Wei Sun

IntroductionThe need for replacement organs and tissue substitutes is on the rise. At present, there is an insufficient amount of tissue replacements for failed or damaged organs, due to the lack of donors. In the USA alone, over twenty million patients per year suffer from some form of tissue- and/or organ-related malady, and are awaiting a replacement. The financial cost of health care for these patients has been estimated to be over $400 billion annually [1, 2]. Computer-aided tissue engineered substitutes are one of the most promising applications of tissue replacements to address this issue. These tissue arrays are fabricated using techniques from a variety of science and engineering disciplines to create the optimum tissue replacement (in terms of the targeted functionality). Additionally, these tissue constructs play a vital role as pre-formed extracellular matrices to which cells can readily attach, whereupon they can rapidly multiply and form new tissue [3, 4]. In recent decades, scientists have proven that these fields are evolving into one of the most promising therapeutic approaches in regenerative medicine [5–9].Three-dimensional tissue scaffolds are designed with a preferred internal architecture, wherein porosity and material connectivity provide the required structural integrity, mass transport, and comprehensive microenvironment for cell and tissue growth. A literature survey has shown that cell survival and proliferation within the tissue scaffold are dependent on oxygen, vital molecules, and the micro-architecture of the scaffolds [10, 11]. The complexity of tissue scaffolds requires novel approaches and computational algorithms to match the desired criteria for internal architecture, permeability, pore size, and connectivity. The dynamics of a tissue scaffold is governed by its structural and topological configuration defined by porosity, pore interconnectivity, tortuosity, and scaffold material permeability and diffusivity [12–14]. The scaffold tortuosity characterizes the diffusion path length of fluid molecules through the scaffold, which shapes the internal architecture of the scaffold and plays a major role in tissue growth and proliferation [15–19]. Many cells respond more favorably to a three-dimensional (3D) microenvironment (than to a two-dimensional (2D) microenvironment) with intricate intracellular architectures where the cell’s morphological shape, behavior, and gene expression are richer, more robust, and more similar to in-vivo responses [20–22].

In their normal in vivo matrix milieu, tissues assume complex well-organized 3D architectures. Therefore, a primary aim in the tissue engineering design process is to fabricate an optimal analog of the in vivo scenario, in which the precise configuration and composition of cells and bioactive matrix components can establish the well-defined biomimetic microenvironments that promote cell cell and cell matrix interactions. With the advent and refinements in microfabricated systems which can present physical and chemical cues to cells in a controllable and reproducible Fashion unrealizable with conventional tissue culture, high-fidelity, high-throughput in vitro models are achieved. The convergence of solid freeform fabrication (SFF) technologies, namely microprinting, along with microfabrication techniques, a 3D microprinted Micro-organ, can serve as an in vitro platform for cell culture, drug screening, or to elicit further biological insights. This chapter firstly details the principles, methods, and applications that undergird the fabrication process development and adaptation of microfluidic devices for the creation of a drug screening model. This model involves the combinatorial setup of an automated syringe-based, layered direct cell writing microprinting process with soft lithographic micropatterning techniques to fabricate a microscale in vitro device housing a chamber of microprinted 3D micro-organ that biomimics the cell's natural microenvironment for enhanced performance and functionality. In order to assess the structural formability and biological feasibility of such a micro-organ, 3D cell-encapsulated hydrogel-based tissue constructs are microprinted reproducibly in defined design patterns and biologically characterized for both viability and cell-specific function. Another key fleet of the in vivo microenvironment that is recapitulated with the in vitro system is the necessary dynamic perfusion of the 3D microscale liver analog with cells probed for their collective drug metabolic function and suitability as a drug metabolism model.

Computer-aided tissue engineering approach was used to develop a novel Precision Extrusion Deposition (PED) process to directly fabricate Polycaprolactone (PCL) and composite PCL/Hydroxyapatite (PCL-HA) tissue scaffolds. The process optimization was carried out to fabricate both PCL and PCL-HA (25% concentration by weight of HA) with a controlled pore size and internal pore structure of the 0 degrees/90 degrees pattern. Two groups of scaffolds having 60 and 70% porosity and with pore sizes of 450 and 750 microns, respectively, were evaluated for their morphology and compressive properties using Scanning Electron Microscopy (SEM) and mechanical testing. The surface modification with plasma was conducted on PCL scaffold to increase the cellular attachment and proliferation. Our results suggested that inclusion of HA significantly increased the compressive modulus from 59 to 84 MPa for 60% porous scaffolds and from 30 to 76 MPa for 70% porous scaffolds. In vitro cell-scaffolds interaction study was carried out using primary fetal bovine osteoblasts to assess the feasibility of scaffolds for bone tissue engineering application. In addition. the results in surface hydrophilicity and roughness show that plasma surface modification can increase the hydrophilicity while introducing the nano-scale surface roughness on PCL surface. The cell proliferation and differentiation were calculated by Alamar Blue assay and by determining alkaline phosphatase activity. The osteoblasts were able to migrate and proliferate over the cultured time for both PCL as well as PCL-HA scaffolds. Our Study demonstrated the viability of the PED process to the fabricate PCL and PCL-HA composite scaffolds having necessary mechanical property, structural integrity, controlled pore size and pore interconnectivity desired for bone tissue engineering.

This paper presents a computer-aided modeling and characterization approach for design and evaluation of mechanical properties and structural heterogeneity of 3D tissue scaffolds. An outline of a computer-aided tissue engineering approach for biomimetic design and freeform fabrication of 3D tissue scaffold, a procedure of computer-aided characterization and a computational algorithm for implementing asymptotic homogenization theory, and its application for predicting the effective mechanical properties of heterogeneous PolY-E-Caprolactone scaffold fabricated through a novel precision extruding deposition process are presented.