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Biomaterials exhibit various degrees of compatibility with the harsh environment within a living organism. They need to
== Background of Polymer Biomaterials ==
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=== Polytetrafluoroethylene (Teflon) ===
[[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
=== Polyetheretherketone (PEEK) ===
[[File:Polyetherketon.svg|thumb|Polyetheretherketone (PEEK) is a thermoplastic, semicrystalline polymer. The backbone consists of ether, ketone, and benzene groups]]
[[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=
== Plasma modification of biomaterials ==
Plasma modification is one way to alter the surface of biomaterials to enhance their properties. During plasma modification techniques, the surface is subjected to high levels of excited gases that alter the surface of the material. Plasma's are generally generated with a [[Radio frequency|radio frequency (RF)]] field. Additional methods include applying a large (~1KV) DC voltage across electrodes engulfed in a gas. The plasma is then used to expose the biomaterial surface, which can break or form chemical bonds. This is the result of physical collisions or chemical reactions of the excited gas molecules with the surface. This changes the surface chemistry and therefore surface energy of the material which affects the adhesion, biocompatibility, chemical inertness, lubricity, and sterilization of the material. The table below shows several biomaterial applications of plasma treatments.<ref>{{cite
{| class="wikitable"
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! Applications of Plasma Treatments!! Devices !! Materials !! Purposes
|-
| Biosensor || Sensor Membranes, Diagnostic biosensors || PC, Cellulose,[[Cuprophane]], PP, PS || Immobilization of biomolecules, non-fouling surfaces
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| Cardiovascular || Vascular grafts, Catheters || PET,PTFE,PE,SiR || Improved biocompatibility, Wettability tailoring, lubricious coatings, Reduced friction, Antimicrobial coatings
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| Dental || Dental Implants || Ti alloys || Enhanced Cell growth
|-
| Orthopedic || Joints,Ligaments || [[Ultra-high-molecular-weight polyethylene|UHMWPE]],PET || Enhance bone adhesion, Enhanced tissue in-growth
|-
| Others || General uses || Example || Sterilization, Surface cleaning, Etching, Adhesion promotion, Lubricity tailoring
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The [[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]] measurements using a version of the [[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=978-0-8412-0044-
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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[[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
==== Plasma Treatment to Reduce Thrombogenesis ====
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 |doi=10.1002/jbm.820270614 |title=Immobilization of high-affinity heparin oligosaccharides to radiofrequency plasma-modified polyethylene |year=1993 |last1=Yuan |first1=Shengmei |last2=Szakalas-Gratzl |first2=Gyongyi |last3=Ziats |first3=Nicholas P. |last4=Jacobsen |first4=Donald W. |last5=Kottke-Marchant |first5=Kandice |last6=Marchant |first6=Roger E. |journal=Journal of Biomedical Materials Research |volume=27 |issue=6 |pages=811–9 |pmid=8408111}}</ref>
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==== Covalent Immobilization by Gas Plasma RF Glow Discharge ====
[[Polysaccharide]]s have been used as [[thin film]] coatings for biomaterial surfaces. Polysaccharides are extremely hydrophilic and will have small [[contact angle]]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]] 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 (NaIO<sub>4</sub>). 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 (NaBH<sub>3</sub>CN) 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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=== Surface Cleaning ===
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
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.<ref name=RFGD>{{cite journal |doi=10.1002/jbm.820290411|title=Effect of parallel surface microgrooves and surface energy on cell growth |year=1995 |last1=den Braber |first1=E.T. |last2=de Ruijter |first2=J.E. |last3=Smits |first3=H.T.J |last4=Ginsel |first4=L.A. |last5=von Recum |first5=A.F. |last6=Jamsen |first6=J.A. |journal=Journal of Biomedical Materials Research |volume=29 |pages=511–518 |pmid=7622536 |issue=1|hdl=2066/21896 |hdl-access=free }}</ref> 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 |issue=1 |pages=49–73 |pmid=9104698
[[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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=== 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 |year= 2013 |work= Paint Research Association |accessdate= 22 May 2013 |url-status= dead |archiveurl= https://web.archive.org/web/20130109105050/http://www.pra-world.com/technical_services/laboratory/testing/surface-tension |archivedate= 9 January 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:
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==== Corrosion Protection ====
Organic coatings are a common way to protect a metallic substrate from [[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 [[Permeation|permeability]] of paint films is very high. It has since been discovered that corrosion protection of steel depends greatly upon the adhesion of a 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|
=== 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]], 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=
==== 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=
==== Hydrophobic Coatings ====
Teflon and [[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,
==== 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|doi-access=free }}</ref>
{| class="wikitable"
|-
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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
|-
| 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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{{Reflist}}
▲[[Category:Biology]]
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