Elsevier

Optics & Laser Technology

Volume 106, October 2018, Pages 157-167
Optics & Laser Technology

Full length article
HAp-functionalized zirconia surfaces via hybrid laser process for dental applications

https://doi.org/10.1016/j.optlastec.2018.03.017Get rights and content

Highlights

  • Hybrid process showed to be an advantageous strategy to functionalize ZrO2 surfaces.

  • HAp adhesion and retention were assessed testing different surface textures.

  • HAp adhesion/degradation was evaluated for different laser powers and scan speeds.

  • A new design with improved bioactivity is proposed for dental applications.

Abstract

The development of new approaches to improve the implant integration and subsequently its long-term maintenance is an actual challenge. In this way and trying to mimic natural bone composition, HAp-functionalized zirconia surfaces were produced by means of hybrid laser technique combining additive (laser sintering) and subtractive (laser machining) processes. Nd:YAG laser-generated textures were created to improve mechanical interlocking of hydroxyapatite (HAp) powder and consequently enhance its adhesion to zirconia surface. Different laser parameters and also different approaches were tested to optimize the textured line-patterning of zirconia surface. The created microtextures were characterized by Scanning Electron Microscopy (SEM). Furthermore, textured zirconia surfaces were functionalized with HAp by means of CO2 laser. Different power and scan speed laser parameters were tested to promote HAp retention inside of line-patterning. The results showed that it is possible to design the textured surface by changing energy density and atmosphere. Furthermore, high amount of retained and sintered bioactive material was found when high laser power and low scan speed were performed.

Introduction

Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) is a ceramic biomaterial has widely been used in biomedical applications such as femoral head [1], [2] and dental crowns or dental implants [3] due to its mechanical properties, wear resistance, biocompatibility, toothlike color [3] and low bacterial affinity [4]. Zirconia was introduced in these applications due to it higher fracture resilience and higher flexural strength [3]. However, zirconia is an bioinert material which means that the bone does not substantially bond to its surface [3]. Continuous effort research has been developed to overcome this problem and to improve the bioactivity of zirconia surfaces.

The modification of the surface is a common strategy [5] since that its topography and physicochemical properties play an important role on their osteoconductive capacity. The morphology of the implants surface and its roughness are two important aspects that affect osseointegration, which consequently influences implant’s performance. Furthermore, the existence of a pattern on the surface has been proven to guide cell growth and orientation [6], [7]. Sandblasting, etching, polishing, ultraviolet (UV) light treatment, lithography, sol-gel processing, laser treatment [8], [9] and coatings (such as plasma method, chemical and physical vapor deposition (CVD and PVD)) are some of techniques that have been used to improve surface properties of zirconia dental implants [5], [10]. Sandblasting and acid-etching are the most widely applied to produce homogeneous roughening [5], [11]. On the other hand, laser treatment is a promising technique used to create a three-dimensional (3D) features at micro-nano scale. In contrast to sandblasting and etching, it is a versatile technique that can remove material quickly and create complex microstructures (regarding surface texture design) with low waste and without surface contamination [10]. Besides this, high speed operation, high precision, local treatment, small heat affected zone and low cost [12], [13] are other advantages of this technique. In addition, laser technique has gained attention due to its ability to improve surface wettability and consequently enhance cell adhesion [5].

Different types of laser such as Nd:YAG, CO2 and Excimer Lasers had been used to machine ceramics. All can be operated in continuous wave (CW) or pulsed mode (PM). The Nd:YAG laser had been reported as the most common used to machine ceramics due to their high energy density and small focused spot [14]. This solid state laser use dopants (Neodinium (Nd3+)) dispersed in a crystalline matrix (complex crystal of yttrium-Aluminum-Garnet (YAG) with chemical composition Y3Al5O12) to generate light through krypton or xenon flash lamps excitation with a wavelength of 1.06 µm [14]. According to Islam and co-workers [15], pulsed lasers are the most suitable for machining ceramics due to the facility to control their processing parameters when compared to continuous mode.

When laser beam is incident on the ceramic surface, absorption, reflection, refraction, transmission and scattering can take place. The majority of energy is absorbed. This adsorption is dependent on the optical properties of material. In turn, depending on the laser parameters and, consequently, energy density, the material can be removed by different physical phenomena: melting, sublimation, vaporization, dissociation, plasma formation and ablation [14]. However, there is no significant work in literature focused on the influence of different laser parameter on the zirconia surface properties [16].

The surface functionalization employing calcium phosphates bioactive materials is another strategy that have been applied to zirconia substrates aiming enhanced bone healing in biomedical applications [17]. Hydroxyapatite (HAp, (Ca10(PO4)6(OH)2)) is the most popular calcium phosphate used in biomedical applications (namely bone substitution and reconstruction) to improve bioactivity due to its similar chemical composition and crystal structure to natural apatite in human bone tissues and teeth [17], [18], [19] as well as due to its good biocompatibility and high osteoconductive and osteoinductive properties [5], [6]. In this field, different methods have been used by the researchers to produce HAp-based coatings on ZrO2 materials. Dip coating [20], sol-gel [21], plasma spraying [22], [23] and electrophoretic deposition [23] are some of these methods. Although, HAp coatings have been performed, if a poor bond strength between coating layer and substrate is created, during the implant insertion the contact shear tresses can lead to the coating detachment and thus compromising bone healing [17]. Nowadays, the laser sintering is a promising technique to promote the adhesion of the HAp powder to the zirconia substrate that can allows to overcome the coating detachment problem.

Guided by the versatility of laser technique and based on documented results related to the role of HAp on bone regeneration the goal of our research is to develop a textured zirconia surface doped with HAp bioactive material produced by hybrid laser process and propose a new design with improved bioactivity for dental applications, combining mechanical resistance provided by inner material and functional properties provided by an HAp-functionalized zirconia surface (see Fig. 1).

Additionally, three different approaches were investigated: laser machining one step; laser machining two steps and laser machining using different laser atmospheres, aiming to study the influence of laser strategy on created texture, which consequently will affect HAp retention.

Section snippets

Materials

In this study, commercial Yttria-stabilized zirconia (3Y-TZP) powder with uniform dispersion of 3 mol% Yttria (Tosoh Corporation), with particle size of 40 nm (agglomerate size of 60 μm) and hydroxyapatite spherical particles (HAp) (nanoXIM.HAp403, purchased from Fluidinova) with particle size distribution (d50) of 10 ± 2 µm were used. The chemical composition of 3Y-TZP is listed in the Table 1.

Sample preparation

The zirconia specimens were produced by Powder Metallurgy (PM) process, namely Press and Sintering.

Results and discussion

Results will be presented and discussed in three sections. The first section starts with an image analysis study of the Nd:YAG laser-generated line structures for three different approaches. The objective of this section is to evaluate the influence of laser parameters as well as the strategy followed on the surface properties (roughness, depth and number of cavities – as open space to introduce bioactive material) of zirconia substrates to find the texture-line that promotes greater retention

Conclusions

  • This study showed that surface texture plays a significant role on adhesion and mechanical interlocking of HAp.

  • The textured lines produced in approach (ii) using a Nd:YAG laser parameters (AW and FS of 5 µm), where cavities with thickness of 20 µm were performed, showed to be the most adequate, in terms of number of cavities (as open spaces to introduce bioactive material) and roughness to promote HAp mechanical interlocking .

  • Laser sintering of HAp on zirconia textured lines was successfully

Acknowledgements

This work was supported by FCT (Fundação para a Ciência e Tecnologia) through the grants SFRH/BD/112280/2015 and SFRH/BPD/112111/2015, the project PTDC/EMS-TEC/5422/2014 and the project UID/EEA/04436/2013, by FEDER funds through the COMPETE 2020 – Programa Operacional Competitividade e Internacionalização (POCI) with the reference project POCI-01-0145-FEDER-006941 and the project with reference NORTE-01-0145-FEDER-000018-HAMaBICo.

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