Periodic Reporting for period 1 - MIDPOINT (Multiscale design of porous implants with a biomimetic functionally graded cellular material)
Reporting period: 2021-11-02 to 2023-11-01
Introduction. Bone implants are now widely used in orthopaedic and dental surgery to restore joint functionality or to replace missing teeth, despite problems regarding their long-term stability. Implant failure is often due to bone resorption resulting from stress shielding, which comes from the mismatch of the mechanical properties between the implant and the surrounding bone. To improve implant stability, a design methodology integrating structural stiffness with fluid flow is needed to allow the implant to have both adequate rigidity to resist physical loading and sufficient permeability to transfer not only cells and nutrients but also to allow cells to grow and proliferate.
Objectives. The objective of MidPoint was to establish an innovative multiscale optimization method for the design of functionally graded porous implants and scaffolds according to i) morphological, ii) structural, iii) permeability, and iv) fatigue requirements. To this end, three specific objectives were pursued:
1. Produce new artificial lattices using the Voronoi tessellation approach, which will eventually be fabricated using AM methods.
2. Conceive a state-of-the-art method making use of damage accumulation models (Miner’s rule) combined with machine learning (artificial neural networks) to predict the fatigue life of the artificial lattices.
3. Adapt the multiscale optimization method previously developed by me to minimize bone resorption and maximize the fatigue life of dental and orthopaedic implants.
Conclusions. The project was completed by the end of the fellowship and the collaboration with the host institution will continue. The conclusions obtained so far are summarized here:
1. Voronoi-based lattices were developed and manufactured using additive manufacturing techniques. The design space was analysed, and it was concluded that this cellular structure can be used for implants in a range that is optimum for bone ingrowth.
2. A damage accumulation method was developed using fast Fourier transform models. This method allows the computation of a database with the fatigue life of the Voronoi-based lattices throughout the whole design space, which will be used to accelerate the implant optimization process.
Objectives. The objective of MidPoint was to establish an innovative multiscale optimization method for the design of functionally graded porous implants and scaffolds according to i) morphological, ii) structural, iii) permeability, and iv) fatigue requirements. To this end, three specific objectives were pursued:
1. Produce new artificial lattices using the Voronoi tessellation approach, which will eventually be fabricated using AM methods.
2. Conceive a state-of-the-art method making use of damage accumulation models (Miner’s rule) combined with machine learning (artificial neural networks) to predict the fatigue life of the artificial lattices.
3. Adapt the multiscale optimization method previously developed by me to minimize bone resorption and maximize the fatigue life of dental and orthopaedic implants.
Conclusions. The project was completed by the end of the fellowship and the collaboration with the host institution will continue. The conclusions obtained so far are summarized here:
1. Voronoi-based lattices were developed and manufactured using additive manufacturing techniques. The design space was analysed, and it was concluded that this cellular structure can be used for implants in a range that is optimum for bone ingrowth.
2. A damage accumulation method was developed using fast Fourier transform models. This method allows the computation of a database with the fatigue life of the Voronoi-based lattices throughout the whole design space, which will be used to accelerate the implant optimization process.
Artificial lattices were designed using the Voronoi tessellation approach. This method allows the creation of personalized biomimetic bone implants with a porous and interconnected microstructure, such as the one that can be observed in natural bone. Furthermore, this strategy produces topologies that can be directly and precisely fabricated using additive manufacturing techniques. The obtained lattices were characterized to obtain the geometric and elastic properties. For this, homogenization analyses were carried out using a dedicated FFT method. A database was built with information on the volume fraction, elastic properties, and tortuosity of the lattice based on its geometric parameters. From the polynomial interpolations of this database, a response surface (RSM) to be used for the optimization method was constructed that allows rapid calculations of the properties. Additionally, Voronoi lattices were additively manufactured using a laser powder bed fusion system with a Ti6Al4V alloy powder to check the manufacturability and analyse the fatigue life. The results of this work were presented at the “8th European Congress on Computational Methods in Applied Sciences and Engineering”.
To determine the fatigue life of the lattices, a method that makes use of damage accumulation models was developed. The procedure can be summarized as follows: i) a fast Fourier transform model of the lattice subjected to constant uniaxial stress is constructed; ii) the model is run, and the distribution of stress is determined; iii) the remaining life is calculated point by point using the Miner's rule; iv) the strut with the minimum remaining life is removed; v) the process is repeated until total failure of the lattice. This procedure must be performed on different lattices and loading states to determine the stress vs. number of cycles to failure curves throughout the whole porosity range.
While working on MidPoint and after learning about the work being done at the host institution, I realized the importance of studying the biomechanical properties of the bone-implant interface for surgical success. Therefore, I decided to develop a multiscale homogenization model to evaluate the effective elastic properties of bone as a function of the distance to the implant, based on tissue structure and composition at lower scales. The model considers three scales: mineral matrix (nanoscale), ultrastructure (microscale), and bone tissue (mesoscale). The elastic properties and volume fraction of the elemental constituents of the bone matrix (mineral, collagen, and water), the orientation of the collagen fibrils relative to the implant surface, and the porosity at the mesoscale constitute the input data of the model. The effect of a spatiotemporal variation in the orientation of collagen fibrils on the anisotropic properties of bone in the vicinity of the implant was investigated. The findings revealed a strong variation of the components of the effective bone elasticity tensor as a function of the distance to the implant. The results of this work were presented at the “X International Conference on Computational Bioengineering”, and a publication was accepted in the journal “Biomechanics and Modelling in Mechanobiology”.
To determine the fatigue life of the lattices, a method that makes use of damage accumulation models was developed. The procedure can be summarized as follows: i) a fast Fourier transform model of the lattice subjected to constant uniaxial stress is constructed; ii) the model is run, and the distribution of stress is determined; iii) the remaining life is calculated point by point using the Miner's rule; iv) the strut with the minimum remaining life is removed; v) the process is repeated until total failure of the lattice. This procedure must be performed on different lattices and loading states to determine the stress vs. number of cycles to failure curves throughout the whole porosity range.
While working on MidPoint and after learning about the work being done at the host institution, I realized the importance of studying the biomechanical properties of the bone-implant interface for surgical success. Therefore, I decided to develop a multiscale homogenization model to evaluate the effective elastic properties of bone as a function of the distance to the implant, based on tissue structure and composition at lower scales. The model considers three scales: mineral matrix (nanoscale), ultrastructure (microscale), and bone tissue (mesoscale). The elastic properties and volume fraction of the elemental constituents of the bone matrix (mineral, collagen, and water), the orientation of the collagen fibrils relative to the implant surface, and the porosity at the mesoscale constitute the input data of the model. The effect of a spatiotemporal variation in the orientation of collagen fibrils on the anisotropic properties of bone in the vicinity of the implant was investigated. The findings revealed a strong variation of the components of the effective bone elasticity tensor as a function of the distance to the implant. The results of this work were presented at the “X International Conference on Computational Bioengineering”, and a publication was accepted in the journal “Biomechanics and Modelling in Mechanobiology”.
Increased life expectancy, stress, and bad nutritional habits are causing diseases that weaken bone tissue, such as osteoporosis and/or the loss of natural teeth. Moreover, intense lifestyles as well as traffic and work accidents frequently result in body injuries. Locomotion system injuries are one of the main causes of disability, crippling around four million people according to the World Health Organization (WHO), mostly because of the failure of the treatments of long bones. This project has managed to provide further insight into the field of bone tissue engineering, which addresses the aforementioned problems. The field of bone tissue engineering (BTE) is rapidly growing and BTE-based products are already being used in clinical practice. Nevertheless, significant challenges and limitations still exist. This project provides further insight towards overcoming these limitations thanks to the development of an optimization method capable of dealing with multi-functional designs on multiple length scales simultaneously. This approach allows us to make a step forward in manufacturing custom implants that will deal with abnormal anatomical structures. At the same time, it provides a microstructure with interconnected pores for improved osseointegration. These implants allow to reduce bone resorption around the implant by improving fixation and, in turn, reducing the need for revision surgery, which will lower social and economic costs.