Oscillator Based Readout System For Inductively Coupled Stretchable Resistive Strain Sensor

Abstract: The development and use of stretchable electronics is a relatively new research field. However, the use of these kinds of electronics could prove fruitful when seeking to improve medical sensors and equipment. Since the substrates and manufacturing methods used when making stretchable electronics allows them to be extremely thin, with a thickness in the μm range. At these thickness levels, the electronics are capable of following and adhering to human skin as it bends and stretches. This project proposes a wireless and stretchable reader and sensor system which uses a simple oscillator circuit with an inductively coupled resonator to de- tect resisitve strain in a fully stretchable RLC sensor circuit. By delving into the malleable nature of these cutting-edge electronics, we determine the most effec- tive circuit parameters to optimize signal behavior for specific applications. The findings showed that the simulated frequency shift sensitivity to variations in sen- sor resistance in a simple NIC based oscillator circuit could (in some cases) be accurately modelled. The simulation was then confirmed via measurement with a reader circuit inductively coupled to a stretchable sensor. Further resistive strain experiments revealed a frequency shift of -0.129% per 1% of strain in the sensor re- sistor, which was high enough to detect the relatively faint skin deformation caused by a pulse wave from the heart. The system was then further capable of measur- ing the heart rate of a person which was verified using a photoplethysmography sensor.