A stretchable strain sensor includes a light-emitting element, an optical structure, and a photo-detective element. The stretchable strain sensor is located in a path of light emitted from the light-emitting element. The optical structure is configured to have optical properties that change in response to stretching of at least a portion of the stretchable strain sensor. The photo-detective element is configured to detect light transmitted through the optical structure or reflected through the optical structure.