Metallic wires with an adherent lubricious and blood-compatible polymeric coating and their use in the manufacture of novel slippery-when-wet guidewires: possible applications related to controlled local drug delivery

Multifunctional, Low Friction, Antimicrobial Approach for Biomaterial Surface Enhancement

Poly(vinyl chloride) (PVC) biomaterials perform a host of life-saving and life-enhancing roles when employed as medical devices within the body. High frictional forces between the device surface and interfacing tissue can, however, lead to a host of complications including tissue damage, inflammation, pain, and infection. We herein describe a versatile surface modification method using multifunctional hydrogel formulations to increase lubricity and prevent common device-related complications. In a clinically relevant model of the urinary tract, simulating the mechanical and biological environments encountered in vivo, coated candidate catheter surfaces demonstrated significantly lower frictional resistance than uncoated PVC, with reductions in coefficient of friction values of more than 300-fold due to hydration of the surface-localized polymer network. Furthermore, this significant lubrication capacity was retained following hydration periods of up to 28 days in artificial urine at pH 6 and pH 9, representing the pH of physiologically normal and infected urine, respectively, and during 200 repeated cycles of applied frictional force. Importantly, the modified surfaces also displayed excellent antibacterial activity, which could be facilely tuned to achieve reductions of 99.8% in adherence of common hospital-acquired pathogens, Staphylococcus aureus and Proteus mirabilis, relative to their uncoated counterparts through incorporation of chlorhexidine in the coating matrix as a model antiseptic. The remarkable, and pH-independent, tribological performance of these lubricious, antibacterial, and highly durable surfaces offers exciting promise for use of this PVC functionalization approach in facilitating smooth and atraumatic insertion and removal of a wide range of medical implants, ultimately maintaining user health and dignity.

Flexible Coils with a Drug-Releasing Hydrophilic Coating: A New Platform for Controlled Delivery of Drugs to the Eye?

Delivery of drugs to the front-side of the eye is routinely done through eye drops. It is known that approximately 80% of each eye-drop is lost, as a result of rapid clearance of the tear fluid via the naso-lacrymal canal. Consequently, repeated administration through several droplets is usually necessary to achieve a desired effect, such as widening of the pupil prior to corneal surgery. A new ocular drug delivery device was studied. The new device is believed to provide a basis for a more convenient and efficient method for ocular drug delivery. The device is a metallic coil with a hydrophilic, drug-containing polymeric coating. The coil is placed in the conjuctival fornix (under the lower eye-lid) and the drug is slowly released by diffusion into the tear fluid. The capacity of the device could be increased by using the lumen of the coils as a depot for the drug to be released. Preliminary experiments with the new device were performed largely in vitro and in vivo. The latter experiments involved the release of a fluorescent dye and atropine (a potent mydriatic agent) in the eye of several healthy volunteers. The first results obtained with the new device indicate its potential utility. More research and development work is required to define the optimal design of the coil in order to minimize the risk of irritation. Furthermore, the parameters that define the kinetics of the intraocular drug release must be defined and optimized with respect to the exact application.

Antimicrobial and anti-thrombogenic features combined in hydrophilic surface coatings for skin-penetrating catheters. Synergy of co-embedded silver particles and heparin

Percutaneous (skin-penetrating) catheters such as central venous catheters (CVCs), are used ubiquitously in the treatment of critically ill patients, although it is known that the risks for serious complications, particularly bloodstream infection and thromboembolism, are high. Materials science and engineering offer important new perspectives regarding further improvement of CVCs. A promising approach is the use of synthetic biocompatible hydrogel coatings with both silver particles and heparin embedded therein. Such formulations combine the well-known broad-spectrum antimicrobial features of silver with the anticoagulant activity of immobilized heparin. Previous work revealed that heparin augments antimicrobial activity of silver, while maintaining its anticoagulant function. This study set out to investigate the synergy of heparin and silver in more detail. Exit-challenge tests, experiments on bacterial killing and adherence, as well as in vitro challenge tests with three Staphylococcus aureus strains (one reference strain, and two clinical isolates) consistently showed the synergistic effect. In addition, the impact of changing the coating's hydrophilicity, and changing the silver concentration in the coatings, were examined. The experimental results, taken together and combined with data from the literature, point out that synergy of heparin and silver is best explained by binding of Ag(+) ions to heparin within the swollen coating, followed by release of heparin-Ag(+) complexes upon immersion of the coatings in an aqueous environment such as blood. Possible implications of this work regarding the development of improved/safer CVCs are briefly discussed.

Improvement of surface lubricity of polymers and metals by a glow-discharge plasma cross-linking process

A plasma cross-linking process was employed to improve the surface lubricity of different types of biomaterials, including stainless steel (SS), nitinol, polyethylene and nylon. To investigate the influence of monomers containing double bonds on top-layer cross-linking of poly(ethylene oxide) compound (PEOC), five different monomers, N-trimethylsilyl-allylamine (TMSAA), ethylene, propylene, allyl alcohol and ethane, were used in the study to produce a cross-linked coating layer on sample surfaces. Before the plasma cross-linking, samples underwent plasma treatment followed by wet chemical coating. The plasma treatment consists of plasma etching in NH(3)/O(2), Tetramethylcyclo-tetrasiloxane (TMCTS) coating and TMSAA grafting. The wet coating process includes dip-coating in a solution of poly(oxyethylene)-compound bis(1-hydroxy-benzotriazolyl carbonate) (HPEOC), then dip-coating in a solution of PEOC. By application of plasma processing, HPEOC and PEOC wet coating to sample surfaces, the lubricity was increased by 83% compared to clean samples. The plasmas of TMSAA, ethylene, propylene and allyl alcohol, all containing a C=C double bond, produced a cross-linking layer on the PEOC surface. Consequently the surface lubricity was improved by 20% to 37% in comparison to no cross-linking. The favorable condition for plasma cross-linking was found to be high power and long time. Ethane plasma also reduced the pulling force although it has no double bond in the molecular structure, which indicated a thin plasma coating from saturated hydrocarbons deposited on HPEOC or PEOC surfaces could also cause cross-linking and improve lubricity. It was found that the TMSAA cross-linking also worked on HPEOC and HEPOC/PEOC, even though the prior plasma coating process was skipped.

Polypropylene meshes to prevent abdominal herniation. Can stable coatings prevent adhesions in the long term?

Abdominal surgery is associated with a significant risk for incisional herniation. Hernia repair is routinely performed by implantation of synthetic meshes. Such meshes may cause serious adhesions between the implanted material and organs leading to intestinal obstruction or enterocutaneous fistulas. This study compares three knitted meshes for their capacity to prevent adhesion formation in an in vivo study. The meshes evaluated are polypropylene (Prolene), polypropylene coated with oxygenated regenerated cellulose-in principle-a biodegradable biomaterial (Proceed, and Prolene coated with a nondegradable copolymer of the hydrophilic building block N-vinyl pyrrolidone (NVP) and the hydrophobic building block n-butylmethacrylate (BMA). The meshes were implanted in the abdomen of rats (follow-up 7 or 30 days). After 7 days, the formation of adhesions decreased in the order: Prolene > NVP/BMA-coated Prolene > Proceed; after 30 days, this order changed into: Proceed > Prolene > NVP/BMA-coated Prolene. Both at 7 and at 30 days, Proceed was the only mesh surrounded by macrophage cells that contained foreign materials, presumably degradation products of the (biodegradable) surface coating. The data indicate that long-term protection of implanted meshes against excessive adhesions may be achieved through stable biocompatible hydrogel surface coatings.

Capacity and tolerance of a new device for ocular drug delivery

A new method to increase the drug-capacity of the OphthaCoil, a flexible and tubular device for delivery of drugs to the tear film of the eye, was explored. Poly(2-hydroxyethyl methacrylate)- and poly(2-hydroxyethyl methacrylate-co-1-vinyl-2-pyrrolidone)-microspheres were prepared by suspension polymerization. The resultant particles were swollen in a highly concentrated solution of either the dye fluorescein sodium or the antibiotic chloramphenicol. The loaded particles were placed in the central cavity of the ocular device. In vitro release profiles showed a six-fold increase of the capacity for the dye fluorescein sodium, but not for the antibiotic chloramphenicol. Flexibility measurements revealed that by introducing microspheres in the central cavity of the device, flexibility did not decrease. Finally, a preliminary in vivo evaluation of the device (n=5) was done for a 2h-period to assess the tolerance of the device in the human eye. Ophthalmologic examinations and photographs of the eye indicated no signs of irritation. Volunteers reported that the presence of the device in the eye could be noticed, but no irritation was reported.

Thrombus Formation at the Surface of Guide-Wire Models: Effects of Heparin-releasing or Heparin-exposing Surface Coatings

PURPOSE

This study was conducted to investigate whether thrombus formation at the surface of guide wires occurs, and--if so--whether this can be suppressed or prevented through incorporation of heparin in the surface coating.

MATERIALS AND METHODS

Five guide wire models were examined; three had a polymeric hydrophilic surface coating (90/10 guide wire), which was either heparin-free, impregnated with sodium-heparin (Na-hep), or impregnated with benzalkonium heparin (BAK-hep). The other two guide wires had a coating of polytetrafluoroethylene (PTFE), either without heparin, or impregnated with BAK-hep. Release of heparin, exposure of heparin at the surface of the guide wires, thrombogenicity (under static and flow conditions) and their propensity to attract blood platelets were investigated.

RESULTS

The guide wire 90/10 Na-hep releases approximately 150-200 mU active heparin per cm coil within the first few minutes after incubation in buffer. The PTFE BAK-hep shows a relatively slow release of 60-70 mU active heparin per cm coil. The 90/10 BAK-hep showed no released heparin but the most exposed heparin. In a static experiment with human full blood excessive thrombus formation occurred at the heparin-free models, whereas the others remained essentially clean. In a thrombin-generation assay under flow the authors observed strong retardation of thrombin formation in the case of the 90/10 Na-hep guide wire.

CONCLUSIONS

The static and dynamic in vitro assays, taken together, show that the 90/10 Na-hep provides a coating with an extremely low level of surface thrombogenicity. Use of a guide wire with a hydrophilic distal coating that releases and exposes sodium heparin may contribute to the safety of diagnostic and therapeutic interventional procedures.

The effect of high-density-lipoprotein on thrombus formation on and endothelial cell attachement to biomaterial surfaces

Cardiovascular implants such as vascular grafts fail frequently because they lack genuine blood-compatibility. The blood-contacting surface should simultaneously prevent thrombus formation and promote formation of a confluent endothelial cell layer, to achieve sustained haemostasis. Contact activation and endothelialization are known to be determined by the plasma proteins which adsorb onto virtually all synthetic surfaces almost immediately upon contact with blood. A common approach in blood-compatibility research is, therefore, to use hydrophilic biomaterials, which are sometimes claimed to be "protein-repellent". We report here that, for synthetic polymeric surfaces, hydrophilicity is by no means synonymous to protein-repellency. We discovered that significant amounts of proteins, especially high-density lipoprotein, adsorb to hydrophilic surfaces. Pre-incubation of hydrophilic synthetic surfaces with high-density lipoprotein provides a blood-biomaterial interface, which inhibits thrombin generation and subsequent thrombus formation, and also accommodates overgrowth with a confluent endothelial layer. This approach may open the way to truly functional small-caliber arterial prostheses, and may also be relevant to cardiovascular tissue engineering in which de novo vascular tissues are cultured on or within a biomaterial scaffold.

Hydrophobicity as a design criterion for polymer scaffolds in bone tissue engineering

Porous polymeric scaffolds play a key role in most tissue-engineering strategies. A series of non-degrading porous scaffolds was prepared, based on bulk-copolymerisation of 1-vinyl-2-pyrrolidinone (NVP) and n-butyl methacrylate (BMA), followed by a particulate-leaching step to generate porosity. Biocompatibility of these scaffolds was evaluated in vitro and in vivo. Furthermore, the scaffold materials were studied using the so-called demineralised bone matrix (DBM) as an evaluation system in vivo. The DBM, which is essentially a part of a rat femoral bone after processing with mineral acid, provides a suitable environment for ectopic bone formation, provided that the cavity of the DBM is filled with bone marrow prior to subcutaneous implantation in the thoracic region of rats. Various scaffold materials, differing with respect to composition and, hence, hydrophilicity, were introduced into the centre of DBMs. The ends were closed with rat bone marrow, and ectopic bone formation was monitored after 4, 6, and 8 weeks, both through X-ray microradiography and histology. The 50:50 scaffold particles were found to readily accommodate formation of bone tissue within their pores, whereas this was much less the case for the more hydrophilic 70:30 counterpart scaffolds. New healthy bone tissue was encountered inside the pores of the 50:50 scaffold material, not only at the periphery of the constructs but also in the center. Active osteoblast cells were found at the bone-biomaterial interfaces. These data indicate that the hydrophobicity of the biomaterial is, most likely, an important design criterion for polymeric scaffolds which should promote the healing of bone defects. Furthermore, it is argued that stable, non-degrading porous biomaterials, like those used in this study, provide an important tool to expand our comprehension of the role of biomaterials in scaffold-based tissue engineering approaches.

Coils and tubes releasing heparin. Studies on a new vascular graft prototype

Coiled metallic guidewires find widespread use, for instance in interventional cardiology. It is known that release of heparin from the surface of guidewires is advantageous to prevent formation of thrombotic emboli. New coiled tubular structures, having larger inner and outer diameter as compared to guidewires, are presented. In theory these tubes can be used as interposition vascular grafts. Ten coiled tubes with an internal diameter of 690 microm were made. Five different adherent polymeric coatings with increasing hydrophilicity were used. Five tubes contained heparin in the coating and the other five were unheparinised controls. The five tubes containing heparin were studied with respect to heparin release in vitro (amount released, kinetics), and immobilised heparin that is exposed at the surface. All tubes were studied with a direct cell contact assay using 3T3 mouse fibroblast cells, a dynamic thrombin generation test, and endothelial cell growth onto the coils. It was found that the heparinised tubes lead to very little thrombin formation. It is argued that this is due to heparin that is immobilised and exposed at the inner surface of such tubes. Furthermore the coils showed to be cytocompatible and endothelial cells adhere and proliferate well onto the coils. This concept is believed to hold promise for further development of small vascular grafts.

Tracheal replacement in rabbits with a new composite silicone-metallic prosthesis

A new composite silicone-metallic prosthesis was tested, studying the potential for respiratory epithelial covering over the biocompatible inner lining, in a rabbit survival model. Seven New Zealand White rabbits underwent near-total excision of their trachea and implantation of a sterile prosthesis. After 2 months, they were sacrificed and the prostheses were retrieved. Specimens were fixed and histologically examined for tissue reaction around the prosthesis, at the anastomotic lines, and particularly for the presence or absence of epithelialization of the inner lumen over the biocompatible surface. All rabbits survived the operation. At 2 months, the outer layer of the prosthesis was consistently covered with fibrosis and neutrophils. The inner layer showed necrotic cells and scant re-epithelialization over the biocompatible lining, up to 5 mm beyond the anastomosis, with no evidence of organized respiratory epithelium in the middle sections. The new prosthesis is a viable temporary solution for airway replacement in rabbits. Granulation tissue was not observed at the anastomosis, and re-epithelialization did occur, but failed to achieve full-length luminal covering. The potential for granulation tissue does not yet make this an ideal long-term solution. Improvements in prosthesis design or biocompatibility are required, and need to be re-evaluated before applicability for chronic use.

A polymeric mini-stent designed to facilitate the vasectomy reversal operation. A rabbit model study

Vasectomy has become popular since it is the safest surgical method for contraception. It is known that approximately 6% of the men that undergo vasectomy will seek reversal (vasovasostomy). This operation is, however, technically demanding and relatively time-consuming. This study was based on the hypothesis that a polymeric mini-stent can facilitate and accelerate vasovasostomies. A mini-stent was manufactured out of a crosslinked hydrogel biomaterial, which was synthesized from N-vinyl-pyrrolidinone (NVP), n-butylmethacrylate, and (triethyleneglycol) dimethacrylate. The device was tested with 28 rabbits, which were divided over two equal groups. In one group, the vasa deferentia were dissected and reanastomosed via microsurgical one-layer technique (end-to-end group). In the other group, the vasa deferentia were dissected and reattached through implantation of the mini-stent. Sperm counts revealed 100% patency in both groups, i.e. all vasovasostomies were successful. It was experienced that the operation was easier and faster in the case of the mini-stent, probably since the mini-stent keeps the lumens of both vas ends exactly in line during suturing. This study demonstrates the feasibility of the mini-stent. Further work is necessary to evaluate the utility of this approach for clinical vasovasostomies.

Human endothelial cell attachment and proliferation on a novel vascular graft prototype

A new vascular prosthesis prototype was assessed for its ability to support an endothelial cell layer in vitro. A coiled tubular structure, constructed from polymer-coated metallic wires, with an internal diameter of 690 microm, was used. Addition of heparin to the surface coating of the coil strongly enhanced the blood compatibility of the device. A series of coils with five different coatings, increasing in hydrophilicity, was studied. Heparin was added to one series, another series did not contain this anticoagulant drug. Upon contact with blood, a vascular prosthesis will instantaneously adsorb plasma proteins on its surface, and these proteins will influence the behavior of cells binding to the device. When coils were treated with human plasma proteins, mimicking the in vivo situation, human microvascular endothelial cells grew well on all coils studied, irrespective of the hydrophilicity of the underlying coating or the addition of heparin. For control coils, only endothelial cell growth on the most hydrophobic surfaces, and a moderate enhancing effect for heparin, were observed. This novel vascular graft prototype seems well suited for the support of an endothelial cell layer, especially when plasma proteins are adsorbed to its surface, and shows promise for in vivo testing.

Heparin release from slippery-when-wet guide wires for intravascular use

Thin metallic wires with an adherent hydrophilic/ lubricious polymeric coating were manufactured in a new extrusion-like procedure. This procedure is part of a novel and efficient way of assembling lubricious guide wires for intravascular interventions, such as percutaneous transluminal angioplasty. It is reported that heparin can readily be incorporated in the hydrophilic coating. A set of heparin-containing guidewire models was made and studied in detail. This showed that (i). immersion of the guide-wire models in an aqueous environment leads to release of heparin from their surface; (ii). the presence of heparin in the coating does not impede the lubricity of the coils; (iii). addition of stearic acid in the coating, next to heparin, does not influence the lubricity of the guide-wire models. Two different charges of heparin (designated heparin-low and heparin-high) were incorporated in the coating. It is discussed that release of heparin from the surface of medical devices (e.g. guide wires and catheters) is much more effective than systemic heparinization, basically because dissolved heparin molecules have a much larger probability of simply passing a medical device's surface (axial convection) rather than contacting it (radial diffusion).