N-Me-N-bis(PEG4-acid) HCl salt is a crosslinker derivative containing two PEG4-acid groups. The two terminal carboxylic acid groups can be reacted with primary amines in the presence of activators such as EDC and DCC to form stable amide bonds. The hydrophilic PEG linkers increase the water solubility of the compound. The water solubility properties of the PEG linker are enhanced with longer PEG chains.