Enhancement of surface properties of polyetheretherketone implant material by fractional laser texturing

Introduction

Polyetheretherketone (PEEK) is a high-performance polymer that has been recently investigated as an alternative biomaterial for metallic materials in dentistry.¹ The increased interest in PEEK as an implant material is due to its biocompatibility and its superior physical and mechanical properties over titanium and other metallic materials.^2,3

Studies have shown that PEEK causes fewer hypersensitive and allergic reactions than titanium.^4,5 Radiolucency is one of the benefits of using PEEK as an implant material since it can be imaged by X-ray without any distortion or artifacts.^6,7 Besides, it has more aesthetic appeal than titanium due to its beige color.⁸ Most importantly, the elastic modulus value of neat PEEK is about 3.6 GPa, which lies between the values of trabecular (1.4 GPa) and cortical bone (14 GPa).^9,10 The close proximity in stiffness between PEEK and human bone causes less stress shielding and bone resorption around the implant in comparison with stiffer titanium (elastic modulus = 110 GPa).¹¹

This polymer is not bioactive due to its hydrophobicity and poor interfacial interaction, meaning it has a limited ability to bind with the surrounding bone tissue. Wettability is a crucial property of an implant material since the first event of implantation is the adsorption of water and protein from the blood on the implant surface, which signals the process of new bone formation.^12,13 However, it is well-established that wettability is affected by the biomaterials' surface roughness and texture, which play important roles in establishing a functional interface with the surrounding tissue in the body.^14,15

The topography of a surface has a direct impact on biological reactions at the cellular level, including cell orientation and migration as well as the development of ordered cytoskeletal structures. Successful osseointegration of implants has been shown to be associated with surface roughness in the nano- and micro-scales.^16,17 Previous studies^18,19 demonstrated that a moderately rough implant surface with an average surface roughness of more than 1 μm allowed for bone ingrowth and provided mechanical interlocking with bone tissue. High bone-to-implant contact was seen with rough surfaces due to the increase in surface area.^18,19

Several methods have been experimented to alter the surface topography of PEEK polymer, including acid etching, grit blasting, plasma treatment, and laser texturing.²⁰ Laser surface texturing (LST) is one of the simplest methods to modify the surface of polymers. It can simultaneously alter the surface roughness at the macro-, micro-, and nano-sized scales without using different tools.²¹ A laser can be used to create surface features like pits and grooves with an equidistance ranging from several nanometers to even micrometers between the pits or grooves.²² These topographic features influence cellular adhesion, migration, proliferation, and differentiation behavior.^23,24

A fractionated laser is a mode of laser delivery in which energy is conveyed in an array of parallel vertical columns of multiple microscopical thermal spots termed microscopic treatment zones (MTZs) with a constant distance between spots.²⁵ Fractional CO₂ was introduced in 2004 in dermatology for skin resurfacing and it may have several advantages in dentistry, especially in the surface treatment of biomaterials.^26–28 The device allows for a precise irradiation area with a more homogenous texturing pattern to be set without the need for manual movement of the device handpiece. Unlike with traditional CO₂ lasers, there is less overall material bulk heating and thermal degradation with fractional lasers.²⁵ Additionally, with the correct selection of laser parameters (power, pulse duration, distance between spots, and spot pattern), the desired melting, ablation, and penetration depth can be achieved.^29,30

The aim of this research was to improve the surface roughness and wettability of a PEEK substrate by fractional CO₂ laser texturing without affecting the material’s microstructure. The null hypothesis is that laser surface texturing by fractional CO₂ laser does not improve the surface roughness nor the wettability of PEEK.

Methods

Results

Optical light microscope

Optical microscope images of control and laser textured samples³⁶ are shown in Figure 1. Samples treated with 100 μs PD did not show any noticeable laser effect for both patterns. With increasing the pulse duration over 200 μs, the dots pattern became more prominent with larger heat-affected zones. The dots of samples with 400 μs PD had more depth than other samples with less PD due to the melting and evaporation of the material as the absorbed heat increased. Carbonization can be seen over the edges of the dots with samples treated with 500 μs PD. The staggered dots pattern seemed to cover more surface area than the straight dots pattern with fewer unaffected zones between the dots due to dots staggering or zig-zag dots formation.

Figure 1. Optical light microscope images of control and laser textured samples at 10× magnification.

FESEM

The surface morphologies of PEEK samples at 200× and 2000× magnifications are shown in Figure 2 and Figure 3, respectively. The FESEM images were consistent with the optical microscope images in terms of the effect of heat on the material. The laser effect became more prominent as the pulse duration was increased. With PD of 200 μs, there was some uniform ablation at the center of the dot and in the heat-affected zone with no signs of melting. There was little melting at the center of the dot with 300 μs PD, but with 400 μs PD complete melting occurred at the center of the dot with formation of uniform edges. There were no cracks caused by laser in all samples.

Figure 2. Field emission scanning electron microscopy images of control and laser textured samples at 200× magnification.

Figure 3. Field emission scanning electron microscopy images of control and laser textured samples at 2000× magnification.

EDX

Elemental analysis with an EDX spectrum of untreated PEEK sample is presented in Figure 4. An area analysis was done for the whole image shown in the figure. For laser treated sample with 400 μs PD, EDX spectra were obtained at three points, as shown in Figure 5 to assess the effect of laser on the microstructure of the material. The elemental analysis of the laser treated samples showed very comparable weight percentages of carbon and oxygen to that of unmodified samples. There was no increase in the percentage of carbon at the center or at the margins of the spot, indicating no presence of carbonization or burning.

Figure 4. Energy dispersive X-ray spectroscopy (EDX) spectrum of control PEEK sample.

Figure 5. Energy dispersive X-ray spectroscopy (EDX) spectra of laser treated PEEK sample.

Surface microroughness

Mean values of surface roughness (R_a) in micrometers are presented in Figure 6. One-way ANOVA showed that there was a high significance among groups (p-value < 0.01). All laser treated samples showed a highly significant increase in surface roughness compared to the control sample. The highest microroughness values belonged to samples treated with 400 μs pulse duration and a staggered dots pattern.

Figure 6. Mean values of surface roughness (R_a) with Tukey’s HSD (honestly significant difference) significance.

AFM

The 3-dimensional topographical images of control and laser textured samples are presented in Figure 7. There was an increase in summits and valleys density in all laser treated samples compared to the control group.

Figure 7. Atomic force microscopy (AFM) 3D topographical images of control and laser treated samples.

Mean values of AFM average surface roughness (S_a) and surface area ratio (S_dr) are presented in Figure 8 and Figure 9, respectively. There were highly significant increases in S_a and S_dr values for all laser samples in comparison to control samples. The samples of 400 μs PD treatment with a staggered dots pattern showed the highest mean values of both S_a and S_dr.

Figure 8. Mean values of atomic force microscopy (AFM) average surface roughness (S_a) with Tukey’s HSD (honestly significant difference) significance.

Figure 9. Mean values of atomic force microscopy (AFM) surface area ratio (S_dr) with Tukey’s HSD (honestly significant difference) significance.

Wettability

Images of water contact angle measurements are shown in Figure 10, while the mean values are presented in Figure 11. One-way ANOVA resulted in a high significance among groups. All laser textured samples showed a highly significant decrease in the water contact angle. There were highly significant improvements in wettability with laser PD of 400 μs especially with a staggered dots pattern, which showed the most improvement in this property.

Figure 10. Water contact angle measurement.

Figure 11. Mean values of water contact angle with Tukey’s HSD (honestly significant difference) significance.

Discussion

This study was conducted to assess the efficiency of a fractional CO₂ laser system used for skin resurfacing in surface texturing and enhancement of surface properties of PEEK implant substrate. PEEK has proven itself to be a valuable material in the field of implantology due to its biocompatibility, high mechanical strength, inherent radiolucency, and exceptional chemical resistance. Unfortunately, PEEK's bioinert surface does not support cellular adhesion and bone tissue bonding, limiting its use in the medical field.¹²

Laser texturing with a fractional laser is easy, fast, and applicable since the whole surface can be treated with a single run without the need for a moving device. When a laser beam of high intensity comes in contact with the surface of a material, some of the beam is absorbed by the material, and the remaining intensity is lost to the surrounding environment through the processes of reflection and scattering. The effect of laser on a polymeric material can be either photothermal or photochemical. A photothermal effect occurs when the absorbed laser energy converts to heat that raises the temperature of the material. When the temperature of the substance exceeds its boiling point, it turns into vapor and is generally ejected from the surface as it boils. This effect induces diverse phenomena, including melting or vaporization (ablation), which modify the topography.³¹

The photochemical effect occurs when the laser used has a wavelength in the UV range (like excimer lasers) in which the photon energy is enough to break molecular bonds at the polymer surface and induce a change in the surface chemistry.

Since the wavelength of CO₂ laser is within the infrared range (10600 nm), the effect induced on the PEEK surface was photothermal by a process called photothermolysis without any chemical change in the microstructure of the material as confirmed by EDX in Figure 4 and Figure 5. The fractional laser causes selective photothermolysis by splitting the laser beam into an array of microbeams with a fixed distance between them. The ablation and texturing pattern of PEEK material became clearer and more distinct as the pulse duration was increased, as can be seen in Figure 1. By increasing the pulse duration, the spot energy increases and causes more heat to be absorbed by the material. With a short pulse duration of 100 μs, no effect was seen because the heat was below the melting threshold of PEEK, while increasing the duration to 500 μs caused burning of the material due to intense heating.

Surface topography, roughness, and wettability have been shown to significantly affect cellular responses to biomaterials in a number of studies.^32–34 The behavior of cells on biomaterial surfaces is determined by the interaction between implant and cells, which is related to surface roughness. Naturally, bone cells encounter and interact with nanostructures in their environment, such as extracellular matrix (ECM) proteins. To improve bone cell adhesion and proliferation, it is thoughtful to mimic the cellular environment by creating materials with nanotopography. While nanotopographic features are important for the recruitment and migration of bone cells to the implant surface, microtopographically complex surfaces are equally vital in increasing the degree of bone-to-implant contact. Microcraters created on PEEK’s surface by the fractional laser could encourage bone ingrowth and enhance the anchorage of implants with bone tissue. The results of this study confirmed that laser texturing with fractional CO₂ laser significantly increase surface roughness on both nano- and micro-level as demonstrated by AFM and microroughness tests. The samples treated with 400 μs pulse duration showed surface roughness values of about 1.3 μm. This value is well within the optimum roughness range (1–2 μm) to enhance bone integration reported by other studies.^32,35

In regards to wettability, it is an essential property for cell spreading, which in turn is an important stage in cell adhesion prior to proliferation and differentiation. The wettability was significantly enhanced after laser texturing, particularly with samples treated with 400 μs PD which showed the least water contact angles among other samples. These results are consistent with Wenzel’s theory, which stated that adding surface roughness and increasing the surface area leads to an enhancement in the wettability.

Since the fractional CO₂ laser surface texturing improved the surface properties of PEEK material, the null hypothesis was rejected.

Conclusion

Although this study has some limitations, it can be concluded that laser texturing of PEEK biomaterial by fractional CO₂ laser significantly enhanced its surface properties in terms of surface topography, roughness, and wettability. The best results were achieved with pulse duration = 400 μs, dot separation distance = 0.2 mm, and a 60° staggered dots pattern. With further in vitro (like cell culture) and in vivo (like animal study) investigations, this method of PEEK modification might have the potential to be used in the implant field.