As illustrated in Fig. , SU-8 polymerization is activated by epoxide ring opening in the presence of protons (acids). When used as a photoresist, SU-8 is mixed with up to 10% Triarylsulfonium hexafluoroantimonate salt as a photoacid generator. Ring opening after protonation of the epoxide groups results in a carbocation. Polymerization is due to the subsequent reaction of the carbocation with other epoxide groups. Each carbocation can participate in polymer-ization reactions with multiple epoxide groups. This makes SU-8 a highly photo-sensitive chemically amplified photoresist, as a single photon generating a single proton leads to multiple polymerization events. Each SU-8 monomer has 8 epoxide functional groups and thus can be linked with up to 8 other SU-8 monomers, yielding a highly cross-linked and dense structure unlike that of most linear polymers. The weakest link in polymerized SU-8 are the C-O-C single (ether) bonds between SU-8 units resulting from epoxide ring opening and crosslinking. These are extremely inert bonds, only slightly weaker than the diamond-like (sp 3 ) and graphene-like (sp 2 ) C-C bonds in the rest of the SU-8 structure.