lic ▁acid ▁concluded ▁the ▁P EG ▁and ▁P EG - am ine ▁surface ▁groups ▁allowed ▁for ▁the ▁greatest ▁pen etr ation ▁of ▁particles . ▁The ▁car box y lic ▁acid ▁co ated ▁particles ▁did ▁not ▁pen et rate ▁past ▁the ▁str atum ▁cor neum . ▁ ▁In cre asing ▁per me ability ▁ ▁Scient ists ▁previously ▁believed ▁that ▁the ▁skin ▁was ▁an ▁effective ▁bar rier ▁to ▁in organ ic ▁particles . ▁Dam age ▁from ▁mechanical ▁stress ors ▁was ▁believed ▁to ▁be ▁the ▁only ▁way ▁to ▁increase ▁its ▁per me ability . ▁ ▁Rec ently , ▁simpler ▁and ▁more ▁effective ▁methods ▁for ▁increasing ▁skin ▁per me ability ▁have ▁been ▁developed . ▁Ult rav io let ▁radiation ▁( U V R ) ▁slightly ▁dam ages ▁the ▁surface ▁of ▁skin ▁and ▁causes ▁a ▁time - dependent ▁defect ▁allowing ▁easier ▁pen etr ation ▁of ▁nan op articles . ▁The ▁U V R ' s ▁high ▁energy ▁causes ▁a ▁rest ruct uring ▁of ▁cells , ▁weak ening ▁the ▁boundary ▁between ▁the ▁str atum ▁cor neum ▁and ▁the ▁e pid erm al ▁layer . ▁The ▁damage ▁of ▁the ▁skin ▁is ▁typically ▁measured ▁by ▁the ▁tr anse pid erm al ▁water ▁loss ▁( TE W L ), ▁though ▁it ▁may ▁take ▁ 3 – 5 ▁days ▁for ▁the ▁TE W L ▁to ▁reach ▁its ▁peak ▁value . ▁When ▁the ▁TE W L ▁reaches ▁its ▁highest ▁value , ▁the ▁maximum ▁density ▁of ▁nan op articles ▁is ▁able ▁to ▁per me ate ▁the ▁skin . ▁While ▁the ▁effect ▁of ▁increased ▁per me