Biomaterial surface modifications: Difference between revisions

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Biomaterials exhibit various degrees of compatibility with the harsh environment within a living organism. They need to un-reactive chemically and physically with the body, as well as integrate when with tissue.<ref name=biomaterial>{{cite journal |doi=10.1007/BF00680113 |title=Biomaterials for abdominal wall hernia surgery and principles of their applications |year=1994 |last1=Amid |first1=P. K. |last2=Shulman |first2=A. G. |last3=Lichtenstein |first3=I. L. |last4=Hakakha |first4=M. |journal=Langenbecks Archiv für Chirurgie |volume=379 |issue=3 |pages=168–71}}</ref> The extent of compatibility varies based on the application and material required. Often modifications to the surface of a biomaterial system are required to maximize performance. The surface can be modified in many ways, including plasma modification and applying coatings to the substrate. Surface modifications can be used to affect [[Surface energy|surface energy]], [[Adhesion|adhesion]], [[Biocompatibility|biocompatibility]], chemical inertness, lubricity, [[Sterilization (microbiology)|sterility]], [[Asepsis|asepsis]], [[Thrombosis|thrombogenicity]], susceptibility to [[corrosion|corrosion]], degradation, and [[hydrophile|hydrophilicity]].

== Background of Polymer Biomaterials ==

=== Polytetrafluoroethylene (Teflon) ===

[[File:PTFE3.pdf|thumb|Polytetrafluoroethylene (PTFE) is a thermoplastic that is used for its non-wetting properties and low friction.]]

[[polytetrafluoroethylene|Teflon]] is a hydrophobic polymer composed of a carbon chain saturated with fluorine atoms. The fluorine-carbon bond is largely ionic producing a strong dipole. The dipole prevents Teflon from being susceptible to Van der Waals forces, so other materials will not stick to the surface. <ref name=teflon>{{cite web |url=http://www.sigmaaldrich.com/technical-documents/articles/material-matters/fluorinated-hyperbranched.html |title= Fluorinated Hyperbranched Polymers |last1= Mueller |first1= Anja |dateyear= 2006 |websitework= Sigma Aldrich |accessdate=19 May 2013}}</ref> Teflon is commonly used to reduce friction in biomaterial applications such as in arterial grafts, catheters, and guide wire coatings.

=== Polyetheretherketone (PEEK) ===

[[File:Polyetherketon.svg|thumb|Polyetheretherketone (PEEK) is a thermoplastic, semicrystalline polymer. The backbone consists of ether, ketone, and benzene groups]]

[[PEEK|PEEK]] is a semicrystalline polymer composed of benzene, ketone, and ether groups. PEEK is known for having good physical properties including high wear resistance and low moisture absorption <ref name=teflon>{{cite web |url= http://www.boedeker.com/peek_p.htm |title= PEEK (PolyEtherEtherKetone) Specifications |dateyear= 2013 |websitework= Boedeker Plastics |accessdate=20 May 2013}}</ref> and has been used for biomedical implants due to it's relative inertness inside of the human body.

== Plasma modification of biomaterials ==

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=== Surface Energy ===

The [[Surface_energy|surface energy]] is equal to the sum of disrupted molecular bonds that occur at the interface between two different phases. Surface energy can be estimated by [[Contact angle|contact angle]] measurements using a version of the [[Young–Laplace equation|Young–Laplace equation]]:

<math> \gamma_{SV} - \gamma_{SL}= \gamma_{LV} cos\theta </math> <ref name=CA>{{cite book |doi=10.1021/ba-1964-0043.ch001 |chapter=Relation of the Equilibrium Contact Angle to Liquid and Solid Constitution |title=Contact Angle, Wettability, and Adhesion |series=Advances in Chemistry |year=1964 |last1=Zisman |first1=W. A. |isbn=0-8412-0044-0 |volume=43 |pages=1–51 |editor1-first=Frederick M. |editor1-last=Fowkes}}</ref>

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=== Surface Functionalization ===

[[Surface modification|Surface modification]] techniques have been extensively researched for the application of adsorbing biological molecules. Surface functionalization can be performed by exposing surfaces to RF plasma. Many gases can be excited and used to functionalize surfaces for a wide variety of applications. Common techniques include using air plasma, oxygen plasma, and ammonia plasma as well as other exotic gases. Each gas can have varying effects on a substrate. These effects decay with time as reactions with molecules in air and contamination occur.

[[File:Argon plasma used for polymer surface functionalization prior to bonding.Argon plasma used for polymer surface functionalization prior to bonding.png|125 × 240 pixels.0px|thumb|right|Argon plasma used for polymer surface functionalization prior to bonding.]]

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==== Covalent Immobilization by Gas Plasma RF Glow Discharge ====

[[Polysaccharide|Polysaccharides]]s have been used as [[thin film|thin film]] coatings for biomaterial surfaces. Polysaccharides are extremely hydrophilic and will have small [[Contactcontact angle|contact angles]]s. They can be used for a wide range of applications due to their wide range of compositions. They can be used to reduce the [[adsorption|adsorption]] of proteins to biomaterial surfaces. Additionally, they can be used as receptor sites, targeting specific biomolecules. This can be used to activate specific biological responses.

Covalent attachment to a substrate is necessary to immobilize polysaccharides, otherwise they will rapidly desorb in a biological environment. This can be a challenge due to the fact that the majority of biomaterials do not possess the surface properties to covalently attach polysaccharides. This can be achieved by the introduction of [[amine|amine groups]] by RF glow discharge plasma. Gases used to form amine groups, including ammonia or n-heptylamine vapor, can be used to deposit a thin film coating containing surface amines. Polysaccharides must also be activated by oxidation of anhydroglucopyranoside subunits. This can be completed with sodium metaperiodate (NaIO₄). This reaction converts anhydroglucopyranoside subunits to cyclic hemiacetal structures, which can be reacted with amine groups to form a Schiff base linkage (a carbon-nitrogen double bond). These linkages are unstable and will easily [[dissociation (chemistry)|dissociate]]. Sodium cyanoborohydride (NaBH₃CN) can be used as a stabilizer by reducing the linkages back to an amine.<ref name=immobilization>{{cite journal |doi=10.1002/(SICI)1096-9918(200001)29:1<46::AID-SIA692>3.0.CO;2-6 |title=Biomedical coatings by the covalent immobilization of polysaccharides onto gas-plasma-activated polymer surfaces |year=2000 |last1=Dai |first1=Liming |last2=Stjohn |first2=Heather A. W. |last3=Bi |first3=Jingjing |last4=Zientek |first4=Paul |last5=Chatelier |first5=Ronald C. |last6=Griesser |first6=Hans J. |journal=Surface and Interface Analysis |volume=29 |pages=46–55}}</ref>

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=== Adhesion of Coatings ===

In general, the lower the surface tension of a liquid coating, the easier it will be to form a satisfactory wet film from it. The difference between the surface tension of a coating and the surface energy of a solid substrate to which a coating is applied affects how the liquid coating flows out over the substrate. It also affects the strength of the adhesive bond between the substrate and the dry film. If for instance, the surface tension of the coating is higher than the surface tension of the substrate, then the coating will not spread out and form a film. As the surface tension of the substrate is increased, it will reach a point to where the coating will successfully wet the substrate but have poor adhesion. Continuous increase in the coating surface tension will result in better wetting in film formation and better dry film adhesion. <ref name=SurfaceTension>{{cite web |url=http://www.pra-world.com/technical_services/laboratory/testing/surface-tension |title= Surface Tension, Surface Energy, Contact Angle and Adhesion |dateyear= 2013 |websitework= Paint Research Association |accessdate=22 May 2013}}</ref>

More specifically whether a liquid coating will spread across a solid substrate can be determined from the surface energies of the involved materials by using the following equation:

<math> S = \gamma_{SA} + (\gamma_{CA} - \gamma_{SC}) </math><ref name=teflonse>< /ref>

Where S is the coefficient of spreading, <math> \gamma_{SA} </math> is the surface energy of the substrate in air, <math> \gamma_{CA} </math> is the surface energy of the liquid coating in air and <math> \gamma_{SC} </math> is the interfacial energy between the coating and the substrate. If S is positive the liquid will cover the surface and the coating will adhere well. If S is negative the coating will not completely cover the surface, producing poor adhesion.

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==== Corrosion Protection ====

Organic coatings are a common way to protect a metallic substrate from [[corrosion|corrosion]]. Up until ~1950 it was thought that coatings act as a physical barrier which disallows moisture and oxygen to contact the metallic substrate and form a corrosion cell. This cannot be the case because the [[permeability|permeability]] of paint films is very high. It has since been discovered that corrosion protection of steel depends greatly upon the adhesion of an noncorrosive coating when in the presence of water. With low adhesion, osmotic cells form underneath the coating with high enough pressures to form blisters, which expose more unprotected steel. Additional non-osmotic mechanisms have also been proposed. In either case, sufficient adhesion to resist displacement forces is required for corrosion protection. <ref name=CoatingsBook>{{cite book| author = Z.W. Wicks; Frank N. Jones; S Peter Pappas; Douglas A. Wicks | title = Organic Coatings Science and Technology | edition = 2nd expanded | ___location = New Jersey | publisher = [[John Wiley & Sons, Inc.]] }}{{pnpage needed|date=June 2013}}</ref>

=== Guide Wires ===

Guide wires are an example of an application for biomedical coatings. Guide wires are used in [[percutaneous coronary intervention|coronary angioplasty]] to correct the effects of [[coronary artery disease|coronary artery disease]], a disease that allows plaque build up on the walls of the arteries. The guide wire is threaded up through the femoral artery to the obstruction. The guide wire guides the balloon catheter to the obstruction where the catheter is inflated to press the plaque against the arterial walls. <ref name=angioplasty>{{cite web |url=http://www.nlm.nih.gov/medlineplus/ency/anatomyvideos/000096.htm |title= Percutaneous transluminal coronary angioplasty (PTCA) |last1= Gandelman |first1= Glenn |date= March 22, 2013 |websitework= Medline Plus |accessdate=19 May 2013}}</ref> Guide wires are commonly made from stainless steel or [[Nickel Titanium|Nitinol]] and require polymer coatings as a surface modification to reduce friction in the arteries. The coating of the guide wire can affect the trackability, or the ability of the wire to move through the artery without kinking, the tactile feel, or the ability of the doctor to feel the guide wire's movements, and the thrombogenicity of the wire.

==== Hydrophilic Coatings ====

Hydrophilic coatings can reduce friction in the arteries by up to 83% when compared to bare wires due to their high surface energy.<ref name=friction>{{cite journal |pmid=8485751 |year=1993 |last1=Schröder |first1=J |title=The mechanical properties of guidewires. Part III: Sliding friction |volume=16 |issue=2 |pages=93–7 |journal=Cardiovascular and interventional radiology |doi=10.1007/BF02602986}}</ref> When the hydrophilic coatings come into contact with bodily fluids they form a waxy surface texture that allows the wire to slide easily through the arteries. Guide wires with hydrophilic coatings have increased trackability and are not very thrombogenic; however the low coefficient of friction increases the risk of the wire slipping and perforating the artery. <ref name=techniques>{{cite journal |first1=Andrejs |last1=Erglis |first2=Inga |last2=Narbute |first3=Dace |last3=Sondore |first4=Alona |last4=Grave |first5=Sanda |last5=Jegere |title=Tools & Techniques: coronary guidewires |journal=EuroIntervention |url=http://www.pcronline.com/eurointervention/tools-and-techniques/coronary-guidewires/download_pdf.php |pmid=20542813 |year=2010 |volume=6 |issue=1 |pages=168–9 |doi=10.4244/ |doi_brokendate=June 16, 2013}}</ref>

==== Hydrophobic Coatings ====

Teflon and [[Silicone|Silicone]] are commonly used [[hydrophobe|hydrophobic]] coatings for coronary guide wires. Hydrophobic coatings have a lower surface energy and reduce friction in the arteries by up to 48%.<ref name=friction/> Hydrophobic coatings do not need to be in contact with fluids to form a slippery texture. Hydrophobic coatings maintain tactile sensation in the artery, allowing doctors full control of the wire at all times and reducing the risk of perforation; though, the coatings are more thrombogenic than hydrophilic coatings.<ref name=techniques/> The thrombogenicity is due to the proteins in the blood adapting to the hydrophobic environment when they adhere to the coating. This causes an irreversible change for the protein, and the protein remains stuck to the coating allowing for a blood clot to form.<ref name=thrombo>{{cite journal |first=Denis |last=Labarre |title=Improving blood compatibility of polymeric surfaces |journal=Trends in Biomaterials and Artificial Organs |year=2001 |volume=15 |issue=1 |pages=1–3 |url=http://www.biomaterials.org.in/ojs/index.php/tibao/article/view/5}}</ref>

==== Magnetic Resonance Compatible Guide Wires ====

Using an [[Magnetic Resonance Imaging|MRI]] to image the guide wire during use would have an advantage over using x-rays because the surrounding tissue can be examined while the guide wire is advanced. Because most guide wires' core materials are stainless steel they are not capable of being imaged with an MRI. Nitinol wires are not magnetic and could potentially be imaged, but in practice the conductive nitinol heats up under the magnetic radiation which would damage surrounding tissues. An alternative that is being examined is to replace contemporary guide wires with PEEK cores, coated with iron particle embedded synthetic polymers. <ref name=MRI>{{cite journal |doi=10.1002/jmri.20486 |title=A polymer-based MR-compatible guidewire: A study to explore new prospects for interventional peripheral magnetic resonance angiography (ipMRA) |year=2006 |last1=Mekle |first1=Ralf |last2=Hofmann |first2=Eugen |last3=Scheffler |first3=Klaus |last4=Bilecen |first4=Deniz |journal=Journal of Magnetic Resonance Imaging |volume=23 |issue=2 |pages=145–55 |pmid=16374877}}</ref>

{| class="wikitable"

|-

! Material !! Surface Energy (mN/m)

|-

| Teflon || 24 <ref name=teflonse>{{cite web |url=http://www.vtcoatings.com/plastics.htm|title= Coating Plastics - Some Important Concepts from a Formulators Perspective |last1= Van Iseghem |first1= Lawrence |websitework= Van Technologies Inc |accessdate=2 June 2013}}</ref>

|-

| Silicone || 22 <ref name=Siliconese>{{cite journal |doi=10.1021/la9906626 |title=Surface Modification of Silicone Elastomer Using Perfluorinated Ether |year=2000 |last1=Thanawala |first1=Shilpa K. |last2=Chaudhury |first2=Manoj K. |journal=Langmuir |volume=16 |issue=3 |pages=1256–60}}</ref>

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{{Reflist}}

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[[Category:Biology]]

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