You will be shown an abstract from a biomedical research paper. Given this abstract, your task is to extract all unique entities of the following types: ["Chemical"].

Please return the output as a JSON object of the format: {"Chemical": ["Arsenic", ...]}. The keys should be entity types and values should be lists of extracted entities belonging to the corresponding type. If you cannot find entities belonging to a specific type, the value should be [].

Only output the JSON object and do not include any additional text.

Abstract:

Optically healable polymers. Polymers that can easily be repaired after being damaged are attractive as this characteristic can improve the reliability, functionality, and lifetime of many products. In the last decade, researchers have thus developed new approaches to create stimuli-responsive polymer systems, which have the ability to autonomously heal or can be repaired upon exposure to an external stimulus. This review summarizes the current knowledge of optically healable or photo-healable polymers. The use of light as a stimulus for healing offers several attractive features, including the ability to deliver the stimulus locally, which opens up the possibility of healing the material under load, as well as the ability to tailor the wavelength of light to selectively address a specific component of the material, e.g. only the damaged parts. So far, two main classes of optically healable polymers have been explored, which are structurally dynamic polymers and mechanically activated reactive systems.
{"Chemical": []}