Cone cracks in tissue-mimicking hydrogels during hypodermic needle insertion: the role of water content

Needle insertion into soft biological tissues is a common process in various surgical procedures. During insertion, soft biological tissues with different water contents undergo large deformation often leading to uncontrollable cracks and tissue damage. Despite the numerous experimental studies and numerical modelling of needle-tissue interaction, the results do not show any consistency mainly due to the heterogeneity of tissue properties and opaqueness. In this context, understanding the fracture behaviour of soft tissues during needle insertion is important for minimally invasive surgeries and other medical procedures. Recently, we showed that the needle insertion into a transparent, tissue-mimicking polyacrylamide (PAAm) hydrogel causes periodic cone cracks. In this work, we systematically varied the water content of the PAAm hydrogel in the preparation state and performed needle insertion experiments using a hypodermic needle at a constant velocity to study the fracture characteristics of the PAAm hydrogel. The results show that the number of peaks, the magnitudes of the insertion forces, and corresponding cone cracks decrease with increasing water content. Furthermore, we discussed the influence of water on cone crack fracture characteristics, cone angle, periodicity, crack speed and fracture energy release rate. These results provide a better understanding of the fracture processes of soft tissues with different water concentrations such as the lung, liver, and brain during needle insertion and the control of tissue damage during needle insertion involved in medical procedures.