Medical Plastics and Biomaterials Magazine
MPB Article Index

Originally published March 1998

HEMOCOMPATIBILITY

Most of today's blood-contacting medical devices are made of synthetic materials. When blood contacts these materials foreign to the body, a number of adverse reactions are triggered—including platelet attachment, platelet activation, and complement activation—that eventually lead to fibrin production and clot formation. These clots can impair the function of the device. More drastically, they can occlude critical vessels at the implant location or be released into the patient's blood stream, where they can obstruct distant blood vessels, potentially leading to strokes or even death.¹

Catheters on a dipping machine are coated to enhance hemocompatibility. Photo: Surmodics, Inc.

A variety of devices can be successfully accepted by most patients through the administration of systemic anticoagulants such as heparin and warfarin. However, the risk of complications or drug intolerance related to these anticoagulants is ever present, and only adds to the inherent risk of device-associated complications. Some patients cannot tolerate any pharmaceutical regimen used to counteract the clotting that can be caused by synthetic materials, and are therefore ineligible to receive certain blood-contacting devices. For these reasons, research into blood-compatible materials has been aggressively pursued.

It is not always feasible to produce a bulk material that can furnish both the desired blood-compatible surface and the mechanical and physical properties necessary for a particular device. Another daunting task is finding a material that can be used across the vast spectrum of medical devices requiring hemocompatible properties. Such devices range from syringe needles to polymer tubing to artificial hearts, and they must retain their compatibility properties over periods that vary from seconds to weeks to years.

Surface modification is another way of providing blood compatibility; however, the diverse array of medical devices demands a corresponding range of hemocompatible surface properties. For example, coatings containing biological agents (such as heparin or prostaglandins) can degrade over time and therefore are not suitable for use on long-term blood-contacting devices such as pacemaker leads or heart valves. Another surface-treatment candidate is polyethylene oxide, which prevents platelet attachment but also repels other cells, making these coatings inappropriate for devices such as vascular grafts or coronary stents, onto which cell overgrowth is desired. To successfully modify the variety of devices requiring blood-compatible surfaces, many different surface-modification agents need to be developed and tested.

The specific surface-modification requirements of a blood-contacting device depend on the length of time the product will be exposed to blood and on the criticality of the device or procedure. Three categories can be delineated: (1) devices requiring a short blood exposure, of a few hours or less; (2) devices in blood contact for longer periods of time (up to 30 days) under more-critical conditions; and (3) life-extending, permanently implantable devices.

ANTIADHERENT COATINGS

For applications requiring short-term blood compatibility, it is important only that the device repel platelets, proteins, cells, or other fouling materials. It may not be necessary to provide heparin or other biologicals on the surface, since the attachment of blood components must be prevented for a limited time only, and the risk of introducing emboli into the blood stream is minimal. In addition, the patient usually receives systemic anticoagulants during such procedures, further reducing the need for a bioactive coating on the device.

Several companies provide nonthrombogenic coatings for short-duration blood-contacting medical devices. STS Biopolymers, Inc. (Henrietta, NY), offers its Slip-Coat hybrid polymer system, and SurModics, Inc. (Eden Prairie, MN), has developed the PhotoLink photoactivated hydrogel surface modification. Other suppliers include Spire Corp. (Bedford, MA), with its SPI-Polymer line, and Hydromer, Inc. (Branchburg, NJ), producer of Hydromer lubricious coatings. Among the products that can benefit from the use of these antiadherent coatings are percutaneous transluminal coronary angioplasty catheters, guide catheters, angiography catheters, dilators, introducers, and drug-infusion catheters.

Many of the short-term blood-contacting devices are catheters or introducers that also require a lubricious surface. Because antiadherent blood-compatible coatings are often hydrogels that are lubricious when wet, they can simultaneously provide the dual surface enhancements of lubricity and hemocompatibility. For example, Target Therapeutics, Inc., a Boston Scientific Corp. (Fremont, CA), uses a photoactivated hydrogel surface modification on its FasTracker microcatheters. Studies have shown that platelet aggregation and clot formation through adherence of blood components to the coated catheters are less than with uncoated catheters or other commercially available coated catheters.² The investigations involved platelet attachment and activation experiments conducted using human platelet-rich plasma. Test samples (polyethylene disks) were placed into six-well plates, one disk per well. The platelet-rich plasma solution was applied to the disks until the entire surface was covered. The plasma was then removed, the surface rinsed, and the disks fixed, dehydrated, and observed under a scanning electron microscope. The number of platelets was assessed visually, and the degree of activation was judged qualitatively on the basis of morphology.²

Table I. Results of adhesion of platelets from platelet-rich plasma onto coated and uncoated polyethylene surfaces.

Results indicated that the surface concentration of platelets on the polyethylene that had been modified with the photoactivated hydrogel was reduced by 99% compared with the uncoated polyethylene (see Table I). In addition, the platelets that did adhere were of a morphology that indicated reduced activation compared with the platelets on the uncoated surface.² The reduction in platelet adhesion with the photoactivated hydrogel was comparable to that achieved on a similarly applied heparinized surface.

BIOLOGICALLY ACTIVE COATINGS

A second method for making a device surface hemocompatible is through the use of biologically active coatings that contain agents such as heparin among their components. These coatings are more effective than a simple repellent surface at inhibiting thrombus development on a device surface. Heparin coatings are also advantageous when the interventional procedure is more critical in nature: for example, when the shedding of tiny clots from the surface of the medical equipment could result in the clots reaching the brain and causing a stroke. Devices benefiting from the use of heparin coatings include coronary-bypass oxygenators, central venous access catheters, coronary stents, and hemodialysis equipment.

Heparin coatings work by preventing platelet adhesion and activation, events that must be minimized to maintain blood compatibility. Heparin is a highly charged molecule that interferes with the blood-clotting mechanisms by catalyzing the inactivation of enzymes—such as thrombin and Factor X_a—in the fibrin cascade. Figure 1 shows the results of an in vitro platelet adhesion study on uncoated versus heparin-coated polyethylene. Coatings containing heparin have also been shown to reduce thrombogenesis on metallic stents in animal models.³ In addition, heparin stent coatings have been associated with a reduction in the rate of acute thrombosis, according to preliminary clinical data.⁴

Figure 1. Scanning electron micrographs of platelet attachment on uncoated (left) and photoheparin-coated (right) polyethylene.

In another study, an acute dog-jugular-vein catheter model was used. Both uncoated polyurethane rods and those treated with a photoactivated heparin coating were implanted for 1 hour in the external jugular veins of dogs that had not received an anticoagulant. Platelet attachment was monitored in real time with spatial resolution, using gamma-camera imaging of Indium 111—labeled platelets. Thrombus formation was quantified by removing and weighing any adherent biological material upon explant of the catheter. To reduce the influence of animal variability, each dog received bilateral implants, with a coated catheter and an uncoated control catheter used in each experiment. Figure 2 shows the results of this study.⁵ Both heparin alone and heparin combined with a hydrogel reduced platelet adhesion and thrombus formation compared with the uncoated surface. The data suggest there may be added benefit to having the hydrogel present, although the differences between the results with the two coatings were not statistically significant.

Figure 2. Platelet attachment and thrombus formation on canine jugular-vein implants.

Heparin coatings for medical devices have been commercialized by several companies. Carmeda AB (Stockholm) produces a heparin coating for blood-contacting devices known as the Carmeda BioActive Surface (CBAS). Baxter International, Inc. (Deerfield, IL), offers a line of products treated with the company's Duraflo technology, a coating that may lessen complications during surgery. Other examples of heparin-containing coatings include PhotoLink photoheparin coatings from SurModics and Medi-Coat hydrogels from STS.

COATINGS TO PROMOTE HOST-CELL OVERGROWTH

A different means of producing blood-compatible coatings is required for devices that will be permanently implanted in the body. In most cases, these devices need to be inert to both the body's immune system and its coagulation system: the implanted device will have to mimic the body to such a degree that it actually becomes invisible to the body's defense mechanisms. Artificial vascular grafts, stent grafts, and heart-valve sewing cuffs are examples of devices that remain in the body permanently and thus require a surface that will retain its hemocompatibility throughout its service life.

One technique that falls into the realm of tissue engineering for hemocompatibility involves promoting endothelial cell growth. Endothelial cells are the cells that line the internal walls of blood vessels, and as such prevent platelet attachment and blood coagulation. In the case of blood-compatible biomimetic coatings, one objective is to encourage the growth of a layer of endothelial cells over the device surface so that the blood is no longer exposed to the foreign material.

An effective method of promoting the integration and adhesion of the cells onto the device is to immobilize agents such as extracellular matrix (ECM) proteins and peptides directly onto the device surface. This technique has been shown to be particularly effective in small-diameter (<6 mm) vascular grafts, for which it is desirable that a layer of endothelial cells cover the entire inner surface of the device to prevent occlusion.

A series of dog implant studies have been conducted to evaluate the effectiveness of photoimmobilized ECM proteins at enhancing endothelial cell ingrowth.⁶ For these studies, fibronectin (FN) and collagen IV (COL IV) were immobilized individually and in combination onto the luminal surfaces of two types of 4-mm vascular grafts, made of either porous polyurethane or standard expanded polytetrafluoroethylene (ePTFE). Coated grafts and uncoated controls 4—5 cm long were implanted intrafemorally in the animals. The patency was evaluated by Doppler echocardiography, and the grafts were retrieved at 30 or 180 days. Each graft was split longitudinally, with one half evaluated by scanning electron microscopy to quantitate endothelial cell coverage on the luminal surface (see Figure 3). The other half of each graft was embedded, sectioned, and examined histologically for intimal hyperplasia.

Figure 3. Neointimal growth on 4-mm-ID expanded polytetrafluoroethylene vascular grafts. Note thrombus formation on uncoated graft (left) and complete endothelial-cell coverage on coated graft (right).

After 30 days, grafts coated with FN/COL IV showed greater patency and endothelial coverage than did uncoated controls or grafts coated with either protein applied individually.⁶ Table II shows that the combination of FN/COL IV produced an approximately threefold improvement in both patency and endothelialization of both types of 4-mm vascular grafts at 30 days. The mechanism of endothelialization was most likely via pannus ingrowth—the migration of endothelial cells onto the graft from the adjacent artery. This is the most probable explanation, since (1) the grafts had wall porosities that were below that which is reported to allow for transmural endothelialization, and (2) the endothelial cells that were observed on grafts that did not achieve 100% endothelialization (uncoated controls and 4 of the 14 coated grafts that stayed patent) were all part of continuous endothelial sheets that extended to the an-astomoses. Because endothelial cell ingrowth seemed to originate from each anastomosis, the rate of ingrowth was about 2 cm/month.

Table II. Patency and endothelial-cell coverage of 4-mm vascular grafts implanted in dogs for 10 days.

A study using the same ePTFE grafts implanted for 180 days showed endothelial coverages of 95% ±6% with the FN/COL IV coating, as compared with 61% ±6% for uncoated controls. This means that uncoated controls at 180 days still have a lower endothelial cell coverage than the 88% observed with the FN/COL IV coating at only 30 days. These results demonstrate that an appropriate combination of covalently immobilized ECM proteins can improve both the patency and endothelialization of 4-mm vascular grafts, with the mechanism of endothelialization most likely being via promotion of pannus ingrowth. The endothelial cell coverage of 61% on uncoated controls at 180 days corresponds to a pannus ingrowth of about 1 cm, which was previously reported to be the maximum pannus ingrowth that occurs on vascular grafts implanted into dogs.^7,8 Histological evaluation of the grafts showed no significant intimal hyperplasia on the ePTFE grafts at either 30 or 180 days. The layer of smooth muscle—like cells present under the endothelial cells was typically 50—200 µm thick at the anastomoses, tapering to 0—50 µm thick at a point 0.5 cm from the anastomoses.

Another study that reported enhanced endothelialization produced by an ECM coating involved applying a strongly adsorbing preparation of RGD (a peptide derived from fibronectin) onto ePTFE patches and then implanting the coated patches in dog carotid or femoral arteries. Upon retrieval at three weeks, three of four patches coated with RGD showed complete endothelial cell coverage, whereas only one of the four uncoated controls showed complete coverage.⁹ Although this implant model is not as rigorous as the previously described experiment, it supports the concept that immobilized ECM components can enhance endothelialization.

OTHER APPROACHES TO HEMOCOMPATIBILITY

Several other prominent technologies are under investigation for possible use in designing hemocompatible devices. For example, phosphorylcholine surfaces have been demonstrated to improve the blood compatibility of medical device materials. Phosphorylcholine is an abundant constituent of the external membranes of cells that circulate in blood; it inhibits protein adsorption and platelet adhesion and activation, which are key events in surface-initiated thrombus formation. Experimental results from in vitro and in vivo studies indicate the potential benefits of immobilized phosphorylcholine.^10—12 (One company that has marketed a phosphorylcholine coating for hemocompatible surfaces is Biocompatibles International plc, located in Farnham, Surrey, UK.)

Albumin-binding coatings are yet another surface-modification candidate for improving blood compatibility. Studies have shown that albumin inhibits fibrinogen adsorption in vitro and in vivo, and also inhibits fibrin formation and platelet aggregation in short-term in vivo experiments.¹³ Other tests have demonstrated that albumin groups can enhance the blood compatibility of polyurethane in vivo.¹⁴

Research also continues on thrombolytic coatings containing plasminogen activators such as tissue plasminogen activator, polylysine, streptokinase, and urokinase. The plasminogen activators induce the body to remove any occurrence of clotting, since activated plasminogen dissolves fibrin as it forms.

CONCLUSION

Just as more and better implantable medical devices are being developed specifically for introduction into the body's circulatory system, more and better hemocompatible surface modifications are being introduced for those devices. Surface modification is growing in interest and in use across a wide range of applications. Now viewed as a competitive necessity rather than a luxury feature, the enhancement of surface characteristics has rapidly become an essential element in the development process for medical devices.

Fortunately, there are today many surface-modification techniques available. Depending on the length of time the device will be implanted and the difficulty and criticality of the procedure, most manufacturers will be able to find the process that best meets their specific requirements. Whether it be a coating that repels fouling materials, a biologically active coating containing heparin, or a coating that induces host-cell growth across the surface of a device, options are available for engineers to consider as they design new and improved medical products.

REFERENCES

1. March J, Advanced Organic Chemistry, 3rd ed, New York, Wiley, pp 216, 1078, 1985.

2. Anderson AB, Chudzik SJ, Hergenrother RW, et al., "Platelet Deposition and Fibrinogen Binding on Surfaces Coated with Heparin or Friction-Reducing Polymers," Amer J Neurolog Res, 17:859—863, 1996.

3. Hårdhammar PA, van Beusekom HMM, Emanuelsson HU, et al., "Reduction in Thrombotic Events with Heparin-Coated Palmaz-Schatz Stents in Normal Porcine Coronary Arteries," Circulation, 93:423, 1996.

4. Serruys PW, Emanuelsson H, van der Giessen W, et al., "Heparin-Coated Palmaz-Schatz Stents in Human Coronary Arteries. Early Outcome of the Benestent-II Pilot Study," Circulation, 93:412, 1996.

5. Anderson AB, Enrico LS, Melchior MJ, et al., "Photochemical Immobilization of Heparin to Reduce Thrombogenesis," Trans Soc Biomat, p 75, 1994.

6. Clapper DL, Hagen KM, Hupfer NM, et al., "Covalently Immobilized ECM Proteins Improve Patency and Endothelialization of 4-mm Grafts Implanted in Dogs," Trans Soc Biomat, 16:42, 1993.

7. Sauvage LR, Berger KE, Wood SJ, et al., "Interspecies Healing of Porous Arterial Prostheses," Arch Surg, 109:698—705, 1974.

8. Williams SK, Schneider T, Kapelan B, et al., "Formation of a Functional Endothelium on Vascular Grafts," J Elec Micros Tech, 19:439—451, 1991.

9. Tweeden KS, Blevitt J, Harasaki H, et al., "RGD Modification of Cardiovascular Prosthetic Materials," in Proceedings of the Cardiovascular Science and Technology Conference, Arlington, VA, Association for the Advancement of Medical Instrumentation, p 124, December 1993.

10. Campbell EJ, Chronos NAF, Robinson KA, et al., "Non-Thrombogenic Phosphorylcholine Coatings for Stainless Steel," Trans Soc Biomat, 28:51, 1995.

11. Chronos NAF, Robinson KA, Kelly AB, et al., "Thromboresistant Phosphorylcholine Coatings for Coronary Stents," Circulation, 92(8):I-685, 1995.

12. Ishihara KR, Aragaki R, Ueda T, et al., "Reduced Thrombogenicity of Polymers Having Phospholipid Polar Groups," J Biomed Mat Res, 24:1069—1077, 1990.

13. Eberhart RC, Munro MS, Frautschi JR, et al., "Influence of Endogenous Albumin Binding on Blood-Material Interactions," Annals of The New York Academy of Sciences, Vol 516—Blood in Contact with Natural and Artificial Surfaces, Leonard EF, Turitto VT, and Vroman L (eds), New York, New York Academy of Sciences, 1987.

14. Grasel TG, Pierce JA, and Cooper SL, "Effects of Alkyl Grafting on Surface Properties and Blood Compatibility of Polyurethane Block Copolymers," J Biomed Mat Res, 21:815—842, 1987.

Aron B. Anderson, PhD, has been with SurModics, Inc. (Eden Prairie, MN), since 1991, and is currently director of hemocompatibility. His responsibilities include developing coatings for improving the compatibility of blood-contacting devices. Director of tissue engineering at SurModics, David Clapper, PhD, has worked at the company since 1986. Current projects include the development of coatings to enhance tissue integration with implant devices and of biodegradable matrices to promote the healing of tissue defects.

Copyright ©1998 Medical Plastics and Biomaterials