  The development of needle-free drug injection systems is of great importance
to global healthcare. However, in spite of its great potential and research
history over many decades, these systems are not commonly used. One of the main
problems is that existing methods use diffusive jets, which result in scattered
penetration and severe deceleration of the jets, causing frequent pain and
insufficient penetration. Another longstanding challenge is the development of
accurate small volume injections. In this paper we employ a novel method of
needle-free drug injection, using highly-focused high speed microjets, which
aims to solve these challenges. We experimentally demonstrate that these unique
jets are able to penetrate human skin: the focused nature of these microjets
creates an injection spot smaller than a mosquito's proboscis and guarantees a
high percentage of the liquid being injected. The liquid substances can be
delivered to a much larger depth than conventional methods, and create a
well-controlled dispersion pattern. Thanks to the excellent controllability of
the microjet, small volume injections become feasible. Furthermore, the
penetration dynamics is studied through experiments performed on gelatin
mixtures (human soft tissue equivalent) and human skin, agreeing well with a
viscous stress model which we develop. This model predicts the depth of the
penetration into both human skin and soft tissue. The results presented here
take needle-free injections a step closer to widespread use.
