Fractal tool paths for layered manufacturing of scaffolds with matched bone properties
Treatment of large bone defects such as cavities that result by removal of benignin tumor using bone cement has several disadvantages (Kenny and Buggy 2003). An autograft, which is the best option for treatment has a limited availability and is thus suitable only for small defects. Tissue engineering has shown great promise in repairing such cavities in bone, diseased and large fractured bone. The bone tissue engineering uses scaffolds that fill the defect, stimulate new bone tissue growth and get resorbed over time as they are replaced by newly formed bone. The success of bone TE techniques depends on the technology to generate reliable, fully integrated, complex, three-dimensional and controlled porous scaffolds of the exact shape and size of the replacement of the bone cavity or defect (Hutmacher 2000, Hollister et al. 2002, Sun et al. 2004a). The scaffold should bear the load imposed during and early recovery period without collapsing and the stiffness of scaffold should be equal or slightly less than the surrounding bone so that stress shielding is avoided These constraints demand that the scaffold have a internal structure with channels and interconnected pores to help suitable mechanical attachment and biological environment for cell proliferation, tissue regeneration and nutrient flow (Vuola 1998, Chu et al. 2002, Taboas et al. 2003, Sun et al. 2004b). Specifically
it is important that the mechanical properties of the scaffold should be well defined and controlled (Cleynenbreugel 2002). Many natural, e.g., collagen and chitin, and synthetic biomaterials, e.g., poly(a-hydroxyesters) and poly(anhydrides), Hydroxyapatite (HA), Tricalcium Phosphate (TCP) ceramics, Polycaprolactone (PCL) and non biodegradable material such as Titanium (Ti) have been widely and successfully used as scaffolding materials because of their good cell-tissue biocompatibility and processability. The complexity of architecture and the variability of properties of bone tissue (e.g., porosity, pore size, mechanical properties, mineralization or mineral density and cell type), as well as differences in age, nutritional state, activity (mechanical loading) and disease status of individuals establish a major challenge in fabricating scaffolds and engineering bone tissues that will meet the needs of specific repair sites in specific patients. The external size and shape of the scaffold should also confirm to the replacement for body part specific to a subject for biological and structural acceptability. Solid-free form fabrication (SFF) also known as layered manufacturing (LM)/rapid prototyping (RP), can potentially be used to fabricate scaffolds with morphological and mechanical properties more selectively designed to meet the specific bone repair needs. The present paper proposes a biomimetic design and LM approach to bone tissue regeneration
ABSTRACT: Tissue engineering has shown great promise to repairing large segmental bone fractures. In this technique, the design of scaffolding material and modeling of porous structure are important issues to be addressed. These porous structures provide initial support for cell adhesion and growth and gradually resorb during bone regeneration. The pore sizes, distribution, interconnectivity, strength of the scaffold are critical factors in the design of scaffolds. Layered Manufacturing (LM) techniques have shown great promise for fabricating tissue scaffolds with specific designed properties. The present work aims at studying the applicability of using space-filling fractal curves as tools paths for LM of porous hydroxyapatite scaffolds with optimized mechanical properties along with the constraints on pore sizes for repair and regeneration of bone. Specifically, theoretical mechanical modulus, porosity and pore sizes that are obtainable in LM using the fractal curves for a chosen bio material have been derived.