RAPID PROTOTYPING OF POLYMERIC MEDICAL DEVICES

工业快速原型技术在加速

Industrial rapid prototyping techniques are invaluable in accelerating the transformation process from design to finished product. Three Dimensional Printing (3DP) is an Solid Freedom Fabrication (SSF) technology which are developed by MIT researchers in 1990. 3DP follows a slicing algorithm from a CAD file, and manufactures three dimensional objects by "'printing" droplets of binder onto a stack of two-dimensional powder layers. Depending on the purpose of the final part, the appropriate liquid binder is selectively sprayed (i.e. printed) sequentially onto each layer of powder through a printhead onto regions where the powder is to be joined. By moving the printhead over each layer of powder in a raster-scan fashion, precise patterns can be produced in two dimensions. This process is repeated in a layer-by-layer fashion until the entire three dimensional object is completed. After the unbound powder is separated from the printed part, the appropriate processing procedures, if any, are performed depending on the materials used, and the ultimate purpose of the printed part. The flexibility and adaptability of 3DP, plus its ability to control microstructure and composition, sets 3DP apart from all other rapid prototyping techniques. Prior to this project, only ceramic and metal powders have been studied in 3DP. Our research term a MIT is attempting to merge two developing technologies (3DP and biomaterials processing). Degradable polymeric materials of interest include poly-caprolactone, polyglycolic acid, polylactic acid, and their copolymers. Poly--caprolactone (PCL) is a semicrystalline polymer with high solubility, low melting point, and exceptional ability to blend with other polymers. PCL is the principal matrix material in Capronor, a one year implantable contraceptive device. Polyglycolic acid (PGA) is the simplest linear, aliphatic polyester with high crystallinity and melting point. It degrades rapidly and is used clinically as a resorbable suture, Dexon. An additional methyl group makes polylactic acid more hydrophobic than PGA. This reduced water uptake decreases the rate of PLA backbone hydrolysis, as compared to PGA. PLA-PGA coplymers are less crystalline than either pure PLA or pure PGA. This decreased crystallinity is associated with increased rate of hydrolysis. These copolymers degrade more rapidly, and their clinical application can be found in sutures (Vicryl). One of our goals is to define the limits and explore the possibilities of using these types of polymers in 3DP. Experiments are performed to study the fundamental processes in powder processing, classification, spreading, solvent solubility, jet stability, powder-solvent binding and printing behavior. Featuresize, dimensional accuracy, and processing distortion are studied with various materials and printing parameters. A range of build strategies using different polymeric and inorganic powders with a number of binders are investigated to establish a fundamental understanding of the specific relationship between material properties and processing parameters. Once the basic science is quantified, our team began preliminary investigations on a revolutionary concept of exploiting 3DP's ability to produce polymeric medical devices with anisotropic microstructures and heterogeneous compositions. Key Words: polymer, PCL, PLA, PGA, biomaterials, rapid prototyping, CAD