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Dr. Alagiriswamy A A, (M.Sc, PhD, PDF)Asst. Professor (Sr. Grade),

Dept. of Physics, SRM-University,Kattankulathur campus,

Chennai

UNIT IIILecture 4

ABCs of Biomaterials

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CLASSIFICATION OF BIOMATERIALS

Biomaterials can be divided into three major classes of materials:

Metals

Polymers

Ceramics (including carbons, glass ceramics, and glasses).

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Changing the chemistry at the surface

Inducing roughness/porosity at the surface

Incorporate surface reactive materials (bioresorbable; helps in slow replacement by tissue)

Should not secrete oxidizing agents

Reduce corrosion rate of biomaterials

Biological responses ; requirements

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METALLIC IMPLANT MATERIALS Stainless steel

Cobalt-chromium alloysTitanium alloys

Metallic implants are used for two primary purposes.

To replace a portion of the body such as joints, long bones and skull plates.

Fixation devices are used to stabilize broken bones

Must be corrosion resistant

Good fatigue properties Other compatible issues

LECTURE 3 5

Type % C %Cr % Ni %Mn % other elements

301 0.15 16-18 6-8 2.0 1.0Si

304 0.07 17-19 8-11 2.0 1-Si

316, 18-8sMo

0.07 16-18 10-14 2.0 2-3 Mo, 1.0 Si

316L 0.03 16-18 10-14 2.0 2.3 Mo, 0.75Si

430F 0.08 16-18 1.0-1.5 1.5 1.0 Si, 0-6 Mo

CONSTITUENTS OF STEEL

less chromium content should be utilized (because Cr is a highly reactive metal)

Make use of austenite type steel (less magnetic properties)

Lowered carbon content

Inclusion of molybdenum helps corrosion resistance

Electroplating technique (increases corrosion resistance)

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Other features

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Devices Alloy Type

Jewitt hip nails and plates 316 L

Intramedullary pins 316 L

Mandibular staple bone plates 316L

Heart valves 316

Stapedial Prosthesis 316

Mayfield clips (neurosurgery) 316

Schwartz clips (neurosurgery) 420

Cardiac pacemaker electrodes 304

APPL

ICAT

ION

S O

F

SS S

TEEL

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COBALT CHROMIUM ALLOYS

Cobalt based alloys are used in one of three forms

•Cast; as prepared •Wrought (fine structure with low carbon contents ; pure forms)

•Forged Cobalt based alloys are better than stainless steel

devices because of low corrosion resistance

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More detailsCast alloy:• a wax model of the implant is made and ceramic shell is built around the wax model

• When wax is melted away, the ceramic mold has the shape of the implant

• Molten metal alloy is then poured in to the shell, cooling, the shell is removed to obtain metal implant.

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Wrought alloy: possess a uniform microstructure with fine grains.Wrought Co-Cr –Mo alloy can be further

strengthened by cold work.

Forged Alloy: produced from a hot forging process. Forging of Co-Cr –Mo alloy requires sophisticated press and complicated tooling. Factors make it more expensive to fabricate a device

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TITANIUM BASED ALLOYSThe advantage of using titanium based alloys as implant materials are

low density good mechano-chemical properties

The major disadvantages o relatively high costoreactivity.

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More details• a light metal • Titanium exists in two allotropic forms,

• The low temperature -form has a close-packed hexagonal crystal structure with a c/a ratio of 1.587 at room temperature

• Above 882.50C -titanium having a body centered cubic structure which is stable

• Ti-6 Al-4V alloy is generally used in one of three conditions wrought, forged or cast

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THREE CLASSES OF CERAMICS (according to their reactivity)

completely resorbable •More reactive (Calcium phosphate) – over a span of times •Yielding mineralized bone growing from the implant

surfacesurface reactive

•Bioglass ceramics ; Intermediate behavior •Soft tissues/cell membranes

nearly inert •Less reactive (alumina/carbons) even after thousands of

hours •how minimal interfacial bonds with living tissues.

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Pyrolitic carbon; •Pyrolysis of hyrdocarbon gas (methane) ≤ 15000 degrees •Low temperature isotropic (LTI) phase•Good bonding strength to metals (10 Mpa – 35 Mpa)•Inclusion of Si with C, wear resistance increases drasticallyVitreous carbon (glassy carbon); •controlled pyrolysis of a polymer such as phenol formaldehyde resin, rayon and polyacrylonitrile •Low temperature isotropic phase•Good biocompatibility, but strength and wear resistance are not good as

LTI carbonsTurbostratic carbon (Ultra low temperature isotropic carbons (ULTI))• Carbon atoms are evaporated from heated carbon source and condensed into a cool substrate of ceramic, metal or polymer. •Good biocompatibility

DIFFERENT VARIETIES OF CARBON (NEARLY INERT CERAMICS)

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Alumina (Aluminium oxide)

•high corrosion resistance•wear resistance • Surface finishing•small grain size•biomechanically correct design•exact implantation technique

Alumina ceramic femoral

component

Natural single crystal alumina known as sapphire High-density alumina ; prepared from purified alumina powder by isostatic pressing and subsequent firing at 1500-17000C. -alumina has a hcp crystal structure (a=0.4758 nm and c=1.2999nm) load bearing hip prostheses and dental implants, hip and knee joints, tibial plate, femur shaft, shoulders, vertebra, and ankle joint prostheses

Porous network ; SEM images

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Glass CeramicsTo achieve a controlled surface reactivity that

will induce a direct chemical bond between the implant and the surrounding tissues.

Also known as 45S5 glass. It is composed of SiO2, Na2O, CaO and P2O5.

45 wt.% of SiO2 and 5:1 ratio of CaO to P2O5. Lower Ca/P ratios do not bond to bone.

Bioglass and Ceravital; fine-grained structure with excellent mechanical and thermal properties

The composition of Ceravital is similar to bioglass in Sio2 content but differ in CaO,MgO,Na2O.

Bioglass implants have several advantages like•high mechanical properties•surface biocompatible properties.

Bioglass

Ceravital

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Resorbable Ceramics (first resorbable implant material-Plaster of Paris).

•Should not have variable resorption rates •Should not have poor mechanical properties.

Two types of orthophosphoric acid salt namely -tricalcium phosphate (TCP) and hydroxyapatite (HAP) (classified on the basis of Ca/P ratio).

The apatite- [Ca10 (PO4)6 (OH)2] crystallizes into the hexagonal rhombic system. The unit cell has dimensions of a = 0.9432 mm and c = 0.6881 nm.

The ideal Ca/P ratio of hydroxyapatite is 10/6 and the calculated density is 3.219 g/ml.

The substitution of OH- with F- gives a greater structural stability due to the fact that F- has a closer coordination than the hydroxyl, to the nearest calcium.

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POLYMERSElastomers; able to withstand large deformations

and return to their original dimensions after releasing the stretching force.

Plastics; are more rigid materials

Thermoplastic (can be

reused, melted) Thermosetting (can’t)

Elastomers include, butyl rubber, chlorosulfonated polyethylene, epichlorohydrin,rubber, polyurethane,natural rubber and silicone rubber.

Polymers toxicity

Residual monomers due to incomplete polymerization/catalyst used for polymerization may cause irritations.

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Polymer Specific Properties Biomedical usesPolyethylene Low cost, easy Possibility excellent

electrical insulation properties, excellent chemical resistance, toughness and flexibility even at low temperatures

Tubes for various catheters, hip

joint, knee joint prostheses

Polypropylene Excellent chemical resistance, weak permeability to water vapors good transparency and surface reflection.

Yarn for surgery, sutures

Tetrafluoroethylene Chemical inertness, exceptional weathering and heat resistance, nonadhesive, very low coefficient of friction

Vascular and auditory

prostheses, catheters tubes

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Polyethylene structures The first polyethylene [PE,(-CH2-CH2-)n] was made

by reacting ethylene gas at high pressure in the presence of a peroxide catalyst for starting polymerization; yielding low density polyethylene (LDPE).

By using a Ziegler-Natta catalyst, high-density polyethylene (HDPE) can be produced at low pressure; (first titanium-based catalysts)

The crystallinity usually is 50-70% for low density PE and 70-80% or high density PE

ultra high molecular weight polyethylene (UHMWPE) …??????

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ACRYLIC RESINS (organic glass)The most widely used polyacrylate is poly(methyl

methacrylate, PMMA) ; The features of acrylic polymers ;

high toughness/strength, good biocompatibility properties brittle in comparison with other polymers excellent light transparency high index of refraction.

Causes allergic reactions

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BONE CEMENT MIXING AND INJECTION

PMMA powder + MMA liquid mixed in a ratio of 2:1 in a dough, to cure

Injected in the femur (thigh bone)

The monomer polymerizes and binds together the preexisting polymer particles.

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HydrogelsInteraction with

H2O, but not soluble

PHEMA; absorbs 60 % of Water, machinable when dry

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HYDROGELSInteresting features

(1) The soft, rubbery nature coupled with minimal mechanical/frictional irritation to the surrounding tissues.

(2) Low or zero interfacial tension with surrounding biological fluids and tissues, thereby, minimizing the driving force for protein adsorption and cell adhesion

(3) Hydrogels allow the permeating and diffusion of low

molecular weight metabolities,waste products and salts as do living tissues.April 24, 2023

LECTURE 5 BIOMATERIALS 25

POLYURETHANES Polyther-urethanes; block copolymers (variable length

blocks that aggregate in phase domains) Good physical and mechanical characteristics Are hydrophilic in nature

Good biocompatibility (blood compatibility)

Hydrolytic heart assist devices Non-cytotoxic therapy

Consists of hard and soft segments

BIOMATERIALS 26

POLYAMIDES (Nylons)Obtained through condensation of

diamine and diacid derivative.Excellent fiber forming properties due to inter-chain hydrogen bonding and high degree of crystallinity, which increases the strength in the fiber direction.Hydrogen bonds play a major roleAs a catheterHypodermic syringes

Diamino hexane + adipic acid

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Changing the chemistry at the surface

Inducing roughness/porosity at the surface

Incorporate surface reactive materials (bioresorbable; helps in slow replacement by tissue)

Should not secrete oxidizing agents

Reduce corrosion rate of biomaterials

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Biological responses ; requirements

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Biosensors (in vitro/in vivo);

analytical devices which convert biological response into a useful electrical signal to determine the concentration of substances either directly or indirectly areas of biochemistry, bioreactor science, physical chemistry, electrochemistry, electronics and software engineering, and others

http://www.lsbu.ac.uk/biology/enztech/

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Principle of biosensors (bio-recognition systems)

LECTURE 6 BIOMATERIALS 30

biocatalyst (a) converts the substrate to product. This reaction is determined by the transducer (b) which converts it to an electrical signal. The output from the transducer is amplified (c), processed (d) and displayed (e).

WORKING PRINCIPLE OF BIOSENSOR

distribution of charges light-induced changes mass difference

output

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Three so-called 'generations' of biosensors; First generation; normal product of the reaction diffuses to the transducer and causes the electrical response.

Second generation; involve specific 'mediators' between the reaction and the transducer in order to generate improved response.

Third generation; reaction itself causes the response and no product or mediator diffusion is directly involved.

Clinical diagnosis and biomedicine Farm, garden and veterinary

analysis Process control: fermentation

control and analysis food and drink production and analysis Microbiology: bacterial and viral

analysis Pharmaceutical and drug analysis Industrial effluent control Pollution control and

monitoring/Mining, industrial and toxic gases

Military applications LECTURE 3 32

Brief applications of biosensor(s)

By restoring, maintaining, enhancing the tissue, and finally functionalize the organs

Tissue can be grown inside or outside

Finally to exploit the living cells in many ways

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Tissue engineering (also referred to as “regenerative medicine)

To create products that improve tissue function or heal tissue defects. Replace diseased or damaged tissueBecause……

Donor tissues and organs are in short supplyWe want to minimize immune system response by using our own cells or novel ways to protect transplant

RegenerateIdentify the cues that allow

for regeneration without scarring

Like growing a new limbRepair

Stimulate the tissue at a cell or molecular level, even at level of DNA, to repair itself.

ReplaceA biological substitute is

created in the lab that can be implanted to replace the tissue or organ of interest

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Tissue engineering

The cells themselves Non-soluble factors within the extracellular matrix

(ECM) such as laminins,collagens,and other molecules Soluble factors such as cytokines, hormones,

nutrients, vitamins, and minerals

cell isolation cell culture scaffold material choice cell scaffold co-culture studies implantation in animals human trials

Normal strategies

SkinBoneCartilageIntestine

SUCCESSFULLY ENGINEERED TO SOME

EXTENT

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Questions?