Failure mechanisms in fibrous scaffolds.

Polymeric fibrous scaffolds have been considered as replacements for load-bearing soft tissues, because of their ability to mimic the microstructure of natural tissues. Poor toughness of fibrous materials results in failure, which is an issue of importance to both engineering and medical practice. The toughness of fibrous materials depends on the ability of the microstructure to develop toughening mechanisms. However, such toughening mechanisms are still not well understood, because the detailed evolution at the microscopic level is difficult to visualize. A novel and simple method was developed, namely, a sample-taping technique, to examine the detailed failure mechanisms of fibrous microstructures. This technique was compared with in situ fracture testing by scanning electron microscopy. Examination of three types of fibrous networks showed that two different failure modes occurred in fibrous scaffolds. For brittle cracking in gelatin electrospun scaffolds, the random network morphology around the crack tip remained during crack propagation. For ductile failure in polycaprolactone electrospun scaffolds and nonwoven fabrics, the random network deformed via fiber rearrangement, and a large number of fiber bundles formed across the region in front of the notch tip. These fiber bundles not only accommodated mechanical strain, but also resisted crack propagation and thus toughened the fibrous scaffolds. Such understanding provides insight for the production of fibrous materials with enhanced toughness.

Failure mechanisms in fibrous scaffolds

Polymeric fibrous scaffolds have been considered as replacements for load-bearing soft tissues, because of their ability to mimic the microstructure of natural tissues. Poor toughness of fibrous materials results in failure, which is an issue of importance to both engineering and medical practice. The toughness of fibrous materials depends on the ability of the microstructure to develop toughening mechanisms. However, such toughening mechanisms are still not well understood, because the detailed evolution at the microscopic level is difficult to visualize. A novel and simple method was developed, namely, a sample-taping technique, to examine the detailed failure mechanisms of fibrous microstructures. This technique was compared with in situ fracture testing by scanning electron microscopy. Examination of three types of fibrous networks showed that two different failure modes occurred in fibrous scaffolds. For brittle cracking in gelatin electrospun scaffolds, the random network morphology around the crack tip remained during crack propagation. For ductile failure in polycaprolactone electrospun scaffolds and nonwoven fabrics, the random network deformed via fiber rearrangement, and a large number of fiber bundles formed across the region in front of the notch tip. These fiber bundles not only accommodated mechanical strain, but also resisted crack propagation and thus toughened the fibrous scaffolds. Such understanding provides insight for the production of fibrous materials with enhanced toughness. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Electrospun fiber - Hydrogel composites for nucleus pulposus tissue engineering

New materials are needed to replace degenerated intervertebral disc tissue and to provide longer-term solutions for chronic back-pain. Replacement tissue potentially could be engineered by seeding cells into a scaffold that mimics the architecture of natural tissue. Many natural tissues, including the nucleus pulposus (the central region of the intervertebral disc) consist of collagen nanofibers embedded in a gel-like matrix. Recently it was shown that electrospun micro- or nano-fiber structures of considerable thickness can be produced by collecting fibers in an ethanol bath. Here, randomly aligned polycaprolactone electrospun fiber structures up to 50 mm thick are backfilled with alginate hydrogels to form novel composite materials that mimic the fiber-reinforced structure of the nucleus pulposus. The composites are characterized using both indentation and tensile testing. The composites are mechanically robust, exhibiting substantial strain-to-failure. The method presented here provides a way to create large biomimetic scaffolds that more closely mimic the composite structure of natural tissue. © 2012 Materials Research Society.

Mechanical behaviour of electrospun fibre-reinforced hydrogels.

Mechanically robust and biomimicking scaffolds are needed for structural engineering of tissues such as the intervertebral disc, which are prone to failure and incapable of natural healing. Here, the formation of thick, randomly aligned polycaprolactone electrospun fibre structures infiltrated with alginate is reported. The composites are characterised using both indentation and tensile testing and demonstrate substantially different tensile and compressive moduli. The composites are mechanically robust and exhibit large strains-to-failure, exhibiting toughening mechanisms observed in other composite material systems. The method presented here provides a way to create large-scale biomimetic scaffolds that more closely mimic the composite structure of natural tissue, with tuneable tensile and compressive properties via the fibre and matrix phases, respectively.

Development of a nanoporous and multilayer drug-delivery platform for medical implants.

Biodegradable polymers can be applied to a variety of implants for controlled and local drug delivery. The aim of this study is to develop a biodegradable and nanoporous polymeric platform for a wide spectrum of drug-eluting implants with special focus on stent-coating applications. It was synthesized by poly(DL-lactide-co-glycolide) (PLGA 65:35, PLGA 75:25) and polycaprolactone (PCL) in a multilayer configuration by means of a spin-coating technique. The antiplatelet drug dipyridamole was loaded into the surface nanopores of the platform. Surface characterization was made by atomic force microscopy (AFM) and spectroscopic ellipsometry (SE). Platelet adhesion and drug-release kinetic studies were then carried out. The study revealed that the multilayer films are highly nanoporous, whereas the single layers of PLGA are atomically smooth and spherulites are formed in PCL. Their nanoporosity (pore diameter, depth, density, surface roughness) can be tailored by tuning the growth parameters (eg, spinning speed, polymer concentration), essential for drug-delivery performance. The origin of pore formation may be attributed to the phase separation of polymer blends via the spinodal decomposition mechanism. SE studies revealed the structural characteristics, film thickness, and optical properties even of the single layers in the triple-layer construct, providing substantial information for drug loading and complement AFM findings. Platelet adhesion studies showed that the dipyridamole-loaded coatings inhibit platelet aggregation that is a prerequisite for clotting. Finally, the films exhibited sustained release profiles of dipyridamole over 70 days. These results indicate that the current multilayer phase therapeutic approach constitutes an effective drug-delivery platform for drug-eluting implants and especially for cardiovascular stent applications.

Evaluation of the functionality of biodegradable polymeric platforms for drug delivery systems

We present the development of a drug-loaded triple-layer platform consisting of thin film biodegradable polymers, in a properly designed form for the desired gradual degradation. Poly(dl-lactide-co-glycolide) (PLGA (65:35), PLGA (75:25)) and polycaprolactone (PCL) were grown by spin coating technique, to synthesize the platforms with the order PCL/PLGA (75:25)/PLGA (65:35) that determine their degradation rates. The outer PLGA (65:35) layer was loaded with dipyridamole, an antiplatelet drug. Spectroscopic ellipsometry (SE) in the Vis-far UV range was used to determine the nanostructure, as well as the content of the incorporated drug in the as-grown platforms. In situ and real-time SE measurements were carried out using a liquid cell for the dynamic evaluation of the fibrinogen and albumin protein adsorption processes. Atomic force microscopy studies justified the SE results concerning the nanopores formation in the polymeric platforms, and the dominant adsorption mechanisms of the proteins, which were defined by the drug incorporation in the platforms. © 2013 Elsevier B.V. All rights reserved.

Mechanical behaviour of electrospun fibre-reinforced hydrogels

Mechanically robust and biomimicking scaffolds are needed for structural engineering of tissues such as the intervertebral disc, which are prone to failure and incapable of natural healing. Here, the formation of thick, randomly aligned polycaprolactone electrospun fibre structures infiltrated with alginate is reported. The composites are characterised using both indentation and tensile testing and demonstrate substantially different tensile and compressive moduli. The composites are mechanically robust and exhibit large strains-to-failure, exhibiting toughening mechanisms observed in other composite material systems. The method presented here provides a way to create large-scale biomimetic scaffolds that more closely mimic the composite structure of natural tissue, with tuneable tensile and compressive properties via the fibre and matrix phases, respectively. © 2014 Springer Science+Business Media New York.