Fabrication of Low-Cost Flexible Superhydrophobic Antibacterial

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Fabrication of Low-Cost Flexible Superhydrophobic Antibacterial Surface with Dual-Scale Roughness Abinash Tripathy, Arvind Kumar, Syama Sreedharan, Girish Muralidharan, Amitava Pramanik, Dipankar Nandi, and Prosenjit Sen ACS Biomater. Sci. Eng., Just Accepted Manuscript • DOI: 10.1021/acsbiomaterials.8b00209 • Publication Date (Web): 08 May 2018 Downloaded from http://pubs.acs.org on May 8, 2018

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ACS Biomaterials Science & Engineering

Article

Fabrication of Low-Cost Flexible Superhydrophobic Antibacterial Surface with Dual-Scale Roughness Abinash Tripathya ( ), Arvind Kumara, Syama Sreedharanb, Girish Muralidharanc, Amitava Pramanikc, Dipankar Nandib and Prosenjit Sena ( ) a

Centre for Nano Science and Engineering, Indian Institute of Science, Bangalore 560012, India

b

Department of Biochemistry, Indian Institute of Science, Bangalore, 560012, India

c Unilever

R&D Bangalore, 64, Main Road, Whitefield, Bangalore 560066, India

Email: [email protected], [email protected]

Abstract In this work, we report a large-area fabrication of a flexible superhydrophobic bactericidal surface decorated with copper hydroxide nanowires. This involves a simple two-step method which involves growth followed by transfer of the nanowires onto the polydimethylsiloxane (PDMS) surface by mechanical peeling. Additional roughness in PDMS is obtained through incomplete wetting of the nanoscale gaps which leads to dual-scale roughness and superhydrophobicity with a contact angle of 169° and hysteresis of less than 2°. The simplicity of the process makes it low-cost and easily scalable. The process allows fabrication of non-planar 3D surfaces. The surface shows blood repellence and antibacterial activity against E. coli with more than 5 log reductions in bacterial colony. The surface also shows hemocompatible behaviour making it suitable for healthcare applications. The fabricated surface is found to be extremely robust against stretching, twisting, sand paper abrasion, solid weight impact, and tape peel test. The surface is found to withstand human weight multiple times without losing its hydrophobicity making it suitable for several practical scenarios in healthcare and household applications.

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Article Keyword: superhydrophobic, antibacterial, nanowire, PDMS, E. coli

Introduction In recent times there has been an alarming loss in the efficacy of traditional or conventional antibiotics in the fight against multi drug resistant microbial infections leading to detrimental implications for the healthcare sector1. Despite the drastic improvements in medical science, antimicrobial-resistant infections currently claim 700,000 lives each year from all across the world and if not stopped this figure will increase alarmingly to 10 million by 20502. It is also distressing to note that the rate of nosocomial infections is estimated to be 1 out of every 10 patients that are admitted. These infections which lead to an increase in the hospital stay by almost 2.5 times3, are often caused by line-associated infections, surgical wound infections, nosocomial pneumonia, catheter-associated urinary tract infection, and gastrointestinal infection4. The seriousness of the problem has accelerated the advancements in the development of biomaterials and surfaces with improved biocompatibility and reduced fouling properties5. Although chemical bactericidal techniques based on silver6 and copper7–10 have proven to be effective, there are always associated uncertainties towards the duration of activity and the specificity of these materials11. This has consequently paved the way for a class of biomaterials that use physical structure to kill any bacteria coming in contact with the surface12. The first step towards prevention of bacterial colonization, however, lies in preventing the adhesion of bacteria to the interface13 which can be attained by superhydrophobic surfaces that repel aqueous suspensions of microbial organisms14. Use of physical structures to repel or kill bacteria has been inspired by nature where there are several excellent illustrations of nanostructured surfaces which show multiple


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Article functionalities including liquid repellence (superhydrophobicity) and the ability to resist bacterial colonization15,16. Some of the popular examples of such natural water repelling surfaces include lotus leaves, gecko foot, shark skin, cicada wings, fish scale, and spider silk11. Due to low surface energy and micro/nanoscale morphology these surfaces also deter bacterial adhesion and slow down biofilm formation by easy removal of the cells17–20. Ability to resist bacteria has been associated with the reduced points of contact in these nanostructured surfaces which reduces protein adsorption17 and hence influencing cell adhesion14. These properties has led to multiple efforts to develop artificial surfaces for diverse applications in both industry and household21. Despite several advances in this area, it is well understood that the micro/nano structures are mechanically fragile. This reduces their reliability and hence increases the cost of use as the surfaces have to be revived periodically. Moreover, several applications such as aprons, gloves, temporary mats etc. require these surfaces to be flexible. The above requirements call for development of flexible superhydrophobic bactericidal surfaces with mechanical reliability. In addition, the surfaces should be lowcost so that they can be periodically replaced to address the issues with reliability. There have been previous demonstrations of metal coated flexible nanostructured surfaces10,22. They have however been limited either due to complex fabrication process or use of specialized equipment. Du et al.23 reported transfer of metal thin films onto a polydimethylsiloxane (PDMS) surface from a periodic nanopatterned surface obtained using lithographic technique which limits the scalability to large areas. Other techniques reported in literature incorporate nanoparticles into polymeric sheets using either by dispersing them in the liquid phase prior to curing or by using swell-encapsulate-shrink



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Article technique7,24. These surfaces however often lack the nanoscale surface texturing which is required to make surfaces multifunctional. Herein, we report a process for physically transferring copper hydroxide nanowires onto a curable silicone polymer PDMS carried out by a two-step process: (i) copper wet etching to form nanostructures and (ii) transfer of the copper based nanostructures onto the PDMS surface by mechanical tearing. This PDMS surface decorated with the copper nanostructures is unique in displaying two functionalities; superhydrophobicity preventing bacterial adhesion and a potent bactericidal effect from the copper nanowires as copper has been regarded as a very good antimicrobial agent from centuries25. As our process does not require any specialised equipment, it can be easily scaled for large area fabrication at low cost for real world applications. Absence of a micro-fabricated template makes this process significantly cheaper and easily scalable as it is not limited by the size of the template. The reported process minimises the use of nanowires as they are only present on the surface of the PDMS. In addition, as the cured polymer strongly holds these nanowires in place, these surfaces shows reliability against abrasion, tape peel and solid weight impact.

Materials and Methods Copper Nanowire Fabrication and Transfer to PDMS Copper foil (30 mm × 20 mm × 0.15 mm, purity 99.98%) was ultrasonically cleaned in acetone, IPA and deionized water for 5 min each respectively, followed by immersing in 1M sulphuric acid for 30 seconds (s). Cleaned copper foil was immersed in an aqueous solution of 2.5 mol/L sodium hydroxide and 0.1 mol/L ammonium persulphate at room temperature for 10 minutes (mins), 15 mins and 20 mins. After the process, the foil was taken out from the solution and rinsed in deionized water and dried with nitrogen air. 4 ACS Paragon Plus Environment

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Article The

copper

nanowires

were

silanized

using

1H,

1H,

2H,

2H-

Perfluorooctyltriethoxysilane. The bond between copper hydroxide and silane is very strong because of the formation of siloxy copper bond (-Si-O-Cu) via a condensation reaction between silanol (-Si-OH) and copper hydroxide. It has been reported to be extremely stable26. PDMS and the curing agent were taken in the proportion 10:1 and mixed vigorously. Then the mixture was poured on the silanized copper nanowire surfaces followed by desiccating the substrate for 45-60 mins to remove the air bubbles. After desiccation, the substrates were allowed to dry at room temperature till the PDMS became hard (6-8 hours (hrs)). After that the PDMS was peeled off from the copper nanowire surface gently (Figure 1(a)).

Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) High-resolution scanning electron microscopy images of the nanostructured surfaces were obtained using a field emission (Carl Zeiss) scanning electron microscope at 5 kV under different magnifications. Nanowires density and dimensions were measured using ImageJ©27 software. For bacteria imaging, samples were first washed with phosphate buffered saline (PBS) and then dipped in 2.5 wt% of Glutaraldehyde in PBS for 5 mins followed by desiccation of samples for 48 hrs in vacuum. Then 15 nm gold was sputtered on the surface using Quorum Techport sputter coater. For energy dispersive spectroscopy (EDS) analysis of the PDMS_Cu surface, the SEM tool was operated at 15 kV. Different locations on the surface were chosen and EDS analysis was performed to identify the elements present on the surface. The presence of Cu(OH)2 after etching was confirmed using Rigaku SmartLab diffractometer (Cu Kߙ radiation (ߣ = 0.154178 nm), ߠ − 2ߠ scan, 2ߠ = 10°-90°, step size - 0.02 (2ߠ), step count - 1.5 s).



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Article

Contact Angle Measurement and Drop Impact Study Static contact angle measurement on all the fabricated surfaces was carried out using a custom-made Goniometer set up. 8 µL water droplet was placed gently on the substrates. Image of the droplet was captured by a CMOS camera (Thor Labs). Then contact angle was measured using ImageJ©27 software. Contact angle hysteresis was measured by tilting the Goniometer stage and capturing the image of the tilted droplet just before its starts to slide. From that image, the advancing (ߠ஺ ) and receding ((ߠோ ) contact angles were measured, and the difference was calculated to obtain the contact angle hysteresis.

All the measurements were repeated three times to ensure

repeatability. Drop impact study was carried out on the fabricated surfaces using a highspeed imaging camera (Photron FastCam SA4) capturing at 10,000 frames per second. Drop impact was studied by allowing a 10 µL water droplet to fall on the surfaces from a height of 7 cm.

Abrasion, Solid Impact and Stretching Test The abrasion test was performed by placing a 50-g weight on the substrate and sliding it over a sandpaper with a grit size of ~40 µm. After sliding for a distance of 60 cm, contact angle was measured to characterize the degradation of the surfaces. This routine was repeated four times to give a total sliding distance of 240 cm for all samples. To test the robustness of the surface against impact, a 450-g weight was dropped on the surface from a height of 5 cm. The change in hydrophobicity was quantified by measuring the contact angle after every 10 impacts. For stretching test, a custom-made mount as shown in Figure 5(a) was used to stretch the sample using a micrometer screw. Stretching was carried out from 0 to 5 mm. Static contact angle and contact time for drop impact was characterised for various amounts of applied strain. 6 ACS Paragon Plus Environment

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Article

Bacterial Viability Assay Isolated single colonies of E. coli K-12 (MG 1655) were used to prepare the preinoculum28. The culture growth was carried out for 8 hrs in Luria Bertani (LB) medium at 37°C with constant shaking at 160 revolutions per minute. A 0.2% pre-inoculum was added to 10 mL of LB and was allowed to grow until 0.3 O.D. at 600 nm. The cells were subsequently pelleted and washed with PBS. 0.01 O.D. cell suspension at 600 nm was used for performing the experiments29. A standard plate assay method was used for the enumeration of bacteria. The samples (1.3 x 1.3 cm2) to be tested for bactericidal activity (flat silicon control, flat PDMS and PDMS_Cu_20 surfaces) were placed in the 12 well plates having 1.5 cm diameter wells (procured from Falcon, BD Biosciences, Belgium) as shown in Figure S16 in supplementary information. PBS was poured on all the samples and kept for 2 hrs to eliminate the effect of any residual chemistry from the fabrication process. A volume of 2 mL from 0.01 O.D. bacterial culture was added to each well containing the samples after discarding the PBS. 100 µL of the culture was collected and a 1:100 dilution of the culture was plated on an LB agar plate after incubating for 0, 3, 18 and 24 hrs. The plates were incubated at 37°C for 24 hrs and the colonies were counted. All the experiments were performed in triplicates (3 samples for each surface were taken). The bactericidal activity of the samples was expressed in CFU/mL.

Copper Ion Concentration Measurement using ICP Copper ion concentration in PBS was measured by ICP (Optima 2000 DV Perkin Elmer). Standard calibration plot was made by measuring the copper ion concentration using copper standard solution (1000 ppm from Sigma). 20 mL of solution was taken and acidified by adding 0.5 mL of Suprapur nitric acid for preparing the ICP sample. The 7 ACS Paragon Plus Environment


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Article samples were filtered by passing through a 0.45 µm syringe filter to remove any particulate impurities from the solution after the acidification process. Based on the calibration plot, the concentrations of the samples were subsequently measured.

Results and Discussion Copper Hydroxide Nanowire Transfer Process Morphology of the prepared samples as observed in the SEM images (see Figure 1(b), (c) & (d), Figure S1 supplementary information) show the formation of the nanowires on the copper surfaces for different growth times. The copper surfaces were uniformly covered by the nanowires which showed randomness in both size and spatial distributions. During the initial 20 mins, the nanowire diameters were observed to increase with the growth time. The mean tip diameter for 20 mins of nanowire growth (Cu_20) was measured to be ~217 nm (see Figure S2 supplementary information). In comparison, for the samples with 10 minutes (Cu_10) and 15 mins (Cu_15) of nanowire growth, the mean tip diameters were measured to be ~104 nm and ~150 nm respectively. Our process shows a linear increase in the nanowire diameter with a growth rate of ~10 nm/min. As evident from the cross-section SEM (see Figure S3 supplementary information) the height of the nanowires varies significantly due to variation in length and orientations. Approximate heights of nanowire surfaces were roughly estimated as ~5 μm for 10 mins dip, ~7 μm for 15 mins dip and ~12 μm for 20 mins dip. Cu(OH)2 nanowire growth process has been studied extensively30. Anisotropic growth rate of Cu(OH)2 crystals and weakness of the interlayer hydrogen bonds in the alkaline medium has been attributed as the cause for the nanosheets to roll up and form tubular nanowires30. The growth is however not continuous. At longer times the Cu(OH)2 8 ACS Paragon Plus Environment

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Article nanowire growth rate decreases and the nanowires get consumed as Cu(OH)2 converts to more stable CuO31,32. Figure 1(e) presents the XRD plot of the nanostructures on the copper surface dipped for 20 mins. Peaks corresponding to Cu(OH)2 were identified, confirming the formation of randomly oriented copper hydroxide nanostructures on the surface (also see Figure S4). Few intensity peaks corresponding to copper were also obtained (shown with red arrows). Figure 2(a) (also see Figure S5, Figure S6 (top view) and Figure S7 (cross-sectional view supplementary information) shows the SEM images of the transferred copper hydroxide nanowires onto the PDMS surface. PDMS was poured and cured at room temperature on the silanized nanostructured samples. For all the surfaces, there was a successful transfer of the nanowires as is evident from the change in colour of the copper surface from blue to black (see Figure S8(a) supplementary information). This change in colour is due to the exposure of the underlying CuO surface after the nanowires were transferred to the PDMS surface which in turn appears blue. SEM of the copper surface shows that a large fraction of the nanowires was transferred to the PDMS surface (see Figure S8(b) supplementary information). Energy dispersive spectroscopy analysis of the PDMS_Cu_20 surface further confirmed the successful transfer (see Figure S9 supplementary information). Even though the nanowires were transferred successfully, for all the three cases there was a large difference in the morphology of the PDMS_Cu_20 surface when compared to the other two surfaces. For the PDMS_Cu_20 we observe in the SEM images that the PDMS surface is significantly rougher than the other two cases. The roughness of the surfaces was further characterised by atomic force microscopy (AFM) as seen in Figure 2(b). The mean feature height (Rz) as measured from the AFM data was ~1 μm for PDMS_Cu_20 which



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Article was significantly higher than ~300 nm and ~50 nm for PDMS_Cu_15 and PDMS_Cu_10 sample. This additional roughness due to the structuring of the PDMS in PDMS_Cu_20 enhances its liquid repellence which was characterised by measuring the wetting properties of these surfaces. The contact angle measured on the PDMS surfaces varied significantly with etch time ranging from 133° ± 9.7° for the PDMS_Cu_10 sample to 169° ± 2.3° for the

PDMS_Cu_20

sample.

The

PDMS_Cu_15

sample

also

showed

good

superhydrophobicity with a contact angle of 163° ± 3.2° (see Figure S10(a) supplementary information). Contact angle hysteresis for the PDMS_Cu_20 was measured to be less than 2°. To examine the maintenance of superhydrophobicity under dynamic conditions, drop impact experiments were performed on the prepared surfaces from a height of 7 cm (ܹ݁ = 49). Water droplet was found to stick on the PDMS_Cu_10 surface during recoil. On the PDMS_Cu_15 surface major part of the droplet bounced off the surface while a small residual droplet was left on the surface. There was a clear droplet recoil without any residues on the PDMS_Cu_20 surface with a contact time of 14.5 ms (see Figure S10(b) supplementary information, Video_S1).

Incomplete Penetration of the Nanostructures by PDMS It is evident from the SEM images and the AFM data that the PDMS is able to completely penetrate the nanowires for PDMS_Cu_10 and PDMS_Cu_15 which in turn leads to only nanoscale roughness on the peeled PDMS surface. Some secondary roughness observed on these surfaces is attributed to the cracks in the PDMS from the mechanical tearing process. The additional μm scale roughness on the PDMS_Cu_20 surface as seen in Figure S6 is attributed to partial penetration of the PDMS. Capillary pressure driven wetting however fails to describe the partial penetration as the approximate time


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Article predicted for complete wetting of the 12 μm tall nanostructures is less than 1 s which is insignificant compared to the 6-8 hrs required for complete curing. Another possible explanation for incomplete penetration could be due to the significantly larger quantity of trapped air for the taller PDMS_Cu_20. Air trapping as the singular reason for the additional roughness was invalidated in the study where a non-silanized nanostructured surface was used for the transfer. Unlike transfer of silanized nanostructures where the underlying CuO surface was found to remain on the copper foil after transfer, for non-silanized nanostructures the underlying CuO layer was completely transferred on to the PDMS base and left a shiny surface on the copper strip (see Figure S11 supplementary information). Consequently, the PDMS surface had a different colour due to the CuO layer. The transfer of the CuO layer was possible due to the complete penetration of the PDMS which in the absence of silanization lead to a strong adhesion of the PDMS with the CuO layer. SEM of the surface shows a predominantly flat topography (Figure S12). This indicates a strong role of the surface wettability (silanization) in partial penetration and subsequent transfer. The dynamics of PDMS filling of the nanowire jungle is a competition between the capillary forces which drives the liquid in and the viscous forces which slows down the flow. Silanization of the surface reduces the surface energy leading to a reduction in the capillary force and hence is expected to slow down the wetting. To study the extent of slowdown in the wetting process, we measured the wetting of the silanized versus a non-silanized nanostructured surface (growth time 20 mins) by a drop of curing PDMS. In a control experiment on a flat silane coated copper surface we obtained PDMS contact angle of ~35°. The minimal change of the contact angle on the flat surface over a period 120 min verified that the surface properties do not change significantly during



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Article this initial phase of curing. Figure 3(a) plots the contact angles with respect to time. For both the nanostructured surfaces the droplets start with high contact angle confirming an elevated fakir state. The PDMS drop contact angle on the non-silanized surface reduces to less than 10° within the first few minutes, indicating a transition to complete wetting (Wenzel state). After that as the wetting area increases the apparent contact angle keeps on decreasing very slowly. In contrast, for the silanized nanostructured surface the progression of wetting was significantly slower. Within the first 20 mins the contact angle reduces rapidly to less than 40°. After that the reduction in contact angle slows down and the contact angle reaches ~12° after 120 mins of spreading (see Table S1 supplementary information). Non-uniform distribution and orientation of the nanowires makes it difficult to calculate the depth of penetration from the contact angle data. There is however a large difference in the rate of wetting which cannot be attributed solely to the contact angle difference arising from silanization. For the copper sample with 20 mins growth time the nanowires have larger diameters (see Figure S2 supplementary information). The nanowire density is also observed to increase significantly with depth. This increase in diameter and density leads to a decrease in the effective gaps through which the PDMS has to penetrate leading to a further slowdown as the wetting progresses. It has been reported that PDMS shows anomalous behaviour in filling of high aspect ratio nanoholes smaller than 100 nm33 even when the viscosity and surface energy are modulated using dilution. We believe that the incomplete wetting of the nanowire jungle by PDMS is partly due to this anomalous behaviour of PDMS in high aspect ratio nanoholes. We further observed that the nanostructure transfer with PDMS diluted in Toluene (1:1) leads to a change in the morphology of the PDMS surface as seen in Figure 3(b). The dilution reduces viscosity, enhances the penetration and hence the PDMS is observed to have smaller roughness. 12 ACS Paragon Plus Environment

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Article We also observe transfer of a larger number of nanowires which is easily understood as a result of enhanced wetting of the nanowires by the diluted PDMS33.

Nanowire Transfer from Pre-Structured 3D Copper Surface To test the feasibility of this process for transfer from pre-structured surfaces, curved copper surfaces with small (wavelength = 0.6 cm - 1 cm, amplitude ~0.5 cm) and large (wavelength = 1.5 cm, amplitude ~1 cm) grooves were used as seen in Figure 3(c). Nanowires were grown on the curved surface following the standard process for 20 mins. Then the nanowires were transferred to PDMS from the curved surfaces using the standard procedure. Even though there are differences in the surface morphology of the crest and the trough as seen in Figure 3(d), both showed water repellence. The developed process is versatile and can be used with pre-structured copper surfaces leading to low-cost fabrication of large area flexible surfaces with hierarchical multiscale roughness. We also demonstrate large area (10 cm x 8 cm) fabrication of the PDMS_Cu_20 surface as seen in Figure S13 supplementary information. Multiple copper strips were used for the fabrication of this large area surface. Due to the simplicity of the process it is highly scalable as it does not require any sophisticated equipment. The process is also low cost as we can reuse the copper surface by removing the residual CuO layer covering the copper surface either by sand paper abrasion or a chemical treatment in acetic acid. The feasibility of nanostructure growth on the used surface was further verified (see Figure S14(a), (b) supplementary information).

Surface Robustness To ensure robustness, abrasion of the fabricated PDMS_Cu_20 surface was carried out against a sand paper of length 30 cm. A 50 g (0.49 Newton force) weight was placed on 13 ACS Paragon Plus Environment


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Article the substrate while performing the abrasion test (Figure 4(a1), see Video_S2). Static contact angle was measured after different number of abrasion cycles to observe the change in wettability. A change in contact angle (~169° without abrasion to ~150° after 30 cycles of abrasion) was observed after 30 cycles (1800 cm) of abrasion (Figure 4(a2)). The surface was also found to be resistant to damage when the rubbing was performed using a softer surface (fingers with gloves) as seen in supplementary video (Video_S3). Tape peeling test was also performed to test the adhesion of the embedded nanowires to the PDMS_Cu_20 surface. Tape was stuck to the surface, pressed and then removed. After the peeling, the blue colour on the PDMS_Cu_20 surface was retained, and the tape showed no visual signs of nanowire sticking to it. Superhydrophobicity of the PDMS_Cu_20 surface was checked by dropping water droplets on the surface and it was found to retain its water repellence (see Video_S3). To check the reliability of the surface due to impact with a solid surface, a weight of 450 g was continuously allowed to hit the PDMS_Cu_20 surface from a height of 5 cm (see Figure 4(b1)). This process was repeated 50 times and the surface was found to retain its water repellent properties with a contact angle of 159° (see Figure 4(b2), Video_S4 supplementary information). To further examine the possibility of using these surfaces on floors and beds where large weights are expected, the retention of hydrophobicity of the surface was confirmed by walking on it multiple times (see Figure 4(c1), Figure 4(c2), inset in Figure 4(c3) and Video_S5 supplementary information). The extreme robustness of the PDMS_Cu_20 surface under different conditions can be attributed to the flexible nature of the substrate with nanostructures on the surface which are strongly adhered to the PDMS after it is cured. Flexibility also prevents the surface from breaking under stress which is not true for rigid surface.


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Article

Hydrophobicity of the Surface under Stretching The flexibility of the substrate was studied by measuring the hydrophobicity of the surface under stretched condition, PDMS_Cu_20 surface was stretched using a custom made set up and contact angle and contact time on the surface were measured. Strain was varied from 0 to 16.7 % (Figure 5(a)). Due to the thickness of the sample it was difficult to put it under more strain. Contact angle was found to decrease, and contact time was found to increase with increase in the stretching of the surface (Figure 5(b)). This can be attributed to the increased distance between both the copper nanowires and the PDMS microstructures present on the surface. Hydrophobicity of the surface was qualitatively tested for bending upwards, bending downwards and in twisted conditions (see Figure 5(c)-(e), Video_S6 supplementary information). In all the three cases PDMS_Cu_20 surface retained its water repelling property.

Blood Repellence and Antibacterial Nature Flexible bactericidal surface has several potential applications in the healthcare industry. In addition, the ability to resist wetting from biological fluids is a desired characteristic. To demonstrate this, we studied the behaviour of these surfaces with blood. Static contact angle as high as 159° was obtained for a 10 μL blood droplet. The contact angle reduced to 151° after 30 mins of continuous exposure (see Figure S15(a)). Blood droplet was observed to reduce in volume with respect to time. After 12 hrs, the droplets were found to have dried without spreading (see Figure S15(b)). The dried blood droplets were removed from the surface with minimal effort. The process of removal however leads to reduction in the blue colour locally. This was because some of the nanostructures were removed with the dried blood leading to partial loss of superhydrophobicity at those points. We also studied the blood repellence in dynamic



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Article conditions for the PDMS_Cu_20 surface. Blood droplets dropped from a height 5 cm were completely repelled without leaving any residue on the surface as seen in Video_S7 of supplementary information. The contact time for the blood drop impact was found to be 20.3 ms which is ~6 ms higher than the contact time for a water droplet. In contrast, blood wets flat glass and flat PDMS surfaces as shown in Figure 6(a1), (a2). A standard viability plate-count method was used to quantify the bactericidal efficacy of a flat silicon, a flat PDMS and the PDMS_Cu_20 surface (see Figure S16 supplementary information). Figure 6(b1) shows the CFU/mL data which represents the number of viable E. coli cells on the surfaces over a 24 hrs incubation period. Values plotted are average from the experiments that were carried out in triplicates. It is clear from the standard plate assay that PDMS_Cu_20 surface has the highest antimicrobial efficiency as compared to the flat silicon and flat PDMS surfaces. Additionally, SEM images of all the samples were taken after the bacterial exposure of 24 hrs. Standard protocol was followed prior to the electron microscopy imaging for sample preparation. Bacteria colony of E. coli was observed on both the flat silicon and flat PDMS surfaces. On the contrary no bacteria colony was observed on the PDMS_Cu_20 surface (Figure 6(b2), (b3), (b4)). The antimicrobial effect of the PDMS_Cu_20 surface can be attributed to the combined effect of multi-scale surface roughness which reduces the adherence of the bacteria on the surface and the copper nanowires releasing Cu2+ ions which ultimately attacks the bacteria cell wall and inactivates them. In order to evaluate the role of copper in the bactericidal action, copper ion leaching form the PDMS_Cu_20 surface in PBS was measured using inductively coupled plasma mass spectrometry (ICP-MS). Silicon with 20 nm copper was used as the control surface.


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Article The samples were first dipped in PBS for 2 hrs and then the copper ion concentration in the PBS solution was measured. After this the samples were again dipped in fresh PBS for 2 hrs and the copper ion concentration was measured. Table 1 shows the copper ion concentration data in PBS for both flat copper and PDMS_Cu_20 surface. In the first 2 hrs copper ion leaching from PDMS_Cu_20 was approximately 60 ppb (11 times less than the flat copper surface). As visible from the SEM images (Figure 2(a)) the area fraction of exposed copper hydroxide nanowires is smaller attributing towards a slower leaching rate. The polymer coating on the nanowires further slows down the dissolution of the copper ions. In the next 2 hrs, the copper ion leaching from the PDMS_Cu_20 surface was found to be 30 ppb (5 times less than the flat copper surface). To verify the effect of long term exposure to wet conditions, the surfaces were exposed to PBS solution for a period of 5 days. Even though the surface retained its blue colour on closer inspection (see Figure S17) the surface morphology was found to have changed completely as shown in Figure 6(c). Copper phosphate nanoflower34 formation was observed on the surface after dipping the surface in PBS for 5 days. EDS analysis confirmed the presence copper on the surface even after a long exposure to liquid medium.

Hemocompatibility of the PDMS_Cu Surface Hemocompatibility investigation is an important toxicity assessment for biomedical applications35. We have performed hemocompatibility studies for our surfaces. RBC suspended in PBS was poured on the PDMS_Cu_20 surface and image of RBC was captured using an optical microscope at different time points (Figure S18(a)). Morphology of the RBC did not change significantly on the PDMS_Cu_20 surface over a period of 3 hrs. The diameter of the RBC was calculated from the optical microscopic



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Article images. Diameter of more than 100 RBCs was calculated at each time point from a set of 7 images each (Figure S18(b)). No significant difference was observed between the size of RBC at different time points (Figure S18(c)). At every time point, the diameter of RBC was found to lie between 6-8μm which is the size range for healthy RBCs. This proves that the PDMS_Cu_20 surface is hemocompatible and can be used for healthcare applications36,37.

Conclusion In this work, we have successfully fabricated a novel flexible PDMS surface decorated with copper hydroxide nanowires which exhibits superhydrophobicity and bactericidal behaviour with multi-scale roughness. The process involves nanowire growth on the copper foils using simple wet etching technique. When PDMS is poured and cured over the silanized nanowires at room temperature, incomplete wetting of the nanostructures leads to structuring of the PDMS. The reported process benefits from incomplete wetting of high aspect ratio nanoholes by PDMS, which have been reported before. Finally, the nanowires are transferred to the PDMS surface using mechanical peeling. The fabricated surfaces demonstrated extremely robust superhydrophobicity with a water contact angle of 169° and a contact angle hysteresis of less than 2°. The simplicity of the process makes it extremely low-cost and easily scalable to larger area for commercial applications. The prepared surface showed good repellence to whole blood and was found to be more efficient in killing E. coli as compared to the control surfaces. The surface also exhibited hemocompatible behaviour making it suitable to be used in healthcare applications. This novel hybrid surface can be used in practical applications to prevent different kinds of infections.


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Article

Supporting Information Table and images describing the details of fabrication and characterization of the PDMS_Cu surface. Video_S1 – high speed imaging of drop impact dynamics, Video_S2 – sand paper abrasion test, Video_S3 – tape peel test and rubbing with soft surface, Video_S4 – weight impact test, Video_S5 – walking test, Video_S6 – stretching test, Video_S7 – blood repellency

Conflict of Interest The authors declare no conflict of interest.

Corresponding Authors Abinash Tripathy, [email protected] Prosenjit Sen, [email protected] ORCID Abinash Tripathy - 0000-0003-3546-2806 Arvind Kumar - 0000-0001-5690-8753 Prosenjit Sen - 0000-0001-6519-1707

Acknowledgement The authors would like to thank Swapnil, Micro/Nano Sensor Lab, CeNSE, IISc, Bangalore for providing the platform to carry out the stretching test. PS acknowledges the financial support from Unilever R&D, Bangalore. AT would like to thank Ministry of Electronics and Information Technology (Meity), Government of India for providing the



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Article financial support. DPN acknowledges the funding by the Department of BiotechnologyIISc partnership program.

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Article

Figures and Tables

Figure 1: (a) Schematic illustration of the main route used to fabricate copper nanostructures and transferring the nanostructures onto the silicone based polymer. (bd) FESEM images of the copper nanostructures prepared in NaOH and (NH4)2S2O8 solutions for 20 mins of etch time (e) XRD of the prepared sample. Red arrows showing the peaks from the copper surface.


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Article

Figure 2: (a) Representative FESEM images (top and cross-section view) of PDMS_Cu surfaces (scale bar – 1μm). Copper nanowires in case of PDMS_Cu_20 surface were coming out of the surface, while they are at the same level as that of the PDMS surface in case of PDMS_Cu_10 and PDMS_Cu_15 surfaces. (b) AFM images of PDMS_Cu surfaces showing the roughness values. PDMS_Cu_20 surface was found to be have the highest roughness.



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Article

Figure 3: (a) Static contact angle of PDMS droplet on different surfaces with respect to time. (b) SEM image of PDMS_Cu surface prepared by diluting PDMS in Toluene with different dilution values. (c) Images showing transfer of copper nanostructures onto PDMS from curved surfaces. (d) Difference in morphology was observed at the trough and crest after the transfer from the SEM images (scale bar – 1μm). Surface looks rougher at the trough as compared to crest part.


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Article

Figure 4: (a) Sand paper abrasion test was performed on the flexible PDMS_Cu_20 superhydrophobic surface to ensure the robustness of the surface. (a1) 50-g weight was placed on the surface and it was dragged against the sandpaper (scale bar of SEM image of sand paper - 20µm) for different cycles. (a2) A change in the contact angle was observed even after 30 abrasion cycles (1800 cm). (b) Weight impact test to check the hydrophobicity of PDMS_Cu_20 surface. Hydrophobicity was retained even after 50 impacts from a height of 5 cm with a 450-g weight. (c) Testing the hydrophobicity of the PDMS_Cu_20 surface by walking on it multiple times. The surface survived the human weight and continued its water repellence behaviour (inset showing the contact angle after walking multiple times on the PDMS_Cu_20 surface). 29 ACS Paragon Plus Environment


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Article

Figure 5: (a) Stretching test performed using a custom-made platform. Stretching length was varied from 0 to 5 mm (strain - 0 to 16.7 %). (b) Change in contact angle and contact time of water droplet on the PDMS_Cu_20 surface with respect to stretching. Increase in contact time and decrease in contact angle was observed with the increase in stretching. (c), (d) and (e) Testing hydrophobicity of PDMS_Cu_20 surface under different stretched conditions: stretched upward, stretched downward and twisted conditions respectively.


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Article

Figure 6: (a) Interaction of the whole blood with the flat PDMS and PDMS_Cu_20 surface. (a1) Blood stuck to the flat PDMS surface when dropped on it. (a2) image showing the blood dropping on the PDMS_Cu_20 surface. Blood could not stick to the PDMS_Cu_20 surface and was repelled by it. Blood could not stay on top of the surface, rolled down and stuck to the edge of the substrate. (b) CFU/mL data on flat silicon control, flat PDMS and PDMS surface decorated with copper nanowires (PDMS_Cu_20). 31 ACS Paragon Plus Environment


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Article After 18 hrs of incubation no E. coli colony was found on the PDMS_Cu_20 surface while bacterial colonies were observed on the flat silicon and PDMS surfaces (scale bar – 1μm). The antimicrobial effect of the PDMS_Cu_20 surface can be attributed to the combined effect of superhydrophobicity which reduces the adherence of the bacteria on the surface and presence of copper nanowires release Cu2+ ions which ultimately interact with the bacteria cell wall and kill them. (c) EDS data of PDMS_Cu_20 surface dipped in PBS solution after 5 days. Copper peak is clearly seen in the EDS spectra. Table 1: Copper ion leaching into the PBS medium. Sample

ppb

PBS

9.2

Flat Copper (2 hrs)

727.4

PDMS_Cu (2 hrs)

63.6

Flat Copper (after next 2 hrs with fresh PBS)

178.4

PDMS_Cu (after next 2 hrs with fresh PBS)

37.5


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Graphic TOC 99x75mm (300 x 300 DPI)

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Fabrication of Low-Cost Flexible Superhydrophobic Antibacterial

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