The doctoral research summarized in this thesis has focused on the study of microflows in Tissue Engineering (TE) scaffolds and microdevices. The thesis is organized in two parts. In the first part, the properties influencing mass transport through scaffold are investigated both experimentally and in silico. In detail: (1) an acoustic measurement system suitable for the evaluation of TE porous scaffolds and based on a single (pressure) transducer is developed; (2) realistic models of irregular porous scaffolds were reconstructed from micro-CT images and fluid transport through them is simulated by applying the Lattice Boltzmann method. In the second part, the issue of mixing of species in microdevices is investigated in depth and a novel low-cost passive microfluidic mixer design is proposed and its performance evaluated both in silico and in vitro. PART I: The performance of porous scaffold for tissue engineering (TE) applications are generally evaluated in terms of porosity, pore size and distribution, and pore tortuosity. However these descriptors are often confounding when they are applied to characterize the mass transport within porous scaffolds. On the contrary, permeability is a more effective parameter in (1) estimating mass and species transport through the scaffold and (2) describing its topological features. Therefore, this first part has focused on the study of TE porous scaffold permeability and on its dependence on the microscopic features of the scaffold. Firstly, an overview of methods applied to evaluate TE scaffold permeability is provided, with an emphasis on both experimental and computational approaches. In detail, after a discussion on the most relevant scaffolds features to be considered in the evaluation of the permeability, the presentation of the theoretical background and the introduction of semi-empirical models relating scaffolds features to permeability, the most widely applied experimental setup for the direct measurement of tissue engineered scaffold permeability are presented. Then, the focus is put on the application of computational methods, useful to verify and compare the experimental measurements of permeability, and to integrate experimental data with a more quantitative analysis which is very effective in supporting the design process of TE porous scaffolds. In conclusion, limitations of the methods and future challenges are pointed out. Successively, an acoustic permeability measurement system to quantify the inter-pore connectivity structure of tissue-engineering scaffolds by using a single (pressure) transducer is presented. The proposed method has been developed keeping in mind the limitations of the permeability measurement system in TE field. Technically, this system uses a slow alternating airflow as a fluid medium and allows at the same time a simple and accurate measurement procedure. The intrinsic permeability has been determined in the linear Darcy’s region, and deviation from linearity due to inertial losses has been also quantified. The structural parameters of a scaffold, such as effective porosity, tortuosity and effective length of cylindrical pores, have been estimated using the modified Ergun’s equation. From this relation, it is possible to achieve a well-defined range of data and associated uncertainties for characterizing the structure/architecture of tissue-engineering scaffolds. This quantitative analysis is of paramount importance in tissue engineering, where scaffold topological features are strongly related to their biological performance. In the last investigation of this part, the permeability of three bioactive glass/polymer composite scaffolds for bone tissue regeneration is evaluated. Structural features such as porosity, specific surface area and tortuosity, and lacunarity have been measured as well. Concerning lacunarity analysis, results confirmed its potential in providing insights into (i) self-similarity, (ii) random structure at some scale (i.e. heterogeneity) and (iii) Representative Elementary Volume (REV) identification. Permeability is evaluated both experimentally and computationally using the novel acoustic permeability system and Lattice Boltzmann Method (LBM), respectively. The advantage of LBM approach is due to their geometric versatility in simulating flows in irregular porous media. Results of the LBM models are in good agreement with the experimental results, even if the permeability values estimated in silico overestimate experimental data. This discrepancy is due to the influence of grid resolution and sample size on permeability calculations. In addition, the lower permeability values obtained in this study than the permeability data of different bone tissue reported in literature confirms the need to optimize the design of these scaffolds in terms of mass transport. PART II: Microfluidic deals with the control and manipulation of fluids at the microscale. A typical microfluidic platform is characterized by several components. One of the most important is the micromixer. Mixing of species is often critical to be achieved, since microfluidics is characterized mainly by very low Reynolds flows, and cannot take advantage of turbulence in order to enhance mixing. A good understanding of the dynamic of mixing becomes crucial to i) improve the effectiveness of and ii) speed up chemical reactions. In order to enhance mixing, several techniques have been developed. In general, mixing strategies can be classified as either active or passive, according to the operational mechanism. Active mixers employ external forces in order to perform mixing, so that actuation system must be embedded into the microchips. On the contrary, passive mixers avoid resorting to external electrical or mechanical sources by exploiting characteristics of specific flow fields in microchannel geometries to mix species, offering the advantage to be easy to be produced and integrated. The aim of this investigation was to develop a new low-cost passive microfluidic mixer design. First, a survey of the passive micromixing solutions currently adopted is provided. In detail, the most widely used microchannel geometries and the metrics used to quantify mixing effectiveness in microfluidic applications has been discussed. Then, a new low-cost passive microfluidic mixer design, based on a replication of identical mixing units composed of microchannels with variable curvature (clothoid) geometry, is shown. The micromixer presents a compact and modular architecture that can be easily fabricated using a simple and reliable fabrication process. The particular clothoid-based geometry enhances the mixing by inducing transversal secondary flows and recirculation effects. The role of the relevant fluid mechanics mechanisms promoting the mixing in this geometry have been analysed using computational fluid dynamics (CFD) for Reynolds numbers ranging from 1 to 110. A measure of mixing potency has been quantitatively evaluated by calculating mixing efficiency, while a measure of particle dispersion has been assessed through the lacunarity index. The results showed that the secondary flow arrangement and recirculation effects are able to provide a mixing efficiency equal to 80% at Reynolds number above 70. In addition, the analysis of particles distribution promotes the lacunarity as powerful tool to quantify the dispersion of fluid particles and, in turn, the overall mixing. On fabricated micromixer prototypes the microscopic-Laser-Induced-Fluorescence (µLIF) technique has been applied to characterize mixing. The experimental results confirmed the mixing potency of the microdevice. In conclusion, the proposed design (i) assures a good mixing efficiency (i.e. comparable, if not superior, to other passive micromixer, (ii) is easy to fabricate (i.e. single layer microfluidic devices) and (iii) is easy to integrate (i.e. high modularity).

Analysis of microscale flows in tissue engineering systems and microfluidic devices / Pennella, Francesco. - (2013).

Analysis of microscale flows in tissue engineering systems and microfluidic devices

PENNELLA, FRANCESCO
2013

Abstract

The doctoral research summarized in this thesis has focused on the study of microflows in Tissue Engineering (TE) scaffolds and microdevices. The thesis is organized in two parts. In the first part, the properties influencing mass transport through scaffold are investigated both experimentally and in silico. In detail: (1) an acoustic measurement system suitable for the evaluation of TE porous scaffolds and based on a single (pressure) transducer is developed; (2) realistic models of irregular porous scaffolds were reconstructed from micro-CT images and fluid transport through them is simulated by applying the Lattice Boltzmann method. In the second part, the issue of mixing of species in microdevices is investigated in depth and a novel low-cost passive microfluidic mixer design is proposed and its performance evaluated both in silico and in vitro. PART I: The performance of porous scaffold for tissue engineering (TE) applications are generally evaluated in terms of porosity, pore size and distribution, and pore tortuosity. However these descriptors are often confounding when they are applied to characterize the mass transport within porous scaffolds. On the contrary, permeability is a more effective parameter in (1) estimating mass and species transport through the scaffold and (2) describing its topological features. Therefore, this first part has focused on the study of TE porous scaffold permeability and on its dependence on the microscopic features of the scaffold. Firstly, an overview of methods applied to evaluate TE scaffold permeability is provided, with an emphasis on both experimental and computational approaches. In detail, after a discussion on the most relevant scaffolds features to be considered in the evaluation of the permeability, the presentation of the theoretical background and the introduction of semi-empirical models relating scaffolds features to permeability, the most widely applied experimental setup for the direct measurement of tissue engineered scaffold permeability are presented. Then, the focus is put on the application of computational methods, useful to verify and compare the experimental measurements of permeability, and to integrate experimental data with a more quantitative analysis which is very effective in supporting the design process of TE porous scaffolds. In conclusion, limitations of the methods and future challenges are pointed out. Successively, an acoustic permeability measurement system to quantify the inter-pore connectivity structure of tissue-engineering scaffolds by using a single (pressure) transducer is presented. The proposed method has been developed keeping in mind the limitations of the permeability measurement system in TE field. Technically, this system uses a slow alternating airflow as a fluid medium and allows at the same time a simple and accurate measurement procedure. The intrinsic permeability has been determined in the linear Darcy’s region, and deviation from linearity due to inertial losses has been also quantified. The structural parameters of a scaffold, such as effective porosity, tortuosity and effective length of cylindrical pores, have been estimated using the modified Ergun’s equation. From this relation, it is possible to achieve a well-defined range of data and associated uncertainties for characterizing the structure/architecture of tissue-engineering scaffolds. This quantitative analysis is of paramount importance in tissue engineering, where scaffold topological features are strongly related to their biological performance. In the last investigation of this part, the permeability of three bioactive glass/polymer composite scaffolds for bone tissue regeneration is evaluated. Structural features such as porosity, specific surface area and tortuosity, and lacunarity have been measured as well. Concerning lacunarity analysis, results confirmed its potential in providing insights into (i) self-similarity, (ii) random structure at some scale (i.e. heterogeneity) and (iii) Representative Elementary Volume (REV) identification. Permeability is evaluated both experimentally and computationally using the novel acoustic permeability system and Lattice Boltzmann Method (LBM), respectively. The advantage of LBM approach is due to their geometric versatility in simulating flows in irregular porous media. Results of the LBM models are in good agreement with the experimental results, even if the permeability values estimated in silico overestimate experimental data. This discrepancy is due to the influence of grid resolution and sample size on permeability calculations. In addition, the lower permeability values obtained in this study than the permeability data of different bone tissue reported in literature confirms the need to optimize the design of these scaffolds in terms of mass transport. PART II: Microfluidic deals with the control and manipulation of fluids at the microscale. A typical microfluidic platform is characterized by several components. One of the most important is the micromixer. Mixing of species is often critical to be achieved, since microfluidics is characterized mainly by very low Reynolds flows, and cannot take advantage of turbulence in order to enhance mixing. A good understanding of the dynamic of mixing becomes crucial to i) improve the effectiveness of and ii) speed up chemical reactions. In order to enhance mixing, several techniques have been developed. In general, mixing strategies can be classified as either active or passive, according to the operational mechanism. Active mixers employ external forces in order to perform mixing, so that actuation system must be embedded into the microchips. On the contrary, passive mixers avoid resorting to external electrical or mechanical sources by exploiting characteristics of specific flow fields in microchannel geometries to mix species, offering the advantage to be easy to be produced and integrated. The aim of this investigation was to develop a new low-cost passive microfluidic mixer design. First, a survey of the passive micromixing solutions currently adopted is provided. In detail, the most widely used microchannel geometries and the metrics used to quantify mixing effectiveness in microfluidic applications has been discussed. Then, a new low-cost passive microfluidic mixer design, based on a replication of identical mixing units composed of microchannels with variable curvature (clothoid) geometry, is shown. The micromixer presents a compact and modular architecture that can be easily fabricated using a simple and reliable fabrication process. The particular clothoid-based geometry enhances the mixing by inducing transversal secondary flows and recirculation effects. The role of the relevant fluid mechanics mechanisms promoting the mixing in this geometry have been analysed using computational fluid dynamics (CFD) for Reynolds numbers ranging from 1 to 110. A measure of mixing potency has been quantitatively evaluated by calculating mixing efficiency, while a measure of particle dispersion has been assessed through the lacunarity index. The results showed that the secondary flow arrangement and recirculation effects are able to provide a mixing efficiency equal to 80% at Reynolds number above 70. In addition, the analysis of particles distribution promotes the lacunarity as powerful tool to quantify the dispersion of fluid particles and, in turn, the overall mixing. On fabricated micromixer prototypes the microscopic-Laser-Induced-Fluorescence (µLIF) technique has been applied to characterize mixing. The experimental results confirmed the mixing potency of the microdevice. In conclusion, the proposed design (i) assures a good mixing efficiency (i.e. comparable, if not superior, to other passive micromixer, (ii) is easy to fabricate (i.e. single layer microfluidic devices) and (iii) is easy to integrate (i.e. high modularity).
2013
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11583/2514479
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