Preparation of macroporous biodegradable poly(L-lactide-co-epsilon-caprolactone) foams and characterization by mercury intrusion porosimetry, image analysis, and impedance spectroscopy.

Two poly(L-lactide-co-epsilon-caprolactone) random copolymers containing 5 and 40 mol % of epsilon-CL, namely P(LA-co-CL(5)) and P(LA-co-CL(40)), respectively, have been made macroporous by freeze-drying solutions in dimethylcarbonate. Most of the freeze-dried foams, prepared by varying polymer concentration and cooling rate, exhibited two main pore populations: (1). longitudinally oriented tube-like macropores with diameters >or=100 microm, and (2). interconnected micropores (10-100 microm). Pore characteristics, including macropore density, mean diameter, and interdistance, as well as micropore density, area, and shape, were determined by image analysis of scanning electron micrographs in order to study the influence of processing and formulation parameters on foam structure and properties. The pore orientation and the 3-D texture also were studied by image analysis and impedance spectroscopy. In the case of the P(LA-co-CL(5)), the macropore diameter increased with the cooling rate while the micropore diameter decreased. The micropores also became more circular when the cooling rate was increased. The pore size and morphology of the P(LA-co-CL(40)) were quite unchanged by varying the cooling rate. All the other conditions being the same, the P(LA-co-CL(5)) foams were better organized than the P(LA-co-CL(40)) foams, and pore orientation was improved at the higher cooling rate. Pore size and morphology also can be controlled by changing the polymer concentration (Cp), as we showed by studying P(LA-co-CL(5)) foams prepared by freeze-drying solutions in the 1-10 w/v % Cp range. Macropore density, average diameter, and interdistance of P(LA-co-CL(5)) foams increased with Cp, but the micropore characteristics remained almost unchanged no matter the Cp. The reliability of the characterization methods has been discussed, with special attention to mercury intrusion porosimetry, which is used primarily for measurement of pore volume and pore size distribution. However, this technique is reported here as a destructive and unreliable method for the characterization of fragile P(LA-co-CL(40)) foams. This study shows that image analysis and impedance spectroscopy can give reliable information relative to the pore morphology and anisotropy of freeze-dried foams.

Development of a three-dimensional transmigration assay for testing cell--polymer interactions for tissue engineering applications.

The ability of synthetic or natural scaffolds to support invasion of cells from surrounding tissue is a key parameter for tissue engineering (TE). In this study, the migration of fibroblasts, chondrocytes, and osteoblasts into biodegradable polymer scaffolds was evaluated using a novel, three-dimensional (3-D) transmigration assay. This assay is based on a cell-populated contracted collagen lattice with a biodegradable polymer scaffold implanted at the center of the collagen gel. Cell migration into the scaffolds was assessed both quantitatively and qualitatively following various time lengths in culture using image analysis. Chondrocytes, incorporated within the collagen lattice, migrated into polymer scaffolds, when cultured both statically or in a rotating bioreactor. However, the bioreactor cultures resulted in a significantly greater cell invasion as compared to static cultures. There was a cell density-dependent osteoblast migration from collagen lattice into polymer scaffold, when tested in the transmigration assay. In addition, polymer scaffolds, treated with or without recombinant human platelet-derived growth factor (rh-PDGF-BB) were evaluated for fibroblast migration. The presence of rh-PDGF-BB resulted in significantly greater fibroblast invasion as compared to untreated scaffolds. Our studies suggest that the transmigration model provides a rapid system for testing cell invasion of potential scaffolds for tissue engineering applications.