Conductive hydrogels have attracted attention because of their wide application in wearable devices. However, it is still a challenge to achieve conductive hydrogels with high sensitivity and wide frequency band response for smart wearable strain sensors. Here, we report a composite hydrogel with piezoresistive and piezoelectric sensing for flexible strain sensors. The composite hydrogel consists of cross-linked chitosan quaternary ammonium salt (CHACC) as the hydrogel matrix, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS) as the conductive filler, and poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) as the piezoelectric filler. A one-pot thermoforming and solution exchange method was used to synthesize the CHACC/PEDOT: PSS/PVDF-TrFE hydrogel. The hydrogel-based strain sensor exhibits very high sensitivity (GF: 19.3), fast response (response time: 63.2 ms), and wide frequency range (response frequency: 5−25 Hz), while maintaining excellent mechanical properties (elongation at break up to 293%). It can be concluded that enhanced strain-sensing properties of the hydrogel are contributed to both greater change in the relative resistance under stress and wider response to dynamic and static stimulus by adding PVDF-TrFE. This has a broad application in monitoring human motion, detecting subtle movements, and identifying object contours and a hydrogel-based array sensor. This work provides an insight into the design of composite hydrogels based on piezoelectric and piezoresistive sensing with applications for wearable sensors.

In recent years, hydrogels have attracted extensive attention in smart sensing owing to their biocompatibility and high elasticity. However, it is still a challenge to develop hydrogels with excellent multiple responsiveness for smart wearable sensors. In this paper, a facile synthesis of carbon quantum dots (CQDs)-doped cross-linked chitosan quaternary/carboxymethylcellulose hydrogels (CCCDs) is presented. Designing of dual network hydrogels decorated with CQDs provides abundant crosslinking and improves the mechanical properties of the hydrogels. The hydrogel-based strain sensor exhibits excellent sensitivity (gauge factor: 9.88), linearity (R 2 : 0.97), stretchable ability (stress: 0.67 MPa; strain: 404%), good cyclicity, and durability. The luminescent properties are endowed by the CQDs further broaden the application of hydrogels for realizing flexible electronics. More interestingly, the strain sensor based on CCCDs hydrogel demonstrates photo responsiveness (𝚫R/R 0 ≈20%) and pH responsiveness (pH range ≈4-7) performance. CCCDs hydrogels can be used for gesture recognition and light sensing switch. As a proof-of-concept, a smart wearable sensor is designed for monitoring human activities and detecting pH variation in human sweat during exercise. This study reveals new possibilities for further applications in wearable health monitoring.

Currently, sensors based on hydrogels are less sensitive at low temperatures, and discarded sensors are prone to generate e-waste. Thus, it is challenging to design and prepare novel hydrogels with good properties for wearable sensors. Herein, we report a sandwich-structure liquid metal organic hydrogel with low-temperature sensitive, recyclable and multiple sensation properties. Liquid metal (LM)/cross-linked chitosan quaternary ammonium salt (CHACC) composite is prepared by evaporating ethanol and water co-solvent as the core layer. CHACC is used as the top and the bottom layer respectively. The optimal performance of the organic hydrogel is obtained when the volume ratio of ethanol to water is 1:3. The LM/CHACC organic hydrogel exhibits excellent stretchability (strain: 428.37%). Importantly, it has fast responsiveness, recyclability and excellent low-temperature sensitivity (GF = 8.29) at À20 C. The sensors possess stress-temperature sensation with high sensitivity. Moreover, LM as the conductive filler is recyclable under acidic conditions. We demonstrate the functionality of the LM/CHACC organic hydrogel to detect human joint movements and fine movements at low temperatures. It is anticipated to open up new sensing applications based on LM organic hydrogel-based wearable sensors.

A series of Eu 0.5 Tb 0.5 (TTA) 3 Phen/PMMA (TTA=thenoyltrifl uoroacetone, Phen=phenanthroline) and Eu 0.5 Tb 0.5 (TTA) 3 Dipy/PMMA (Dipy=2,2'-dipyridyl) were prepared by in-situ polymerization. The structures of the composites were characterized by IR spectra and electron spectrum. Photoluminescence properties were investigated by UV-Vis spectra and fl uorescence spectra. Meanwhile, the energy transfer models were set up. The results indicated that polymer parts were attached with the rare-earth molecular parts in the composite luminescent materials. Eu 0.5 Tb 0.5 (TTA) 3 Phen/PMMA and Eu 0.5 Tb 0.5 (TTA) 3 Dipy/PMMA emitted mostly characteristic fl uorescence of europium ion and intense red fl uorescence with peak wavelength at 611.8 nm and bandwidth of 10.4 nm under UV excitation at 365 nm. Fluorescence intensity of Eu 0.5 Tb 0.5 (TTA) 3 Phen/PMMA was found to be infl uenced with the content of MMA. The fl uorescence emission of europium ions was greatly sensitized by terbium ions and the enhancement of red emission was most likely due to the energy transfer enhancement from Tb 3+ to Eu 3+ .