Precise stimulation of near-infrared light responsive shape-memory polymer composites using upconversion particles with photothermal capability

The incorporation of thermally-induced shape-memory polymer (SMP) with photothermal fillers has been widely used in creating photoresponsive SMP composites (SMPCs). Near-infrared (NIR) light, which is safer for human tissues and naked eyes, has been widely used to trigger such intelligent SMPCs containing carbon nanomaterials, metal nanoparticles, or rare earth organic complexes. There is still a need to aim invisible NIR light beam remotely onto SMPCs to realize the precise shape recovery of their featured areas. Here, NaYF4:99.5%Yb3+, 0.5%Tm3+ particles presenting both upconversion and photothermal capabilities were utilized as multifunctional fillers in a crosslinked copolymer of methyl methacrylate and butyl acrylate, enabling the prepared SMPCs to transfer the NIR light at 980 nm simultaneously into both visible light and heat. The particles with a low content up to 1 phr did not vary the crosslinking level and glass transition temperature of the SMPC. With the aid of upconversion at a relatively low power density, the position of laser beam on SMPC surface was detected easily, facilitating the aim towards the area anticipated to be triggered without inducing a shape deformation. The subsequent increase in power density successfully resulted in the precise shape recovery with the recovery ratio higher than 90%. The concept of precise location before stimulation was demonstrated in the cases of multiple shape deformations, remote activation in the darkness, and microscale structured surfaces. The reported multifunctional nanoparticles truly executed the remote and precise trigger of SMPCs using invisible NIR light, which can be further exploited in other thermally-induced smart polymer composites.