Review articles

By Dr. Luke R Ocone
Corresponding Author Dr. Luke R Ocone
none, 8513 Widener Road - United States of America 19038-7530
Submitting Author Dr. Luke R Ocone

Prostheses, Stents, Transfusions, Plastics, Organ Implants

Ocone LR. Factors Affecting The Biocompatability Of Plastics. WebmedCentral GENERAL SURGERY 2011;2(2):WMC001519
doi: 10.9754/journal.wmc.2011.001519
Submitted on: 01 Feb 2011 02:51:30 PM GMT
Published on: 02 Feb 2011 05:34:22 PM GMT


Many otherwise chemically inert plastics or coatings are not biocompatible because triboelectrically and piezoelectrically generated positive charges destabilize colloids, disrupt cell membranes and promote oxidative reactions, which can result in promotion of tissue growth, fatty deposits, mineralization of implant surfaces, hemolysis and blood clot formation. Similar deleterious reactions can also occur in blood, tissue and organs stored or handled in plastics. It is likely that these destabilizing charges can be eliminated by electrically poling the plastics or coatings in order to make the plastic surfaces uniformly electron-rich. The locked-in dipoles created and re-oriented by poling should change both the triboelectric and piezoelectric properties of the plastic surfaces so that generation of positive charge is reduced or eliminated. It is also likely that the wettability of plastics will be affected by poling. Poling procedures are reviewed, and the application of these to implants is discussed.
The ability to control triboelectric and piezoelectric properties should allow designers to improve the long-term biocompatibility of articles constructed from the plastics currently used in devices, and it should also facilitate greater use of other plastics, such as polytetrafluoroethylene (PTFE), which, unpoled, have generally unfavorable properties in many biomedical applications.


Major problems are experienced with many implanted polymeric materials, including promotion of tissue growth, fatty deposits, mineralization of implant surfaces, hemolysis and activation of blood coagulation factors leading to clot and emboli formation. Some of these adverse reactions can be experienced even when the implants are completely surrounded by fibrous tissue, but they are most serious in cardiovascular prostheses and in blood handling and storage. Therefore, the following discussion is focused on blood storage and stents. The mechanisms discussed are also responsible for adverse reactions in other prostheses, and the potential utility of poling for these prostheses will be clear from these examples.
Many biological structures consist of colloids and gels separated and contained by membranes, and the vitality of organisms depend critically on the physical integrity of these structures. Coagulation of these colloidal suspensions and gels and adhesion of these colloids to membranes is inhibited in the same way that coagulation of a simple colloid is inhibited. A biological colloidal particle, like those of most colloids encountered in nature and commerce, consists of a central volume with an electron-rich (that is, anionic or negative) surface surrounded by an electrically equivalent, diffuse array of cations in solution. Agglomeration of the colloidal particles and cells is inhibited by the mutual repulsion of like charges.
Biological membranes have an electron-rich surface because of aligned molecular dipoles, and there is a diffuse array of counter cations in solution close to the interface. The vascular surface, for example, has an oriented molecular structure like that of a colloidal particle, which inhibits destabilization of serum colloids and blood cells in contact with these membranes and adhesion of these to the membranes.
These systems can be destabilized (coagulated) at an anode, but also at an electron-poor surface. It was shown many years ago [1] that an inflamed vascular interface can become electron-poor, which leads to deposition of colloidal particles, adhesion and lysis of cells, clotting of blood and blockage of blood vessels, which very likely is the result of loss of the normal molecular orientation of the vascular membrane. This has been confirmed in more recent work [2], which also demonstrated that oxidative reactions result in the destruction of the endothelial cell orientation that is responsible for the existence of the electron-rich vascular interface. The role of positive charge in the promotion of oxidative reactions has been explored in a recent publication [3].
It is possible to demonstrate the gross physical effect of positive charge in a system analogous to biological colloids and cells in contact with electron-poor plastic surfaces. Colloids whose electrical polarity is the opposite of that commonly encountered can be prepared in the laboratory. The surfaces of such colloid particles are positively charged (electron-poor), and they are surrounded by a diffuse array of anions in solution. When this colloid is added to a colloid having the more common, opposite polar structure, the latter is coagulated. (More accurately, each colloid destabilizes the other.) A positive charge over a sufficiently large area of an implant surface will destabilize a contacting biological colloid or disrupt a cell membrane in a similar way. The positively charged implant surface will, of course, be covered with a diffuse array of counter anions.
Electron-poor plastic surfaces exist or are generated in several ways with the consequences described above. Destabilizing, electron-poor areas can exist at an implant surface because of a specific orientation of dipoles in the plastic, but transient destabilizing positive potentials can also be generated piezoelectically. One consequence of the specific orientation of their oriented polar molecules is that all cell membranes are piezoelectric. Even bone surfaces are piezoelectric. These piezoelectrically generated voltages are likely to be small because they are produced by single lipid layers or bi-layers with opposing dipoles. However, much larger destabilizing positive voltages can be produced piezoelectrically in plastics. These phenomena and their development in polymers are discussed below.
The second way that destabilizing positive voltages may be generated can be thought of as a contact potential, although it is known as the triboelectric effect. This, also, is discussed below. Dipoles and Piezoelectricity in Plastics
There are three kinds of dipoles in solids: polar molecules and lattice and molecular defects. When the charge-separation distance of a dipole is changed by physical or thermal stress, negative and positive compensating voltages will be generated at opposite ends of the dipole. If the polymer is completely amorphous and the arrangement of these dipoles is random, all the unstressed-dipole (static) charges will cancel each other over very small molecular distances, and the same is true of voltages generated piezoelectrically. The net charge measured over sufficiently large areas of the surfaces of such plastics will be zero.
However, many polymers are not random at a molecular level. So-called crystalline polymers consist of sizeable zones of ordered molecules (crystallites) in an amorphous matrix, and crystallites with aligned dipoles can have a piezoelectric response over surface areas large enough to affect a colloidal particle or cell in the vicinity. Even if they are randomly oriented, some of the crystallites may present positive surface charges over large enough areas to destabilize contacting biological structures and promote oxidation.
But, in addition, the crystallite orientation is seldom random in fabricated plastic shapes. There is often some degree of net dipole orientation in plastics manufactured by commonly used procedures. A non-random orientation of dipoles can result from the shear forces on molten polymer during extrusion and injection molding. Some commercial grades of polyvinylchloride, extruded over copper wire are piezoelectric and can be employed as transducers without further treatment. This dipole-orienting effect may be more noticeable in crystalline polymers in which the polymer chain folds so that regularly spaced dipoles along the molecular backbone tend to become aligned in the same direction. Furthermore, aggregates of these polar zones or crystallites can be aligned with each other, and this alignment can be increased by some standard plastics fabrication procedures. For example, orientation of the ordered zones of some polymers is increased by uniaxial or biaxial stretching of extruded sheet. Some commercial thin-film transducers are made from biaxially stretched polyvinylidenefluoride. The polar crystalline aggregates, some of considerable size, are re-oriented in both directions normal to the stretched film.
Orientable dipoles can also exist in amorphous plastics because of molecular and lattice defects that are commonly present or are created by post-fabrication treatment. This will be discussed further below.
In summary, it is clear that the surface of an implant in contact with biological tissue can be a mosaic of areas with different piezoelectric responses at the plastic surface. Some areas will respond to physical stresses such as changes in blood pressure, circulation turbulence forces, peristalsis and body movement with a probably benign negative voltage and others with a possibly destabilizing positive voltage. The orientation of the underlying dipoles also affects the triboelectric potential of each area of the mosaic: some will be electron-rich and others will be electron-poor. Electron-poor areas will accept electrons more strongly from biological tissue, which will promote oxidative reactions and destabilize oriented bio-structures.
When two materials are brought into contact, one of the materials can gain electrons at the expense of the other. On separation, the surface of one material is negatively charged and the other is positively charged. A ranking of materials according to gain or loss in such encounters is, of course, the familiar triboelectric series. The phenomenon is functionally similar to the contact potential observed when different metals are in contact, and metals are included in some published triboelectric series. Table 1, which is based mainly on a published series [4] includes many materials likely to be of interest for implants.
Table 1: Triboelectric Series
Most positive acquired charge
Hair (human)
Cellulose acetate
Silicone rubber
Most Negative Acquired Charge
On contact, electrons are transferred from each material in the series to those below it, and the further apart two materials are in the series, the greater is the charge-transfer propensity. For a number of reasons, Table 1 and other, published triboelectric series [,5,6,7] may not be completely dependable guides to the behavior of specific materials of medical interest. Triboelectric charge transfer measurements are affected by temperature, humidity, surface roughness and force of contact, which may account for the slightly different order of materials in published series, especially if they are close neighbors in the series. The original sources for many of the rankings are obscure, and the experimental conditions are not known. Furthermore, many of the plastics in these series are inadequately described. For example, there are many kinds of nylon and silicones with very different compositions, and the specific compositions tested were not specified. Also, commercial compositions of many other plastics such as polypropylene and polyvinylchloride contain a variety of plasticizers, antioxidants and other additive that may affect their triboelectric properties. Therefore, these series should be considered only as rough guides to the relative charge transfer propensity of materials relatively close to each other in the lists. However, we can reliably expect that wool, silk, and human hair and skin and other biological structures that they resemble molecularly will lose electrons to those materials at the bottom of the Table 1. This may be one of the reasons that unmodified polytetrafluoroethylene (PTFE), is not an ideal implant material, even though it resists chemical attack.
Poling Procedures
Thermal Poling
Thermal poling is the best known and most widely used method, and transducers made by various thermal processes are available commercially. It may also be the most useful method for biocompatibilizing plastics. The common feature of the processes used commercially is that plastic film or sheet, sometimes oriented, is heated to a temperature at which there is sufficient molecular mobility to allow dipoles to become oriented in a DC field applied across the film with two electrodes. Plastics do not have sharp melting points and, in general, dipoles will have mobility at temperatures well below the level at which there will be significant deformation of a shaped article. The film is then quenched or allowed to cool in the field so that the field-imposed orientation is frozen in. Commercial poling is generally done at field strengths just below breakdown at maximum allowable temperatures and times in order to achieve the highest possible dipole orientation and transducer response. The lower degree of orientation required for the applications under discussion can be achieved by careful control of temperature, time and voltage, without appreciable dimensional change of the article.
Corona and Plasma Poling
Some plastics can be poled by exposing them to a corona discharge or plasma. For example, polyvinylidenefluoride (PVF2) film has been poled by exposing one side to a plasma with the other side at a positive potential. It is possible that dipoles with a net orientation are created as a result of defect production by energetic electrons. However, it is also possible that the surfaces are modified by oxidation if the processes are carried out in air. Electrical discharges, generated in various ways, but generally without a large accelerating potential, have been used on both metal and plastic implants to clean surfaces and also to improve printability and adhesion in non-implant applications. At their current state of development, these electrical discharge processes are poorly understood and probably hard to control. They are also limited to line-of-sight applications and may not be as useful for implants as the thermal methods described above. However, it is possible that dipoles and consequent piezoelectric activity may be created inadvertently in implants by such treatments intended to achieve other objectives.
Beta-irradiation Poling
It has been demonstrated that PTFE can be poled by a beam of energetic electrons. Dipoles with a net orientation may be the result of defects created by the radiation, which is strongest at the surface and diminishes according to the Beer-Lambert Law as it penetrates the solid. However, it may also be due to surface oxidation if oxygen was present. In either case, the result is a charge gradient or dipole. Many plastics, including PVF2 and high-density polyethylene, should be polable by this method. However, the procedure will be limited to line-of-sight surfaces. Beta irradiation has been used to sterilize surgical supplies, some of which contain plastics, so the piezoelectrical characteristics of these plastics have probably been changed.


It should be possible to mimic the normal, stabilizing dipole structure of biological membranes by poling the surfaces of plastic implants. Poling should eliminate destabilizing electron-poor areas on the tissue- and fluid-contacting surfaces of implants and of tissue and fluid handling and storage equipment. Poling should also eliminate destabilizing triboelectric effects. There is a correlation between triboelectric activity and wettability [9] so poling should also change the wetting properties of a prosthesis.
For reasons discussed above, thermal poling may be the most useful method for biocompatibilizing plastics. The modified thermal poling procedures suggested below may be useful for biocompatibilizing polytetrafluoroethylene and other inert fluoroplastics that are currently not widely used in implants.
The thermal poling procedures summarized above were developed for transducers, which produce a positive potential on one side and a negative potential on the side of the film when stressed. They should be applicable to the treatment of power leads, stents, catheters, and, other, similar structures in which only one side of the poled plastic will be in contact with tissue or fluid. However, it may be possible to uniformly modify all surfaces of plastics articles of any shape with a modified thermal poling procedure.
If a shaped plastic article is totally immersed in a charged, conductive medium, such as a saline solution or a molten salt bath at a sufficiently high temperature, there will be a net orientation, normal to the surface, of the underlying molecular and crystalline dipoles in response to the field. The bath is, in effect, a Faraday cage, and the field will be effective over the entire surface in contact with the conductive fluid. Then, if the bath is allowed to cool while charged, or alternatively, if the plastic shape is quickly removed from the hot, charged bath and quenched in a cold liquid, the field-imposed molecular polarization should be frozen in. The latter procedure may help to control the depth from the surface to which dipoles are oriented and also limit the piezoelectric response to desired levels. Lengths of tubing can be treated on both inner and outer surfaces by inserting a close-fitting electrode into the lumen and inserting this assembly into a sleeve at the same potential as the other electrode. A counter-electrode is not required in these procedures, and no electron-poor surface areas are produced.
The thermal poling procedure described above should be applicable to most polymers useful for implants or biological fluid handling, but some may be difficult to pole without affecting the dimensions of the piece. Another poling alternative can be employed where changes in the dimensions of the piece during poling can not be tolerated, for example, in a heart valve. If an injection molding of such a part is allowed to cool in an electrically charged mold, the dipoles under the entire surface of the article will be oriented. Alternatively, the molded piece can be put back into the mold after the latter is removed from the machine, and the entire assembly can then be re-heated and re-cooled while the metal mold is connected to one terminal of a power source in order to achieve the desired orientation.
The suggested modifications of thermal poling should be the most generally applicable for biocompatibilizing plastics, but beta-ray poling may also be useful, especially for plastics with very low crystallinity, that is, those that consist almost entirely of amorphous phase. Novel combinations of both methods might be effective for treating these amorphous plastics. For example, the plastic item could be exposed to soft X-rays or beta radiation, in order to create dipoles by creating defects, followed by the suggested thermal poling modifications to orient the defect dipoles in the desired direction. X-rays are more penetrating and would be less effective at producing defects, but it is easier to generate x-rays economically in-house. Therefore, “soft” (low-energy) x-rays may have utility for defect production. The proposed combination should be useful for all plastics, even those that are highly crystalline, and complex shapes will be easily treated.
Blood Storage
Stored blood is considered to have a 42-day shelf life at 4oC. However, many recent studies have shown that whole blood and blood fractions deteriorate significantly long before 42 days. According to a recent retrospective study [10], morbidity and mortality of patients who received red blood cells were associated with duration of storage. Changes in the concentrations of many important blood components have been reported in a large number of publications. For example, Stamler and McMahon and their associates at Duke University [10] have shown that nitric oxide (NO) in stored blood disappears completely within three days. Loss of NO is only one of many changes, including complement activation [12], that occur in blood stored under standard blood-bank conditions. Oxidative reactions have been identified as the cause of almost all of these changes, and antioxidants are widely used to preserve ingredient levels, especially those of specific disease markers in samples drawn for clinical analysis.
Significant deterioration also occurs when blood is handled; for example, in dialysis equipment. It is widely believed that mechanical effects, such as turbulence and shear forces, are responsible, but this remains unproven and is perhaps unlikely since blood is subjected to equivalent or greater forces in normal flow.
Deterioration of blood occurs in storage containers of all kinds so it is not probable that materials leached from container surfaces are responsible for degradation on storage in state-of-the-art containers. The tentative conclusion that exposure to positive charge on plastic container surfaces may be responsible for the observed oxidative changes is entirely consistent with the literature, and this reasonable speculation can be easily tested by comparing the compositional changes in poled and un-poled plastic containers under standard blood-storage conditions.
The failure rate of arteries treated by balloon angioplasty alone is about 50%. Restenosis of the arteries is reduced to about 25% if bare-wire stents are inserted at the treated sites and reduced further to 5% if drug-eluting stents are used. However, there have been reports of later clotting at the drug-eluting stent site. A meta-analysis of 14 studies involving a total of 6,675 patients [13] determined that there was a 4- to 5-fold increase in late thrombosis compared with patients that had bare-metal stents. It has been estimated that deaths from the drug-eluting stents exceed 2,000 a year, and some people believe that yearly deaths may become many times higher because the increased risk of late thrombosis seems to persist. The thrombosis rate is 1.2% for both bare- metal and drug-eluting stents 30 days after placement [14]. However, thereafter the thrombosis rate at bare-metal stent sites decreases with time while the rate at drug-eluting stent sites increased annually. This persistence and increase of the thrombosis risk is consistent with the possibility that electron-poor plastic stent surfaces are destabilizing blood cells and colloids and are causing oxidation and complement activation.
It should be relatively easy to pole the plastic coating on drug-eluting stents in order to test this central speculation. The plastic coating on a wire stent can be poled in a heated and charged bath as described above. The wire of the stent can be used as a counter-electrode, but this may not be necessary. Alternatively, the wire can be coated while charged.


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Source(s) of Funding

The author was not employed by a commercial organization or associated with any teaching or research institution during the period in which this study was conducted, and he has no personal relationships with any researcher in relevant fields. This study was unsponsored and unfunded.

Competing Interests

The author does not have any competing interests.


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