# Orthopedics-Related Applications of Ultrafast Laser and Its Recent Advances
## 4 Surface Modifications of Bone Implants  
Another vastly studied area is surface modification of bone implants by ultrafast laser for improved osteogenesis and osteointegration. The major clinical issue present today, despite well-established implant techniques, is implant loosening owing to inadequate integration, fibrogenesis and infection [73]. Surface modification thus plays an important role to enhance cell-material interactions, and various properties can be manipulated via ultrafast laser irradiation such as surface topography, roughness and wettability [30; 74]. Figure 2: Morphology of laser-drilled bone tissues under different environmental cooling conditions. Vertical panels (**a**-**c**) correspond to without cooling, gas cooling and water cooling; horizontal panels (subscript: none, 1, 2) correspond to white light, SEM and histology images. Adapted from [42]. Different nanostructures of identical materials can initiate different cellular responses and behaviors including cellular adhesion, motility, orientation and signaling pathways [75]. These factors, in short, greatly affect the success of bone implant performance, which is essentially dependent on the outcome from the complex interaction between surface characteristics and biological response. The detailed description of bone-implant interface can be found in review articles, for example, Refs [76; 77]. In general, the goal of surface modification is to enhance the beneficial effects exerted on osteogenic differentiation and osteointegration of the implant [73; 78]. Specifically, increasing hydrophilicity and roughness supports improved cell adhesion on an enlarged surface area as well as strong biomechanical anchorage of the implant [79; 80]. Some commonly used techniques for surface roughening include grinding, polishing, sputtering and laser texturing [73]. Similar to orthopedic bone ablation, the use of ultrafast pulsed lasers (pico- or femtosecond) for surface modifications of bone implants has become attractive because of the relatively faster processing time, simpler technical requirements and minimal damage to adjacent surfaces [74; 81; 82]. In terms of the materials for biomedical implants, a particular attention has been paid to titanium and its alloys owing to their great biocompatibility, high resistance to temperature and corrosion as well as exceptional strength-to-weight ratio [83; 84]; however, due to bio-inertness of titanium alloys, bioactivity of the material can sometimes be difficult to achieve [85]. Therefore, in this section, we briefly review recent studies on ultrafast laser surface texturing of titanium-based implants for orthopedic applications. In recent years, femtosecond lasers have been explored extensively for surface modifications of bone implants. For example, [PERSON] et al., investigated the ability of femtosecond lasers texturing to improve bioactivity of three types of titanium surfaces by assessing apatite deposition, a key indicator of enhanced biocompatibility and osteointegration, and found that femtosecond laser texturing could promote apatite precipitation and therefore better clinical outcomes [85]. [PERSON] et al. examined, opposite to increasing cell adhesion, micro- and nanostructures fabricated by femtosecond laser processing to reduce cell adhesion [86]. The authors demonstrated that post-processing by electrochemical anodization following femtosecond laser treatment was able to significantly reduce cell adhesion to titanium-based flat plates and screws while promoting extracellular matrix production for osteoblasts. Figure 3 shows the SEM images of differences in osteoblast attachment to the screw and the growth on the surface between femtosecond laser processing with and without anodization. The finding suggested that the technique with its reversal could be applied to bone fixture implants that needed to be removed from the body after a period of time, i.e., less integration with the bone matrix preferred, as well as the ones that remained in the body permanently, i.e., promoted matrix growth [86].