Abstract
We demonstrate the fabrication of novel reduced graphene oxide (rGO)-based double network (DN) hydrogels through the polymerization of poly(N-isopropylacrylamide) (PNIPAm) and carboxymethyl chitosan (CMC). The facile synthesis of DN hydrogels includes the reduction of graphene oxide (GO) by CMC, and the subsequent polymerization of PNIPAm. The presence of rGO in the fabricated PNIPAm/CMC/rGO DN hydrogels enhances the compressibility and flexibility of hydrogels with respect to pure PNIPAm hydrogels, and they exhibit favorable thermoresponsivity, compressibility, and conductivity. The created hydrogels can be continuously cyclically compressed and have excellent bending properties. Furthermore, it was found that the hydrogels are pressure- and temperature-sensitive, and can be applied to the design of both pressure and temperature sensors to detect mechanical deformation and to measure temperature. Our preliminary results suggest that these rGO-based DN hydrogels exhibit a high potential for the fabrication of soft robotics and artificially intelligent skin-like devices.
Highlights
Hydrogels—wet and soft materials that consist of water and cross-linked three-dimensional (3D) networks—have been proven to possess several unique properties, such as stimuli response, shock absorption, swelling, and conductivity [1,2,3]
It is clear that the graphene oxide (GO) nanosheets exhibited an extended thin film with a wrinkled surface, which prevented the collapse of GO back into a graphite structure [23]
We suggest that the increased thickness of the graphene sheets indicates that GO nanosheets were covered by carboxymethyl chitosan (CMC)
Summary
Hydrogels—wet and soft materials that consist of water and cross-linked three-dimensional (3D) networks—have been proven to possess several unique properties, such as stimuli response, shock absorption, swelling, and conductivity [1,2,3]. The created smart hydrogels can be utilized in various fields, including photochemistry, sensors, tissue engineering, biomedical engineering, and materials science [4,5,6]. Hydrogels are mostly brittle, with a fracture energy of. 1–10 J/m2 and an elastic modulus of around 10 kPa [7,8] Both values are much lower than those of cartilage in the body—which has a fracture energy of 16 J/m2 [9] and an elastic modulus of around 0.32 GPa [10]. The formation of double network (DN) hydrogels is one of the potential ways to significantly improve the mechanical properties of hydrogels. Gong and co-workers developed DN hydrogels (PAMPS-PAAm) including poly(2-acrylamido-2-methylpropanesulfonic acid) (PAMPS) and polyacrylamide (PAAm) with enhanced mechanical strength [11,12]. The fabricated DN hydrogels exhibited fracture toughness, fracture tensile stress, and fracture tensile strain of 102 –103 J/m2 , Sensors 2018, 18, 3162; doi:10.3390/s18093162 www.mdpi.com/journal/sensors
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