(203f) Soft, Resorbable Nanofibrous Mesh for Pelvic Organ Prolapse

Pelvic organ prolapse (POP) is a medical condition faced by menopausal women wherein pelvic organs like uterus, bladder and rectum descend from their anatomical position. There are four stages or degrees of prolapse as mentioned by POP-Q system, an accepted grading system by multiple professional societies. First degree prolapse can be cured by modifying lifestyle, reducing weight and performing Kegel exercises. But higher degree prolapse requires a mesh that is surgically inserted to give mechanical support of the pelvic organs.

Mechanical support to the pelvic floor can be provided by biological grafts (autografts and allografts, xenografts) and synthetic meshes. Biological grafts have advantages because they support tissue remodelling, but they have disadvantages like immunogenicity (for allografts and xenografts), limited availability, donor site morbidity, disease transmission, mechanical property mismatch, etc. Therefore, there is a need for synthetic meshes that overcomes the disadvantages of biological grafts. To overcome these disadvantages, non-degradable meshes of polypropylene, a synthetic polymer, in the past were widely used by direct implantation to treat POP, but they have come into disrepute recently due to adverse surgical implantation sequelae.

The surgical route is either transabdominal or transvaginal, chosen by the surgeons based on the compartment that got prolapsed. In the past, to treat this condition, non-biodegradable polypropylene hernia meshes were implanted to provide support to pelvic organs, but due to mesh stiffness and mechanical property mismatch, this mesh perforated the neighbouring organs, caused bacterial mesh infection, dyspareunia etc., and compromised women’s quality of life. As a result, in April 2019, regulatory agencies like USFDA banned all synthetic meshes that were used to treat POP, from the market.

To overcome all these disadvantages posed by biological grafts and non-biodegradable polypropylene mesh, there is a need for biodegradable, biocompatible, bacteriostatic mesh that mimics the mechanical property and extracellular matrix of the native tissue namely, vaginal tissue. Here we report a novel nanofibrous mesh that was exclusively designed and developed as a mechanical support for the treatment of pelvic organ prolapse.

The properties that need to be considered while designing a nanofibrous biodegradable pelvic mesh are i) biocompatibility, ii) biodegradability, iii) ability to enhance collagen production, iv) mimics the mechanical properties of the vaginal wall (ultimate load, ultimate tensile strength, strain at break and modulus) and v) bacteriostatic ability.

To obtain the above property we chose biodegradable polymers along with additive. The polymers we used to design our biodegradable mesh are polycaprolactone (PCL) and citric acid functionalised polyethylene glycol (PEGC). PCL and polyethylene glycol (PEG) are FDA approved biocompatible and biodegradable polymer. PCL is used for its ease of processibility, good tissue integration, negligible inflammation and slow degradation, which gives enough time for the tissues to laydown extracellular matrix (ECM). PEGC is prepared by functionalising citric acid with PEG through thermal polycondensation reaction. Citric acid is a multifunctional acid, metabolic product of Krebs cycle, has a role in cell proliferation and metabolism. Citric acid is known to promote collagen production in-vivo, therefore we hypothesis that functionalisation of PEG with citric acid to give PEGC will enhance the collagen production, which is essential for pelvic floor repair and regeneration. PEGC addition in the mesh formulation imparts elasticity to the mesh, improves hydrophilicity and tunes mesh degradation. To this formulation zinc oxide nanoparticles (ZnO) are added to improve mechanical properties (ultimate load, ultimate tensile strength, strain at break and modulus) and collagen production. ZnO also imparts antibacterial property whose addition into mesh imparts bacteriostatic ability.

The polymer, additive and nanoparticles are electrospun into nanofibrous mesh. The composition of PCL-PEGC-xZnO meshes produced through electrospinning are 90%PCL, 10%PEGC and ZnO (x=0, 0.1, 0.5 and 1 wt% ZnO w.r.t PCL). With increasing ZnO loading into the nanofiber, the fiber diameter of mesh increased from 495 nm to 900 nm, which improved the strain at break of the mesh. The design criteria for mechanical property of mesh is chosen based on the mechanical property of the vaginal wall: the minimum ultimate load- 1N, minimum ultimate tensile strength- 1 MPa, minimum strain at break- 0.7 mm/mm and modulus range 5 to 11 MPa. The mechanical properties of mesh even after degradation up to 28 days and terminal sterilization using gamma irradiation was similar to that of vaginal wall. The mechanical properties of the commercial polypropylene mesh were also measured, and it was observed that their mechanical properties were not similar to that of a vaginal wall: the ultimate load is 102x higher, ultimate tensile strength is 29x higher, modulus is 7x times higher and strain at break is 35x times lower than the vaginal tissue which is not desirable as a mesh for pelvic organ prolapse, whereas our mesh perfectly matched the mechanical properties of vaginal wall.

The biological mesh performance in-vitro was tested by using L929, mouse fibroblast cells. The mesh formulation PCL-PEGC-x ZnO (x=0, 0.1, 0.5 and 1 wt% ZnO w.r.t PCL) was tested for biocompatibility, collagen secretion by L929 and the effect of ECM secretion on the mechanical properties of the nanofibrous mesh. The formulations PCL-PEGC and PCL-PEGC-0.1 ZnO mesh were biocompatible, haemocompatible and enhanced the collagen production in-vitro in comparison with commercial mesh. Out of which PCL-PEGC-0.1 ZnO mesh supported the highest collagen production. Mesh formulations containing high loading of ZnO like PCL-PEGC-0.5 ZnO and PCL-PEGC-1 ZnO were found to be not favourable for L929 growth due to higher ZnO loading. Therefore, PCL-PEGC-0.1 ZnO was chosen as the optimum formulation.

When deployed, the mesh would be infiltrated with fibroblast that would laydown ECM. Effect of ECM on the mechanical properties of the meshes were studied by culturing the L929 on the meshes for a period of 28 days. The collagen deposition on the PCL-PEGC-x ZnO (x=0, 0.1 wt% ZnO w.r.t PCL) improved their mechanical properties (ultimate load, ultimate tensile strength, strain at break and modulus) and their values were in the correct range of the mechanical design criteria for the mesh.

The meshes were evaluated for their bacteriostatic ability in-vitro using E. coli and S. aureus. In comparison to the commercial mesh, PCL-PEGC-x ZnO (x=0, 0.1, 0.5 and 1 wt% ZnO w.r.t PCL) meshes exhibited 2 log reduction in bacterial adhesion.

Based on extensive testing and analysis presented above, the nanofibrous mesh that we formulated and fabricated possessed all the criteria such as softness, elasticity, proper mechanical property and ability to promote ECM production to give a natural fibrous support membrane as the scaffold mesh degrades, all of which are essential for its application in pelvic organ prolapse repair, making it an appropriate alternative solution for the USFDA banned non-degradable polypropylene mesh. With these better matched properties the mesh developed here can be taken up for preclinical studies.