A Wireless, Passive pH Sensor Based on Magnetic Higher-Order Harmonic Fields

A new wireless sensor was fabricated for remote pH monitoring. The sensor was made of a magnetically soft film (sensing element) and a permanent magnetic film (biasing element) sandwiching a reversibly swelling hydrogel. When subjected to a low frequency magnetic AC field, the sensing element generated higher-order harmonic magnetic fields that could be detected with a remotely located detection coil. In the presence of a magnetic DC field (biasing field), such as that generated from the biasing element, the pattern of the higher-order harmonic magnetic fields varied depending on the field strength of the biasing field. The hydrogel, which acted as a transducer that converted variations in pH into changes in its own dimension, physically varied the separation distance between the sensing and biasing elements. This caused a change in the biasing field experienced by the sensing element, changing its higher-order harmonic field pattern allowing remote pH measurement. The advantage of this sensor technology is the wireless and passive nature, which allows its application for long-term pH monitoring in the human body. In this study, a pH-sensitive hydrogel was used for pH measurement. Nevertheless, this technology can be applied to a wide range of chemical and biological targets, provided the appropriate sensing coatings are employed.