think of actually using [spider silk] as a biocompatible material."
Humans often look to Mother Nature for inspiration to solve important engineering problems, or even straight-up pilfer her designs. Zhang, Li, Gong and their colleagues’ work on single-cell thermometry may be only the tip of the iceberg for the optical applications of silk-based fibers.
"It is very exciting!" said Zhang, who plans to continue working with spider silk despite his arachnophobia.
Tow agrees, though he acknowledges that there still are significant challenges that scientists need to overcome before spider silk can be practical enough for widespread use in the human world. Like Zhang, Tow can't help but admire the futurism of spider silk as a material. "The fact that we're using [spider silk] as fiber … it's a kind of a Hollywood thing," he said.
Editor's Note: This story was produced in collaboration with the NPR Scicommers program.