The impact of different low-pressure plasma types on the physical, chemical and biological surface properties of PEEK Qian Fu , Matthias Gabriel , Franziska Schmidt , Wolf-Dieter Müller and Andreas Dominik Schwitalla Dental Materials, 2021-01-01, Volume 37, Issue 1, Pages e15-e22, Copyright © 2020 The Academy of Dental Materials Abstract Objective Plasma treatment can be used as surface treatment of PEEK (poly-ether-ether-ketone) to increase the bonding strength between veneering composite and dental prosthetic frameworks of PEEK or enhance biocompatibility of PEEK implants. These improvements are probably based on chemical changes of the PEEK surface. However, the aim of the study was to evaluate the impact of different low-pressure plasma treatments on surface properties of PEEK, such as roughness, hydrophilicity, micro-hardness, crystallinity and biological activity of PEEK. Methods Due to different plasma treatments, 143 disc-shaped specimens of pure implantable PEEK were divided into 4 groups: PEEK (no plasma treatment, n = 29), H-PEEK (hydrogen plasma treatment, n = 38), O-PEEK (oxygen plasma treatment, n = 38), H/O-PEEK (hydrogen/oxygen plasma treatment with a gas mix ratio of 2:1, n = 38). Subsequently, surface roughness, surface contact angle, surface crystallinity, surface micro-hardness and human osteoblast cell coverage area of each group were examined. Results The hydrophilicity, crystallinity and micro-hardness of the plasma-treated groups increased significantly compared to the untreated group, whereas significant differences in the results of the micro-hardness tests could be shown between all groups up to a test force of 0.02N. Cell density was significantly higher on treated vs. untreated PEEK surfaces. Oxygen and H/O plasma treatments revealed to be most effective, whereas H/O plasma worked ten times faster to achieve the same effects. Significance The hydrogen-oxygen, 2/1-mixed plasma treatment combines the effect of hydrogen and oxygen plasma which strongly improve the surface properties of PEEK implant material, such as hydrophilicity, crystallinity, surface micro-hardness and HOB cell adhesion. 1 Introduction PEEK (polyether ether ketone) is a poly-aromatic semi-crystalline thermoplastic polymer, which represents a viable metal-free alternative implant material, and as such it has been extensively used in recent years [ ]. Due to its elastic modulus of 3–4 GPa, which is closer to that of bone compared to metals, PEEK is being used as implant material, e.g. in the fields of orthopedics, trauma and maxillofacial surgery [ ]. This makes it also interesting for dental applications, whereas it is currently mostly being used as framework material for dental prostheses [ ]. Dental implants of PEEK are of growing interest and are being investigated in different studies [ , ]. Although the mechanical properties of PEEK, which can be adjusted additionally, e.g. by reinforcing carbon fibers [ ], are suitable for manufacturing implants, the surface of PEEK is biologically inert due to its low surface energy, limiting its osseointegration ability [ ]. Therefore, surface modification of PEEK implants is necessary. Commonly used surface modifications include surface coating, and chemical and physical treatments [ ]. Among these treatments, plasma treatment has also been employed in the recent years [ , ], e.g. for surface conditioning prior to veneering of PEEK dental prostheses frameworks. It has been found that using a low-pressure plasma treatment with argon/oxygen gas mix or only oxygen gas leads to a significantly enhanced shear bond strength between veneering composite and PEEK [ , ]. In contrast to chemical treatments with hazardous solutions (e.g. concentrated sulfuric acid) and complicated surface coating technologies, plasma treatment is safe and simple to handle [ ]. Plasma treatment causes several changes on the PEEK surface. Primarily, the surface contact angle is substantially reduced, which means that the hydrophilicity is significantly increased, for example after the plasma treatment with argon, nitrogen, oxygen, ammonia, methane and water vapor [ ]. After plasma immersion ion implantation (PIII) treatment, where the particles of the plasma are being implanted into surfaces by accelerating them over a high voltage towards the surfaces, the adhesion and proliferation of osteoblasts on the surface of PEEK were significantly improved [ , ]. Nonetheless, the effects on osteoblasts due to low-pressure plasma treatment of PEEK surfaces have not been studied yet. Additionally, surface hardness of PEEK was increased after H 2 , N 2 , He or Ar plasma treatment [ , ]. In theory, a higher hardness of PEEK is associated with a higher degree of crystallinity [ ]. The crystallinity of PEEK is generally up to 47%, depending on the cooling rate during the manufacturing process, whereas a fast cooling process causes rather amorphous PEEK with less crystallinity [ ]. In which way the changes of the surface hardness could be based on changes in the degree of crystallinity due to plasma treatment is unclear. Therefore, the aim of the present study was to investigate physical, chemical and biological properties of PEEK surfaces after different low-pressure plasma treatments using hydrogen, oxygen and hydrogen/oxygen process gases. 2 Materials and methods A round PEEK rod (diameter: 14 mm, length: 1000 mm; Vestakeep ® i4R, Evonik Industries AG, Essen, Germany) was cut into 3 mm thick discs (n = 143) using a precision saw (IsoMet1000 Precision Cutter, Buehler, Lake Bluff, USA) under water cooling. The discs were polished on one side with sand paper of 320, 800, 1200, 2500 and 4000 grit (Hermes Schleifmittel GmbH, Hamburg, Germany) for 10 min each using a polishing machine (Exakt 400CS, EXAKT Advanced Technologies GmbH, Norderstedt, Germany). All specimens were rinsed with deionized water afterwards. For the plasma treatment, a low-pressure plasma system (Femto PCCE, Diener electronic GmbH & Co KG, Ebhausen, Germany) was used, whereas the specimens were divided into four groups according to plasma treatments: 1 Untreated-PEEK: no plasma treatment (n = 26), 2 H-PEEK: plasma treatment with hydrogen gas (n = 35), 3 O-PEEK: plasma treatment with oxygen gas (n = 35), 4 H/O-PEEK: plasma treatment with 67% hydrogen and 33% oxygen gas (n = 35). Plasma treatment was performed at 0.4 mbar, 70 °C and 200 W for 10 s, 20 s, 1 min, 5 min, 10 min and 30 min. The number of specimens used for each plasma treatment and the subsequent analyses are listed in Table 1 . Table 1 Number of samples used for the different plasma treatments and subsequent analyses. Untreated PEEK H-PEEK O-PEEK H/O-PEEK SUM Duration of plasma treatment (s) 0 10 20 60 300 600 1800 10 20 60 300 600 1800 10 20 60 300 600 1800 Roughness 3 3 3 3 12 Contact angle-hydrophilicity 9 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 63 FTIR and hardness 5 5 5 5 20 Cell culture 9 9 9 9 36 Contact angle after sterilization with 60% isopropanol for 1 h 3 3 3 3 12 143 After the pre-treatment, the surface roughness (R a ) was measured using the Alicona infinite focus system (Alicona Imaging GmbH, Raaba/Graz, Austria). This system records a 3D picture of the surface at 20-fold magnification. The mean surface roughness (R a ) along two perpendicular lines, each with a length of 4 mm, was calculated based on the obtained 3D images. Water contact angles were measured after the pre-treatment procedures with the sessile drop method using a H 2 O droplet of 10 μL. For the contact angle measurement a digital microscope (Keyence VHX-5000, Keyence GmbH, Neu-Isenburg, Germany) was used, whereby the optical axis was set horizontally and aligned parallel to the specimen surface. The measurement was taken at room temperature and 10 s after the initial contact between droplet and surface. To evaluate the change in crystallinity of the PEEK surfaces after 30 min plasma treatment, Fourier-transform infrared spectroscopy (FTIR) measurements were performed of the spectra range between the wavenumbers 1800 cm −1 and 600 cm −1 using a FTIR microscope (FTIR Microscope Hyperion 3000, Bruker, Rheinstetten, Germany). In order to analyze the crystallinity of the PEEK surface, the peaks on the absorption bands at the wavenumbers 1305 cm −1 and 1280 cm −1 were recorded for each group, since these peaks are known to be sensitive to crystallinity [ ], whereas the ratio of these peaks (1305 cm −1 /1280 cm −1 ) defines the crystallinity index (CI [%]). The percentage of crystallinity was calculated as follows (ASTM Standard F2778-09): Crystallinity[%]=CI−0.7281.549*100 C r y s t a l l i n i t y [ % ] = C I - 0.728 1.549 * 100 Micro-hardness was detected immediately after 30 min plasma treatment according to the ISO standard 6507 using a hardness tester with a force of 0.005, 0.01, 0.02, 0.2 and 0.5N (Q10M, Qness GmbH, Golling, Austria). The indentation depth was measured indirectly using the integrated microscope, whereas the length of the groove “L” was recorded. Since the apex angle of the diamond was 136°, the depth “D” could be calculated with the formula: D = L / 2 tan68°. For the cell culture tests, the specimens were plasma-treated for 30 min. After plasma treatment, all the specimens were sterilized with 60% isopropanol/water for 1 h before cell seeding. To reveal any influence of the sterilization process on the water contact angle, contact angle measurements of n = 3 additional specimens per group were recorded after plasma treatment and subsequent sterilization with 60% isopropanol for 1 h. Human osteoblasts (HOB; Provitro AG, Berlin, Germany) were seeded on the specimens at a density of 5 × 10 4 cells per well in 24-well tissue culture plates. The cultures were incubated in a humidified atmosphere of 5% CO 2 at 37 °C and the osteoblast growth medium (Provitro AG, Berlin, Germany) was refreshed after three and six days. After 1, 3 and 7 days, the area of the surface of the specimens covered by HOB was measured by means of fluorescence microscopy. Specimens were washed with sterile PBS and incubated in 1 mL PBS containing 5 μL of fluorescein diacetate (FDA, Sigma, Germany) stock solution in acetone (5 mg/mL) for 30 min at 37 °C. The stained specimens were again washed with PBS and immediately examined under a fluorescence microscope (Vanox-T, Olympus, Hamburg, Germany) at excitation/emission wavelengths of 490/526 nm. Three pictures of a surface area of 3.72 mm 2 (2.24 × 1.66 mm) of each specimen were recorded, positioned at the corners of an imaginary regular triangle around the center of the specimen′s surface and the percentage of cell coverage was calculated by the software Image J (National Institutes of Health, Bethesda, USA). A commercial software (OriginLab Corporation, Northampton, MA, USA) was used for statistical analysis and one-way ANOVA to reveal significant differences of the results. The significance level for differences in the findings was set at p < 0.05. 3 Results After 30 min of plasma treatment, there was no statistically significant difference in the surface roughness (Ra) between the treated (H-PEEK: 0.42 ± 0.07 μm; O-PEEK: 0.40 ± 0.07 μm; H/O: 0.43 ± 0.06 μm) and untreated (0.41 ± 0.07 μm) PEEK surfaces. The changes of the water contact angle after the different plasma treatments are shown in Fig. 1 , whereas untreated PEEK showed a contact angle of 80 ± 1.12°, corresponding to the result at 0 s. The highest changes of the surface contact angles occurred within the first minute of plasma treatment, whereas after 10 min the contact angles remained rather stable at almost 0° for the H/O-PEEK group and the O-PEEK group and 41.67 ± 1.15° for the H-PEEK group. After 30 min, the contact angle of the H-PEEK was 41.00 ± 2.65°. Fig. 1 The contact angles of each group depending on the duration of the plasma treatment. Concerning the treatment time less than 600 s, the contact angles were ordered as following: H-PEEK > O-PEEK > H/O-PEEK. Using a treatment time of 10, 20 and 60 s, the H-PEEK, O-PEEK and HO-PEEK were significantly different (p < 0.05). Using a treatment time longer than 300 s, there was no statistically significant difference between O-PEEK and HO-PEEK (p > 0.05), but there were significant differences between each of these two groups and H-PEEK (p < 0.05). FTIR results showed different heights of the “a-peak” for all groups between wavelength 1250 cm −1 and 1350 cm −1 ( Fig.2 ), resulting in a CI of 87.43 ± 0.95% for untreated PEEK, 91.39 ± 0.96% for H-PEEK, 100.41 ± 2.11% for O-PEEK and 110.44 ± 2.51% for H/O-PEEK. Therefore, the crystallinity amounted 9.45 ± 0.61% for untreated PEEK, 12.00 ± 0.62% for H-PEEK, 17.83 ± 1.36% for O-PEEK and 24.30 ± 1.62% for H/O-PEEK. All groups differed significantly from each other (p < 0.05). Fig. 2 FTIR measurements of the different groups after 30 min plasma treatment each between wave length 1250 cm −1 and 1350 cm −1 . The surface micro-hardness increased parallel to the surface crystallinity (untreated PEEK < H-PEEK < O-PEEK < H/O-PEEK), whereas the results showed significant differences between all tested groups up to a loading force of 0.02N ( Table 2 ). Using a force of 0.2N, no significant difference between untreated PEEK and H-PEEK and between O-PEEK and H/O-PEEK was observed. Table 2 Summary of the micro-hardnesses and the indentation depths. Test forces Untreated PEEK H-PEEK O-PEEK H/O-PEEK Micro-hardness [N/mm 2 ] 0.005 N 182.67 ± 2.4 a 229.90 ± 6.8 a 268.12 ± 6.8 a 289.49 ± 7.5 a 0.01 N 183.26 ± 1.5 a 213.83 ± 2.8 a 231.67 ± 2.6 a 258.52 ± 2.8 a 0.02 N 183.26 ± 1.5 a 200.90 ± 2.2 a 212.27 ± 2.6 a 228.14 ± 1.9 a 0.2 N 182.28 ± 2.7 b,c 183.26 ± 2.6 d,e 187.96 ± 2.3 b,d 188.75 ± 3.2 c,e 0.5 N 182.67 ± 2.4 182.48 ± 2.1 182.67 ± 2.6 182.67 ± 2.3 Indentation depth [μm] 0.005 N 1.41 ± 0.02 a 1.20 ± 0.04 a 1.16 ± 0.03 a 1.02 ± 0.03 a 0.01 N 2.01 ± 0.02 a 1.83 ± 0.02 a 1.75 ± 0.02 a 1.62 ± 0.02 a 0.02 N 2.82 ± 0.02 a 2.70 ± 0.03 a 2.61 ± 0.03 a 2.51 ± 0.02 a 0.2 N 10.13 ± 0.15 b,c 10.07 ± 0.14 d,e 9.81 ± 0.06 b,d 9.78 ± 0.16 c,e 0.5 N 14.05 ± 0.18 14.06 ± 0.16 14.05 ± 0.20 14.05 ± 0.17 a significant difference between all groups (p < 0.05). b significant difference between untreated PEEK and O-PEEK (p < 0.05). c significant difference between untreated PEEK and H/O-PEEK (p < 0.05). d significant difference between H-PEEK and O-PEEK (p < 0.05). e significant difference between H-PEEK and H/O-PEEK (p < 0.05). Using higher forces between 0.2N and 0.5N, no statistical difference between all tested groups could be shown. Accordingly, the indentation depths showed significant differences between all groups up to a test force of 0.02N ( Table 2 ). Untreated PEEK showed the highest indentation depth, followed by H-PEEK, O-PEEK and H/O-PEEK. After using a test force of 0.5N, the indentation depth was 14 μm on all specimens. After sterilization with isopropanol, the contact angles were significantly increased, but showing the same pattern compared to untreated PEEK ( Fig.3 ). Fig. 3 Contact angles of the different groups before and after sterilization with 60% isopropanol for 1 h. *significantly different to untreated PEEK, ** significantly different to H-PEEK, *** significantly different to the results after sterilization. Representative fluorescence microscopy pictures of the HOB cell culture tests at days 1, 3 and 7 are shown in Fig.4 . The calculated HOB cell density is shown in Fig.5 . The cell density on plasma treated PEEK surfaces was higher than on the untreated PEEK surface. Within the plasma treated PEEK O and H/O plasma showed the highest cell densities with around 40% surface area covered by HOB cells after 7 days. H-PEEK showed a lower cell density with around 20% surface area covered by HOB cells after 7 days. The results of the untreated PEEK were significantly different to the plasma treated PEEK surfaces. There was no significant difference between O-PEEK and H/O-PEEK, whereas these two groups and H-PEEK and untreated PEEK differed significantly from each other after 1, 3 and 7 days. Fig. 4 Representative fluorescence microscopic pictures of the cultivated human osteoblasts on the PEEK samples. Fig. 5 PEEK surface areas covered by human osteoblasts. 4 Discussion Within the limits of the present study it could be shown, that the three different low-pressure plasma treatments of PEEK had a significant impact on the hydrophilicity, the crystallinity, the micro-hardness, and the HOB cell coverage area, which was in accordance with the literature [ ]. The literature indicates that surface roughness plays an important role in cell adhesion and osteogenesis [ ]. In the present study, no obvious change in surface roughness was observed after plasma treatment. Therefore, the reason for the significantly increased cell coverage area after plasma treatment might have rather been chemical changes of the surface of PEEK, especially for the O-PEEK and H/O-PEEK groups, which might have caused enhanced cell adhesion. This demonstrates that despite the increased contact angle due to the sterilization process with isopropanol, oxygen plasma treatment and hydrogen-oxygen low-pressure plasma treatment could significantly improve the biological activity of PEEK, indicating that this treatment could represent a promising method for clinical application when thinking of endosseous implants of PEEK. Theoretically, the sterilization step could have been neglected, as plasma itself has sterilizing effects [ ], thus causing even higher cell adhesion rates. Considering the contact angle of the O and H/O-PEEK groups, a plasma treatment time of 10 min. might have led to the same results of the cell culture test, as no changes of the contact angles could be observed compared to a treatment time of 30 min. A limitation of the study was that titanium as positive control was not included into the cell culture tests, as it still represents the gold standard of implant materials. Furthermore, the influence of the modified surfaces on other cell culture parameters such as cell viability and cell proliferation rate should be investigated in the future. The effectivity of plasma treatment was clearly depending on the process gases, which could be ranked in the following order: H-PEEK < O-PEEK < H/O-PEEK. It seems that low-pressure plasma treatment not only had an etching effect also cleaning the surface by removing adhered particles, but also an impact on the chemical structure of PEEK modifying the performance of its surface. During plasma treatment, the bombardment of the surface with the highly energetic ions and radicals can cause local heating-up at the nano-scale and activation of chemical reactions [ , ]. In the present study, ions of different sizes were being used, which derived from small H-atoms and bigger O-atoms, causing different effects in relation to ablation and chemical modification. Using a humid-air plasma treatment of polypropylene, it could be shown that an increasing energy deposition increased the densities of alcohol, carbonyl, acid, and peroxy radicals on the polypropylene surface [ ]. Brennan et al. have shown newly formed O C O (carbon acid) bonds after oxygen plasma treatment of PEEK, [ ]. Comyn et al. reported that besides the COOH groups, PEEK can also show new OH groups after oxygen plasma treatment [ ]. Based on the afore mentioned literature, it could be assumed that during hydrogen (H) plasma treatment, not only the C O C bond of PEEK, but also the C O bond may have been broken to form C OH. A small amount of benzene rings may have been cleaved and volatilized ( Fig.6 ). Consequently, the PEEK surface would have contained more C OH groups, which could have been a reason for improved hydrophilicity. However, the moderate changes in hydrophilicity after H-Plasma could be explained by the absence of polar groups resulting from plasma based on O atoms. Fig. 6 Possible chemical changes of PEEK due to plasma treatment. In case of the oxygen plasma treatment, an O-atom/radical may have been added to form C O O C after the cleavage of C O C bonds. The bond between a benzene ring and the C O group may have been broken forming unstable radicals, such as O C O and C O ( Fig.6 ). These unstable oxides could have reacted with the humidity of the air after the plasma treatment forming O C OH and C OH. These new functional polar groups on the PEEK surface could have been the reason for the increased hydrophilicity with a significantly smaller contact angle than after hydrogen plasma treatment. Consequently, the resulting groups from the afore mentioned secondary reaction with the air humidity after oxygen plasma treatment could have been already formed during the treatment with the hydrogen/oxygen plasma, as H and O radicals were present simultaneously to react with the PEEK surface. Theoretically, this gas mixture is highly explosive, but not at the low pressure at which the gas mixture was used for this study. Therefore the use of this gas mixture inside the plasma chamber is officially approved by the manufacturer, also because the low pressure plasma system has the necessary safety valve. After 1 min of plasma treatment, the contact angle of the H/O-PEEK group was reduced to 3°, while the O-PEEK group showed a contact angle of 16°, which was reduced to 3° after 5 min oxygen plasma treatment. After 10 min treatment time, the contact angles of O-PEEK and H/O-PEEK were both 0°. This shows that the hydrogen-oxygen mixed treatment could form more hydrophilic groups in a shorter time period. In addition, the C OH and O C OH groups generated during the plasma treatment could have been reacted to generate new chemical bonds and chains. This could be a reason for the increased crystallinity and thus the micro-hardness of the surface of PEEK. This effect could be interesting, e.g. for PEEK based dental implants, to stabilize the surface layer of the outer threads for the insertion procedure. However, when illustrating the relationship between surface crystallinity and micro-hardness after plasma treatment, the resulting graph of the correlation between these properties showed a quite exponential relationship ( Fig.7 ). Therefore, as an approximation, the mathematical dependency could be used to calculate the crystallinity (y) [%] from a given micro-hardness (H) [N/mm²]: Fig. 7 The relationship between micro-hardness and crystallinity using a test force of 0.005N. y = 1.8449e 0.0086H , whereas this equation showed a high coefficient of determination R² = 0.9265. As FTIR is a rather complex method to detect surface crystallinity, measuring surface hardness to calculate the crystallinity using the equation could be a simple and convenient way. Therefore, the test force of 0.005 N has been proven to be ideal. In theory, the crystallinity of PEEK depends on the cooling rate of the manufacturing process [ ]. But also the processing temperature, such as the mold temperature on compression molding seems to have an influence on the crystallinity, as found by Conrad et al. [ ]. According to them a mold temperature of 340 °C causes a crystallinity of 23%, whereas a higher mold temperature of 390 °C decreased the crystallinity to 14% [ ]. For the untreated PEEK the detected crystallinity of 9.18% was relatively low possibly due to the manufacturing process. Maybe despite the water-cooling used for the different processes (cutting, polishing) during specimen preparation, the generation of local heat could not have been completely avoided. Further, different plasma process times should be performed in the future, not only to underline the current findings but also to evaluate the maximum plasma process time causing a maximum surface crystallinity. Also tribological tests should be performed to show whether the higher surface hardness of PEEK has a positive effect on the wearing behavior, e.g. of a dental implant of PEEK, especially during insertion [ ]. Furthermore, a potential positive effect on the osseointegration of PEEK implants due to the plasma treatments has to be proven in vivo . 5 Conclusion The evaluated low-pressure plasma treatments had a significant impact on the hydrophilicity, crystallinity and thus micro hardness of PEEK surfaces and caused a substantially increased adhesion and proliferation rate of human osteoblasts seeded on the plasma-treated PEEK surfaces. Among those treatments, a plasma atmosphere of H/O with a ratio of 2/1 seems to be most effective. Acknowledgements The authors would like to thank Evonik Industries AG for donating the PEEK material. The present study was conducted as part of a project supported by the Central Innovation Program for SMEs (ZIM) of the German Federal Ministry for Economic Affairs and Energy (Project management agency: A iF Projekt GmbH, Berlin, Germany ; grant number ZF4133201AK5) . Q.F. has a scholarship from the Chinese Scholarship Council. References 1. 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