Microelectromechanical Systems Research Unit

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Organizational Unit

Publications

Tribological behavior of bioactive multi-material structures targeting orthopedic applications

Publication . Costa, M.M.; Bartolomeu, F.; Alves, N.; Silva, F.S.; Miranda, G.

The following study proposes a multi-material solution in which Ti6Al4V cellular structures produced by Selective Laser Melting are impregnated with bioactive materials (hydroxyapatite or β-tricalcium phosphate) using press and sintering technique. To assess the tribological response of these structures, an alumina plate was used as a counterpart in a flat-on-flat reciprocating sliding test. Ti6Al4V cellular structures impregnated with bioactive materials displayed the highest wear resistance when compared with the unreinforced structures. Among the bioactive structures, Ti6Al4V cellular structures impregnated with βTCP were the ones with higher wear resistance, having the lowest weight loss. Hence, these structures are promising multifunctional solutions for load-bearing applications by gathering suitable mechanical properties (strength and stiffness); bioactive properties and in addition an improved wear performance.

2019Journal article

Open access

Corrosion behaviour of PEEK or β-TCP-impregnated Ti6Al4V SLM structures targeting biomedical applications

Publication . Costa, M.M.; Dantas, T.A.; Bartolomeu, F.; Alves, N.; Silva, F.S.; Miranda, G.; Toptan, F.

Ti6Al4V cellular structures were produced by selective laser melting (SLM) and then filled either with beta-tricalcium phosphate (β-TCP) or PEEK (poly-ether-ether-ketone) through powder metallurgy techniques, to improve osteoconductivity and wear resistance. The corrosion behavior of these structures was explored considering its importance for the long-term performance of implants. Results revealed that the incorporation of open cellular pores induced higher electrochemical kinetics when being compared with dense structures. The impregnation of β-TCP and PEEK led to the creation of voids or gaps between the metallic matrix and the impregnated material which also influenced the corrosion behavior of the cellular structures.

2019-12Journal article

Open access

Engineering the elastic modulus of NiTi cellular structures fabricated by selective laser melting

Publication . Bartolomeu, F.; Costa, M.M.; Alves, N.; Miranda, G.; Silva, F.S.

Nickel-titanium (NiTi) cellular structures are a very promising solution to some issues related to orthopaedic implant failure. These structures can be designed and fabricated to simultaneously address a combination of mechanical and physical properties, such as elastic modulus, porosity, wear and corrosion resistance, biocompatibility and appropriate biological environment. This ability can enhance the modest interaction currently existing between metallic dense implants and surrounding bone tissue, allowing long-term successful orthopaedic implants. For that purpose, NiTi cellular structures with different levels of porosity intended to reduce the elastic modulus were designed, modelled, selective laser melting (SLM) fabricated and characterized. Significant differences were found between the CAD design and the SLM-produced NiTi structures by performing systematic image analysis. This work proposes designing guidelines to anticipate and correct the systematic differences between CAD and produced structures. Compressive tests were carried out to estimate the elastic modulus of the produced structures and finite element analyses were performed, for comparison purposes. Linear correlations were found for the dimensions, porosity, and elastic modulus when comparing the CAD design with the SLM structures. The produced NiTi structures exhibit elastic moduli that match that of bone tissue, which is a good indication of the potential of these structures in orthopaedic implants.

2020Journal article

Restricted access

Additive manufacturing of NiTi-Ti6Al4V multi-material cellular structures targeting orthopedic implants

Publication . Bartolomeu, F.; Costa, M.M.; Alves, N.; Miranda, G.; Silva, F.S.

The amount of hip revision surgeries is significantly increasing due to the loss of fixation between implant and bone, that leads to implant failure. The stiffness mismatch between Ti6Al4V hip implants and bone tissue, the non-uniform implant-bone contact pressure, and the poor wear resistance of Ti6Al4V are pointed as three critical issues that contribute to these implant’s failure. In this study, a multi-material design and fabrication concept was exploited aiming to change traditional manufacturing paradigms, by allocating different biomaterials in a single component targeting a multi-functional hip implant. Selective Laser Melting technology was explored to fabricate NiTi-Ti6Al4V multi-material cellular structures with a Ti6Al4V inner region and a NiTi outer region. This work was focused on the SLM fabrication and processing parameters validation on a commercial SLM equipment. The morphological analyses allowed to assess a successful solidification and bond between NiTi and Ti6Al4V materials in the transition region. The shear tests revealed a high bond strength of the transition region with an average strength of 33 MPa. The nano-indentation results showed that the Ti6Al4V region exhibits a higher hardness and elastic modulus when compared with the NiTi region. This work is a part of a broader objective that aims to create a NiTi-Ti6Al4V multi-material and cellular structured hip implant capable to provide customized stiffness, superior wear resistance and a controlled NiTi outer region volume change.

2020Journal article

Restricted access

Additive manufactured porous biomaterials targeting orthopedic implants: A suitable combination of mechanical, physical and topological properties

Publication . Bartolomeu, F.; Dourado, N.; Pereira, F.; Alves, N.; Miranda, G.; Silva, F. S.

Orthopedic implants are under incessant advancement to improve their interactions with surrounding bone tissue aiming to ensure successful outcomes for patients. A successful biological interaction between implant and surrounding bone depends on the combination of mechanical, physical and topological properties. Hence, Ti6Al4V cellular structures appear as very promising solutions towards the improvement of conventional orthopedic implants. This work addresses a set of fundamental tools that allow improving the design of Ti6Al4V cellular structures produced by Selective Laser Melting (SLM). Three-point bending tests were carried out to estimate the elastic modulus of the produced structures. Morphological analysis allowed to evaluate the dimensional differences that were noticed between the model CAD and the SLM structures. Finite element models (adjusted CAD) were constructed with the experimentally obtained dimensions to replicate the mechanical response of the SLM structures. Linear correlations were systematically found for the dimensions of the SLM structures as a function of the designed model CAD dimensions. This has also been observed for the measured porosities as a function of the designed CAD models. This data can be used in further FE analyses as design guidelines to help engineers fabricating near-net-shape SLM Ti6Al4V cellular structures. Besides, polished and sandblasted surface treatments performed on the Ti6Al4V cellular structures allowed to obtain suitable properties regarding roughness and wettability when compared to as-produced surfaces. The capillarity tests showed that all the analyzed Ti6Al4V structures are able to transport fluid along its structure. The cell viability tests demonstrate Ti6Al4V cellular structures SLM produced did not release toxic substances to the medium, indicating that these structures can assure a suitable environment for cells to proliferate and attach. This study proposes a design methodology for Ti6Al4V cellular structures, that owe suitable mechanical properties but also provide a proper combination of porosity, roughness, wettability, capillarity and cell viability, all of them relevant for orthopedic applications. A Ti6Al4V cellular structured hip implant prototype gathering the suitable features addressed in this study was successfully SLM-produced.

2020-02Journal article

Open access