Interfacial strength measurement of soft hydrogels with needle insertion and pressurization*

Biological interfaces are critical for maintaining the structure and performing functions; however, current knowledge of interfacial strength is limited, especially for soft tissues. Here, we introduce a method of measuring interfacial strength by needle insertion and pressurization to provide quantitative insight into the criteria of interfacial damage initiation. We demonstrate this method and develop a quantitative analysis by measuring the strength of ultra-soft polyacrylamide (PAM) hydrogel interface. In order to make the PAM self-interface, plastic sheet separators are inserted before crosslinking and removed when the hydrogel is fully cured. The residual stress on the interface is tuned by the thickness of the separators. The critical pressure is observed to open the PAM self-interface and this critical pressure scales with the thickness of the separator and the size of the needle. We propose a quantitative relationship between the critical pressure and the energy release rate for the PAM self-interface. This pressurization method of adhesive strength measurement can provide information about internal interfacial properties at desired locations, making it helpful for in vivo measurements of biological tissues.