On the fabrication of bioactive glass implants for bone regeneration by laser assisted rapid prototyping based on laser cladding

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Abstract

The processing of bioceramic materials is a topic of great interest for bone regeneration; bioceramic implants are specifically appropriate for low-load applications, such as cranioplasty. In the present study, we investigated the capabilities of rapid prototyping based on laser cladding to generate three-dimensional bioactive glass implants without moulds or preplaced powder bed. 45S5 bioactive glass and lower crystallization tendency S520 bioactive glass particles were successfully injected and melted to obtain glass-derived implants with similar mechanical properties to the precursor materials. The role of processing parameters in the process outcome was analysed: optimization of the assist gas volumetric flow, the precursor glass mass flow, the substrate preheating, and the optical power of the CO2 infrared laser beam, allowed to adjust the material cooling rates to preclude extensive crystallization or cracking. The assessment of calcium hydroxyapatite precipitation ability and ion release in simulated body fluid conclude the potential osteoconductivity of the produced implants.

Introduction

The regenerative treatment for bone defects due to trauma or intended surgery often requires the insertion of a resorbable implant, with a cosmetic warped geometry, capable to guide and support new bone ingrowth. Due to limited autograft availability and its associated complications, synthetic bone reconstruction implants are employed to treat critical-size defects and to shorten rehabilitation times. Among the available synthetic materials applied for bone regeneration, bioactive glasses are especially interesting because of their advantaged osteoconductive and osteoinductive properties. Silicate based bioactive glasses are reported to up-regulate several bone growth factors through the release of ionic species during implant resorption, thus increasing osteoblast activity and bone growth [1], [2]. These materials are already employed in low load-bearing craniofacial prostheses [3], [4], [5], [6], [7], [8], [9], [10], [11].

Due to the particular geometrical characteristics of the patient bone defect, customized manufacturing of the implant is required, which is difficult to solve by conventional glass casting methods. In the customized implant fabrication procedure, imaging techniques, such as computed tomography or magnetic resonance imaging, are used to obtain three-dimensional data on the bone defect geometry [12], [13], [14], [15]. Such information is then employed to design a computer model, which can be used to fabricate an implant with the appropriate geometry through a rapid prototyping technique. A number of techniques can be applied to produce polymeric templates and moulds subsequently replicated by bioactive glass coating or infiltration [16], [17], but some other manufacturing methods pursuit the direct generation of the bioactive glass part. Glass–ceramic samples were produced by tape casting of slurries composed by 45S5 bioactive glass, ethanol, toluene and polyvinyl butyral, followed by a sintering step at high temperature [18], [19]. 3D-printing was employed to manufacture bone implants using calcium phosphate and bioactive glass blends as precursor powder, and binders composed by orto- and pyrophosphoric acids [20]. Similarly, fused deposition modeling, also referred as ink-jet printing or robocasting, was applied to bioactive glass processing through the use of polyol or carboxymethyl cellulose solvents and a high temperature sintering stage [21], [22], [23], [24]. Freeze extrusion fabrication was employed for free-form 13–93 bioactive glass manufacturing from slurries composed by glass, water and polymeric additives [25]. Regarding the laser assisted techniques, selective laser sintering has been applied to 13–93 bioactive glass by using a polymeric binder and a burn out sintering stage [26]. Nevertheless, the last techniques present weaknesses pendant to overcome, as the use of toxic additives or the requirement of post-processing stages. Moreover, regardless the deposition nature of each rapid manufacturing technique, the high temperature processing stage required for sintering the bioactive glass, promote them to the group of thermal manufacturing processes. Consequently, these thermal rapid prototyping techniques involve sintering of particles by increasing the temperature above the glass transition point, thus allowing the glass to flow. Bioactive glass compositions frequently present a very narrow temperature interval where the viscosity allows sintering without undesired structural changes, also referred as sintering window or glass working range [27], [28]. Hence, glass devitrification and the impact on the manufactured implant bioactivity must be carefully assessed.

In the field of biomaterials processing, the laser cladding technique was applied to produce bioceramic coatings on titanium implants [29], [30], [31], [32], [33]. Moreover the feasibility of this technique to fabricate three-dimensional bioceramic implants for low load-bearing applications has been demonstrated [34], [35], [36]. The present work tackles the assessment of bioactive glass implant fabrication by rapid prototyping based on laser cladding. Here we present, for the first time, the analysis of the influence of the process parameters on important implant properties for its overall performance in the regeneration of low load bearing bone such as implant microstructure, or toughness.

Section snippets

Rapid prototyping based on laser cladding

The technique consists of the injection of bioactive glass microparticles over a substrate that is moving beneath the powder flow and the laser beam, as outlined in Fig. 1a. The laser source employed was a CO2 laser (Rofin SCx10) emitting at the fundamental wavelength of λ=10.6 µm and characterized by a M2 quality factor lower than 1.25. The pulse frequency is adjustable between 0 and 100 kHz with a maximum power of 140 W. The laser beam polarization is converted from linear to circular by means

Deposited layer dimensions

Rapid prototyping based on laser cladding makes use of a predesigned computer model to manufacture the bioactive glass implant; therefore, the analysis of the geometrical dimensions of the deposited layers and their correlation with the processing parameters are important for the feasibility of this laser assisted technique. The layer dimensions are influenced by the convection flow generated within the molten pool during processing. Due to the smooth glass viscosity–temperature dependence and

Conclusions

Due to the low thermal conductivity, high surface tension and viscosity–temperature behavior, the deposited layer dimensions are mainly governed by the laser power and powder mass flow. Very high deposition efficiency can be achieved by rapid prototyping based on laser cladding under viable processing conditions. The obtained material efficiency and the material injection specific feature support the technique feasibility from the point of view of bioactive glass consumption.

The assist gas flow

Acknowledgements

This work was partially funded by the European Union (Project MARMED-2011-1/164), the Spanish Government and FEDER (CICYT MAT2006-10481) and by Xunta de Galicia (CN2012/292, POS-A/2013/161). The aid of Dr. Benito Rodríguez and Dr. Eugenio Solla from CACTI (University of Vigo) is gratefully acknowledged.

We would like to thank Dr. Julian Jones from Imperial College, London for his support and fruitful discussions.

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