Surface treatments :Coatings on

Metallic Materials for Bone

Implantation
Dr .Nida Iqbal
Surface treatments
• Surface treatments are normally carried out to modify yet maintain
desirable properties of the substrate materials especially implant
industry.
• The surface area can be increased remarkably by using proper
modification techniques, either by addition or subtraction procedures
• A surface treatment can also be classified into mechanical, chemical,
and physical methods
 Introduction to Metallic Implants
• Metals play an important role in the human body as implants. Metallic alloys
are most commonly used in all bone joint replacements and dental implants.
• Most metallic implants find application in orthopedic surgery: Titanium,
Stainless Steel 316L, Nickel-Titanium, Magnesium, Cobalt-Based
Alloys.
•Properties: High mechanical strength, corrosion resistance, wear
resistance.
•Applications: Fracture plates, hip nails, screws, joint caps, wires.
•Challenges:
•Biologically inert (except magnesium).
•Magnesium alloys: Bioactive, biodegradable, biotolerant.
•Requirement: Coatings enhance biocompatibility and promote
osseointegration
Various factors responsible for implant failure.
• Surface modification techniques
Suitable surface modification technique with nontoxicity for specific
biomedical applications, corrosion resistance, modulus of elasticity,
fatigue strength and controlled degradability have been recognized as
basic properties.
The rationale behind surface modification of an implant is that the
surface of a material determines the response of the biological
environment to implanted materials.
Surface modification of implants is broadly classified into two
approaches: accelerated bone healing and enhanced bone bonding of
an implant.
 Surface topography
• In the enhanced bone-bonding approach, the surface topography of
an implant material is modified using a suitable approach that
enables the modified surface to increase the mechanical interlocking
of the bone with the implanted material.
• The modified surface automatically increases the surface area and
surface energy of the material, which leads to enhanced matrix
protein absorption, cell adhesion and proliferation, and finally, to
better osteointegration of the implant with the bone.
• Mechanical processing is simple physical treatment and shaping of a
material surface by cutting, blasting, grinding and polishing to
produce an improved surface topography and roughness, remove
contaminated materials and reinforce its bonding strength by way of
increased adhesion.
Bioactive coating
In accelerated bone healing, inorganic or
composite bone materials are incorporated into
the bone’s surface to enhance the bone-forming
capability of the cells and cause biochemical
interlocking of the implant with the adjacent bone.
Incorporation of organic molecules (biochemical)
such as proteins and peptides into the surface of
the implant is also an accelerated bone-healing
method.
Introduction to Adhesive Bond Theory
• Key Characteristics:
• Adhesion: Attractive force between layers resisting separation.
• Cohesion: Internal force holding molecules of the coating film together.

(A) adhesion and (B) cohesion forces between the adhesive layer and substrate
 Types of Adhesive Forces
• Specific Adhesion: Attractive forces between dissimilar molecules.
• Mechanical Adhesion: Adhesive penetration into substrate
microstructures.
• Efficient Adhesion: Combination of specific and mechanical adhesion.
• Failure: Results from breaking bonds between adhesive and
substrate.
 Theory Behind Adhesion
• Adhesives: Non-metallic compounds binding surfaces.
• Mechanisms:
• Physical adsorption.
• Mechanical interlocking.
• Chemical bonding (e.g., hydrogen bonds and Van der Waals forces).
• Classification:
• Reactive adhesives: Chemically interact with substrates.
• Non-reactive adhesives: Physical forces dominate.
• Surface Preparation: Rough surfaces enhance adhesion by increasing
interfacial contact.
Coating Challenges in Metallic Biomaterial
•Role of Adhesion: Critical for determining surface and
mechanical properties of metallic biomaterials.
•Issues with Poor Coating:
•Increases risk of implant failure due to mechanical stress.
•Leads to exfoliation, affecting tissue healing and surrounding
body parts.
 Coating Techniques for HAp
Methods:
•Thermal Spray Coating: Uniform, efficient, widely used.
•Sol-Gel: High purity, smooth finish, closed conditions required.
•Dip Coating: Cost-effective, high finishing temperature.
•Plasma Spraying: Multilayer capability, costly.
•Pulsed Laser Deposition (PLD): Versatile, uniform coating,
expensive.
Challenges:
•Adhesion issues on metallic surfaces.
•Crystalline HAp film prone to failure.
 Sol-Gel and Dip Coating Technique
• A widely used method for coating metallic implants, involving calcium and
phosphate-based salts, and solvents like water and ethanol.
• Dip Coating Process: Coating is applied by immersing metallic substrates into a
sol-gel solution, followed by removal and drying
 Sol-Gel and Dip Coating Technique

Advantages
•Low operating temperatures
•Cost-effective
•Ability to coat irregular geometries
•Uniform thickness control
•Improved wear and corrosion resistance due to reduced
cracks
•Heat Treatment: Applied after coating to enhance strength,
density, and apatite formation.
 Literature on Sol-Gel Hap Coating
• Studies on Sol-Gel HAp Coating:Materials Used: Various calcium and
phosphorus compounds, solvents like water and ethanol.
• Operating Conditions: Varying temperatures (25–500°C), speeds, and
dipping times for optimized coating performance.
• Outcomes:
• Enhanced bioactivity and osteoconductivity in titanium implants
• Reduced metal ion release and corrosion in alloys like Ni-Ti and Mg alloys
• Improved bone integration, with studies showing 70.9% increased bone area
ratio and enhanced healing with HAp coatings.
 Biomimetic Deposition
•What is Biomimetic Deposition?
•Mimics natural bone-building processes
to enhance implant osseointegration.
•Uses Hydroxyapatite (HAp) to promote
the adhesion and proliferation of
osteoblast cells.
•Key Process:
•Pre-treatment of the implant surface with
acid or alkali to introduce hydroxyl groups.
•Facilitates the CaP nucleation and
crystallization process for apatite
formation.
 Key Biomimetic Coating Materials &
Parameters
Coating Materials:
• Calcium phosphate, Hydroxyapatite (HAp), and related compounds.
Operating Parameters:
• Temperature: 37°C (standard for biomimetic coatings).
• pH: Often around 7.1 to 7.3.
• Time: Typically 24 hours for coating formation.
• Surface treatments like immersion in Simulated Body Fluid (SBF) or sodium
hydroxide (NaOH) for surface activation.
Biomimetic Coating Examples and Outcomes
Example 1:
•Coating: Calcium phosphate and Tobramycin.
•Outcome: Prevents post-surgical infections.
•Operating Conditions: 37°C, pH 5 or 7.3, 24h (2002).

Example 2:
•Coating: Calcium phosphate (SBF).
•Outcome: Facilitates rapid bone formation, reducing recovery time.
•Operating Conditions: 37°C, pH 7.1, stirring at 250 rpm, 24h (2004).
 Chemical Vapor Deposition (CVD)
What is CVD?
•A coating technique using volatile precursors to
coat substrates.
•Decomposition or reaction occurs on the
substrate surface to form coatings.

Key Features of CVD:

•Control of crystal phases and microstructure
formation.
•Uniform coating on complex metallic shapes.
•Widely used for coating metal implants with
Hydroxyapatite (HAp) and calcium phosphate-
based coatings.
 Materials & Operating Conditions in CVD
• Materials Used:
• Calcium diketonate and tri-methyl phosphate for HAp coatings.
• Fluorine-containing carbonated hydroxyapatite for bone-like agglomerated
structures.
• Bis-dipivaloylmethanocalcium for mechanical biocompatibility.
• Operating Conditions:
• Temperature: Ranges from 500°C to 1073 K.
• Pressure: Typically in the range of 0.8 kPa to 10 Torr.
• Deposition time: From 0.3 ks to 3 hours.
• Growth Rate: Can be as high as 15 nm/min.
 CVD Coating Outcomes and Examples
Example 1:
•Coating: HAp from calcium diketonate and tri-methyl phosphate.
•Outcome: Dense, crack-free coatings with Ca/P ratios of 1.5 ± 0.5 and 1.0 ± 0.5.

Example 2:
•Coating: Fluorine-containing carbonated hydroxyapatite.
•Outcome: Cauliflower-like agglomerated structure with similarities to human bone.

Example 3:
. Coating: Calcium dipivaloylmethanate and Titanium di(i-
propoxy)bis(dipivaloylmethanate).
•Outcome: Excellent mechanical properties and biocompatibility with a uniform coating
on titanium implants.
•Year: 2023.
 Electro-Chemical Deposition
•Electro-chemical deposition is a widely adopted technique
for coating biomaterials using a two-electrode system:
anode and cathode.
•Two Main Procedures:
1.Electrophoretic Deposition (EPD):
•Uses suspended ceramic particles.
2.Electrolytic Deposition (ELD):
•Utilizes metallic salts from saturated salt solutions.
•Typical Substrate Material:
•Titanium implants are most commonly used in both ELD
and EPD techniques for coating.
•Key Features:
•High production rates, uniform thick coatings, and strong
bonding to substrates.
•Lower operating temperatures compared to other coating
techniques.
•Energy consumption is relatively high due to electricity
requirements.
Electro-Chemical Deposition
 Research Studies & Examples of Electro-
Chemical Deposition
•Study 1:
•Electrolyte: Calcium nitrate, Ammonium dihydrogen phosphate, Sodium nitrate, Hydrogen peroxide,
Zirconium oxide.
•Implant Material: Nickel-Titanium.
•Operating Conditions:
•pH: 6.0, Temperature: 65°C, Current density: 0.5 mA/cm² for 40 min.
•Drying at room temperature.
•Outcome:
•Zirconia enhances bonding strength, corrosion resistance improved by 60 times.
•Study 2:
•Electrolyte: Calcium nitrate, Sodium hydrogen phosphate, Tris-hydroxy-methyl-amino-methane.
•Implant Material: Cobalt-Chromium-Molybdenum (CoCrMo).
•Operating Conditions:
•Electrolyte stirring at 250 rpm, pH: 6.0, CoCrMo as cathode and platinum as anode.
•Outcome:
•Strong mechanical bonding strength compared to other methods.
 Recent Advances & Enhanced Materials in
Electro-Chemical Deposition
•Recent Reinforced Materials for Enhanced Mechanical Properties:
Reinforcements Used:
•Zirconia oxide (ZrO2), Carbon nanotubes (CNTs), Titanium oxide (TiO2).
Example:
•HAp + Single-Walled Nanotubes (SWNTs):
•Enhanced crystallinity and homogeneity of the coating.
•Increased bond strength from 15.3 to 25.7 MPa.
•Defect-free surface and improved cell attachment/proliferation.
•Year: 2019【155】.
Key Advantages of Electro-Chemical Deposition:
•High adhesion and compact coating formation due to nucleation and growth mechanisms.
Annealing & Heat Treatment:
•Post-deposition heat treatment improves coating density, adhesion, and bond strength
•Ideal for orthopedic implants due to enhanced bioactivity and tissue integration.
 Plasma Spraying

Definition: Plasma spraying is a technique to coat bio-

active hydroxyapatite (HAp) on biomedical implants.
Mechanism:
•Uses an electric arc of high temperature and pressure.
•Melts and sprays HAp onto the implant surface.
•Process:
•Dried HAp is converted into plasma via a thermal
plasma jet.
•High-temperature plasma adheres to the substrate
surface.
•Variations:
•Air plasma spraying.
•Vacuum plasma spraying.
 Improving Plasma Spraying Outcomes

•Advantages:
•Produces stronger coatings with superior properties.
•Enhances osteoconductivity due to better bonding of HAp with
metal.
•Applicable to various biomedical materials.
•Limitations:
•High temperatures may deform the HAp structure.
•Reduced adhesive strength between HAp layer and metal surface.
 Advantages and Limitations
• Post-heat treatment:Annealing at 400°C for 90 hours improves
crystallinity.
• Heat treatment at 700°C for 1 hour:
• Removes impurities.
• Enhances coating purity and adhesive strength.
• Reduces fatigue stresses and alters coating thickness.
 Pulsed Laser Deposition (PLD)
• Definition: A coating technology that
uses a highly accelerated laser beam
in a vacuum to coat substrates.
• Process:
• Laser beam vaporizes coating material
into high-temperature plasma.
• Plasma strikes the substrate, forming a
thin and uniform coating.
• Vacuum atmosphere and substrate
heating ensure defect-free coatings.
• Applications: Used in bio-implants
for precise and uniform coatings.
 Features and Limitations
• Features:
• Uniform coatings with micro-porous structures.
• Surface roughness up to a few nanometers promotes cell adhesion and
proliferation.
• Enhances wettability, aiding body fluid interaction and tissue regeneration.
• Supports osteoconductivity (bone growth along the implant) and
osteoproductivity (stimulus generation for tissue growth).
• Limitations:
• Residual stresses due to:
• High-temperature operations.
• Mismatch in crystal structure between HAp and metallic substrates.
• Thick coatings (>1 µm) may mask defects.
 Enhancements and Biological Response
Post-treatment techniques:
•Heat treatment minimizes stresses and improves coating properties.
Impact on bio-implants:
•Promotes rapid healing and bone regeneration.
•Enhances body response due to bioactive coatings that bond with tissues.
•Coating surface topography is crucial for osseointegration (implant-bone
bonding).
Biological process:
•Coatings facilitate faster clot formation and phagocytic cell response.
•Stimulates bone growth along and away from the implant surface.
 Flame Spraying (FS)
• Process Overview:Utilizes a
combustion flame (oxygen
and acetylene) to melt HAp
powder.
• Melted powder sprayed onto
metallic surfaces, forming
porous and composite
coatings.
• Typical combustion
temperature: ~2600°C.
• Particle velocity: 200–300
m/s.
 Advantages and Limitations
Advantages:
•Economical and easy to operate for commercial applications.
•Effective for coating bioactive materials (e.g., zinc-doped HAp on Ti-
6Al-4V).
•Enhances biocompatibility and provides antibacterial properties (e.g.,
against E. coli).
Limitations:
•Results in larger microstructure, pores, and cracks in coatings.
•Coatings developed in wet conditions contain nano-size cracks (~100
nm).
•Lower mechanical strength compared to advanced thermal spray
methods.
 Bioactive Coatings and Applications
• Examples of Applications:
• Coating 316L steel and titanium alloys with bioactive glass.
• Enhancing bioactivity by forming hydroxy carbonate apatite (HCA) layer in
SBF solution.
• Improving cell proliferation and differentiation (e.g., pre-osteoblast cells).
• Role of Magnesium and Thermal Conductivity:
• Higher magnesium content → increases crystallinity, promotes pore
formation.
• Titanium alloys → slower cooling rate → fosters crystalline phases.
• Bioactivity Indicators:
• Formation of HCA layer on immersion in simulated body fluid (SBF).
Introduction to Innovative Coating Methods
Importance of Innovation:
• Enhances biocompatibility,
adhesion, and implant reliability.
• Reduces cytotoxic effects and
improves biological performance.
Key Techniques:
• Electro-Deposition with Bi-Layer
Coating:
• Used on SS 316L substrates.
• Heat treatment at 800°C (vacuum) or
600°C (air) for bonding. A) Oxide layer in between metallic implant and HAp and (B) Bi-
• Bi-layer minimizes implant-fluid contact, layer Metallic oxide coating
ensuring high uniformity and bioactivity.
• Oxide Layer Introduction:
• Protects metallic surfaces and prevents
toxic ion release.
• Enhances adhesion between HAp and
oxide.
 Advanced Coating Techniques
• Ceramic Layers (e.g., Zirconium):
• Acts as a strong bond and composite material.
• Ideal for implants under cyclic stresses.
• Super-High-Speed (SHS) Blasting:
• Improves bond strength and prevents HAp layer exfoliation.
• Yields HAp films with superior adhesion and wetability properties.
• Combination Coating Methods:
• Micro-Arc + Sol-Gel Processes:
• Micro-arc: Improves metallic implant biocompatibility.
• Sol-gel: Further enhances bioactivity of anodized Ti.
 Benefits of Innovative Methods
Key Advantages:
•Enhanced uniformity and bond strength.
•Improved bioactivity and osteointegration.
•Protection against toxic ion release from metallic substrates.
•Suitability for complex stresses and demanding applications.
•Future Implications:
•Combining techniques allows for tailored coatings.
•Potential for novel composite materials with superior performance.
 Conclusion

•Hydroxyapatite (HAp) coatings: Enhance biocompatibility and mimic

natural bone.
•Common coating methods:
•Thermal Spray Coatings: Efficient, uniform, high strength, with
controlled porosity.
•Sol-Gel Method: Suitable for complex shapes, uses aqueous precursors.
•Electrochemical Deposition: Effective for complex geometries but
limited by substrate conductivity.
•Innovative techniques: Intermediate oxide layers and post-treatment
enhance adhesion and implant performance.
 Future Perspective
•Nanoparticle-Coated Titanium Alloys:
•Increased surface area for enhanced adhesion and biocompatibility.
•Porous Titanium Implants Coated with HAp:
•Address stress shielding and improve osseointegration.
•Biphasic Coatings:
•Combine HAp with calcium magnesium phosphates like whitlockite for
improved osteogenesis.
•Explore combinations of HAp, whitlockite, and ions for optimized tissue
healing.
•In Vivo Studies:
•Comprehensive testing of novel coatings and materials for clinical
applications.