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.