Fig. 1: Sensor-based classification of SSIs and their deployment form factors.

a, Sensor modalities in SSIs can be categorized, by the proximity of signal acquisition to the human body, as either off-body device interfaces (for example, optical cameras or ultrasound probes), on-body sensor interfaces (for example, surface EMG, strain sensors, EEG and IMUs) or in-body neural interfaces (for example, ECoG, sEEG or MEAs). These categories represent a continuum between user comfort and signal specificity, ranging from general-purpose wearable devices to highly personalized clinical systems. b, Representative deployment examples for each category, aligned with the same taxonomy: off body (smartphones, earbuds and smart glasses), on body (facial patch/tattoo, choker/throat patch, mask and EEG hat) and in body (ECoG, sEEG and MEA implants). The asterisks denote technologies with clinical validation.