Failure mechanisms in fibrous scaffolds
Introduction
Polymeric fibrous scaffolds have been studied extensively, owing to their promise as tissue engineering scaffolds for load-bearing soft tissues [10] such as the annulus fibrosus of the intervertebral disc [2], cartilage, blood vessels [4] and amniotic membrane [5]. Their material structure mimics the hierarchical structure of load-bearing soft tissues at three levels: polymer chains in a single fiber at a molecular scale; fibrous networks at a microscopic scale; and bulk membranes at a macroscopic scale.
The characteristics of microscopic fibrous network architecture such as fiber orientation [6], fiber density [7] and cross-link density [8] influence the deformation properties of the material. Through an understanding of the relationship between microstructural features and deformation, useful guidelines can be provided to reproduce various aspects of mechanical behavior for tissue engineering. For instance, highly anisotropic materials with similar tensile stiffness to annulus fibrosus have been produced by controlling fiber orientation [2], and other fibrous networks approaching the biaxial stiffness of blood vessels have been reproduced by mimicking the network, with fibers aligning in the helical direction [9].
An understanding of failure mechanisms is crucial in the study of fracture [10]. The toughness of a material depends on the ability of the microstructure to dissipate energy without propagation of a crack. Therefore, an understanding of the failure mechanisms presented in fibrous networks can provide insights into the production of tissue-engineering scaffolds with improved toughness. It also provides basic physical understanding of structural failure in diseases and conditions that involve soft tissue failure.
The incomplete knowledge of toughening mechanisms in fibrous networks is due to fact that the microstructure evolution during failure process is difficult to visualize. Current imaging techniques such as small angle light scattering (SALS) [16], [17], polarized light microscopy [18], confocal microscopy and digital image correlation [19] have been used to characterize microstructural morphology, including the fiber distribution. There are two limitations to these studies. First, the detailed microstructural fibrous network features, including fiber diameter and network bonding, are hard to see. For instance, mean scattered light distribution identifies fiber orientation, but is unable to show detail at the level of an individual fiber. Second, it is difficult to maintain the deformed configuration of the microstructure during visualization, and special apparatus is needed to fit within imaging tools to stretch the sample in situ.
The objective of the work presented here is to examine the failure mechanisms of fibrous scaffolds at both macroscopic and microscopic scales. Uniaxial tensile tests and fracture tests were first performed. The detailed toughening mechanisms at the notch front were then examined by scanning electron microscopy (SEM) and optical microscopy. A novel and simple method, namely a sample-taping technique, was developed to capture the deformed microstructures by SEM. Further, in situ fracture testing by SEM was also performed to compare with the sample-taping technique. This study provides a basic understanding of the toughening mechanisms present in fibrous networks, by considering the non-linear and non-affine deformation of the fibers present in the network. Such understanding facilitates the production of engineering fibrous materials with enhanced toughness.
Section snippets
Sample preparation
The three types of polymeric fibrous scaffolds studied in this paper are gelatin electrospun scaffolds, polycaprolactone (PCL) electrospun scaffolds and nonwoven fabrics; these three materials have different material length scales and network properties. Gelatin, PCL and nonwoven fibers have diameters of ∼80 nm, 1and 20 μm, respectively. The undeformed scaffolds of both gelatin and PCL electrospun scaffolds are random networks, while the undeformed nonwoven fabric is slightly oriented. The
Toughening at macroscopic scales
During fracture tests, cracks in electrospun gelatin remained small (Fig. 2). Once cracks propagated, the samples broke within 1 s (an increase in strain Δε = 0.25%). The small red markers on the samples show qualitatively the local deformation of the material. From examination of the markers on samples corresponding to an increasing strain, little vertical deformation occurred.
Notches in PCL electrospun scaffolds and nonwoven fabrics blunted instead of propagating (Fig. 2) during fracture tests.
Visualization techniques
A novel visualization technique, namely a sample-taping technique, was developed to visualize the deformation of fibrous microstructures. This technique is based on the assumption that the random morphology of undeformed fibrous scaffolds is consistent across samples. Instead of stretching one sample in an imaging device, multiple samples were stretched to the assigned single strains. Their deformation configurations were retained by adhesive tape prior to visualization. The sample-taping
Conclusions
Electrospinning has emerged as a leading technique for producing tissue engineering scaffolds, owing to its ability to mimic random fibrous networks on the order of nanometers found in natural collagenous soft tissues. Both brittle and ductile failure has been demonstrated here in electrospun scaffolds. Brittle cracking in gelatin scaffolds illustrated poor toughness, leading to material failure. In contrast, toughening mechanisms in PCL scaffolds provide inspiration for the design of tough
Acknowledgements
The authors acknowledge the Ministry of Higher Education Malaysia and the Cambridge Commonwealth Trust for funding support, Dr. Michael Sutcliffe for providing the nonwoven fabric, Anne Bahnweg for the support in the scanning electron microscope visualization, Alan Heaver for making the custom-built tensile test device used in the in situ testing, and Prof. Vikram S. Deshpande for helpful discussion.
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