Comparing the endothelialisation of extracellular matrix bioscaffolds with coated synthetic vascular graft materials

INTRODUCTION

Existing synthetic vascular grafts have unacceptably high failure rates when replacing below knee arteries. In vitro endothelialisation is a technique, which has been shown to enhance the patency rates of below knee vascular grafts. Synthetic materials are however poor cellular substrates and must be combined with coatings to promote cellular growth and attachment. The most common coating clinically is fibrin-coated ePTFE. The aim of our study was to compare the endothelialisation of fibrin-coated ePTFE with novel extracellular matrix (ECM) biomaterials that we hypothesise will provide a superior substrate for cell growth.

METHODS

Human endothelial cells were cultured on ECM scaffolds and fibrin-coated ePTFE. Uncoated Dacron and ePTFE acted as controls. The cells were examined for viability, phenotype, adhesion and proliferation. Cell morphology was accessed using scanning electron microscopy.

RESULTS

Cells remained viable and produced von Willebrand factor on all substrates tested. ECM scaffolds and fibrin-modified ePTFE achieved statistically higher attachment efficiency when compared to both uncoated synthetic graft materials (p ≤ 0.001). At 90 min 80 ± 3.6% of cells had attached to the ECM scaffold compared to Dacron (30 ± 4.5%, n = 3) and ePTFE (33 ± 2.5%, n = 3). There was no difference in adhesion rates between ECM scaffolds and fibrin-coated ePTFE (p = 1.00). Endothelial cells proliferated fastest on ECM scaffolds when compared to all other materials tested (p < 0.001) and reached confluency on day seven.

CONCLUSION

ECM bioscaffolds offer an improved substrate for promoting rapid endothelialisation compared to fibrin-coated ePTFE by combining firm cellular anchorage and superior cell expansion.

Immobilized nerve growth factor and microtopography have distinct effects on polarization versus axon elongation in hippocampal cells in culture

Cell interfacing with biomaterial surfaces dictates important aspects of cell behavior. In particular, axon extension in neurons is effectively influenced by surface properties, both for the initial formation of an axon as well as for the maintenance of axon growth. Here, we investigated how neurons behaved on poly(dimethyl siloxane) (PDMS) surfaces decorated with biochemical and physical cues presented individually or in combination. In particular, nerve growth factor (NGF) was covalently tethered to PDMS to create a bioactive surface, and microtopography was introduced to the material in the form of microchannels. Embryonic hippocampal neurons were used to investigate the impact of these surface cues on polarization (i.e., axon initiation or axogenesis) and overall axon length. We found that topography had a more pronounced effect on polarization (68% increase over controls) compared to immobilized NGF (0.1 ng/mm(2)) (27% increase). However, the effect of NGF was negligible when both types of stimuli were simultaneously presented on the biomaterial surface. In addition to axon formation, chemical and physical cues are also involved in axon growth following the initiation process. Interestingly, for the same studies described above, the effects of microchannels and NGF were opposite from the effects on polarization; the most evident effect was for the immobilized growth factor (10% increase in axon length with respect to controls) whereas there was no effect in general for the microtopography. More importantly, when the two surface stimuli were presented in combination, a synergistic increase in axon length was detected (25% increase with respect to controls), which could be a result of faster polarization triggered by topography plus enhanced growth from NGF. Additionally, axon orientation was also analyzed and we found the well-known tendency of perpendicular or parallel axonal alignment to be dependent on the width and depth of the channels. This investigation thoroughly compared and distinguished the individual and combined impact of material surface properties (chemical and physical) on axogenesis from the effects on axon length. Overall, topography dominated polarization mechanisms, whereas NGF, and particularly a synergy of immobilized NGF plus topography, dominated axon length. These results could be potentially applied for the design of biomaterials in applications were axon growth is critical.

Fabricated hyalS micropatterns and surface guidance of NCTC 2544 continuous cell line: an in vitro study

Surface topography is important in establishing tissue organisation adjacent to implants, smooth surfaces generally being associated with fibrous encapsulation. By virtue of its large hydrated molecular volume and its capacity to form molecular matrix, hyaluronic acid can expand the interfibrillar collagen spaces to allow the movement of cells, although it can also hamper their locomotion. Low molecular-weight hyaluronan can also stimulate cell proliferation, especially at low concentrations. The aim of the present work was to evaluate in vitro the growth and migratory behaviour of NCTC 2544 keratinocytes cultured on different materials microstructured with hyaluronic acid or sulfated hyaluronic acid to assess the possibility of using these devices in the repair process of soft tissues. Ultrastructural morphological analyses, morphometric evaluations and detection of cytoskeletal elements were performed. Our observations provide evidence that micrometer-size parallel grooves of hyaluronic acid can influence cell growth behaviour since cells seeded onto the microstructured substrate arranged themselves according to a shape and an orientation that clearly reflected the chemotropism exerted on them by the two forms of acid. These data also highlight the importance of accurate microtexture fabrication. We intend to follow up these in vitro studies with in vivo experimental applications using PET and gelatin substrates structured with HyalS to evaluate wound healing responses, and to extend our investigations of the cytoskeletal modifications induced by different microstructures.

Immobilized liposome layers for drug delivery applications: inhibition of angiogenesis

Liposomes were immobilized onto the surface of perfluorinated polymer tape samples and tissue culture polystyrene well-plates using a multilayer immobilization strategy. In the first step, a thin interfacial bonding layer with surface aldehyde groups was deposited from a glow discharge struck in acetaldehyde vapour. Polyethylenimine was then covalently bound onto the aldehyde groups by reductive amination, followed by covalent binding of NHS-PEG-biotin molecules onto the surface amine groups by carbodiimide chemistry. Next, NeutrAvidin protein molecules were bound onto the PEG-biotin layer. Finally, liposomes containing PEG-biotinylated lipids were docked onto the remaining binding sites of the surface-immobilized NeutrAvidin molecules. AFM was used to image surface-bound liposomes and revealed a density well below close packing. The release characteristics of the surface-bound liposomes were measured by the fluorescence intensity changes of carboxyfluorescein upon release. Liposomes filled with sodium orthovanadate were surface immobilized and used in two in vitro angiogenesis assays. Marked differences compared to various control samples were observed, demonstrating the utility of drug-filled, surface-bound liposomes for evoking localized, controlled biological host responses proximal to an implanted biomedical device.

Beneficial use of fibroblast growth factor 2 and RGTA, a new family of heparan mimics, for endothelialization of PET prostheses

We have studied the endothelialization of polyethylene terephtalate (PET) prostheses coated with collagen by adult human saphenous endothelial cells (EC) under various in vitro conditions. Collagenous PET was impregnated either by Fibroblast Growth Factor 2 (FGF2), heparin, a synthetic heparan sulfate mimic named RGTA 11 (for ReGeneraTing Agent), or combinations of these products. RGTA 11 belongs to a new family of drugs, which have been previously described as stabilizer and protector of heparin binding growth factors (HBGF), and to act in vivo as to stimulate wounded tissue repair. As endothelialization of prosthesis can be obtained in vivo after EC seeding and/or by transanastomotic, as well as by transprosthetic EC migrations, we have designed in vitro models to study the growth of EC seeded on PET, the EC colonization of an acellular area on PET, and the migration of EC from a collagen gel through the prosthesis. The combinations of either RGTA11 or heparin with FGF2 enhanced after a week by 5-fold the growth of seeded EC compared to RGTA or heparin alone and by 3-fold compared to FGF2 alone (p < 0.05). More than 80% of the colonization of an acellular area was achieved within 6 days when FGF2 was combined with RGTA11 or heparin. In contrast, colonization was only of 20% promoted in presence of FGF2 alone and not promoted in the presence of RGTA or heparin alone (p < 0.05). In addition, transprosthetic migration of EC and endothelialization of the luminal side were observed only when gel contained RGTA11 or heparin in combination with FGF2. The present work did strongly indicate that RGTA11 could be used in vivo as to improve endothelialization and should be the focus of continued investigation.

Tissue reactions to polypyrrole-coated polyesters: A magnetic resonance relaxometry study

The electrically conductive properties of polypyrrole (PPy) as a coating on polyester material are very attractive for the manufacture of small diameter blood conduits. However, before these PPy-coated materials can be investigated for their capacity to generate endothelialized luminal surfaces, they must first be studied for their innocuousness in a living environment. The specific goal of the present study was to investigate the in vivo interactions of PPy-coated and noncoated woven polyester materials implanted subcutaneously in rats for prescheduled periods of 2, 5, 10, 20, and 30 days. The in vivo magnetic resonance (MR) relaxation times were computed for a small area of muscle tissue adjacent to the implants. A correlation was concurrently attempted with blood monocyte activation studies as well as histological observations of the tissue-material interface. The progressive pattern of the slower transversal relaxation time (T2s) values revealed a more persistent tissue reaction for the most conductive PPy-coated materials and a shorter acute tissue response as the surface resistivity increased. Similarly, the blood monocyte activation studies indicated that the thickness of the PPy coating, which correlated with the conductivity, was directly related to tissue response. Furthermore, both the MR and biological studies showed that the PPy-coated material with a high surface resistivity displayed the lowest tissue reaction over the entire period of implantation. The results obtained from the blood monocyte activation studies and histological observations correlate well with the noninvasive MR measurements of the body's healing process. The conductive materials with high surface resistivities must be further investigated. Finally, the noninvasive nature of MR relaxometry reveals its outstanding potential for future in vivo investigations of the body's tissue interactions with polymers and nonferromagnetic biomaterials.

Biomaterials and cardiovascular devices

In the field of cardiovascular surgery there is presently a lack of biomaterials possessing essential characteristics of the native tissue or organ which is to be replaced. This paper describes various biomaterials that have been introduced into the circulatory system and the complex reactions that subsequently occur. The risk of infection is also discussed as well as prevention and treatment regimes that can be used. Examples of future biomaterial development are outlined in an attempt to achieve biocompatibility.

Transplantation of human endothelial cell monolayer on artificial vascular prosthesis: the effect of growth-support surface chemistry, cell seeding density, ECM protein coating, and growth factors

The failure rates of synthetic vascular grafts, when placed in low blood flow environments in humans, are not acceptable. Thus, endothelial cell (EC) seeding technology of vascular grafts was developed to prepare prostheses lined with a human monolayer expressing optimal thromboresistant properties. In a clinical setting, endothelialization of a graft can be achieved using higher cell seeding densities, or by creating a surface on which EC can adhere and grow to confluence. But, human endothelial cells show little or no proliferation on the currently available graft materials. In this study, surface modification of PTFE and ePTFE by ammonia plasma treatment was carried out to enhance its interactions with ECM protein, EC growth factors, and with EC harvested from human umbilical vein (HUVEC), and from human saphenous veins (HSVEC). Our data shows that various vascular graft materials generated from ammonia plasma treated PTFE and ePTFE exhibited statistically significant improvements in HUVEC and HSVEC growth when compared to their respective controls (p values < 0.001). Growth of HSVEC on ammonia plasma treated ePTFE without ECM protein coating was also found to be statistically significant in comparison to that on fibronectin coated ePTFE (p < 0.001). The final HSVEC cell densities found on various ePTFE surfaces prepared from ammonia plasma treated ePTFE, suggests that transplantation of HSVEC monolayers on vascular prostheses can be established within clinically relevant times. Ammonia plasma treatment process provides an unique opportunity to surface modify prosthetic materials of various construct to transplant mammalian cells including those that have undergone ex vivo gene transfer, and to deliver angiogenic molecules to a target area for tissue development.