A flap mechanics testbed for skin reconstructive surgery: Evaluating the mechanical interaction between flap design and anisotropy

Minimizing tension in skin tissue is one of the key factors in reconstructive surgery, as residual stress after defect closure directly affects tissue regeneration and wound healing. To achieve the required skin mobility and thereby reduce tension, careful flap design is essential. Because skin exhibits intrinsic in-plane anisotropy arising from the preferred orientation of collagen fibers in the dermis and its mechanical properties vary across anatomical regions, incorporating skin mechanics into surgical planning is therefore crucial. To investigate the interaction between flap design and skin anisotropy, we utilized a silicone-based skin surrogate fabricated by 3D printing with a uniform stacking direction and developed a flap mechanics testbed that enables the controlled application of pretension. Three canonical flap designs, advancement, rotation, and transposition, were examined under different anisotropy orientations, with circular defects introduced into the skin surrogate. The kinematics during the flap closure were acquired by tracking marked control points using 3D scanning, and the deformation and maximum principal stress fields were in turn reconstructed from these kinematic measurements using a B-spline tensor product surface representation and Kirchhoff-Love shell kinematics, without solving a boundary-value problem. The resulting stress fields show that both flap type and its orientation relative to the underlying anisotropy influence residual stress distributions. These findings underscore the importance of considering both skin anisotropy and proper flap design to minimize post-operative tension and demonstrate that the proposed computational and experimental framework can guide the optimal flap orientation for a given defect geometry and anisotropy.