The
effect of strontium incorporated Zn-Ca-P coating on the biodegradability of
AZ31 alloy was evaluated. The Sr content and deposition time were optimized and
coated on AZ31 alloy by chemical conversion technique. The coating formed with
1.5 wt.% Sr and 20 min phosphating time at 50° C with pH 2.5 completely covers
the surface of the alloy. Sr doped coated sample showed evolved hydrogen volume
and pH value three times lower than Zn-Ca-P coatings which implied the
controlled degradation of the coating. On immersion in Simulated Body Fluid,
this surface exhibits high bioactivity with the deposition of calcium phosphate
phases with Ca/P ratio of 1.55 which is close to that of hydroxyapatite,
mineral component of bone. Cytotoxicity evaluation with L969 cells showed that
Sr doped coatings exhibited 72% cell viability on resorbable magnesium alloys.

Key
words: Magnesium, Zinc Calcium Phosphate,
biocompatible, corrosion, Strontium, cell viability.

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1. Introduction

Magnesium
alloys have been evolving as a promising biodegradable material due to its favourable
mechanical properties, biocompatibility and lightweight 1. Particularly, the
elastic modulus of magnesium is closer to that of natural bone which avoids
stress shielding effect 2. Moreover, magnesium is an important element found
in human body and involved in body metabolic activities such as protein
synthesis, muscle contraction and relaxation, energy transport etc. 3, 4. Low
levels of magnesium lead to Alzheimer’s disease,
asthma, attention deficit hyperactivity disorder (ADHD) 5. Even though
magnesium alloys have many favourable properties, electrochemical potential of
magnesium is in the active region in the EMF series leading to rapid corrosion,
increase in local pH, hydrogen evolution, loss of their mechanical integrity
before bone healing process 6, 7. The necessary requirement of biodegradable
implants is that the degradation rate should be matched with the healing rate
of bone 8. For new bone formation, magnesium alloy should maintain its
mechanical integrity at least for 3 months. At present, magnesium alloys are
used for biodegradable implant applications such as screws, pins and
stents. 

Mainly
three approaches have been developed to control the degradation rate of
magnesium such as alloying, surface modification and coating 9. Coating is
one of the key solutions to overcome the corrosion rate of magnesium alloy in
chloride containing environment and provides some barrier effect between the
material and the environment. Various surface treatment techniques such as
physical vapor deposition, electrodeposition, anodization, chemical vapour
deposition, microarc oxidation are available to achieve desired coating 10.
Long treatment time, high cost, high temperature, complex procedures are the
limitations of these techniques. In contrast, conversion coating technique is a
simple technique and can produce a highly adherent coating 11. Particularly,
phosphate conversion coatings are regarded as suitable alternative compared to
other conversion coating techniques to improve surface properties due to low
toxicity 12.

Zinc phosphate, calcium phosphate
(CaP) and zinc calcium phosphate (Zn-Ca-P) coatings are attractive for
biomedical applications, as the elements present in the coatings are essential
for human health 13-17. However, to be best of available literature, no
attempt has been made to study the effect of ions in the Zn-Ca-P coating to
further improve the bioactivity and corrosion resistance.
Among various dopant ions, strontium is of our interest due to its biological
performance and bioactivity. Hence Sr doped HA, bioactive glass, bone cement
have been synthesized 18-21. Sr doped CaP coatings have been investigated as
an implant coating material 22. In view the advantages of both the coating
material and coating technique, we have attempted the feasibility of Sr doped
Zn-Ca-P coating by conversion coating method. By adding strontium precursor in
the phosphating bath, strontium incorporated PCC coatings can be developed.
Hence the present work deals with the optimization of strontium content in the
zinc calcium phosphate conversion coating on magnesium AZ31 alloy and its
evaluation for bioactivity, corrosion resistance and cytocompatibility.

2. Experimental Procedure

2.1. Substrate preparation

The
substrate used in the present study is AZ31 magnesium alloy which was purchased
from Exclusive Magnesium, Hyderabad, India. The chemical composition of the
substrate is 2.9% Al, 0.88% Zn, 0.001% Fe, 0.02% Mn, and balance magnesium. The
substrate was polished with SiC paper up to 1200 grit. The substrate was washed
with double distilled water and ultrasonically cleaned and degreased with
acetone.

Zn-Ca-P
and Sr doped Zn-Ca-P coatings were prepared on the surface of AZ31 alloy in the
phosphating bath. The phosphating temperature was maintained at 50° C. The pH of
the bath was adjusted to 2.5 with the help of phosphoric acid. The composition
of the phosphating bath is 10g/L diammonium hydrogen phosphate ((NH4)2HPO4),
7 g/L zinc nitrate (Zn (NO3)2), 3 g/L calcium nitrate
(Ca(NO3)2), 3 g/L sodium nitrate (NaNO2), 1g/L
sodium fluoride (NaF). Sr doped zinc calcium phosphate coating was deposited on
the substrate by varying the strontium content (precursor used is strontium
nitrate) as 0.5, 1 and 1.5 wt.% with optimized pH of 2.5 at 50° C (optimized
temperature) for various deposition times i.e. 5, 10, 15, 20 and 30 min. Zinc
calcium phosphate coating was also deposited for comparison.

2.2 Surface characterization

 Fourier Transform Infrared (FTIR) spectra of
Zn-Ca-P and Sr doped Zn-Ca-P coatings of various compositions were recorded on
an FTIR spectrometer in the range of 400-4000 cm-1 with a single
reflection ATR accessory (Perkin Elmer Spectrum two, USA). Chemical composition
and phases of the compounds were analyzed using X-Ray powder diffractometer
(XRD, D8 DISCOVER, Bruker, USA) using Cu k? radiation at 40kV and
30mA at a scan rate of 0.02°. Scanning Electron Microscopy with
Energy-Dispersive X-ray spectroscopy (SEM, FEI, QUANTA 200, NETHERLANDS) was
used to characterize the surface morphology and elemental composition of
Zn-Ca-P and Sr doped Zn-Ca-P coatings. Wettability of undoped and doped samples
was measured using contact angle instrument (Easy Drop KRUSS, Germany) and SBF
is used as contact liquid at different locations.

2.3 Adhesion characterization

Films
with good adhesion strength are considered as a protective overcoat and are very
vital for the protection against corrosion. Hence adhesion of the coating was
tested as per ASTM (American Standards for Testing and Materials) D3359-09
using tape adhesion test 23. 25 squares were developed by cross cutting the
coating in both directions using cross hatch cutter and adhesive tape was
applied on the cross-cut area and pulled rapidly. Percentage of the adhesion
remaining was calculated using the equation                  

Adhesion
remaining (AR) %= (n/25) × 100                  
————————– Eq.1

Evaluation
was made by substituting the number of peeled squares (n) in Eq.1 as per the
ASTM standard (0%-5B,

x

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