Numerical simulation of heat transfer from a transonic jet impinging on skin for needle-free powdered drug and vaccine delivery

In this paper, gas dynamics and temperature distribution of a transient impinging jet, typically used to deliver powdered drug or vaccines ballistically to the skin, have been numerically simulated. Calculations were performed with the unsteady compressible Navier-Stokes equations, solved using a modified implicit flux vector splitting (MIFVS) difference scheme with a modified k-epsilon model. After validation against an experimental underexpanded jet-flow test cast, the code was applied to two biolistic configurations. Firstly, calculated pressure histories were compared with corresponding measurements in an unsilenced biolistic device, with good agreement. Secondly, this calculation was extended to a silenced geometry, with an emphasis on the gas properties immediately above a skin target. A peak stagnation gas temperature of 420 degrees C is revealed within 175 mu s of diaphragm rupture. The temperature of the following driver helium gas suddenly drops to below - 100 degrees C before approaching ambient temperature within 1 ms. The effect of this impinging jet on the human skin is subsequently explored by a one-dimensional convection heat transfer model. On the skin surface, a peak fluctuation of [+5.5 degrees C, -4.5 degrees C] is recorded during the operation process. Beyond the outer skin layer, called the stratum corneum (i.e. > 10.6 mu m), temperature deviations from the normal condition are trivial. Therefore, the heat transfer between the biolistic jet and skin is negligible. This result is consistent with pain-free clinical trial observations.