Electrolytes, such as potassium, calcium, magnesium, and other salts are essential in carrying nerve impulses that tell the heart and other muscles when to contract or relax. A proper balance of them in the human body is required for the normal function of cells and organs. Realizing that laboratory analyses can be expensive, and that patients, athletes, and soldiers would prefer a pain-free, wrist-worn diagnostic device that can analyze, continuously record, and even remedy low electrolyte levels, researchers at Sandia National Laboratories, Albuquerque, NM, have developed and are patenting a prototype model that can analyze various electrolyte levels on the spot, yet fit in a palm or be worn on a wrist. The device, when commercially available, could decrease the time users would spend in emergency rooms, lab testing facilities, or doctors’ offices.

Fig. 1 – Ronen Polsky holds a prototype of the microneedle fluidic chip device. The visible rectangular gold pads are electrical contacts. (Credit: Randy Montoya)
The device features nine sampling needles, each only 800 millionths of a meter (microns) in height, and beneath them, a fluidic channel that can draw interstitial fluid over nine gold disk electrodes. Each disk can be tailored to detect a different analyte. The microneedles are so tiny that they don’t traumatize nerves when pressed into the skin. It also samples only interstitial fluid, the liquid between skin cells, so, it could be used for long-term, noninvasive use, they said.

“This is the future of personalized health care,” said Ronen Polsky, a Sandia investigator. “We’re proposing a minimally invasive way to move away from centralized laboratory testing.”

In a paper published in Advanced Healthcare Materials, Polsky, Sandia colleagues, and University of North Carolina and North Carolina State University graduate student Phil Miller describe using a laser to create strong hollow microneedles that suck nearly infinitesimal amounts of colorless fluid from just beneath the skin’s surface. (See Figure 1)

Their research demonstrates that tiny amounts of potassium passed through the microneedle pores into a fluidic cartridge containing carbon electrodes. These measure the amount of potassium without being confused by the presence of other electrolytes in the fluid.

Miller said it’s easy to change the selectivity of the carbon electrodes to detect and measure other electrolytes like sodium or calcium in the same device. “We want to make the device wearable, noninvasive, and with real-time readout to constantly measure things a doctor might normally order for laboratory tests,” he said.

Eventually the researchers envision a “sense-respond” device where some needles can read electrolytes while other needles, on demand, send electrolytes to make up for deficiencies.

University of New Mexico physician and researcher Justin Baca, who will lead the human testing of the device, said that he has initial approval from the university’s Human Research Protection Office review board to start tests. He’s interested, he said, because “it’s hard, using traditional methods, to take blood samples continuously.” Interstitial fluids should be far easier to sample, Baca said.