Programmable Soft Actuation in Plasmonic Biopolymer Films

Biopolymer soft actuators capable of programmable, directional motion are essential for the advancement of adaptive soft robotics and intelligent responsive systems. However, existing soft actuators often suffer from weak directional control, poor spatial programmability, limited environmental adaptability, and an inability to operate under confined conditions. Here, we present a sustainable multi-stimuli-responsive soft actuator based on free-standing cassava starch films embedded with plasmonic gold (Au) and silver (Ag) nanoparticles that combines the intrinsic hygroscopicity of starch with plasmonic photothermal actuation. The hybrid films exhibit strong humidity-responsive deformation together with reproducible bending under green and blue laser irradiation. Notably, the nanoparticle-embedded films demonstrate excellent cyclic stability and reproducible light-responsive actuation over 1000 consecutive cycles. We show that laser-induced heating generates localized moisture gradients within the film that serve as the primary driving force for actuation. By engineering an asymmetric structural hierarchy with a polydimethylsiloxane (PDMS) layer, we achieve predictable bending pathways and spatially resolved shape transformations through multispot illumination. Notably, we demonstrate a trilayer Ag-PDMS-Au architecture that exhibits wavelength-selective bidirectional actuation: blue light induces bending toward the source, whereas green light drives bending away. We show that these actuators retain robust performance under confined environments and can be directly integrated into electrical circuits. This chromatically programmable mechanical response, combined with the ability to function in confined spaces, establishes plasmonic starch multilayers as a versatile and sustainable system for programmable soft actuation.