Biomaterials for heart repair

12.2.4 Biomaterials for heart repair

12.2.4.1 Heart valves

Heart disease is one of the major killers in the developed world. Many of the serious conditions arise from the strain imposed on the heart by obstructions to the flow of blood either in the main passages of the circulatory system or as a result of valvular disease. The heart acts as a blood pump with four valves which open in response to a unidirectional flow of blood. Two of the valves allow the blood into the heart and the other two control the blood leaving the heart. Problems can arise with the heart valves if they become structurally damaged by disease affecting the opening and closing mechanisms.

The early prostheses developed in the 1960s for valve replacement were based on a stainless steel ball-in-cage or polysiloxane ball in a Co–Cr alloy cage (see Figure 12.7). These restrict the blood flow, even in the open position, and were superseded by a tilting disk device which opens and closes to the beat of the heart to allow the required blood flow. The major problem with valves arose from the tendency to initiate a blood clot and thus it was found necessary to give valve recipients anticoagulant

Case examination of biomaterials, sports materials and nanomaterials 591

Patella bearing

Femoral

surface

bearing surface Femoral guide surface

(FGS) Tibial

guide surface

(TGS) Mobile

bearing

Smooth metal tibial plate

A total knee replacement joint. (a) Schematic diagram (after Walker and Sathasiwan, 1999). (b) Photograph of a stainless steel prosthesis (courtesy R. Grimer, Royal Orthopaedic Hospital, Birmingham).

(a)

(b)

Figure 12.7 Photographs of replacement heart valves. (a) Ball-in-a-cage type (courtesy Institute of Materials, Minerals and Mining). (b) Pyrolitic carbon disk type (courtesy P. Marquis, Dental School, Birmingham).

drugs. Nowadays, advanced valves are made with Co–Cr or titanium bodies with metal or graphite disks, or occluders, coated with pyrolytic carbon. These coatings are made by heating a hydrocarbon, such as methane, to about 1500 ◦

C, depositing the carbon vapor on the graphite surface to a thickness of about 1 mm. Small amounts of silane mixed with the CH 4 add Si to the deposit, increasing its strength. Pyrolytic carbon is strong and wear resistant but, more importantly, resists the formation of blood clots on its surface. The disks are attached by a coated Ti metal arm to a fabric ring made of polymer (PTFE), which is sewn to the tissue of the heart valve opening. Dacron cloth has also been used and encourages tissue growth with better anchorage and thromboembolic resistance.

The prosthesis has moving parts and thus catastrophic failure by fracture is a finite possibility. As an alternative, the construction of artificial valves from biological tissue has been developed. Collageneous tissue from the heart wall of cows and heart valves from pigs have been used to

592 Physical Metallurgy and Advanced Materials make these ‘bioprosthetic’ valves. These valves are naturally biocompatible with a reduced risk

of thrombosis. Unfortunately, two problems have hindered this development. The first is that the collagen valve material suffers from slow calcification, whereby hard deposits form on the surface of the valves, causing them to stick and tear. The use of anticalcification drugs is a possibility, but the second problem has thrown the whole area of implant surgery using animal tissue in doubt. This problem is the emergence of BSE (mad cow disease) in cattle, which has led to restrictions and worries in the use of animal tissue for reconstructive implant surgery because of the fear of transmission of viral illness from animal tissue to humans.

12.2.4.2 Pacemakers

Other developments in improving heart performance include the use of cardiac pacemakers which produce a 5 V electrical impulse for 1/500 second at regular heartbeat rate. These devices have been available for some time but have improved significantly over recent years. The basic requirement is to provide electrical signals to the heart at the appropriate level to stimulate the patient’s own electrical activity to produce the proper physiological change, normally linking the pacemaker into the cardiac system so that it works when needed. The biomaterial aspects of the pacemaker are, however, also important, not only in overcoming the problems introduced by any device/body environmental interaction, but also in designing the proper electrical supply and insulation. Power supplies have advanced considerably in the last few years and lithium cells are now exclusively used. Titanium is again the most common biomaterial to encase the device, manufactured and electron beam welded to seal it hermetically. Polymers have been used for encapsulation, e.g. epoxy resin or silicone rubber, but these materials do not completely prevent moisture from entering the pacemaker, shortening the lifetime of the device. These have now been superseded by titanium alloys because of their better strength and environmental properties. These are sutured into the aorta with a Dacron sleeve.

Another problem area is that provided by the electrodes which have to flex with every heartbeat and hence are liable to fatigue failure (see Chapter 6). Good design and choice of electrode materials can minimize this problem. The electrical supply passes through the titanium casing via a ceramic insulator and the leads to the heart are insulated with a polymer (polyetherurethane). Degradation with time is still a possibility and has to be considered in an effective design.

12.2.4.3 Artificial arteries

In branches of surgery, particularly cardiovascular surgery, there is often a need to replace arteries blocked by atherosclerosis. Sometimes this can be achieved by using tissue grafts from the patients, thereby avoiding any immune response. In other cases it is necessary to use artificial arteries made from polymers; such arteries must be tough and flexible enough to avoid kinking, with the added requirement of avoiding the formation of blood clots. In modern surgery, the blood clotting tendency can be removed by anticoagulants, such as heparin, but the ultimate goal is to provide artificial arteries with natural clot resistance. Several different polymers have been used to make blood vessels, but none is entirely satisfactory.

The polyester Dacron can make small tubes but these have porous walls which have to be sealed. This is achieved by treating with the protein albumin and heating it to form a coagulated coating. In the body, the albumin degrades and is replaced by the natural protein collagen, forming a smooth lining (pseudointima). This process leads to some initial inflammation, which is one disadvantage of this biomaterial. Woven Dacron is quite rigid and unsuitable for small arteries and is difficult to suture; it is mostly used for resected aortic aneurysms. Knitted Dacron is easier to suture. It may be coated with polyurethane, tetrafluoroethylene or heparin to reduce the thrombogenic tendency. The use of poly(hydroxyethyl methacrylate) coating establishes an endothelial-like cell layer in a few weeks.

Case examination of biomaterials, sports materials and nanomaterials 593 Other arterial polymers include PTFE and polyurethanes. PTFE is used as an expanded foam

to form the porous tubes. These rapidly develop a smooth neointima layer and thus acquire blood compatibility. Polyurethanes have a natural compatibility with blood, and are tough and flexible in tube form, but unfortunately slowly degrade in the body, producing toxic products. PTFE coatings on polyester and polyurethane vessels have also been tried. Silicone-lined tubing has been used for extracorporeal circulation during open-heart surgery.

To produce artificial arteries with built-in clot resistance, heparin molecules have been attached to their surface either directly by chemical bonds or by cross-linking to form a polymerized heparin film. To mimic total thrombosis resistance, however, requires not only anticoagulation, but also avoidance

of platelet deposition normally achieved by the endothelial cells lining the blood vessel releasing the protein prostacyclin. Ideal artificial arteries should have both of these anticlotting agents attached at their surface.