4arm-PEG-NH2 (4arm-PEG-Amine) is an amine-functionalized multi-arm PEG polymer that can be cross-linked into PEG hydrogels. It consists of an amine group combined with a carboxyl group (-COOH) or other amine-reactive chemical groups. PEG hydrogels have a variety of applications in medical devices and regenerative medicine, especially for controlled release of drugs, 2D and 3D cell culture, and wound closure and healing. It reacts with active esters such as NHS, TFP and PFP.