Biocompatible Soft Fluidic Strain and Force Sensors for Wearable Devices

Abstract

Fluidic soft sensors have been widely used in wearable devices for human motion capturing. However, thus far, the biocompatibility of the conductive liquid, the linearity of the sensing signal, and the hysteresis between the loading and release processes have limited the sensing quality as well as the applications of these sensors. In this paper, silicone based strain and force sensors composed of a novel biocompatible conductive liquid (potassium iodide and glycerol solution) are introduced. The strain sensors exhibit negligible hysteresis up to 5 Hz, with a gauge factor of 2.2 at 1 Hz. The force sensors feature a novel multi-functional layered structure, with micro-cylinder-filled channels to achieve high linearity, low hysteresis (5.3% hysteresis at 1 Hz), and good sensitivity (100% resistance increase at a 5 N load). The sensors’ gauge factors are stable at various temperatures and humidity levels. These bio-compatible, low hysteresis, and high linearity sensors are promising for safe and reliable diagnostic devices, wearable motion capture, and compliant human-computer interfaces.

Wearable and Stretchable Strain Sensors: Materials, Sensing Mechanisms, and Applications

“…Applications include smart rehabilitative devices, gesture recognition based on haptic feedback, soft robotic grippers, and in building artificial electronic skin (e-skin). [29,30,78,[201][202][203][204][205][206][207] For example, Guo et al built a stretchable optical strain sensor that can further be used for soft rehabilitative gloves and biomechanics research for ultrasensitive physiological activities. [20] Similarly, in minimally invasive surgery, there is a great demand to combine biocompatible soft sensors with surgical instruments for better navigation and control.…”

Flexible Hybrid Sensors for Health Monitoring: Materials and Mechanisms to Render Wearability

“…Liquid metals (e.g., Galinstan, eutectic gallium-indium (EGaIn)) have been widely utilized in wearable electronics (as interconnects or physical sensing elements) due to high conductivity, good electrical stability, high surface tension, and low toxicity. [15,[85][86][87][88][89][90][91][92] As an example of interconnect, EGaIn was used in a stretchable antenna which possesses reversible resonance frequency tuning while retaining high efficiency in radiation. [93]…”

“…Another conductive liquid of low toxicity, potassium iodide and glycerol solution, was introduced for strain and force sensor demonstrations. [91] This mixture exhibits good stability due to low evaporation rate, and the long chains of glycerol molecules, which prevent them from penetrating through the substrate matrix. In addition, this material hybrid possesses high thermal stability and low hysteresis owing to stable viscosity, which are important to the performance of wearable sensors.…”

Highly Filled Glycerol/Graphite Suspensions as Fluidic Soft Sensors and Their Responsive Mechanism to Shear

“…Besides the aforementioned soft solid sensors, flexible sensors fabricated with conductive liquid display the advantages of low cost, ease for large‐scale production and biocompatibility, which have been applied in wearable sensing devices as well. [ 10 ] Traditional liquid‐encapsulated flexible sensors mainly contain three types of conductive liquids: electrolytes made of soluble salts (such as KI‐glycerol [ 11 ] ), liquid metals [ 12 ] and ionic liquids. [ 13 ] The electrical resistance of the above sensors varies with the changed size of the conductive path during extension, twisting, or other forms of deformation.…”