The practical application of a technology that differentiates a product from its competition isn’t always the most novel approach. In today’s world, OEMs must consider using technologies to make products that prove their relevance based on both increased regulatory scrutiny and a cost-benefit ratio. Plasma spray and additive manufacturing technologies, which are used to manufacture macroporous surfaces for orthopaedic implants, can accomplish this mission.

Part 1: Plasma Spray Coatings

Figure 1. The plasma spray process must be controlled to avoid excessive heat transfer to a surface.

The use of plasma spray coatings in the manufacture of medical devices began in the early 1980s with Biomet’s introduction of the titanium porous plasma spray coating. Another breakthrough came later that decade with the introduction of hydroxyapatite (HA) coatings for dental implants. HA is a naturally occurring mineral form of calcium apatite with the formula Ca₁₀ (PO4)₆ (OH)₂. The material is synthetically derived into a powder form and applied using an air plasma system (see Figure 1). Unlike many technologies in use during the 1980s, plasma spray was a special process technology that was not easily verifiable without destructive testing. However, within the last five years, the technology has proven itself reliable, safe, and effective, with new engineered process controls and automated equipment that is all validated as a complete system.

When applied to a substrate, the plasma spray process depends on thermal conditions (the temperature must be at the melting point) and kinetic energy. The material melted in the plasma is simultanously accelerated by high-velocity plasma-gas stream. The molten or semimolten droplets attach to the roughened substrate and splats of particles. Inert gases (argon-nitrogen) flow over a cylindrical copper anode and a tungsten cathode. A dc arc is maintained, which creates gas plasma with a core temperature that can reach up to 30,000°C. Powder metal or ceramics are injected into the plasma stream internally or externally and are then melted and accelerated at the substrate. The entire process is carefully automated and controlled to prevent the application or transfer of excessive energy—specifically heat—to the substrate.

The air plasma spray is used to process an HA coating but is not typically used for processing titanium metal powder. Controlled atmospheric plasma spray (CAPS) or vacuum plasma spray (VPS) is used to process metallic powders that exhibit oxygen-sensitive properties. In the orthopaedics market, plasma-sprayed titanium coatings are commonly used to make titanium alloy hip implants. Titanium coatings are being applied to knee, shoulder, elbow, and ankle implants, along with other products made from titanium and cobalt-chromium-
molybdenum alloys. Titanium is extremely sensitive to oxygen pick-up in high-temperature environments. CAPS and VPS methods process titanium powder using plasma spray technology but perform the same process in a different manner with different equipment. The CAPS unit processes the coating and the implant in a positive-pressure inert atmosphere, while the VPS unit processes at a controlled low-vacuum pressure. Both processes can successfully produce porous coatings that are beneficial for bone up-growth and in-growth.

A porous coating is defined as one that exhibits bone in-growth into the coating. Bone will not grow into pores that are less than 100 µm.^1,2 Today, plasma sprayed titanium coatings exhibit the minimum pore size characteristics needed to be defined as a porous coating. The porosity profile is not an ordered structure like porous beads but instead is a completely random structure that is rough and porous. The thickness of the coating determines the predictable amount of pore size, volume, and overall porosity that can be achieved.

Figure 2. This chart shows the resulting data from the tight process control of a porous titanium coating onto PEEK implants. Click chart for larger image.

Tight Process Control

What are the distinct advantages to plasma-sprayed titanium porous coatings, and how do we understand how to best use the technology? The answer is multifaceted and depends on the device market.

Advances in process control have enabled the application of thick coatings (up to 1000 µm), which allows for high porosity and interconnecting porosity that is suitable for joint replacement components. Ti-Growth is a VPS coating that eliminates the pore-size advantage of sintered beads. However, the VPS coating process introduces considerable heat. This process normally requires the use of metallic hard masking to protect parts and can raise the  overall cost of production.

The CAPS coating process maintains a low-temperature coating substrate interface that avoids metallurgical bonding or elemental migration. It offers mechanical performance benefits and allows for parts to be moved in and out of the chamber. The ease of manufacturing allows for greater overall output, which in turn gives a service manufacturer more flexibility on delivery times and overall cost.

Figure 2 shows the results that can be obtained with tight process control over the application of a porous titanium coating onto PEEK implants. PEEK is used in fusion devices that are designed to be inert, high strength, and radiolucent. The spinal device market will continue to use PEEK in a range of applications, though the material does have one limitation. Although noted for its bioinert properties, PEEK has little direct bone attachment. This means that bone treats the polymer as inert but does not want to attach to it mechanically or chemically. The search for a process that improves the uses of this material has led OEMs to work with a variety of surface-modification technologies. The basic concept is simple. Apply a bioactive coating to a bioinert material to enhance bone up-growth or osseointegration. This application fits into many of the uses for PEEK, especially when applied to fusion-type devices.

An HA coating is applied via the plasma spray process onto a femoral hip stem.

However, manufactures face challenges when applying thermally sprayed materials to PEEK. The physical and thermal properties of polymeric materials are very different from those of metallic or ceramic materials. The chemical structure of PEEK exhibits outstanding chemical and radiation resistance. It can also withstand structural degradation at high temperatures. Nevertheless, the material must be kept at a temperature that does not exceed the glass transition temperature of 143°C. To address the challenges related to these characteristics, the titanium coating can be applied using specially developed air or vacuum plasma spray processes that provide low-temperature stability during processing. CAPS has also shown superior capability in enabling a porous titanium coating on PEEK without chemical or mechanical deterioration during the plasma-spray process.

The plasma spray process has evolved from a misunderstood technology method to a mainstream technology for manufacturing porous coatings. These coatings can be applied to almost any material and geometry. Thus, the plasma spray process is used extensively in the device market. In addition, rapid or additive manufacturing (AM) technologies, although not new to the world, are showing that they can compete with traditional coating technologies in manufacturing porous titanium surfaces and offer the added benefit of a complete near-net part construct. Improved software and process controls enable these technologies to produce devices with porous structures that compete directly with those produced by alternative machining and porous coating methods of manufacturing. By eliminating several manufacturing steps and inventory costs, AM has created a loyal following among suppliers.

Part 2: Additive Manufacturing

For at least two decades, there has been technology that can build up solid parts from 3-D computer-aided design models. Starting materials are in liquid or powder form and are subjected to consolidation upon local energy supply, stimulating a specific chemical or physical reaction. Technologies that produce components according to this strategy are categorized under AM.^3,4

An acetabular cup is serially manufactured for clinical use in Europe.

The second part of this article focuses on two specific AM technologies
—electron beam melting (EBM) and direct metal laser manufacturing (DMLS). These technologies can produce parts made of implantable metals such as titanium alloy (Ti6Al4V).

The common feature of these AM technologies is that they melt and rapidly solidify portions of a metal powder layer according to the specific drawing being replicated. Under specific process conditions, solidification results in a fully dense solid body.

In an EBM machine, an electron beam gun preheats the powder layer using a relatively low beam current and a relatively high scan speed. This process has two effects: first, the partial sintering of the powder, which holds it in place during the subsequent melting; and second, the high temperature (about 600°C) maintained during the process reduces the thermal gradient between the just-melted layer and the already built-up body of the part. This approach substantially reduces residual stresses. A typical layer thickness is 70–100 µm.⁵

A DMLS machine for titanium alloys uses a laser beam with a higher focus capacity than EBM. Each layer can be as thin as 30 µm. The process leads to improved resolution and accuracy in the pieces manufactured compared to those made via EBM. However, tolerances against nominal design are about 0.1 mm and are a long way from typical values obtained by machining. Productivity for the DMLS process as defined (melted material per hour) is lower when compared to the high-energy EBM process.⁶

Designing and Manufacturing Macroporous Surfaces

When a contract manufacturer uses both EBM and DMLS technologies in serial production, it can establish a process surveillance strategy. One critical concern is that titanium is chemically prone to oxidation. If the oxide content in the manufactured parts exceeds standard requirements (i.e., < 0.2% per ISO 5832-3), it may decrease material ductility. The production cycle of Ti-6Al-4V alloy-based parts presents challenges. Part of the powder loaded into the equipment that is not melted to form the solid parts will eventually be reused in the next manufacturing cycle. The powder feeder tanks are also routinely refilled with new powder so that a blend between reused and new powder is permanently in place. Numerous analyses were performed to obtain a statistical picture of the oxygen and carbon rate variation against powder reuse, both for the powder in the tanks and for the final parts. Results revealed a high capability to maintain the material within specification when the processes are performed correctly.⁷

If porous structures are manufactured on the pieces, a critical postprocess step such as titanium debris cleaning may be required. The porous structures, as manufactured, contain a number of loosely adhered surface beads and it is necessary to remove these particles. The most practiced particle-removal methods are thermal treatments, sandblasting, and chemical etching, or a combination of these methods.⁸

Once components with their porous coating surface are additive manufactured, they may be subjected to further critical postprocess steps such as thermal treatments, hipping, or machining, when high dimensional accuracy is required for coupling components. When additional mechanical work is applied, it is crucial to avoid or remove contaminants such as emulsions and lubricant residuals from porous structures.

Both technologies release the Ti-6Al-4V alloy with a specific acicular microstructure that cannot be modified by heat treatment and can be considered a building process fingerprint. It does not find a correspondence in the international standard dealing with titanium alloy microstructures for biomedical applications (ISO 20160). Nevertheless, a number of studies have demonstrated how the mechanical properties of the titanium alloy obtained by AM may fully satisfy the applicable standards and fatigue resistance requirements.^5,6

When using AM technologies, a number of different topographic structures can be achieved with excellent performances. The ultimate goal of porous titanium structures is to host living bone tissue in a manner that guarantees device fixation by osseointegration. Such data has been proven successful through in vitro and animal testing. Properly engineered and EBM-manufactured surface structures allow human mesenchemal stem cells to attach to the structures and spread.⁹ Studies also showed that Ti alloy macroporous structures that were EBM-manufactured and surgically placed in immediate stable conditions promoted a bone in-growth of about 1.5 mm after six weeks of implantation time in goats.¹⁰ These results show favorable conditions to enable device fixation. 

Applying Additive Manufacturing

AM has a host of possibilities. The main advantage is design freedom. The method enables complex surface components to be manufactured in a cost-effective and timely manner. It enables the delivery of intricate geometrical components completed with designed lattice surface topography in one manufacturing step. Using the process with titanium produces orthopaedic components with a surface trabecular (porous) structure that promotes bone colonization and in growth, which improves device fixation strength. The method can also be used in the rapid manufacture of complex-shaped custom implants and thousands of acetabular components for hip arthroplasty. Implant makers are examining ways to meet inventory volume needs while adhering to regulatory requirements. AM technologies are proving to be the way of the future.

References

Leo Glass is president of Surface Dynamics (Cincinnati). Pierfrancesco Robotti is responsible for scientific marketing in Eurocoating s.p.a (Ciré-Pergine, Italy).

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