Technology employed in the manufacture of Fusion Implant products

Introduction

Additive Manufacturing (AM) is a term used to describe the production of tangible products made using a growing set of digitally controlled machine tools. Often referred to as 3D printing, the approach differs radically from more traditional manufacturing methods, in that products are produced through the selective addition of materials layer-upon-layer, rather than through machining from solid, moulding or casting.

A ‘tool-less’ and digital approach to manufacturing, AM presents companies and consumers with a wide and expanding range of technical, economic and social benefits. AM technologies have the potential to change the paradigm for manufacturing, away from mass production in large factories with dedicated tooling, with high costs, to a world of mass customisation and distributed manufacture. AM can be used anywhere in the product life cycle from pre-production prototypes to full scale production, as well as for tooling applications or post production repair. AM processes are stimulating innovation in component design, enabling the manufacture of parts that cannot be made by traditional methods and are stimulating alternative business models and supply chain approaches.
The layer-wise nature of AM enables the manufacture of highly complex shapes with very few geometric limitations compared to traditional manufacturing processes. This freedom-of-design has led to the technology being used to manufacture topologically optimised shapes with improved strength to weight ratios for example, an important consideration in both aerospace and automotive design to reduce vehicle weight and fuel consumption.

As a digital technology, AM is progressively being integrated with the internet, enabling consumers to engage directly in the design process, and allowing true consumer personalisation. The recent introduction of home based 3D printing has now enabled consumers to also engage in the manufacture of products, using digital data bought or shared online, circumventing much of the traditional manufacturing and retail value chain.

Specific AM process

Of the many types of AM processes available, Fusion Implants uses “Selective Laser Melting” (SLM) to convert titanium-type powders directly into product, employing commercially available equipment and propriety approach. The principle of this technology is illustrated in Figure 1:

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Figure 1: Schematic demonstrating principle of the selective laser melting process.

The following sequences outline the process that is involved in the manufacture of a part by SLM irrespective of the machine used.

  • A high power laser beam is focused through a series of optics and directed onto mirrors mounted on high speed scanning galvanometers, which enables the laser beam to be scanned over the powder bed, with focus being maintained through the F-theta lens.
  • The powder layer is distributed evenly over a flat substrate plate which is securely fastened to the build piston.
  • The laser and build facilities are enclosed in a chamber, and the oxygen level in the build chamber is reduced by purging with argon until the level falls below 0.2%.
  • A uniform layer of powder is deposited on the substrate by the recoating mechanism
  • The laser is then scanned over the powder to melt and fuse the power together to form parts
  • The piston moves down by the chosen layer thickness. This process of powder deposition and laser exposure is repeated until the component parts are completed.
  • Unfused powder is then removed from around the parts on the build plate and the assembly is removed from the build chamber.
  • The parts are then removed from the build plate and any post-manufacturing processes can be completed

The powder to be melted is defined by the 3D model of the desired part. This is sliced into layers of
thickness corresponding to the powder bed layer thickness, to produce slice data. The area of the slice that is designated to be solid or porous is then exposed to the laser beam for a pre-determined time in a pattern defined by an appropriate scanning strategy.

Part properties

Biology

The components produced by Fusion Implants may be all solid, all porous or a combination of porous and solid structures. The material of choice is cp-titanium, a metal with superior properties in terms of strength and biocompatibility. When porous and solid elements form the implant, both structures are perfectly integral with each other, unlike other porous coated structures where there is a defined interface between the two phase types. In our case, the porous structures have been modelled around the architecture of trabecular bone in terms of pore size, pore size distribution and permeability attributes. This provides ideal conditions for the porous structure to encourage early bone cell adhesion and consequential differentiation into bone. The ensuing matrix is well vascularised and with the open structure supports the long-term osteogenic process, and maintains the security of the device in the host bone. The following series of images captures the sequence of this process.

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Figure 2: Images showing the structure of the porous region of the implant, where LHS is a rendering of a µ-CT scan of the structure, the central image is a photo of the actual porous part, and RHS is an SEM capture of a section of the part. In all cases the scale bar represents 1000µm.

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Figure 3: Assessment of bone ingrowth into a porous implant fitted into a bone defect in a rat leg as visualised by µ-CT imaging. Images provided by Prof P. Lee and Mr. H. Geng, University of Manchester.

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Figure 4: Bone quantification data as a function of implantation time derived from µ-CT data

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Figure 5: SEM images showing bone ingrowth data for SLM implants (A, B, C – after 2, 4 & 8 weeks) compared with a sintered bead implant (D, E and F – after 2, 4 & 8 weeks) at the same time intervals. Data extracted from a rabbit model with implants positioned in the tibial portion of the knee.

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Figure 6: Actual bone ingrowth (grey), soft tissue (blue) for 8 week samples were the level of ingrowth corresponds to 54 and 35% for SLM and beaded surface respectively.

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Figure 7: Examples of vascularity. Vascular casted tibia of male Wister rat, with PU4ii resin used to reveal blood vessels.  Images at two magnifications show periosteal vessels surrounding the struts of the implant. Feeder vessels going into the bone are clearly visible in the LHS image. Images provided by Prof. C Mitchell and Miss. A Devlin, University of Ulster.