Evaluation of Protein Adsorption on Atmospheric Plasma Deposited Coatings Exhibiting Superhydrophilic to Superhydrophobic Properties
© The Author(s) 2012
Received: 2 March 2012
Accepted: 5 April 2012
Published: 3 May 2012
Protein adsorption is one of the key parameters influencing the biocompatibility of medical device materials. This study investigates serum protein adsorption and bacterial attachment on polymer coatings deposited using an atmospheric pressure plasma jet system. The adsorption of bovine serum albumin and bovine fibrinogen (Fg) onto siloxane and fluorinated siloxane elastomeric coatings that exhibit water contact angles (θ) ranging from superhydrophilic (θ < 5°) to superhydrophobic (θ > 150°) were investigated. Protein interactions were evaluated in situ under dynamic flow conditions by spectroscopic ellipsometry. Superhydrophilic coatings showed lower levels of protein adsorption when compared with hydrophobic siloxane coatings, where preferential adsorption was shown to occur. Reduced levels of protein adsorption were also observed on fluorinated siloxane copolymer coatings exhibiting hydrophobic wetting behaviour. The lower levels of protein adsorption observed on these surfaces indicated that the presence of fluorocarbon groups have the effect of reducing surface affinity for protein attachment. Analysis of superhydrophobic siloxane and fluorosiloxane surfaces showed minimal indication of protein adsorption. This was confirmed by bacterial attachment studies using a Staphylococcus aureus strain known to bind specifically to Fg, which showed almost no attachment to the superhydrophobic coating after protein adsorption experiments. These results showed the superhydrophobic surfaces to exhibit antimicrobial properties and significantly reduce protein adsorption.
When a biomaterial is introduced into the body interactions take place between the first few nanometres of the material surface and the surrounding tissue or body fluid . The adsorption of a protein layer is the first stage in this response which can then function to mediate cellular adhesion and as a result is an important issue when considering the design of implant materials. The formation of a protein layer can induce implant failure by both promoting bacterial adhesion and facilitating the formation of thrombin [2, 3]. In these instances protein attachment initiates failure cascades which can lead to inflammation and immune reaction, causing a loss of biocompatibility and functionality . A number of approaches have been applied to tailor polymer and metal surfaces for biomedical applications and to study the adhesion of proteins. Some of these include the use of graft polymerisation , and plasma modification techniques such as ion beam implantation and plasma polymerisation [6, 7]. The atmospheric pressure plasma technique examined in this study can be employed as a post production process for surface modification of sensitive materials under low temperature ambient conditions.
Protein adsorption is influenced by the particular physico-chemical properties of the biomaterial surface which include chemistry, wettability; charge, and surface morphology . The wettability of a material surface is considered to be one of the most influential parameters affecting protein adsorption with numerous studies in particular investigating protein adsorption and cellular adhesion on surfaces with a designed wettability gradient [9–12]. It is generally considered that proteins tend to adsorb more favourably onto hydrophobic than hydrophilic surfaces. Both Lee  and Xu  treated polyethylene material with a glow discharge plasma to produce surfaces with a wettability gradient for the study of albumin and fibrinogen (Fg) adsorption. These studies showed an increased amount of protein adsorption on hydrophobic surfaces and in the latter case, higher protein adhesion forces as surface hydrophobicity increased. Malmsted  and Nygren  used spectroscopic ellipsometry to monitor the adsorption of serum proteins onto hydrophilic and hydrophobic modified silica surfaces. The study by Malmsted showed higher levels of protein adsorption on hydrophobic substrates which included albumin and fibrinogen, while Nygren et al. also showed higher levels of protein binding on hydrophobic surfaces.
While most literature suggests that protein adsorption tends to occur more favourably on hydrophobic surfaces or on surfaces with an intermediate wettability (60–90°), other investigations have demonstrated more favourable protein adsorption on hydrophilic surfaces . This conflicting data with regard to the influence of surface wetting on protein adsorption is considered to be the result of the variety of factors which influence protein adsorption, including surface charge, roughness, environmental pH, etc. An investigation of albumin and fibronectin (Fn) adsorption by Tamada et al.  on polymeric substrates with water contact angles between 20° and 120° observed the highest level of adsorption in the region of 60–80°. A significant drop in protein adsorption was observed as contact angles reduced from 60 to 20°, while surfaces with water contact angles approaching 120° also exhibited a reduction in protein adsorption. There has, however, been relatively few reports on protein adsorption measurements on highly hydrophobic or superhydrophobic (θ > 150°) surfaces. Some researchers report on the adsorption of protein on superhydrophobic surfaces  while some report on non-adsorbent properties on these surfaces [19, 20]. Roach et al. studied the interaction of proteins on hydrophilic and superhydrophobic porous thermally modified silica . In their study a reduction in the level of BSA adsorption was observed on the superhydrophobic silica substrate material when compared with hydrophilic substrates. Both hydrophobic and hydrophilic surfaces offer diverse polar interactions for the investigation of protein adsorption. However, the interfacial boundary structure formed when a superhydrophobic surface is contacted with water offers a very different arrangement through which protein molecules must diffuse in order to adsorb to the underlying material surface.
The adsorption of proteins at a biomaterial interface is a dynamic process with attachment, detachment and conformational changes all often taking place in a flowing aqueous environment. Under laboratory test conditions it is beneficial to replicate this in vivo flow environment where possible. There are many methods of measuring this adsorption, such as surface plasmon resonance (SPR), quartz crystal microbalance (QCM) and radiolabelling techniques. Spectroscopic ellipsometry allows real time in situ monitoring of the process of protein adsorption under flow conditions and has been used to study the thickness, adsorption and desorption kinetics of serum proteins on a range of surface chemistries [14, 21, 22].
The objective of this study is to quantify the adsorption of serum proteins and bacterial attachment on nanometre thick polymer coatings exhibiting water contact angles between <5° and 155°. Analysis of both BSA and Fg was carried out as they are two of the most abundant proteins in blood plasma and have significantly different molecular weights and shapes. Albumin with a molecular weight of 66 kDa is the most abundant plasma protein (50–60 %) and is associated with the transportation of other proteins . It has a heart shaped structure consisting primarily of α-helixes 67 %. Fg with a molecular weight of 340 kDa, is a much larger protein, 47 nm in length and consists of three globular domains connected by thin identical sequences . Fg is investigated as it is the protein most associated with the coagulation cascade  and is reported to be the dominant ligand promoting attachment of Staphylococcus aureus (S. aureus), the primary bacteria associated with biomaterial implant related infection .
Plasma polymerised siloxane coatings were deposited on one-side polished, p-type, boron doped silicon wafers, resistivity 0–100 Ω cm (450 μm thick), supplied by Compart Technology Ltd and onto titanium grade 5 coupons (Ti6Al4V—medical grade). The wafers and coupons were ultrasonically cleaned in methanol followed by acetone and propanol, air dried and pretreated with He/O2 plasma prior to coating deposition using the PlasmaStream™ system .
Siloxane monomers were investigated as these chemistries are widely used in plasma polymerisation deposition studies and are considered to be biochemically inert, while offering structural stability and flexibility. Siloxane coatings were deposited from tetramethylethosilicate (TEOS) (C8H20O4Si) (Fluka 99 %) and from hexamethyldisiloxane (HMDSO) O(Si(CH3)3)2 (Aldrich 98 %), while fluorinated siloxane coatings (TCFS) were deposited from an equal volume mixture of tetramethylcyclotetrasiloxane (TMCTS) ((HSiCH3O)4) (Aldrich 99 %) and perfluorooctytriethoxysilane (PFOTES) (C14H19F13O3Si) (Aldrich 98 %).
Bovine fibrinogen (Fg, type I S, lyophilized powder) and bovine serum albumin (BSA, fraction V, lyophilized powder) were obtained from Sigma and used as received. Phosphate buffered saline (PBS) was freshly prepared using sodium salts: NaH2PO4 and Na2HPO4 (200 mmol phosphate) and NaCl (100 mmol) obtained from Aldrich to give pH 7.4 at 25 °C. BSA protein solution was prepared at a concentration of 10 mg ml−1 prior to adsorption experiments. Fg protein solutions were prepared at a concentration of 0.1 mg ml−1 by heating the PBS solution to 37 °C and gently stirring after addition of Fg until a slightly hazy solution was obtained.
2.2 Plasma Polymerisation of Functional Coatings
The coatings were deposited using a non-thermal atmospheric plasma jet system which has been described in detail elsewhere . Briefly, the system is configured with a dielectric head housing two pin electrodes either side of a pneumatic nebuliser (Burgener Ari Mist nebuliser) through which liquid chemical precursors are introduced at 80 psi. The chemical precursor interacts with plasma species generated by gas carriers from either a He/O2 or He/N2 gas mixture inside a 75 mm long by 15 mm wide Teflon tube. This interaction which initiates polymerisation reactions results in the deposition of cross-linked polymerised coatings downstream of the plasma jet onto substrates positioned beneath the plasma plume. Low frequency electrical power is delivered to both electrodes from a modified PTI 100 W rf power supply at a frequency of approximately 15–25 kHz. Voltage measurements obtained using a custom-built HV probe. The depositions reported in this study were carried out at approximately 13.5 kV. The entire plasma device was moved over the surface of the substrate in a raster pattern (XY directional scan) using a CNC device with a line speed of 15 mm/s and a step interval of 2.5 mm.
2.3 Coating Characterisation
Static water contact angle and surface energy measurements were carried out using the sessile drop technique at room temperature (OCA 20 from Dataphysics Instruments). Deionised water, diiodomethane and ethylene glycol were used for surface energy measurements. Contact angles were determined at three different locations per sample. These were averaged and the OWRK (Owens, Wendt, Rabel and Kaelbe) method was then used to calculate the surface energy of the deposited coatings [21, 22]. The water contact angle and surface energy of the deposited coatings were determined on five different sample substrates for each coating process condition. The quoted contact angle values (Table 2) represent the mean of these five measurements, and a typical deviation from the mean value of 3° was determined.
The coating surface morphology was examined using a Wyko NT1100 optical profilometer operating in vertical scanning interferometry (VSI) mode. This system was used to calculate the average surface roughness, Ra (arithmetic average roughness) and Rq (root mean square roughness). The thickness of superhydrophobic coatings was determined by step height measurements using this technique. These measurements were facilitated by masking part of the wafer surface with scotch tape prior to coating deposition; this was then removed after coating deposition to obtain a defined coating edge profile. The quoted roughness values (Table 3) represent the average of five measurements, with a typical deviation of 1 and 5 nm for siloxane and fluorinated siloxane, respectively, determined.
Average coating thickness of non-superhydrophobic samples was also measured using an M-2000® variable angle spectroscopic ellipsometer from J. A. Woollam Co., utilising an FLS 300 75W Xenon arc lamp operating within a wavelength range of 270–1,700 nm. Three measurements were taken on each sample at incident angles of 65°, 70° and 75°. Analysis of spectroscopic data was carried out using CompleteEase™ analysis software. It was not possible to use the ellipsometry technique to obtain thickness measurements of the superhydrophobic coatings as the signal from the relatively rough surface morphology was depolarised and did not allow for an accurate model fit for coating thickness.
Fourier transform infrared spectroscopy (FTIR) measurements were carried out on each of the coatings using a Bruker Vertex-70 system. The sample chamber was purged by N2 gas before the scans were obtained. Spectra were collected in the range of 400–4,000 cm−1 using a spectral resolution of 4 cm−1. The transmission spectra of the coated silicon substrates were obtained by the overlay of 64 scans to increase the signal to noise ratio.
2.4 Protein Adsorption by Spectroscopic Ellipsometry
Protein adsorption analysis was performed by spectroscopic ellipsometry using a specifically designed 5 ml LiquidCell™ (TLC-100-02.04) supplied by J. A. Woollam. The samples were sealed inside the liquid cell and positioned on the ellipsometry stage. Variation in polarised light was monitored at a fixed incident angle of 70°.
2.5 Bacterial Attachment Assay
The attachment of S. aureus SH1000  to sterile plain titanium (PT) coupons and TCFS coated superhydrophobic (SH) titanium coupons was performed using the method described previously [31, 32]. Where indicated, underwent protein adsorption with bovine fibrinogen (Fg) using the solution flow cell. Attachment assays were performed in 24 well plates in which coupons were immersed overnight in 3 ml of SH1000 cultures adjusted to A600 = 0.2 and subsequently incubated statically at 37 °C for 1 h. The coupons were then removed from the plates and rinsed gently in 1× PBS to remove loosely adhered bacteria. To quantify the remaining attached bacteria, the coupons were placed in 1 ml sterile 1× PBS, vortexed for 5 min, sonicated gently for 2 min and then vortexed again for 2 min. The combination of vortexing and sonication was designed to detach bacterial cells from the surface of the coupon and to disrupt bacterial cell aggregates prior to serial dilution, plating onto brain heart infusion agar (Oxoid) and enumeration of colony forming units (CFUs). The number of CFUs in the inoculum was also determined and attachment retention was expressed as the percentage CFUs attached to the coupons relative to the number of bacteria in the inoculum. Each experiment was repeated three times and standard deviations are shown.
3 Results and Discussion
3.1 Plasma Polymerised Coatings
Deposition parameters used to form plasma polymer films with varying surface wetting properties
Monomer flow rate (μl/min)
Substrate to source distance (mm)
3.2 Surface Analysis
Water contact angle and surface energy of siloxane, fluorosiloxane and SiO2 substrates used for protein adsorption studies
Water contact angle (°)
Surface energy (mJ m−2)
Polar (mJ m−2)
Dispersive (mJ m−2)
Average thickness and surface roughness of coatings
TCFS coatings deposited at a flow rate of 5 μl min−1 do not exhibit the same smooth morphology as HMDSO coatings deposited at the same flow rate. The TCFS surface exhibits an inhomogeneous surface roughness. This may be explained by the greater volatility (higher vapour pressure) of the TCFS mixed monomer chemistry which also has a heavier molecular weight and greater chain length. These factors might more readily enable the formation and nucleation of larger particulates in the plasma discharge when compared to HMDSO. The superhydrophobic TCFS* coating deposited at a flow rate of 3 μl min−1 also exhibits a greater variation in surface roughness than the superhydrophobic HMDSO* coatings. This coating exhibits protrusion features with peak to trough heights as high as 250 nm surrounded by smaller features with peak to trough heights of approximately 40 nm.
3.3 Coating Chemistry
To confirm retention of monomer chemistry in the plasma polymerised films, FTIR analysis on each of the coatings was performed. The infrared spectra of the coatings deposited from the HMDSO precursor were dominated by a feature around 1,050 cm−1 which can be attributed to the asymmetric Si–O–Si stretch . In the case of the superhydrophobic coating, this peak was shifted to higher wavenumbers which may be a result of an increase in SiOx stoichiometry. Peaks at 1,265, 1,350 and 2,965 cm−1 were identified and assigned to Si–(CH)x stretch and CH3 asymmetric stretch, respectively. A broad peak between 3,200 and 3,550 cm−1 was attributed to the SiOH functional group. The relative intensity of the CH3 peak and SiOH band with respect to the Si–O–Si was reduced in the superhydrophobic coating. The infra-red spectra of the atmospheric plasma deposited coatings of TMCTS, PFOTES and combination TCFS were also determined. Coatings deposited from each of the monomers showed retention of all major spectral bands after plasma polymerisation. Spectral bands present in coatings deposited from each of the monomers were identified in the copolymer spectrum also and were found to be in agreement with those reported previously concerning plasma polymerisation of both the TMCTS monomer  and the PFOTES monomers .
3.4 In Situ Ellipsometry Analysis of Protein Adsorption
Protein adsorption on each of the surfaces was monitored by spectroscopic ellipsometry analysis. The measurements were carried out in situ using the liquid flow cell. The stability of the plasma deposited coatings was assessed by passing the PBS solution through the cell for 10 min prior to the introduction of the protein solution. After introduction of the protein solution there was an immediate shift from the baseline phase (Δ) signal which relates directly to a change in thickness at the substrate . Spectroscopic data relating to both the change in Δ and change in Ψ were recorded. A Cauchy model was fit to these changes and a determination of the adsorbed protein film thickness (del) was made. The adsorbed amount of protein per unit area, or surface concentration, Γ was then calculated.
Typical adsorption profiles of BSA onto coated and uncoated wafer substrates are compared in Fig. 4. The adsorption of BSA is a relatively fast process, influenced by the surface binding affinity and diffusion rate of the protein through the solution. Immediately after introduction of the protein at 10 min, nearly full coverage on each of the surfaces is observed. A slow, further increase in adsorption is then observed to occur over a further 30 min period. The data shows an increase in Γ with increasing water contact angle, with respect to the TEOS, SiO2 and HMDSO substrates. The highest level of adsorption is seen to occur on the hydrophobic HMDSO surface where there is a large interfacial free energy between the substrate and water molecules, which protein molecules readily displace, reducing in the interfacial free energy. The adsorbed amount of 0.15 μg cm−2 is in agreement with results obtained by Lok et al. who measured the same surface saturation concentration for BSA on polymerised polydimethylsiloxane (PDMS) surfaces by total internal reflection fluorescence spectroscopy analysis . Protein surface concentration on the TCFS surface, however, is lower than any of the other substrates despite its hydrophobic properties. As indicated by surface energy data, the dispersive component of surface energy is lower in the hydrophobic TCFS coating. This indicates that weak Van der Waals interactive forces may play a more significant role than polar interactions at the lower surface energy regimes. These short range forces also play an important role in stabilising protein molecules when they interact with other molecules or surfaces in the aqueous environment. The addition of fluorocarbon groups has the effect of reducing these interactions which may explain this reduction in protein binding on these hydrophobic surfaces.
3.5 Analysis of Protein Adsorption and Bacterial Attachment on Superhydrophobic Surfaces
Figures 4 and 5 do not include adsorption dynamics on SH surfaces as it was not possible to fit an optical model to calculate a value for del due to high variation in spectral data resulting from the textured nature of the SH surface morphology. A comparison, however, of the adsorption profiles on hydrophobic and superhydrophobic surfaces can be obtained by monitoring changes in phase signal (Δ) over time. The graphs shown in Fig. 8 compare changes in Δ at a fixed wavelength of 508 nm during adsorption experiments for BSA and Fg onto hydrophobic and superhydrophobic TCFS surfaces. The Δ change relates directly to a change in layer thickness and so can be used to give an indication of the amount of protein adsorbing to the substrate.
Protein adsorption experiments performed on SH surfaces using a QCM technique did not provide comparative data, as the structure and viscoelastic properties of the textured SH surface resulted in a decoupling of the coating from the driven sensor surface. Alternative confirmation of results obtained by ellipsometry which indicated minimal protein attachment to superhydrophobic surfaces was facilitated by bacterial attachment studies. Bacterial interactions with implanted biomaterials represent the first step in the development of biofilm-associated, device-related infections . Surface hydrophobicity plays an important role in these interactions as does the conditioning film of serum and tissue proteins such as Fg and Fn, which is rapidly deposited on implanted biomaterials . Bacterial pathogens express surface proteins that promote binding to Fg, Fn and a range of other serum and extracellular matrix proteins . Thus, bacterial attachment experiments were carried out using an S. aureus (SH1000), which is known to bind specifically to Fg before and after protein adsorption experiments to identify any potential increase in bacterial attachment due to an adsorbed protein layer. It is hypothesised that SH coatings may both directly and indirectly (by limiting the deposition of serum and tissue proteins) reduce bacterial interactions with implanted materials.
This educed protein and bacterial attachment may be attributed to the combined low surface energy chemistry and nano-textured morphology of the superhydrophobic coating. This chemistry and structure creates a barrier to wetting by trapping pockets of air in the nano-scale morphology, which in effect presents a reduced surface area onto which protein molecules can diffuse from the flowing solution.
In situ adsorption of serum proteins has been investigated on plasma polymer surfaces deposited with wetting properties ranging from superhydrophilic to superhydrophobic. While reduced protein adsorption on superhydrophilic siloxane coatings was observed when compared to hydrophobic surfaces, much greater reductions were observed on superhydrophobic coatings. Hydrophobic siloxane coatings (θ = 95°) deposited from the HMDSO monomer showed the highest level of protein adsorption. Hydrophobic fluorinated copolymer siloxane coatings (θ = 105°), however, were shown to reduce the adsorption of both BSA and Fg. A comparison of surface energy data showed reduced dispersive interactions (Van der Waals forces) on the copolymer fluorinated siloxane coating. While the data confirms that protein adsorption tends to occur more favourably on hydrophobic surfaces than on hydrophilic surfaces, this result indicates that the specific surface chemistry also plays an important role in determining protein surface interaction as protein structural stability after adsorption is influenced by forces other than polar interactions. Analysis of protein adsorption on superhydrophobic surfaces indicated a significant reduction when compared to all other surfaces. A study of bacterial interactions on these coatings also showed a resistance to bacterial attachment. This study also confirmed resistance to protein adhesion on the superhydrophobic surface as indicated by minimal bacterial attachment both before and after protein adsorption trials.
This work is supported by Science Foundation Ireland under Grant 08/SRC/I1411.
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