Stretchable, Skin‐Mountable, and Wearable Strain Sensors and Their Potential Applications: A Review

Abstract

wileyonlinelibrary.comSkin-mountable and wearable sensors can be attached onto the clothing or even directly mounted on the human skin for the real-time monitoring of human activities. [ 10 ] Besides their high effi ciency, they must fulfi ll several minimum requirements including high stretchability, fl exibility, durability, low power consumption, biocompatibility, and lightweight. These demands become even more severe for epidermal electronic devices where mechanical compliance like human skin and high stretchability ( ε > 100% where ε is the strain) are required. [ 11,12 ] Recently, several types of skin-mountable and wearable sensors have been proposed by using nanomaterials coupled with fl exible and stretchable polymers. Indeed, nanomaterials are utilized as functional sensing elements owing to their outstanding electrical, mechanical, optical, and chemical properties while polymers are employed as fl exible support materials thanks to their fl exibility, stretchability, humanfriendliness, and durability. [ 13 ] Examples of those innovative sensors include strain sensors, [ 10,[12][13][14] pressure sensors, [ 5,[15][16][17] electronic skins (e-skins), [ 3,[16][17][18][19][20] and temperature sensors. [ 2,21,22 ] Particularly, various skin-mountable and wearable strain sensors have been developed because of their broad applications in personalized heath-monitoring, human motion detection, human-machine interfaces, and soft robotics. [ 10,12,[23][24][25][26][27][28][29][30][31] This paper aims to survey fabrication processes, working mechanisms, strain sensing performances, and applications of stretchable strain sensors. The article is organized as follows: fi rst, common operation mechanisms of stretchable strain sensors are described. Here, we summarize novel functional nanomaterials and techniques for the fabrication of stretchable strain sensors in details. Second, mechanisms involved in the strain-responsive behavior of resistive-type and capacitive-type sensors are explained. We show that the infl uence of traditional mechanisms like geometrical changes and piezoresistivity of materials on the strain sensing performance of fl exible strain sensors are very small whereas mechanisms such as disconnection between sensing elements, crack propagation in thin fi lms, and tunneling effect can potentially be employed for highly stretchable and sensitive strain sensing. Third, we emphasize the performance parameters of stretchable strain sensors in terms of stretchability, sensitivity, linearity, hysteresis behavior, response time, overshooting, and durability. We demonstrate

3D Printing of Transparent and Conductive Heterogeneous Hydrogel–Elastomer Systems

“…[5][6][7][8] Although many stretchable conductors exist, including liquid metals, [9,10] nanowires, [11,12] nanoribbons [13] , pre-stretched elastomer fibers with conductive coatings, [14] and micro-cracked metals, [15,16] these materials have generally been unable to achieve high levels of optical transparency while maintaining high conductivities and stretchability; a feature that would enable their use in optogenetics [17] or allow optical imaging of the underlying substrate. Conventional strategies of incorporating metallic components with elastomers to attain stretchability also yield non-trivial failure modes such as liquid metal leakage [8] and hard-soft material interfacial failure [18] .…”

“…If the field is to progress, advanced manufacturing techniques that integrate dielectric elastomers and hydrogels need to be developed. [7] Fabrication techniques specific to hydrogels have already been developed, including extrusion three-dimensional (3D) printing, [24,[32][33][34][35] digital projection based techniques, [36] and screen printing [37,38] . Extrusion printing techniques in particular are most easily capable of multi-material printing at high resolution and low costs.…”

Nanomaterial‐Enabled Wearable Sensors for Healthcare

“…[41] Capacitive strain sensors possess good linearity with low hysteresis, fast response, and are less susceptible to overshoot and creep. [107] While capacitive strain sensors exhibit smaller GFs than the resistive strain sensors, they are ideal for applications where the strain is relatively large. In addition, the GFs of capacitive strain sensors remain constant in the entire strain range.…”

“…While most resistive strain sensors exhibit large stretchability and high sensitivity, many suffer from nonlinear response, large hysteresis, and irreversibility. [107] When the applied strain is released, it is difficult for the nanomaterials to slide back to the initial position or for the cracks to be completely closed. Due to the viscoelasticity of polymers and the friction between the nanomaterials and the polymer matrix during loading and unloading, rearrangement of nanomaterials Polyaniline doped AuNW/Latex rubber [68] Resistive 2-12 for 0-100% 149.6% -Human finger-controlled robotic arm system AuNW/Latex rubber [44] Resistive 9.9 (0-5%), 6.9 (5-50%) 300% 0.01% Data glove, monitoring of forearm muscle movement, cheek movement, phonation, and wrist pulse AgNW/Ecoflex/AgNW [41] Capacitive 0.7 50% -Monitoring of finger bending and knee motion CNT/Ecoflex/CNT [25] Capacitive 0.4 50% --CNT/silicone/CNT [127] Capacitive 0.99 100% -Robotic linkage CNT/Dragon Skin/CNT [129] Capacitive 1 300% -Date glove, monitoring of balloon inflation and chest movement PVDF-TrFE with graphene FET [134] Piezoelectric 389 ≈0.3% 0.008% Monitoring of hand movement ZnO NW array [132] Piezoelectric 1813 0.8% --Carbon fiber-ZnO NW [217] Piezoelectric 60-80 1.2% 0.2% -www.advancedsciencenews.com www.advhealthmat.…”

“…For strain sensors based on nanomaterial composites, the resistance change originates from four mechanisms: (1) the change in the geometry of the nanomaterials themselves and the composites; (2) change in resistivity of nanomaterial fillers due to the intrinsic piezoresistive effect (second term in Equation (1)); (3) strain induced separation of neighboring nanomaterials and the resulting modified tunneling resistance for nanomaterials not electrically conducting with each other; [105,106] (4) decreased number of percolation pathways between nanomaterials, mainly due to disconnection (or sliding) between nanomaterials or propagation of microcracks under strain, which leads to increased contact resistance. [107] Separation and/or sliding of nanomaterials under strain (especially large strain) results in small strain experienced by the nanomaterials themselves. In this case the resistance change due to the first two mechanisms is relatively small, while that due to the last two mechanisms becomes dominant.…”