Comparison of polylactic acid/polycaprolactone membrane-coated composite meshes for repairing pelvic floor defects fabricated by two processing methods

Abstract

Applying mesh to reconstruct the pelvic floor is a new surgery method developed in recent years. The composite mesh property is expected to reach perfection by taking advantage of absorbable material and non-absorbable material. This work adopts the dipping method and the electrospinning method to produce polylactic acid/polycaprolactone (weight ratio = 7/3) membrane-coated composite meshes (D-PP1, D-PP2, E-PP1, E-PP2). The effect of coating methods on the mesh structure, such as thickness and weight, is first measured. The electrospinning membrane is tested for only around 3% of the mesh weight, while the dipping membrane is tested for more than 30%. The composite meshes then experience mechanical testing, including tensile strength, bursting strength and flexibility. The coating process is demonstrated to increase elongation at the break in tensile testing: E-PP1 (76.7%) > D-PP1(60%) > PP-1(41.2%); D-PP2 (143.3%) > E-PP2(112.3%) > PP-2(56.7%). It also strengthens the bursting strength: D-PP1 (179.8 N) > E-PP1(157.3 N) > PP-1(149.8 N); D-PP2 (183 N) > E-PP2(174.8 N) > PP-2(154.3 N). However, the dipping method shows weaken composite mesh flexibility, with larger initial modulus for D-PP1 and D-PP2 meshes. Hydrophilicity is represented by the water contact angle and absorption rate. Even though the result of the hydrophobic surface on the composite mesh is reported to easily cause shrinkage, the polypropylene mesh base in this study is considered to prevent this phenomenon. E-PP1 mesh degrades in 20 weeks; D-PP2 mesh degrades in 34 weeks with an induction period of 20 weeks. The optical density value tested, measuring biocompatibility, proves the non-toxicity of both E-PP1 and D-PP2 meshes.

Keywords

polylactic acid/polycaprolactone membrane-coated pelvic floor repairing dipping electrospinning

Pelvic floor function (PFD) is a type of normal gynecological condition caused by pathologic and structural change of pelvic floor supportive tissues. Pelvic organ prolapse (POP), stress urinary incontinence (SUI) and chronic pelvic pain are the main representations of PFD. The morbidity of PFD affects 20–30% of postmenopausal women,¹ which even reaches up to 40% in China.² Applying mesh to reconstruct the pelvic floor is a new surgery method developed in recent years,³ making up for its high recurrence rate of traditional hysterectomy.⁴ Mesh should play a supportive role to effectively reinforce, bridge and overhang weak tissues, which is defined as the optimum target for prosthesis repairing pelvic floor tissues.

Different materials have been developed and widely used in repairing or reconstructing the pelvic floor structure. Synthetic materials began to be used in reconstructive surgery in the 20th century, and are divided into absorbable and non-absorbable types, according to material degradability.⁵ Both absorbable mesh and non-absorbable mesh have different advantages and disadvantages. For example, absorbable mesh is good for preventing infection, but it also easily leads to scar formation by stimulating connective tissues. Moreover, insufficient mesh mechanical strength caused by degradation may increase the risk of recurrence. In contrast, non-absorbable material has a stable property through the whole implanting period, but its biocompatibility is inferior, easily leading to infection, erosion and other complications. Based on this, composite mesh is proposed to combine advantages of both absorbable and non-absorbable mesh, while avoiding their disadvantages.

Currently, several composite meshes prepared by the coating method have been reported. Schug-Pass et al.⁶ coated a titanium layer on the surface of macroporous polypropylene (PP) mesh and a significantly lower shrinkage rate, lower adhesion and better biocompatibility were proven. Gemma et al.⁷ studied and analyzed several composite meshes consisting of different membrane materials; the result proved a reduced seroma rate in the short-term implantation period, and a low shrinkage rate was found at each observation point. QuininoI et al.⁸ fabricated a polyethylene glycol hydrogel/PP coating mesh for the purpose of anti-adhesion; its validity was proved. Even though much research has been conducted on composite mesh, the fabrication process has not been discussed and analyzed in detail. Existing studies prefer to target the material composition of composite mesh.

This paper adopts and compares composite meshes prepared by two coating methods. The purpose is to achieve a more suitable processing method for preparing pelvic floor repairing mesh. The coating membrane part of composite mesh is polylactic acid (PLA) and polycaprolactone (PCL) polymers; two elements are mixed using a weight ratio of 7:3. Two macroporous lightweight PP meshes with different structures are chosen as the base part of the composite mesh. The structural parameters, crystalline structure, thermal analysis, mechanical property, hydrophilicity, hot water shrinkage, in vitro degradation and biocompatibility are tested and analyzed. The effect of the coating method on the composite mesh property is evaluated basing on these experiment results.

Materials and methods

Materials

PLA (Mw = 10⁵) and PCL (Mw = 8 × 10⁴) polymers were both purchased from Yisheng New Materials Co., Ltd (Shenzhen, China). Dichloromethane (DCM) and N,N-dimethylformamide (DMF) are both of the analytical pure type provided by Damao Chemical Reagent Factory (Tianjing, China). The two different structure PP meshes used in this paper are fabricated by the State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University. The rhombus-pore-type PP mesh is named PP-1 in the paper; mesh with hexagon-pore-type is PP-2. Both meshes have been tested in our previous study; the structure and parameters are shown in Table 1.

Table 1.

Parameters and patterns of PP-1 mesh and PP-2 mesh

	Weight (g/m²)	Thickness (mm)	Porosity (%)	Pattern
PP-1	41.19	0.412	68.53
PP-2	35.97	0.387	63.4

PLA and PCL are chosen for their good biocompatibility and non-toxicity.⁹ PLA is reported to be of high strength modulus and good biocompatibility. However, its tenacity, tensibility and impact resistance are worse than that of PCL.¹⁰ This means that PLA material is strong but not flexible enough. On the other hand, PCL has excellent flexibility, machinability and stability, which make up for PLA’s defects.¹¹ Several researches have demonstrated superior anti-adhesion and reduced inflammation of PLA membrane-coated PP composite mesh.^12–14 The addition of PCL into PLA is expected to modify the mechanical property of the composite mesh, especially improving its flexibility in this work.

Fabrication of PLA/PCL membrane-coated composite mesh

PP-1 and PP-2 are used as the base parts in composite meshes; each structure PP mesh is coated by both the dipping method and the electrospinning method. PLA/PCL solution is used to form the coating membranes; the preparation parameters and methods of solutions have been discussed in our previous research.¹⁵ The solvent is a DCM and DMF mixture with weight ratio of 4:1. PLA and PCL polymers are weighed as 7:3 and dissolved in this solvent.

The fabrication process of composite mesh by the dipping method is as follows. Firstly, PP meshes were soaked in PLA/PCL solutions for 10 s. Then the meshes are taken out to volatilize to dry at room temperature (25 ± 3℃). It is noteworthy that meshes have to be kept horizontal in the whole volatilizing process to prevent non-uniform thickness, because liquid PLA/PCL solution tends to deposit in the bottom, caused by gravity, if the mesh is kept vertical.

The electrospinning technique is another way to form membrane-coated mesh in this experiment. The PLA/PCL spinning solution was stretched to superfine microfibers; these microfibers deposited on the surface of PP mesh under high-voltage pressure. The electrospinning membrane consisted of numerous microfibers. The processing parameters have also been discussed in our previous study:¹⁵ the flow rate of the PLA/PCL solution is 0.6 ml/h; the voltage is 12 kV; the received distance is 15 cm; aluminum foil with PP mesh stuck on is used as the receive device.

Four kinds of composite meshes (D-PP1, D-PP2, E-PP1, E-PP2) are obtained eventually. D-PP1 mesh combines PP-1 mesh with a PLA/PCL membrane and is fabricated by the dipping method. D-PP2 also consists of a PLA/PCL dipping membrane, but with a PP-2 base part. E-PP1 mesh consists of PP-1 mesh and a PLA/PCL membrane prepared by the electrospinning method. The same coating method is used for E-PP2 mesh but with a PP-2 base part.

Structural parameters

The structural parameters of the composite mesh are presented by weight, thickness, membrane ratio and mesh surface observation. Meshes are balanced in a standard state for 24 h before testing (the temperature is 20℃ ± 2℃, the humidity is 65% ± 2%). The results are expressed with average and standard deviations for 10 replicates. (1) Weight: samples were cut into 50 mm × 50 mm squares; the testing device was a type FA 2004A electronic balance. (2) Thickness: samples were cut into 150 mm × 150 mm and tested using a YG141N digital fabric thickness gage; the pressure foot area was 2000 ± 20 mm²; 1 ± 0.01 kPa pressure was put on samples for 30 ± 5 s. (3) The membrane ratio means the weight proportion of the PLA/PCL membrane in composite mesh; this data was calculated according to equation (1)

MembraneRatio (%) = \frac{W_{c} - W_{p}}{W_{c}} \times 100

(1)

where W_c refers to the weight of the composite mesh and W_p refers to the weight of the PP mesh base.

The observation of the composite mesh membrane was tested using an HITACHI S-3000 scanning electron microscope. For the electrospinning membrane, diameters of 80 fibers were read by Photoshop CS3 and calculated. For the dipping membrane, pore size was measured using an automatic aperture measuring device (Quantachrome Ins, USA).

Crystalline structure and thermal property

X-ray diffraction characterization

X-ray diffraction (XRD) measurements were recorded using Type XRD-6000 diffractometer (Shimadzu, Japan). The prepared samples were operated at 40 kV and 200 mA. A Cu-Kα radiation source was used to scan samples in a 2θ range from 0° to 60° with a scan rate of 0.06°/s. D-spacing was determined from Bragg’s law [nλ = 2d sinθ], where θ is the diffraction angle and λ is the wavelength [λ = 1.54056 Å for a Cu target]. The degree of crystallinity (D_c) is calculated according to equation (2)

D_{c} (%) = \frac{\sum I_{c}}{\sum I_{c} + \sum I_{a}}

(2)

where I_c is intensity of the crystallization peak and I_a is intensity of the amorphous peak.

Differential scanning calorimetry

Thermal analysis of samples is obtained by differential scanning calorimetry (DSC) testing. Dry samples (5 mg) were heated from 20℃ to 200℃ at a scanning rate of 10℃/min using a Netzsch DSC 204F1 (Phoenix, USA) under nitrogen atmosphere.

Mechanical property

Samples were all balanced in a standard condition for 24 h before testing (the temperature was 20℃ ± 2℃, the humidity was 65% ± 2%).

Tensile strength

A YG(B)026 G-500 universal testing system (Darong Textile Ins, China) was used to measure tensile strength. Samples with size of 50 mm × 30 mm were clamped in two chucks placed in both ends. The distance between the two chucks was 30 mm; the tensile speed was 100 mm/min; 1 N tension was applied before the drafting process. Five samples were tested along the longitudinal direction and five samples were tested along the transverse direction. The final result is expressed with an average value and a standard deviation of five replicates.

The tensile strength method here is aimed at woven fabric; the PP mesh used as the base part in this paper is knitting fabric, which is mainly assessed by bursting strength. However, a difference between the two directions of meshes is expected to be observed that cannot be obtained by bursting testing alone. Therefore, tensile strength testing is considered essential and useful. The original standard (GB T 3923.1-1997) is not suitable for porous knitting mesh; the above-mentioned testing parameters (sample size, tensile speed, distance, pre-tension) were chosen after several trials in order to make sure there is an effective fracture of mesh. This means that breakage should be located in the middle of the sample instead of being concentrated around the clamped position.

Bursting strength

Bursting strength was tested according to GBT 19976-2005 using an electronic fabric strength tester HD026H200 (Darong Textile Ins, China). Samples were cut into circle shape (the diameter was 60 mm); the bursting ball diameter was 20 mm; the descending rate was 100 mm/min; the distance between the bursting ball and the sample was 100 mm. The final result was expressed by an average and a standard deviation of 10 replicates.

Flexibility

The initial modulus was used to represent flexibility. It was obtained by analyzing the tensile curve and calculated using equation (3) as follows

InitialModulus (MPa) = \frac{E_{1}}{d \times t} \times 100

(3)

where E₁ (N) is the load corresponding to 1% elongation of the original length, d (mm) is the thickness of the testing sample and t (mm) is the sample width.

Hydrophilicity

The water contact angle and water absorption rate were both used to evaluate hydrophilicity. An OCA15EC water contact angle measuring instrument (Defei Instrument Co. Limited, China) was used to test the value of the contact angle. The experiment was operated at room temperature (22–26℃) under application of a yellow-light source. Deionized water (4 μl) was dropped on the sample surface for 1 min; then the angle between the water drop and the sample surface was measured under a microscope. Three measuring points more than 5 mm apart were tested in each sample. The final average contact angle is calculated by these three.

As for the absorption rate, samples of size 50 mm × 50 mm were soaked in deionized water for 24 h under an environment of 22–26℃. After being taken out of the water, the samples were immediately wiped and weighed. The absorption rate was calculated according to equation (4)

Waterabsorption (%) = \frac{W_{a} - W_{b}}{W_{b}} \times 100

(4)

where W_b refers to the weight of the sample before absorbing and W_a is the weight after absorbing.

Hot water shrinkage

Samples of size 50 mm × 50 mm were soaked in deionized water under different temperatures ranging from 35℃ to 55℃. These temperatures were selected according to the limited temperature of the human body. After being soaked for 24 h, the samples were taken out and their surface area was measured. The shrinkage rate was calculated according to the following equation

Hotwatershrinkagerate (%) = \frac{S_{b} - S_{a}}{S_{b}} \times 100

(5)

where S_b is the mesh surface area before being soaked in hot water and S_a is the surface area after being soaked.

In vitro degradation

Samples were cut into a rectangular shape with dimensions of 50 mm × 50 mm for the degradation test. Specimens were placed in a Petri dish containing phosphate buffer solution (PBS, pH = 7.4 ± 0.2) and incubated at a temperature of 37℃, CO₂ volume of 5%. Specimens were taken out and weighed every 14 days.

In vitro biocompatibility (CCK8 test)

In vitro biocompatibility was assessed by testing cytotoxicity.

Materials

Extract solutions were prepared by immersing samples into Dulbecco’s Modified Eagle’s Medium (DMEM) at a ratio of 6 cm²/ml for 72 h at 37℃ temperature. Negative control only contained DMEM and L-929 fibroblast cells cultured for 24 h. Cell suspension was prepared by twice passage culturing L-929 cells for 24 h at 37℃, 5% CO₂, until an apparent logarithmic phase existed. Pancreatin with concentration of 0.025% was then added to trypsinize solution in order to form single-cell suspension. The concentration was adjusted to 1 × 10⁵cells/ml eventually.

Method

The suspension (100 µL) was transferred into 96 well culture plates and incubated at 37℃, 5% CO₂. The initial culture solution was discarded before adding 100 ml extract solution and negative control. Each sample was transferred in four holes in each culture plate. Cell proliferation was observed under a bio-microscope after incubating for 24, 48 and 72 h, respectively.

Cell viability was measured and read under application of the ELISA reader (Infinite F50, Tecan). Dimethylsulfoxide (DMSO) (100 µL) was added after discarding the initial culture solution, before oscillating at 600 r/min for 10 min. Absorbance at 595 nm was quantified after that. The relative growth rate (RGR) was calculated according to equation (6)

RGR (%) = {OD}_{m} / {OD}_{p} \times 100

(6)

where OD_m is the average absorbance of the sample group and OD_p is the absorbance of the negative control group.

The cell toxicity system (CTS) was set according to the RGR value at different observation points. Samples were graded as 0 degree when RGR ≥ 100%, I degree for a RGR of 75–99%, II degree for a RGR of 50–74%, III degree for a RGR of 25–49%, IV degree for a RGR of 1–24% and V degree for a RGR of <1%. 0 degree and I degree were both deemed as non-toxicity; II degree was deemed as minor toxicity; III degree and IV degree were considered of medium cell toxicity; V degree was considered of severe cell toxicity.

Statistical analysis

The statistical significance was determined by analysis of variance (ANOVA) and the Tukey post hoc test at the significance level of 0.05 using SPSS software version 16.0.

Results and discussion

Structural parameters

Figure 1 shows micrographs of E-PP1 and D-PP2 surfaces on the membrane side. It can be seen that the membranes on composite meshes produced by the two coating methods are similar under the macro-scale. The meshes are both bi-layer products consisting of a white opaque membrane layer and a mesh-like layer. However, a significant difference is observed after magnification reaches to 3 K. Disorder microfibers are distributed on the surface of the electrospinning membrane-coated mesh; the average diameter of these fibers is 412.34 ± 57.8 nm. In contrast, micropores are found on the surface of the dipping membrane-coated mesh; the average pore size is 4.89 ± 2.55 µm.

Figure 1.

Macrographs (×4) and micrographs (×3 K) of E-PP1 and D-PP2 composite mesh: (a) E-PP1; (b) D-PP2.

Table 2 summarizes the thickness, weight and membrane ratio of the composite meshes. The electrospinning and dipping methods are both found to be available to form a super thin layer of membrane on the surface of PP mesh. To be specific, the thicknesses of composite meshes are all less than 0.5 mm; only a small increase occurs after the coating process compared with the thickness of PP-1 and PP-2 mesh (0.412 and 0.387 mm, respectively).

Table 2.

Thickness, weight and membrane ratio of four composite meshes

	Thickness/mm	Weight/g·m⁻²	Membrane Ratio/%
D-PP1	0.462 ± 0.026	79.96 ± 2.07	48.49 ± 1.26
D-PP2	0.437 ± 0.029	65.34 ± 2.15	36.96 ± 1.09
E-PP1	0.439 ± 0.018	42.56 ± 1.28	3.22 ± 0.08
E-PP2	0.393 ± 0.010	36.87 ± 1.02	2.44 ± 0.12

On the other hand, a significant increase is observed in the weight property of composite meshes produced by the dipping method. D-PP1 and D-PP2 meshes are both much heavier than E-PP1 and E-PP2 meshes. The PLA/PCL membrane on the dipping membrane-coated mesh occupies more than 30% of the whole mesh in weight. However, the electrospinning membrane mesh is only about 3% of the mesh weight.

The two coating methods used in this paper both achieve the aim of completely covering PP macroporous mesh; a relative smooth membrane-like surface is formed as a result. The coating methods have almost no significant effect on the thickness property. However, a sharp increase in the weight property is easily induced by the dipping method (D-PP1, D-PP2). This can be explained by analyzing the dipping coating process (Figure 2). The process can be divided into two parts. Firstly, PLA/PCL solution fully fills the pores of PP mesh. Secondly, the extra solution exceeds the pore scope and forms a layer of membrane covering knitting structure. The sharp increase in weight is mainly due to the high porosity (nearly 70%) of PP meshes.

Figure 2.

Fabrication process of the dipping membrane-coated mesh. PP: polypropylene; PLA: polylactic acid; PCL: polycaprolactone.

As for the electrospinning membrane-coated mesh, a PLA/PCL membrane is formed by spraying PLA/PCL microfibers on the surface of PP mesh (Figure 3). It is known that the electrospinning membrane has a high specific surface area.⁹ Moreover, it can be observed from Figure 2 that microfibers align loosely in E-PP1 mesh, while the dipping membrane seems more dense. The fabrication process and inherent property of the dipping membrane both cause a large increase in the weight property of D-PP1 and D-PP2 composite mesh.

Figure 3.

Fabrication process of the electrospinning coated-membrane mesh. PP: polypropylene; PLA: polylactic acid; PCL: polycaprolactone.

Crystalline structure and thermal property

Figure 4 shows the XRD pattern of E-PP1 mesh and D-PP2 mesh. The dipping membrane-coated mesh shows three sharp peaks at 14.08°, 17.00° and 18.4°, whereas the electrospinning membrane-coated mesh obtains wider peaks at similar positions. There only exists a small difference between the two curves because of a low content of PLA/PCL in composite mesh. However, a relatively less crystalline structure of E-PP1 mesh still can be confirmed. The degree of crystallinity of the two samples is calculated based on the ratio of the crystalline area obtained from the XRD pattern. The degree of crystallinity of E-PP1 and D-PP2 are 44.39% and 63.85%, respectively. The low crystallinity of the electrospinning membrane is attributed to the rapid forming process during its preparation. To be specific, the electrospinning process leaves insufficient time for chains to be stretched, ordered and highly oriented.

Figure 4.

X-ray diffraction pattern of the electrospinning membrane-coated mesh (E-PP1) and dipping coated-membrane mesh (D-PP2).

Figure 5 shows DSC curves of E-PP1 and D-PP2 mesh. It can be seen that both samples have three melting peaks, which correspond to the melting points of composition materials (PP, PLA, PCL) in composite mesh.

Figure 5.

Differential scanning calorimetry of electrospinning membrane-coated mesh (E-PP1) and dipping coated-membrane mesh (D-PP2).

In conclusion, E-PP1 has a less crystalline structure but similar thermal property to D-PP2 mesh. There is no obvious difference of the mesh crystalline structure and thermal property between products fabricated by the dipping method and the electrospinning method.

Mechanical property

Tensile strength

Figure 6 presents the tensile property of composite meshes in the longitudinal direction. It can be observed that meshes with the same base structure (PP-1 or PP-2) have a similar peak value. However, there is still a difference among the tensile curves of meshes, even with the same base (E-PP1, D-PP1 and PP-1; E-PP2, D-PP2 andPP-2). Therefore, the coating method is proven to affect the tensile strength of composite meshes, but it has only a small effect on the value of tensile breaking strength.

Figure 6.

Tensile property of composite meshes in the longitudinal direction.

On the other hand, elongation at break of meshes with the same base part is significantly different. Take PP-1 based meshes for example (Figure 6(a)): the elongation at break of both E-PP1 (76.7%) and D-PP1 meshes (60.0%) are larger than that of PP-1 (41.2%) mesh. The same phenomenon happens even more obviously in PP-2-based meshes (elongation: E-PP2 = 112.3%, D-PP2 = 126.6%, PP-2 = 34.3%) (Figure 6(b)). Therefore, it is certain that coating processing strengthens the extensibility of composite meshes. The meshes can be stretched longer until breakage.

Figure 7 shows the tensile property of composite meshes in the transverse direction. Curves of meshes in the transverse direction have the same feather as those in the longitudinal direction, with similar breaking strength value but significantly larger elongation than PP-1 or PP-2 meshes. This confirms the above assumption of the coating effect on the composite mesh tensile property.

Figure 7.

Tensile property of the composite meshes in the transverse direction.

However, it was found that the same composite mesh presents different characteristics in two directions. E-PP1, D-PP1 and PP-1 meshes are much stronger in the longitudinal direction, with breaking strength of more than 60 N, while the breaking strength of these meshes in the transverse direction is less than 30 N. In contrast, there is no great difference of strength values for E-PP2, D-PP2 and PP-2 meshes between the transverse direction and the longitudinal direction.

Therefore, it is proved that the tensile property of membrane-coated composite meshes mainly relies on the corresponding PP mesh base part. Both the electrospinning method and the dipping method play a role in improving mesh extensibility; meshes are able to be stretched longer.

Figure 8 provides the stretching process of composite meshes. From Figure 8, significantly different situations during and after-mesh breakage are observed. The PLA/PCL membrane on dipping membrane-coated mesh was cracking to slices and then fell along with the stretching process; many membrane fragments were found surrounding the mesh after experimentation. PLA/PCL was even found to separate from PP mesh; composite mesh lost its completely smooth surface after tensile testing. In contrast, the electrospinning membrane bonded to PP mesh during the stretching process; no membrane cracking or falling down phenomenon happened. After testing, the electrospinning membrane-coated composite mesh still retained a smooth surface.

Figure 8.

Stretching process for (a) D-PP2 mesh and (b) E-PP1 mesh; meshes after being stretched for (c) D-PP2 mesh and (d) E-PP1 mesh.

In clinical use or animal experimentation, the tension effect caused by muscle movement inevitably occurs. The tension leads mesh to be repetitively stretched. Harmful effect may happen when fragments left by the dipping membrane enter into body. Blood vessels are possibly plugged up, which causes a risk of thrombus. Pelvic function is also damaged by migration of these fragments. Moreover, composite meshes fabricated in this work are aimed to promote tissue growth and prevent severe adhesion by taking the advantage of a smooth surface characteristic. Therefore, the electrospinning method is thought to be more suitable for producing composite mesh for the pelvic repair area.

Bursting strength

Figure 9 presents the bursting strength of meshes. The bursting strength values of both PP-1 and PP-2 mesh are found to be the smallest. Compared with pure PP mesh, composite meshes after the coating process have larger bursting strength. Considering the difference between composite meshes produced by the electrospinning method and the dipping method, the bursting strength value of D-PP1 is found to be larger than that of E-PP1, the same as PP-2-based composite meshes. Therefore, the dipping method is assumed to strengthen the bursting property more than the electrospinning method.

Figure 9.

Bursting strength for (a) PP-1-based composite meshes and PP-1 mesh and (b) PP-2-based composite meshes and PP-2 mesh.

The bursting property is used to assess the mesh’s ability to sustain vertical force. In this paper, pelvic floor repairing mesh is required to support procident organs in their original position. The force happens in the vertical direction, which is closely related to bursting strength. PP meshes in the composite meshes obtain large and numerous pores, which easily induced fragility or even rupture in the surrounding pore area. The coating method covers the pores with super thin membranes, which retards the rupture of the repairing mesh.

Flexibility

The initial modulus is used to evaluate the flexibility of repairing meshes. The value is calculated from the tensile stress–strain curve. The larger the initial modulus value is, the more the mesh presents as less flexible and stiffer. Figure 10 shows the flexibility of meshes. The initial modulus values of D-PP1 and D-PP2 meshes are found to be much larger than others. This demonstrates that the dipping method causes composite mesh to become stiffer and have less flexibility. In contrast, the electrospinning method does not bring about such significant change of mesh flexibility compared with the PP mesh base without experiencing the coating process.

Figure 10.

Flexibility for (a) PP-1-based composite meshes and PP-1 mesh and (b) PP-2-based composite meshes and PP-2 mesh.

The electrospinning method is of benefit to maintain the soft characteristic of origin macroporous PP mesh. However, the dipping method induces composite mesh flexibility to decrease, which is not expected for pelvic repairing mesh. The difference of flexibility between the two coating methods also corresponds to the weight property. E-PP1 and E-PP2 meshes are much lighter because of the high specific surface area of microfibers with diameters of less than 600 nm.

Overall, the coating methods have different impacts on the mesh mechanical property. The coating membrane contributes to strengthen the bursting property, while it has almost no influence on tensile breaking strength. The effect of the coating method on the tensile property is mainly concentrated on mesh elongation at break. Composite meshes can be stretched longer than PP mesh under a similar largest applied load. These changes are expected for repairing mesh, because mesh needs to be extensible to fit flexible soft tissues. No significant difference is found in the bursting property and the tensile property between the dipping method and the electrospinning method. However, the two coating methods present obvious different situations in mesh flexibility. The dipping method leads mesh to be stiffer, while the electrospinning mesh maintains the soft characteristic of the original macroporous PP mesh.

Hydrophilicity

The water contact angle of the mesh surface and mesh absorption rate are adopted to evaluate hydrophilicity. Figure 11 presents the water contact angle of composite meshes. E-PP1, E-PP2 and D-PP1 were found to have a hydrophobic surface (contact angle >90°), while D-PP2 has a hydrophilic surface but its hydrophilicity is not obvious (contact angle = 79.17 ± 1.88℃). It is reported that polyesters and polyethers are not highly hydrophobic materials; PLA and PCL cast film are reported to have contact angles of 84℃ and 89℃,^16,17 corresponding to the contact angle results of D-PP1 and D-PP2 (both around 90℃). The greater hydrophobic property of the electrospinning membrane has also been proved. Teoh et al.¹⁸ coated PCL nanofibers on the surface of a PCL membrane; the contact angle (124.58℃) was much larger than that of a single PCL membrane (77.42℃). The increase of contact angle along with a more hydrophobic property is due to the PCL inherent hydrophobic group (CH₃).

Figure 11.

Contact angle of composite meshes.

Figure 12 summarizes the absorption rate of composite meshes. The electrospinning membrane-coated mesh absorbed much more water, while the dipping membrane-coated mesh barely absorbed. The result of the absorption rate seems contradict that of the contact angle; the electrospinning method proved to be less hydrophilic with a larger contact angle value. This can be explained by analyzing testing principles. The contact angle evaluates the ability of the membrane surface to refuse or accept water drops, while the absorption rate assesses the ability of the bi-layer composite mesh to absorb a large amount of water. The difference between these two testing methods is that the membrane surface plays a leading role in the contact angle experiment, while the whole mesh structure has the main effect in the absorption rate experiment.

Figure 12.

Water absorption of composite meshes.

The reason for the larger absorption rate for the electrospinning coating mesh is the bi-layer structure. The electrospinning method forms a layer of PLA/PCL membrane on the surface of the macroporous PP mesh. A large space is able to be left between the membrane part and PP mesh part, which makes it possible for water to be stored inside the composite mesh. In contrast, the dipping method forms a layer of membrane, but also completely fills PP mesh pores. Little space is left for water storage, which leads a small absorption rate.

The hydrophilicity of implanting materials partly decides the cell viability and proliferation. Franco et al.¹⁹ compared membrane-like ePTFE mesh with ePTFE/PP composite mesh, and concluded that the high hydrophobicity of ePTFE mesh may lead to capsule formation and introduce severe shrinkage. A hydrophilic surface is considered to be of benefit to relieve severe shrinkage of mesh. Although the composite meshes in this work do not have a hydrophilic surface, the knitting PP mesh used as the base plays has the effect of supporting and fixing the soft membrane, which is able to prevent severe shrinkage caused by the hydrophobic surface.

Hot water shrinkage

The composite mesh is aimed to be implanted into the human body. Hot water shrinkage is used to assess the possibility of mesh shrinkage caused by the human internal environment. The selected temperatures are 35℃, 45℃ and 55℃, ranging a little larger than the limit temperature of the human body (35–46.5℃).

Figure 13 shows the shrinkage rate of composite meshes. It can be observed that E-PP1 and E-PP2 meshes shrunk more obviously than D-PP1 and D-PP2 meshes in each testing temperature, and the shrinking area enlarged along with the temperature increasing. The electrospinning membrane-coated mesh is proved to shrink more than the dipping membrane-coated mesh. The difference between composite meshes fabricated by the two methods can be explained as follows. The dipping membrane is stiffer, and integrates to PP mesh more closely because of the complete filling in the pore area. In contrast, the electrospinning membrane only covers the surface of the PP mesh; the soft and super thin membrane is easily affected by water. The higher temperature may accelerate the attack of water, and the membrane part easily shrinks as a result.

Figure 13.

Shrinkage of composite meshes under temperatures of 35℃, 45℃ and 55℃.

In general, all composite meshes in this work did not shrink under 35℃; it shrunk less than 3% of the original area under 45℃. The upper limit temperature of the human body is 46.5℃; therefore, both coating methods are able to sustain sufficient area to a great extent. The result of hot water shrinkage also provides important experimental evidence for studying the in vivo shrinkage mechanism of medical mesh.

In vitro degradation

E-PP1 and D-PP2 meshes were used as experimental objectives in the degradation test, because only membrane parts can be degraded and the different PP structure makes it easier to distinguish them in PBS solution. Figure 14 presents the weight loss of composite meshes. E-PP1 mesh is found to degrade completely in 20 weeks. D-PP2 mesh degrades in the 32nd week; a little membrane part still remains, not disappearing even after extending the experimental time for another 5 weeks.

Figure 14.

Weight loss of composite mesh.

The electrospinning membrane only occupies about 3% of the whole mesh weight. The weight decreases slowly along with degradation time. Surprisingly, a sharp increase in weight loss is found in D-PP2 mesh in the 20th week. Before that, D-PP2 mesh had no change at all, no matter the weight value or the surface condition (Figure 15). In the 20th week, bulk membrane parts began to fall from mesh under the application of deionized water washing. In addition, the membrane continued to depart from the mesh in the next degradation time. You et al.²⁰ reported a similar phenomenon in PLGA degradation; the weight loss is slow and accelerates suddenly. The slow degradation period is called the induction period. Water needs to attack the chemical bond first, and a high specific area is of benefit for water entering into the molecular inner. The microfiber in the electrospinning membrane is much easier for water to enter into, with no induction period in E-PP1 mesh degradation. While dipping membrane is more dense and difficult for water to attack, the former 20 weeks are defined as the induction period.

Figure 15.

Micrographs of composite meshes after degradation at different time intervals: (a) E-PP1; (b) D-PP2.

PLA and PCL materials are both reported to degrade in a relative long period, which needs at least more than 1 year for PLA²¹ and 2 years for PCL.²² However, the PLA/PCL membrane in the paper take less than 1 year to degrade. This can be explained as follows. The degradation test in this work is aimed at composite mesh instead of a single PLA/PCL membrane. The weight loss not only contributed to material degradation, but also separation between the membrane and PP mesh. For normal degradation, chemical bond breakage makes the molecule weight of PLA or PCL small enough; the polymers then can be dissolved in water. However, the adhesion between the membrane and PP mesh becomes weak after the chemical bond is attacked; the separation may happen before the bond rupture is complete. Strictly speaking, the membrane disappearance phenomenon cannot be defined as degradation in this work. However, it is still deemed that degradation is easy.

In vitro biocompatibility

Figure 16 summarizes the optical density (OD) value of composite mesh. Only E-PP1 and D-PP2 meshes are chosen as the research objective, because the cytotoxicity is mainly affected by material type. It is known that active cells are accompanied with a large OD value. No significant different is observed between E-PP1 and D-PP2 meshes (P = 0.2). The OD value increases along with experiment time, which proves that cells grow well in the sample extract solution.

Figure 16.

Optical density (OD) of composite meshes.

Table 3 presents the RGR and toxicity classification of composite mesh. Both composite meshes are classified as 0-I degree, meaning they are not toxic at all. Figure 17 provides the cellular morphology of the three experimental groups. Cells have a similar morphology and are all presented in a fusiform shape, confirming a satisfactory condition of cells. Along with experimental time, more cells are incubated with a significant proliferation phenomenon; cells grow most densely on the fourth day.

Figure 17.

Cellular morphology of the three experimental groups.

Table 3.

Relative growth rate and toxicity classification of composite mesh

Time	Type	RGR (%)	CTS (degree)
1-day	E-PP1	93.28	I
1-day	D-PP2	91.16	I
2-day	E-PP1	101.62	0
2-day	D-PP2	96.22	I
3-day	E-PP1	98.14	I
3-day	D-PP2	96.58	I
4-day	E-PP1	89.79	I
4-day	D-PP2	90.02	I

RGR: relative growth rate; CTS: cell toxicity system.

Conclusion

This paper fabricated composite meshes using two coating methods, the electrospinning method and the dipping method. The two methods are both available to form a super thin smooth membrane overlapping macroporous PP mesh. No obvious difference was found in the mesh crystalline structure and thermal property. The coating process was proven to strengthen mesh bursting strength and extensibility. However, a large increase in the weight property as well as weakened flexibility is caused by the dipping method.

The bonding structure between the membrane and PP mesh contributes to the earlier disappearing of PLA/PCL compared with degradation of PLA or PCL single material; the electrospinning membrane-coated mesh showed a shorter degradation period. The cytotoxicity test guarantees the safety of composite meshes, which is an important precondition for usage in the human body.

Therefore, the electrospinning membrane-coated mesh, with a more flexible property and shorter degradation period, seems more suitable for application in repairing pelvic floor defects.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Key Technology R&D Program (grant number 2012BAI17B05) and the project of Shanghai Committee of Science and Technology, China (grant number 12DZ1940300).

References

Renaud

Xavier

Amelie

. Long-term anatomical and functional results after trans-vaginal cystocele repair using a tension-free polypropylene mesh. Int Urogynecol J 2006; 17: 483–488.

Yan

. Short term effects of Prolift and Gynemesh mesh repairing pelvic organ prolapsed. Ningxia Med J 2011; 33: 626–626.

Xia

. Application of mesh in pelvic floor suspension. J China Med Uni 2008; 37: 402–404.

Wang

Zhu

Lang

. Applications of replacement materials in pelvic floor reconstructive surgery. Am J Obstet Gynecol 2005; 40: 789–790.

. Clinical application and biocompatibility in obstetrics and gynecology diseases. J Clin Rehab Tissue Eng Res 2009; 13: 8369–8372.

Schug-Pass

Tamme

Tannapfel

. a lightweight polypropylene mesh (Timesh) for laparoscopic intraperitoneal repair of abdominal wall hernias. Surg Endosc 2006; 20: 402–409.

Gemma

Sandra

Marta

. Behaviour of a new composite mesh for the repair of full-thickness abdominal wall defects in a rabbit model. Plos One 2013; 8(11): e80647–e80647.

QuininoI

RME

Araujo

Lima

. Adhesion prevention in reabsorbable polyethylene glycol hydrogel (Coseal®) coated polypropylene mesh in rabbits. Acta Cir Bras 2013; 28: 807–814.

Xia

. Electrospinning of nanofibers: reinventing the wheel. Adv Mater 2004; 16: 1151–1170.

10.

Dorgan

Lehermeier

Palade

. Polylactides: properties and prospects of an environmentally benign plastic from renewable resources. Macromol Biosci 2001; 175: 55–66.

11.

Anders

Mikael

. Properties of lactic acid based polymers and their correlation with composition. Prog Polym Sci 2002; 27: 1123–1163.

12.

Fujino

Kinoshita

Saitoh

. Novel technique of overlaying a poly-L: -lactic acid nanosheet for adhesion prophylaxis and fixation of intraperitoneal onlay polypropylene mesh in a rabbit model. Surg Endosc 2011; 25: 3428–3436.

13.

Tanaka

Mutter

Inoue

. In vivo evaluation of a new composite mesh (10% polypropylene/90% poly-l-lactic acid) for hernia repair. J Mater Sci Mater M 2007; 18: 991–999.

14.

Zhang

. Preparation of poly (L-lactic acid)-modified polypropylene mesh and its antiadhesion in experimental abdominal wall defect repair. J Biomed Mater Res B 2013; 102: 12–21.

15.

Chen

Zhang

. A membrane coated composite mesh for repairing pelvic floor defects using electrospinning method. Ind Textila 2015; 66: 192–199.

16.

Chen

Lee

Teoh

. Nanofiber modification on ultra-thin poly(epsilon-caprolactone) membrane via electrospinning. Mat Sci Eng C-Mater 2006; 27: 325–332.

17.

Gombotx

Guanghui

Horbett

. Protein adsorption to poly(Ethylene Oxide) surfaces. J Biomed Mater Res 1991; 25: 1574–1562.

18.

Teoh

Chen

Lee

. Nanofibrous modification on ultra-thin poly(e-caprolactone) membrane via electrospinning. Mat Sci Eng C-Mater 2007; 27: 325–332.

19.

Franco

Min

Yang

. Fabrication and biocompatibility of novel bilayer scaffold for skin tissue engineering applications. J Biomater Appl 2013; 27: 605–615.

20.

You

Min

Lee

. In vitro degradation behavior of electrospun polyglycolide, polylactide, and poly(lactide-co-glycolide). J Appl Polym Sci 2004; 95: 193–200.

21.

La Mantia

Mistretta

Palermo . Thermomechanical degradation of PLA-based nanobiocomposite. Polym Adv Technol 2016; 3: 308–308.

22.

Rodenas-Rochina

Vidaurre

Castilla Cortázar

. Effects of hydroxyapatite filler on long-term hydrolytic degradation of PLLA/PCL porous scaffolds. Polym Degrad Stabil 2015; 119: 121–131.