Capacitive Sensors with Hybrid Dielectric Structures and High Sensitivity over a Wide Pressure Range for Monitoring Biosignals

Abstract

Various dielectrics with porous structures or high dielectric constants have been designed to improve the sensitivity of capacitive pressure sensors (CPSs), but this strategy has only been effective for the low-pressure range. Here, a hierarchical gradient hybrid dielectric, composed of low-permittivity (low-k) polydimethylsiloxane (PDMS) foam with low Young's modulus (low-E) and highpermittivity (high-k) MWCNT/PDMS foam with high Young's modulus (high-E), is designed to develop a CPS for monitoring biosignals over a wide force range. The foam-like structure with hybrid permittivity (low-k + high-k) is facilitated to improve the sensitivity, while the hierarchical structure with gradient Young's modulus (low-E + high-E) contributes to broadening the pressure sensing range. With the hierarchical gradient hybrid structure, the flexible pressure sensor achieves an enhanced sensitivity of 2.155 kPa −1 , a wide pressure range (up to 500 kPa), a minimum detection limit (50 Pa), and an excellent durability (>2500 cycles). As a demonstration, a venous thrombosis simulation and smart insole system are established to monitor venous blood clots and plantar pressures, respectively, which reveal potential applications in wearable medicine, sports health prediction, athlete training, and sports equipment design.

All‐Organic Smart Textile Sensor for Deep‐Learning‐Assisted Multimodal Sensing

“…For pressure sensing, the sensing principle is pressure‐dependent capacitance. The capacitance changes of our textile sensor under pressure are expressed as: [ 38 , 39 ] $$\[ \begin{array}{*{20}{c}}{\frac{{\Delta C}}{{{C_0}}}\; = \;\frac{{C'}}{C}\;\; - \;1\; = \;\frac{{\frac{{\varepsilon '{\varepsilon _0}S}}{{d'}}}}{{\frac{{\varepsilon {\varepsilon _0}S}}{d}}}\;\; - \;1\;\; = \;\;\frac{{\varepsilon 'd}}{{\varepsilon d'}}\; - \;1}\end{array} \]$$where C 0 Δ C , and C ′ denote the sensor capacitance under zero pressure, the sensor capacitance variation under pressure, and the sensor capacitance under pressure, respectively. Moreover, ε′ and ε denote relative permittivity of the dielectric layer under pressure and zero pressure, d and d ′ are the thickness of the dielectric layer without and with pressure, and S is the sensing unit area.…”

“…For pressure sensing, the sensing principle is pressure-dependent capacitance. The capacitance changes of our textile sensor under pressure are expressed as: [38,39]…”

Bioinspired Engineering of Fillable Gradient Structure into Flexible Capacitive Pressure Sensor Toward Ultra‐High Sensitivity and Wide Working Range

“…Furthermore, we addressed the low compression characteristics of the structural patterns of the dielectric layer in the above study by designing fillable structures, and there is no significant hardening of the dielectric layer as the pressure of the filled structure increases. To date, various elastic dielectric substrates have been extensively studied for their dielectric properties on sensor performance, such as polydimethylsiloxane, [26][27][28][29][30] silicone rubber, [31] hydrogel, [32][33][34] and other super-elastic materials. Both structural and material level options are chosen to balance the contradiction between high sensitivity and a wide range of flexible sensors.…”

Microstructured Polyfluoroacrylate Elastomeric Dielectric Layer for Highly Stretchable Wide-Range Capacitive Pressure Sensors

“…For example, Wu and Yang and co-workers used a layered gradient mixed medium composed of low dielectric constant polydimethylsiloxane (PDMS) foam and high dielectric constant MWCNT/PDMS foam as the dielectric layer of CPS. This design allows the sensor to have higher sensitivity in the low-pressure range and a wider pressure sensing range in the high-pressure range . Lu and co-workers used a nickel foam template to fabricate an Ecoflex conductive porous nanocomposite material doped with carbon nanotubes as the dielectric layer and fabricated sensors with improved sensitivity …”

“…This design allows the sensor to have higher sensitivity in the low-pressure range and a wider pressure sensing range in the high-pressure range. 20 Lu and co-workers used a nickel foam template to fabricate an Ecoflex conductive porous nanocomposite material doped with carbon nanotubes as the dielectric layer and fabricated sensors with improved sensitivity. 21 Besides, the capability to withstand large tensile strain is often more valuable than flexibility for a soft electronic device in practical applications.…”