Engineering Microneedle Patches for Improved Penetration:Analysis,Skin Models and Factors Affecting Needle Insertion

Transdermal microneedle(MN)patches are a promising tool used to transport a wide variety of active compounds into the skin.To serve as a substitute for common hypodermic needles,MNs must pierce the human stratum corneum(~10 to 20μm),without rupturing or bending during penetration.This ensures that the cargo is released at the predetermined place and time.Therefore,the ability of MN patches to sufficiently pierce the skin is a crucial requirement.In the current review,the pain signal and its management during application of MNs and typical hypodermic needles are presented and compared.This is followed by a discussion on mechanical analysis and skin models used for insertion tests before application to clinical practice.Factors that affect insertion(e.g.,geometry,material composition and cross-linking of MNs),along with recent advancements in developed strategies(e.g.,insertion responsive patches and 3D printed biomimetic MNs using two-photon lithography)to improve the skin penetration are highlighted to provide a backdrop for future research.