Linus H. Leung

Comparison of morphology and mechanical properties of PLGA bioscaffolds

In this study, bioscaffolds using poly(DL-lactide-co-glycolide) acid (PLGA) were fabricated and studied. The gas foaming/salt leaching technique in a batch foaming setup was employed, and the effects of material composition of PLGA on the morphology and mechanical properties using this process were investigated. Two material compositions of PLGA 50/50 and 85/15 were used, and characterization of scaffolds fabricated with these materials showed that a lower relative density can be achieved with an increasing poly(DL-lactide) acid (PDLLA) content; however, higher open-cell porosity was obtained with lower PDLLA content. Furthermore, the effect of PLGA composition on modulus of the scaffolds was minor.

A Parametric Study on the Processing and Physical Characterization of PLGA 50/50 Bioscaffolds

The ability to control the characteristics of scaffolds is important such that scaffolds can be fine tuned for specific applications. In this study, the effects of processing parameters on cell morphology and mechanical properties of PLGA 50/50 bioscaffolds for tissue engineering applications were investigated. Specifically, the effects of salt particle sizes and salt-to-polymer mass ratios on the scaffold relative density, average pore size and density, open-cell porosity, and mechanical properties were examined. The PLGA samples were processed using a salt leaching technique in a batch-foaming setup. Experiments showed that pore size and density were dependent on the salt particle size used, and that as the salt-to-polymer mass ratio increased, the porosity increased while the relative density decreased. The results showed that by varying the salt parameters during fabrication, the scaffold characteristics and morphology can be controlled.

A study of the mechanics of porous PLGA 85/15 scaffold in compression

Constitutive modelling of the stress-strain relationship of open-celled PLGA 85/15 bioscaffolds under compression was studied. A constitutive model for compressive behaviour was directly derived from the morphology of a unit cubic cell. These constitutive equations describe the stress-strain relationship as a function of the foams material properties and cell morphology, such as elastic modulus, yield stress, relative density, cell strut thickness, and cell size. To verify this model, uniaxial compression testing was performed on scaffold samples. Using the gas foaming/salt leaching method, the samples were prepared by using different foaming parameters such as salt/ polymer mass ratio, saturation pressure, and saturation time. The comparisons of theoretical and experimental data demonstrate that the constitutive model using a cubic unit cell accurately describes the behaviour of PLGA foams with low relative densities under compression.

Characterizing the viscoelastic behaviour of poly(lactide-co-glycolide acid)–hydroxyapatite foams

The viscoelasticity of poly(lactide-co-glycolid acid) 75/25 and hydroxyapatite composite open-pore foams were examined in this study. The foams were fabricated using the gas foaming/salt leaching process, and two specific experiments were performed. The first was to test the viscoelastic dependency of the foams on the load frequency, and the second was to measure the creep of the foams under a static load. The experiments showed that the viscoelasticity of the foams did not vary with the addition nano-hydroxyapatite particles, but the environment of testing significantly affected these properties. These results can be used to better understand the behaviour of the scaffolds when in the physiological environment.

Novel Fabrication Technique for 3-Dimensional Electrospun Poly(DL-Lactide-Co-Glycolide) Acid Scaffolds

Current tissue engineering scaffolds created by electrospinning techniques are mostly limited to a 2D membrane. In this study, a novel method to fabricate 3D fibrous scaffolds is presented, and a parametric study to identify the optimal processing parameters was performed. The fabrication technique uses a batch foaming setup to saturate the fibrous scaffolds with CO2 to lower the glass transition temperature of PLGA to allow for the sintering of the fibers. When a mechanical pressure was applied to multiple layers of the thin films in the presence of gas, the layers of PLGA scaffolds sinter to form a thick 3D fibrous structure. This study was divided into three parts. First, the effect of gas saturation on the scaffolds was examined. Three saturation pressures of 200, 300, and 400 psi and multiple saturation times of 1, 3, 5, 30, 60, and 120 minutes were used. At the lowest pressure of 200 psi, the morphology and mechanical properties of the scaffolds were not affected. As saturation pressure and time were increased, the fibers sintered, and eventually the fibrous structure was lost because the polymer was over-sintered. The second part of the study was to determine the adhesion properties of the scaffolds using gas pressure. The same processing pressures and times were used for this set of experiments, and a higher pressure was found to better adhere the layers of PLGA films together. From the first two parts of this study, the optimal combination of processing parameters was determined to be saturating the samples under 400 psi of pressure for three minutes. This set of parameters was then used in the third part of the study to fabricate 3D fibrous scaffolds. The demonstration of this ability to fabricate 3D scaffolds improves on current electrospinning techniques while maintaining a desirable fibrous structure for tissue engineering.© 2010 ASME

Physical and Mechanical Properties of Poly(E-Caprolactone)–Hydroxyapatite Composites for Bone Tissue Engineering Applications

Tissue engineering using bioscaffolds is a promising technique that may provide new treatments for various diseases and injuries. These bioscaffolds can be temporary or permanent materials that can be implanted in a patient for tissue repair. Poly(e-caprolactone) (PCL) is a biocompatible and biodegradable polymer, and hence suitable for usage in tissue engineering applications. Depending on the targeted tissue to be repaired, scaffold properties need to be altered to match that of the tissue. In load bearing applications, such as bone repair, the mechanical properties need to be sufficiently high to prevent material failure. To strengthen the scaffold, various composites have been proposed in the literature, and one of these composites includes PCL with hydroxyapatite (HA). To be able to control the processing of these materials into scaffolds, the characterization of fundamental material properties need to be investigated. In this study, the physical, thermal, mechanical, and viscoelastic properties of PCL:HA at three different weight compositions of 80:20, 70:30, and 60:40 wt% were characterized and compared to neat PCL. PCL/HA composites were fabricated by blending using a twin-screw compounder, and disc-shaped samples were fabricated by compression molding at an elevated temperature. Analysis using a differential scanning calorimeter demonstrated that the glass transition and melting temperatures of the composites remained nearly unaffected by the HA content at −56 °C and 56 °C, respectively; however, depending on the cooling method used for processing, the degree of crystallinity can be controlled. Thermogravimetric analysis was also performed to study the thermal degradation profile. PCL-HA composite samples were tested in compression to determine the effects of HA content on the mechanical properties. Compared to neat PCL, incorporating HA at 40 wt% increased the modulus nearly twofold from 85 to 155 MPa. Lastly, to study the viscoelastic properties of the solid materias, frequency dependency and creep experiments were performed using a dynamic mechanical analyzer. The composites at high HA concentrations were more compliant to creep and other viscoelastic effects. The results found in this study are important in developing novel processing techniques or scaffolds and in controlling final scaffold properties such that any desired properties may be fabricated.Copyright

Constitutive Modeling for Porous PLGA 85/15 Scaffold in Compression

Constitutive modeling of stress-strain relationship of open-celled PLGA 85/15 foams under compression was studied. A constitutive model for compressive behavior was directly derived from the morphology of a unit cubic cell. These constitutive equations describe the stress-strain relationship as a function of the foams material properties and cell morphology, such as elastic modulus, yield stress, relative density, cell strut thickness, and cell size. To verify this model, uniaxial compression testing was performed on foam samples. Using the gas foaming/salt leaching method, the samples were prepared by using different foaming parameters such as salt/polymer mass ratio, saturation pressure, and saturation time. The comparisons of theoretical and experimental data demonstrate that the constitutive model using a cubic unit cell accurately describes the behavior of PLGA foams with low relative densities under compression.Copyright

Comparative Study on the Effect of Filler Size on the Mechanical Properties of Poly(DL-Lactide-Co-Glycolide) Acid – Hydroxyapatite Composite Scaffolds

Tissue engineering is an emerging alternative treatment for various diseases and injuries. The use of hydroxapatite composites for bioscaffolds has shown to improve the osteoconductivity. Studies in the literature have been performed to examine the advantages of using this composite in the biological aspects of tissue engineering; however, few studies have been on the synthesis and mechanical properties of these bioscaffolds. These properties are especially important in load bearing applications like in bone tissue engineering. In this study, poly(DL-lactide-co-glycolide) acid (PLGA) and hydroxyapatite (HA) were used to fabricate gas-foamed/salt leached scaffolds. Micro and nano-scaled HA particles were melt-compounded with PLGA using a twin-screw compounder. The composite material was dry blended with salt particles at a salt-to-polymer mass ratio of 5:1. From observations of micrographs taken with a scanning electron microscope, both the micro and nano-HA particles were shown to be well dispersed within the polymer matrix. The thermal properties were examined using differential scanning calorimetry, and the effect of the compositions on the glass transition temperature, Tg , was studied. The Tg was not significantly changed with the addition of HA particles from 0 to 20 wt%. Similarly, thermogravimetric analysis showed that the thermal degradation temperature remained at approximately 320 °C. The mechanical properties of the scaffolds were tested in compression. At the same composition, the scaffolds fabricated with the nano-HA showed an increase in modulus and strength. The results found in this study are important in developing novel biodegradable scaffolds with a focus on improving the mechanical properties of the scaffolds for bone tissue engineering applications.Copyright