Sensor Sensibility


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Researchers drew sensors capable of detecting pollutants on a leaf. Image: UC San Diego

The chemical sensors of the world are a clinical looking bunch. Be they for skin, air, or airport, they resemble Band-Aids with wires, conspicuous and ungainly prototypes of the gadgets of yesteryear, generic litmus strips stamped out by the millions.

But the days of the unattractive sensor have come to an end. Thanks to researchers at the University of California, San Diego, those hoping to measure their glucose level, gauge the concentration of heavy metals on a tire swing, or determine whether or not a suspicious personal electronic device is made of a plastic explosive, can simply draw a sensor onto the surface in question, as intricate, elegant, and artistic as the assessor’s hand will allow.

The invention, known as the biocompatible enzymatic roller pen for direct writing of biocatalytic materials, “basically has enzymes loaded in it and the enzymes react only with specific types of chemicals,” says Amay Bandodkar, the lead researcher on the project.

Enzyme Survival

Creating an ink that would harm neither enzymes nor flesh was not as simple as it is sounded. “The biggest challenge was how to stabilize the enzyme in the aqueous media,” says Bandodkar. Typically, enzymes start to deteriorate when they hit water. To keep them as durable as the blue in your Bic, Bandodkar and his team turned to a common sweetener. “If you jut go to any super market, or CVS, or something like that, you can buy this sugar substitute, called xylitol,” he says. “It comes between the enzyme molecules and the water molecules—it forms a protective layer and doesn’t allow the water molecules to denature the enzyme.” And it’s already FDA approved.

The glucose sensor strips can be drawn directly on the skin. Image: UC San Diego

Suspended thusly, the enzymes can survive temperatures as high as 120 degrees, whereas, unprotected by xylitol, the delicate enzymes would start to fall apart at room temperature.

But enzyme survival was only one element of the ink that needed to be engineered. The pen also had to—you know—write nice. So the team examined the makeup of many of today’s inks. “We went through the ingredients that people usually use,” says Bandodkar, “but most of the ingredients we had to reject because they are certainly not biocompatible.” In addition to being innocuous to the environment and safe for an enzyme, Bandodkar’s ink had to be conductive. So the team used graphite among the many ingredients that “nobody has tried before.”

As a result, electrodes can be drawn wherever electrodes might be wanted. “For something like explosive detection, where what you need is a yes/no sensor, you don’t need to be very specific about the distance between electrodes or the shape you have drawn,” says Bandodkar. But if you’re after the exact amount of glucose on the skin, or the precise level of a pesticide on a leaf of kale, or any other particular chemical concentration you want to measure, then the electrode area needs to be consistent, whatever the shape or design.

Wearable and Wireless

When the sensor generates the signal, a wearable device will receive and display the results. Likely a watch or a band, “it will be completely wireless, so you basically need to wear it on top of the sensor, that’s it,” says Bandodkar.

The wireless component is still in development. Once complete, those in need of a sensor will no longer have to compromise their fashion sense. “The prefabricated sensors you have on the market have a fixed design—but maybe you don’t want the shape or design the companies are offering,” says Bandodkar. “This is giving freedom to the end user. I can draw any shape, any form. Tomorrow, if I get bored, I can draw it in a different way, so it becomes more camouflaged in the surroundings. It doesn’t seem obvious that you are wearing some sensor. It can be like a cool body art.” At long last, the hipness of the sensor will reside in the hipness of the sensor pen wielder.

Michael Abrams is an independent writer.

Learn about the latest trends in biomedical devices at ASME’s 2015 4th Global Congress on NanoEngineering for Medicine and Biology.

This is giving freedom to the end user. I can draw any shape, any form. Tomorrow, if I get bored, I can draw it in a different way.

Amay Bandodkar, University of California, San Diego

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April 2015

by Michael Abrams, ASME.org