Modulation of anisotropy in electrospun tissue-engineering scaffolds: Analysis of fiber alignment by the fast Fourier transform

Abstract

We describe the use of the fast Fourier transform (FFT) in the measurement of anisotropy in electrospun scaffolds of gelatin as a function of the starting conditions. In electrospinning, fiber alignment and overall scaffold anisotropy can be manipulated by controlling the motion of the collecting mandrel with respect to the source electrospinning solution. By using FFT to assign relative alignment values to an electrospun matrix it is possible to systematically evaluate how different processing variables impact the structure and material properties of a scaffold. Gelatin was suspended at varying concentrations (80, 100, 130, 150 mg/ml) and electrospun from 2,2,2 trifluoroethanol onto rotating mandrels (200-7000 RPM). At each starting concentration, fiber diameter remained constant over a wide range of mandrel RPM. Scaffold anisotropy developed as a function of fiber diameter and mandrel RPM. The induction of varying degrees of anisotropy imparted distinctive material properties to the electrospun scaffolds. The FFT is a rapid method for evaluating fiber alignment in tissue-engineering materials.

Fig. 1

Representative SEM images of a random (A) and an aligned matrix (D). FFT output images (B and E) and radial projection (B). Pixel intensity plots against the angle of acquisition for a random matrix (C) and an aligned matrix (F). Note the distinctive peak produced by the image containing aligned information (F).

Fig. 2

Fiber diameter (A) and pore area (D) as a function of mandrel RPM. Bars at 200 RPM indicate three classes of fibers (A) and pore areas (D) detected by statistical analysis (P<0.001). Inset (B), second-order modeling of fiber diameter as a function of starting concentration at 200 RPM, R² = 0.56. Inset (C), regression analysis of the relationship between starting concentration and solution viscosity, first-order R² = 0.98, second-order R² = 0.99 (depicted). Inset (E), first-order modeling of pore area as a function of fiber diameter at 200 RPM, R² = 0.33. Error = bars B and E = ±standard deviation.

Fig. 3

FFT analysis of scaffold structure as a function of mandrel RPM. Scaffolds from suspensions of 80 mg/ml (A), 100 mg/ml (B), 130 mg/ml (C) and 150 mg/ml (D). Total area under the curve for frequency plots (E).

Fig. 4

FFT analysis of brightfield, confocal Z-stack average and confocal Z-stack maximum projections of similar image fields (A). FFT analysis of SEM images as a function of SEM magnification (B). Cross sectional analysis of random (C and D) and aligned (E and F) electrospun scaffolds. Scale Bar D = 5 μm, F = 10 μm.

Fig. 5

Material properties as a function of mandrel rotation. Ratio (in the parallel to perpendicular orientations) of stress (A) and strain (B) at failure. Average stress (C) and strain (D) at failure in scaffolds deposited at 200 RPM. * = bias in the parallel orientation, P = bias in the perpendicular orientation (P<0.05). Error bars = ± standard error. Table E = average stress values, F = average strain values as a function of starting concentration and direction of testing.