Crosslinking agents include chemicals such as glutaraldehyde, dithiobis(succinimidyl) propionate, carbodiimide, and N-succinimidyl-3-(2-pyridyldithio) propionate (SPDP), 1,4-Bis(acryloyl)piperazine, N-Hydroxysulfosuccinimide, as well as electromagnetic radiation such as UV radiation.