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SUPERCRITICAL CO2 FOAMING OF PLLA WITH LIQUID CO2-PHILIC CELL NUCLEATORS

2018-09-01
Çulhacıoğlu, Yağmur
Hasırcı, Nesrin
Dilek Hacıhabiboğlu, Çerağ
Normal 0 false false false EN-US X-NONE X-NONE /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Calibri",sans-serif; mso-bidi-font-family:"Times New Roman";} There is an increasing trend towards replacing synthetic polymers with their natural counterparts and applying environmentally friendly processing techniques [1]. For both industrial and biomedical applications, natural originated biodegradable polymers are intensely studied due to their environmentally benign and biocompatible nature. Supercritical carbon dioxide (scCO2) processing of polymers is a green approach since it reduces or eliminates the use of organic solvents and thus emissions of volatile organic compounds [2,3]. ScCO2, which is non-toxic, non-flammable and inert with relatively low critical temperature compared to other supercritical solvents, has a plasticizer effect on polymers having CO2 affinity by decreasing the glass transition temperature of the polymer. Therefore, it can be used as a foaming agent for such polymers. In the first step of the foaming process, the polymer is saturated with scCO2 under high pressure [4]. Next, rapid depressurization and thus supersaturation of the polymer-scCO2 system is achieved, which leads to nucleation and growth of the pores in the polymer matrix. Poly(L-lactic acid) (PLLA) is a biodegradable and biocompatible polymer that can be replaced with petroleum based polymers [4,5]. PLLA foams can be used in biomedical applications such as drug delivery, scaffold for bone tissue or artificial vasculature systems. Due to its highly crystalline structure, high saturation temperatures close to the melting temperature of PLLA (about 180°C) and pressures over 20 MPa are necessary for the foaming of the polymer with scCO2, especially when the crystallinity of the polymers are greater than 30% [4,6,7]. In this study, foaming of highly crystalline PLLA composite films (>40%) was achieved only at 40oC and 21 MPa with the help of two novel CO2-philic components used as cell nucleators. These cell nucleators are liquid polyhedral oligomeric silsesquioxanes (POSS), which are monofunctional isooctyl POSS and bifunctional methacrylisooctyl POSS. These molecules have recently been identified to be CO2-philic with different levels of CO2-philicity and physicochemical properties [8]. When PLLA film was processed with scCO2 at the same conditions, pore formation was not observed within the polymer matrix. Therefore, POSS molecules act as not only cell nucleator but they also allow dissolution of CO2 in the matrix. The foam properties were analyzed with various techniques. The EDX analyses show that the majority of the cell nucleator is extracted by the scCO2 during the process. The mechanical tests show that the tensile moduli of the films increased by at least 80% after the CO2 processing. Both films reached a tensile modulus of around 1100 MPa. The nano-indentation analyses also revealed similar trend in the indentation hardness and moduli. The glass transition of the films increased by at least 40% after the CO2 treatment and reached a value about 57 oC while the crystallinity of the CO2 processed films increased by 3%. The water contact angles of the films show that the process did not change the hydrophobicity of the polymer. The drug release studies are currently carried out and the results will be presented. References [1] Babu, R. P., O'connor, K., & Seeram, R. (2013). Current progress on bio-based polymers and their future trends. Progress in Biomaterials, 2(1), 8. [2] Duarte, A. R. C., Mano, J. F., & Reis, R. L. (2009). Perspectives on: supercritical fluid technology for 3D tissue engineering scaffold applications. Journal of Bioactive and Compatible Polymers, 24(4), 385-400. [3] Garcia-Gonzalez, C. A., Concheiro, A., & Alvarez-Lorenzo, C. (2015). Processing of materials for regenerative medicine using supercritical fluid technology. Bioconjugate chemistry, 26(7), 1159-1171. [4] Novendra, N., Hasirci, N., & Dilek, C. (2016). Supercritical processing of CO2-philic polyhedral oligomeric silsesquioxane (POSS)-poly(L-lactic acid) composites. The Journal of Supercritical Fluids, 117, 230-242. [5] Saini, P., Arora, M., & Kumar, M. R. (2016). Poly (lactic acid) blends in biomedical applications. Advanced drug delivery reviews, 107, 47-59. [6] Liao, X., Nawaby, A. V., & Whitfield, P. S. (2010). Carbon dioxide‐induced crystallization in poly (L‐lactic acid) and its effect on foam morphologies. Polymer international, 59(12), 1709-1718. [7] Kiran, E. (2010). Foaming strategies for bioabsorbable polymers in supercritical fluid mixtures. Part I. Miscibility and foaming of poly(L-lactic acid) in carbon dioxide+ acetone binary fluid mixtures. The Journal of Supercritical Fluids, 54(3), 296-307. [8] Kanya, B., & Dilek, C. (2015). Effects of functional groups on the solubilities of polyhedral oligomeric silsesquioxanes (POSS) in supercritical carbon dioxide. The Journal of Supercritical Fluids, 102, 17-23.