Surface modification techniques in biomedical sector

Surface modification techniques in biomedical sector

Term paper presentation for

Surface Modification

Metallurgical and Materials Engineering

IIT Kharagpur

Introduction

● A biomaterial is any matter, surface, or construct that interacts with biological systems.

● Natural or synthetic.● Metallic, ceramic or polymer.

Metallic steel, Cobalt and Titanium based alloys

Ceramic titania (TiO2), titanium carbide (TiC), titanium nitride (TiC), bioglass, hydroxyapatite (HA), silicon carbide (SiC)

Polymer polyethylene terephthalate, polytetrafluoroethylene, ultrahigh molecular weight polyethylene (UHMWPE) and lactide-co-glycolide

Surface modification of biomaterials

● Bio-integration is the ideal outcome expected of an artificial implant.

● Phenomena occurring at the interface between the implant and host

tissues does not induce any deleterious effects.

● Bulk properties for mechanical strength

● Surface properties for biocompatibility

Objectives of surface modification of biomaterials

As biomedical devices are subject to extremely high clinical requirements, a thorough surface modification process is needed prior to implantation process into the human body. The objectives of surface modification are:

1.Increase bioactivity, cell growth and tissue attachments after implantation

2.Increase hardness of implant to reduce wear rate especially in articulationjoint applications

3.Introduce passive layer to prevent excessive ion release into bodyenvironment

4.Promote antibacterial effect

5.Increase fatigue strength of implants

Biomaterial applications

Hard tissue replacements

hip and knee joint implant replacement.Schematic diagram of

the screw-shaped

artificial tooth.

Cardiac and cardiovascular applications

Artificial heart valve: Ring and struts made up of Ti alloys, disc made of

pyrolitic carbon

Artificial vascular stents: Ni-Ti alloys used due to shape memory effect

Osteosynthesis

Bone screw and bone plate

Electron Beam Deposition

● Electron beam deposition of Ti on Co-Cr substrate.● Co-Cr has better mechanical properties compared to Ti.● But Ti has better biocompatibility.● So surface coating of Ti on Co-Cr substrate

● 10 × 10 × 2 mm3

● Coating thickness:

10microns

● Coating rate: 0.1nm/sec

● substrate holder: 10rpm

Electron beam Deposition of Ti on Co-Cr

Co-Cr substrate Ti coating on Co-Cr substrate

Electron Beam Deposition

Micro-Arc Oxidation

● Anodic oxidation technique.● Plasma modifies oxide structure.● The excellent biocompatibility of Ti is due to a thin TiO2 layer which

forms spontaneously in air.● Ti coated Co-Cr substrate is oxidised electrochemically by MAO.● Electrolyte containing Ca & P.● 0.15M calcium acetate monohydrate and 0.02M glycerol phosphate

calcium salt.● MAO requirements: Passivation at anode side & oxide should be stable in

electrochemical environment.

Ti coated Co-Cr MAO treated after Ti coating

MAO

TiO2 by MAO on Ti coated on Co-Cr substrate

SEM secondary electron cross sectional image

BSE images of MAO treated specimen.

● 3 to 5 microns

● Repeated dielectric

breakdown makes the TiO2

layer rough and porous

● Ca and P ions incorporated

into the TiO2 layer

Micro-Arc Oxidation

Ion implantation process

● Ion implantation is a procedure in which ions of a material are accelerated in an electric field and bombarded into the solid substrate surface.

● Ions implanted: Nitrogen, oxygen, carbon

● Two types of ion implantation process are:

(a)Conventional beam line ion implantation (b)Plasma immersion ion implantation (PIII) method

● In beam line ion implantation, the target is totally isolated from the ion

beam generation.

● In PIII, the target is an active part of the ion generation through bias

voltage. ● Specimen is surrounded by a high-density

plasma and pulse biased to a high negative

potential. Ions generated in the overlying

plasma are accelerated and implanted into

the surface.

● Energy of ions keV to MeV.

● Low substrate temperature.

● metal and non-metallic ion simultaneously

implanted on titanium alloy.

● Ca and Mg ion implanted into titanium

alloy for increasing the bone integration.

schemiatic of PIII

Ion implantation process

Ion-beam assisted deposition

● Ion-beam assisted deposition (IBAD) is a vacuum deposition process that combines physical vapor deposition (PVD) with ion-beam bombardment.

Schematic drawing of the polyfunctional IBAD system and the process of IBAD

IBAD

● Biocoating with higher adhesive strength than traditional coating

techniques.

● Low substrate temperature.

Applications:

● hydroxyapatite coating preparation.

● DLC film and C–N film: chemically inert, extreme high hardness and low

friction coefficient are their attractive.

● silver coating is that it prevents bacteria attachment to the biomaterial

surface.

Ion-beam assisted deposition

Laser Surface Modification

● A 3kW CO2 laser emitting with a wavelength of 10.6 mm● series of optical units deliver the CO2 laser beam to the workpiece

through the laser head

● The defocused CO2 laser beam was traversed a single time along the x axis

● The fumes produced were removed with an extraction system● CO2 process gas with 2 bar pressure was used to shield the laser optics

and assist the surface treatment.

The three requirements generally expected of biomaterials coating are:1) crystallinity,2) porosity3) adhesion.

Common advantages of laser surfacing compared to alternatives are :● chemical cleanliness● controlled thermal penetration and, therefore, distortion● controlled thermal profile and, therefore, shape and location of the heat● affected region● less after-machining, if any, is required● remote non contact processing is usually possible● relatively easy to automate

Laser Surface Modification

At present, the lasers are being used in the following surface modifications ofthe biomaterials:

● Laser patterning and microfabrication● focusing an intense laser beam at certain spots on a surface, where the

high beam intensity causes evaporation of the material.● By this approach, pits can be produced down to 1 mm, in the size range

of interest to match cell sizes.

● Pulsed laser deposition (PLD) of biocompatible ceramics

● thin films of biocompatible ceramics . Pulsed laser deposition isespecially well suited to the

● deposition of bone-like ceramics (e.g. hydroxyapatite (HA) and calciumphosphates) on to metal, ceramic, semiconductor or polymer substratesfor potential application in medical implants, prosthetic devices andbiocompatible probes or sensors.

Laser Surface Modification

● Laser surface treatment for improving corrosion● improvement resistance by a combination of the homogenisation of the

surface by melting, the hardening due to N incorporation and thethickening of the oxide layer.

● improvement in pitting corrosion resistance for 316LS biograde stainlesssteel.

● eliminate carbides and second phases alike, while also serving thefunction of homogenising the microstructure.

● N2 induced into the laser treated surface could promote newprecipitates and as a result lowered the corrosion resistance of 316LSstainless steel and Ti–6Al–4V alloy.

● Laser grafting● improved surface hydrophilicity and biocompatibility of ethylene–

propylene rubber, 2-hydroxyethyl methacrylate (HEMA) and N-vinylpyrrolidone (NVP) have been grafted on to the surface of this polymerusing a CO2 pulsed laser at different fluence (output power J/cm2) as theexcitation source

Laser Surface Modification

High Velocity Oxygen Fuel (HVOF) coating

● thermal spray coating process● used to improve or restore a component’s surface properties or

dimensions, thus extending equipment life● increasing erosion and wear resistance, and corrosion protection.

The application of hydroxyapatite (HA) coatings on Ti-6Al–4V based

prosthetics has been widely used due to the unique biocompatibility of HA.

For a long term usage, an HA coating must exhibit a high biocompatibility and

adequate mechanical properties, such as a high bond strength and an elastic

modulus value close to that of the bone. The biocompatibility and the

mechanical properties will depend on the coating microstructure, crystallinity

and phase composition.

Cross-section of the HA coating after a 7-day incubation in the SBF solution.

High Velocity Oxygen Fuel (HVOF) coating

Sputtering

1. creating a gaseous plasma

2. accelerating the ions from this plasma into some source material thesource material is eroded by the arriving ions via energy transfer

3. ejected in the form of neutral particles - either individual atoms, clustersof atoms or molecules

4. As these neutral particles are ejected they will travel in a straight lineunless they come into contact with something - other particles or anearby surface.

5. If a "substrate" such as a Si wafer is placed in the path of these ejectedparticles it will be coated by a thin film of the source material

Schematic presentation of an apparatus for Sputtering

Calcium ion implantation where calcium ions are implanted intobiomedical titanium alloys, calcium ion mixing method where Ca is sputteredon the surface of biomedical titanium alloys followed by Ar ion implantation,etc. CaP precipitation is enhanced on the surface of biomedical titaniumalloys conducted with these treatments when they are implanted into livingbody.

Sputtering

Gas Nitriding

● Process-

● the specimen is set in a furnace equipped with a chamber

● by using a rotary vacuum pump, the atmosphere in the chamber is

exchanged three times from air to nitrogen; purity of nitrogen gas is

higher than 99.9995%

● furnace is heated up to 1023, 1073, 1123, or 1223 K at a reduced

pressure below 0.001 MPa

● nitrogen gas is introduced into the chamber at a pressure of up to

0.100 MPa and the recording of the nitriding time is started

● specimen is kept in the nitrogen atmosphere at each temperature

for 21.6 ks

● at last furnace is cooled down to the room temperature while

maintaining the nitrogen atmosphere in the chamber.

Schematic drawing of gas nitriding process

Gas Nitriding

● Developed –A biomedical, β-type titanium alloy, Ti–29Nb–13Ta–4.6Zr (TNTZ), in order to

achieve a lower Young's modulus similar to that of human hard tissues in addition to excellent mechanical properties and good corrosion resistance for use as structural biomaterial

● Problem –When the titanium alloys are utilized as a material for artificial hip joints,

bone plates, etc., one of the possible risks due to wear includes the loosening of these tools. Thus, the improvement of wear resistance is required for biomedical titanium alloys.

Gas Nitriding

● Effect of Al on diffusion rate of O in TiO2 in gas nitriding -o Depending on the oxygen partial pressure and ambient temperature,

the main point defect in TiO2 are probably oxygen vacancies under the experimental conditions

o the inward diffusion of O is dominant in TiO2. A Ti atom is present as Ti4+

in TiO2, while Al3+ is the stable state of an Al atom. According to the point defect theory, when Ti4+ is substituted with Al3+, an oxygen vacancy is generated in order to satisfy the electroneutrality as follows equation Al2O3=2Al′Ti+VO..+3OO

o it is expected to increase the diffusion rate of O ions

Gas Nitriding

Vacuum plasma sprayed● used to manufacture HA coatings with an approximate thickness of 40 lm● VPS coatings were sprayed onto Ti—6Al—4V strips (80]20]2 mm) which had been

previously grit blasted with alumina grit and coated with a pure titanium bond layer

● particles used in the manufacture of the VPS coatings were angular in nature and had a wide size distribution

● linear increase of pushout failure load with increasing surface roughness for VPS HA coatings

● coatings had a comparably moderate roughness which should encouragecoating dissolution due to the large surface area exposed to the body’senvironment and allow good mechanical interlocking with bone, withoutimpairing the mechanical strength of the surface

● found that there is higher crystallinity and lower residual stress in theVPS coatings, which will result in a slow rate of dissolution in vitro and invivo relative to the DGUN coatings

Vacuum Plasma Sprayed

Detonation Gun (DGUN)

● used to manufacture HA coatings with an approximate thickness of 40 lm● coatings were sprayed directly onto grit blasted Ti—6Al—4V with no

intervening bond layer● Coatings were applied to one side of the substrate only● The higher temperature which the powder particles reached during

detonation spraying should impose a higher degree of melting on thestarting powder creating a more amorphous coating

● better adhesion of the hydroxyapatite to the substrate● higher velocity, higher energy DGUN process imposed a greater degree

of melting on the powder; but this process may be counterbalanced bythe extremely short dwell time of the particles in the plasma

● effects should combine to produce a more amorphous, more densecoating which, despite being better adhered to the substrate willundergo a more rapid dissolution in vitro than the VPS coatings

● detonation process resulted, to some degree, in the degradation of purehydroxyapatite to beta-tricalcium phosphate in the final coating

● process is a higher temperature, higher velocity technique which isthought to impose a higher degree of melting on the ceramic startingpowder

● process producing a denser coating which had a higher proportion of theamorphous phase with some evidence for the appearance of beta-tricalcium phosphate

VPS vs DGUN - There was a considerable difference in the crystallinity of the

VPS and DGUN coating types

Detonation Gun (DGUN)

Conclusion

● The current applications of surface modification techniques in the field of

biomaterials and bioengineering have been described.

● It is observed that the overall trends of surface modification methods has

shifted from the use of conventional source (chemical, induction heater

and gas) to the application of advanced technology (electrolyte based,

laser, plasma and ion).

● The works on surface modifications has expanded from focusing on

tribological issues such as wear resistance, corrosion resistance and

hardness of modified layer to clinical issues such as cell growth, cell

attachment and antibacterial effects.

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