Fibrication Of The3D Poly Lactic Acid Bone Tissue Engineering Scaffold Using Microcellular Injection Molding

Microcellular injection molding produces components with excellent dimensionalstability while using lower injection pressures, shorter cycle time, and less material. Duringthis process, supercritical fluid （SCF） CO₂or N₂was used as the physical blowing agent.The microcellular injection molding technology is being applied to the fabrication of the3Dinterconnected biodegradable bone tissue engineering scaffold, taking advantage of thecontrolled weight saving and significant reduction s in cycle time without chemical solventinvolved. Most notably, the process is being applied in the bone regeneration in thebio-medical area.Poly lactic acid is a compostable polymer that can be derived completely fromrenewable resources. It has been used for the medical and biodegradability. One of thecritical steps in the production of microcellular polymers is the dissolution of gas into apolymer matrix. For gas and semi-crystalline thermoplastic systems, the solution formationprocess is notably more complex. The crystallization results in a solution that is relativelydifficult to microcellular process requiring relatively high temperature compared toamorphous polymer/gas solutions. In addition, the crystallization of the solution results in alower solubility, increased matrix stiffness, and a lower diffusivity. This paper is to present asystematic study of the gas dissolution process in semi-crystalline polymers in an attempt toprovide further insight and engineering analysis into the roles of gas dissolution withinduced crystallization, viscoelastic behavior, and crystallniity in microcellular processing.Firstly, the mechanism of the cell nucleation and growth theories were introduced tofurther investigate the effect of the crystallinity and rheological properties on the cellmorphology. To study the solution’s gas dissolution and crystallization characteristics, ouranalysis include an experimental characterization of the carbon dioxide-inducedcrystallization occurring during microcellular polymer processing indicating a critical gasconcentration is required for crystallization and an experimental estimation of theviscoelastic behavior of semi-crystalline solution, and an experimental investigation of the effects of crystallinity on microcellular processing and the resulting cell morphology.Secondly, the dispersion of the nano-particle in the nanocomposites is a knownproblem either in the conventional or microcellular injection molding process. To improvingthe morphology and structure of the nanocomposites, the pre-foam technology wasintroduced to the microcellular injection molding to produce a single polymer-gas solutionand uniform foaming parts with uniform properties. This method involves producing thepellets with gas laden by extrusion foaming to improve the dispersion of the nano-particle inthe microcellular injection molding specimens and also decrease the skin-core structureunder the shear flow.Thirdly, based on the requirements of3D interconnected bone tissue engineeringscaffold, the thermal, dynamic thermo-mechanical property, and the cell morphology of thePLA blend were studied to tune the mechanical properties and biodegradable rate to accessthe optimal processing condition. One kind of bio-model cell structure was produced withhigh porosity. The cell culture experiments were conducted on the PLA/PHBV scaffold, itwas found that the cell can attach on the scaffold very well, and the cell proliferation wassignificant, indicating the scaffold was suitable for the cell attachment and proliferation.Finally, the effect of the nanoclay on the nucleation of crystallinity and cell was studied.Based on the viscoelastic behavior characterization of polymer-gas solutions, this study hasfound that solutions of crystalline polymer and gas tend to have higher storage modulicompared with their amorphous counterparts. The crystallization and the resulting change inviscoelastic behavior tend to play a major role in microcellular processing. It was also foundthat crystallization influences microcellular processing through its effects on cell nucleationmechanisms resulting in larger cell densities and cell growth mechanisms resulting insmaller cell sizes.