Abstract Hypodermic needles for the injection of fluids generally have annular cross-sections. The preference for round shapes is, besides manufacturing reasons, due to the ideal ratio between the cross-section circumference and area, which leads to optimal flow properties. However, when additional parameters are considered in the design, other forms can prove to be better: the healing of the wound induced by the needle is highly individual and effects such as skin tension can make non-round shapes more suitable. When reimagining the shape of hypodermic needles, relevant factors such as the buckling of the needle during piercing/injection, and the fluid flow through the needle must be considered. The current work demonstrates an approach for the identification of different geometries within the framework of multi-objective optimization. Conflicting aspects of microfluidic flow, buckling under piercing/injection conditions, as well as (simplified) tissue damage suggest the notion of Pareto-optimal solutions. Equipped with more advanced models, the proposed take on hypodermic needles can lead to a better standard geometry and even concepts of ad-hoc manufacturing of individually optimal geometries through additive methods in the context of personalized medicine.

Abstract Since multi-neck Helmholtz resonators (HRs) have attracted considerable research interest in recent years in the context of leakage, and both extended tunability and broader frequency response can be achieved through the targeted use of such resonators, the development of a precise analytical model for predicting the acoustic behaviour of individual multi-neck HRs or those embedded in complex systems is of central importance. In this study, an analytical model based on the parallel transfer matrix method (PTMM) is developed to predict the acoustic behaviour of multi-neck HRs under plane-wave excitation. The analytical PTMM approach is validated by means of impedance tube measurements according to ISO 10534-2 on seven carefully selected symmetrical and asymmetrical specimens and by numerical comparisons of the acoustic parameters predicted by the PTMM approach with the results of the finite element method (FEM) and the predictions of two established analytical models. The results show that the PTMM approach can predict the acoustic behaviour of both symmetrical and asymmetrical samples with high accuracy and has clear advantages in terms of accuracy over one of the established models, the transfer impedance-based model. A comparison with the analytical approach of the model for micro-perforated plates adapted in this article shows that the proposed PTMM approach delivers identical results, but in certain cases is superior in terms of the accuracy of the acoustic prediction. To summarise, the analytical PTMM approach proposed in this study shows superior performance in predicting relevant acoustic parameters compared to the established analytical models as well as FEM and offers significant potential for further applications and extensions in a wider range of contexts and variants.