The feasibility of fabricating custom frameworks for tissue regeneration utilizing
three-dimensional inkjet printing technology followed by slip casting was investigated. A
CAD solid model of mold was created with structures within the mold cavity to provide
variable, customized porosity in the cast framework. The polymer mold was printed using
a 3D ink-jet printer. The mold was then infiltrated by an aqueous suspension of
hydroxyapatite to produce porous frameworks that were tested for mechanical properties.Mechanical Engineerin

Gomez, J.R.

Jones, R.E.

Nambiar, R.V.

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CUSTOM FABRICATION OF HARD TISSUE RECONSTRUCTIVEFRAMEWORKSR. V. Nambiar, R. E. Jones and J. R. GomezThe University of Texas-Pan American, Edinburg TX 78539AbstractThe feasibility of fabricating custom frameworks for tissue regeneration utilizingthree-dimensional inkjet printing technology followed by slip casting was investigated. ACAD solid model of mold was created with  structures within the mold cavity to providevariable, customized porosity in the cast framework. The polymer mold was printed usinga 3D ink-jet printer. The mold was then infiltrated by an aqueous suspension ofhydroxyapatite to produce porous frameworks that were tested for mechanical properties.IntroductionPorous materials have been used as non-resorbable intermediate structures forbone repair and when pore size is properly controlled such repairs can integrate into newbone, even permitting the development of vascularity [1].  A promising approach for hardtissue reconstruction is the integration of a custom mechanical framework with biologicalagents to promote development of the desired tissue [2].   Such a structure must havethree characteristics.   It must fit the patient, thus requiring a unique structure for everyrepair.   It must have specified mechanical properties to permit proper function duringregeneration. Finally, it must be able to carry the required precursor cells and/or growthfactors and direct their development.  Thus it will require a controlled internal structurethat will probably not be uniform if it is to guide development of all of the subcomponents539of functioning bone or cartilage.   These requirements make solid free form fabrication thebest hope for cost effective fabrication of the required structures.Research to date on rapid fabrication of tissue frameworks has utilized eitherselective laser sintering [3], laminated object manufacturing [4] or fused depositionmodeling [5]. In each case, the part must be sintered after the polymer binders are burnedout. The ink-jet based rapid prototyping technology proposed for this study is well suitedto the fabrication of precision frameworks for reconstruction.  The system prints a layer ofbuild material using an ink-jet head and maintains accuracy in the z-direction by milling thelayer to a precise height before the next layer is deposited.  The result is one of the mostaccurate and affordable commercially available solid free-form fabrication system.  Modelswith pore feature size of 0.5 mm repeatability accurate to 0.05 mm have been produced.This process is not suitable for very large parts that may be required in orthopedicreconstruction, but it is capable of producing many of the common components needed formaxillofacial work.In this work, a CAD solid model of a mold is built with the inner surface createdusing imported surface data from a CT scan. Structures are created within the mold cavityto provide variable, customized porosity in the cast framework. The polymer mold isprinted using a 3D ink-jet printer. The mold is then infiltrated by an aqueous suspension ofhydroxyapatite mixed with binder. The part is dried then processed in an oven to burn outthe polymer binder and then sintered to produce porous framework.The ability of tissue growth into the framework is requires a well connected network of macropores.A number of different designs for the internal geometry were considered. A repeated hexagonalpattern of axial cylinders interconnected like a honeycomb was found to give the most uniform540mesh as shown in Figure 1. The pattern is repeated at uniform axial increments throughout thestructure. Cross section sizes ranging from 0.25 mm2 to 0.7 mm2 were used, the smaller sectionsshowing a higher rate of damage in processing. These structures in the wax mold will providepredictable sized pores in the cast structure.Figure 1. CAD model of mold with internal structureProcessingHydroxyapatite powder (Aldrich) was mixed with water and compressed to form coupons.Samples of the coupons were analyzed in a differential thermal analyzer (DTA).The graph in Figure 2 shows the decrease in weight of the sample as thetemperature increases. The initial downward slope is attributed to the loss of water fromthe specimen up to 600C.541Figure 2. DTA weight loss analysis of HAThe graph in Figure 3 shows the temperature difference of the sample from thecontrol pan in the chamber due to exothermic or endothermic processes. This shows thatthe loss of the water can be completed at below 600C, but that the sintering temperaturecan be set below 1200C.Hydroxylapatite - Weight % vs. Temperature9495969798991001010 200 400 600 800 1000 1200 1400 1600Temp. (deg C)Weight %H y d r o x y l a p a t i t e  -  T e m p e r a t u r e  D i f f e r e n c e  v s .  T e m p e r a t u r e-0.500.511.522.50 200 400 600 800 1000 1200 1400 1600T e m p .  ( d e g .  C )Temp. Difference542Figure 3. DTA temperature analysis of HAFigure 4. DTA weight loss analysis of HA with binderH y d r o x y l a p a t i t e  +  B i n d e r  -  T e m p e r a t u r e  D i f f e r e n c e  v s .  T e m p e r a t u r e-0.500.511.522.50 200 400 600 800 1000 1200 1400 1600T e m p .  ( d e g .  C )Temp. DifferenceFigure 5.  DTA temperature analysis of HA with binderFigures 4  and 5 show the corresponding graphs for hydroxyapatite mixed with DuramaxB-1020 binder. Figure 4 shows the loss of water from the specimen at 500C. The graph inH y d r o x y l a p a t i t e  +  B i n d e r  -  W e i g h t  %  v s .  T e m p e r a t u r e0204060801001200 200 400 600 800 1000 1200 1400 1600T e m p .  ( d e g .  C )Weight %543Figure 5 shows binder is burnt off at 700C. This establishes the burn off temperature at600C followed by sintering at 1100C, thus the temperatures for processing the specimenshave been identified.Molds with the outer body of the mold and the internal structures for the pores allin one piece were built using the Sanders ink-jet printing machine. Hydroxylapatitepowder was combined with Duramax B-1020 organic binder and water to make a slurry.The slurry was poured into the mold and the water allowed to seep out of the base to formsolid ceramic specimens. The specimens fabricated above were all found to have cracks and internal cavities. Thesemake compression strength tests unreliable. The cause of these defects appears to beuneven drying. Experimentation to eliminate these defects by modifying the binder contentand controlling the humidity is continuing.SEM scans of the surface reveal as shown in Figure 6. that the sintering processleads to relatively smooth surfaces. This may indicate that the surface needs to be etchedto create micropores to enhance osteoinductivity.544Figure 6.   SEM picture of part surface 4000X magnification545ConclusionThe results of the studies conducted this year suggest that we can design molds forframeworks with specified pore size and distribution in a CAD system. These molds canbe fabricated using ink-jet printing rapid prototyping technology. The materials to beused for casting of the frameworks in these molds have been selected and analyzed toidentify the processing temperatures.Bibliography1. C.J. Damien, J.R. Parsons, “Bone graft and bone graft substitutes: a review of currenttechnology and applications,” J. Appl. Biomaterials, (1991) 2(3) p187.2. D.L. Cochran, J.M. Wozney, “Biological mediators for periodontal regeneration,”Periodontal 2000 (1999) Feb: 19 p40.3. G. Lee, J.W. Barlow, “Selective Laser Sintering of Calcium Phosphate Powders,”Solid Freeform Fabrication Symposium Proceedings, 1993 UT Austin, TX.4. Steidle, C., et al., Automated Fabrication of Nonresorbable Bone Implants UsingLaminated Object Manufacturing (LOM), M.S. Thesis, Chemical Engineering,University of Dayton, May, 1998.5. Richter, P.W., Thomas, M.E., Van Deventer, T., Macroporous hydroxyapatiteBioceramics by solid Freeform Fabrication: Towards Custom Implants. South AfricanJournal of Science, vol. 95, 1999.6. Schenker, R., De Beer, D.J., Du Preez, W.B., Thomas, M.E., Richter, P.W., NovelCombination of Reverse Engineering and Rapid Prototyping in Medicine.  SouthAfrican Journal of Science, vol. 95, no. 8, August, 1999, pp. 327-328.

Custom Fabrication of Hard Tissue Reconstructive Frameworks

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