7. Cobalt based & shape memory alloys, … based & shape memory alloys, biodegradable metals Dieter...

Dieter ScharnweberMaterials in Biomedicine 1

7. Cobalt based & shape memory alloys, biodegradable metals


Groups of materials – CoCr based alloys

1.1. Classifications + Applications1.2. Chemical compositions1.3. Mechanical properties for cast/wrought materials


The Co-Cr-phase diagram

austenite

hcp … hexagonal closed packed


Classifications

Typically 60 – 70 % Co, 20 – 30 % Cr, (+ Ni, Mo, W, …)


Alloy Application

CoCrMo (cast) Joint implants (hip, knee, elbow, shoulder, angle, finger)

Bone plates and screwsArtificial heart valves

CoCrMo (wrought) Joint implants

CoCrWNi (wrought) Joint implantsArtificial heart valvesWiresInstruments for surgery

CoNiCrMo (wrought) Hip stems (too soft for joint balls)

Start in 1929 with applications in dental surgery

Applications


Chemical compositions

CoCr-MoCast alloy

CoCrWNiwrought alloyHS 25

CoNiCrMowrought alloyMP 35 N

Composition (mass%)


alloy structure Tensile strenght Elongation at fracture [%]

Tensile Fatigue strenght

cast

wroughtsintered

Cold workedannealed

annealedCold worked

Cold worked and aged

Physical and mechanical properties


Groups of materials – Shape memory alloys

1. Materials2. The basic effect

1. Shape memory2. Superelasticity

3. Properties4. Applications


Binary phase diagram Ni-Ti

> 90 % of application with NiTi, NiTi-Cu, NiTi-Nb

Materials


The basic effect – phase transformation as f (T)


The basic effect - crystal structures

austenite austenite Twisted martensite


The effects in their combination

1

2

3

4

5


Supereleastic behaviour - limited to a few tens of °C above Af (up to MD)

Transformation starts at critical stress p-m (p-m increases linearly with temperature with ~ 2 - 15 MPa K-1, with p-m = 0 for Ms) – reversible strains up to 8 %!!!

Superelasticity


Transformation mechanism as f (T)

With increasing stressbelow As

elastic strain of (twinned) M / (plastic) detwinning of M / elastic strain of M / plastic strain of M / fracture

between As and Af

(plastic) detwinning of M + elastic strain of A / transformation of A fraction to detwinned M / elastic strain of M / plastic strain of M / fracture

between Af and MD

elastic strain of A / transformation of A to detwinned M / elastic strain of M / plastic strain of M / fracture

above MD

elastic strain of A / plastic strain of A / fracture

Superelasticity irreversible

MD


Properties – comparison with other actuator principles


Summary of physical parameters

Property Unit Value

Density g cm-3 6.5

Thermal conductivityof austeniteof martensite

W m-1 K-1

18 8.6

Transformation enthalpie J kg-1 15 000

Resistivityof austeniteof martensite

10-6 m1.00.8

Transformation temperature °C -150 to 100

Hysteresis °C 5 – 50

One way memory strain % 3 – 8

Superelastic strain % 6 – 8

Work output J g-1 4

Maximum recovery stress MPa 600 - 900


Function and application categories

• free recovery applications• clamping and fixation devices• actuation applications• biomedical and other superelastic applications• damping applications (in cold shape up to 90 % not

yet used for biomedical applications)• https://www.youtube.com/watch?v=e2f29Sw7UVc• https://www.youtube.com/watch?v=XPrg8EZlD1E

17


Superelastic applications










galvanic corrosion by electron-conducting connection of different materials

dental surgery - more craftsman-like techniques in dental laboratory additional disadvantageous changes in materials microstructure caused by heat treatment during welding or soldering

Short-circuit potentials and local current investigations of the combining of titanium with several other alloys in a 1 % NaCl solution (pH 6.3), in part with 10 % lactic acid (pH 1.81) showed:

1. Using Ti/gold-based alloys, Ti/palladium-based alloys and Ti/non-precious alloys, titanium was under either cathodic or anodic control.

2. In combination with noble metal alloys: titanium acts as local anode because of stability of the ion-conducting passive layer no increased corrosion takes place

3. In combination with nickel-based alloys: titanium acts as the local cathode because of the n-semiconducting properties of the titanium passive layer increased corrosion appears. Causes corrosion attack of nickel-based alloys + in case of unfavourable surface ratios damage to titanium due to hydrogen embrittlement.

An alloy is potentially useable for superstructures in galvanic coupling with titanium if:in a coupling titanium has a weak anodic polarisationthe galvanic cell current density is also weakthe crevice corrosion potential is markedly higher than the coupling potential.

Combinations of metallic biomaterials


Goal: development of alloys being able to replace amalgams Gallium based alloys

(typical) composition (Galloy®):solid component: 60% Ag, 28% Sn, 12% Culiquid component: 62% Ga, 25% In, 13% Sn (Fp 10 °C, (Fp for Gallium 29.8 °C))

Investigations in deoxygenated Ringer’s solution at 37 °C showed:

For uncoupled gallium-based alloy at low overpotentials: selective dissolution of divalent tin ions, followed by a dissolution of GaFor uncoupled gallium-based alloys and gallium alloy coupled with amalgam: anodic current densities 103-104 times higher than that of an uncoupled amalgam very poor corrosion stability.

Comparison of the cytotoxicity of gallium and indium ions with that of mercuric ions in a concentration range of 1 µM to 1 mM using L-929 mouse fibroblasts:

mercuric ions: 50 % inhibition at concentration of 0.35 mM

gallium and indium ions:did not significantly inhibit dehydrogenase activity in either growing or the confluent phase

Special Materials


Effects of metal ions used in dental materials on conversion of amorphous calcium phosphate to hydroxyapatite in vitro:

According to effects on both the rate of hydroxyapatite transformation and induction time:

inhibitory (in the order: Ni, Sn, Co, Mn, Cu, Ga, Th, Mo, Cd, Sb, Mg, Hg)

ineffective (Cs, Ti, Cr, Fe2+)

stimulatory (Fe3+, In)

Summary• similar or better mechanical properties than modern (Hg) amalgams• difficult manipulation• low corrosion resistance• lack of gallium’s biological impact• → restricted use

Results from in vitro experiments


Magnesium Alloys for use as degradablemetallic implants in musculo-skeletal surgery

Why?

Mg?• Essential to human metabolism – co-factor for many enzymes & stabilizes DNA and RNA• Fourth most abundant cation in human body: 25 g in adult body, about 50 % in bone• Extracellular fluid level 0.7 to 1.05 mM• effectively excreted in urine

historyFirst use in human 1907 (Mg plate with gold plated steel nails to secure bone fracture in lower leg

failed by disintegration after 8 days + large amount of H2 gas

During WW II in Russia to secure fractures and to treat gun shot woundsno increased levels of serum Mg, no distinct inflammatory reactions, but gas cysts – treated by drawing gas with subcutaneous needle

from Staiger et al. Biomaterials 27(2006) 1728


Magnesium Alloys for use as degradable metallic implants in musculo-skeletal surgery

Current status

New alloys to improve mechanical properties & corrosion resistancea) 2 – 10 wt% Al + traces of Zn + Mgb) 0.4 - 4 wt% mixture of rare earth elements + (traces) of Zn, Ag, Zr,

In vivo results

from Witte et al. Biomaterials 26(2005) 3567

ACP within corrosion product layerImproved mineralization of surrounding bone



Results:In vitro corrosion rates in artificial seawater four orders of magnitude higher than in vivo rateTendency of corrosion rates in vivo and in vitro was in the opposite direction

from Witte et al. Biomaterials 27(2006) 1013

AZ AlZn91 LAE LiAlRE442



Open questions

Behaviour / toxicity of alloying elementsFormation of H2 gasAdaptation of corrosion rate

summaryBiodegradable material with mechanical (+ biological) properties superior to most biodegradable polymeric materials


Iron - hMSC on Fe(CPP) scaffolds

Fe13-Brushit Fe13-HAp2McCoys DMEM McCoys DMEM

3d

14d

7d

21d

Fe13 Fe13-HAp2 Fe13-Brushit3 Fe13-MgCPC

O2 ↓↓ ↓ ≈ control ≈ control

Fe ↑↑ ↑ - -

H2O2 ↑↑ ↑ - -

Ca ≥ control ↓ ≥ control

pH ≈ control ↑ ≈ control

Cytotoxicity f (concentration) f (concentration) no no

Adhesion already difficult with SaOs-2

Proliferation (static) ↓ +/- 0 ↑↑Differentiation (static)

Proliferation (dynamic) ↓ ↑ ↑↑Differentiation (dynamic) ↑


Degradable cardiovascular iron stents

from Peuster et al. Heart 86(2001) 563

Material: > 99.8 % Fe (third most abundant cation in human body)Animal experiment in rabbits – native descending aorta – 6…18 month

Result:No thromboembolic complicationsNo adverse effectsNo significant neointimal proliferationNo pronounced inflammatory responseNo systemic toxicity

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7. Cobalt based & shape memory alloys, … based & shape memory alloys, biodegradable metals Dieter...

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