Biomaterial surface modifications: Difference between revisions

Revision as of 11:44, 16 June 2013

Revision as of 11:49, 16 June 2013

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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 f�r Chirurgie |volume=379 |issue=3 |pages=168}}</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 ==

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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 journal |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}}</ref>

Where <math>\gamma_{SV}</math> is the surface tension at the interface of solid and vapor, <math>\gamma_{SL}</math> is the surface tension at the interface of solid and liquid, and <math>\gamma_{LV}</math> is the surface tension at the interface of liquid and vapor. Plasma modification techniques alter the surface of the material, and subsequently the surface energy. Changes in surface energy then alter the surface properties of the material.

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===== Plasma Treatment to Reduce Thrombogenisis =====

Ammonia plasma treatment can be used to attach amine functional groups. These functional groups lock on to anticoagulants like Heparin decreasing thrombogenicity.<ref>{{cite journal |pmid=8408111 |year=1993 |last1=Yuan |first1=S |last2=Szakalas-Gratzl |first2=G |last3=Ziats |first3=NP |last4=Jacobsen |first4=DW |last5=Kottke-Marchant |first5=K |last6=Marchant |first6=RE |title=Immobilization of high-affinity heparin oligosaccharides to radiofrequency plasma-modified polyethylene |volume=27 |issue=6 |pages=811–9 |doi=10.1002/jbm.820270614 |journal=Journal of biomedical materials research}}</ref>

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

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[[Polysaccharide|Polysaccharides]] have been used as [[thin film|thin film]] coatings for biomaterial surfaces. Polysaccharides are extremely hydrophilic and will have small [[Contact angle|contact angles]]. 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 posses 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}}</ref>

=== Surface Cleaning ===

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There are many examples of contamination of biomaterials that are specific to the preparation or manufacturing process. Additionally, nearly all surfaces are prone to contamination of organic impurities in the air. Contamination layers are usually limited to a monolayer or less of atoms and are thus only detectable by surface analysis techniques, such as XPS. It is unknown whether this sort of contamination is harmful, yet it is still regarded as contamination and will most certainly have an impact on surface properties.

Glow discharge plasma treatment is a technique that is used for cleaning contamination from biomaterial surfaces. Plasma treatment has been used for various biological evaluation studies to increase the surface energy of biomaterial surfaces, as well as cleaning. Plasma treatment has also been proposed for [[sterilization (microbiology)|sterilization]] of biomaterials for potential implants.<ref name=glow>{{cite journal |doi=10.1002/(SICI)1097-4636(199704)35:1<49::AID-JBM6>3.0.CO;2-M |title=Glow discharge plasma treatment for surface cleaning and modification of metallic biomaterials |year=1997 |last1=Aronsson |first1=B.-O. |last2=Lausmaa |first2=J. |last3=Kasemo |first3=B. |journal=Journal of Biomedical Materials Research |volume=35 |pages=49–73 |pmid=9104698 |issue=1}}</ref>

[[File:Glow Plasma Discharge Schematic Polymer Chemistry.png|framed|center|Schematic of cleaning of a polymer surface using glow plasma discharge. Note the removal of adsorbed molecules and presence of dangling bonds.]]

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==== 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}}</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/}}</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=1-31–3 |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>

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| 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 |website= Van Technologies Inc |accessdate=2 June 2013}}</ref>

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| Silicone || 22 <ref name=Siliconese>{{cite journal | author = Shilpa K. Thanawala, Manoj K. Chaudhury | journal = Langmuir| year = 2000 | volume = 16 | pages = 1256-12601256–1260 | doi = 10.1021/la9906626 | title = Surface Modification of Silicone Elastomer Using Perfluorinated Ether | issue = 3}}[http://www.lehigh.edu/~mkc4/our%20papers/shilpa_modification%20of%20pdms%20with%20PFE.pdf Link text.] </ref>

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| PEEK || 42.1 <ref name=PEEKse>{{cite web |url=http://www.surface-tension.de/solid-surface-energy.htm|title= Solid surface energy data (SFE) for common polymers |date= November 20, 2007 |accessdate=2 June 2013}}</ref>

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| Stainless Steel || 44.5 <ref name=SSse>{{cite web |url=http://www.kruss.de/en/theory/substance-properties/solids.html|title= Selected literature values for surface free energy of solids |accessdate=5 June 2013}}</ref>

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| Nitinol || 49 <ref name=nitinolse>{{cite journal |doi=10.1016/j.biomaterials.2006.09.040 |title=The influence of surface energy on competitive protein adsorption on oxidized NiTi surfaces |year=2007 |last1=Michiardi |first1=Alexandra |last2=Aparicio |first2=Conrado |last3=Ratner |first3=Buddy D. |last4=Planell |first4=Josep A. |last5=Gil |first5=Javier |journal=Biomaterials |volume=28 |issue=4 |pages=586–94 |pmid=17046057}}</ref>

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