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:

Patchy surfaces stabilize dextran-polyethylene glycol aqueous two-phase system liquid patterns. This paper analyzes surface chemistry effects to stably pattern aqueous two-phase system (ATPS) droplets on chemically modified poly(dimethylsiloxane) (PDMS). Polyethylene glycol (PEG) and dextran (DEX) are used as phase-forming polymers for the ATPS. PDMS surface modifications studied include primary amine groups, carboxylic acid groups, and neutral polymer surfaces. The PDMS surfaces were characterized by fluorescent measurement, water and DEX contact angle measurements, and X-ray photoelectron spectroscopy (XPS) analysis to confirm surface properties. While homogeneous surfaces of different functional groups affect DEX droplet pinning somewhat, the most stable patterns were realized using surfaces with chemical heterogeneity. Arbitrary DEX solution patterning was achieved on a chemically patchy surface.
{"Chemical": ["PDMS", "primary amine", "Polyethylene glycol", "carboxylic acid", "poly(dimethylsiloxane)", "polyethylene glycol", "PEG"]}