A Novel Approach to Measure Needle Insertion Response and the Overlooked Impact of Insertion on Injection Outcomes

Abstract

Needle-based injection techniques are widely used in drug delivery, diagnostics, and soft material characterization, yet the mechanical influence of the insertion process on the ensuing injection behavior remains poorly understood. Here, we demonstrate that both the morphology of the expanded cavity and the resisting pressure are not only governed by material properties, but can be critically influenced, and even reliably modulated, by the preceding needle insertion and retraction program. To investigate the insertion process, we measure the pressure developed in the droplet that is initially suspended at the tip of the needle and then driven through the material to obtain pressure-depth curves. This offers a local measure of tearing resistance that is not governed by frictional forces along the needle shaft. By systematically varying insertion and retraction depths and speeds in two contrasting soft materials, we find that features in the pressure-depth curve reliably indicate expected outcome of the injection procedure, as defined for different use cases. These findings reveal the insertion phase as a critical yet previously underutilized control in drug injection and needle-based mechanical testing, and establishes pressure-depth monitoring as a realtime diagnostic tool. By eliminating reliance on visual confirmation, this approach can improve the robustness, scalability, and automation potential of needle-based injection methods, particularly in opaque, biological, or high-throughput environments.