Dual Cross-Linked Ion-Based Temperature-Responsive Conductive Hydrogels with Multiple Sensors and Steady Electrocardiogram Monitoring

Abstract

To balance the requirements of transparency, mechanical strength, stable conductivity, and biocompatibility of traditional electronic conductive hydrogels in intelligent devices is still a formidable challenge. The increase of ionic conductive gels has provided decent transparency, stretchability, and wearability in artificial skins but the dilemma still exists between stability and functionality. This article reports a facile strategy to develop a visual thermosensitive physically and chemically dual cross-linked ion-based conductive hydrogel through in situ free-radical copolymerization, achieving robust mechanical properties, an obvious response, and a multiple sensing process. As an archetypical template, the ion-based conductive hydrogel offers an evaluation and monitoring of electrocardiogram (ECG), which is comparable to commercial electrodes. Intriguingly, such kinds of conductive gels exhibit tunable upper critical solution temperature (UCST) behaviors. Taking advantage of their high temperature responsive accuracy, the visualized qualitative observation for smart response and digitized measurement and calibration for thermal stimulus can be simultaneously achieved. We therefore believe that this work will inspire the design of skinlike sensing materials, promote the preparation of biocompatible and multiple sensing abilities and the synthesis of intelligent hydrogels, and realize their applications in biosensors, wearable devices, and biomedicine.

Highly Sensitive On‐Skin Temperature Sensors Based on Biocompatible Hydrogels with Thermoresponsive Transparency and Resistivity

“…The ionic conductivity of the TRHs with increasing the content of PAAc polymer was increased from 0.1189 to 0.2523 S m −1 due to an increase in the number of a mobile abundant proton, and showed a similar value (0.41 S m −1 ) to previous literature. [ 25 ] To be able to make accurate measurements at various temperatures, a temperature sensor should be insensitive to humidity. The various TRHs stored at various fixed humidity levels indeed did not show any noticeable changes in bulk resistance (Figure S8, Supporting Information), confirming its insensitivity to humidity.…”

“…This −0.0289°C −1 sensitivity value was found to be several times higher than previously reported values using other hydrogels displaying intrinsic conductivity and similar to those using hydrogels with ionic additives or volume changes (Table S1, Supporting Information). [11][12][13][14][15][16][17][18]24,25] The temperature sensor of TRH 1-1 showed a reversible change in resistance under temperature cycles with no significant hysteresis, as shown in Figure S9 (Supporting Information) (less than 1% change in resistance corresponding to each temperature during heating and cooling).…”

“…Hydrogels have been recently regarded as promising candidates for use in temperature and physicochemical sensors capable of being mounted on human skin because of their biocompatibility and intrinsic flexibility. [10] Numerous previous works have been devoted to fabricating wearable temperature sensors based on hydrogels that can vary their electrical resistance as a result of changes in ion conductivity [11][12][13][14][15]24,25] or volume due to swelling/contraction with temperature. [16][17][18] Thermoresponsive hydrogels (TRHs) have played a crucial role in a variety of temperature-adaptive applications for actuators, [19] membranes, [20] displays, [21] and electrolytes [22] due to their intelligent thermo-responsive transitions.…”

“…[24] Tan et al fabricated a dual cross-linked hydrogel temperature sensor and ECG monitor consisting of a UCST hydrogel and ionic salts. [25] However, some ionic molecules may influence the function of epidermal keratin, and hence lead to an inhibition of the secretion of sweat and occlusion of sebaceous gland ducts. [26] So far, a demonstration of a highly sensitive onskin temperature sensor made of biocompatible conductive hydrogels and displaying simultaneous tunability of transparency and resistivity has not been reported.…”

“…Hydrogels have been recently regarded as promising candidates for use in temperature and physicochemical sensors capable of being mounted on human skin because of their biocompatibility and intrinsic flexibility. [ 10 ] Numerous previous works have been devoted to fabricating wearable temperature sensors based on hydrogels that can vary their electrical resistance as a result of changes in ion conductivity [ 11–15,24,25 ] or volume due to swelling/contraction with temperature. [ 16–18 ]…”