Bone-Like Hydrogel Boosts Robotics and Sensor Performance
Hydrogels have long been valued for their elasticity, but their relatively low mechanical strength has limited applications in demanding environments. To address this, researchers integrated graphene nanosheet-embedded carbon (GNEC) into a hydrophobic associative polyacrylamide (HAPAAm) hydrogel, creating a composite with bone-like mechanical properties. The process began with fabrication of the GNEC film using an electron cyclotron resonance (ECR) sputtering system under an 80 V substrate bias. Low-energy electrons facilitated vertical growth of graphene nanosheets on a SiO? substrate, as confirmed by transmission electron microscopy of focused ion beam-prepared cross sections. The film was then mechanically peeled, ground into ~10 ?m particles via high-frequency vibration, and dispersed into the hydrogel precursor.
Dispersion relied on sodium dodecyl sulfate (SDS) to adsorb onto the GNEC surface, exposing hydrophilic sulfate groups, while lauryl methacrylate (LMA) acted as a hydrophobic cross-linker. Acrylamide monomers were added, followed by ammonium persulfate (APS) and TEMED initiators to copolymerize the network. The resulting porous composite, filled with inorganic GNEC particles, mimicked the strain characteristics of bone and substantially improved tensile strength.
Microstructural analysis revealed a hierarchical architecture. Scanning electron microscopy showed an internal pore network, while TEM confirmed nanoscale graphene embedded in amorphous carbon. Raman spectroscopy indicated high-density edge structures, with the D-to-G peak ratio reflecting defect-rich graphene edges. Electron energy loss spectroscopy detected a ?* peak near 286 eV, confirming sp² bonding, and a ?* peak near 295 eV, indicating crystalline regions.
Mechanical testing demonstrated the composite’s capabilities. Increasing GNEC content raised stress limits from 1.4 kPa to 6.5 kPa and strain from 95% to 1685%. At 2.0 wt% GNEC and 40% water content, tensile strength reached 171 kPa—about 30 times higher than the freshly fabricated hydrogel—and supported a 500 g load. Water retention also increased with GNEC content due to enlarged pore structures. Extended finite element simulations showed that embedded GNEC films dissipated energy and prevented crack propagation under load.
These properties enabled practical integration into an underwater robot. The hydrogel served as an elastic retraction device, contracting after actuator-driven expansion to push water and generate thrust. The robot swam while carrying a 100 g payload. Fatigue testing over 100 cycles at 50% strain showed minimal degradation in tensile stress.
Electrical performance was equally notable. Stretching reduced electron mobility in the conductive network, but the hydrogel maintained a clear, rapid response—up to 150 ms—to strain changes. I–V measurements at varying strain confirmed piezoresistive behavior, and stability tests over ~500 cycles at 10% strain showed consistent resistance change rates. In a demonstration, the hydrogel connected to an LED exhibited visible brightness variation during stretching, underscoring sensitivity.
Wearable sensing applications were explored by mounting the hydrogel on fingers. Resistance changes tracked bending angles from 0° to 90°, returning to baseline upon straightening. I–V data at discrete angles validated accurate angle discrimination. Installing sensors on all five fingers enabled real-time gesture recognition, with distinct resistance profiles for each gesture.
By combining a bone-inspired microstructure with high-density graphene edges, the GNEC/HAPAAm hydrogel achieved a rare balance of mechanical robustness, fatigue resistance, and electrical responsiveness. Its performance in underwater robotics and wearable sensing highlights its potential for advanced actuation and monitoring systems where durability and sensitivity are paramount.
