Novel Mixing Device for Polyurethane Foam Scaffolds

1. Novel Mixing Device for Polyurethane Foam Scaffolds Michael Scherer1, Dustin Dowell1, Andrew Solomon2, Scott Guelcher, Ph.D.,3 1Biomedical Engineering Department, 2Mechanical Engineering Department, 3Chemical Engineering Department Vanderbilt University School of Engineering, Nashville, Tennessee 37235 INTRODUCTION IMPELLER DESIGN RESULTS CONT’D Frequency Sweep • Rushton Turbine designed to create an radial flow pattern • Outer Diameter: D0=.75”, Height: H=.25” • Container Diameter: Dc=1” • Radial Clearance Ratio: D0/Dc= .75 • Minimal container bottom separation Polyisocyanates, when exposed to water, react to produce carbon dioxide. This reaction has recently been implemented in the synthesis of polyurethane (PUR) foams used for tissue scaffolding. Derived from an isocyanate and polyester triol combination, the resulting foam supports the attachment, proliferation, and differentiation of bone marrow stromal cells. Its porous structure allows these cells to propagate quickly through the scaffolding, accelerating the recovery process. One current obstacle in the polymer's development is the creation of a novel mixing process for the polyol and isocyanate. Once mixed, the reaction begins immediately; the foam swells to more than 5 times its original volume within 10 minutes. As the window of operation for the foam is so small, it is essential that the new mixing system be simple and expedient while still producing a homogeneous product. Our solution to this mixing issue requires the use of a small rotary driven turbine to evenly mix both elements in the operating room setting. Employing fluid mixing theory and mechanical design, we would develop a mixing device and test its feasibility in a laboratory setting. RESULTS DISCUSSION • Density and Porosity Characteristics • Young’s Modulus • Compressive Stress at 50% Deflection • Stress Relaxation • The primary goal of this project was to test a prototype axial mixer as a valid substitute for the current co-axial vortex method currently employed. • Our results indicate that: • The rushton-turbine driven axial mixing is an effective, convenient, and cost-effective way to homogeneously mix PUR foams. • The Young's Modulus for these foams are within an acceptable error range compared to accepted values. Both the 1800/HDIt and 900+50PEG are similar in stiffness, with the 900DHIt within an acceptable percentage deviation. • The compressive stresses of the foams were very close to their accepted values, with no deviation greater than 17 Kpa. • Novel mixing produced foams with nearly identical Storage and Loss Modulus. • •Foams prepared with the stirred tank agitation method behave similarly over different compressive stress conditions. MATERIALS AND METHODS Components: 1. Hardener a. Polyester (PCLG) triol b. Water c. TEGOAMIN 33 (catalyst) d. Turkey red oil (stabilizer) e. Calcium stearate (pore opener) 2. Isocyanate a. DN3300A (HDI trimer) Porous PUR scaffolds are synthesized when the polyol and isocyanate react, with CO2 acting as a blowing agent to create pores. Operating Room Procedure 1. Add hardener to isocyanate in sterile canister 2. Insert canister into fixture loading tray 3. Lower impeller into canister at preset mixing height 4. Mix for 40 seconds at level 2 mixing (11,000 RPM) 5. Turn off rotary device and withdraw the shaft from the canister 6. Remove canister from loading tray 7.Inject polyurethane CONCLUSION The stirred tank agitation technique utilizing a Rushton turbine impeller provided adequate mixing as compared to the co-axial vortex method.  Radial mixing proved to be effective, producing foams with comparable densities and reflexive characteristics vital to the materials performance in practice. Moreover, the new process delivered comparable mixing in a more compact device with decreased cost.  Pending further evaluation, stirred tank agitation is a promising method for mixing PUR scaffolds in the operating room setting.  REFERENCES Guelcher, Scott A., AbiramanSrinivasan, and Jerald E. Dumas. "Synthesis, Mechanical Properties, Biocompatibility, and Biodegradation of Polyurethane Networks From Lysine Polyisocyanates." Biomaterials 29 (2008): 1762-1775. Guelcher, Scott A. "Biodegradable Polyurethanes: Synthesis and Applications in Regenerative Medicine." Tissue Engineering: Part B 14 (2008): 3-17. Guelcher, Scott, AbiramanSrinivasan, and Andrea Hafeman. "Synthesis, in Vitro Degradation, and Mechanical Properties of Two-Component Poly(Ester-Urethane)Urea Scaffolds: Effects of Water and Polyol Composition." Tissue Engineering 13 (2007): 2321-2333. King, Paul H., and Richard C. Fries. Design of Biomedical Devices and Systems. New York: Marcel Dekker, 2003. Oldshue, James Y. Fluid Mixing Technology. New York: McGraw Hill, 1983. Perry, R H., and D W. Green. Perry's Chemical Engineers' Handbook. 8th ed. New York: McGraw Hill, 2007. Tatterson, Gary B. Fluid Mixing and Gas Dispersion in Agitated Tanks. New York: McGraw Hill, 1991.