Design and Development of Multi-Material Soft Vacuum Actuator for Humanoid Facial Expression Generation

Abstract Biomimetic humanoid robots are increasingly recognized for their potential in the medical and social domains. Achieving Action Unit (AU)-geometric approximated lifelike facial expressions in such robots requires small-scale in-place biomimetic soft actuators capable of replicating the motion patterns of corresponding human facial muscles. Mimicking human facial muscles requires a thin actuator capable of producing linear contraction to function effectively within constrained anatomical spaces; to address this, this work presents a multi-material soft vacuum actuator (M-SVA) inspired by the localized buckling behavior of beams. Under negative pressure, the actuator undergoes controlled buckling, causing its internal pneumatic cavities to collapse in a directed manner, resulting in longitudinal contraction. The development process involves a 3-D printing of the actuator core, elastomer molding of the skin cover, and their assembly. The numerical and experimental findings demonstrate that M-SVA provides several advantages, including high flexibility, large actuation strain (0.3–0.5), substantial actuation stress (0.053 MPa), and a notable blocked force of 5.53 N. Furthermore, M-SVA is readily scalable, allowing its deformation and output force to be tuned for targeted expressions by varying the core length and thickness, respectively. To demonstrate practical applicability, the actuators were placed in muscle-analog positions beneath a silicone facial skin on a humanoid head, generating intensities of AU12 and AU15 for happy and sad expressions, which support natural human–robot interaction.