Fluctuation of puncture force in medical needle puncture of soft solids

Puncture of soft solids involves the process of piercing through a soft solid material using a sharp object, which is of great interest in biomedical and industrial applications. In this work, we investigate the puncture of soft solids by a commercial medical needle using polyacrylamide (PAAm) hydrogel as the model material. We measure the relationship between the depth of puncture and force exerted on the needle as it penetrates and retracts from the PAAm hydrogel at different speeds, and interpret the results of the experiments. We find that the puncture force fluctuates with the increasing puncture depth during needle penetration, while the retraction force decreases almost linearly with the retraction depth. Both the puncture and retraction forces, as well as the amplitude and period of the puncture force fluctuation, are positively related to the loading speed. We estimate the size of the crack, which is difficult to directly recognize from experiments, induced by puncture using the critical puncture depth at which the needle first pierces the hydrogel. We find that the size increases with the puncture speed. Moreover, we obtain a work balance relationship between the energy done by the puncture force minus friction (Fp-Fr) and the sum of the energy required for crack propagation and the elastic energy stored in the hydrogel bulk. These findings from our research contribute to a comprehensive understanding of deep indentation and puncture in soft materials. These results carry important implications for the design of biomedical and industrial devices, where the puncturing of soft solids is a critical procedure.