Smart Core‐Shell Nanostructures for Force, Humidity, and Temperature Multi‐Stimuli Responsiveness

Abstract

stimuli and transmits the information to the brain. [1] In the last decade, a substantial understanding of how this complex system behaves has been gained. [2][3][4] Nevertheless, its replication to form artificial skins is still a relatively new field with massive potential. Relying on advancements in functional materials, structural design, and state-of-art production/deposition techniques, a wide variety of single/ multi-stimuli responsive sensory systems, suitable for electronic skin (e-skin) applications, have been reported. [5][6][7][8] An efficient e-skin design requires a combination of functional materials with suitable mechanical and electrical properties, [9] in addition to the micro/nanoscale control of the layer's thickness and dimensions, which is optimized by the choice of suitable fabrication techniques.For pressure and force detection, the most common methods exploit piezoelectric, piezoresistive, or capacitive sensing. [10][11][12][13] Bao's group investigated an e-skin design based on flexible pressure-sensitive organic thinfilm transistors deploying a force-sensitive gate dielectric capacitance. The sensor has a maximum sensitivity of 8.4 kPa −1 and a fast response time of 10 ms. This was realized with a combination of microstructured polydimethylsiloxane (PDMS) gate dielectric and a high-mobility semiconducting polymer in a transistor design. The sensor relies on capacitance change due to mechanical excitations. [14] Another pressure-sensitive e-skin design, investigated by Bao's group, is realized by a composite piezoresistive material consisting of an organic polymer and nickel nanostructured microparticles. [15] Park and Jang investigated hybrid piezoelectric/piezoresistive pressure sensors based on a nanohybrid material from graphene with free-standing nanofibers of PEDOT/P(VDF-HFP). Their e-skin device impresses with a gauge factor as high as 320 under tensile strain thus showing high sensitivity to pressure with a low limit of detection of 0.5 Pa only. [16] Humidity sensors for e-skin applications have also been investigated. [17,18] Guo et al. demonstrated that a tungsten sulfite (WS 2 ) film combined with graphene electrodes and PDMS substrate exhibits a high humidity response (up to 90% relative humidity or RH) due to the change in the WS 2 conductivity. [19] Similarly, e-skin sensitivity to changes in surrounding temperature is desired and has been investigated. [20][21][22] Chen et al.A force, humidity, and temperature-responsive electronic skin is presented by combining piezoelectric zinc oxide (ZnO) and poly-N-vinylcaprolactamco-di(ethylene glycol) divinyl ether hydrogel into core-shell nanostructures using state-of-the-art dry vapor-based techniques. The proposed concept is realized with biocompatible materials in a simplified design that delivers multi-stimuli sensitivity with high spatial resolution, all of which are prerequisites for an efficient electronic skin. While the piezoelectricity of ZnO provides sensitivity to external force, the thermoresponsiveness of the hydrogel...

Perspectives on recent advancements in energy harvesting, sensing and bio-medical applications of piezoelectric gels

“…Piezoelectric hydrogels with exceptional structural functionality and highly programmable properties have accelerated the development of sensing devices. 147,267–276 Tian and coworkers recently developed a self-powered motion sensor comprised of PHEMA, graphene oxide (GO), and single-walled carbon nanotubes (SWCNTs). 267 The degree of piezoelectricity and sensing properties of the nanocomposite hydrogel was determined by the surface area of the GO and the high strength of the SWCNTs.…”

“…Other interesting examples of polymeric gel-based PiezoGels were demonstrated for strain, humidity, force, and temperature sensing. 273–275 These polymeric gels, PVDF-polypyrrole-gelatin; 273 p(NVCL- co -DEGDVE)-ZnO, (poly- N -vinylcaprolactamco-di(ethylene glycol) divinyl ether, p(NVCL- co -DEGDVE); 274 PVDF-TrFE-CHACC (chitosan quaternary ammonium salt, CHACC), 275 displayed superior piezoelectric properties and sensing capabilities. In addition to these excellent reports, Wu and coworkers described the metal-ion-induced hierarchical structure and biological tissue-mimicking properties of polymer gels composed of P(AM-AN-MA)-Fe 3+ (acrylamide, AM; maleic acid, MA; ferric ion (Fe 3+ ).…”

Ultraflexible Organic Active Matrix Sensor Sheet for Tactile and Biosignal Monitoring

“…[9][10][11][12][13][14][15] These special properties allow unprecedented integration possibilities on complex-shaped objects, [16] and pave the way for the next generation of electronic skin (e-skin) like multi-stimuli responsive e-skins. [17,18] Meanwhile, e-skin is the basis for intuitive human-machine interfaces, [19] soft robotics, [3,20] prosthetics, [21] implantable, wearable, and physiological sensors, [22][23][24][25] and sensor networks for the internet of things [8] or structural health monitoring. [26] The use of e-skin sensors to monitor vital parameters such as pulse wave or respiratory rate at the point of care, that is, in hospitals, nursing homes, therapy wards, at home, or during sports ideally improves patient treatment, facilitates data acquisition and complements the work of the therapist.…”

Enhancement of the Sensing Performance of Devices based on Multistimuli-Responsive Hybrid Materials

“…The strong volume change stemming from the sharp LCST transition can be easily detected and converted into an electrical output with the use of electrical transduction, and therefore, it is extremely useful for sensors to achieve large signal amplitude and a fast response. Hydrogels based on biocompatible polymers with an LCST in the physiological temperature range are of particular interest for applications in tissue engineering, , biotechnology, or wearable electronics …”

“…In a previous work, we successfully combined a piezoelectric semiconductor material, namely, zinc oxide (ZnO), and a multistimuli-responsive hydrogel, namely poly( N -vinylcaprolactam- co -di(ethylene glycol) divinyl ether) or p(NVCL- co -DEGDVE) in core–shell nanorod structures capable of detecting force, humidity, and temperature. As depicted in Figure a, the stimuli detection is achieved through measuring the piezoelectric current generated upon deformation of the ZnO shell due to the hydrogel core swelling in response to humidity (max sensitivity S H = 1.2 nC % –1 to relative humidity (RH) in the range of 85–96% at 25 °C) and temperature (max sensitivity S T = 0.14 nC °C 1– response in the range 30–50 °C at 96% RH) . Additionally, the piezoelectric properties of ZnO allow direct detection of the applied force (max sensitivity S F = 36 pC N 1– ), with site-specific force sensing and a resolution down to 0.25 mm 2 .…”