Samuel VanGordon

Computational modeling of flow-induced shear stresses within 3D salt-leached porous scaffolds imaged via micro-CT

Flow-induced shear stresses have been found to be a stimulatory factor in pre-osteoblastic cells seeded in 3D porous scaffolds and cultured under continuous flow perfusion. However, due to the complex internal structure of porous scaffolds, analytical estimation of the local shear forces is impractical. The primary goal of this work is to investigate the shear stress distributions within Poly(l-lactic acid) scaffolds via computation. Scaffolds used in this study are prepared via salt leeching with various geometric characteristics (80-95% porosity and 215-402.5microm average pore size). High resolution micro-computed tomography is used to obtain their 3D structure. Flow of osteogenic media through the scaffolds is modeled via lattice Boltzmann method. It is found that the surface stress distributions within the scaffolds are characterized by long tails to the right (a positive skewness). Their shape is not strongly dependent on the scaffold manufacturing parameters, but the magnitudes of the stresses are. Correlations are prepared for the estimation of the average surface shear stress experienced by the cells within the scaffolds and of the probability density function of the surface stresses. Though the manufacturing technique does not appear to affect the shape of the shear stress distributions, presence of manufacturing defects is found to be significant: defects create areas of high flow and high stress along their periphery. The results of this study are applicable to other polymer systems provided that they are manufactured by a similar salt leeching technique, while the imaging/modeling approach is applicable to all scaffolds relevant to tissue engineering.

Enhanced osteoblastic differentiation of mesenchymal stem cells seeded in RGD-functionalized PLLA scaffolds and cultured in a flow perfusion bioreactor

The present study combines chemical and mechanical stimuli to modulate the osteogenic differentiation of mesenchymal stem cells (MSCs). Arg–Gly–Asp (RGD) peptides incorporated into biomaterials have been shown to upregulate MSC osteoblastic differentiation. However, these effects have been assessed under static culture conditions, while it has been reported that flow perfusion also has an enhancing effect on MSC osteoblastic differentiation. It is clear that there is a need to combine RGD modification of biomaterials with mechanical stimulation of MSCs via flow perfusion and evaluate its effects on MSC differentiation down the osteogenic lineage. In this study, the effect of different levels of RGD modification of poly(L‐lactic acid) scaffolds on MSC osteogenesis was evaluated under conditions of flow perfusion. It was found that there is a synergistic enhancement of different osteogenic markers, due to the combination of flow perfusion and RGD surface modification when compared to their individual effects. Furthermore, under conditions of flow perfusion, there is an RGD surface concentration optimal for differentiation, and it is flow rate‐dependent. This report underlines the significance of incorporating combined biomimesis via biochemical and mechanical microenvironments that modulate in vivo cell behaviour and tissue function for more efficient tissue‐engineering strategies. Copyright

Distribution of flow-induced stresses in highly porous media

Simulation results for the distribution of flow-induced wall stresses within 36 different porous scaffolds with porosity larger than 80% indicate that the normalized wall stress follows a single gamma distribution. Experimental and computational results for scaffolds prepared via different techniques from different materials and by other laboratories follow the same distribution. The form of this universal distribution is offered, as well as the methodology to obtain it for laminar flow through high porosity materials.

Predicting the stress distribution within scaffolds with ordered architecture

Current tissue engineering technologies involve the seeding of cells on porous scaffolds, within which the cells can proliferate and differentiate, when cultured in bioreactors. The flow of culture media through the scaffolds generates stresses that are important for both cell differentiation and cell growth. A recent study [Appl. Phys. Lett. 97 (2010), 024101] showed that flow-induced stresses inside highly porous and randomly structured scaffolds follow a three-point gamma probability density function (p.d.f.). The goal of the present study is to further investigate whether the same p.d.f. can also describe the distribution of stresses in structured porous scaffolds, what is the range of scaffold porosity for which the distribution is valid, and what is the physical reason for such behavior. To do that, the p.d.f. of flow-induced stresses in different scaffold geometries were calculated via flow dynamics simulations. It was found that the direction of flow relative to the internal architecture of the scaffolds is important for stress distributions. The stress distributions follow a common distribution within statistically acceptable accuracy, when the flow direction does not coincide with the direction of internal structural elements of the scaffold.

Mechanical and in vitro investigation of a porous PEEK foam for medical device implants

Purpose Implantable-grade polyetheretherketone (PEEK-OPTIMA®) is a high-performance thermoplastic that has been used in implant devices such as spinal-fusion cages since its introduction in 1999. Here, a new porous PEEK version was investigated. Methods Porous PEEK was fabricated using industrial scale relevant methods of compounding with porogen filler, extrusion, and subsequent extraction with water at supercritical temperatures and pressures. Mechanical properties were assessed according to ISO standards. Marrow stromal cells were cultured on porous PEEK samples and in vitro cytocompatibility was assessed by total DNA, alkaline phosphatase activity, osteopontin, calcium, and cell morphology to indicate stages of proliferation, differentiation, and mineralization. Compressive strength was assessed statically on 21 day cell cultures and media-soaked samples and dynamically within a medical device application specific context for interbody fusion cages (ASTM F2077). Results Manufacturing resulted in a biomaterial with ∼50% porosity and a mean pore size of 100 microns. The porous PEEK was found to have: tensile strength (14.5MPa), strain at break (3.5%), impact strength (3.6 kJ/m2), fexural strength (21.6MPa), and fexural modulus (0.8GPa). Production of extracellular mineralized matrix occurred very early in the culture period, indicating a preferred surface for differentiation. SEM images revealed polygonal cell morphology supporting a differentiated osteoblastic-like phenotype. EDS analysis detected levels of carbon, phosphorus, and calcium coinciding with assay results for the proliferation and differentiation stages. Conclusion Previous observations of cytocompatibility and calcification on the PEEK biomaterial could be carried through to this new porous form of the PEEK biomaterial. This helps porous PEEK to potentially offer more design options for implant devices requiring reduced modulus and/or increased tissue ingrowth aspects at the surface.

Abnormalities in Expression of Structural, Barrier and Differentiation Related Proteins, and Chondroitin Sulfate in Feline and Human Interstitial Cystitis

PURPOSE We analyzed the urothelium of cats diagnosed with feline interstitial cystitis to determine whether abnormalities in protein expression patterns could be detected and whether the expression pattern was similar to that in patients with human interstitial cystitis/bladder pain syndrome. The proteins analyzed are involved in cell adhesion and barrier function, comprise the glycosaminoglycan layer or are differentiation markers. MATERIALS AND METHODS Formalin fixed biopsies from 8 cats with feline interstitial cystitis and from 7 healthy control cats were labeled by immunohistochemistry and scored with a modified version of a system previously used for human samples. Cluster analysis was performed to investigate relationships between markers and samples. RESULTS Of the feline interstitial cystitis bladders 89% showed abnormal protein expression and chondroitin sulfate patterns while only 27% of normal tissues showed slight abnormalities. Abnormalities were found in most feline interstitial cystitis samples, including biglycan in 87.5%, chondroitin sulfate, decorin, E-cadherin and keratin-20 in 100%, uroplakin in 50% and ZO-1 in 87.5%. In feline interstitial cystitis bladders about 75% of chondroitin sulfate, biglycan and decorin samples demonstrated absent luminal staining or no staining. Cluster analysis revealed that feline interstitial cystitis and normal samples could be clearly separated into 2 groups, showing that the urothelium of cats with feline interstitial cystitis is altered from normal urothelium. CONCLUSIONS Feline interstitial cystitis produces changes in luminal glycosaminoglycan and several proteins similar to that in patients, suggesting some commonality in mechanism. Results support the use of feline interstitial cystitis as a model of human interstitial cystitis.

Increased bladder permeability in interstitial cystitis/painful bladder syndrome

The definition of interstitial cystitis (IC) has evolved over the years from being a well-defined entity characterized by diagnostic lesion (Hunner’s ulcer) in the urothelium to a clinical diagnosis by exclusion [painful bladder syndrome (PBS)]. Although the etiology is unknown, a central theme has been an association with increased permeability of the bladder. This article reviews the evidence for increased permeability being important to the symptoms of interstitial cystitis/painful bladder syndrome (IC/PBS) and in treating the disorder. Recent work showing cross-communication among visceral organs is also reviewed to provide a basis for understanding IC/PBS as a systemic disorder of a complex, interconnected system consisting of the bladder, bowel and other organs, nerves, cytokine-responding cells and the nervous system.