Endothelialization of polymer surfaces

Vascular Tissue Engineering: Challenges and Requirements for an Ideal Large Scale Blood Vessel

Since the emergence of regenerative medicine and tissue engineering more than half a century ago, one obstacle has persisted: the in vitro creation of large-scale vascular tissue (>1 cm³) to meet the clinical needs of viable tissue grafts but also for biological research applications. Considerable advancements in biofabrication have been made since Weinberg and Bell, in 1986, created the first blood vessel from collagen, endothelial cells, smooth muscle cells and fibroblasts. The synergistic combination of advances in fabrication methods, availability of cell source, biomaterials formulation and vascular tissue development, promises new strategies for the creation of autologous blood vessels, recapitulating biological functions, structural functions, but also the mechanical functions of a native blood vessel. In this review, the main technological advancements in bio-fabrication are discussed with a particular highlights on 3D bioprinting technologies. The choice of the main biomaterials and cell sources, the use of dynamic maturation systems such as bioreactors and the associated clinical trials will be detailed. The remaining challenges in this complex engineering field will finally be discussed.

Surface functionalization of acrylic based photocrosslinkable resin for 3D printing applications

The limited number of resins, available for stereolithography applications, is one of the key drivers in research applied to rapid prototyping. In this work an acrylic photocrosslinkable resin based on methyl methacrylate (MMA), butyl methacrylate (BMA) and poly(ethylene glycol) dimethacrylate (PEGDA) was developed with different composition and characterized in terms of mechanical, thermal and biological behaviour. Two different systems have been developed using different amount of reagent. The influence of every components have been evaluated on the final characteristic of the resin in order to optimize the final composition for applications in bone tissue engineering. The crosslinked materials showed good mechanical properties and thermal stabilities and moreover cytotoxicity test confirms good biocompatibility with no cytotoxic effect on cells metabolism. Moreover two different treatments have been proposed, using fetal bovine serum (FBS) and methanol (MeOH), in order to improve cell recognition of the surfaces. Samples threatened with MeOH allow cell adhesion and survival, promoting spreading, elongation and fusion of C2C12 muscle myoblast cells.

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Photocrosslinkable biocompatible resin for application in tissue engineering.

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Surface treatment to improve materials wettability.

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Myoblast spreading and elongation on photocrosslinked modified surfaces.

Synthesis and characterization of polycaprolactone urethane hollow fiber membranes as small diameter vascular grafts

The design of bioresorbable synthetic small diameter (<6mm) vascular grafts (SDVGs) capable of sustaining long-term patency and endothelialization is a daunting challenge in vascular tissue engineering. Here, we synthesized a family of biocompatible and biodegradable polycaprolactone (PCL) urethane macromers to fabricate hollow fiber membranes (HFMs) as SDVG candidates, and characterized their mechanical properties, degradability, hemocompatibility, and endothelial development. The HFMs had smooth surfaces and porous internal structures. Their tensile stiffness ranged from 0.09 to 0.11N/mm and their maximum tensile force from 0.86 to 1.03N, with minimum failure strains of approximately 130%. Permeability varied from 1 to 14×10(-6)cm/s, burst pressures from 1158 to 1468mmHg, and compliance from 0.52 to 1.48%/100mmHg. The suture retention forces ranged from 0.55 to 0.81N. HFMs had slow degradation profiles, with 15 to 30% degradation after 8weeks. Human endothelial cells proliferated well on the HFMs, creating stable cell layer coverage. Hemocompatibility studies demonstrated low hemolysis (<2%), platelet activation, and protein adsorption. There were no significant differences in the hemocompatibility of HFMs in the absence and presence of endothelial layers. These encouraging results suggest great promise of our newly developed materials and biodegradable elastomeric HFMs as SDVG candidates.

Cellular behavior of human adipose-derived stem cells on wettable gradient polyethylene surfaces

Appropriate surface wettability and roughness of biomaterials is an important factor in cell attachment and proliferation. In this study, we investigated the correlation between surface wettability and roughness, and biological response in human adipose-derived stem cells (hADSCs). We prepared wettable and rough gradient polyethylene (PE) surfaces by increasing the power of a radio frequency corona discharge apparatus with knife-type electrodes over a moving sample bed. The PE changed gradually from hydrophobic and smooth surfaces to hydrophilic (water contact angle, 90° to ~ 50°) and rough (80 to ~120 nm) surfaces as the power increased. We found that hADSCs adhered better to highly hydrophilic and rough surfaces and showed broadly stretched morphology compared with that on hydrophobic and smooth surfaces. The proliferation of hADSCs on hydrophilic and rough surfaces was also higher than that on hydrophobic and smooth surfaces. Furthermore, integrin beta 1 gene expression, an indicator of attachment, and heat shock protein 70 gene expression were high on hydrophobic and smooth surfaces. These results indicate that the cellular behavior of hADSCs on gradient surface depends on surface properties, wettability and roughness.

Carbon nanotubes impregnated with subventricular zone neural progenitor cells promotes recovery from stroke

The present in vivo study was conducted to evaluate whether hydrophilic (HL) or hydrophobic (HP) carbon nanotubes (CNTs) impregnated with subventricular zone neural progenitor cells (SVZ NPCs) could repair damaged neural tissue following stroke. For this purpose, stroke damaged rats were transplanted with HL CNT-SVZ NPCs, HP CNT-SVZ NPCs, or SVZ NPCs alone for 1, 3, 5, and 8 weeks. Results showed that the HP CNT-SVZ NPC transplants improved rat behavior and reduced infarct cyst volume and infarct cyst area compared with the experimental control and the HL CNT-SVZ NPC and SVZ NPCs alone groups. The transplantation groups showed an increase in the expression of nestin (cell stemness marker) and proliferation which was evident with the increased number of doublecortin and bromodeoxyuridine double-stained immunopositive cells around the lesion site. But, these effects were more prominent in the HP CNT-SVZ NPC group compared with the other transplantation groups. The HP CNT-SVZ NPC and HL CNT-SVZ NPC transplants increased the number of microtubule-associated protein 2 (marker for neurons) and decreased the number of glial fibrillary acidic protein (marker for astroglial cells) positive cells within the injury epicenter. The majority of the transplanted HP CNT-SVZ NPCs collectively broadened around the ischemic injured region and the SVZ NPCs differentiated into mature neurons, attained the synapse morphology (TUJ1, synaptophysin), and decreased microglial activation (CD11b/c [OX-42]). For these reasons, this study provided the first evidence that CNTs can improve stem cell differentiation to heal stroke damage and, thus, deserve further attention.

Regulation of endothelial cells migration on poly(D, L-lactic acid) films immobilized with collagen gradients

To investigate the effect of protein surface-density gradient on the motility of endothelial cells, we developed a novel approach for the fabrication of a collagen density gradient onto poly(d, l-lactic acid) (PDLLA) films in this study. The approach involves a sequential alkali hydrolysis of PDLLA films to produce a density gradient of -COOH moieties onto the films, which were activated and then covalently linked with collagen. A collagen surface-density gradient onto PDLLA films was thus generated by this approach. Contact angle measurement and confocal laser scanning microscopy (CLSM) were employed to confirm the formation of -COOH gradient and collagen gradient, respectively. All results proved the feasibility of the fabrication of a collagen density gradient onto PDLLA films via the approach. Endothelial cells cultured on the gradient areas with low and moderate collagen surface-densities displayed a strong motility tendency, with the values such as net displacement, total distance, chemotactic index, migration rate and cell trajectories in parallel to the gradient. However, endothelial cells grew on the gradient area with high collagen density demonstrated a reverse response to the collagen gradient clue. These results suggest that cell motility is regulated by the collagen gradient with a surface-density dependent manner. This study provides an alternative for the fabrication of protein surface-density gradient onto biodegradable substrates to investigate chemical stimuli induced cell directional motility. It is potentially important for understanding the controlled angiogenesis for implantation of tissue-engineered constructs.

Physico-chemical and thermodynamic aspects of fibroblastic attachment on RGDS-modified chitosan membranes

In the present study, the cell attachment/spreading behaviour of L929 mouse fibroblasts on chitosan membranes was evaluated by using physico-chemical properties. For this purpose chitosan membranes were prepared and then photochemically modified with the cell adhesive peptide RGDS (Arg-Gly-Asp-Ser). The physico-chemical properties of unmodified (CHI) and RGDS-modified chitosan (CHI-RGDS) membranes were evaluated by calculating surface free energy (gamma(sv)) and interfacial free energy (gamma(sw)) values using captive bubble contact angle measurements and harmonic mean equation. The cell attachment experiments were performed both in 10% FBS containing and serum-free media with CHI and CHI-RGDS membranes. Eventually, it was not possible to predict a direct relationship between the change in physico-chemical properties and L929 cell attachment behaviour. The experimental results obtained from cell attachment agree with the theoretical prediction for the free energy of adhesion except for the cell attachment on CHI membrane in serum-free medium. Although a negative interfacial free energy of adhesion was calculated for CHI membrane in serum-free medium (DeltaF(adh)=-2.19 ergs/cm(2)), the cell attachment was poor (approximately 70%) compared to CHI-RGDS (approximately 90%) and none of the cells were spread on CHI surface to gain a fibroblastic morphology. Negative energy of adhesion was calculated for CHI and CHI-RGDS in 10% FBS medium, in which approximately 100% of cells were attached on the membranes correlating with the thermodynamic approach. It can be suggested that, adsorption of serum proteins strongly affected the cell attachment meanwhile the presence of biosignal RGDS molecules triggered the cell spreading in serum medium.

Adhesion behavior of human bone marrow stromal cells on differentially wettable polymer surfaces

An appropriate cellular response to implanted surfaces is essential for tissue regeneration and integration. In this study, we investigated how human bone marrow stromal cells (hBMSCs) respond to scaffold substrates. We prepared wettable polymer surfaces by exposing polymer sheets to radio frequency plasma discharge, which gradually oxidizes the polymer surface, increasing the roughness and greatly reducing the hydrophobicity. We found that hBMSCs adhered better to highly hydrophilic and rough surfaces than to hydrophobic and smooth surfaces. In addition, the cells flattened extensively on hydrophilic surfaces. Further, c-fos gene expression increased in parallel with the degree of hydrophilicity, whereas the expression of the c-myc gene was higher on hydrophobic than on hydrophilic surfaces. Finally, p53 gene expression was higher on more hydrophobic or hydrophilic surfaces than on moderately hydrophobic or hydrophilic surfaces. These results indicate that the biological signals induced by cell adhesion depend on the wettability of the surface to which the cells attach.

Biodegradable polyurethane cytocompatibility to fibroblasts and staphylococci

Biodegradable polyurethanes have potential for use as implantable devices (orthopedic, maxillofacial, cardiovascular, wound dressing and plastic surgery) because of their controllable elasticity, and the possibility of changing their chemistry and structure. Studying bacterial and cell adhesion to polyurethanes helps to determine surface cytocompatibility and suitability for in vivo trials. Staphylococcus aureus, Staphylococcus epidermidis and hTERT human fibroblast cells were used to determine the cytocompatibility of experimental biodegradable polyurethanes (PUs) with different hydrophobic-to-hydrophilic (pho:phi) content ratios (100% pho, 70% pho, and 30% pho). Poly(L/DL-lactide) 70/30% (PLDL) and Thermanox were used as control surfaces. Surface characterization using noncontact profilometry, contact angles, and scanning electron microscopy (SEM) showed that the three PU surfaces, PLDL, and Thermanox have different properties. On the 100% PU and 30% PU surfaces, hTERT cells spread less in comparison to the 70% PU, PLDL, and Thermanox surfaces. The adsorption of fibronectin to the surfaces had no effect on the adhesion and spreading of hTERT cells when compared to the uncoated surfaces. The trend for S. aureus was the most adhered on the 70% PU and 30% PU, then Thermanox, followed by 100% PU and PLDL, respectively. The amount of S. epidermidis adhesion followed the trend of the most on 70% PU, then 100% PU, then 30% PU and PLDL, and the least on Thermanox. These results suggest that the 70% PU surface is cytocompatible to hTERT fibroblasts, while the 100% PU and 30% PU were not. All surfaces encouraged S. aureus and S. epidermidis colonization, particularly the 70% PU.

Basic Fibroblast Growth Factor Coating and Endothelial Cell Seeding of a Decellularized Heparin-coated Vascular Graft

The objective of this study was to determine the effect of basic fibroblast growth factor (bFGF) coating on endothelial cell seeding and proliferation on a decellularized heparin coated vascular graft and to determine the retention of seeded cells on the graft under flow conditions. Disks of heparin coated decellularized grafts were incubated for 24 h as controls or with bFGF. Human microvascular endothelial cells (HMECs) or canine peripheral blood endothelial progenitor cells (CEPC) were seeded onto the disks and incubated for 96 h or 48 h, respectively. HMECs were also seeded onto the luminal surfaces of two heparin-coated decellularized grafts for 3 h. One graft was placed in a perfusion culture system and cultured for an additional 6 h with flow and pressure. After culturing, there were 4.7 +/- 1.4 cells/mm(2) HMECs on control grafts and 11.4 +/- 1.4 cells/mm(2) in bFGF treated grafts (P < 0.05). Likewise, with CEPCs, there were 14.8 +/- 4.8 cells/mm(2) in control grafts and 33.3 +/- 7.3 cells/mm(2) in bFGF treated grafts. After only 3 h of cell attachment, 60% of HMECs were retained in the intact graft exposed flow relative to the static control graft, which is an acceptable level. These data demonstrate that bFGF coating on the heparin bound decellularized grafts significantly increases both HMEC and dog EPC proliferation and that seeded cells are stable under perfusion conditions.

Tissue engineering therapy for cardiovascular disease

The present treatments for the loss or failure of cardiovascular function include organ transplantation, surgical reconstruction, mechanical or synthetic devices, or the administration of metabolic products. Although routinely used, these treatments are not without constraints and complications. The emerging and interdisciplinary field of tissue engineering has evolved to provide solutions to tissue creation and repair. Tissue engineering applies the principles of engineering, material science, and biology toward the development of biological substitutes that restore, maintain, or improve tissue function. Progress has been made in engineering the various components of the cardiovascular system, including blood vessels, heart valves, and cardiac muscle. Many pivotal studies have been performed in recent years that may support the move toward the widespread application of tissue-engineered therapy for cardiovascular diseases. The studies discussed include endothelial cell seeding of vascular grafts, tissue-engineered vascular conduits, generation of heart valve leaflets, cardiomyoplasty, genetic manipulation, and in vitro conditions for optimizing tissue-engineered cardiovascular constructs.

Effect of pre-adsorbed proteins on attachment, proliferation, and function of endothelial cells

As certain proteins control cell adhesion, it has been hoped that cell transplantation and tissue engineering could be augmented by pre-adsorption of specific proteins to biological or synthetic surfaces. The questions that remain, however, are whether such proteins can affect cell production as well as adhesion, and if so, whether in a protein-specific manner. We examined the adhesion and the biochemical secretion of bovine aortic endothelial cells (BAEC) on tissue culture polystyrene (TCPS) discs coated with fibronectin (Fn), laminin (Ln), or gelatin. The three coating proteins nonspecifically promote sub-confluent and post-confluent endothelial cell production of total protein up to 2.5-fold of the reference value. Total soluble glycosaminoglycan (GAG) production slightly increased with the different coatings only at low cell density. In contrast, Ln and Fn, not gelatin, drastically enhanced post-confluent BAEC production of prostaglandin (PGI2). However, antibody-blockage of the alpha5 integrin, constituent of the Fn receptor in BAEC, appeared to inhibit the upregulation of PGI2 production observed on Fn-coated surfaces. The results indicate that the cell adhesion mediators used as coating agents dictate cell biological production as well as adhesion and proliferation.

Binding and release of basic fibroblast growth factor from heparinized collagen matrices

Endothelial cell seeding is a promising method to improve the performance of small-diameter vascular grafts. Growth of endothelial cells seeded on the luminal surface of synthetic vascular grafts, coated with a matrix suitable for cell seeding (e.g. collagen), can be accelerated by local, sustained release of basic fibroblast growth factor (bFGF). In this study two potential matrices for in vivo endothelial cell seeding were studied with respect to bFGF binding and release: collagen crosslinked using N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide (EDC) and N-hydroxysuccinimide (NHS), as well as heparinized EDC/NHS-crosslinked collagen. bFGF binding was determined after incubation of circular samples (10 mm diameter) with 0.25 ml bFGF solution for 90 min. Immobilization of increasing amounts of heparin, also using EDC and NHS, to crosslinked collagen containing 14 free primary amino groups per 1000 amino acid residues (E/N14C) resulted in binding of increasing amounts of bFGF. A plateau in bFGF binding was observed for heparinized E/N14C containing approximately 2.0-3.0 wt% of immobilized heparin which was obtained using a molar ratio of EDC to heparin-carboxylic acid groups of 0.4 during heparin immobilization (E/N14C-H(0.4)). At concentrations up to 840 ng bFGF/ml, 10% of the added bFGF bound to E/N14C, while binding of bFGF to E/N14C-H(0.4) amounted to 22%. Both E/N14C and E/N14C-H(0.4) pre-loaded with bFGF showed sustained bFGF release. A burst release of 30% in endothelial cell culture medium (CM) was observed for E/N14C during the first 6 h, compared to 2% release from E/N14C-H(0.4). After 28 days, the bFGF release from E/N14C and E/N14C-H(0.4) in CM amounted to 100 and 65%, respectively. Combined results of binding and release of bFGF indicate that compared to E/N14C, E/N14C-H(0.4) is the substrate of choice for bFGF pre-loading and subsequent endothelial cell seeding.

Immobilization of heparin to EDC/NHS-crosslinked collagen. Characterization and in vitro evaluation

In the present study, heparin immobilization to a non-cytotoxic crosslinked collagen substrate for endothelial cell seeding was investigated. Crosslinking of collagen using N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide (EDC) and N-hydroxysuccinimide (NHS) resulted in a material containing 14 free primary amino groups per 1000 amino acid residues (E/N14C). At a fixed molar ratio NHS:EDC of 0.6, the amount of heparin covalently immobilized to E/N14C increased with increasing molar ratios of EDC to heparin carboxylic acid groups (Hep-COOH), to a maximum of approximately 5-5.5 wt% at a ratio of 2. Upon incubation in cell culture medium of endothelial cells, 4 to 7% of the immobilized heparin was released during 11 days. Immobilization of increasing amounts of heparin to E/N14C progressively reduced activation of contact activation proteases. Optimal anticoagulant activity, as measured by thrombin inhibition, was obtained after heparin immobilization using a ratio of EDC to Hep-COOH of 0.2-0.4 (14-20 mg heparin immobilized per gram of collagen). Platelets deposited to (heparinized) E/N14C showed only minor spreading and aggregation, although heparin immobilization slightly increased the number of adherent platelets. The results of this study suggest that heparin immobilization to EDC/NHS-crosslinked collagen may improve the in vivo blood compatibility of this material.

Endothelial cell seeding of (heparinized) collagen matrices: effects of bFGF pre-loading on proliferation (after low density seeding) and pro-coagulant factors

Endothelial cell seeding to improve the performance of small-diameter vascular grafts requires a suitable substrate, such as crosslinked collagen. In addition to providing a suitable substrate for adhesion and growth of endothelial cells, proliferation of seeded endothelial cells can be enhanced by local, sustained release of basic fibroblast growth factor (bFGF, a heparin-binding growth factor for endothelial cells). We have previously shown that collagen crosslinked using N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide (EDC) and N-hydroxysuccinimide (NHS) supports adhesion and proliferation of human umbilical vein endothelial cells (HUVECs). In the present study, HUVECs were seeded on (heparinized) EDC/NHS-crosslinked collagen, pre-loaded with bFGF. Proliferation of HUVECs on (heparinized) crosslinked collagen increased with increasing amounts of pre-loaded bFGF. The minimal cell-seeding density required for proliferation proved to be very low after pre-loading the substrates with bFGF, and was 4-fold lower for heparinized crosslinked collagen compared to crosslinked collagen (250 versus 1000 cells/cm(2)). Pro-coagulant properties (von Willebrand factor secretion and tissue factor expression) of HUVECs seeded on (heparinized) crosslinked collagen, with or without pre-loading of bFGF, were comparable to those of HUVECs on TCPS. It is concluded that heparinized, EDC/NHS-crosslinked collagen pre-loaded with bFGF is a candidate matrix for in vivo endothelial cell seeding of synthetic vascular graft materials.

Interaction of fibroblasts on polycarbonate membrane surfaces with different micropore sizes and hydrophilicity

Surface topography appears to be an important but often neglected factor in implant performance. In this study, fibroblasts were cultured on a range of porous polycarbonate (PC) membranes with well defined surface topography (track-etched micropores, 0.2-8.0 microm in diameter) and wettability gradients. The wettability gradient on the PC membrane surfaces was produced by treating the surfaces with corona from a knife-type electrode whose power increased gradually along the sample length. The PC membrane surfaces were characterized by scanning electron microscopy (SEM) and the water contact angle measurement. Fibroblasts were cultured on the corona-treated PC membrane surfaces with different micropore sizes for 1 and 2 days. The cells attached on the membrane surfaces were examined by SEM and the cell density on the surfaces was estimated by counting the number of attached cells along the wettability gradient. It was observed that the cells were adhered and grew more on the hydrophilic positions of the membrane surfaces than the more hydrophobic ones, regardless of micropore size. It was also observed that cell adhesion and growth decreased gradually with increasing micropore size of the membrane surfaces. It seems that the cell adhesion and growth were progressively inhibited as the membrane surfaces had micropores with increasing size, probably due to surface discontinuities produced by tract-etched pores. On the membrane surfaces with smaller micropore sizes, the cells seemed to override these surface discontinuities.

Endothelial implants inhibit intimal hyperplasia after porcine angioplasty

The perivascular implantation of tissue-engineered endothelial cells around injured arteries offers an opportunity to study fundamental vascular physiology as well as restore and improve tissue function. Cell source is an important issue because the ability to implant either xenogeneic or allogeneic cells would greatly enhance the clinical applications of tissue-engineered grafts. We investigated the biological and immunological responses to endothelial cell xenografts and allografts in pigs 4 weeks after angioplasty of the carotid arteries. Porcine or bovine aortic endothelial cells were cultured within Gelfoam matrices and implanted in the perivascular space of 42 injured arteries. Both porcine and bovine endothelial cell grafts reduced the restenosis index compared with control by 54% and 46%, respectively. Perivascular heparin release devices, formulated to release heparin at twice the rate of release of heparan sulfate proteoglycan from endothelial cell implants, produced no significant reduction in the restenosis index. Endothelial cell implants also reduced occlusive thrombosis compared with control and heparin release devices. Host immune responses to endothelial implants were investigated by immunohistochemical examination of explanted devices and by immunocytochemistry of serum samples. The bovine cell grafts displayed infiltration of leukocytes, consisting primarily of lymphocytes, and caused an increase in antibodies detected in serum samples. Reduced cellular infiltration and no humoral response were detected in animals that received allografts. Despite the difference in immune response, the biological effects of xenografts or allografts did not differ significantly.

Effects of amide and amine plasma-treated ePTFE vascular grafts on endothelial cell lining in an artificial circulatory system

We sought to examine whether surface modification of expanded polytetrafluoroethylene (ePTFE) vascular grafts might extend graft patency without modifying the graft structure. Amide and amine plasma (butylamine) were applied to graft surfaces using radio frequency glow discharge. Surface analyses by Fourier transform infrared spectroscopy-attenuated total reflectance, X-ray photoelectron spectroscopy, and dynamic contact angle measurements revealed the presence of nitrogen-containing functional groups on the plasma modified graft surfaces, along with an increased surface hydrophilicity. Bovine aortic endothelial cells were seeded on amide and amine plasma coated ePTFE vascular grafts and placed inside an artificial circulatory system under well-defined flow conditions. The seeded endothelial cells were exposed to either constant or pulsatile flow condition for 5 days. Their corresponding maximum wall shear stresses were 1 dyn/cm2 under constant flow and 65, 108, and 259 dyn/cm2 under various pulsatile flows. Plasma modified ePTFE vascular grafts enhanced the endothelial cell lining under constant and pulsatile flow conditions. Fluorescence nuclear staining, scanning electron microscopy, and histological staining indicated the formation of an endothelial cell monolayer on the plasma coated graft surfaces.

Adhesion of Staphylococcus epidermidis and transposon mutant strains to hydrophobic polyethylene

Staphylococcus epidermidis capsular polysaccharide adhesin (PS/A) and slime were studied as possible mediators of bacterial adhesion to NHLBI polyethylene (PE) under dynamic flow. This putative interaction was examined by quantifying the adhesion of M187 (PS/A+, slime+) parent strain and isogenic transposon mutant strain sn3 (PS/A-, slime-) to polyethylene (PE) under a range of physiologic shear stress conditions in both phosphate-buffered saline (PBS) and 1% platelet poor plasma (PPP). No significant differences in adhesion were noted between the M187 and sn3 strains in either test medium. However, adhesion of both strains in 1% PPP was decreased 75-95% compared to adhesion in PBS. In PBS, adhesion was shear stress dependent from 0-15 dyne/cm2, after which adhesion was comparatively shear stress independent. Adhesion in 1% PPP was independent of shear stress. Epifluorescent imaging of both strains labeled for slime confirmed the presence of slime on the surface of M187 and suggested that PS/A and slime promote the formation of large aggregates, as aggregates were totally absent in the images of the sn3 strain. The results suggest that PS/A and slime do not mediate S. epidermidis adhesion to bare PE or PE with adsorbed plasma proteins, but may be necessary for intercellular adhesion, which is important for biofilm formation.

Blood and cell compatibility of gelatin-carrageenan mixtures cross-linked by glutaraldehyde

Mixtures of gelatin and iota-carrageenan cross-linked by glutaraldehyde were prepared and their physical properties and blood and cell compatibility were compared to gelatin as a control material. According to scanning electron microscopic observation of fracture surfaces, the mixtures were composed of dispersed and continuous domains which might be generated by phase separation of carrageenan. The thermal degradation temperature of iota-carrageenan in the mixtures rose with increasing gelatin content. The swelling process in the mixtures proceeded slower than in gelatin. Tensile strengths of the mixtures, except that containing 50% iota-carrageenan, increased with increased amounts of iota-carrageenan in the mixtures. The iota-carrageenan contents at the surfaces of the mixtures were generally higher than those admixed originally. Static friction coefficients of the mixtures were lower than that of gelatin. Plasma recalcification times of the mixtures were longer than that of gelatin. Platelet adhesion of the mixtures was lower than that of gelatin, while cell adhesion and growth assays using Chinese hamster ovary cells showed that cell adhesion and growth were not dependent on adding iota-carrageenan. It was concluded that blood compatibility of the mixtures increased and cell compatibility did not decrease, compared to gelatin.

Adhesion and spreading of human skin fibroblasts on physicochemically characterized gradient surfaces

In this study, adhesion and spreading of human skin fibroblasts on gradient surfaces of dichlorodimethylsilane (DDS) coupled to glass was investigated. Gradient surfaces were prepared by the diffusion technique and characterized by the Wilhelmy plate technique and characterized by the Wilhelmy plate technique for their wettability and by scanning x-ray photoelectron spectroscopy for their chemical surface composition. A linear relation between the gradient length, based on advancing water contact angles, and the square root of the diffusion time of DDS was observed. XPS analysis and the cellular experiments were carried out on gradient surfaces prepared using a diffusion time of 3 h. A continuously varying chemical composition with Si/O elemental surface concentration ratio being highest on the hydrophobic end of the gradient surfaces. In the presence of serum proteins, human skin fibroblasts seeded on these gradient surfaces showed a preferential adhesion onto the steepest part of the gradient, probably due to an optimal local wettability and/or local chemistry. Furthermore, it was shown that the spread area of human fibroblasts increased over the length of the gradient surface when going from the hydrophobic to the hydrophilic end. Summarizing, this study shows that the use of gradient surfaces to study cellular responses to materials surface properties, like wettability, yields more-convincing conclusions than the use of a variety of materials with different wettabilities due to the control of the specific surface chemistry of gradient surfaces.

Effect of parallel surface microgrooves and surface energy on cell growth

To evaluate the effect of surface treatment and surface microtexture on cellular behavior, smooth and microtextured silicone substrata were produced. The microtextured substrata possessed parallel surface grooves with a width and spacing of 2.0 (SilD02), 5.0 (SilD05), and 10 microns (SilD10). The groove depth was approximately 0.5 microns. Subsequently, these substrata were either left untreated (NT) or treated by ultraviolet irradiation (UV), radiofrequency glow discharge treatment (RFGD), or both (UVRFGD). After characterization of the substrata, rat dermal fibroblasts (RDF) were cultured on the UV, RFGD, and UVRFGD treated surfaces for 1, 3, 5, and 7 days. Comparison between the NT and UV substrata revealed that UV treatment did not influence the contact angles and surface energies of surfaces with a similar surface topography. However, the contact angles of the RFGD and UVRFGD substrata were significantly smaller than those of the UV and NT substrata. The dimension of the surface microevents did not influence the wettability characteristics. Cell culture experiments revealed that RDF cell growth on UV-treated surfaces was lower than on the RFGD and UVRFGD substrata. SEM examination demonstrated that the parallel surface grooves on the SilD02 and SilD05 substrata were able to induce stronger cell orientation and alignment than the events on SilD10 surfaces. By combining all of our findings, the most important conclusion was that physicochemical parameters such as wettability and surface free energy influence cell growth but play no measurable role in the shape and orientation of cells on microtextured surfaces.

A novel microporous polyurethane blood conduit: biocompatibility assessment of the UTA arterial prosthesis by an organo-typic culture technique

An organotypic culture assay has been used to assess the biocompatibility and cytotoxicity of an arterial prosthesis developed at the University of Texas-Arlington (the UTA graft) from a structurally modified polyurethane (PU) elastomer (Tecoflex). The cell culture test was applied to the UTA graft after sterilization by ethylene oxide and by gamma radiation in two separate series. First, small specimens of the prosthesis were incubated for 7 days on a semisolid nutrient medium with their luminal surface in direct contact with endothelium explanted from the aorta of chick embryos. Second, the possibility of cytotoxic contaminants being leached from the polyurethane was assessed by immersing the biomaterial in the liquid culture medium for 5 days at 37 degrees C prior to conducting the organo-typic culture assay on a standard control surface. The structure of the UTA polyurethane prosthesis is porous, but the graft wall is impervious because it contains closed (i.e., noncommunicating) pores. In addition, four other vascular prostheses were included in the study for comparison. They were the Hydrophilic Mitrathane PU graft with a similar impervious, closed pore structure, an experimental Hydrophobic Mitrathane PU graft with a fibrous, open pore structure, and the commercial Impra and Reinforced Goretex expanded PTFE grafts. Following 7 days of cell culture, the biocompatibility and cytotoxicity of the various biomaterials were measured in terms of the area of migrating cells, the density of cells surrounding the explants, and the level of cell adhesion. Comparison of the results against control cultures demonstrated that the UTA graft, along with the other four prostheses, does not release cytotoxic extractables. Microscopic observations of its cultured surface indicated that the UTA graft promotes a high density of cell growth over a limited area, similar to the Hydrophilic Mitrathane graft. This level of biocompatibility is considered inferior to that of the two PTFE and the Hydrophobic Mitrathane prostheses, which promote more extensive cell migration, greater cell adhesion, and cell growth in a continuous single layer.

Attachment and proliferation of bovine aortic endothelial cells onto additive modified poly(ether urethane ureas)

To better understand endothelial cell interactions with poly(ether urethane urea) (PEUU) materials, and to assess bovine aortic endothelial cell attachment, films were incubated for 24 h with BAEC in media containing 5% fetal bovine serum. Other films were allowed to incubate for 4 more days in media containing 5% fetal bovine serum without cells to assess BAEC proliferation. The assay was performed on PEUU films modified with acrylate and methacrylate polymer and copolymer additives that spanned a wide range on the hydrophobicity/hydrophilicity scale. Tissue culture polystyrene (TCPS) was used as a control. The assay showed that PEUU films loaded with Methacrol 2138F [copoly(diisopropylaminoethyl methacrylate [DI-PAM]/decyl methacrylate [DM]) (3/1)] or with its hydrophilic component, DIPAM, in homopolymer form (i.e., h-DIPAM), significantly enhanced BAEC attachment (approximately 80% of TCPS values) and proliferation (approximately 80%) when compared to unloaded PEUU films (attachment 73%; proliferation, 47%) or to PEUU films loaded with the more hydrophobic acrylate or methacrylate polymer additives (attachment, 32-69%; proliferation, 18-57%). The assay also showed that PEUU films coated with homopoly(diisopropylaminoethyl acrylate) (h-DIPAA) significantly enhanced BAEC attachment and proliferation when compared to PEUU films coated with h-decyl acrylate (h-DA); films coated with the copolymer of these two acrylates (i.e., co-[DIPAA/DA] [3/1]) showed intermediate behavior. To explain the enhancement of BAEC interaction with films loaded with Methacrol 2138F or h-DIPAM, when compared to unmodified PEUU films or to PEUU films loaded with more hydrophobic acrylate and methacrylate polymer additives, it was assumed that the additives near the surface region of the solvent swollen PEUU matrix may have migrated to, or near to, the PEUU-air interface during film formation, creating an additive enriched PEUU surface region. It is suggested that, once at this surface region, dynamic reorientation in response to an aqueous medium ensured the additives were able significantly to influence protein adsorption, and concomitant endothelial cell behavior, but only if they interacted with aqueous media more favorably than the PEUU. The ability of Methacrol and h-DIPAM additives to enhance endothelial cell behavior is argued to be the result of increased hydrophilicity. This is the result of exposed, hydrogen-bonding DIPAM moieties and increased surface flexibility, which is itself due to the hydration of unhindered Methacrol chains, which may create an additive enriched PEUU-water interfacial zone.

Endothelial cell seeding: a review

The concept of endothelial cell seeding, designed to provide vascular grafts with a nonthrombogenic lining, has progressed from crude animal experiments during the past two decades to detailed in vitro functional studies using human cells. Although favorable results have been obtained in animal studies this has yet to be translated to humans, where current application of these techniques has been limited to a very few clinical trials. The history, current status and future directions are reviewed herein.

Molecular approaches to the characterization of cell and blood/biomaterial interactions

In order to address questions related to cell/biomaterial interactions with respect to cell function and production of extracellular matrix proteins that support or maintain cell/tissue specific properties, we have developed molecular approaches for analysis of in vivo implanted materials and in vitro studies. In an explant of a human left ventricular assist device (LVAD), intact total cellular RNA could be isolated in sufficient quantities for hybridization analyses with gene-specific probes to evaluate cell growth, cytoskeletal organization, and production of extracellular matrix proteins. Cells harvested from a 132-day implanted LVAD exhibited proliferative activity and expressed genes for fibronectin and collagen types I, III, and IV. In vitro studies revealed that endothelial cells cultured on two different segmented polyurethane biomaterials (Biomer and Tecoflex 60D) exhibited different patterns of gene expression that reflected differences in cell growth rates, morphology, and composition of the extracellular matrix. These methodologies provide a valuable approach for a detailed evaluation of: (1) the biocompatibility of cells colonizing implanted cardiac assist devices; and (2) the functionality of cells seeded onto biomaterials.

Influence of substratum wettability on the strength of adhesion of human fibroblasts

To determine the strength of adhesion and the detachment mechanisms of fibroblasts from substrata with different wettability, the behaviour of adhered cells was studied in a parallel-plate flow chamber during exposure to shear. Adhered cells were observed in situ, i.e. in the flow chamber, by phase-contrast microscope and images were analysed semiautomatically. Detachment was found to be dependent both on shear stress and time, although a critical shear stress can be found below which no detachment occurs. On all substrata, cells round up before detachment and are approximately spherical immediately before detachment. The strength of adhesion calculated ranged from 0.6-3.5 x 10(-10) N per cell on FEP-Teflon (the least wettable material included) to 9.4 x 10(-9) N per cell for glass (the most wettable). Ease of detachment seemed to decrease with increasing wettability. However, cells reacted more strongly with tissue culture polystyrene (TCPS) than expected on the basis of its wettability, probably due to surface chemistry.

Proliferation of normal human keratinocytes on silicone substrates

Several polydimethylsiloxane elastomers and gels were tested as culture substrates for proliferating normal human epidermal keratinocytes. Growth kinetics of normal human keratinocytes (NHK) and dermal fibroblasts were compared on 'very soft', 'soft' and 'hard' silicone gels, as well as on standard cell culture polystyrene dishes. Water contact angles and chemical compositions (IRFT-HATR) of the different silicone surfaces were found to be equivalent, although very different from standard cell culture polystyrene. The topography of the surfaces as well as the shape of the keratinocytes and fibroblasts grown on the different substrates were visualized by scanning electron microscopy, and compared. Although the surface softness and topography of the substrates differed markedly, dermal fibroblasts proliferated in serum-containing medium in equivalent manner on all substrates. Again no correlation could be found between the characteristics and the attachment of the substrates and rapid proliferation of the epidermal keratinocytes in defined medium. The epidermal keratinocytes spread, secreted a structured extracellular matrix network and grew up to confluence on all silicone substrates (elastomers and gels), except the relatively 'hard' silicone gel; this could be due to a direct interference by the waves observed on the silicone gel surfaces.

Adhesion of endothelial cells and adsorption of serum proteins on gas plasma-treated polytetrafluoroethylene

From in vitro experiments it is known that human endothelial cells show poor adhesion to hydrophobic polymers. The hydrophobicity of vascular prostheses manufactured from Teflon or Dacron may be the reason why endothelialization of these grafts does not occur after implantation in humans. We modified films of polytetrafluoroethylene (Teflon) by nitrogen plasma and oxygen plasma treatments to make the surfaces more hydrophilic. Depending on the plasma exposure time, modified polytetrafluoroethylene surfaces showed water-contact angles of 15-58 degrees, versus 96 degrees for unmodified polytetrafluoroethylene. ESCA measurements revealed incorporation of both nitrogen- and oxygen-containing groups into the polytetrafluoroethylene surfaces, dependent on the plasma composition and exposure time. The thickness of the modified surface layer was approximately 1 nm. The adhesion of cultured human endothelial cells from 20% human serum-containing culture medium to modified polytetrafluoroethylene surfaces with contact angles of 20-45 degrees led to the formation of a monolayer of cells, which was similar to the one formed on tissue culture polystyrene, the reference surface. This was not the case when endothelial cells were seeded upon unmodified polytetrafluoroethylene. Surface-modified expanded polytetrafluoroethylene prosthesis material (GORE TEX soft tissue) also showed adhesion of endothelial cells comparable to cell adhesion to the reference surface. The amounts of serum proteins, including fibronectin, adsorbed from serum-containing medium to modified polytetrafluoroethylene surfaces were larger than those adsorbed to unmodified polytetrafluoroethylene. Moreover, the modified surfaces probably allow the exchange of adsorbed serum proteins with cellular fibronectin.

Effect of critical surface tension on retention of oral microorganisms

The effect of critical surface tension on the initial retention of microorganisms from unstimulated human saliva was tested in a flow cell system. Prior to each experiment the total numbers and the morphotypes of microorganisms present in saliva were recorded. The test surfaces were prepared to display known increasing critical surface tensions, as verified and standardized by contact angle measurements. Surfaces of initially low (20-22 mN/m), medium (35-38 mN/m) and high (greater than 50 mN/m) critical surface tension were exposed to saliva at a flow rate of 1 ml/min. Microbiota and biofilm material associated with the test surface after 15 min of salivary exposure, were then subjected to standard detachment forces, by introducing a cell-free rinsing fluid at two different shear rates. Both the attachment and the detachment phases were executed at room temperature or 37 degrees C. The retained population was counted in three different zones of the test surfaces with a light microscope and statistically tested for correlation to the main variables (critical surface tension, flow rate and temperature) and interactions. Retention success was significantly dependent on the initial critical surface tension and the flow rate. Surfaces of medium critical surface tension, representative of human tooth surfaces and most restorative dental materials, retained the highest numbers of microorganisms in comparison with the other surfaces tested, with no statistically verified selectivity in proportions of retained coccoid and rodshaped microorganisms for any surface. A 30-fold increase of the flow rate resulted in a 70-80% reduction of the retention success, with a higher relative number of cocci present on all the test surfaces. These results demonstrate that initial retention of microorganisms to surfaces is non-specific with regard to morphotypes, but is strongly related both to the mechanical removal forces and the surface energetic state of the solid surface exposed. Retention of microbial populations at interfaces might, therefore, be controllable by advance selection of the critical surface tensions and predicted if shear forces at given sites are known.