Microfluidic jet impact: spreading, splashing, soft substrate deformation and injection

Injecting with needles causes fear, pain and contamination risks. Billions of injections every year also cause environmental burden in terms of material consumption and waste. Controlled microfluidic-jet injection systems offer a needle-free alternative. However, understanding the relation between jet parameters and resulting injection depth are needed to enable targeting specific skin layers, and enhance the pharmacokinetics of various therapeutic compounds. The complexity of skin, its opacity and non-linear mechanical properties, pose a technological challenge. Hence the use of surrogates is instrumental to understand how to inject without needles. In particular, reducing undesired splashing upon jet impact and liquid squeeze-out after injection are needed to minimize infection risks and ensure accurate dosage. Therefore, in this paper we explore how microfluidic jet characteristics influence the impact outcome on a range of materials as skin surrogate. Jets with velocities between 7 - 77 m/s and diameters 35 - 130 $\mu$m were directed at substrates with shear moduli between 0.2 kPa and 26 GPa. We found seven different regimes depending on jet inertia and substrate shear modulus. Furthermore, three distinct transition regions were identified as the thresholds between regimes: i) spreading/splashing threshold, ii) dimple formation threshold, and iii) plastic/elastic deformation threshold. These thresholds allow predicting the required jet velocity and diameter to inject substrates with known shear modulus. We found that jet velocity is a better predictor for the injection depth compared to the Weber number, as the jet diameter does not influence the injection depth. Our findings are relevant for advancing needle-free injection research, because the shear modulus of skin depends on multiple factors, such as ethnicity, body part and environmental conditions.