Biomaterials Science: Processing, Properties and Applications IV

Susmita Bose, Amit Bandyopadhyay, Roger Narayan, Susmita Bose, Amit Bandyopadhyay, Roger Narayan

• Publisher: Wiley-American Ceramic Society
• Year: 2014
• Print ISBN: 9781118995204
• eBook ISBN: 9781118995259
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

This CT Volume contains 11 contributed papers from the following 2013 Materials Science and Technology (MS&T'13) symposia:

• Next Generation Biomaterials
• Surface Properties of Biomaterials

POROUS TITANIUM IMPLANTS FABRICATED BY A SALT BATH SINTERING PROCESS FOR BONE REPAIR APPLICATIONS

M.N. Rahaman¹, B.S. Bal², T. Huang¹

¹Department of Materials Science and Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USA

²Department of Orthopaedic Surgery, University of Missouri, Columbia, MO 65212, USA

ABSTRACT

A salt bath sintering method was developed for fabricating porous titanium constructs that potentially could be used as implants in bone repair. Advantages of the method include low-cost fabrication, control of the porosity and pore size, and ease of fabricating anatomically relevant shapes. Mixtures of titanium and sodium chloride (salt) particles were compacted to form a cylindrical shape, and sintered for 2 hours in a salt bath at 1200 °C. After dissolution of the salt, porous titanium constructs with controllable porosity in the range 36 to 65% and pore sizes of ~30 μm to ~200 μm were obtained. The compressive strength and elastic modulus of the cylindrical constructs decreased from 216 to 36 MPa and from 9.8 to 1.8 GPa, respectively, with an increase in the porosity from 36 to 65%, and showed an exponential dependence on the porosity. The porous constructs supported the proliferation of murine MLO-A5 cells (an osteogenic cell line), showing their cytocompatibility. Prototypes of a porous Ti insert that could be used to augment the fixation of femoral and tibial stems in total joint arthroplasty were produced to show the applicability of the process.

INTRODUCTION

Titanium (Ti) is widely used as an implant material in dental and orthopedic applications because of its biocompatibility, corrosion resistance, and mechanical durability [1–4]. Of the metallic materials often used as implants in the biomedical industry, commercial-purity, dense Ti has one of the smallest elastic modulus (100–110 GPa) [5], but its modulus is still far larger than that of human cortical bone (10–20 MPa). It is well documented that the use of implants with elastic modulus values far larger than bone can lead to bone resorption resulting from stress shielding [6,7].

Modification of the microstructure and macro-shape is a widely-used approach for controlling the mechanical properties of solids. For a given macro-shape, modification of the pore characteristics such as the porosity, pore size, and distribution of the pore sizes provides a method for tailoring the mechanical properties. Porosity is also important in implant applications. An implant should have the requisite pore characteristics to support tissue ingrowth and integration with host tissues [8–11]. Generally, interconnected pores of size larger than 100 μm have been reported to be beneficial for supporting bone ingrowth [10,12–14].

Because of the importance of porosity in implant applications, the production of porous Ti has been the subject of several investigations in the last few decades [8,15,16]. The methods include conventional powder metallurgy [6,13,17], solid freeform fabrication (e.g., selective electron beam melting and three-dimensional printing) [18,19], sintering of powders [20], tape casting [21], and foam replication techniques [22,23]. More recently, hierarchically-structured Ti foams have been produced by first forming an oxide precursor by a gelcasting method, followed by electrochemical reduction [24].

The production of most active metals by a powder metallurgy route involves a controlled-atmosphere sintering step in which a compacted mass of particles is heated in a vacuum or high purity inert gas atmosphere furnace to bond the particles. However, the use of a vacuum or inert gas atmosphere furnace leads to high fabrication costs. In this work, the sintering step is performed in a molten salt bath which eliminates the need for a vacuum or a high-purity inert atmosphere furnace. A salt bath is widely used in the metals industry for the heat treatment of metals that have been already produced by common fabrication methods; the salt bath serves to protect the metal from oxidization and de-carbonization during the heat treatment.

In the present work, sintering in a salt bath was used for the time for the production of porous Ti implants by a powder metallurgy route. The mechanical properties in compression and the cytocompatibilify of the fabricated Ti constructs were evaluated. To show the applicability of the process developed in this work, prototypes of a porous Ti insert that could be used to augment the fixation of femoral and tibial stems in total joint arthroplasty were produced.

EXPERIMENT PROCEDURE

Materials and processing

The starting materials consisted of commercially-available Ti sponge powder (average particle size <45 μm; Atlantic Equipment Engineers; Bergenfield, NJ, USA) and sodium chloride particles of size 200–500 μm, obtained by sieving biomedical grade material (Fisher Scientific; Pittsburgh, PA, USA) through stainless steel sieves. Different ratios of Ti and salt by volume were used to fabricate final Ti constructs with porosity values in the range ~35–65%.

The Ti and salt particles were first mixed in the dry state by tumbling the mixture in a Nalgene bottle for 12 hours, followed by wet mixing by stirring the mixture for 1 min in acetone to improve the homogeneity of the mixture. The ratio of the solid phase to acetone was 100:1 by weight. The moist mixture was pressed uniaxially in a 6.35 mm diameter stainless steel die under a pressure of 20 MPa. The acetone was allowed to evaporate almost completely, after which the constructs were further compacted by pressing in a cold isostatic press under a pressure of 250 MPa to increase their strength.

The salt bath sintering step was performed in a chamber furnace. An Al₂O₃ crucible was partially filled with salt particles and heated at 10 °C/min to the desired sintering temperature (1200 °C). The size of the Al₂O₃ crucible and the amount of salt particles used were dependent on the dimensions of the constructs to be sintered. The as-formed constructs were placed in the salt bath after the salt had melted (above 800 °C, the melting point of NaCl). The system was held for 2 h at 1200 °C, and then cooled at 10 °C/min to room temperature.

The salt surrounding the sintered constructs and within the pores of the constructs was removed by dissolution in water. The completion of the dissolution process was determined by testing for Cl⁻ ions in the water using a few drops of silver nitrate solution. When there was no observable precipitation of AgCl, the constructs were removed from the water, washed with deionized water, and dried for 24 h at 65 °C.

Microstructural evaluation, chemical analysis, and mechanical testing

The microstructure of the surface and polished sections of the fabricated constructs were examined in a scanning electron microscope (SEM) (S-4700; Hitachi, Tokyo, Japan) using an accelerating voltage of 15 kV and a working distance of 12 mm. The porosity of the open pores in the constructs was measured by the Archimedes method, using the procedure described in ASTM C 830. (Water was used as the liquid medium.) Selected constructs were sent to a commercial laboratory (Porous Materials Inc., Ithaca, NY, USA) for measurement of the pore size distribution using mercury porosimetry. The oxygen content of the starting Ti powder and the fabricated constructs was measured using the inert gas fusion principle at a commercial laboratory (LECO Corp., St. Joseph, MI, USA).

Cylindrical Ti constructs (5 mm in diameter × 12 mm), fabricated as described above, were ground on a diamond wheel to make their ends nearly parallel, and tested in compression at a deformation rate of 0.2 mm/min using an Instron testing machine (Model 4205; Instron; Norwood, MA). The yield strength in compression was determined from the load at an offset strain of 0.2% and the cross sectional area of the construct; the elastic modulus was determined from the slope of the linear portion of the stress vs. strain curve. At least 5 samples were tested under the same conditions for each group.

Cell culture

The ability of the porous Ti constructs to support cell proliferation was evaluated using osteogenic MLO-A5 cells, an established murine cell line, kindly provided by Professor Lynda F. Bonewald, University of Missouri-Kansas City. The procedure is described in detail elsewhere [25], Briefly, disk-shaped constructs (porosity = 65%; 10 mm in diameter × 2 mm; n = 6 for each group) were sterilized in an autoclave, and seeded with 60,000 MLO-A5 cells suspended in 100 μl of complete medium. After incubation for 4 h to permit cell attachment, the cell-seeded scaffolds were transferred to a 24-well plate filled with 2 ml of complete medium per well. All cell cultures were maintained at 37°C in a humidified atmosphere of 5% CO₂, with the medium changed every 2 days.

After incubation for 2, 4, and 6 days, the scaffolds were removed, gently washed twice with warm phosphate-buffered saline (PBS), and placed in 2.5% glutaraldehyde in PBS to fix the cells. The samples were soaked overnight in glutaraldehyde, washed with PBS, dehydrated thoroughly using a graded series of ethyl alcohol, followed by two soaks in hexamethyldisilazane (HMDS) for 10 min each, and allowed to fully dry. The surfaces of the samples were sputter-coated with Au/Pd, and examined in the SEM (Hitachi S-4700) at an accelerating voltage of 5 kV and a working distance of 12 mm.

To visualize the metabolically active cells on and within the scaffolds, the scaffolds were placed in 0.2 ml serum-free medium containing 0.1 mg of the tetrazolium salt MTT for the last 4 h of incubation. After incubation, the scaffolds were briefly rinsed in PBS, blotted, and allowed to dry. Images of the scaffolds were obtained using a stereomicroscope fitted with a digital camera to qualitatively assess the distribution of insoluble purple formazan, a product of mitochondrial reduction of MTT by viable cells.

Fabrication of prototype titanium inserts for total joint arthroplasty

The fabrication of prototype Ti inserts was undertaken to show the feasibility of the method for producing practical implants with a relevant size and geometry for applications in hip and knee joint replacement. The prototypes consisted of a truncated hollow cone (upper external diameter = 50 mm; lower diameter = 35 mm; wall thickness = 5 mm) which could be used as tibial or femoral inserts. The fabrication procedure was similar to that described previously for producing the porous cylindrical Ti constructs, except that a different stainless steel die with the requisite geometry was used to provide the desired shape.

RESULTS

Microstructure of fabricated Ti constructs

Figure 1a shows examples of porous Ti cylinders (average diameter = 5.8 mm; height = 13 mm) which were fabricated in this work. The constructs shown had a porosity of 65%, but...

Table of contents

• Cover
• Half Title page
• Title page
• Copyright page
• Preface
• Bioactive Glass-Ceramic Scaffolds with High-Strength for Orthopedic Applications
• Metallurgical Characterization of Laser-Sintered Cobalt-Chromium Dental Alloy
• Mechanical Properties, Microstructures, and Biocompatibility of Low-Cost β-Type Ti-Mn Alloys for Biomedical Applications
• Microstructural Characteristic of Nano Calcium Phosphates Doped with Fluoride and Titanium Ions
• Development of Implants Composed of Hollow Hydroxyapatite Microspheres for Bone Regeneration
• Porous Titanium Implants Fabricated by a Salt Bath Sintering Process for Bone Repair Applications
• Navigating the Uncharted Waters of the New AIA U.S. Patent Law
• Comparative Analysis of Hydroxyapatite and Titanium-Based Bioscaffolds Fabricated Via Adaptive foam Reticulation
• Laser Processing of Tricalicum Phosphate Reinforced Cobalt – Chrome Alloy Coatings
• Effect of Degree of Deacetylation of Chitosan on Macrophage Function
• PECVD SiOx Accelerates Hydroxyapatite Surface Formation for Enhanced Early Osteogenic Differentiation
• Author Index