Liquid Touch: Soft and Wearable Vibrotactile Actuators Based on Room Temperature Liquid Metals

The human sense of touch is vital to how we understand the world, and interfacing with this sense through haptic technologies has become increasingly important in applications ranging from entertainment to medical rehabilitation. Although wearable versions exist for many common haptic actuators, current technologies are often either heavy and bulky or tethered to external actuation systems such as pumps limiting long‐term wearability. Here, we demonstrate a wearable vibrotactile electromagnetic actuator based on room temperature liquid metal coils. Fine‐gauge silicone tubing is used to create the liquid metal coil using wire‐winding combined with silicone molding and filled with a nontoxic gallium alloy. Initial acoustic measurements of the actuator, intentionally focused in the 100–200 Hz range commonly studied in vibrotactile stimulation in human wrist, suggested a resonance near 140 Hz. To more fully capture the mechanical response, we subsequently performed laser displacement measurements with broadband chirp excitations, which revealed a fundamental resonance at approximately ∼40 Hz in agreement with an analytical membrane model, while also confirming the presence of higher order resonant modes in the 100–200 Hz range relevant for human perception. The actuator is also capable of operating under mechanical strains of up to 40% and to remain within safe thermal limits (heating to no greater than 35°C) under operation. Human testing of the minimum sensing threshold was performed on ten human volunteers and the haptic device was found to follow a typical U‐shaped profile showing a promising capability for almost fully soft electromagnetic haptics.