Ultrashort pulse lasers have significant advantages over conventional continuous wave and long pulse lasers for the texturing of metallic surfaces, especially for nanoscale surface structure patterning. Furthermore, ultrafast laser beam polarization allows for the precise control of the spatial alignment of nanotextures imprinted on titanium-based implant surfaces. In this article, we report the biological effect of beam polarization on human mesenchymal stem cell differentiation. We created, on polished titanium-6aluminum-4vanadium (Ti-6Al-4V) plates, a laser-induced periodic surface structure (LIPSS) using linear or azimuthal polarization of infrared beams to generate linear or radial LIPSS, respectively. The main difference between the two surfaces was the microstructural anisotropy of the linear LIPSS and the isotropy of the radial LIPSS. At 7 d post seeding, cells on the radial LIPSS surface showed the highest extracellular fibronectin production. At 14 days, qRT-PCR showed on the same surface an increase in osteogenesis-related genes, such as alkaline phosphatase and osterix. At 21 d, mineralization clusters indicative of final osteoinduction were more abundant on the radial LIPSS. Taken together, we identified that creating more isotropic than linear surfaces enhances cell differentiation, resulting in an improved osseointegration. Thus, the fine tuning of ultrashort pulse lasers may be a promising new route for the functionalization of medical implants.

Ultrafast laser ablation is widely used as a versatile method for accurate micro-machining of polymers, glasses and metals for a variety of industrial and biomedical applications. We report on the use of a novel process parameter, the modulation of the laser pulse energy during the multi-scan texturing of surfaces. We show that this new and straightforward control method allows us to attain higher and lower roughness ($R_a$) values than the conventional constant pulse energy irradiation sequence. This new multi-scanning laser ablation strategy was conducted on metals that are commonly used in the biomedical industry, such as stainless steel, titanium, brass and silver samples, using a linear (increasing or decreasing) gradient of pulse energy, i.e., varying the pulse energy across successive laser scans. The effects of ablation were studied in terms of roughness, developed interfacial area ratio, skewness and ablation efficiency of the processed surfaces. Significantly, the investigation has shown a global trend for all samples that the roughness is minimum when a decreasing energy pulse sequence is employed, i.e., the irradiation sequence ends up with the applied laser fluences close to threshold laser fluences and is maximum with increasing energy distribution. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) analysis on single craters with the three different energy deposition conditions revealed a chaotic and random material redistribution in the cases of uniform and increasing energy distributions and the presence of regular laser-induced periodic surface structures (LIPSS) at the bottom of the ablation region in the case of decreasing energy distribution. It is also shown that the ablation efficiency of the ablated surfaces does not significantly change between the three cases. Therefore, this novel energy control strategy permits the control of the roughness of the processed surfaces without losing the ablation efficiency.

We describe the formation of LIPSS by fs laser irradiation on polished titanium or steel samples, from which polymer replicas can be produced. The irradiation of inclined samples allows a variation in the periodicity of the LIPSS in a range between about 500 and 1000 nm, depending on the angle of incidence and the orientation of the laser polarization relative to the plane of incidence, either parallel (p-polarization) or perpendicular (s-polarization). For p-polarization, a second larger-size LIPSS feature with periodicities between about 1300 and 2200 nm is observed at medium angles. LIPSS lines are oriented perpendicular to the light polarization, except for s-polarization on steel samples, where a rotation of up to 35° is observed. In a two-step process, the LIPSS are replicated in polymers. We investigate the attachment of Schwann cells and fibroblasts seeded thereon, which show no direct dependence on the variation in the LIPSS periodicities.

We demonstrate a novel method for fabricating densely packed, high-aspect-ratio cavities (depth to diameter ratio > 1) in silicon wafers coated with diamond-like carbon (DLC). The process combines femtosecond laser irradiation with subsequent wet etch (potassium hydroxide-based) or plasma etch (chlorine- and oxygen-based). The femtosecond laser is used to pattern precise openings in the DLC layer, exposing the underlying silicon, which is then rapidly etched to form well-defined cavities. This combined laser ablation and etching approach enables high-throughput fabrication of densely packed micrometer cavities and offers scalability toward industrial production. The resulting hybrid DLC-coated silicon structure, featuring densely packed cavities, may exhibit antireflective properties in the far-infrared spectrum range.