The biomechanics of autoinjector-skin interactions during dynamic needle insertion

Autoinjector devices are rapidly becoming the preferred method of drug delivery for a wide array of pharmaceuticals such as monoclonal antibodies. Yet, our understanding of injection biomechanics is limited, but is crucially important to create autoinjectors that lead to the least amount of pain, penetrate the skin to a desired depth, produce small lesions that minimize back flow of drug, and operate robustly even given the variability in the skin mechanics among individuals. We propose a finite element model of needle insertion coupled to the dynamic model of an autoinjector. The finite element model is embedded with a cohesive zone plane to capture crack initiation and propagation within an energy-based fracture mechanics framework. The cohesive zone model is supported by experimental observations of a mode I crack during the needle insertion into the soft tissue. Model calibration against force curves from needle insertion experiments leads to estimated material and fracture properties that match values reported in independent experiments from the literature. With the calibrated model we explore the effect of change in the material properties and device parameters on the insertion dynamics. One of the most interesting findings is that pre-compression of skin from the autoinjector base plate can regulate the stress field near the skin surface and add strain energy that is available for crack formation.