PROCESS OPTIMIZATION FOR THE RAPID FABRICATION OF CELLULOSE ACETATE SCAFFOLDS BY SELECTIVE LASER SINTERING

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1 PROCESS OPTIMIZATION FOR THE RAPID FABRICATION OF CELLULOSE ACETATE SCAFFOLDS BY SELECTIVE LASER SINTERING Gean V. Salmoria 1, Priscila Klauss 1, Janaina L. Leite 1, Rodrigo A. Paggi 1, Alexandre Lago 2 1 Universidade Federal de Santa Catarina Departamento de Engenharia Mecânica, Laboratório CIMJECT Caixa Postal 476, Florianópolis - SC - Brasil - CEP gsalmoria@cimject.ufsc.br; 1 Laboratório CIMJECT, UFSC pklauss@cimject.ufsc.br; rodrigo@cimject.ufsc.br; 2 Laboratório de Materiais (LABMAT), UFSC lago@materiais.ufsc.br. Abstract: The cellulose acetate (CA) is a thermoplastic produced from cellulose, which presents mechanical and thermal resistance, thermal and dimensional stability, low cost and it is also a renewable feedstock. Biomaterials based on cellulose and its derivates have been used as coating materials for drugs and scaffolds drug delivery. The Selective Laser Sintering is a Rapid Prototype process that creates 3D objects, layer-by-layer, by the sintering of powdered materials using a CO 2 laser beam. This work presents the process optimization for the rapid fabrication of cellulose acetate scaffolds by selective laser sintering using different particles size. The specimens were characterized by density measurements, scanning electronic microscopy (SEM) and flexural test. The specimens with lower particles size presented higher sinter degree and a significant level of close porous as indicated by the density measurements and mechanical tests. The fine process parameters optimization was very important to garanty the processability of biodegradables polymers such as cellulose acetate by SLS. The results obtained showed that is possible to fabricate scaffold structures using cellulose acetate material by the selective laser sintering. Introduction The cellulose acetate (CA) is a thermoplastic produced from cellulose, which presents mechanical and thermal resistance, thermal and dimensional stability, low cost and it is also a renewable feedstock (1). Biomaterials based on cellulose and its derivates have been used as coating materials for drugs and scaffolds for drug delivery (2). Investigation of hydrophilic polymers, in particular cellulose derivatives, has attracted considerable attention for the development of controlled release technology in the formulation of pharmaceutical matrix products. Matrix tablets swell on contact with water, and the release of drug depends on interactions between water, polymer and drug (3). Hydrogen bonds from the hydroxyl group, holding the cellulose chains together, account for the high degree of cristallinity, low solubility and poor cellulose degradation in vivo. The presence of amorphous regions can increase biodegradability in tissue (4). The cellulose acetate is a semi crystalline material with melting temperature 226 o C and glass transition temperature 190 o C (5). The Selective Laser Sintering is a Rapid Prototype process that creates 3D objects, layer-bylayer, by the sintering of powdered materials using a CO 2 laser beam (6). In SLS process, it is important to obtain a better understanding of the relationship between the powder material properties, fabrication parameters and the properties of the final part (7). The powder properties such as particle shape/size, laser absorption, melting flow behavior at process condition and decomposition temperature are important factors to the fabrication parameters optimization in the

2 SLS (8). The optimization of the process will define the microstructure, mechanical properties and surface qualities for SLS parts (8, 9). This work presents the process optimization for the rapid fabrication of cellulose acetate scaffolds by selective laser sintering using different particles size. The specimens were characterized by density measurements, scaninng electronic microscopy (SEM) and flexural test. Experimental The acetate of cellulose (AC) used in this work contains about 40 % of groups acetate, was purchased from Fluka, with Mn = g mol -1. The material can be processed from 200 to 240 o C. The material was grinded in a mechanical grinder to obtain particles in ranges of size: µm and µm. A combination of parameters such as laser scanning speed and laser power were tested to the process of fabrication of the specimens that were tested mechanically and analyzed by microscopy. After the optimum parameters determination, the specimens were produced with J/mm 2 of density energy. The chamber temperature was 60 o C and laser scan speed was 39.8 mm/s. The specimens of the cellulose acetate were prepared SLS with 35 x 5 x 1.75 mm dimensions using 250 µm of layer thickness. The apparent density was obtained in a picnometry of the 50 cm 3. The average value was obtained from four samples. The immersion time was two hours in iso-propanol. The volumetric density was calculated using the size and mass of the specimens. The specimens were tested in a single cantilever clamp, force controlled mode with rate of 1 N/min, using a DMA Q800. The stress versus strain curves were obtained at 30 o C. The morphology of the specimens was examined with a scanning electron microscope (SEM), Phillips XL30. The specimen surfaces and the flexural fracture surface of the samples were investigated. The specimens were coated with gold in a Bal -Tec Sputter Coater SCD005. Results and Discussion Table 1 shows processing parameters used to optimizate the SLS processing of the cellulose acetate based material. After pre-tests, an intermediated value of laser scan speed was used (39.8 mm/s) to determine the power required to the process. The power energy of 1.5 to 2.0 W was required to sinter the specimens without degradation of the material. The final optimization found 1.7 W of laser power energy to processing the particles material.

3 Table 1: Processing parameters and physical characteristics of the specimens sintered. Laser Power (W) Energy Density (J/mm²) Mechanical strength No No Yes Yes Yes Yes Degradation No No No Yes Yes Yes Figure 1 shows the SEM micrographics of surface of cellulose acetate sintered specimens. The SEM analyses showed that the sintered specimens with particles size of µm and µm present a high level of sintering with the coalescence of particles and an interconnected pore distribution along it structure µm µm Figure 1 The surface micrographs of cellulose acetate sintered specimens. The Table 2 presents the values of theoretic, volumetric and apparent density of the cellulose acetate specimens prepared with different size particles. The measured volumetric densities were and g/cm 3 for cellulose acetate samples having respectively average particles size of and µm, that indicate a significant level of porosity (~68%) for all specimens. The measured apparent densities were and g/cm 3 for cellulose acetate samples having respectively average particles size of and µm, that indicate that the specimens with larger particles size presented higher open porosity, i.e. the a significant level of interconnected porous.

4 Table 2 Values of theoretic, volumetric and apparent density to the cellulose acetate specimens prepared with different particles size. Particle size ρ volumetric (g.cm -3 ) ρ apparent (g.cm -3 ) ρ theoretic (g.cm -3 ) µm µm Figure 2 shows curves of stress versus strain for the specimens sintered with different particles size. The average value for the elastic modulus decreased from to MPa to specimens with µm particles size comparing to the specimens with µm particles size. Analogous behavior was observed for ultimate strength for all samples studied, as shown in Table 3. On the other hand, the average strain at failure of the µm particles size specimen was higher than that of the µm particles size specimen. The µm particles size specimen presented a higher average elastic modulus and tensile strength than µm particles size specimen, as expected due to the sinter area distribution, i.e., the number of neck per area. The results suggest that strength of the samples decrease with the increase of the particles size. Particles with smaller size have bigger superficial contact area, contributing with sintering degree (union degree), forming more numbers of the necks. 4,0 3, µm µm 3,0 Stress (MPa) 2,5 2,0 1,5 1,0 0,5 0,0 0,0 0,5 1,0 1,5 2,0 2,5 3,0 Strain (%) Figure 2 The stress versus strain curves for the cellulose acetate specimens prepared with different particles size.

5 Table 3- Average mechanical properties of the cellulose acetate specimens prepared with different particles size. Particles size Elastic modulus Ultimate Failure deformation (%) (MPa) strength (MPa) µm µm µm µm Figure 3 The fracture micrographs of cellulose acetate sintered specimens. The greater particle size specimens present poor mechanical properties and high level of porosity as shown in the fracture surfaces (Figure 3). The pore size presented in the specimens microstructure were function of the particles size used to the preparation of these specimens, result in different values to mechanical properties. Conclusions The fine process parameters optimization was very important to garanty the processability of biodegradables polymers such as cellulose acetate by SLS. The study concerning the processing parameters showed that the best combination is 1.70 W of laser power and scan speed of 39.8 mm/s to process the cellulose acetate material. The specimens with lower particles size presented higher sinter degree and a significant level of close porous as indicated by the density measurements and mechanical tests. On the other hand, the mechanical properties of specimens prepared with bigger particles presented lower values due to the limited number of union, i.e. low density of unions (low necks number/area). The results obtained showed that is possible to fabricate scaffold structures using cellulose acetate material by the selective laser sintering. Information such as specimens prepared with bigger

6 particle size present higher level of porosity and lower mechanical properties are very important to the design of scaffold with mechanical functions and or drug delivery functions. Acknowledgements The authors would like to acknowledge FAPESC, FINEP, CNPq and CAPES for the financial support. References 1. A. G. S. Prado, E.A. Faria, J.R, DeSouza, J. D. Torres, Journal of Molecular Catalysis. 2005, 237, S. D. Desai, J. Blanchard. J. Pharm. Sci.1998, 87, B. Sasa, P. Odon, S. Stane, K. Julijana. European Journal of Pharmaceutical Science, 2006, 27, E. Entcehva, H. Bien, L. Yin, C. Chung, M. Farrell, Y. Kostov. Biomaterials, 2004, 25, Meier, M.M., Kanis, L.A., Lima, J.C., Pires, Alfredo T.N., Soldi Valdir, Polymer Advance and Technology, S. M. Hur, K. H. Choi, S. H. LEE, P. K. Chang, Journal of Materials Processing Technology, 2001, 112, I. Gibson and D. Shi, Rapid Prototyping Journal, 1997, v. 3, n. 4, G.V. Salmoria, J.L. Leite, C.H. Ahrens, A. Lago and A.T.N. Pires, Polymer Testing, DTM Corporation, Sinterstation 2000 System User s Guide 1995.