Novel Slippery Liquid-Infused Porous Surfaces (SLIPS) Based on Electrospun Polydimethylsiloxane/Polystyrene Fibrous Structures Infused with Natural Blackseed Oil

Hydrophobic fibrous slippery liquid-infused porous surfaces (SLIPS) were fabricated by electrospinning polydimethylsiloxane (PDMS) and polystyrene (PS) as a carrier polymer on plasma-treated polyethylene (PE) and polyurethane (PU) substrates. Subsequent infusion of blackseed oil (BSO) into the porous structures was applied for the preparation of the SLIPS. SLIPS with infused lubricants can act as a repellency layer and play an important role in the prevention of biofilm formation. The effect of polymer solutions used in the electrospinning process was investigated to obtain well-defined hydrophobic fibrous structures. The surface properties were analyzed through various optical, macroscopic and spectroscopic techniques. A comprehensive investigation of the surface chemistry, surface morphology/topography, and mechanical properties was carried out on selected samples at optimized conditions. The electrospun fibers prepared using a mixture of PDMS/PS in the ratio of 1:1:10 (g/g/mL) using tetrahydrofuran (THF) solvent showed the best results in terms of fiber uniformity. The subsequent infusion of BSO into the fabricated PDMS/PS fiber mats exhibited slippery behavior regarding water droplets. Moreover, prepared SLIPS exhibited antibacterial activity against Staphylococcus aureus and Escherichia coli bacterium strains.


Introduction
Biofouling refers to the phenomena of the aggregation and growth of live organisms on wet surfaces and membranes [1,2]. This has a significant impact on liquid transportation, non-fouling marine devices, desalination membranes, and health care devices [3][4][5][6][7][8].
Coating nanoparticles (NPs) on the side in contact with the liquid is one method to prepare an anti-biofouling surface. Ag, TiO 2 , ZnO, and Cu NPs are found to exhibit antimicrobial properties, which primarily kill the microorganisms settled on the surface, acting as a biocide. The synthesis of these NPs is costly and increases the overhead in production for possible commercialization. Furthermore, the performance is reported to decline over time, mainly due to the poor addition of the coat with the substrate. Studies have been conducted on NPs on a graphene oxide (GO) matrix to mitigate some of the concerns encountered with coating pristine metal or metal oxide NPs on membranes. Ag NPs on GO [9,10] and Fe NPs on GO systems [11,12] have been extensively studied in this regard and have been and conductivity [41]. To produce uniform, bed-free fibers, several process parameters need to be adjusted during the electrospinning process, including the distance between the syringe and the collector, applied voltage, and solution concentration. An increase in the voltage beyond a critical value typically results in the formation of beads and an increase in the fiber diameter due to the increase in the jet velocity [42]. Moreover, when the polymer solution concentration is low, the entanglements of the polymeric chains will break into small fragments before reaching the collector [43]. Similarly, the distance between the needle tip and the collector plays a vital role in the properties of the electrospun fibers, specifically the morphology. The surface morphology mainly depends on the deposition time and evaporation rate; hence, a proper distance is required to produce uniform fibrous matrices [44]. The fibers can be tailored depending on the properties/specifications required for their usage.
In this study, novel SLIPS have been fabricated by electrospinning polystyrene PS/PDMS fiber mats on polyethylene PE and polyurethane PU substrates with subsequent BSO infusion. BSO is commonly used in medicine because it demonstrates antimicrobial, anti-inflammatory and antioxidant effects and no cytotoxicity towards human peripheral blood mononuclear cells [45]. The process conditions were optimized to yield a homogenous fibrous mat with the desired properties. The anti-wetting/hydrophobic properties of the prepared fibers, as well as morphology, mechanical properties, and chemical composition, have been thoroughly analyzed. The expected end-use of these SLIPS is on health care products made up of PE or PU. In addition, a biocompatible lubricant like BSO with its natural character and antimicrobial properties, containing 32 compounds, including 9-eicosyne (63.04%), linoleic acid (13.48%) and palmitic acid (9.68%) as major constituents [46,47], in addition to its multiple medicinal properties encourages the implementation of such systems in commercial practice. Antibacterial properties of prepared SLIPS on PU and PE have been investigated and shown to exhibit significant antibacterial activity against Gram-positive S. aureus and Gram-negative E. coli bacteria.

Surface Morphology/Topography Analysis
Electrospun fiber mats composed of PDMS and PS were utilized to create hydrophobic porous structures for oil infusion. The use of PS in THF with a concentration of 10% w/v (Figure 1a) led to the creation of well-shaped thick fibers. A combination of PDMS with PS overcomes problems with the electrospinning process of pure PDMS [48]. In order to improve the fiber mats' production processes, several concentrations of the PS/PDMS/THF solution were tested (conditions are provided in Table 1). Using a 1:1:8 ratio of PS/PDMS/THF resulted in the formation of fibers with different diameters and led to the formation of a glue-like fibrous matrix. Nevertheless, SEM images (Figure 1b) of the fibers confirmed the formation of fibrous structures, which proves the good integrity between PS and PDMS in forming a homogenous spinnable polymeric solution. The water contact angle (WCA) showed a hydrophobic behavior (WCA = 124.0 ± 3.6 • ) ( Table 2). Upon increasing the THF solvent content in the polymer mixture (1:1:10 PS/PDMS/THF), the viscosity of the mixture decreased, which resulted in the formation of well-defined fibers. This resulted in fiber mats with larger diameters (rougher surface); hence, the hydrophobicity increased (the contact angle of water for PS/PDMS/THF with a ratio of 1:1:8 was 124 ± 3.6 • in comparison with 149.4 ± 3.2 • for the PS/PDMS/THF mixture with a ratio of 1:1:10). Decreasing the ratio of PS to PDMS (0.5:1:10) (Figure 1) led to the formation of very thin fibers surrounded by flocs. Moreover, increasing the PS content in the PS/PDMS solution (1.5:1:10) led to the formation of beaded fibers. The formation of flocs might be caused by the low net charge density of the prepared polymer solution [31]. However, the water contact angle was 141 ± 3.8 • , which was relatively close to 1:1:10 PS/PDMS/THF (149 ± 3.2 • ) ( Table 2). 124.0 (±3.6) 1.5 g:1 g PS: PDMS in 10 mL THF 141.3 (±3.8)    [31]. However, the water contact angle was 141 ± 3.8°, which was relatively close to 1:1:10 PS /PDMS/THF (149 ± 3.2°) ( Table 2).   [31]. However, the water contact angle was 141 ± 3.8°, which was relatively close to 1:1:10 PS /PDMS/THF (149 ± 3.2°) ( Table 2).   [31]. However, the water contact angle was 141 ± 3.8°, which was relatively close to 1:1:10 PS /PDMS/THF (149 ± 3.2°) ( Table 2).   [31]. However, the water contact angle was 141 ± 3.8°, which was relatively close to 1:1:10 PS /PDMS/THF (149 ± 3.2°) ( Table 2).    Figure 2b confirm the homogenous distribution of PDMS within the electrospun fiber mats. The cross-section view of the fibers in Figure 2a demonstrates that the formed fiber is homogeneous and uniform. The optimized electrospun fibers prepared using the mixture of PS/PDMS in the ratio of 1:1:10 (g/g/mL) using THF solvent on Al foil was used for the formation of fiber mats on the plasma-pre-treated PE and PU substrates, which were used as SLIPS for oil infusion. Figure 2 shows SEM and EDX mapping of the cross-section of electrospun fiber mats containing PS/PDMS/THF-1:1:10 (g/g/mL), confirming the presence of C, O, N and Si elements. Both polymers, PDMS and PS, contribute to C and O elements due to their chemical structures. Distinguishable N and Si elements in Figure 2b confirm the homogenous distribution of PDMS within the electrospun fiber mats. The cross-section view of the fibers in Figure 2a demonstrates that the formed fiber is homogeneous and uniform. The optimized electrospun fibers prepared using the mixture of PS/PDMS in the ratio of 1:1:10 (g/g/mL) using THF solvent on Al foil was used for the formation of fiber mats on the plasma-pre-treated PE and PU substrates, which were used as SLIPS for oil infusion. AFM was performed to provide detailed information about surface morphology/topography of the optimized electrospun fiber mats from the 80 × 80 µm 2 surface area ( Figure 3). This analysis provides information on fiber diameter and the structural features of the fiber mats. It can be observed that the fiber mats produced using neat PS/THF solution (1:10 g/mL) are homogeneous and well-packed; hence, they are expected to possess a relatively small pore size. Smaller pore sizes cause an enhancement in surface hydrophobicity due to the decreased ability of water to penetrate through porous structures. Moreover, it can be remarked that the surface roughness represented by the Ra value (arithmetic mean of line profile) was 501 nm, while surface roughness is a significant factor affecting the hydrophobicity of the surface. The diameter of the fibers can be estimated by measuring the dz of particular peaks in the line profile graphs obtained from ZSensor AFM images. The diameter of PS fibers was approximately 300-700 nm. In the case of the fiber mats fabricated using the polymeric mixture PDMS/PS/THF-1:1:10 (g/g/mL), the fabricated fiber mats excelled with rougher structures, while Ra achieved 1989 nm, caused by the larger diameter of particular fibers around 2-2.5 µm. This increased roughness was responsible for the high water contact angle value (WCA = 149.4 ± 3.2°). The optimized PS/PDMS samples were subsequently used for the preparation of SLIPS on PE and PU substrates, which is discussed later. AFM was performed to provide detailed information about surface morphology/ topography of the optimized electrospun fiber mats from the 80 × 80 µm 2 surface area ( Figure 3). This analysis provides information on fiber diameter and the structural features of the fiber mats. It can be observed that the fiber mats produced using neat PS/THF solution (1:10 g/mL) are homogeneous and well-packed; hence, they are expected to possess a relatively small pore size. Smaller pore sizes cause an enhancement in surface hydrophobicity due to the decreased ability of water to penetrate through porous structures. Moreover, it can be remarked that the surface roughness represented by the Ra value (arithmetic mean of line profile) was 501 nm, while surface roughness is a significant factor affecting the hydrophobicity of the surface. The diameter of the fibers can be estimated by measuring the dz of particular peaks in the line profile graphs obtained from ZSensor AFM images. The diameter of PS fibers was approximately 300-700 nm. In the case of the fiber mats fabricated using the polymeric mixture PDMS/PS/THF-1:1:10 (g/g/mL), the fabricated fiber mats excelled with rougher structures, while Ra achieved 1989 nm, caused by the larger diameter of particular fibers around 2-2.5 µm. This increased roughness was responsible for the high water contact angle value (WCA = 149.4 ± 3.2 • ). The optimized PS/PDMS samples were subsequently used for the preparation of SLIPS on PE and PU substrates, which is discussed later. 6

Characterization of Mechanical Properties Characterization
The investigation of the mechanical properties was done thoroughly using the nanoindentation measurements by AFM with an MFP-3D Nanoindenter. Hardness and reduced Young's modulus (Ec) were measured and calculated as penetration of indenter tip into the fiber mats' surfaces in different areas and taking an average of 10 points. As the indenter penetration depth was in the range of 2-4 µm, it is assumed that the obtained mechanical properties are attributed to the fiber mats and not only particular fibers. This allowed average values of hardness and Ec with standard deviations to be obtained, as shown in Table 3. The relation between stiffness and hardness can vary based on the properties of each material. In practice, the Ec of the produced PS fibers prepared in THF solvent was 3.4 ± 0.7 MPa, and their hardness was 0.4 ± 0.2 MPa. In coexistence with PDMS, the Ec and hardness of PS/PDMS even increased. The Ec value was greater than in the neat PS, achieving a value of 7.2 ± 1.2 MPa, and hardness was increased around 3 times, achieving a value of 1.1 ± 0.3 MPa. Compared to the literature, the mechanical properties of the produced fibers were similar to neat PDMS [49].

Chemical Composition Analyses
The chemical composition of the fabricated electrospun fiber mats was analyzed using FTIR-ATR to confirm the presence of certain functional groups. The results are shown in Figure 4. PS in its pure form contains a styrene aromatic ring, which is in agreement with the literature [50]. The main peaks of the pure form shown in Figure 4 are -CH stretching at 3081 cm -1 -3001 cm −1 of the aromatic ring resonation. The stretching of -CH at 2923 cm −1 and CH2 at 2850 cm −1 originates from the asymmetric and symmetric stretching vibrations. The mono-substitution of the benzene ring of styrene occurs at 1943-

Characterization of Mechanical Properties Characterization
The investigation of the mechanical properties was done thoroughly using the nanoindentation measurements by AFM with an MFP-3D Nanoindenter. Hardness and reduced Young's modulus (E c ) were measured and calculated as penetration of indenter tip into the fiber mats' surfaces in different areas and taking an average of 10 points. As the indenter penetration depth was in the range of 2-4 µm, it is assumed that the obtained mechanical properties are attributed to the fiber mats and not only particular fibers. This allowed average values of hardness and E c with standard deviations to be obtained, as shown in Table 3. The relation between stiffness and hardness can vary based on the properties of each material. In practice, the E c of the produced PS fibers prepared in THF solvent was 3.4 ± 0.7 MPa, and their hardness was 0.4 ± 0.2 MPa. In coexistence with PDMS, the E c and hardness of PS/PDMS even increased. The E c value was greater than in the neat PS, achieving a value of 7.2 ± 1.2 MPa, and hardness was increased around 3 times, achieving a value of 1.1 ± 0.3 MPa. Compared to the literature, the mechanical properties of the produced fibers were similar to neat PDMS [49].

Chemical Composition Analyses
The chemical composition of the fabricated electrospun fiber mats was analyzed using FTIR-ATR to confirm the presence of certain functional groups. The results are shown in Figure 4. PS in its pure form contains a styrene aromatic ring, which is in agreement with the literature [50]. The main peaks of the pure form shown in Figure 4 are -CH stretching at 3081 cm -1 -3001 cm −1 of the aromatic ring resonation. The stretching of -CH at 2923 cm −1 and CH 2 at 2850 cm −1 originates from the asymmetric and symmetric stretching vibrations. The mono-substitution of the benzene ring of styrene occurs at 1943-1728 cm -1 , and the deformation between -CH 2 and C-C in the aromatic ring occurs at 1500 cm -1 . PDMS in its pure form contains three main peaks [51]: an Si-CH 3 peak at 1270 cm −1 , O-Si-O at 1160 cm −1 , and a methyl group -CH at 2980 cm −1 . When both the polymers were mixed, PS and PDMS shared only the same solution, and their peaks were merged but not reacted. This was observed from the peaks of the PS/PDMS spectrum. The peaks of O-Si-O and -Si-CH 3 were shown with high intensity at 1160 cm -1 and 1270 cm -1 . Interestingly, the whole peaks of PS were observed in the mixed fibers with lower intensity than the pure form, but no shifting in the wavenumber was observed. The stretching of the peaks of -CH (PS and methyl groups in PDMS) was observed at 3030 cm -1 and 2960 cm -1 . The aromatic C-C was found at 1500 cm -1 in the FTIR spectrum of PS/PDMS, and it confirmed the successful incorporation of PS with PDMS. 1728 cm -1 , and the deformation between -CH2 and C-C in the aromatic ring occurs at 1500 cm -1 . PDMS in its pure form contains three main peaks [51]: an Si-CH3 peak at 1270 cm −1 , O-Si-O at 1160 cm −1 , and a methyl group -CH at 2980 cm −1 . When both the polymers were mixed, PS and PDMS shared only the same solution, and their peaks were merged but not reacted. This was observed from the peaks of the PS/PDMS spectrum. The peaks of O-Si-O and -Si-CH3 were shown with high intensity at 1160 cm -1 and 1270 cm -1 . Interestingly, the whole peaks of PS were observed in the mixed fibers with lower intensity than the pure form, but no shifting in the wavenumber was observed. The stretching of the peaks of -CH (PS and methyl groups in PDMS) was observed at 3030 cm -1 and 2960 cm -1 . The aromatic C-C was found at 1500 cm -1 in the FTIR spectrum of PS/PDMS, and it confirmed the successful incorporation of PS with PDMS.

Adhesion Investigation
The plasma treatment was used to improve the wettability and adhesion characteristics of PE and PU substrates prior to the deposition of the PS/PDMS fiber mats. Analyses of adhesion between polymeric substrates and produced electrospun fiber mats were carried out by peel test measurements using Scotch tape. Figure 5 shows peel resistance (peel force per entire width) between PS/PDMS fiber mats and the PE and PU substrates. Peel resistance of untreated PE was relatively low, achieving a value of 3.9 ± 1.5 N/m. The plasma treatment was responsible for an improvement in the adhesion of PE, while peel resistance increased to 6.2 ± 0.8 N/m. More visible differences were observed for PU. The peel resistance of untreated PU was much lower compared with untreated PE, achieving 0.1 ± 0.1 N/m because of the lower surface free energy of PU. Plasma treatment led to a remarkable increase in adhesion, while peel resistance increased to 0.4 ± 0.2 N/m. The improvement of the adhesion of the plasma-treated polymeric surface was caused by the improvement in wettability as a result of changes in the functionalization and roughness [52,53].

Adhesion Investigation
The plasma treatment was used to improve the wettability and adhesion characteristics of PE and PU substrates prior to the deposition of the PS/PDMS fiber mats. Analyses of adhesion between polymeric substrates and produced electrospun fiber mats were carried out by peel test measurements using Scotch tape. Figure 5 shows peel resistance (peel force per entire width) between PS/PDMS fiber mats and the PE and PU substrates. Peel resistance of untreated PE was relatively low, achieving a value of 3.9 ± 1.5 N/m. The plasma treatment was responsible for an improvement in the adhesion of PE, while peel resistance increased to 6.2 ± 0.8 N/m. More visible differences were observed for PU. The peel resistance of untreated PU was much lower compared with untreated PE, achieving 0.1 ± 0.1 N/m because of the lower surface free energy of PU. Plasma treatment led to a remarkable increase in adhesion, while peel resistance increased to 0.4 ± 0.2 N/m. The improvement of the adhesion of the plasma-treated polymeric surface was caused by the improvement in wettability as a result of changes in the functionalization and roughness [52,53].

Slippery Behavior Investigation
As mentioned, slippery behavior observed in SLIPS provides an environmentally way to combat the adhesion of pathogens or any particle of that sort on a surface. In this concern, PDMS/PS/THF-1:1:10 (g/g/mL) fiber mats fabricated on plasma-treated PE or PU substrates were converted into a slippery surface by modification with natural-based

Slippery Behavior Investigation
As mentioned, slippery behavior observed in SLIPS provides an environmentally way to combat the adhesion of pathogens or any particle of that sort on a surface. In this concern, PDMS/PS/THF-1:1:10 (g/g/mL) fiber mats fabricated on plasma-treated PE or PU substrates were converted into a slippery surface by modification with natural-based BSO via spin coating. Visual examination of the BSO layer revealed it to be stable and sufficiently wet within the fiber mat substrate. Pathogens are more easily transported by water than any other liquid encountered during daily activities. Hence, water was employed as the model-impinging liquid to evaluate the potential liquid-repellent performance of BSO-infused PDMS/PS fiber mats. PDMS/PS fiber mats on substrate exhibited a WCA of 149 • , which was reduced to~78 • for PDMS/PS/BSO-PU and~67 • with PDMS/PS/BSO-PE. The key requirements of the lubricating liquid (BSO) are to have a higher affinity for the fiber mat surface compared to that of the impinging liquid (water) and for the two liquids to be immiscible [14]. Immiscibility is confirmed by the satisfactory water contact angle values depicted in Figure 7. As can be seen, BSO-infused fiber mats prepared on the PU substrate demonstrated faster sliding behavior than on the PE substrate. Additionally, water droplets of 3 µL, used to avoid the line pinning effect, had a more globular structure on the PU substrate (Figure 7a), and the shape was maintained throughout the sliding period. Conversely, the droplet was more spread on the PE substrate (Figure 7b), as observed by the CCD camera connected to the contact angle measuring device. Precise analysis shows that the translation of the water droplet was similar on both substrates after the initial 50 s. It is also noteworthy that the water droplet was dispensed on the surface at 0 • , followed by tilting to 10 • at a gentle base to avoid vibrational impacts on the droplet during the sliding stage. At 50 s, the stage reaches a 10 • inclination, and the translation of the impinging droplet afterward is distinctly two-fold faster with the BSO-infused fiber mat on the PU substrate. However, PDMS/PS/BSO on both the PU and PE substrates demonstrate favorable sliding behavior with water as the impinging liquid. Thus, the use of a biocompatible lubricant like BSO with multiple medicinal properties encourages the implementation of such systems in commercial practice.

Antibacterial Activity Investigation
The antibacterial properties of fabricated SLIPS on the polymeric substrates were evaluated using S. aureus (Gram-positive) and E. coli (Gram-negative) bacterium strains. Increases in the bacterial colonies of the PS/PDMS samples are summarized in Table 4, and representative photos are shown in Figures 8 and 9 for S. aureus and E. coli, respectively. The fabrication of PS/PDMS fiber mats on the PE or PU substrate demonstrated poor antibacterial activity against S. aureus and E. coli, with poor bacterial proliferation inhibition caused by their chemical nature. However, subsequent fabrication of SLIPS using the BSO infusion into the PS/PDMS porous structures led to a significant improvement in antibacterial activity, as BSO has a strong antibacterial effect. PS/PDMS/BSO-PE samples demonstrated an increase of zero bacterial colonies for both S. aureus and E. coli, and PS/PDMS/BSO-PU samples were characterized by zero colonies for S. aureus and zero or single colonies detectable for E. coli. This analysis proved the

Antibacterial Activity Investigation
The antibacterial properties of fabricated SLIPS on the polymeric substrates were evaluated using S. aureus (Gram-positive) and E. coli (Gram-negative) bacterium strains. Increases in the bacterial colonies of the PS/PDMS samples are summarized in Table 4, and representative photos are shown in Figures 8 and 9 for S. aureus and E. coli, respectively. The fabrication of PS/PDMS fiber mats on the PE or PU substrate demonstrated poor antibacterial activity against S. aureus and E. coli, with poor bacterial proliferation inhibition caused by their chemical nature. However, subsequent fabrication of SLIPS using the BSO infusion into the PS/PDMS porous structures led to a significant improvement in antibacterial activity, as BSO has a strong antibacterial effect. PS/PDMS/BSO-PE samples demonstrated an increase of zero bacterial colonies for both S. aureus and E. coli, and PS/PDMS/BSO-PU samples were characterized by zero colonies for S. aureus and zero or single colonies detectable for E. coli. This analysis proved the significant antibacterial effect of prepared SLIPS. Table 4. Antibacterial activity of samples.

Material
For the preparation of the polymeric substrate for modification, commercial-grade low-density polyethylene (LDPE) FE8000 pellets kindly provided by Qatar Petrochemical Company (QAPCO, Doha, Qatar) were used. Compression molding was used to create thin homogeneous films with a thickness of around 0.41 mm using an industrial mounting press machine (Carver, Wabash, IN, USA). To produce a flat surface, the pellets were melted at 160 • C and compressed for 2 min with a force of 2 tons while keeping the temperature constant. Water was used to cool the LDPE films to room temperature.
The above-mentioned LDPE and PU films were cleaned with acetone and ethanol, respectively, to remove any dust or potential contamination from the manufacturing process that might impact the surface characteristics, and then air-dried for 20 min at room temperature (RT).

Plasma Treatment
A low-temperature plasma treatment of the PE and PU substrates was used before the preparation of electrospun fibers with infused oils on them. For this purpose, the radiofrequency (RF) plasma system Venus75-HF (Plasma Etch Inc., Carson, CA, USA) working at reduced pressure (~25 Pa) was employed. The reactive species, such as electrons and ions responsible for the treatment of polymeric substrates, were generated by an application of a high voltage at a typical frequency of 13.56 MHz. The nominal power was adjusted to 80 W for both, PE and PU substrates. The applied treatment time based on the previous optimizations was 120 s for PE [52] and 180 s for PU [53].

Electrospinning Procedure
Neat PS fiber mats were fabricated using PS/THF solution with a ratio of 1:10 (g/mL). The PS/PDMS fiber mats were fabricated using a NaBond (Shenzhen, China) electrospinning device. Aluminum foil was placed on a plate or drum and was used as a collector of fiber mats to ensure a conductive substrate. Then, the plasma-treated PE and PU substrates (with improved adhesion as a result of increased wettability) were adhered to the aluminum foil in order to create porous structures on polymeric substrates. For the electrospinning process, 5 mL syringes were used, and the electrospinning time was approximately 2 h in multi-jet mode. The procedure for the fabrication of PS/PDMS fiber mats was carried out at a voltage of 12.5-15 kV, a needle/collector distance of 15 cm, and a flow rate of 2.5 mL/h. PDMS with the curing agent was used in a fixed ratio (10:1), and as a mixture, they were used in different concentrations with THF, as shown in Table 1.

Oil Infusion
To create a slippery surface, natural-based BSO was infused into the electrospun fiber mats using the spin coating technique. This technique allows the production of a uniform thin film layer by depositing a small drop of BSO onto the center of a substrate and then spinning the substrate at high speed. Moreover, this ensures the removal of oil excess from the substrate. The oil infusion into the electrospun fibers was carried out using Spin coating WS 650-23 (Spincoater-Laurell, North Wales, PA, USA) at a speed of 500 rpm for 60 s.

Surface Energy and Slippery Effect Investigation
The changes in the hydrophobicity and slippery behavior of the prepared electrospun fibers with infused oil treated by plasma or heated were evaluated by static water contact angle measurement. The optical contact angle measuring system OCA35 (DataPhysics, Filderstadt, Germany) was used for this study, which is equipped with a high-resolution CCD camera. Ultrapure water was used as a testing liquid. A water droplet of 5 µL was dispensed at ambient air conditions. The contact angle was calculated approximately after 3 s (when a thermodynamic equilibrium was reached between the liquid and the sample interfaces), and then the slippery effect of the created electrospun fiber mats with infused BSO was analyzed using a tilted stage at an angle of 10 • . The water droplet was placed at 0 • on the sample, and 10 • was reached by slowly tilting the whole equipment with the stage and camera (~50 s time duration). A minimum of 5 readings were recorded from different surface areas in order to obtain average values of contact angles and standard deviations. Moreover, a homogeneity of fabricated SLIPS was first confirmed by analyzing different surface areas in terms of sliding behavior, and then representative images were recorded.

Surface Morphology/Topography Analysis
The surface morphology after plasma treatment was studied using scanning electron microscopy (SEM). SEM microscope (Nova NanoSEM 450, FEI, Hillsboro, OR, USA) was used to obtain 2D images of the analyzed surfaces. To get higher resolution images, thin Au layers a few nanometers thick were sputter-coated onto prepared fiber mats.
Detailed information about 3D changes in the surface topography of the fibers was obtained using atomic force microscopy (AFM). The AFM device MFP-3D (Asylum Research, Oxford Instruments, Santa Barbara, CA, USA) was employed in these experiments. Scanning was carried out under ambient conditions by a silicon probe (Al reflex-coated Veeco model-OLTESPA, Olympus) in the tapping mode in air (AC mode), allowing images to be obtained from a surface area of 20 × 20 µm 2 . A homogeneity of different surface areas was first pre-checked by scanning different surface areas, and subsequently, representative high-resolution images were recorded. Moreover, the surface roughness parameter (Ra) representing the arithmetical mean height was evaluated.

Investigation of Mechanical Properties
To identify changes in the mechanical properties of the prepared fiber mats, a nanoindentation technique was used. This technique allows the measuring of indentation depths on the surface of thin sections by an application of very low forces to the indentation tip. The MFP-3D Nano Indenter (Asylum Research, Oxford Instruments, Santa Barbara, CA, USA) using a standard diamond three-side pyramidal-shaped Berkovich indenter tip (TB15773) with a tip radius~100 nm and a spring constant of 3940 N/m (calibration with sapphire standard) was applied. Different areas (10 indentations in order to evaluate average values and standard deviations of mechanical properties) of fiber mats were examined with a loading force from 0 to 50 µN at a loading rate of 10 µN/s. The indentation force was kept for 5 s, and then unloading from 50 µN to 0 µN occurred at an unloading rate of 10 µN/s. The hardness (H) (Equation (1)) and the reduced modulus (E c ) (Equation (2)) of prepared fibers were then calculated using the Oliver-Pharr method [54], considering an elastic behavior during the initial stages of unloading.
where P max is the peak indentation load and A is the projected area of the hardness impression.
where E and ν are Young's modulus and Poisson's ratio for the specimen, and E i , and ν i are the same parameters for the indenter tip.

Adhesion Investigation
A standard test technique ASTM D6862 was used to investigate the adhesion properties at the macro level. A Peel Tester LF-Plus (Lloyd Instruments, West Sussex, UK) equipped with a 1 KN cell was used to test the peel resistance at 90 • of peeling between the PS/PDMS fiber mats and the PE or PU substrate. To achieve a constant peeling, the adhesive joints (19 mm width) were delaminated at a rate of 10 mm/min. To achieve average values for peel resistance, six independent measurements were taken.

Chemical Composition Analyses
To assess the chemical composition changes in plasma-treated fiber mats and their surfaces, Fourier-transformed infrared spectroscopy with an attenuated accessory (FTIR-ATR) was used. The Spectrum 400 (Perkin Elmer, Waltham, MA, USA) was used to identify the functional groups introduced after the plasma treatment and to characterize the chemical composition of prepared fiber mats. All measurements were obtained through eight scans with a resolution of four after background subtraction. Qualitative information for the absorbance of chemical groups in the middle infrared region (4400-550 cm −1 ) was obtained.

Antibacterial Tests
The antibacterial activity of modified polymeric materials was evaluated using ISO 22196, which was adapted to investigate the antibacterial effects of the produced samples. The samples were inoculated with standardized bacterial suspensions of Staphylococcus aureus (CCM 4516-2.3 × 10 5 cfu/mL) or Escherichia coli (CCM 4517-7.1 × 10 6 cfu/mL) 100 mL and covered with ethanol-disinfected polypropylene foil with dimensions of 20 mm by 20 mm. Following that, the infected samples were incubated for 24 h at 100% relative humidity and 35 • C. After removing the polypropylene foil, samples were imprinted on agar (three times from various regions) with lecithin 0.7 g/L and Tween 80 5 g/L as neutralizers and incubated at 35 • C for 24 h. For each sample, antibacterial activity was analyzedthree times in order to obtain statistical information. The number of bacterial colonies was then assessed on a scale of 0-5, with 0 being the best antibacterial efficacy without bacteria growing and five representing no antibacterial effect with bacteria growing.

Conclusions
SLIPS have been fabricated on polyethylene PE and polyurethane PU substrates by electrospinning polystyrene PS/PDMS fiber mats with subsequent BSO infusion. Various surface morphologies, including beads with different sizes and shapes, bead-on-string structures, as well as well-defined fibers with various diameters, were formed during the electrospinning process. Results showed that all the prepared fibrous structures showed high resistance to wetting due to their surface hydrophobicity (contact angle > 100 • ). Upon using PS in PDMS as a carrier polymer, well-defined bead-free fibers were obtained by a direct electrospinning solution containing PS/PDMS/THF (1:1:10). Plasma treatment acted as an adhesion promoter between the polymeric substrate and PS/PDMS electrospun fiber mats, which was confirmed by peel resistance measurements. The subsequent infusion of natural BSO into the porous structures caused slippery behavior, as was confirmed by sliding angle measurements at 10 • of titling angle. Moreover, the antibacterial properties of prepared SLIPS on PU and PE have been investigated and found to exhibit significant antibacterial activity against Gram-positive S. aureus and Gram-negative E. coli bacteria.