Advanced Materials Research Vol. 409

Abstract: It is well-known that cellular behavior can be guided by chemical signals and physical interactions at the cell-substrate interface. The patterns that cells encounter in their natural environment include nanometer-to-micrometer-sized topographies comprising extracellular matrix, proteins, and adjacent cells. Whether cells transduce substrate rigidity at the microscopic scale (for example, sensing the rigidity between adhesion sites) or the nanoscopic scale remains an open question. Here we report that micromolded elastomeric micropost arrays can decouple substrate rigidity from adhesive and surface properties. Arrays of poly (dimethylsiloxane) (PDMS) microposts from microfabricated silicon masters have been fabricated. To control substrate rigidity they present the same post heights but different surface area and spacing between posts. The main advantage of micropost arrays over other surface modification solutions (i.e. hydrogels) is that measured subcellular traction forces could be attributed directly to focal adhesions. This would allow to map traction forces to individual focal adhesions and spatially quantify subcellular distributions of focal-adhesion area, traction force and focal-adhesion stress. Moreover, different adhesion intracellular pathways could be used by the cells to differentiate toward a proliferative or a contractile cellular phenotype, for instance. This particular application is advantageous for vascular tissue engineering applications, where mimicking as close as possible the vessels dynamics should be a step forward in this research field.

Abstract: The three dimensional scaffold of the bone marrow (BM) niches is composed of various elements including extracellular matrix proteins and cell types, such as collagen type I (Col I) and stroma cells. Interaction of stem cells with their microenvironment is important for their regulation. In the marrow, Col I is mostly localized in the endosteal regions. The objective of this work was to investigate the role of Col I in the regulation of Hematopoietic Stem Cells (HSC) and Mesenchymal Stem Cells (MSC) growth. Col I was extracted from rat tail tendons and its purity confirmed. Human BM MSCs and umbilical cord blood (UCB) CD34⁺ cells were used as Stem Cell sources. MSCs were cultured in medium with serum while CB CD34⁺ cells were cultured without serum with cytokines. The impact of increasing concentrations of Col I (0-50 µg mL^-1 for coating) on the growth of Hematopoietic Progenitor Cells (HPC) and MSCs was investigated by cytometry, microscopy and clonogenic progenitor assays. Only a minority of CD34⁺ cells expressed the Col I receptor α₂β₁ prior to culture, while the opposite was observed when hematopoietic cells were placed in culture. Col I coated surfaces reduced the expansion of hematopoietic cells by 25% compared to control, while expansions of myeloid and MK progenitors were either unchanged or negatively affected by Col I, respectively. The differentiation of HPCs was also affected on Col I as demonstrated by differences in the frequencies of various cell lineages, such as CD34⁺ cells, megakaryocytes (MK), erythrocytes and others. In contrast to HPCs, Col I surfaces increased MSCs proliferation but had little impact on osteoblasts derived from MSCs. Taken together, this study provides new insights into the regulatory activities of Col I on Stem Cells residing in the marrow.

Abstract: As intravascular biomedical devices, metallic stents are particularly susceptible to corrosion induced by the physiological environment, causing the degradation of mechanical properties and leading to the release of toxic and carcinogenic ions from the SS316L bulk. Therefore, several works have been focused on the development of an ultra-thin fluorocarbon coating that could act both as a drug-carrier for in-stent restenosis and as an anti-corrosion barrier. However, the increase of the corrosion performance was limited by the inevitable permeability of the coating, which exposed some of the sensitive interfacial region to the corrosive environment. Indeed, in previous works, adhesion and growth rate of the film were promoted by the removal of the native oxide layer of the stainless steel which is inhomogeneous, brittle and mechanically unstable. Further refinements of the interface are therefore required in order to enhance the overall corrosion performance without compromising the fluorocarbon film properties and adhesion. Hence, the aim of this work was to enhance the corrosion behaviour of coated SS316L by the creation of a controlled interfacial oxide layer. The native oxide layer was first removed under vacuum and the bare metal surface was subjected to a plasma-reoxidation treatment. Tafel measurements were used to assess the corrosion rates of the specimens. Coated and uncoated modified interfaces were also characterized by X-Ray Photoelectron Spectroscopy (XPS) and Atomic Force Microscopy (AFM).

Abstract: Functional vascular tissue engineering aims to produce blood vessels in vitro in a controlled environment named bioreactor. In order to control the growth and remodeling of vascular tissues, suitable measurements should be made on the construct in situ, i.e. during the growth. These measurements will feed the controller with information in order to take efficient control decisions. The non-destructive measurement of compliance or elastic modulus in vitro is a potential indicator of the vascular construct maturity. This work shows that compliance and elastic modulus are related: they can be estimated during the growth of constructs in a bioreactor, and thus provide useful feedback information to the controller.

Abstract: Several embolizing agents have been tested for minimally invasive treatment of intracranial aneurysms, and more recently to prevent or treat persistent blood flow (endoleaks) in abdominal aortic aneurysms. However, frequent recurrence of endoleaks was seen in most studies, suggesting that current embolization agents are not satisfying yet. Here we report rheological studies of a radiopaque chitosan hydrogel as an embolizing agent. The aim is to provide an agent that would be visible during x-ray based guided interventions. In this study, a commercial contrast agent (iopamidol) was associated to chitosan at different concentrations and its influence on the rheological behavior of chitosan thermogel was evaluated. The resulting hydrogels have a homogenous coherent structure. The addition of iopamidol leaded to an initially more viscous solution. To have a good visibility of hydrogel via x-ray, an optimum iopamidol concentration of 20% v/v was chosen. The addition of 20% v/v iopamidol increased the gelation time. The use of a high βGP concentration constitutes a solution to overcome the slowing down of gelation by 20% v/v iopamidol. Formulations containing around 16-20% βGP provides viscous solutions which rapidly gel and could be promising injectable radiopaque hydrogels for embolization.

Abstract: When problems with the spine of the human body occur, symptoms such as pain and numbness arise, causing impediments to everyday life. In addition, when these symptoms are severe, surgery is required. In recent years, the number of spinal operations performed has tended to increase, and accordingly, there has been an increase in the number of the spinal spacers used in fusion surgeries.In this study, we focused on zirconia as a spacer material because it is superior in terms of strength, toughness, and frictional wear characteristics and does not cause a foreign body response such as metal allergy. Therefore, our purpose was to make adjustments for mechanical characteristics that were suitable for use in spacers by combining a dense structure of zirconia with a porous structure. A further purpose was to fabricate a spinal spacer capable of strong, direct bonding with vertebrae. During the fabrication we packed a powdered mixture of zirconia powder and carbon, the source of the pores, and sintered this using a spark plasma sintering method. Thereafter, the source of the pores was eliminated by heating, and bioactivity was provided by immersion in an aqueous solution of NaOH. Observations and analysis made using a scanning electron microscope (SEM), energy dispersive X-ray spectrometry (EDS), and X-ray diffraction (XRD) allowed us to fabricate materials suitable for our purposes. We are reporting this method as well as the stress occurring at the interface of the layers.

Abstract: Recently, more attention has been devoted to porous implants to avoid stress-shielding effects and facilitate anchor effects. In addition, our previous research revealed that uniaxially aligned pores promoted early recovery of bone tissue with high bone quality similar to that of intact bone. In this study, Ti-based implant materials with uniaxially aligned pores were fabricated using the electron beam melting (EBM) method with 2 types of grid spacing, 0.5 and 1.0 mm. Although grid spacing was varied, the constituent phase and microstructure of the products were homogenous regardless of the grid spacing. Uniaxially aligned pores were created when the grid spacing was 1.0 mm, whereas almost solid structures with random pores were formed when the grid spacing was 0.5 mm. Young’s modulus of the products with the grid spacing of 1.0 mm was 34 GPa; this value is close to that of the bone. It is concluded that the porous material with aligned pores is suitable as a bone implant to reduce stress-shielding effects and to induce bone regeneration with good bone quality.

Abstract: The assessment of mechanical properties of highly hydrated natural materials remains a challenge because, in general, their mechanical evaluation implies invasive and finally destructive methods. Acoustic-based tests may represent the appropriate tools to investigate the mechanical properties of such materials, particularly collagen gels, whose acoustic properties are poorly understood. The objective of this work is to develop two experimental setups for the assessment of acoustic properties of such a hydrogels. In the first one, a typical pulse echo reflectometer was implemented. The acoustic parameters were measured at controlled temperature in an especially designed chamber. In the second one, the previous configuration was combined with a setup for compressive tests, allowing to interrogate simultaneously both the acoustic and mechanical properties of the sample under test. The frequency of the acoustic transducer was 10MHz. The acoustic and mechanical properties of collagen gels prepared according to different experimental conditions (pH and collagen concentration) were evaluated. The first set of experiment was useful to accomplish estimation of the speed of sound, attenuation and acoustic impedance. The second one allowed us to monitor the speed of sound during the evolution of the compression test. This approach could be a potential tool to study the changes in hydrogels mass density and bulk compressibility.

Abstract: Diseases occurring to blood vessel are preferentially solved by replacing the vessel by an autologous graft. When it is not available, a synthetic graft is used which has low patency rates for small diameter (<6 mm) vessels. Tissue engineering of blood vessel aims to improve the performance of vascular substitutes. Bioreactors are used in vascular tissue engineering to mimic the mechanical and biochemical environment of blood vessel. A 2D bioreactor was custom made in order to impose a dynamical strain to silicone membrane receiving the collagen cell-based construct. Collagen gels with vascular smooth muscle cells cultured inside were subdued to maturation under dynamical uniaxial stretch regimes at 1Hz for 48 hours. The percentage of deformation encountered by the silicone membrane was measured by ImageJ. Collagen fibrils and porcine smooth muscle cells (PSMC) orientations were assessed by scanning electron microscopy (SEM). Results show that the study of mechanical conditioning on cell activity is an important issue for enhancing the alignment of collagen fibrils.

Abstract: Collagen gels constitute an adequate scaffold for supporting the adhesion, proliferation and tissue regeneration of vascular cells inside a bioreactor. However, their mechanical properties should be enhanced not only for their manipulation but also to resist the mechanical constraints applied in the bioreactor. Actually, assessing the mechanical properties of a hydrogel requires many precautions since they are very sensitive to the environmental conditions (temperature, ionic strength, aqueous environment, etc). Whereas mechanical properties are usually measured directly in the air, the aim of this work was to evaluate the effects of a pseudo-physiological environment (PPE) on the mechanical properties of collagen gels. Furthermore, reinforcement was also tested using UV treatments (λ = 254 nm, 20 J/cm²), known to induce crosslinking. Irradiated samples were more resistant to enzymatic degradation and swelling tests showed that the crosslink density was increased by a factor of 30. This increase was thereafter correlated to the mechanical properties. Results showed that the UV-treated samples were stiffer and more brittle than the non-treated ones when tested in air. However, a 20% decrease and 40% increase were respectively measured on the linear modulus and strain at rupture when the gels were tested in the PPE. In the perspective of vascular tissue regeneration, these results show that the mechanical properties of a hydrogel should be performed in PPE in order to take into account the plasticization phenomenon that will occur in a bioreactor.