Sandwich-like Polypyrrole/Reduced Graphene Oxide Nanosheets Integrated Gelatin Hydrogel as Mechanically and Thermally Sensitive Skinlike Bioelectronics

Currently, conductive composite hydrogels are promising alternatives for skinlike sensors. Generally, graphene as well as its derivatives and conducting polymers (CPs), such as polypyrrole (PPy), are employed as functional fillers, which critically hinge on simultaneously comprising skinlike mechanical compliance, high sensitivity for multistimuli, and ease of processing. Herein, to fully couple the superiorities of graphene nanosheets and CPs, sandwich-like PPy–rGO–PPy nanosheets were synthesized via a facile universal interface self-assembly reaction, which was an auto-redox reaction with Py as reductants and graphene oxide (GO) as oxidants, and were facilitated by hydrothermal treatment. The as-synthesized nanohybrids came out to be ultrathin (5 nm), flexible, surface wrinkled, and compliant with a large specific surface area, containing PPy-rich (over 50 wt %) and reduced GO, which are highly conductive (100 S m–1). Natural gelatin was used as the flexible supporting matrix to enable the good dispersion of PPy–rGO–PPy nanosheets into the composite hydrogel, which exhibits skinlike mechanical compliance. Indeed, the incorporated PPy–rGO–PPy nanosheets provide the composite hydrogel with a stable conducting network and can perceive and instantly convert multistimuli into electric signal variations. As expected, these hydrogel-based sensors delivered high strain sensitivity (gauge factor ≥ 1.98) within the strain range of 0–200%, low limit of detection (≤0.1% strain), excellent pressure perception (S = −3.573%/kPa) within the pressure range of 0–10 kPa, and superior thermosensation (temperature coefficient of resistance = −1.288%/°C) within the range of human body temperature variations (35–40 °C) and perceptible temperature range (20–80 °C). Moreover, these sensors applied to monitor human body movements and simulate human body thermoperception display highly reliable electrical signal read-outs. Finally, the self-structured sandwich-like PPy–rGO–PPy nanosheets as functional fillers in the conductive composite hydrogel are relatively economical in methodology, novel in material combination, and processed with ease, which will allow for relatively low cost and scaled-up preparations of flexible bioelectronics and biosensors.