Vascular/Cardiac
CathetersEnsure strong binding of bio medical coatings to catheters
To increase biocompatibity in vivo, the issue of thrombogenisis (the propensity of a surface to form or initiate blood clotting) must be addressed. Many unmodified materials encourage protein binding and thus initiate the process of clot formation. To combat this process, anti-thrombin coatings are applied to in vivo devices, however, such coatings often fail to bind to the polymer surface. Plasma treatment significantly improves the binding affinities of the coatings by specifically modifying surfaces. This is achieved by chemically functionalizing an otherwise inert surface. The treatment process is unique for specific base materials, composition of the anti-thrombin, and expected product lifetime. When catheters are implanted into the body, blood clots may develop and lead to premature catheter replacement. Much work has been applied to this application and results show that plasma treatments are able to maintain residence in the body longer than their untreated counterparts. Animal test results on polyurethane catheters that were surface modified by plasma and then heparin coated revealed no protein attachment after 30 days indwelling. Simultaneous testing of polyurethane catheters treated but left uncoated, revealed only slight protein attachment while the untreated and uncoated control catheters displayed severe thrombus formation.
Guide wires
Improve the durability of lubricious coatings on guide wires and micro catheters
Lubricious coatings are best described as slippery when wet yet non-slippery when dry. They facilitate the insertion and manipulation of guide wires and catheters into the body, reducing patient trauma and risk of infection. Hydrophilic materials, such polyvinylpyrrolidone, hydrate profusely in aqueous media. It is this hydration ability that gives these hydrophilic materials their lubricious properties.
For a lubricious coating to be effective on a medical device it must adhere to the substrate well enough that it doesn't come off during usage. Herein lays the technology. Plasma surface treatment provides the means to chemically bond hydrophilic molecules to various materials. Plasma treatment conditions the surface by cleaning it from contaminants and activating it by providing energy that can e used in bonding. Following this, plasma can be used to chemically functionalize the surface for selective chemical reaction with the "anchoring" end of a hydrophilic molecule. Alternatively, the plasma process can polymerize a supporting polymer onto the surface, then chemically functionalize this plasma polymer to bond with the "anchoring" end of the hydrophilic molecule. Plasma should be regarded as an enabling technology for creative researchers putting theory into practice.
Cardiac rhythm devices
Plasma treatment of pacemakers and implantable cardioverter defibrillators ensures hermetically sealed devices and reliably bonded subcomponents.
Artificial cardiac rhythm systems are complex devices with sub components requiring robust integration. Pacemakers and defibrillators are composed of electrode wires and sensors, a titanium housing for a computer, battery and a single- or multiple-pulse generator. It is critical that all bonded subcomponents preserve their adhesion reliability to avoid device failure. Plasma treatment greatly enhances adhesion of these subcomponents by cleaning and activating surfaces prior to laser welding, adhesive bonding and potting. For example, the electrode leads that are intravenously fed to the heart are coupled to the titanium housing via a polyurethane feedthrough. Plasma treatment not only provides excellent bondability between the polyurethane and the titanium housing, it also promotes potting of the leads ensuring the entire unit is hermetically sealed from the body.
Vascular grafts
Materials used in vascular grafts can be plasma surface modified to promote endothelialization, while retaining non-fouling characteristics
Expanded polytetrafluoroethylene (ePTFE) is a commonly used material for prosthetic implant applications. It's mechanical strength, impermeability to blood and inertness to bio fouling make ePTFE ideal for such in vivo applications. Its flexibility aids in healing, and catheter tubing doesn't kink very easily and generally has good compression resistance. For biomedical applications the ability to modify PTFE surfaces is important to promote interfacial biocompatibility. For example, the adhesion of anti-thrombogenic enzymes such as thrombomodulin, urokinase and heparin requires the PTFE surface to be first modified with chemical anchors such as carboxylic groups that provide covalent immobilization of these enzymes. Grafting of carboxylic groups to PTFE has also been used for the covalent immobilization of chitosan, for example, a membrane forming compound useful for the purification of biomaterials.
Epithelial cell growth can be encouraged on PTFE enabling this material to be used for blood contacting devices such as vascular grafts, stents synthetic heart valves, and other in vivo devices. The the use of biologically active collagen IV (CNIV) as a synthetic scaffold to promote the adsorption of endothelial cells to PTFE devices used for in vivo vascular implants, has been investigated. In order to couple the CNIV to PTFE, the surface must first be activated by grafting hydroxyl functionality, followed by the covalent attachment of N-(3-dimethylaminopropyl) N ethylcarbodiimide, while at the same time retaining the hydrophobic properties of the base PTFE.
Retention of hydrophobic properties are important for surfaces that have intimate contact with blood since they are non-activating to platelet adsorption (leading to the adsorption of fibrin). It is possible to graft polar functional groups to PTFE by plasma activation with only a modest loss in hydrophobicity. At PVA TePla we plotted surface energies against the percentage surface OH functionality following plasma induced alcohol copolymerization on PTFE.
We determined that the surface energy can be controlled by limiting the degree of functionalization. This is simply controlled by the amount of time the substrates are exposed to the plasma. For example, after 30 seconds plasma we obtain a surface OH concentration of 2% while the surface energy remains less than that of polyetheretherketone (PEEK), circa 41 dynes/cm.
PTFE can be surface modified with amino functionality as a first step in the eventual covalent conjugation of peptides that promote in vivo endothelialization. This has particular relevance to arterial prosthesis.
