Joseph Cadman

On design of multi-functional microstructural materials

The design of periodic microstructural composite materials to achieve specific properties has been a major area of interest in material research. Tailoring different physical properties by modifying the microstructural architecture in unit cells is one of the main concerns in exploring and developing novel multi-functional cellular composites and has led to the development of a large variety of mathematical models, theories and methodologies for improving the performance of such materials. This paper provides a critical review on the state-of-the-art advances in the design of periodic microstructures of multi-functional materials for a range of physical properties, such as elastic stiffness, Poisson’s ratio, thermal expansion coefficient, conductivity, fluidic permeability, particle diffusivity, electrical permittivity and magnetic permeability, etc.

Cuttlebone: Characterisation, Application and Development of Biomimetic Materials

Cuttlebone signifies a special class of ultra-lightweight cellular natural material possessing unique chemical, mechanical and structural properties, which have drawn considerable attention in the literature. The aim of this paper is to better understand the mechanical and biological roles of cuttlebone. First, the existing literature concerning the characterisation and potential applications inspired by this remarkable biomaterial is critiqued. Second, the finite element-based homogenisation method is used to verify that morphological variations within individual cuttlebone samples have minimal impact on the effective mechanical properties. This finding agrees with existing literature, which suggests that cuttlebone strength is dictated by the cuttlefish habitation depth. Subsequently, this homogenisation approach is further developed to characterise the effective mechanical bulk modulus and biofluidic permeability that cuttlebone provides, thereby quantifying its mechanical and transporting functionalities to inspire bionic design of structures and materials for more extensive applications. Finally, a brief rationale for the need to design a biomimetic material inspired by the cuttlebone microstructure is provided, based on the preceding investigation.

Design of cellular porous biomaterials for wall shear stress criterion

The microfluidic environment provided by implanted prostheses has a decisive influence on the viability, proliferation and differentiation of cells. In bone tissue engineering, for instance, experiments have confirmed that a certain level of wall shear stress (WSS) is more advantageous to osteoblastic differentiation. This paper proposes a level‐set‐based topology optimization method to regulate fluidic WSS distribution for design of cellular biomaterials. The topological boundary of fluid phase is represented by a level‐set model embedded in a higher‐dimensional scalar function. WSS is determined by the computational fluid dynamics analysis in the scale of cellular base cells. To achieve a uniform WSS distribution at the solid–fluid interface, the difference between local and target WSS is taken as the design criterion, which determines the speed of the boundary evolution in the level‐set model. The examples demonstrate the effectiveness of the presented method and exhibit a considerable potential in the design optimization and fabrication of new prosthetic cellular materials for bioengineering applications. Biotechnol. Bioeng. 2010;107:737–746.

Design Optimization of Scaffold Microstructures Using Wall Shear Stress Criterion Towards Regulated Flow-Induced Erosion

Tissue scaffolds aim to provide a cell-friendly biomechanical environment for facilitating cell growth. Existing studies have shown significant demands for generating a certain level of wall shear stress (WSS) on scaffold microstructural surfaces for promoting cellular response and attachment efficacy. Recently, its role in shear-induced erosion of polymer scaffold has also drawn increasing attention. This paper proposes a bi-directional evolutionary structural optimization (BESO) approach for design of scaffold microstructure in terms of the WSS uniformity criterion, by downgrading highly-stressed solid elements into fluidic elements and/or upgrading lowly-stressed fluidic elements into solid elements. In addition to this, a computational model is presented to simulate shear-induced erosion process. The effective stiffness and permeability of initial and optimized scaffold microstructures are characterized by the finite element based homogenization technique to quantify the variations of mechanical properties of scaffold during erosion. The illustrative examples show that a uniform WSS is achieved within the optimized scaffold microstructures, and their architectural and biomechanical features are maintained for a longer lifetime during shear-induced erosion process. This study provides a mathematical means to the design optimization of cellular biomaterials in terms of the WSS criterion towards controllable shear-induced erosion.

Bioinspired lightweight cellular materials - Understanding effects of natural variation on mechanical properties

Cuttlebone is a natural marine cellular material possessing the exceptional mechanical properties of high compressive strength, high porosity and high permeability. This combination of properties is exceedingly desirable in biomedical applications, such as bone tissue scaffolds. In light of recent studies, which converted raw cuttlebone into hydroxyapatite tissue scaffolds, the impact of morphological variations in the microstructure of this natural cellular material on the effective mechanical properties is explored in this paper. Two extensions of the finite element-based homogenization method are employed to account for deviations from the assumption of periodicity. Firstly, a representative volume element (RVE) of cuttlebone is systematically varied to reflect the large range of microstructural configurations possibly among different cuttlefish species. The homogenization results reveal the critical importance of pillar formation and aspect ratio (height/width of RVE) on the effective bulk and shear moduli of cuttlebone. Secondly, multi-cell analysis domains (or multiple RVE domains) permit the introduction of random variations across neighboring cells. Such random variations decrease the bulk modulus whilst displaying minimal impact on the shear modulus. Increasing the average size of random variations increases the effect on bulk modulus. Also, the results converge rapidly as the size of the analysis domain is increased, meaning that a relatively small multi-cell domain can provide a reasonable approximation of the effective properties for a given set of random variation parameters. These results have important implications for the proposed use of raw cuttlebone as an engineering material. They also highlight some potential for biomimetic design capabilities for materials inspired by the cuttlebone microstructure, which may be applicable in biomedical applications such as bone tissue scaffolds.

Computer-Aided Design and Fabrication of Bio-Mimetic Materials and Scaffold Micro-Structures

Computer-aided design (CAD) has proven effective in enabling novel approaches for tissue engineering applications. This paper demonstrates the applicability of various mathematical methods to design and fabricate bio-mimetic materials via two illustrative examples. Firstly, CAD models of cellular biomaterials that mimic the micro-structure of cuttlefish bone are designed based on the principles of the homogenization method. Secondly, a three-dimensional bi-objective topology optimization approach based upon the inverse homogenization method is used to design scaffold micro-structures with tailored effective stiffness and permeability properties. Consequently, solid free-form fabrication is used to fabricate such cellular bio-mimetic materials, which show a great potential in tissue engineering applications.

Creating Biomaterials Inspired by the Microstructure of Cuttlebone

The microstructure of cuttlebone is investigated using Scanning Electron Microscopy (SEM). A graded aspect ratio of the base cells between layers is evident in some samples. A method for designing graded biomaterials mimicking this cuttlebone microstructure is developed. Simplified 3D biomaterial samples are created using CAD software. These biomaterials are fabricated using a stereolithographic apparatus (SLA). The homogenisation technique is used to evaluate the mechanical properties of the original cuttlebone sample and the fabricated biomaterial sample. Good agreement is found between the Young’s moduli of corresponding layers. However, it is inconclusive whether the Young’s moduli have a proportional relationship to the aspect ratio of the base cell at this stage of the study.

Assessing the effects of natural variations in microstructure for the biomimetic modeling of cuttlebone

Cuttlebone is a natural material possessing a unique microstructure providing a high compressive strength to weight ratio. It is potentially desirable to use cuttlebone directly in engineering applications or to design new biomimetic materials based on the microstructural features of cuttlebone. A finite element based homogenization method can be used for characterizing the mechanical properties of such a biomaterial and for the design of biomimetic materials. However, this method assumes a periodicity of microstructure, which does not reflect the variation present in natural or fabricated materials. The method can be extended to investigate the effect of natural variation and manufacturing tolerance by enlarging the base cell domain to include a number of representative volume elements (RVEs) and applying a random displacement vector to the nodes at the internal intersections of the RVEs. As the boundary of the base cell domain is not modified, the homogenization method can still be employed to calculate the bulk mechanical properties. It is found that the number of RVEs in the base cell has an impact on the decrease in mean stiffness tensor components, while the length of the introduced variation seems to influence both the mean and the standard deviation of stiffness tensor components.

Effect of Scaffold Architecture on Tissue Regeneration

As a fast developing technology for tissue engi- neering, scaffolding has drawn tremendous attention in the recent years. Experiments have consistently demonstrated a significant role of mechanical stimuli on the process of tissue regeneration. However, the relationship between tissue in- growth and scaffold architecture remains inconclusive. It is of great importance to understand how scaffold provides a proper biomechanical environment, thereby affecting the tissue differentiation and growth. In this regard, this paper investigates some examples of tissue regeneration based on a mechanobiological model within different scaffold architec- tures. Optimal base-cell topologies of scaffold are sought for a range of effective permeability and stiffness criteria, in which a so-called inverse homogenization technique is applied. Based on the obtained optimal architecture, simulation of tissue regeneration is conducted. It is found that, the remodeling pathway and neo-tissue formation could be distinct when the scaffold structures vary. It is revealed that, as the key factors of scaffold properties, the interplay between stiffness and permeability could be more influential in affecting the tissue regeneration process. The numerical results presented in this paper help us better understand the mechanisms of skeletal mechanobiology and, more importantly, bring significant insights into the design optimization of tissue scaffold for solid free-form fabrication (SFF).