Lee Wright

Fabrication and mechanical characterization of 3D electrospun scaffolds for tissue engineering

Electrospinning is a polymer processing technique that produces fibrous structures comparable to the extracellular matrix of many tissues. Electrospinning, however, has been severely limited in its tissue engineering capabilities because this technique has produced few three-dimensional structures. Sintering of electrospun materials provides a method to fabricate unique architectures and allow much larger structures to be made. Electrospun mats were sintered into strips and cylinders, and their tensile and compressive mechanical properties were measured. In addition, electrospun materials with salt pores (salt embedded within the material and then leached out) were fabricated to improve porosity of the electrospun materials for tissue engineering scaffolds. Sintered electrospun poly(D,L-lactide) and poly(L-lactide) (PDLA/PLLA) materials have higher tensile mechanical properties (modulus: 72.3 MPa, yield: 960 kPa) compared to unsintered PLLA (modulus: 40.36 MPa, yield: 675.5 kPa). Electrospun PDLA/PLLA cylinders with and without salt-leached pores had compressive moduli of 6.69 and 26.86 MPa, respectively, and compressive yields of 1.36 and 0.56 MPa, respectively. Sintering of electrospun materials is a novel technique that improves electrospinning application in tissue engineering by increasing the size and types of electrospun structures that can be fabricated.

Tissue Engineering of the Anterior Cruciate Ligament: The Viscoelastic Behavior and Cell Viability of a Novel Braid–Twist Scaffold

The anterior cruciate ligament (ACL) is the most commonly injured ligament of the knee; it also contributes to normal knee function and stability. Due to its poor healing potential severe ACL damage requires surgical intervention, ranging from suturing to complete replacement. Current ACL replacements have a host of limitations that prevent their extensive use. Investigators have begun to utilize tissue-engineering techniques to create new options for ACL repair, regeneration and replacement. In this study we tested novel braid–twist scaffolds, as well as braided scaffolds, twisted fiber scaffolds and aligned fiber scaffolds, for use as ACL replacements composed of poly(L-lactic acid) fibers. Scaffolds were examined using stress relaxation tests, cell viability assays and scanning electron microscopy. The behaviors of the braid–twist scaffolds were modeled with Maxwell and quasi-linear viscoelastic (QLV) models. In stress relaxation tests, the braid–twist scaffolds behaved similarly to native ACL tissue, with final normalized stresses of 87% and 83% after an 8 N load. There was agreement between the experimental data and the Maxwell model when the model included an element for each structural element in the scaffold. There was also agreement between the experimental data and QLV model, scaffolds with similar braiding angles shared constants. In cell proliferation studies no differences were found between fibroblast growth on the braided scaffolds and the braid–twist scaffolds. SEM images showed the presence of new extracellular matrix. Data from this and previous tensile studies demonstrate that the braid–twist scaffold design may be effective in scaffolds for ACL tissue regeneration.

Rapid mineralization of electrospun scaffolds for bone tissue engineering.

We investigated different techniques to enhance calcium phosphate mineral precipitation onto electrospun poly(L-lactide) (PLLA) scaffolds when incubated in concentrated simulated body fluid (SBF), 10×SBF. The techniques included the use of vacuum, pre-treatment with 0.1 M NaOH and electrospinning gelatin/PLLA blends as means to increase overall mineral precipitation and distribution throughout the scaffolds. Mineral precipitation was evaluated using environmental scanning electron microscopy, energy dispersive spectroscopy mapping and the determination of the mineral weight percents. In addition we evaluated the effect of the techniques on mechanical properties, cellular attachment and cellular proliferation on scaffolds. Two treatments, pre-treatment with NaOH and incorporation of 10% gelatin into PLLA solution, both in combination with vacuum, resulted in significantly higher degrees of mineralization (16.55 and 15.14%, respectively) and better mineral distribution on surfaces and through the cross-sections after 2 h of exposure to 10×SBF. While both scaffold groups supported cell attachment and proliferation, 10% gelatin/PLLA scaffolds had significantly higher yield stress (1.73 vs 0.56 MPa) and elastic modulus (107 vs 44 MPa) than NaOH-pre-treated scaffolds.

PDLA/PLLA and PDLA/PCL nanofibers with a chitosan‐based hydrogel in composite scaffolds for tissue engineered cartilage

Osteoarthritis (OA) is the most prevalent musculoskeletal disease in humans, causing pain, loss of joint motility and function, and severely reducing the standard of living of patients. Cartilage tissue engineering attempts to repair the damaged tissue of individuals suffering from OA by providing mechanical support to the joint as new tissue regenerates. The aim of this study was to create composite three dimensional scaffolds comprised of electrospun poly(D,L‐lactide)/poly(L‐lactide) (PDLA/PLLA) or poly(D,L‐lactide)/polycaprolactone (PDLA/PCL) with salt leached pores and an embedded chitosan hydrogel to determine the potential of these scaffolds for cartilage tissue engineering. PDLA/PLLA‐hydrogel scaffolds displayed the largest compressive moduli followed by PDLA/PCL‐hydrogel scaffolds. Dynamic mechanical tests showed that the PDLA/PLLA scaffolds had no appreciable recovery while PDLA/PCL scaffolds did exhibit some recovery. Primary canine chondrocytes produced both collagen type II and proteoglycans (primary components of extracellular matrix in cartilage) while being cultured on scaffolds composed of electrospun PDLA/PCL. As a result, a composite electrospun embedded hydrogel scaffold shows promise for treating individuals suffering from OA. Copyright

Fabrication and characterization of three-dimensional electrospun scaffolds for bone tissue engineering†

When traumatic injury, tumor removal, or disease results in significant bone loss, reconstructive surgery is required. Bone grafts are used in orthopedic reconstructive procedures to provide mechanical support and promote bone regeneration. In this study, we applied a heat sintering technique to fabricate 3D electrospun scaffolds that were used to evaluate effects of mineralization and fiber orientation on scaffold strength. We electrospun PLLA/gelatin scaffolds with a layer of PDLA and heat sintered them into three-dimensional cylindrical scaffolds. Scaffolds were mineralized by incubation in 10× simulated body fluid for 6, 24, and 48 h to evaluate the effect of mineralization on scaffolds compressive mechanical properties. The effects of heat sintering hydroxyapatite (HA) microparticles directly to the scaffolds on mineral deposition, distribution and mechanical properties of the scaffolds were also evaluated. We found that orientation of the fibers had little effect on the compressive mechanical properties of the scaffolds. However, increasing the mineralization times resulted in an increase in compressive mechanical properties. Also, the direct addition of HA microparticles had no effect on the scaffold mechanical properties, but had a significant effect on the mineral deposition on PLLA/gelatin scaffolds.

Poly(d-lactide)/poly(caprolactone) nanofiber-thermogelling chitosan gel composite scaffolds for osteochondral tissue regeneration in a rat model

Macroporous nanostructured scaffolds that can be made to closely mimic skeletal tissue extracellular matrix as well as have the potential to support bone and cartilage tissue regeneration. Porous poly(d-lactide)/poly(caprolactone) nanofiber scaffolds were prepared by electrospinning respective polymer solutions along with salt crystals, which were sintered into fiber mats into cylindrical shape of 1.5 mm diameter and cut into 2–3 mm length followed by salt leaching in distilled water. The poly(d-lactide)/poly(caprolactone)–chitosan composite scaffolds were prepared by impregnating the porous structure of the electrospun scaffold with a thermosensitive chitosan solution. For in vivo evaluation, the scaffolds with and without chitosan gel were press fitted into osteochondral defects in a rat model. Hematoxylin and eosin staining 6 weeks post implantation showed new bone formation within the porous scaffolds with and without chitosan gel. Significant bone formation was observed within both the scaffolds at 15 weeks post implantation compared to the control group. The results show that macroporous poly(d-lactide)/poly(caprolactone) nanofiber scaffolds can be prepared with and without chitosan hydrogel and can serve as an osteochondral scaffold. The porous scaffolds showed the ability to promote new bone formation at the defect site, and incorporation of chitosan within the pores did not adversely affect the tissue in-growth. However, the scaffolds did not support significant cartilage formation even after 15 weeks, which indicates the need for the addition of cells or bioactive molecules within the scaffold to support effective osteochondral tissue regeneration.

Fusion of Electrospun Mats

Electrospinning has become a popular technique investigated for tissue engineering applications. Electrospinning is unique because it is capable of producing small fibers (diameters ranging from several nanometers to several microns) in a nonwoven mat, which is similar to the architecture of the extracellular matrix (ECM) within tissues [1]. The microenvironment of the cells should be analogous to that of native tissue so that cellular growth and function are improved.Copyright

A Novel Braid-Twist Scaffold for ACL Repair: Control of Stress-Relaxation Properties

The anterior cruciate ligament (ACL) is critical for knee stability when walking or running. Unfortunately, it does not heal well after significant tearing or rupture and surgery is often necessary to reconstruct the injured ligament. Though ACL ruptures are quite common, the surgical repair of this ligament has inconsistent success rates [1]. The goal of this study was to characterize a biomimetic tissue engineered ACL scaffold using a novel combination braid-twist technique. The braid-twist scaffolds were made using the following procedure:• Nine groups of six 160 mm length PLLA fibers were selected.• Each group of six fibers was twisted in a counter-clockwise manner to form a fiber bundle (a total of nine fiber bundles/scaffold).• Three of these bundles were twisted around one another counter-clockwise to form a yarn (a total of three yarns/scaffold).• These three yarns were braided together to form one scaffold. This technique is based on the structure of ACL tissue and is designed to reduce scaffold fatigue and accurately mimic ACL behavior.© 2008 ASME