A Wearable Capacitive Sensor Based on Ring/Disk‐Shaped Electrode and Porous Dielectric for Noncontact Healthcare Monitoring

Abstract

Wearable sensors are gradually enabling decentralized healthcare systems. However, these sensors need to be closely attached to skin, which is unsuitable for long‐term dynamic health monitoring of the patients, such as infants or persons with burn injuries. Here, a wearable capacitive sensor based on the capacitively coupled effect for healthcare monitoring in noncontact mode is reported. It consists of a ring‐shaped top electrode, a disk‐shaped bottom electrode, and a porous dielectric layer with low permittivity. This unique design enhanced the capacitively coupled effect of the sensor, which enables a high noncontact detectivity of capacitance change. When an object approaches the sensor, its capacitance change (ΔC/Ci = −38.7%) is 3–5 times higher than that of previously reported sensors. Meanwhile, the sensor is insensitive to the stretching strain and pressure (ΔC/Ci < 5%) due to the unique ring‐shaped electrode and the incompressible closed cells of the porous dielectric material, respectively. Finally, various human physiological signals (pulse and respiratory) are recorded in noncontact mode, where a person wears loose and soft clothes implanted with the sensor. Thus, it is promising to build smart healthcare clothes based on it to develop wearable decentralized healthcare systems.

Flexible and Stretchable Capacitive Sensors with Different Microstructures

“…In addition, the porous structure has good permeability because of which the normal excretion functions of human skin (e.g., excretion of sweat and body fluids) are not affected and the occurrence of allergies is reduced. [130] Therefore, polymer foams imitating natural structures have greater mechanical deformability than bulk polymers, and can be employed for heat insulation, vibration damping, and pressure absorption in the automotive and construction industries. [131] The manufacture of molds with microstructures is not suitable for low-cost large-area production.…”

“…For example, micropyramids usually require complex photolithography, chemical etching, [38,[134][135][136] and subsequent processes with multiple steps, such as demolding. In contrast, the methods for preparing porous structures are usually simple; for example, pores left by sugar dissolved in water (Figure 5B); [52,71,137] by water volatilized at a high temperature, which does not affect other materials such as the polymer matrix (Figure 5C); [47,130] and by heating ammonium bicarbonate (NH 4 HCO 3 ), the foaming agent commonly used in the food industry, which produces carbon dioxide (CO 2 ) after heating (Figure 5D). [132] These methods are [121] Copyright 2012, The Authors, published by MDPI.…”

“…After drying the soaked sensor, its performance could still be restored to the its original state with excellent mechanical circulation (Figure 11E). [130] Based on the 3D mesh structure, adding other functional materials into the composite materials can provide the sensor with other ideal functions. Park et al mixed spiropyran, PDMS, and silica nanoparticles (SNPs) in a hydrophilic cosolvent (water and ethanol) to prepare a porous mechanical color-changing composite material with a graded nanoporousmicroporous (NP-MP) structure, which could be used to visualize the magnitude of strain (Figure 11F).…”

“…Although the 3D mesh structure can be employed for strain sensing, the maximum deformation achieved in currently research is smaller than that of other structures, usually no more than 50% and only ≈10% for some structures. [33,37,130] This greatly limits its practical application. In addition, as with the porous structure of the pressure sensor, the uniform size and distribution of the holes remain problems to be solved.…”

Recent Progress in Flexible Pressure Sensors Based Electronic Skin

“…Therefore, capacitive-type pressure sensors' performance based on their components such as active materials and nanostructures have been reported. The active materials such as carbon-based materials (e.g., CNT, [94] and graphene [95] ), hydrogel, [96] metal-based materials (e.g., liquid metals, [97] metal particles, [98] and NWs [99] ), and natural materials [100,101] have been investigated for flexible capacitive sensors. Carbon nanomaterials are carbon nanotubes (CNTs), graphene and its derivatives, and carbon black.…”