RMIT Centre for Additive Manufacturing – Printing biomedical implants

AM refers to methods that generate three-dimensional structures layer by layer. Each AM technology is compatible with the specific form of the raw input material, which may be in liquid, powder and sheet form. AM processes are not subject to the constraints associated with traditional manufacturing methods and provide significant opportunities for the design of novel geometries and complex structures, such as cellular structures in particular.

Cellular structures possess a number of properties compared to solid structures. For example, the design freedom offered by AM may be used to enhance the strength-to-weight ratio of structural components by transforming solid geometry into a cellular structure or space-filling hollow sections of the model with a cellular structure. The cellular structure within the model may be useful in distributing the loads evenly compared to a hollow model while conforming to the geometric boundaries of the object.

This article discusses some of the basic concepts behind the metallic lattice structures manufactured using selective laser melting (SLM) technology and their application to biomedical implants in particular at the RMIT Centre for Additive Manufacturing.

Cellular structures

Cellular structures are categorised as aperiodic (or stochastic) and periodic. Aperiodic cellular structures are characterised by random orientations of structural elements and can be found in nature in abundance, observedin bones, sponges, bird beaks and animal horns, for example. Such cellular structures are often surrounded by a thick plate structure, such as cortex bone, and hence are referred to as sandwich structures.

Periodic structures, on the other hand, consist of a plurality of a single unit cell arranged in a specific pattern such as honeycomb structures and engineered lattice structures. A lattice structure, from a structural engineering context, is an array of struts that may be pin-jointed or rigidly bonded at their intersection and have been shown to have up to 300% greater strength compared with aperiodic cellular structures under similar circumstances.

The generation of periodic lattice structures is computationally efficient as it involves the repetition of predefined unit cell configurations. Furthermore, local and global properties of the lattice structure can be tailored by using applicable unit cells from a large unit cell library. This enables greater user-control over lattice configurations and density, making periodic lattice structures ideal for functionally graded materials. Additionally, the structural optimisation of periodic lattice structures is computationally efficient compared with aperiodic structures.

Structural performance of lattice structures

The failure of a lattice structure is a complex phenomenon that can be attributed to several associated failure modes and loading conditions, including yield and buckling failure modes. Structurally, lattice structures can be broadly classified into bending-dominated and stretch-dominated structures. In both cases, the lattice structure undergoes plastic consolidation initially, after which the load starts to increase rapidly.

In stretch-dominated structures at peak load, the struts start to yield plastically and failure occurs either by buckling or fracture, resulting in a steep drop in load carrying ability. In bending-dominated structures, however, the collapse of the structure occurs at nearly constant stress, making bending dominated structures more suitable for energy absorbing applications and for the emulation of trabecular bone whereas stretch-dominated are desirable for high-load carrying applications. Lattice densification occurs after the structure is fully crushed and the structure behaves as a solid material.

Design and manufacture of lattice structures

Generating lattice structures for regular geometries, including orthogonal, prismatic, or symmetric objects is relatively easy due to the object’s predictable geometry. However, irregular, free-form geometries such as orthopaedic implants are challenging to tessellate with periodic lattices, typically requiring the designer to trade-off structural integrity with associated lattice geometric conformity.

Additionally, AM technologies make use of support structures to increase the scope of manufacturable geometries. In SLM, support structures are used as structural supports, as well as providing a thermal path for conducting heat of fusion from the melt pool, thereby increasing cooling of the part and managing residual stresses.

Although support structures are useful for increasing AM manufacturability, they introduce challenges for part quality and cost. For example, in SLM, support structures are fused to the build part and are either frangible or removed by mechanical grinding, posing the risk of physical damage or deformation of the manufactured object. Secondly, support structures used within internal or hollow features (including lattice structures) cannot be removed with non-destructive methods, and thereby introduce further design constraints to AM. Thus, overhang geometries are considered undesirable due to the requirement of support structures and should be avoided.

However, elimination of overhangs is not feasible, especially in complex structures such as lattice structures, and may result in structural instability if critical structural elements are eliminated. Thus, designing geometries that are manufacturable without the use of support structures either results in extended modelling time or structurally unstable components.

Hence, it is complex to determine the internal structural configuration for space filling conformal lattice structures that are manufacturable using SLM as well as being structurally appropriate for the intended application. Research as RMIT University has shown that support-free manufacture of lattice structures is feasible with identification and accommodation of manufacturing constraints of the applied SLM technology.

Lattice structures as orthopaedic implants

Orthopaedic implants are required to reconstruct the natural form and function of a bone that may be compromised due to disease, deformity or trauma. Lattice structures that conform to a free-form geometry and are optimised for the manufacturing and loading constraints are desirable. The design and manufacture of patient-specific biomedical implant applications overcome several major limitations of conventional orthopaedic implants, includuing:

• Standard implants are not customised to individual patients.
• Heavy structural design of the implants for bone stability requires surgeon to remove significant amount of hard as well as soft tissue from patient.
• The biomechanical engineer has to work according to the implant design to decide on placement strategy instead of the anatomical function of the bone. This results in significant rehabilitation and recovery time for the patient
• Stress shielding due to stiffness mismatch results in bone resorption (degradation) and subsequent implant loosening

RMIT University researchers have demonstrated that use of lattice structures can overcome these limitations by optimising the structural configuration of lattices to not only recover load-carrying capacity of the bone but also could promote bone ingrowth to ensure rapid and rigid biological fixation. Implant geometry that is generated using this method also conforms to the resected tissue to ensure that robust surface contact is achieved at the bone-implant interface. Curvilinear struts that conform to the natural contours of the bone avoid stress concentrations that often contribute to fretting wear, i.e. the loss of material owing to the rubbing or contact of components. Recent RMIT research builds on all the identified markers and constraints for design optimisation and manufacture of orthopaedic implants (Shidid, Leary, Choong, & Brandt, 2016). In summary, RMIT University’s patent-pending methodology ensures

• The implants with periodic lattice configuration conform to the bone’s shape
• Their geometry is optimised so that it is manufacturable without the need for internal support structures
• Lattice configuration consists of functionally varying pore size and shape to simultaneously enhance bone-ingrowth and strength to weight ratio.
• And is structurally stable to withstand patient-specific loading conditions

Implant designs manufactured at RMIT for patients with osteosarcoma bone cancer were mechanically tested against the healthy bone geometry using synthetic bones. The results show that the implants designed using RMIT methodology not only recover the load carrying capacity of the bone, but they also mimic the stiffness of the substrate bone yielding a superior option to conventionally designed implants.

The algorithms used in this methodology are computationally highly efficient and can yield optimised designs within minutes making iterative design feasible even with very tight design time schedules.

The RMIT Centre for Additive Manufacturing is involved in a number of research projects with collaborators at Stryker South Pacific, St Vincents Hospital in Melbourne, the Peter MacCallum Cancer Centre and UTS, and supported by the IMCRC and ARC Training Centre in Additive Biomanufacturing to research and develop new biomedical products using AM technology.

By Darpan Shidid, Martin Leary, Maciej Mazur, Eric Yang, Matthew McMillan, Bill Lozanovski, Ma Qian and Milan Brandt.

www.rmit.edu.au