Control of pore size and structure of tissue engineering scaffolds produced by supercritical fluid processing

Abstract

Tissue engineering scaffolds require a controlled pore size and structure to host tissue formation. Supercritical carbon dioxide (scCO 2 ) processing may be used to form foamed scaffolds in which the escape of CO 2 from a plasticized polymer melt generates gas bubbles that shape the developing pores. The process of forming these scaffolds involves a simultaneous change in phase in the CO 2 and the polymer, resulting in rapid expansion of a surface area and changes in polymer rheological properties. Hence, the process is difficult to control with respect to the desired final pore size and structure. In this paper, we describe a detailed study of the effect of polymer chemical composition, molecular weight and processing parameters on final scaffold characteristics. The study focuses on poly(DL-lactic acid) (P DL LA) and poly(DL-lactic acid-coglycolic acid) (PLGA) as polymer classes with potential application as controlled release scaffolds for growth factor delivery. Processing parameters under investigation were temperature (from 5 to 55 o C) and pressure (from 60 to 230 bar). A series of amorphous P DL LA and PLGA polymers with various molecular weights (from 13 KD to 96 KD) and/or chemical compositions (the mole percentage of glycolic acid in the polymers was 0, 15, 25, 35 and 50 respectively) were employed. The resulting scaffolds were characterised by optical microscopy, scanning electron microscopy (SEM), and micro X-ray computed tomography (µCT). This is the first detailed study on using these series polymers for scaffold formation by supercritical technique. This study has demonstrated that the pore size and structure of the supercritical P DL LA and PLGA scaffolds can be tailored by careful control of processing conditions. Key Words: poly(DL-lactic acid) (P DL LA), poly(lactic acidco-glycolic acid) (PLGA), supercritical carbon dioxide (scCO 2 ), plasticization, foaming, scaffolds *Address for correspondence: Steven M. Howdle School of Chemistry The University of Nottingham University Park Nottingham, NG 7 2RD, UK Email: steve.howdle@nottingham.ac.uk IntroductionIn tissue engineering, a porous scaffold is required to act as a template and guide for cell proliferation, differentiation and tissue growth. Scaffolds may also act as controlled release devices that deliver growth factors with rates matching the physiological need of the regenerating tissue (Langer, 1998). Poly(lactic acid) (PLA) and associated poly(lactic acid-co-glycolic acid) (PLGA) copolymers are commonly used biodegradable polymers for fabricating tissue engineering porous scaffolds. PLGA copolymers with various polymer compositions (the ratio of lactic acid and glycolic acid content in the polymer) degrade at different rates. Therefore, it is of great interest using PLGA copolymers to make scaffolds for various applications. These polymers degrade in vivo and eventually disappear at a desired rate while the native tissues grow and the degradation residues are discharged through rental filtration. Moreover, the release of encapsula...

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