PEGSSDA™
In the past, large amounts of proteases and enzymes were required to digest Extracel™ and HyStem™ hydrogels for several hours to recover encapsulated cells. These harsh conditions can potentially damage sensitive cells. In addition, introduction of animal-based enzymes may hinder clinical applications of the cells where growth in an animal-free milieu is important. PEGSSDA™ now allows a researcher to quickly and gently dissolve Extracel or HyStem non-enzymatically, thereby easily recovering cells either grown on top or within the hydrogel. PEGSSDA contains two internal disulfide bonds which are rapidly reduced with small amounts of reducing agent (such as N-acetyl cysteine or glutathione), causing gel liquefaction.
PEGSSDA is a pentablock PEGDA molecule composed of four PEG molecules (MW 2000 Da) and two interdigitated disulfide bonds. Only the external PEG molecules are acrylated (see below). The final molecular weight is approximately 8500 Da1. Like PEGDA, PEGSSDA is a thiol-reactive crosslinker that covalently reacts with the thiol groups in Glycosil™, HyStem™, Heprasil™, and Gelin-S™ to form viscoelastic hydrogels1. PEGSSDA is lyophilized in water.
Gelation
PEGSSDA is used to chemically crosslink Glycosil, HyStem, Heprasil, and Gelin-S. The gelation time ranges from as short as 5 minutes1 to as long as a couple of hours, depending upon the amount of PEGSSDA used and the concentration/dilution of the Glycosil, HyStem, Gelin-S and /or Heprasil. While we use thiol chemistry to crosslink our hydrogels, other researchers have used photoinitiators such as Darocur 1173. This process adds an extra level of complexity; however, as many photoinitiators are toxic to cells and the UV light used to crosslink the hydrogel can damage cells.