Influence of Femtosecond Laser Modification on Biomechanical and Biofunctional Behavior of Porous Titanium Substrates

Abstract

Bone resorption and inadequate osseointegration are considered the main problems of titanium implants. In this investigation, the texture and surface roughness of porous titanium samples obtained by the space holder technique were modified with a femtosecond Yb-doped fiber laser. Different percentages of porosity (30, 40, 50, and 60 vol.%) and particle range size (100-200 and 355-500 μm) were compared with fully-dense samples obtained by conventional powder metallurgy. After femtosecond laser treatment the formation of a rough surface with micro-columns and micro-holes occurred for all the studied substrates. The surface was covered by ripples over the micro-metric structures. This work evaluates both the influence of the macro-pores inherent to the spacer particles, as well as the micro-columns and the texture generated with the laser, on the wettability of the surface, the cell behavior (adhesion and proliferation of osteoblasts), micro-hardness (instrumented micro-indentation test, P-h curves) and scratch resistance. The titanium sample with 30 vol.% and a pore range size of 100-200 μm was the best candidate for the replacement of small damaged cortical bone tissues, based on its better biomechanical (stiffness and yield strength) and biofunctional balance (bone in-growth and in vitro osseointegration).

Keywords: cellular behavior; femtosecond laser; instrumented micro-indentation; porous titanium; scratch test; surface modification; wettability.

Figure 1

Scheme of the procedure followed by sample fabrication, modification, and characterization. Abbreviations: PM (Pulvimetallurgy), SH (Space Holder), IA (Image Analysis), SEM (Scanning Electron Microscopy), CLM (Confocal Laser Microscopy), ALP (Alkaline Phosphatase) and P-h (micro-indentation curve Power or Force applied vs. penetration deep).

Figure 2

(a) SEM (Scanning Electron Microscopy) and (b) 3D-CLM (Confocal Laser Microscopy) images of the fully-dense c.p. Ti substrates after femtosecond laser treatment.

Figure 3

SEM images after the femtosecond laser treatment of the different porous c.p. Ti substrates. Images acquired using the topography view configuration gather both material and topographic contrast with the unique segmented in-lens backscattered electron detector (BSE). Common scale bar for all subfigures.

Figure 4

CLM images after the femtosecond laser treatment of the different porous c.p. Ti substrates. Common scale bar for all subfigures.

Figure 5

Sa and S_q values measured from the CLM images for all the studied c.p. Ti substrates (fully-dense and 100–200 µm for porous substrates).

Figure 6

Wetting analysis of the porous c.p. Ti substrates: (a) Water Contact Angle (WCA) values of surfaces with different porosity percentages and pores range size before and after the FSL (Femto Second Laser) treatment; (b) bovine serum contact angle (BSCA) values of porous surfaces before and after the FSL treatment; (c) images representing the wetting of water and bovine serum droplets deposited on c.p. Ti porous surfaces (355–500 µm pores range size) before and after FSL treatment.

Figure 7

Static loading–unloading test (P–h curves) parameters of the different FSL modified substrates studied: (a) maximum penetration depth; (b) absolute elastic recovery; (c) micro-hardness (HV); and (d) Young’s modulus (E). Note: c and d are calculated following the Oliver and Pharr method.

Figure 8

Scratch tests parameters on FSL-modified titanium substrates: (a) permanent penetration depth; and (b) absolute elastic recovery (difference between scratch in situ tests and permanent penetration depth).

Figure 9

(a,b) SEM and (c,d) CLM images of scar (due to the scratch test) on fully-dense and porous c.p. Ti substrates. Inset SEM: zoomed details of area marked in subfigures.

Figure 10

Effects of the percentage of porosity and pores range size on cell viability in MC3T3–E1 cells. Results are represented as % cell viability.

Figure 11

Cell differentiation of osteoblasts, in vitro evaluation of alkaline phosphatase enzyme (ALP) activity measured as U/mL. Statistical differences are indicated at * p < 0.05.

Figure 12

SEM images of the cell adherence of the fully-dense and porous substrates after femtosecond laser treatment. Cell–cell interactions and filopodium (yellow arrow) are indicated in the images.