▁three ▁spatial ▁dimensions . ▁One ▁study ▁compared ▁the ▁two ▁shapes ▁and ▁recorded ▁data ▁that ▁showed ▁spher ical ▁particles ▁located ▁deep ▁in ▁the ▁e pid erm is ▁and ▁der mis ▁whereas ▁elli ps oid al ▁particles ▁were ▁mainly ▁found ▁in ▁the ▁str atum ▁cor neum ▁and ▁e pid erm al ▁layers . ▁Nan or ods ▁are ▁used ▁in ▁experiments ▁because ▁of ▁their ▁unique ▁flu ores cent ▁properties ▁but ▁have ▁shown ▁medi oc re ▁pen etr ation . ▁ ▁Nan op articles ▁of ▁different ▁materials ▁have ▁shown ▁skin ' s ▁per me ability ▁limitations . ▁In ▁many ▁experiments , ▁gold ▁nan op articles ▁ 4 0   nm ▁in ▁diameter ▁or ▁smaller ▁are ▁used ▁and ▁have ▁shown ▁to ▁pen et rate ▁to ▁the ▁e pid erm is . ▁T itan ium ▁ox ide ▁( T i O 2 ), ▁z inc ▁ox ide ▁( Z n O ), ▁and ▁silver ▁nan op articles ▁are ▁in effect ive ▁in ▁pen etr ating ▁the ▁skin ▁past ▁the ▁str atum ▁cor neum . ▁Cad m ium ▁s elen ide ▁( C d Se ) ▁quantum ▁d ots ▁have ▁proven ▁to ▁pen et rate ▁very ▁effectively ▁when ▁they ▁have ▁certain ▁properties . ▁Because ▁C d Se ▁is ▁to xic ▁to ▁living ▁organ isms , ▁the ▁particle ▁must ▁be ▁covered ▁in ▁a ▁surface ▁group . ▁An ▁experiment ▁comparing ▁the ▁per me ability ▁of ▁quantum ▁d ots ▁co ated ▁in ▁poly eth yl ene ▁g ly col ▁( PE G ), ▁P EG - am ine , ▁and ▁car box y