Polyether-ether-ketone (PEEK) as a synthetic polymer, offers many advantages in biomedical implant applications, due to its appropriate mechanical, chemical and biocompatibility properties. However, this polymer is bioinert, requiring surface modification to overcome its weakness in adhesion to the surrounding bones. Surface modification such as roughness and hydrophilicity have been shown vital parameter for the enhancement of adhesive properties of biomaterials. In this regard, laser treatment has been performed for surface modification, however, further studies are needed to fully understand the effect of laser on the PEEK surface. Therefore, the aim of this paper is to investigate the effect of different laser parameters such as fluency and wavelength on the surface hydrophilicity to determine the best option to improve PEEK surface. Surface average roughness (Ra) and surface topography were investigated by atomic force microscopy (AFM). Surface morphology was observed with Eclipse E200-LEDmicroscope. The hydrophilicity of the surface was investigated with static contact angle test with the help of SSC-DC318P color video and measured by Image J software. At last, the result showed that the wavelength of 532nm with a fluency of 8J/Cm2 is the best choice for the enhancing of surface properties of PEEK for biomedical application. The use of a MedLite6ND: YAG laser with different wavelengths and fluencies to evaluate and compare the effect of each of these parameters on the modification of the sample surface and, ultimately, the selection of superior modification conditions, is the novelty of this project.
Keywords: PEEK, Roughness, Laser, wettability
Today, many people suffer from the defect of bone due to an accident, congenital diseases, age increasing, etc. Therefore, it is necessary to find a suitable alternative for treatment. Despite the advantages that natural alternative has, they also have some disadvantages that limit their use. Therefore, the use of artificial replacements has found a special attention. For the biomaterial being used for the bone implant, osseointegration and bone connection of artificial implant to their surrounding tissue are very important. On the other hand, it has been shown that desirable cell adhesion and spreading on the surface of the implant is needed at cellular level, to achieve this goal(1). Therefore, modifying surface parameters such as roughness and wettability (2) become important.
Recently Polyether ether ketone (PEEK) is taken into consideration due to its interesting properties such as physical properties(radiolucency), and mechanical properties (elastic modulus close to bone modules, high strength wear resistance), forming process (Ability to prepare a three-dimensional implants customized to fit the exact size of the lesion y laser sintering method), and less local inflammation and stress shielding problem compare to metal implant as biomaterial specially in the field of bone implant and orthopedic application (3, 4). Unlike the desirable characteristics of this polymer, its ability to bond with the surrounding bone tissue is low due to its low bioactivity and poor interfacial biocompatibility. On the other hand, it is known that the surface of biomaterials is a major factor in successful connection with surrounding tissue in the body. It is believed that surface properties such as topography and surface chemistry (including surface composition, charge, functional group and density of functional groups) are two important factors for cell adhesion to the surface of biomaterials and causing the bone-implant interactions to be successful (5-7).
To achieve this goal several methods have been used such as acid etching, chemical modification, plasma modification, composite structure and laser treatment (8). Laser treatment is an appropriate choice for enhancing surface bioactivity because of its advantages such as high operation speed, low cost, easy operation, good repeatability(9) flexibility and the ability to modify small areas without affecting other areas(10, 11).
One of the ways to improve surface properties of materials is surface engineering by the laser. The input of radiation from the laser to the solid surface, including electron stimulation and return to non-intrusive mode, is very short on time. In other word, the total energy input to the surface is not enough to change the material’s bulk temperature. This makes it possible to modify surface layer in the acute conditions, without changing of the properties of bulk, which is, of course, one of the other advantages of using the laser in surface modification(11). Laser surface engineering has many uses for increasing surface properties such as hardness, wettability, fracture, fatigue, wear resistance, corrosion, and so on. Therefore, phenomena caused by laser interaction with the surface, such as melting, evaporation and slipping depth are an integral relation to the optical parameters of the laser (pulse energy, pulse width, pulse number, irradiation time), and chemical, physical and optical properties of the metal. What is certain is that the modification by laser depends on the energy density of the laser and its optical-kinetic conditions which effect on their wettability and ultimately their biocompatibility(10, 12).
Scientists try to enhance the bioactivity of PEEK through physical, chemical and biological methods to achieve successes in the field of implant applications. But despite a lot of valuable research that has already been done, an appropriate implant has not been obtained yet and more research is needed. Therefore, the aim of this study is to modify the surface of PEEK to improve its interaction with tissue environment and bone interaction in the next step. For this purpose, a micro-nano structured surface (with specified parameters) will be created by laser treatment. We hope to enhance the surface wettability and roughness to improve its bioactivity by applying different parameter of the laser to choose the optimum condition.
Materials and methods
The PEEK polymer which has used in this study has been purchased from IRAN. With the melting point of 340? C and glass transition temperature of 143?C with an amorphous form. PEEK sample was cut into circular plates with a diameter of 2 cm and a thickness of 2 mm and then the surface of PEEK cleaned with ethanol.
The ND: YAG laser (Med Lite C 6) was used for surface modification of PEEK. Two wavelengths 1064 and 532 nm were used for modification. The pulse width was 5-20 nsec. Treatment has done in atmospheric pressure in the air with at 25ºC.
In this study, six PEEK samples were exposed to the laser to investigate the effect of different laser parameters such as wavelength and fluency (indicated in table 1) on the surface. As shown in Table 1, six samples which are nominating with number 1 to 6, are exposed to three different fluencies (4-8-12 J/Cm2) and two different wavelengths (532-1064 nm) with one pulse.
Table 1: Different laser parameters investigated in this study
Spot size (mm)
An optical microscope (Eclipse E200-LEDmicroscope) was used for analysis of surface morphology with two magnifications including 4x and 10x. For measuring the surface roughness an Atomic force microscopy (AFM), (Ambios tech. USA) in the non-contact mode with 2 Hz scan rate and frequency of 15 kHz along with silicon cantilever were used. For evaluation of roughness parameter, the average roughness (Ra) was reported. Finally, for investigation of surface wettability, the static contact angle was measured. The water drop was poured on a PEEK sample surface with a very accurate syringe. The images of water droplets were stored with the help of SSC-DC318P color video and the contact angle was measured by means of image J software. Serval measurements were performed to estimate accurate contact angles.
For this study, three samples were exposed to a wavelength of 532 nm with different fluencies of 4, 8, and 12 J/Cm2, and the remaining 3 samples were exposed to a wavelength of 1064 nm at the same fluencies of 4-8-12 J/Cm2, a control sample was also considered for comparison.
Optical microscopicy observations:
For the study of the morphology of PEEK surface after laser treatment, optical microscopy has been used. When laser radiation heats the biomaterial surface, the surface may be melt or ablate or sometimes burn that Each of these events depends on the laser condition. As shown in Fig. 1, by increasing laser fluency surface alteration become more obvious and also by the increased wavelength on the other hand, sample was burned as can be observed in sample 6.
Figure 1: Optical microscopic image of the PEEK samples treated with the wavelength of 532nm, respectively, with flounces: 1) 4 J/Cm2, 2) 8 J/Cm2, 3) 12 J/Cm2 and wavelength of 1064nm, fluencies: 4) 4 J/Cm2, 5) 8 J/Cm2, 6) 12 J/Cm2 shown in two magnifications: 10x (Shown with letter b) and 4x (Shown with the letter a)
AFM imaging and roughness analysis
The above, six laser treated samples along with an untreated sample (as a control) were analyzed by AFM. The AFM result was reported in Fig 3 in both topography and phase mode. The average roughness (Ra) was calculated with software with high accuracy. A common roughness index, which can be calculated from the topography image of the AFM with the software were presented in table 2 with their definition.
Table 2: Roughness parameters and their definitions
Average roughness (commonly reported roughness)
Peak roughness (maximum peak height)
Valley roughness (maximum valley depth)
Mean square roughness
Maximum Height of the Profile
Rmax= Rp + Rv
Figure 3: It shows an AFM image of six laser modified samples in phase image (1-6 (a)) and 3D topographic image (1-6 (b)) and a control sample in 2D topographic (Non-a) 3D topographic (Non-b)
Contact angle analysis
For analysis of contact angle, the behavior of the water drop was observed after first 5 seconds and 1 minute later. It was found, that the effect of the surface in the shape of the water drop was determined in the first few seconds and did not change significantly over time. So the image of drops after 5 seconds on the sample surface was shown in Fig.4. It depicted that the contact angle is higher in sample 6 and lower in sample 2 and 3 and the rest samples are nearly the same.
It is believed that a moderate contact angle (about 50-60 degree) or surface wettability gives higher cell attachment. Surface wettability is the result of surface energy. When the surface energy is high the contact angle decrease and when it is low, contact angle increase. The Surface characteristic such as chemical composition, charge, density, roughness, affect surface energy, as a result, surface wettability. It has been shown that in some cases laser radiation produces functional groups on the surface, in addition to the surface roughness(9). Functional groups like hydroxyl which enhance surface hydrophilicity, shown by the chemical analysis of surface via XPS and FTIR.
Figure4: Water droplet images on the laser treated specimens: 1)532 nm- 4 J/Cm2 ,2) 532 nm-8J/Cm2 ,3)532nm-12J/Cm2 ,4)1064nm-4J/Cm2, 5)1064nm-8J/Cm2, 6)1064 nm-12J/Cm2
As mentioned before, there is a relationship between roughness and wettability but this relation is not completely clear and there are several different reports about their relation. In the document has reported the range of the roughness for commercial implant in 0.08-1.30 micrometer (5) Some of the studies has claimed that increasing of the roughness cause decreases in contact angle and followed by enhancement of wettability which can induce enhancement of cell adhesion and osteointegration.(13)Some researchers have indicated the threshold of optimum roughness. Some of them have said roughness more than 1 micrometer is optimum and other one mentioned that roughness under 1- micrometer is good effect and some of them expressed micro and Nano scale effect on surface wettability and cell behavior. Some of them claim that the increment of the roughness cause decrease of the surface contact if the surface is hydrophilic and increment of the roughness cause increase of contact angle if surface hydrophobic.
One study showed that the proliferation of cell is becoming higher if roughness lower than 1-micrometer and has conferred roughness higher than 1 micrometer cause decreases of cell proliferation and claimed the suitable range of roughness (0.08-1 micrometer) for cell adhesion. (14)also surface wettability is effective issue in cell adhesion and cell spreading and besides this issue, it has showed that with decreasing contact angle cell attachment will increase but moderate range of contact angle which reported 40-70centigard is suitable choice and desired(15-17).about roughness it has reported with increasing of roughness due to increment of the surface area, cell adhesion has enhanced but range of roughness has presented .(18-21)one study has used several titanium surface with different roughness ranges from 0.5 -1.20 micrometer and conclude roughness of 0.15 micrometer is the best choice for cell attachment and proliferation.
As shown in figure 4, sample 2 and 5 have less contact angle than the others, which indicates higher wettability than other samples. On the other hand, the roughness (Ra) of each sample was calculated from AFM results and the range of roughness was in 250-800 nanometer which is good for cell behavior, (14, 22). Sample 2 showed the highest roughness value which is a reason for the increment of its hydrophilicity. While the least roughness value was for sample 6 which was the result of the disappearance of the surface grooves due to the melting of the surface. The results of roughness have shown that, with increasing of fluencies in the range of 4-8, the surface roughness has increased, but with the increment of flounces from 8 to 12, the surface roughness has decreased, this result was shown in both 532 and 1064 nm.
It can be justified with the fluency increment, the PEEK sample will melt and burned and then the groove of the surface which has been created in lower fluency were deleted in this range so the roughness decrease. On the other hand, the AFM result has shown higher roughness in samples treated with 532 nm compared with 1064 nm.
In this study, the effeteness of laser irradiation wavelength and fluency on surface roughness and wettability has investigated. The results show that among of six samples with different parameter the sample which exposure to a wavelength of 532 nm and fluency of 8 j/cm2 has the most adequate roughness and also less contact angle and therefore is the best choice for in vivo usage as biomedical implant due to the most adhesion to surrounding bone and osteoblast adhesion.