Some mesh material used in surgical treatment of stress urinary incontinence (SUI) and pelvic organ prolapse (POP) is associated with serious complications in patients (1). Increasing amount of evidence suggests that PPL material is not biomechanically compatible with the dynamic environment of the female pelvic floor. All meshes implanted in this site will undergo continuous deformation for decades. Materials which undergo irreversible plastic deformation under repeated loading may lead to material failure and/ or change in the mechanical properties of the mesh. 
Although the uniaxial mechanical testing of mesh materials is reported (2), we were unable to find published studies of fatigue testing of these materials. 
We hypothesized that a test as simple as three-day fatigue testing would be sufficient to distinguish between materials which have already been found to cause complications clinically and newer materials yet to be tested clinically. We propose this as an easy way to eliminate materials will are mechanically inappropriate for implantation in the pelvic floor. 
Our aim was to test the hypothesis that a very simple fatigue testing regime would be sufficient to identify those materials which have previously caused problems clinically in a comparison of 4 commercial meshes and two new electrospun materials not yet evaluated in the clinic. Uniaxial tensile properties were compared before and three days of continuous 25% dynamic distension applied using a dynamic bioreactor.

Two electrospun materials made of polyurethane (PU) and ureidopyrimidinone-polycaprolactone (UPy) that have not yet been evaluated in the clinic and four other commercially available meshes were included in the study. Two of the meshes were fabricated from PP, a heavy weight PP mesh,  Gynemesh® (Johnson & Johnson) and a new ultra-light weight PP mesh, Restorelle® (Coloplast, Humlebaek, Denmark). The two other commercially available materials were made of polyvinylidene fluroride (PVDF) DynaMesh®-ENDOLAP and Dynamesh®-PR4 (Dynamesh, Aachen, Germany). 
A tensiometer (BOSE Electroforce test instruments, Minnesota, USA) was used to perform the uniaxial tensile test and cyclic testing (Figure 1). Samples (n=6, for each material) were cut keeping uniform width and length (30mm x 5mm). For the cyclic test, the material was stretched for 10 cycles at a rate of 1mm/s and the displacement was adjusted to 25% of its original length. Plastic deformation was measured as the initial % of strain for each sample at the tenth cycle without an increase in stress due to this plastic deformation. For uniaxial tensile testing, a force was applied at a rate of 0.1 mm/s and a displacement of 7mm. Stress vs strain plots are shown as strength (y axis, MPa) by % of displacement (x axis, %). The initial linear gradient of each plot was taken as the Young’s modulus (N/mm2), which is used to measure the stiffness of the material. The failure point was used to measure the ultimate tensile strength (UTS) (y axis) and the maximum strain (x axis). 
Simulated fatigue testing was performed using a TC-3 load bioreactor (Ebers Medical Technology SL, Zaragoza, Spain) and samples were subjected to cyclic uniaxial distension using 25% elongation, 0.1mm/s rate and 18 cycles per minute over 3 days in phosphate buffered solution at 37oC, 5% CO2. Samples were then re-assessed using the uniaxial tensile test as above.

In uniaxial testing, the first thing to note is that most of the materials were able to be stretched until the end of the test (an 80% distension) without significant breaks (Figure 2). Both of the electrospun materials, PU and UPy coped with this 80% deformation whilst maintaining a constant resistance to applied force. Cyclic testing showed that the main change in the mechanical behavior of the different materials happened between cycle 1 and cycle 2 for all materials with permanent mechanical deformation especially for the 4 meshes. 
After fatigue testing for three days, all commercial meshes failed before they reached 80% strain while the two electrospun meshes as before coped with 80% displacement (Figure 2).

Our study demonstrates that the currently used PP and PVDF meshes are strong but rigid materials which plastically deform after just 3 days of stretching on a regular dynamic cycle whereas the 2 new materials, each based on electrospun materials, have more elastic properties with much less deformation.

We suggest that a test as simple as this three-day fatigue testing is sufficient to distinguish between materials which have already been found to cause complications clinically and newer materials yet to be tested clinically which will hopefully prove more mechanically appropriate for implantation in the pelvic floor.

Milani, A. L., Damoiseaux, A., IntHout, J., Kluivers, K. B. and Withagen, M. I. J. Long-term outcome of vaginal mesh or native tissue in recurrent prolapse: a randomized controlled trial. Int. Urogynecol. J. (2017). doi:10.1007/s00192-017-3512-3.
Shepherd, J.P., Feola, A. J., Abramowitch, S.D., and Moalli, P. A. Uniaxial biomechanical properties of 7 different vaginally implanted meshes for pelvic organ prolapse. Int. Urogynecol. J. (2012). doi:10.1007/s00192-011-1616-8.