By Antonio Riveiro, Ramón Soto, Rafael Comesaña, Mohamed Boutinguiza, Jesús del Val, Félix Quintero, Fernando Lusquiños, Juan Pou
One of the main requirements of a bone implant is to be able to withstand severe mechanical conditions during the required lifetime of the component. Nevertheless, the success of a bone implant relies upon the quality of the bone-implant reaction, which is markedly influenced by the surface topography and chemistry of the implant material. Biomaterials commonly used in implants only satisfy part of these requirements (see Fig. 1); for example, biomaterials such as Titanium or PEEK (poly aryl-ether-ether-ketone), have a high strength, good wear resistance, and excellent chemical resistance but their inferior bioactivity may lead to poor bone-implant interactions. Therefore, an intervening fibrous tissue layer occurs around the contact area between the bone and the implant.
Figure 1: Desirable properties of representative biomaterials used in implants.
In order to reconcile surface and bulk properties, one technologically and economically viable option is the modification of the surface properties of these materials to enhance their performance when implanted in a living body. Among the surface modification techniques used to engineer biomaterials, laser-based treatments are excellent candidates due to their ability to keep the bulk properties of the material unaltered, their high resolution, high operating speed, and low operation cost. Laser treatments available for modifying the surface of a potential biomaterial range from the coating with bioactive materials (such as calcium phosphate or bioglass) to the production of highly controlled macro and microstructures (by means of the so called laser texturing technique). Utilization of lasers in the last manner, for varying the surface chemistry and topography of biomaterials, allow for flexibility, accuracy, clean and fast processing.
Laser texturing of two common materials used in implants, such as titanium and PEEK has been investigated. The possibility to tune the processing parameters in order to tailor the physical structure of both materials for biomedical applications was studied. Different laser wavelengths (λ = 10600, 1064, 532, and 355 nm) have been used in this study and the impact on surface properties, relevant to promote a good quality of the bone-implant reaction, were studied by means of a statistically planned experimentation.
Results indicates that the surface wettability of PEEK, which can be beneficial to promote cell adhesion “in vivo”, is promoted by performing the laser treatment with UV (355 nm) laser radiation as seen in Fig. 2; however, this treatment does not substantially increases the surface roughness. Laser treatments performed by means of 1064 and 532 nm laser radiation are more indicated to increment the roughness, but negatively affects the wettability of the material; indeed, 532 nm laser radiation tends to produce highly hydrophobic surfaces as depicted in Fig. 2.
Figure 2: Optical images of water drops on laser treated PEEK surfaces with 1064, 532, and 355 nm laser radiation.
In the case of titanium, the application of successive laser pulses can create macrostructures with the required surface topography for biomedical applications, as depicted in Fig. 3. Results reveal that cavities produced with Nd:YAG lasers have a larger aspect ratio compared to the CO2 laser treatment for the same processing conditions.; however, Nd:YAG laser treatment was revealed to be more efficient for this application because dross-free surfaces can be obtained.
Figure 3: Surface engineered titanium surface by means of pattern created by the application of successive Nd:YAG laser pulses (pulse energy Ep=1.3-7 J, pulse length t=1-5 ms, pulse separation 0.5 mm).