In their proof-of-concept study, Zabow and fellow NIST researcher Mark Ferris clamped to a cellphone a tiny well containing the solution to be tested and a strip of hydrogel — a porous material that swells when immersed in water. The researchers embedded tiny magnetic particles within the hydrogel, which they had engineered to react either to the presence of glucose or to pH levels (a measure of acidity) by expanding or contracting.