Scientists have developed disposable paper-based sensors that can measure glucose concentrations in saliva, paving the way for a pain-free alternative to diabetics for monitoring their blood sugar levels daily. Strips of pH-sensitive paper are commonly used to test whether a liquid is acidic or alkaline. Researchers are now working to apply similar principles to create paper sensors that quickly indicate disease biomarkers.
Key to this approach is replacing traditional electronic circuitry in the sensors with low-cost plastics that can be manufactured quickly and in large quantities.
A team from the King Abdullah University of Science and Technology (KAUST) in Saudi Arabia used inkjet technology to produce sensors sensitive to small sugar concentrations in biofluids.
Utilising a commercial ink made from conducting polymers, the team printed microscale electrode patterns onto glossy paper sheets. They printed a sensing layer containing an enzyme, glucose oxidase, on top of the tiny electrodes.
The biochemical reaction between available glucose and the enzyme creates electrical signals easily correlated to blood sugar levels.
“Paper is porous, which makes it challenging to print conducting and biological inks that are dissolved in water,” said Eloise Bihar, a postdoctoral researcher at KAUST. “Printing the enzyme is tricky, as well — it’s sensitive to variations of temperature, the voltage applied at the cartridge, and the pH of the ink,” said Bihar.
While fluids, such as sweat or saliva, contain enough sugar for monitoring purposes, they also contain molecules, such as ascorbic acid, that interfere electrically with conducting polymers.
Coating the sensor with a nafion polymer membrane that repels the negative charges present in most interfering species enabled measurement of only the relevant glucose levels in saliva samples from volunteers.
Experiments showed the top coating gave the sensor an unprecedented shelf life — the enzyme could be kept alive and active for a month if stored in a sealed bag. These results are encouraging the team to expand the capabilities of this approach by incorporating different enzymes into the sensing layer.