Three-dimensional wound flattening method for mapping skin mechanical properties based on finite element method.

Clinically, skin flap transplantation was often used to repair skin wounds. However, the flap design process with sample cloth is rough and easy to cause infection and necrosis. So an accurate and individual shape design of preoperative flap should be solved. Therefore, a 3D wound flattening method for mapping skin mechanical properties based on finite element method was proposed. Firstly, the 3D point cloud of skin wound was obtained by 3D scanner, and the hierarchical structure of wound model was established. Then a geometric flattening method of wound surface was proposed based on the existing surface flattening theory. The concept of deformed point was introduced according to the special shape of wound surface, and the corresponding modification was given to the original flattening process. Secondly, the mechanical properties of pig skin samples with different orientations were measured by static tensile test. Finally, based on the morphological flattening of wound model and the mechanical parameters of pig skin, a unit material model based on material deformation energy was established. The unit deformation was attributed to the equivalent load acting on the node, and a finite element optimization method of wound unfolding shape based on material deformation energy was proposed. In order to optimize the overall deformation energy, the flap shape was optimized and adjusted to achieve the preoperative design. Clinical examples were selected for verification and analysis. The results show that the proposed method can provide a reasonable and reliable preliminary guide for preoperative flap shape design in clinical wound repair.

Analysis of material parameter uncertainty propagation in preoperative flap suture simulation.

Skin flap transplantation is the most commonly used method to repair tissue defect and cover the wound. In clinic, finite element method is often used to design the pre-operation scheme of flap suture. However, the material parameters of skin flap are uncertain due to experimental errors and differences in body parts. How to consider the influence of material parameter uncertainty on the mechanical response of flap suture in the finite element modeling is an urgent problem to be solved at present. Therefore, the influence of material parameter uncertainty propagation in skin flap suture simulation was studied, Firstly, the geometric model of clinical patient's hand wound was constructed by using reverse modeling technology, the patient's three-dimensional wound was unfolded into a flat surface by using curved surface expansion method, yielding a preliminary design contour for the patient's transplant flap. Based on the acquired patient wound geometry model, the finite element model of flap suture with different fiber orientations and different sizes was constructed in Abaqus, and the uncertainty propagation analysis method based on Monte Carlo simulation combined with surrogate model technology was further used to analyze the stress response of flap suture considering the uncertainty of material parameters. Results showed that the overall stress value was relatively lower when the average fiber orientation was 45°. which could be used as the optimal direction for the flap excision. when the preliminary design contour of the flap was scaled down within 90%, the stress value after flap suturing remained within a safe range.

Preoperative design of flap based on computer-aided design technology: morphological flattening and analysis of three-dimensional wound.

The shape of skin flaps is only described by swatches for preoperative design because of the irregular shape of skin wounds caused by trauma in the clinic. The method is rough, and the flaps cut often cannot match the wounds and affect their appearance. Computer-aided design technology helps to attain precise results in less time. This study proposes a skin wound morphological flattening algorithm based on hierarchical values. First, the skin wound is scanned by three-dimensional (3D) scanning technology to obtain a spatial mesh model consisting of triangular cells, and the mesh model is layered with a hierarchical value. Then, the layered mesh is topologically mapped to the plane. Subsequently, the stress and strain of the skin are simulated using a mechanical method, and the shape of the flattened skin wound is optimized layer by layer. Finally, the multiresolution smoothing technique is used to smooth the developed boundary contour and fit the curve to obtain a guidance plan for preoperative flap design. The results of the study showed that this method can accurately determine the shape of the skin wounds and quantitatively analyze the preoperative design results of the skin flaps. The average error of the area and edge length after flattening was reduced to less than 10%. The work is designed to realize the precise and individualized design of flap schemes before operation and to help standardize the preoperative design process.

Effect of Machining-Induced Subsurface Defects on Dislocation Evolution and Mechanical Properties of Materials via Nano-indentation.

Subsurface defects have a significant impact on the precision and performance of nano-structures. In this paper, molecular dynamics simulation of nano-indentation is performed to investigate the effect of machining-induced subsurface defects on dislocation evolution and mechanical properties of materials, in which the specimen model with subsurface defects is constructed by nano-cutting conforming to reality. The formation mechanism of subsurface defects and the interaction mechanism between machine-induced defects and dislocation evolution are discussed. The hardness and Young's elastic modulus of single crystal copper specimens are calculated. The simulation results indicate that there exist stable defect structure residues in the subsurface of workpiece, such as atomic clusters, stacking fault tetrahedral, and stair-rod dislocations. Secondary processing of nano-indentation can restore internal defects of the workpiece, but the subsurface damage in the secondary processing area is aggravated. The nano-indentation hardness of specimens increases with the introduction of subsurface defects, which results in the formation of work-hardening effect. The existence of subsurface defects can weaken the ability of material to resist elastic deformation, in which the mutual evolution between dislocations and subsurface defects plays an important role.