Skin-Contactable and Antifreezing Strain Sensors Based on Bilayer Hydrogels

Abstract

Currently, most hydrogel sensors require an isolation layer to prevent the current from damaging the skin. However, the mismatch of the mechanical property between the isolation layer and the hydrogel sensor may affect the accuracy of the sensing. Herein, a bilayer hydrogel sensor consisted of a nonconductive layer and a conductive layer, which were prepared through twice freeze–thawing methods. The nonconductive layer which could directly come in contact with the skin was composed of polyvinyl alcohol (PVA) and glycerin (GL), and the conductive layer for conducting current and perceiving strain was composed of PVA, GL, and polyaniline (PAni). Unlike ions, PAni was difficult to diffuse in the hydrogels, which had two layers and exhibited significant difference in conductivity. When the voltage was applied, the bilayer hydrogel could protect the skin from irritation and injury. More importantly, the mechanical property of the two layers was close, enabling the bilayer hydrogel to detect strain effectively. Compared with a single-layer hydrogel sensor, the response and recovery time of the bilayer hydrogel were reduced by 14.8 and 8%, and the accuracy was improved by 32.1%. This unique strategy provides novel inspiration for the development of fast responsive and skin-protective hydrogel strain sensors.

Gradient adhesion modification of polyacrylamide/alginate–calcium tough hydrogels

“…25–27 Moreover, the strains of the adhesive layer and tough layer are different under the same stress, which will affect the sensitivity of the hydrogel strain sensor. 28 Compared with a bilayer structure, a gradient structure can avoid these problems. 29 Mooney et al reported gradient adhesive–tough hydrogels by introducing chitosan and other polymers into polyacrylamide/alginate–calcium tough hydrogels to form a positively charged adhesive surface (about 40 μm depth).…”

An Ultra-Stretchable Sensitive Hydrogel Sensor for Human Motion and Pulse Monitoring

“…Thus, we need to find a balance between conductivity and mechanical properties by tuning the ratio of HEDP according to specific application requirements. We further compared our results with previous studies that demonstrate hydrogels with excellent mechanical toughness or conductivity [ 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 ], as shown in Figure 2 e,f. It is also worth mentioning that our PVA-HEDP hydrogels exhibited excellent conductivity and mechanical performance simultaneously.…”

“…( d ) Compressive stress–strain curves of the hydrogels with various HEDP contents. ( e , f ) Comparison of electrical and mechanical properties with previous research work [ 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 ].…”

Recent Progress in Bionic Skin Based on Conductive Polymer Gels

“…Delocalized 𝜋-electrons act as charge carriers moving freely along the unsaturated backbone. Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), [31,33,34] polyaniline (PANi), [69,70] and polypyrrole (Ppy) [35] are typical conjugated conductive polymers, with conductivities comparable to those of semiconductors and metals. Conventional conjugated conductive polymers with modulus in order of 1 GPa are not much stretchy, thus, mechanically mismatch skin.…”