BIOMATERIALS ENT 311/4 Lecture 9 Blood Contacting Implants or Devices Prepared by: Nur Farahiyah Binti Mohammad Date: 26 th August 2008 Email : [email protected]

BIOMATERIALSENT 311/4

Lecture 9 Blood Contacting Implants or

Devices

Prepared by: Nur Farahiyah Binti Mohammad

Date: 26th August 2008

Email : [email protected]

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Teaching Plan

COURSE CONTENT

Define and identify blood contacting implant.Describe and recommend primary requirements for biomaterials for blood contacting implant.Discuss the development of biomaterials for long term implantIdentify common problems for heart valve prosthesis, total artificial hearts and pace makers

DELIVERYMODE

LectureSupplement

LEVEL OF COMPLEXITY

KnowledgeRepetitionApplicationAnalysisEvaluation

COURSE OUTCOMECOVERED

Ability to describe the concept of biocompatibility & basic concepts of materials used in medical application

Ability to select biomaterials that can be used for different medical applications and explain the criteria that will lead to a successful implants

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Blood contacting implants or devices have a direct contact with the blood.

Blood comes in contact with foreign materials either for a short term or long term.

1.0 Introduction

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1.0 Introduction

Short term extracorporeal devices (outside the body): Dialyzers Blood oxygenator Tubes and catheters for

transport the blood

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1.0 Introduction

Long term blood contacting implant: Heart valves prostheses Vascular grafts Cardiac pacemakers Implantable artificial organs

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2.0 Primary requirement

The primary requirement for biomaterials for long-term implants are: Blood compatibility (blood compatible) Non-toxicity Durability Non-irritating to tissue Resistant to platelet and thrombus deposition Nondegradable in physiological enviroment Do not absorb blood element Do not release foreign substance

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3.0 Design consideration

The implant should mimic the function of organ that it replace without interfering with the surrounding anatomical structures.

Must be suitable size and weight Biomaterial chosen must be easily

available, inexpensive, easily machinable and sterilizable.

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3.0 Design consideration

As an example: artificial heart valve is required to open and close on an average once every second (valves open and close 30 million times per year). The biomaterial chosen must be such that the valve is durable and will not fail under fatigue stress after implantation in patient.

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Revision

“ Biocompatibility is the ability of a material to perform with an appropriate host response in a specific application” (William, 1987).

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Revision

In vivo test for tissue compatibility1.Sensitization2. Irritation3. Intracutaneous reactivity4.Systemic toxicity (acute toxicity)5.Subcronic toxicity (subacute toxicity)6.Genotoxicity7. Implantation8.Hemocompatibility (Blood compatibility)

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4.0 Blood compatibility

Blood compatibility can be defined as the property of material or device that permits it to function in contact with blood without inducing adverse reactions.

Implant should not Induce coagulation (blood clotting) Damage blood cells

Should not induce Hemolysis (the breaking open of red blood cells and the release of hemoglobin into the surrounding fluid)

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4.1 Blood Coagulation Coagulation is a complex process by

which blood forms clots.

4.1.1 Mechanism: Intrinsic

Initiated by blood contact with either a damaged portion of the blood vessel wall or another thrombogenic (clot causing) surface.

Takes 7-12 minutes to form a soft clot

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Extrinsic Result of the presence of a foreign body or

tissue damage (other than blood vessel) Takes 5-12 seconds to form a soft clot

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4.1.2 Factor affect the blood compatibility of a material

i. Surface roughness Rough surface have a greater surface

area and contact surface with blood compared to smooth surfaces

Result in faster coagulation

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ii. Surface Charge The tunica intima (the innermost layer of artery or vein)

of a normal blood vessel has a negative surface charge due to proteins at surface of the cell membrane.

Formed blood element (red cells, white cells, and platelets) also have a negative charge.

Natural repulsive force between intima and cells minimizes cell damage and coagulation

iii. Low surface tension Blood cells less likely to adhere to a surface with a low

surface tension

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iv. Heparinized surfaces Heparin is a polysaccharide with negative

charge. Heparin is a naturally-occurring anticoagulant

produced by basophils and mast cells. Heparin acts as an anticoagulant, preventing the

formation of clots and extension of existing clots within the blood.

it allows the body's natural clot lysis mechanisms to work normally to break down clots that have already formed

Attempt made to attach heparin chemically to the surface of the implant to prevent blood clot.

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4.2 HEMOLYSIS Motion at a blood-surface interface may

damage red and white blood cell resulting in cell death.

Damage of cell occurs with shear stresses on the cells of less than 500dyn/cm2 .

Chronic and accumulated damage of red blood cells and leakage of the cellular contents can result in: Anemia Kidney Failure Toxemia

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Blood contacting implant or devices

BLOOD IN CONTACT

Long term Short termHeart valve prosthesesVascular graftsCardiac pacemakers

Blood oxygenator of heart lung machineDialyzer of hemodialysis machineTubes and catheters for transport the blood

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5.0 Heart valve prostheses

HEART VALVES Heart valves are very important, as they

prevent the backflow of blood, which ensures the proper direction of blood flow through the circulatory system.

Without these valves, the heart would have to work much harder to push blood into adjacent chambers. The heart is composed of 4 valves: tricuspid, pulmonary, mitral, and aortic.

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HEART VALVE PROBLEMS There are numerous complications and

diseases of the heart valves that prevent the proper flow of blood.

Heart valve diseases fall into two categories, Stenosis

The stenotic heart valve prevents the valve from opening fully, due to stiffened valve tissue. Hence, there is more work required to push blood through the valve

Incompetence. the incompetent valves cause inefficient blood

circulation by permitting backflow of blood in the heart

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Stenosis Incompetence

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TREATMENT OPTIONS On a large scale, medication is the

best alternative, although in some cases defective valves have to be replaced with a prosthetic valve in order for the patient to live a normal life

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MAIN PROSTHETIC HEART VALVE Heart valve prostheses can be classified

into two type: 1. Mechanical prostheses : made of non-biological

materials. 2. Biological heart valve: made of biological tissue

Heart valves are designed to fit the peculiar requirements of blood flow through the specific chambers of the heart, with emphasis on producing more central flow and reducing blood clots.

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1. MECHANICAL PROSTHESIS

a) Caged ball This valve uses a small

ball that is held in place by a welded metal cage.

The ball in cage design was modeled after ball valves used in industry to limit the flow of fluids to a single direction

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b. Tilting disc Have a polymer disc held in

place by two welded strut The disc floats between the

two struts in such a way, as to close when the blood begin to travel backward and then reopen when blood begin to travel again.

The titling-disc valves open at an angle of 60° and close shut at rate of 70 times/minute

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Advantages: Provide improved central flowwhile still

preventing backflow Reduce mechanical damage to blood cells Reduce blood clotting and infection

Problem: Have a tendency for the outlet strut to

fracture as a result of fatigue from the repeated ramming (smash into) of the struts by the disc.

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c. Bileaflet valves Consist of two semicircular

leaflets that pivot on hinges Advantages:

Provide the closest approximation to centarl flow achieved in natural heart valve.

Disadvantages: They do not close completely,

which allows some backflow. Since backflow is one of the

properties of defective valves, the bileaflet valves are still not ideal valves.

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d. Trileaflet heart valve Afford true central flow

characteristic with reduced back flow

Good wear characteristic. Significantly improve patient’s

quality of life. This will be achieved due to reduced

consumption of anticoagulants by the patients, reduced noise, low blood hemolysis, and the elimination of the need for repeated implantations because of high reliability of the mechanical design.

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BIOMATERIAL USED IN MECHANICAL HEART VALVE

Heart valve type Component Biomaterial

Caged ball Ball/occluder

Cage

Suture ring

Silicone rubber (Silastic)

Cobalt-chromium alloy (Stellite 21®) or titanium

Silicone rubber inser under knitted composite Teflon and polypropylene cloth

Tilting disc Leaflet

Housing/strut

Suture ring

Polyacetal (Delrin®),pyrolytic carbon, ultra height molecular polyethylene (UHMWPE)

Cobalt-chromium alloy (Haynes 25®) or titanium

Teflon® or Dacron®

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Bileaflet Leafleat

Housing

Suture ring

Pyrolytic carbon

Pyrolytic carbon

Double velour Dacron® tricot knit polyester

Trileaflet Leaflet

Ring

Pyrolytic carbon

Titanium alloy coated with high-density turbostratic carbon

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5.0 Heart valve prosthesesAdvantages of mechanical

heart valveDisadvantages of

mechanical heart valve

High durability- typically last for the lifetime of the patient

1. The increased risk of blood clotting

2. When blood clots of any kind occur in the heart, there is a high probability of a heart attack or stroke.

3. Patient need to take anti-coagulant drug

4. Anti-coagulant caused birth defects in the first trimester of fetal development

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2. BIOLOGICAL/ PROSTHETIC TISSUE HEART VALVE

i. Human tissue valvesii. Animal tissue valve

Advantages: Design of valve are closer to the design of

the natural valve. Do not require long term anticoagulant Do not cause damage to blood cells Do not suffer from many of structural

problems experienced by the mechanical heart valve

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Human Tissue valve

Homograft: valves that are transplanted from another human being

Autograft: valves that are transplanted from one position to another within the same person.

Dysfunctional aortic valve (exit of the left ventricle) is removed, patient’s pulmonic valve is then transplanted to the aortic position.

A homograft pulmonic valve is usually used to replace the patient’s pulmonic valve.

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Human Tissue valve

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Animal tissue valve

Refereed as heterograft or xenograft valves.

The two common prosthesis valve from animal tissue are: PORCINE VALVES BOVINE PERICARDIAL VALVE

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Animal tissue valve

PORCINE VALVES Valve tissue from pig Valve tissue is sewn to a metal wire stent

made of cobalt-nickel alloy. The wire is bent to form three U-shaped

prongs. A Dacron cloth sewing skirt is attached to the

base of the wire stent, and then the stents themselves are also covered with cloth.

Porcine valves have good durability and usually last for ten to fifteen years.

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Animal tissue valve

BOVINE PERICARDIAL VALVE Bovine pericardial valves are similar to porcine

valves in design. The major difference is the location of the

small metal cylinder which joins the ends of the wire stents together.

In the case of pericardial valves, the metal cylinder is located in the middle of one of the stent post loops.

Pericardial valves have excellent hemodynamics and have durability equal to that of standard porcine valves after 10 years.

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Porcine valve

Leafleat

Stent

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Pericardial valve

Leafleat

Suture ring Stent

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Biological tissue valves


Porcine bioprosthesis

Leaflets

Stent

Suture ring

Porcine aortic valve fixed by stabilized gluteraldehyde

Polypropylene stent covered with Dacron, Elgiloy wire covered with porous knitted Teflon cloth

Dacron, soft silicone rubber insert covered with porous Teflon cloth

Pericardial bioprosthesis

Leaflets

Stent

Suture ring

Bovine pericardial tissue fixed by stabilized gluteraldehyde

Polypropylene stent covered with Dacron, Elgiloy wire covered with porous knitted Teflon cloth

PTFE fabric over silicone rubber filter

http://cape.uwaterloo.ca/che100projects/heart/files/pigvalve.jpg

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Biological tissue valves

Advantages of biological heart valve

Disadvantages of biological heart valve

1. Design of valve are closer to the design of the natural valve.

2. Do not require long term anticoagulant

3. Do not cause damage to blood cells

4. Do not suffer from many of structural problems experienced by the mechanical heart valve

1. Stiffening of the tissue due to the build up calcium.

2. Calcification can cause a restriction of blood flow through the valve (stenosis) or cause tears in the valve leaflets.

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Common problem with implanted heart valve

Mechanical valve Biological tissue valve

1. Thrombo-embolism2. Structural failure3. Red blood cell and platelet

destruction4. Tissue overgrowth5. Damage to endothelial

lining6. Tearing of sutures7. Paravalvular leakage8. Infection

1. Tissue calcification (build up of calcium around the tissue

2. Leaflet rupture3. Paravalvular leakage4. Infection

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Prosthetic heart valve type

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Conclusion

The future for replacement heart valves lies in tissue engineering.

The most ideal replacement would be formed from the patient's tissue, and tailored to the right shape and dimensions.

This would improve the biocompatibily factor, and increase the life expectancy of the heart valve.

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6.0 VASCULAR GRAFT

BLOOD VESSELS Blood vessel are the channels through

which blood is distributed to body tissue.

Blood vessel are classified as either: Arteries (carry blood away from the heart) Capillaries Veins (carry blood to the heart)

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6.0 VASCULAR GRAFT

BLOOD VESSELS PROBLEMS Vascular graft is needed to replace diseased

blood vessel such as atherosclerosis blood vessel and aortic aneurysm .

Atherosclerosis is a disease in which plaque (plak) builds up on the insides of your arteries.

Aneurysm is blood-filled dilation (balloon-like bulge) of a blood vessel caused by disease or weakening of the vessel wall.

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Blood vessel problem

Atherosclerosis

Aneurysm

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6.0 VASCULAR GRAFT

TREATMENT OPTIONS The main treatment for atherosclerosis

is lifestyle changes. You also may need medicines and medical procedures.

For aortic aneurysms or aneurysms that happen in the vessels that supply blood to the arms, legs, and head (the peripheral vessels), surgery involves replacing the weakened section of the vessel with an artificial tube.

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6.0 VASCULAR GRAFT

Type DescriptionBIOLOGICAL GRAFT

Autograft Graft transplanted from part of a patient’s body to another.Example: saphenous vein graft for pheripheral bypass

Allograft Homograft. Transplanted vascular graft tissue derived from the same species as recipient.

Xenograft Heterograft. Surgical graft of vascular tissue derived from animal. Example: moddified bovine heterograft

SYNTHETIC GRAFTDacron PTFE (polytetrafluoroethylene) Other

Woven, knittedExpanded, knitted

Nylon, polyurethane

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6.0 VASCULAR GRAFT

Polyurethane vascular graft photographed in situ in carotid artery

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Synthetic graft - Dacron

Dacron grafts are manufactured in either a woven or knitted form.

woven

knitted

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Synthetic graft - Dacron

Woven grafts have smaller pores and do not leak as much blood.

To reduce the blood loss knitted grafts should be pre-clotted prior to insertion.

They are less frequently used than woven grafts.

Dacron grafts have recently been manufactured coated with protein (collagen/albumin) to reduced the blood loss and antibiotics to prevent graft infection.

Dacron grafts are frequently used in aortic and aorto-iliac surgery.

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Synthetic graft - PTFE

Polytetrafluoroethylene (PTFE) is a knitted graft.

Its smooth surface is less thrombogenic than Dacron.

Its smooth wall is prone to kinking as it passes around joints necessitating it to be externally supported.

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Synthetic graft-stent graft

A stent graft is a tubular device, which is composed of special fabric supported by a rigid structure, usually metal.

The rigid structure is called a stent. An average stent on its own has no

covering, and therefore is usually just a metal mesh.

Although there are many types of stent, these stents are used mainly for vascular intervention.

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Synthetic graft-stent graft

The device is used primarily in endovascular surgery. Stent grafts are used to support weak points in arteries, commonly known as an aneurysm.

Stent grafts are most commonly used in the repair of an abdominal aortic aneurysm, in a procedure called an EVAR (Endovascular Aneurysm Repair ).

The theory behind the procedure is that once in place inside the aorta, the stent graft acts as a false lumen for blood travel through, instead of into the aneurysm sack.

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Problem

The commonest complications associated with the use of vascular grafts are: Graft occlusion (blockage) Graft infection (Graft infection is thankfully

rare (1-2%)) True and false aneurysms at the site of

anastomosis Distal embolisation (blocking of a graft) Erosion in to adjacent structures

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Conclusion

Most of the vascular graft are stiffer compared to the host artery.

Development with more compliant grafts and in modifying the surface interaction of the graft with blood may result in reducing the problems with loss of patency.

Recent advance is to engineered vascular graft from recipients own tissue. This will provide better biocompatibility and performance.

Documents

BIOMATERIALS ENT 311/4 Lecture 9 Blood Contacting Implants or Devices Prepared by: Nur Farahiyah Binti Mohammad Date: 26 th August 2008 Email : [email protected]