Mechanical evaluation of a new bilayer mesh prosthesis for ventral hernia

Ventral hernias are common complications in abdominal surgery, with a 10–20% incidence. A hernia occurs when viscera protrude through the abdominal wall, because of trauma or conditions increasing intra-abdominal pressure. These protrusions can cause chronic pain, impair daily movement, and, in severe cases, surgery is required to repair the defect. The most used repair method involves implantation of a mesh prosthesis to reinforce the abdominal wall and reduce the risk of hernia recurrence. One of the main limitations of traditional monolayer prosthesis is the risk of visceral adhesion, leading to bowel obstruction and infections. To mitigate this, bilayer meshes have been developed, combining a macroporous layer for abdominal wall reinforcement and a smooth layer to prevent organ adhesion. While these reduce adhesions, issues such as recurrence, infection, seroma, chronic pain, and postoperative discomfort still negatively affect outcomes and quality of life. In this context, the objective of this thesis was to investigate the mechanical performance of a new bilayer prosthesis made of a non-resorbable polypropylene (PP) monofilament mesh combined with a resorbable (9–12 months) polylactic acid (PLA) film, produced by cast film extrusion. Two manufacturing methods were evaluated for fabrication: compression molding and continuous lamination. For both manufacturing processes, different parameter sets (i.e. pressure, temperature, contact time) were tested to identify the most suitable combination to ensure effective interfacial adhesion. Prototypes were produced as sheets (press moulding) or linear meters (continuous lamination). Visual inspection and manual delamination were first carried out to assess the quality of interfacial adhesion. Tensile and burst strength tests were then performed following company protocols. Tensile tests were conducted in directions A and B. To evaluate the rupture of PLA film, minimum force values were extracted from force-displacement curves of the composite. Subsequently, an analysis of variance (ANOVA) was conducted on mean maximum stress values of the PLA to assess process repeatability. For press-molded prototypes, stresses in direction B showed lower variability than in direction A. This is due to the knitting pattern of PP substrate, where horizontal reinforcements in direction B adhere to PLA film and, under tension, the PLA fails by irregular tearing rather than brittle fracture. Mean tensile strength values of 0.6 N/mm (A) and 0.7 N/mm (B) were recorded, both below the 3.2 N/mm, corresponding intrabdominal wall stress peak during coughing or jumping. Burst strength tests on press-molded prototypes returned a mean value of 5.6∙10^2 kPa, comparable to 5.5∙10^2 kPa measured for laminated prototypes. Early delamination was significant in laminated samples, so burst tests measured only PP mesh behavior. In contrast, press-molded samples showed inconsistent adhesion, with some delaminating and others remaining intact during testing. Limited adhesion from the lamination method caused delamination and PLA rupture, making statistical analysis unreliable. While compression molding showed superior adhesion, its scalability for industrial use remains limited. Overall, the combination of PLA and PP presents challenges in mechanical and thermal compatibility, affecting the reproducibility and performance of the bilayer mesh. Further material optimization may be necessary to improve adhesion and functionality.