Biomaterials, biocompatibility and its importance in medical devices

Biocompatibility is most important while dealing with medical devices

1. Biomaterials

2. Terms Biomaterial: A synthetic material used to make devices to replace part of a living system or to function in intimate contact with living tissue. Biological material: A material produced by a biological system. Biocompatibility: Acceptance of an artificial implant by the surrounding tissues and by the body as a whole .

3. Early biomaterials Gold: Malleable, inert metal (does not oxidize); used in dentistry by Chinese, Aztecs and Romans Iron, brass: High strength metals; rejoin fractured femur Glass: Hard ceramic; used to replace eye (purely cosmetic) Wood: Natural composite; high strength to weight; used for limb prostheses and artificial teeth Bone: Natural composite; uses: needles, decorative piercings Sausage casing: cellulose membrane used for early dialysis

4. A biomaterial is a nonviable material used in a medical device, intended to interact with biological systems is used to make devices to replace a part of a function of the body in a safe, reliable, economic, and physiologically acceptable manner. is any substance (other than a drug), natural or synthetic, that treats, augments, or replaces any tissue, organ, and body function.

5. The need for biomaterials replacement of body part that has lost function (total hip, heart) correct abnormalities (spinal rod) improve function (pacemaker, stent) assist in healing (structural, pharmaceutical effects: sutures, drug release)

6. EXAMPLES OF USES OF BIOMATERIALS

7. MATERIAL ATTRIBUTES FOR BIOMEDICAL APPLICATIONS

8. Classes of Biomaterials Metals stainless steel, cobalt alloys, titanium alloys Ceramics aluminum oxide, zirconia, calcium phosphates Polymers silicones, poly(ethylene), poly(vinyl chloride), polyurethanes, polylactides Natural polymers collagen, gelatin, elastin, silk, polysaccharides

9. General Criteria for materials selection Mechanical and chemicals properties No undersirable biological effects carcinogenic, toxic, allergenic or immunogenic Possible to process, fabricate and sterilize with a godd reproducibility Acceptable cost/benefit ratio

10. Material Properties Mechanical Properties elasticity, viscoelasticity, brittle fracture, fatigue Surface chemistry

11. Surface Energy Interface boundary between 2 layers significance protein adsorption to materials blood coagulation/thrombosis due to material contact cellular response to materials

12. Surface Chemistry At the surface (interface) there are intermolecular forces and intramolecular forces of attraction and repulsion. van der Waals forces Hydrogen Bonds Coulombic

13. Surface Electrical Properties surface may become charged by adsorption of ionic species present in sol’n or preferential adsorption of OH - ionization of -COOH or -NH 2 group

14. Critical Surface Tension, g c The critical surface tension is the surface tension of a liquid that would completely wet the solid of interest. Material g c (dyne/cm) Co-Cr-Mo 22.3 Pyrex glass 170 Gold 57.4 poly(ethylene) 31-33 poly(methylmethacrylate) 39 Teflon 18

15. Sequence of local events following implantation in soft tissue Injury Actute inflammation Granulation tissue Foreign body reaction fibrosis

16. Testing of Biomaterials Physical and mechanical Biological In vitro assessment in vivo assessment Functional assessment Clinical assessment

17. Metallic biomaterials

18. Metals are used as biomaterials due to their excellent electrical and thermal conductivity and mechanical properties. Since some electrons are independent in metals, they can quickly transfer an electric charge and thermal energy . The mobile free electrons act as the binding force to hold the positive metal ions together. This attraction is strong, as evidenced by the closely packed atomic arrangement resulting in high specific gravity and high melting points of most metals. Since the metallic bond is essentially nondirectional , the position of the metal ions can be altered without destroying the crystal structure resulting in a plastically deformable solid.

19. Some metals are used as passive substitutes for hard tissue replacement such as total hip and knee joints , for fracture healing aids as bone plates and screws, spinal fixation devices, and dental implants because of their excellent mechanical properties and corrosion resistance. Some metallic alloys are used for more active roles in devices such as vascular stents, catheter guide wires, orthodontic archwires , and cochlea implants.

20. The first metal alloy developed specifically for human use was the “vanadium steel” which was used to manufacture bone fracture plates (Sherman plates) and screws. Most metals such as iron (Fe), chromium (Cr ), cobalt (Co), nickel (Ni), titanium (Ti), tantalum (Ta), niobium ( Nb ), molybdenum (Mo ), and tungsten (W) that were used to make alloys for manufacturing implants can only be tolerated by the body in minute amounts. Sometimes those metallic elements, in naturally occurring forms, are essential in red blood cell functions (Fe) or synthesis of a vitamin B12(Co ), but cannot be tolerated in large amounts in the body

21. Disadvantage The biocompatibility of the metallic implant is of considerable concern because these implants can corrode in an in vivo environment. The consequences of corrosion are the disintegration of the implant material, which will weaken the implant, and the harmful effect of corrosion products on the surrounding tissues and organs.

22. Metals used as biomaterials Stainless Steels CoCr Alloys Ti Alloys( titanium) TiNi Alloys( titanium–nickel ) Dental amalgam ( alloy made of liquid mercury and other solid metal particulate alloys made of silver , tin, copper, etc ) Tantalum Platinum

23. Ceramic biomaterials

24. Ceramics are defined as the art and science of making and using solid articles that have as their essential component inorganic nonmetallic materials. Ceramics are refractory, polycrystalline compounds , usually inorganic, including silicates, metallic oxides, carbides and various refractory hydrides , sulfides , and selenides . Their relative inertness to the body fluids, high compressive strength, and aesthetically pleasing appearance led to the use of ceramics in dentistry as dental crowns.

25. Some carbons have found use as implants especially for blood interfacing applications such as heart valves . Due to their high specific strength as fibers and their biocompatibility, ceramics are also being used as reinforcing components of composite implant materials and for tensile loading applications such as artificial tendons and ligaments.

26. Unlike metals and polymers, ceramics are difficult to shear plastically due to the (ionic) nature of the bonding and minimum number of slip systems. These characteristics make the ceramics nonductile and are responsible for almost zero creep at room temperature. Consequently, ceramics are very susceptible to notches or microcracks because instead of undergoing plastic deformation they will fracture elastically on initiation of a crack. Ceramics are generally hard; in fact, the measurement of hardness is calibrated against ceramic materials .

27. Desired Properties of Implantable Bioceramics 1. Should be nontoxic 2. Should be noncarcinogenic 3. Should be nonallergic 4. Should be noninflammatory 5. Should be biocompatible 6. Should be biofunctional for its lifetime in the host

28. Ceramics as biomaterials Alumina ( Al2 O3) Zirconia Carbons Calcium Phosphate Aluminum –Calcium–Phosphate Coralline(natural made by marine i nvertebrates ) Tricalcium Phosphate Zinc–Calcium–Phosphorous Oxide Zinc– Sulfate –Calcium–Phosphate etc

29. Dental implant

30. 3-principles in dental implant design Initial retention Anti-rotation mechanics No sharp-edges

31. Tooth fillings materials Amalgam Dental composite Ceramics Other metals

32. General criteria for tooth filling materials Non-irritation to pulp and gingival Low systemic toxicity Cariostatic Bonding to tooth substance without marginal leakage (20 u) Not dissolved or erode in saliva Mechanical strength, wear resistance, modules matching. Good aesthetic properties Thermal propertiesy (expansion & conductivity) Minimal dimensional changes on setting and adequate working time and radio opacity

33. Contact lens Optical properties Chemical stability Oxygen transmissibility Tear film wettability Resistance to lipid/protein deposition Easy to clean