Prestress as an Optimal Biomechanical Parameter for Needle Penetration and Formulation Injection

Intradermal and subcutaneous needle injections have long been used as a means to deliver vital medication subcutaneously across a range of applications from vaccines to insulin [1]. In order to optimize the efficacy of these injections, the needle must penetrate precisely to a targeted depth. However, the actual net penetration of the needle beneath the surface of the skin is confounded by the deflection of the skin. With the rise of the use of auto-injectors in such fields as diabetes care, achieving the actual required penetration becomes exceedingly important. There have been previous attempts to characterize this deflection [2–4], but these were limited in that they did not account for a range of factors that play a role in the penetration mechanics, and many were based on mathematical models, synthetic skin substitutes, or other tissues. The purpose of our work was to investigate the multiple factors that govern needle penetration and injection, including velocity, needle gauge, depth of insertion, and skin prestress.