Công nghệ vật liệu trong y sinh học biom aterials chapter 2 3

Metals1 Advanced Program Biomedical Engineering – HUST, Vietnam Fabrication of metallic implants Note:purity of metals 2 Advanced Program Biomedical Engineering – HUST, Vietnam Raw met

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Chap.2 – 2.9 Metals

1 Advanced Program Biomedical Engineering – HUST, Vietnam

Fabrication of metallic implants

Note:purity of metals

Raw metal products to stock metal shapes

- Bulk raw metal products were processed into “stock” bulk shapes such as bars, wire, sheet, rods,

plates, tubes, or powder and supplied to implant manufacturers

- Processes used: remelting and continuous casting, hot rolling, forging, cold drawing through dies

- May include heat-treating step (e.g annealing) to facilitate further working or shaping of the stock and

to relieve the effect of prior plastic deformation

- Or produce a specific microstructure and properties in the stock materials (Note: increase of the cost

for vacuum or inert condition due to high reactivity of metals at high temperature)

- The stock metal shapes often are chemically and metallurgically tested to ensure that the chemical

composition and microstructure of metals meet the standards for surgical implants (ASTM standards)

Stock metal shapes to preliminary and final metal devices

- Implant manufacturers fabricate preliminary and final form of the devices from stock materials

- Specific steps depend on many factors, including final geometry of implant, the forming and machining

properties of the metals, and the cost of alternative fabrication methods

- Fabrication methods:

Casting, CAD/CAM, forging, powder metallurgical processes (hot isostatic pressing

Surface treatment: macro- or microporous coating on implants, or deliberate production of certain

degree of surface roughness (this surface modification is very popular in recent years to improve fixation

of implants in bone) Methods for surface treatments: plasma or flame spraying a metal onto implants;

ion implantation, nitriding (commonly increase the surface hardness or wear properties)

cuu duong than cong com

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- Finishing steps (vary with metals and manufacturer): chemical cleaning and passivation in

appropriate acid, or electrolytically controlled treatment to remove machining chips or impurities on

the implant surfaces

This step can be extremely important to the overall biological performance of the implants

SEM image of a titanium plasma spray coating on an oral implant

Micro structures and properties of implant metals

Properties depend on:

(1) Chemical and crystallographic indentities of the phases present in the microstructures

(2) The relative amounts, distribution, and orientation of these phases

(3) The effects of the phases on properties

Stainless steels

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• Most common in practice: 316L (ASTM F138, F139), grade 2

- L: low carbon content; this steel has less than 0.030%w carbon in order to reduce the possible in vivo

corrosion (if C content >0.03%w, it will increases the danger of formation of carbide, such as Cr23C6

(carbide precipitations deplete the adjacent grain boundary regions of Cr, which in turn has the effect of

diminishing formation of the protective Cr2O3

- 60-65% Fe with significant alloying addition of

- 17-20% Cr (permit to development of corrosion resistance by forming a strongly adherent surface

oxide Cr2O3)

- Ni (12-14% ): to stabilize austenitic phase (FCC)

- Small amount of nitrogen, manganese, molybdenum and silicon (feritte stabilizers), phosphorus and

sulfur

- In microstructure: should be no free ferritic (BCC) or carbide phases, should also be free of inlussions

or impurity phases such as sulfide stringers

- Grain size: very important, recommended ASTM#6 or finer:

ASTM grain size number n: N = 2 n-1 (N: number of grains/in2at magnification X100 (0.0645mm2

actual area)

Hallo-Petch-type relationship between mechanical yield stress and grain diameter:

ty= ti+ kd-m ; ty& ti: yield & friction stress, k: constant, d: grain diameter, m: 0.5

- Bone screws

Mechanical properties

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Cobalt-based alloys

•ASTM F75:

- Corrosion resistance in chloride environments (related to its bulk composition and surface oxide

(Cr2O3)

- melt at 1350-1450oC, casting, Co- rich matrix (alpha phase) plus interdendritic and grain boundary

carbide (M23C6 - M:Co, Cr or Mo) Relative between alpha &carbide phases: 85% and 15%; problem of

fatigue due to high processing temperature

- Grain size: about 8 µm

- Applications: hip stem

•ASTM F799:

- modified F75 mechanical processed by hot forging at about 800oc after casting

Microstructure of Co-Cr-Mo ASTM F75 alloys made via hot isostatic pressing (HIP)

•ASTM F90: Co-Cr-W-Ni (difference: tungsten &

Ni for improvement of machinability and fabrication)

• ASTM F562 (MP35N) : strongest available for implant applications

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Titanium-based alloys

F67 & F136 are two most common Ti-based implant biomaterials

* ASTM F67 (commercial pure –CP- titanium): many current dental implants

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Chap.2 – 2.10 Surface modification of Biomaterials

•Surface can be modified by using biological, mechanical, or physicochemical methods

•Two categories of surface modifications:

(1) Chemically or physically altering the atoms, compounds, or molecules in the

existing surface (chemical modification, etching, mechanically roughening

(2) Overcoating the existing surface with a material having a different composition

(coating, grafing, thin film deposition)

•Thin surface modifications are desirable, as thin as possible, ideally 3-10 Angstrom, but

in practice a little bit thicker (Note: problems of thick films)

•The surface-modified layer should be resistant to delamination and cracking (by

covalently bonding the modified region to substrate, intermixing the components of

substrate and surface film at interfacial zone, apply a compatibilizing – “primer” layer at

the interface, or incorporating appropriate functional groups for strong intermolecular

adhesion between a substrate and an overlayer)

•Surface rearrangement can readily occur (immigration or diffusion of chemistries from

surface to bulk or from bulk to surface)

•Need methods for surface analysis (chemistry of the surface and surface energy)

•Need to minimize the steps of modification process to catch commercializability and to

design each step to be relatively intensive to small changes in reaction condition

Chap.2 – 2.10 Surface modification of Biomaterials

Surface modification possibility

Methods to modify surface

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ionized

neutral plasma

What is PLASMA?

(High energy + Light)

Typical conditions within low-pressure cold plasma:

Electron energy: 1-10 eV Gas temperature: 25-60oC Electron density: 10-9to 10-12/cm3 cuu duong than cong com

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Plasma treatment, what happen?

Etching

• Clean

• Roughness

Plasma treatment, what happen?

Activation the surface

* Forming new functional groups

PLASMA

O2 c

c

c c

c

c c

c c c

OH O COOH

PLASMA

N2 c

c

c c

c

c c

c c c

NH 2 NO 2 NH

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Grafting monomer on the substrate surface: Ar plasma activates the surface forming

radicals in the molecule backbone and monomer react and graft on the surface

How can monomers graft onto the surface?

Activate the surface, forming radicals Monomer react and graft on surface

METHODS FOR MODIFYING SURFACE OF MATERIALS Chemical reaction

-Nonspecific reactions leave a distribution of different functional groups at the surface (chromic acid

oxidation of PE surface, corona discharge modification of material in air, RFGD treatment of surface…)

- Specific surface chemical reactions change only one functional groups into an other with a high yield

and few side reactions

Radiation grafting and photografting

-Starting in the late 1960s, focused on attaching chemically reactable groups (-OH, -COOH, -NH2…) to

the surface of relative inert hydrophobic polymers

-Strong depend on source energy, radiation dose rate and the amount of dose absorbed

-Graft layer thickness: > 1µm, well bond to surface

-Photoinitiated grafting (usually with visible or UV light): subcategory of surface modification with

growing interest: phenyl azide to nitrene (by UV), nitrene can react quickly with many organic groups,

immobilization of polymer containing phenyl azide to the substrate surface

RFGD plasma deposition and other plasma gas process

•Advantages in biomedical application

- They are conformal

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-They are free of voids and pinholes

- Plasma-deposited polymeric films can be placed upon almost any solid substrate, including metals,

ceramics, semiconductors

- Very good adhesion to the substrate

- Unique film chemistry can be

produced They can serve as excellent barrier films (pinholeproduced free, crossproduced linked nature)

- These films are easily prepared

- Mature technology

- Can be characterized (by IR, ESCA, SIMS, NMR

- Plasma-treated surface are sterile when remove from the reactor

- Apparatus can be expensive ( Note: air-pressure process!)

Silanization

- Silane reactions are most often used to modify hydroxylated surfaces

- the advantages are simplicity and stability (attributed to their covalent, cross-linked structure)

- Silanes can form twp types of surface film structures (only surface reaction occur or in presence of

water-thicker layer can form)

Chap.2 –

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Chap.2 2.10 Surface modification of Biomaterials

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Chap.2 –

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