Fig. 3: Representative sensor modalities used in SSIs.

a–i, Schematics illustrating key sensing technologies enabling silent speech decoding, categorized by sensing principle: optical sensing (a; a smartphone-based front camera detects articulatory motion, such as lip and jaw movements); ultrasonic sensing (b; an ultrasound imaging probe beneath the jaw visualizes tongue motion in real time); IMU sensing (c; IMUs distributed at the head, lip and chin track multi-point facial kinematics during articulation); triboelectric sensing (d; self-powered wearable sensors detect facial motion through contact electrification); EMG (e; tattoo-like epidermal electrodes acquire facial myopotentials associated with silent articulation); strain sensing (f; textile strain sensors embedded in a smart choker capture throat deformation patterns); EEG (g; in-ear conformal bioelectronics measure brain activity associated with speech imagery); ECoG (h; implanted cortical arrays decode neural activity from speech-generating regions in patients with brainstem injury); and MEA (i; intracortical electrodes implanted in the motor cortex record neural activity associated with fine motor intention, enabling high-resolution decoding of attempted handwriting and speech imagery). PI, polyimmide; PVC, polyvinyl chloride. Panels adapted from: d, ref. ³³, CC BY 4.0; e, ref. ³⁶, CC BY 4.0; f, ref. ⁴², CC BY 4.0; g, ref. ⁴⁷, CC BY 4.0.