Micropatterned Pyramidal Ionic Gels Capacitance Change Pressure Sensors
Authors: Hwang Ihn , Kim Kang Lib , Park Cheolmin 
Published: February 2020

Pressure sensors have been of great interest in Internet of Things (IoTs) applications including smart window, displays, security systems, mobile phones, and prospective electronics skin (e-skin). In particular, sensing high pressures (> 2 kPa) as well as low pressures (< 30 Pa), such as blood pressure, human touch, and sound pulse, is of great importance for low-powered IoT sensors. These pressure sensing capabilities have been dramatically accelerated through the developments of both resistive and capacitive sensing elements. The resistive sensors are based on the change in electrical resistance upon pressure, and recently, 1-D or 2-D nanomaterials employed for the efficient pressure-sensitive percolation of their networks have allowed for high sensitivity of pressure sensors. Although the advance in the resistive pressure sensors is significant with their simple device construction and high pressure sensitivity, they still suffer from high power consumption much greater than that of the capacitive pressure sensors with the operation voltage of approximately 1 V.

The capacitive sensors utilizing the capacitance change upon pressure on the other hand are advantageous owing to their low power consumption, fast response time and compact circuit layout with vertically stacked device architecture. The low pressure sensitivity of a capacitive sensor, however, is detrimental because most of pressure responsive materials such as elastomers and gels have low dielectric constants, which leads to low capacitance change, and thus less responsive to small pressure. Sensors adopting field effect transistor architecture show the high sensitivity because small capacitance change was readily converted into large alteration in channel current of the device, in particular, combined with topologically micropatterned gate dielectric layer. The high driving voltage over 50 V and complicated device architecture including semiconducting channel and three terminal electrodes still limit their usefulness.

Ionic gels of polymer composites with ionic liquids (ILs), non-volatile positive and negative charged ion pairs, exhibit high specific capacitance (> 5 μF/cm2) arising from nanometer-thick electrical double layer (EDL) at ionic liquid/electrode interface, giving rise to a pressure sensor with high sensitivity of approximately 1 kPa−1. We envisioned that an ionic gel with high dielectric constant could become more sensitive to pressure when it is topographically micropatterned. A topographically patterned ionic gel sandwiched between two electrodes contains periodic air gap that is likely to decrease when pressurized, making an effective capacitance of the device drastically increase upon even small pressure.

Here we present highly sensitive capacitive pressure sensors with topologically patterned arrays of pyramidal ionic gels between two Indium Tin Oxide (ITO) electrodes deposited on mechanically flexible poly(ethylene terephthalate) (PET) substrates. Micropatterned pyramidal ionic gels (MPIGs) exhibits significant variation in capacitance as a function of pressure, giving rise to a pressure sensor with its sensitivity of approximately 40 kPa−1 at the pressure range lower than 400 Pa and 13 kPa−1 at the range between 0.5 kPa and 5 kPa. Our device is suitable for sensing the broad pressure range up to 50 kPa with its sensitivity greater than 2 kPa−1, allowing for efficient detection of various pressure sources from a few Pa to tens of kPa including sound wave, light weight object, jugular venous pulse, radial artery pulse and finger touch.

Figure 1