Underwater adhesion represents a huge technological challenge as the presence of 
water compromises the performance of most commercially available adhesives. 
Inspired by natural organisms, we have designed an adhesive based on complex 
coacervation, a liquid-liquid phase separation phenomenon. A complex coacervate 
adhesive is formed by mixing oppositely charged polyelectrolytes bearing pendant 
thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) chains. The material fully 
sets underwater due to a change in the environmental conditions, namely 
temperature and ionic strength. In this work, we incorporate silica 
nanoparticles forming a hybrid complex coacervate and investigate the resulting 
mechanical properties. An enhancement of the mechanical properties is observed 
below the PNIPAM lower critical solution temperature (LCST): this is due to the 
formation of PNIPAM-silica junctions, which, after setting, contribute to a 
moderate increase in the moduli and in the adhesive properties only when 
applying an ionic strength gradient. By contrast, when raising the temperature 
above the LCST, the mechanical properties are dominated by the association of 
PNIPAM chains and the nanofiller incorporation leads to an increased 
heterogeneity with the formation of fracture planes at the interface between 
areas of different concentrations of nanoparticles, promoting earlier failure of 
the network-an unexpected and noteworthy consequence of this hybrid system.