Surface Modification of Artificial Implants by Hybrid Nanolayers: Antimicrobial Surface Finishing and Strength Tests Open Access

Nowadays, artificial hernia implants (meshes) are used in a facultative way for supporting soft tissues during hernia repair surgeries. Different types of meshes are standard; they differ especially by material, shape, weight, structure, flexibility, shape memory, and others [1‒6]. A number of producers have developed complete lines of hernia meshes aimed at different parts of abdominal walls and surgical techniques, with a different emphasis on certain properties of implants with various preferential usages [7, 8]. The multitude of different implants (around 300 types at the moment!) proves an implant of ideal properties for all possible surgical solutions has not been developed yet [1‒35]. An increased risk of infection development on the implant surface is one of the weaknesses of implant usage. So-called hiding of microorganisms from the effect of the immune system can happen in the material from which the hernia meshes are made, i.e., persistence of inflammation or its onset within the range of several months or years after implanting the mesh [36‒40]. Infection can be solved only by definitive removal of the implant in many cases. A problem with using the implants under primarily non-sterile conditions, where the situation cannot be solved differently (fistulas, chronic defects, stomia, etc.) – has not been solved sufficiently yet. Implant producers normally do not offer antibacterial modification in their portfolio for different reasons [33]. Other preventive measures than direct implant modification appear to be significantly less efficient in these cases [41‒43].

Titanium, titanium alloys, and titanium dioxide (TiO₂) are also widely used for the realization of surfaces in medicine. A big advantage is that titanium and its other forms have minimal or no interaction with living tissues. When evaluating the biocompatibility of titanium on different types of cell lines, it was found that the material is not cytotoxic to human tissue [44]. In experimental studies, it has been demonstrated that the application of an additional coating of atomic titanium to the polypropylene (PP) filaments has resulted in further improvement of the biocompatibility and in significantly lower shrinkage rates compared with an identical PP mesh without a titanium coating (TiMesh™, PFM, Kőln, Germany and TiO₂Mesh™, BioCer, Bayreuth, Germany) [45]. The comparison between coated and uncoated meshes showed that there was no significant difference between titanium-coated PP and uncoated PP, while silver-chlorhexidine coating significantly reduced the bacterial adherence, highlighting the role of the antiseptic agent [46].

The titanium surface treatment is able to ensure, to a lesser extent, that the pathogen that adheres to the surface does not have the possibility of further uncontrollable multiplication. But it does not have significant antibacterial power by itself. If the titanium substrate on the joint replacement is subsequently modified with silver nanoparticles, it is already able to significantly inhibit the multiplication of bacteria and thus prevent the formation of a biofilm, which is a very common phenomenon [44].

TiO₂ in the form of a thin layer is suitable for applications on the surfaces of joint or dental implants. For joint implants, it is able to increase osseointegration and thereby shorten the time required for the implant to heal in the bone [47].

The use of TiO₂ nanoparticles is a separate chapter. The highly active effect of photoactive TiO₂ nanoparticles is tied to the presence of UV radiation, which is not fulfilled in the body. The ineffectiveness of TiO₂ nanoparticles without the presence of UV radiation is described in the work of Vargas-Reus et al. [48], in which testing was carried out with different types of nanomaterials.

The antibacterial effects of titanium, or its oxide, i.e., TiO₂, are very well known and are published in a number of other works. The work of Tsuang et al. [49] describes the antibacterial effects of 20 nm TiO₂ nanoparticles. Five bacterial strains (Escherichia coli, Staphylococcus aureus, Enterococcus hirae, Pseudomonas aeruginosa, and Bacteroides fragilis) were selected for the experiment. Irradiation was performed using ultraviolet C (UV C) radiation. After 60 min of exposure, all tested bacterial strains had 100% inhibition, i.e., to death, and when compared to the effect of UV radiation alone (without the presence of TiO₂), there was no inhibition after 60 min.

Nanoparticles of TiO₂ have also been investigated in many works from the point of view of their accumulation and adverse effects in the bodies of various animals. The model organism for these tests is usually Daphnia magna. In these animals, which were exposed to TiO₂ concentrations of 80 and 250 mg·L⁻¹, the accumulation of TiO₂ particles in their digestive system was recorded [50]. TiO₂ can react with a whole range of other substances that are present in the tested organism. It was found that at a TiO₂ concentration of 2 mg·L⁻¹, there was an increased accumulation of copper in the organism. Accumulated TiO₂ particles together with copper were detected in the intestines of the tested animals. Increased concentrations of the copper complex and TiO₂ are toxic and with their accumulation, increased mortality of individuals was observed. At the same time, the attenuation of detoxification mechanisms, which are used to cope with the increased concentration of metals, was recorded [51, 52].

The use of TiO₂ nanoparticles was banned by the European Food Authority (EFSA) in May 2012, as TiO₂ can no longer be considered a safe additive in the food industry, as no safe daily intake limit can be established for TiO₂ (E 171). It is allowed to remain on medication for the time being, as there is no full-fledged replacement. Once a replacement is found, the use of TiO₂ in medicine will also be banned [53]. Due to the accumulation of TiO₂ in animal species (see above), a ban on its use is being considered also in other products.

The issue of the increase of pathogenic bacterial strains would be possible to eliminate or at least limit by suitable disinfection with long-term effect. However, as current information shows, the presence and resistance of pathogenic microorganisms in both health-care and social facilities is still a significant problem. In spite of the relatively fast development and introduction of more and more new and effective disinfection means with both long- and short-term effect to the market, resistant strains keep proliferating and cause new questionable long-term and uneasily cured infections, treatment of which is often financially more costly than the treatment for which the patient was accepted to the medical facility. Unfortunately, mesh infections occur in clinical practice despite the best efforts to implant them sterilized under sterile conditions.

Surface modification is the most frequent and researched method for achieving long-term antimicrobial properties. Antimicrobial surfaces are able to restrict snapping and growth of bacterial cells (so-called biofilm creation which precedes bacterial proliferation). Surfaces can be divided into two basic types based on their effects on bacteria. Surface prevention from pollution is the first type, which prevents or precedes snapping of bacteria. A bactericidal surface which destroys the bacteria is the second type. Surfaces can be of natural origin, nature-inspired, or artificial. Lotus leaves, shark skin, insect wings, etc. are an example of natural surfaces. Furthermore, artificial surfaces are divided based on the method of preparation. These include, for example, surface polymerization, functionalisation by plasma, or surfaces modified by layers, i.e., hybrid layers with antimicrobial effect [1, 23, 54‒65].

Sol-gel method is a chemical method for obtaining special nanolayers for technical usage. The method involves the preparation of a solution of initial ingredients in a waterless solvent (alcohol) and then its transfer into sol (colloid solution of oligomer) by adding water under acid catalysation. The prepared sol is applied on to the substrate and is left to evaporate and finishing the hydrolysis with air humidity. Finally, the gel is firmed by increased temperature.

At the Technical University of Liberec, a method of preparing hybrid organic-inorganic nanolayers was developed on 3-(trimethoxysilyl)propyl methacrylate (TMSPM) which shows outstanding chemical resistance towards all organic solvents and also water solutions besides concentrated hydroxides and hydrofluoric acid and also relatively good mechanical resistance. Concerning thermal stability, these nanolayers are long-term stable at 170°C.

The basic hybrid polymer of the nanolayer consists of two components chemically linked to a 3D network, of which one is a silicate and the other polymethylmethacrylate. This basic polymer can be modified by suitable addition of other raw materials into an initial solution on the hydrophobic nanolayers (i.e., by adding hexadecyltrimethoxysilane with functional hexadecyl group) or by adding cations of suitable metals (silver– Ag, copper– Cu, and zinc –Zn) on antimicrobial (and also antiviral and fungicide) nanolayers. Preparation and properties of these nanolayers have been published [66‒71] and also patented [72‒74].

Special attention of numbers of studies is given to the antibacterial efficacy of individual ions Ag, Cu, and Zn and also nanoparticles of TiO₂ [69, 75, 76]. As an example, Jaiswal et al. [76] prepared hybrid antibacterial layers from methyltriethoxysilane into which metal ions (Ag, Cu, Zn) of nitrate salts were fixed. The results showed that an Ag⁺-doped hybrid layer displays the highest antibacterial activity, followed by a layer doped by Zn²⁺ cations; a layer containing only Cu²⁺ cations proved to be the least efficient [76]. Hybrid layers doped by silver in water solution release Ag⁺ slowly, causing a bactericide effect. Ag⁺ cations can interact with cell walls containing peptidoglycan, bacterial DNA in the cytoplasm, or with bacterial proteins necessary for life important processes of a bacterial cell [77]. Properties of antibacterial layers are used in numbers of medical applications (care of catheters, implants, breathing devices) where prevention of nosocomial infection occurrence and increased bacterial resistance are necessary [69, 75, 76].

A basic hybrid organic-inorganic network of nanolayer does not contain any substances which would be potentially dangerous for application into a human organism from the biochemical point. Cations of silver, copper, and zinc which are carriers of antiseptic influence, are bound on a silicate of polymer through the weakly acid Si-O⁻ groups of this network. Their release is very slow and thus has a long-term effect. The concentration of metal cations is very low, however sufficient for achieving a long-term bactericide effect, in the environment of the hybrid polymer surface. Exact concentrations are not known, however, concerning the fact that it was proved that hybrid nanolayer applied on the textile contains at least tens of percent of original content of silver, copper, and zinc even after 50 cycles of washing, and the speed of releasing heavy metal cations is unmeasurable using common analytical methods [76].

Concerning the fact that final applied nanolayers are only 40–200 nm thick (based on the degree of diluting basic sol) and overall contents of metal cations (Ag⁺, Cu²⁺, Cu⁺, Zn²⁺, only a small amount of silver is in an atomic reduced form Ag⁰) reach individually only about 2% mass (expressed as metal) in them, the overall amount of heavy metals in nanolayer mass applied to the implant is so small that it would not represent a significant burden even during prospective fast release into an organism.

Long-term maintenance of very low concentrations of silver, copper, and zinc cations on the surface and in the close environment of the nanolayer without temporary high concentrations which could be toxic is the main effect of using a hybrid nanolayer. Long-term maintenance of the contained silver in the Ag⁺ form, which is the main carrier of antimicrobial properties, is the result of oxidation-reduction balances with the Cu⁺/Cu²⁺ system. A part of the reduced silver (as a result of reduction by organic substances or radiation as in photography) is gradually transformed back into an active form of cation by this mechanism and thus the nanolayer activity is prolonged within an order of magnitude. It was confirmed experimentally that this hybrid nanolayer also has antiviral properties [66]. Zinc cation expands its (nanolayer) function both on moulds and candida so that the nanolayer also has fungicide properties.

Alongside antimicrobial, antiviral, and fungicide functioning of silver, copper, and zinc cations, it is potentially possible to also immobilize other organic substances into the nanolayer mass or its surface (pharmaceuticals, antibiotics, antiseptics, etc.) which can act over the short- or long-term in the area of an implant based on requirements. Due to the fact that production of initial sol is neither technologically nor economically nor ecologically demanding and is a typical example of a technical solution with high know-how requirements, a product with high added value is the result.

Oxygen plasma creates functional groups containing oxygen on the surface of plastics. Bichler modified the properties of the non-polar surface of PP with oxygen plasma, and the resulting oxygen-containing functional groups improved the adhesion of the layers [78]. Information about a significant reduction in the mechanical strength of plastics after short-term exposure to oxygen plasma was not found in the literature.

A different situation was found when some plastics were exposed to UV C radiation. PP samples were experimentally exposed to accelerated ageing by UV C radiation. Irradiation with a UV C germicidal lamp was carried out for 196 h and changes in properties were monitored. After this time, the tensile strength decreased to 20% of the original value (dynamic mechanical thermal analysis) and the presence of hydroxyl and carboxyl functional groups was confirmed by Fourier transform infrared spectroscopy. The decrease in tensile strength was very significant, but the irradiation time was extreme [79]. Grause et al. [80] provided an overview of the chemical changes of polyolefins in the presence of UV radiation and oxygen. The result is the formation of carboxyl compounds by photooxidation and a reduction in their mechanical strength.

The authors state that during the UV polymerization of the layers on the polyvinylidenefluoride (PVDF) substrate, its properties did not deteriorate [81]. Muchtar et al. [82] confirmed a reduction in tensile strength of a pure PVDF membrane after UV C irradiation. However, the resistance of PVDF was considerable, as a reduction to 60% of the original value was achieved only after 24 h of intensive irradiation.

Mechanical properties of the implant are defined as strength, stiffness, elasticity, and compliance [16, 18]. Tensile strength as a material constant is defined as maximum strength which solid material has to resist on a unit of area before its disruption or tearing occurs, and it is measured in Pa (or N/cm²) [83‒87]. Namely, it is a property of the product itself in case of implants and thus the maximum value of strength of the implant is defined with respect to its width in N/cm. Naturally, the implant must resist maximum physiological stresses which are created in the abdominal wall. Usual intra-abdominal pressure is 240 Pa (1.8 mm Hg) at rest and 22,660 Pa (170 mm Hg) during, for example, on landing following a jump. Laboratory data show prosthetic material should have a minimum limit of strength of 16 N/cm in any direction and 32 N/cm is the best in the maximum [19, 88‒92]. Most commercial implants, according to the manufacturers’ information, exceed these limits significantly. Different stiffness leads to differences in shape memory of the implant. Elasticity is defined as the tendency of the material to return to its original shape after deformation and it is measurable by elastic modules. Natural elasticity of the abdominal wall is up to 38% at 32 N/cm. It is naturally a little bit less in men than in women. An implant whose elasticity is greater than 30% at 32 N/cm is not suitable for surgery (chewing gum effect). On the other hand, too little elastic implants can limit distention of the abdominal wall, cause pain, and lead to failure of the surgery (for example, by pulling out the anchoring suture). The lowest elasticity should be between 4 and 15% at 16 N/cm. The mesh must have similar mechanical features (at least strength and also ductility) as the abdominal wall. Significant differences in mechanical features of the mesh and the abdominal wall often leads to the feeling of the presence of a foreign body [1, 16, 18].

The aim of this part of the study was to evaluate the optimal conditions for the surface modification of various mesh implants (PP, polyester [PES], PVDF) with hybrid nanolayers using the sol-gel method and to confirm the minimal effect of this treatment on their mechanical properties.

PP is the most widely used material for hernia mesh production. Also, other materials, like PES or PVDF are used commercially to a lesser extent.

Representatively chosen implants (group representative) were used for testing: Bard^® Soft Mesh and Bard^® Mesh (both produced by C.R. Bard Davol Inc., NJ, USA) – PP knit monofilament mesh is a representative of the most widespread type of meshes in a materially light version Soft (so-called lightweight mesh) and in the classic version (so-called heavyweight mesh) of sizes 15 × 10 cm (light – 44 g/m² with macropore the size of 2.5 mm) and 15 × 15 cm (heavy – 90 g/m² with micropores the size of 0.46 mm).

Premilene^® Mesh (produced by B Braun, Melsungen, Germany) – PP microporous (pore size 0.8 mm) is a heavy, non-absorbable mesh (82 g/m²), size 15 × 15 cm. Parietex^® Hydrophilic 3-Dimensional Mesh (produced by Medtronic, Minneapolis, USA) is a PES, very elastic, hydrophilic, multifilament, large porous (size of 1.0–1.6 mm), middle weight (78 g/m²) mesh for open surgery as 3D hexagonal knit, size 15 × 10 cm. DynaMesh^® Endolap (produced by FEG Textiltechnik, Aachen, Germany) is a PVDF monofilament mesh with a good shape memory leading to a lower creation of granuloma tissue and enabling usage of knits with a smaller porosity (biologically very stable). Some fibres are tinted green or, prospectively, black for manipulation and local identification, size 15 × 10 cm.

It should be noted that the most widely used classification of meshes according to weight distinguishes materials into heavy (over 80 g/m²), medium/middle (50–80 g/m²), or light (below 50 g/m²). Some authors and manufacturers also use the term ultralight for meshes with a defined density below 35 g/m². For PP material, we divide meshes into macroporous (over 1 mm pore diameter) and microporous (below 1 mm). It is the limit when the organism’s reaction to a foreign material during healing leaves part of the pore free or completely overgrows it with connective tissue (the so-called granuloma), which is currently considered undesirable for a number of reasons [93‒96]. For the PVDF material, a pore size of 0.6 mm is sufficient for its lower reactivity.

Antimicrobial sol labelled as AD30 was used for surface mesh modification. This sol is prepared by sol-gel method from TMSPM (supplier Sigma Aldrich, St. Louis, USA) and tetraethoxysilane (supplier Sigma Aldrich, St. Louis, USA) in a solvent of isopropyl alcohol (IPA, 0.03 wt% H₂O, supplier Lach:Ner, Neratovice, Czech Rep.) and it contains bound cations of silver, copper, and zinc (primarily as nitrate, supplier Fisher Scientific, Pardubice, Czech Rep.) in the amount of 2% of dry mass (calculated as metals) of antimicrobial nanolayer. Nitric acid (supplier Fisher Scientific, Pardubice, Czech Rep.) was used as a polycondensation catalyst and dibenzoyl peroxide (with 25 wt% H₂O, supplier Sigma Aldrich, St. Louis, USA) as a polymerization catalyst.

The specific starting composition of the sol was as follows: TMSPM 26 mL, tetraethoxysilane 10 mL, AgNO₃ 0.755 g, Cu(NO₃)₂.3H₂O 1.83 g, Zn(NO₃)₂.6H₂O 2.19 g, HNO₃ (c = 2 mol.dm⁻³) 2.0 mL, H₂O (deionized) 2.2 mL, and dibenzoyl peroxide 1 g. The total amount of IPA is 460 mL. Total concentration of dry mass (of resulting nanolayer weight) was 4.8 g/100 mL in the concentrated sol. Preparation and properties of this sol and resulting nanolayer have been published [66‒71], and the synthesis procedure was described in patents [72‒74].

Samples of meshes were cleaned by multiple soaking in clear IPA in advance. After IPA, evaporation sol was applied on the mesh surface by simple soaking (for 1 min) and removing surplus sol by dripping. The simple soaking method appears to be optimal for applying sol to the surface of fabrics/textiles from previous research. Other methods such as spraying or dip coating (immersion with instrumental pulling out at various defined speeds) have proven to be inappropriate or unnecessary.

After dripping of surplus sol from hanging samples finished, followed by the evaporation of any solvent residue from the applied nanolayer in a laboratory environment and finishing polycondensation reactions of silicate network by air humidity (about 30–60 min), the nanolayer was polymerized (stabilization) in a preheated dryer at 85°C for 3 h. Higher temperatures were not applied due to the temperature resistance of used materials.

According to previous research, a shorter stabilization time would be insufficient at these limit temperatures. It was also proven that a longer time (up to 5 h) does not lead to significantly better results.

The first experiments with sol application were performed on meshes cleaned only in IPA. However, the adhesion of the nanolayers was unsatisfactory (see the results chapter). Therefore, further work was carried out with surface-treated meshes (increasing of hydrophilicity of the mesh surface by forming surface C-OH groups) by physical plasma treatment or UV C radiation.

It should be noted that, for plastics, from a long-term point of view, the upper temperature of use is usually stated as the temperature at which the plastic retains at least 50% of its typical properties after 10,000 (up to 20,000) hours of exposure. By increasing the temperature, polymers first soften and eventually melt. When the temperature is further increased, the structure of the polymer already changes, its decomposition – degradation occurs. Polymer degradation is an irreversible process. The melting point of the polymers used is 160–176°C for PP, 240–275°C for PET, and 141–178°C for PVDF [97].

Physical plasma modification of surfaces was tested with all 5 samples of meshes. Testing was performed in semi-operational laboratories of SurfaceTreat, Turnov, Czech Republic, on equipment they themselves developed.

A plasma nozzle using gliding electric discharge (GlidArc) was used for the first series of experiments. The electrode system of a device working with a sliding discharge consists of a pair of identical electrodes shaped in such a way that the inter-electrode (and discharge) area expands in the direction of the working gas flow. After the working voltage is applied to the electrodes, an electrical breakdown occurs in the region of the minimum inter-electrode distance and the resulting discharge channel/filament is carried away (blown out) in the direction of the flowing working gas. Compressed air is used as the working gas in this case.

The modification conditions were chosen based on the temperature and mechanical resistance of the modified material. Compressed air flow is 12.7 dm³/min, and the distance between the substrate and the nozzle is 20 mm. The speed of movement of the substrate under the nozzle flame is 100 mm/s, and the displacement of the nozzle in the y direction is 10 mm/s. The displacement in the y direction was realized outside the modified substrate.

The second series of experiments was performed in a vacuum in LA_400 apparatus. Oxygen was added under low pressure for increased efficacy. Flow rate of the oxygen was 2.5 × 10⁻⁸ kg.s⁻¹ (200 sccm) under a pressure of 100 Pa. The height of the table was 240 mm, rotation at 7 revs/min (rpm) was left standard and duration of plasma treatment was set to 5, 10, 30, or 60 s.

Consequently, sol AD30 in two dilutions 1:1 and 1:3 was applied to the prepared samples. The application of the sol must be carried out immediately after the activation of the surface, as the activation wears off within minutes. Thermal polymerisation of the hybrid nanolayer was finished in the dryer at 85°C for 5 h after evaporating of the solvent and final hydrolysis in a laboratory environment (30 min).

Radiation using a UV C lamp (UV-C Hg lamp TUV 15W G15T8 UV-C long-life made by Philips, 1 min on each side of the sample) was tested as another possibility for the activation of the mesh surface. However, it is necessary to expect certain problems during this method, as UV C radiation disturbs the mass of the meshes as well and thus it can decrease their mechanical properties. Therefore, a check of the mechanical properties of selected samples of the modified meshes was later added as well.

An ultra-high-resolution field emission scanning electron microscope (SEM) (Carl Zeiss ULTRA Plus) equipped with an energy dispersive spectroscopy detector (Oxford X-Max20), was used for checking the quality of the nanolayers on the surface of the meshes and for confirming their chemical composition. Before the evaluation of the quality of layers, the sample surface was conducted by deposition of 1 nm of Pt using sputter coater Quorum Q150R ES. The samples were observed at accelerating voltage of 2 kV, aperture 10 μm, extractor voltage 3 kV (probe current ≈1–2 pA), and working distance ≈2.5–3.0 mm; the topographic signal was collected using highly sensitive integrated InLens secondary electron detector. For the local chemical analysis, following parameters were set: accelerating voltage 10 kV, aperture 60 μm, extractor voltage 4.26 kV (probe current ≈1 nA), working distance 8.5 mm, activated platinum coating correction. The quality of the prepared nanolayers was monitored for all prepared samples (5 mesh materials, various surface modifications) in comparison with the original samples (without the application of nanolayers).

For samples with physical plasma modification, there is no need to worry about a significant decrease in strength because plasma acts only superficially. Another situation occurs with samples modified by UV C radiation, which can disrupt the inner layers of the sample material. The published results showed that a decrease in the mechanical properties due to UV C radiation can be expected for PP, on the contrary, PVDF should be stable [79‒82]. Information on the stability of PES against UV C radiation has not been found. Therefore, Bard^® Soft Mesh and Bard^® Mesh (both produced by C.R. Bard Davol Inc.) and Parietex^® Hydrophilic 3-Dimensional Mesh (produced by Medtronic) samples were selected for objective strength measurements.

Structural and mechanical properties (porosity, shape stability, elasticity/flexibility) of original and modified samples after the activation of UV C radiation and nanolayer from AD30 sol (1:1) were compared optically and manually by members of the scientific team subjectively. It was not possible to differentiate the original samples from the modified ones in a blind test.

Strength and ductility of selected samples (Bard mesh, Bard soft mesh, and Parietex) were measured objectively on a universal tensile testing machine Testometric M350-5CT of the Technical University of Liberec, Faculty of Textile Engineering. The size of samples was 30 × 75 mm, loading was performed in the direction of the longer side, loading speed was 100 mm/min (Fig. 1). For each supplied sample, five independent tests of the mesh sample with the nanolayer and five original samples (without modification) were performed for comparison. Statistical evaluation of the results obtained was performed with the help of statistical software QCExpert 3.3 (with a level of significance of 5%).

After the evaluation of the first series of microbiological tests, the dilution of the antimicrobial sol in further tests was chosen in a ratio of 1:1 and 1:3 (the sol was diluted with IPA p.a.). The reason was that the concentrated sol after thermal polymerization cracked on the fibres of the material and the dilution ratio of the sol 1:5 showed negligible antibacterial activity.

The ISO 20645 agar plate propagation method was used for qualitative evaluation, according to which the zone of inhibition and the growth of bacteria under the sample are evaluated [98]. The AATCC 100 (2019) method was used to evaluate the quantitative microbiological efficiency [99]. Pathogenic bacterial strains from the Czech Collection of Microorganisms of Masaryk University in Brno were used for the tests. E. coli – CCM 2024 (AATCC 9637) was used as a representative of the Gram-negative rod-shaped bacterium and Staphylococcus aureus – CCM 2260 (AATCC 1260) as a typical Gram-positive coccal bacterium.

Necessary concentration of applied sol (concentrated sol and diluted by IPA in the following ratios: 1:1, 1:3, and 1:5) and furthermore the quality of nanolayers of the mesh surface were observed during preliminary experiments using SEM. In these experiments, it was found that concentrated sol remained stuck to the fine lugs of the meshes and the layer was too thick and cracked under mechanical stress. On the other hand, diluted sol in the ratio of 1:5 did not make a good-quality nanolayer. Therefore, only sols diluted by IPA in the ratios 1:1 and 1:3 were chosen for other experiments.

Another finding was the lack of adhesion of the applied nanolayer on the mesh materials, as can be seen from Figure 2b. For comparison, the image of the surface of the mesh material without the applied nanolayer is shown (Fig. 2a). Similar results were observed by SEM on all samples of modified mesh materials. For this reason, activation of their surfaces with the help of physical plasma or UV C radiation was performed for subsequent experiments.

The quality of all prepared nanolayers on activated substrates was checked by SEM and their antibacterial activity. The use of atmospheric plasma turned out to be inappropriate because the samples were destroyed during the experiments. The most suitable duration of generating vacuum plasma was evaluated as 10 s on the Bard^® Soft Mesh and Bard^® Mesh (both produced by C.R. Bard Davol Inc.) samples, neither longer (30 and 60 s) nor shorter time (5 s) showed any significant effect in better antibacterial activity.

A difference in surface properties between times of plasma treatment for 10, 30, and 60 s were not found for the Premilene^® Mesh (produced by B Braun) sample, however, a time of 5 s was insufficient. The result is that achieving the effect of a good wetting of the surface by sol (activation) for this sample, plasma treatment of 10 s is sufficient.

A time of 60 s for the plasma treatment was evaluated as the most suitable for the Parietex^® Hydrophilic 3-Dimensional Mesh (produced by Medtronic) sample. Shorter times were not sufficient.

The conditions for plasmatic modification of the surface were insufficient for all the times for the DynaMesh^® Endolap (produced by FEG Textiltechnik) sample. It was only until a time of 60 s plasma treatment that the first hints of adhesion was observed, yet they were not optimal. An even longer time of plasma modification would be necessary for good activation of the surface. Meshes made of PVDF are thus the least suitable of the investigated materials for this method of surface treatment.

The results of the above-mentioned tests can be summarized in the way that optimal plasma modifications in the vacuum apparatus are about 10 s for the meshes based on PP (under stated working conditions), about 60 s for a mesh based on PES, and evidently significantly longer than 60 s for meshes based on fluoropolymer PVDF. Particular conditions for plasma modifications of plastic surgery meshes will be necessary to optimize on particular production devices.

The quality of all prepared nanolayers on activated substrates was checked by SEM and their antibacterial activity. If the nanolayer is of good quality, it is practically not visible in the SEM image (possibly covers the irregularities, see comparison of Fig. 2a and c).

Nanolayers applied on the meshes modified by UV C radiation (1 + 1 min) were very good for sol wetting and created a quality nanolayer (Fig. 2c, d). Meshes modified by UV C were practically indifferentiable from those modified in a plasmatic way during manipulation.

The presence of the applied nanolayer and also its chemical composition corresponding to the sol dry mass (ratio of Si, Ag, Cu, Zn) were confirmed on all five modified meshes by chemical analysis of the surfaces of the original and modified meshes (energy dispersive spectroscopy detector in Carl Zeiss ULTRA Plus). Regarding the fact that a nanolayer is very thin, elements from the corresponding original mesh (every time increased contents of carbon, increased oxygen with PES, and increased fluorine with PVDF) were always evident during chemical analysis. The final quality control of the nanolayers on all prepared samples was performed by their antibacterial activity [98, 99].

The results of microbiological tests according to ISO 20645 have clearly shown that the most appropriate dilution of the sol is in a ratio of 1:1. Dilution of the sol in a ratio of 1:3 proved to be insufficiently effective against both tested bacterial strains according to the above-mentioned standard. Evaluation of the antibacterial effectiveness according to the AATCC 100 standard also confirmed the suitability of diluting the antibacterial sol in a ratio of 1:1. According to this standard for the bacterial strain S. aureus, the effectiveness in terms of the R factor ranged from 79 to 100% in the tested samples. For the bacterial strain E. coli, the efficiency ranged from 99.5 to 100%.

Strength of implants is normally tested by traction uniaxially, by vertical strokes biaxially, by protrusion (pulling a little ball through clamped mesh), and by tugging [19]. Evaluation on the basis of measuring of strength of meshes in tugging and values of strength and ductility limits for comparison were used for the purposes of this work (Fig. 3-5).

It was proved by statistical comparison of measured values of the limits of strength of meshes with the help of QCExpert 3.3 program (two selections comparison module) that while comparing limits of strength of original and corresponding modified samples of meshes, ranges of values were similar, however, means were different (at a level of significance of 0.05). There was a decrease of resulting maximum limit of strength in all three cases, from 72 up to even 86% of the value of the original samples. The survey of the results of statistical processing is shown in Table 1.

Also, experimentally determined values of mesh ductility at the maximum limit of strength were processed statistically in a similar way. Ranges of measurement values were the same in this case as well, however, the means – except for the values for Bard^® Soft Mesh (produced by C.R. Bard Davol Inc.) where the result was statistically inconclusive as it was borderline – are different (at the level of significance of 0.05). There was a decrease in the resulting ductility up to 83 to even 91% of the original sample values (Table 2) in all three cases.

Mechanical properties of meshes (strength limit, ductility) were decreased slightly (to 72–86% of the value of original samples at the strength limit and to 83–91% at the ductility) by mesh modification before applying the antibacterial nanolayer with the help of UV C radiation. However, this decrease does not play practically any role concerning the usage of meshes in surgery and above-mentioned limits. Sterilization of treated samples before packaging and commercial use (ethylene oxide, etc.) has no effect on these nanolayers.

At present, modification of surfaces of medical implants appears to be the most effective method to fight infection and spreading of nosocomial agents. The use of hybrid nanolayers with immobilized antiseptic substances/ions through the sol-gel method, which would ensure protracted release of these substances and this way increase the implant resistance towards infection, appears to be promising. Our research showed this technology is suitable for surface modification of all basic implants/meshes commonly used in surgery today (especially on the base of PP and PES). Pre-treating the surface by vacuum plasma or UV C radiation appeared to be a necessary step for applying and sufficient adherence of nanolayers from an antibacterial sol.

Mechanical properties of meshes (strength limit, ductility) were decreased slightly by mesh modifications before the application of UV C radiation and application of the antibacterial nanolayer. However, this decrease does not have any negative clinical effects regarding the way meshes are used in surgery and the above-mentioned limits.

Therefore, this could be a method easily applicable during production, financially undemanding, and leading to the obtaining of materials with significant added value after confirming improved antibacterial properties and screening out implant toxicity with the mentioned surface modification by our other research. Also, research on the change in the anti-adhesive properties of the material after treatment with the sol-gel method could provide interesting results [46, 100‒111].

The authors are thankful to Mr. P.Kejzlar – operator of scanning electron microscope, and Mr. L.Čapek – operator of Testometric tensile testing machine.

Ethics approval is not required due to local and also national guidelines (neither humans nor animals were subjects of the research).

The authors have no conflict of interest to declare.

This work was supported by internal research grant of Regional Hospital Liberec.

J.S. and I.S. designed the research, performed preliminary experiments, and completed the manuscript. H.S. performed sol-gel modification and plasmatic and UV C activation of surfaces. P.E. performed statistics. P.E. and R.G. supervised and edited the manuscript.

All data generated or analysed during this study are included in this article or are available upon request from the corresponding author. Data from our previous and follow-up research on this topic beyond the content of this article have been published or will be published elsewhere. Further enquiries can be directed to the corresponding author.