One Man's Trash Is Another Man's Sensor

The last time you purchased a compact disc (CD) you were probably wearing cargo pants and oversized sunglasses, and if word got out about what album you were buying back then, you might feel a bit embarrassed to admit it now. There is no doubt that the heyday of compact discs is well behind us now, but there are still a lot of discs out there. Many hundreds of billions of them have been sold worldwide to date. But now these discs of plastic, metal, and acrylic are filling up landfills rather than sweet CD changers because their useful lifespan has been exceeded.

And so it goes. Technologies are introduced, advancements make the previous technologies obsolete and they are discarded. Making matters worse, most modern technologies have short lifespans, leading to lots of electronic waste. It is difficult to find a use for a technology that is no longer relevant. But in the case of compact discs, a group of researchers from Binghamton University believe that they can give this technology a second life. Using a simple, inexpensive process, they have shown that the metal layer from CDs can be extracted and transformed into wearable biosensors.

Biointegrated electronics present many opportunities for continuous monitoring of biological signals that can help to detect pathological conditions, improve athletic performance, or serve a wide variety of other purposes. Manufacturing soft, flexible sensors to serve this purpose has proven to be expensive and time-consuming, however. These manufacturing technologies, often printing or lithography-based, can cost up to $95 per gram to produce the materials for these sensors, with additional processing costs on top of that figure. And depending on complexity, the lead time for production can be several days, which makes rapid prototyping a challenge.

To address these problems, the team developed a method to extract the metal layer from compact discs and shape them into biosensors. First, they soak a CD in acetone for 90 seconds to break down the polycarbonate substrate. Once dissolved, the metal layer can be lifted away with a polyimide adhesive tape. The metal layer was then transferred to tattoo paper as a temporary, durable substrate for the next step in the process.

The freed metal layer was then adhered to the cutting mat of an inexpensive Cricut mechanical cutter of the sort that might be found in any hobbyist's toolkit. By importing AutoCAD drawings into the Cricut Design Space software, the researchers were able to cut intricate patterns into the thin metal extracted from the CD. By introducing various patterns (serpentine, wavy, etc.) into the design, the resulting sensors can be made to conform to the body, be stretchable, and have other qualities like strain relief.

To test the suitability of the recycled material for use as a biosensor, the researchers created a pair of electrodes for use in collecting electromyography (EMG) measurements. These electrodes were attached to the skin, then compared with commercial gel electrodes. A two-channel EMG was recorded synchronously, and it was observed that the signals from the recycled metal electrodes, and the commercial gel electrodes, looked very similar. In another trial, they created a wearable temperature sensor which was benchmarked against an infrared camera. Again, the performance of the recycled sensors was found to be quite good.

The process to go from CD to sensor only costs about $1.50, and as a bonus, CDs are kept out of the landfill. The team is hoping to set up drop boxes for unwanted CDs so that they can be upcycled into sensors. They also have plans to distribute step-by-step instructions so that others can use their technique — they note that anyone can do it, and it does not require any special engineering skills.