Evaluation of the relationship between the viscoelastic stress and strain of fetal rat skin as a guide for designing the structure and dynamic performance of a manipulator for fetal surgery

Purpose. To design an endoscopic manipulator for fetal surgery. The viscoelastic properties of fetal skin were estimated from both the viewpoint of mechanical structure and data collection for controlling the device. Methods. The skin of fetal Wistar rat (19.5 days old) was set on a rheometer and the relationship between stress and strain was examined. Morphological damage was assessed histologically. Results. The stress-strain curve was nonlinear and sigmoidal throughout the process. The skin fracture point was estimated to be over 4 kPa. After multiple challenges of low-level loading under 150 Pa, the curve showed no detectable change due to mechanical fatigue. Histologically, the basement membrane was not damaged even at the fracture point; however, severe damage to the dermis was observed. Conclusion. The viscoelastic properties of the fetal rat skin were mainly caused by the dermis and the value of the shear stress that causes skin fracture was estimated to be 4 kPa. To design a robotic stabilizer, limit of mechanical loading was thus tentatively set at 400 Pa, with a 1/10 fracture point.