Enhancing Ossiointegration And Corrosion Properties of Ti6Al4V Alloy By Coating With Chitosan – Collagen- Hydroxyapatite For Biomedical Applications


 In this research, Ti-6Al-4V alloy samples were coated with 4 gm strontium hydroxyapatite with 2 gm from chitosan and (2,4,6) gm from collagen and the samples heat treated at 150 ºC in muffle furnace for one hour under air atmosphere. The sample were tested by XRD,FTIR,SEM and corrosion test was also achieved. The samples were immersed in a laboratory prepared simulated body fluid (SBF) solution for two weeks, the samples treated at 150 ºC in muffle furnace for one hour under air atmosphere to get more bonding for new layer . The samples tested by XRD,FTIR,SEM and corrosion test was also achieved after immersing . The sample coated with 6 gm collagen showed maximum growth of hydroxyapatite formed from simulated body fluid SBF and corrosion characteristics was much improved.

In this research, strontium hydroyaptite SrHA prepared by wet chemical method, by reacting calcium nitrate (Ca((NO 3 ) 2 .4H 2 O) and Diammonium hydrogen phosphate (NH 4 ) 2 HPO 4 . Then Ti6Al4V alloy was coated with composite consisting of SrHA ,collgen and chitosan by dip coating in order to improve the corrosion characteristics and osseointegration with bone. The coated samples was immersed in synthesized simulated body uid SBF solution which it's component listed in table (1) below for two weeks in order to study the effect of collagen addition on the osseointegration. Germany) to 250 ml dionized water. The solution was mixed using ultrasonic bath for 30 minutes. 20 gm o orthophosphoric acid 85% (Merck, Germany) was added to 250 ml deionized water and mixed gentle using magnetic stirrer. The phosphoric acid solution was added dropwise to the rst mixture for 30 minutes under mixing in the ultrasonic bath. After the addition was complete the mixture in ultrasonic bath was continued for another 30 minutes, then the solution was ltered using lter paper and dried under 80 ºC for 24 hr. and then milled in mortar to get ne powder. The ne powder as heat treated at 600 ºC for one hour under air atmosphere using mu e furnace. 1 gm of phosphorus pentoxide P 2 O 5 (BDH, England) was added to 50 ml of absolute ethanol 96% (scharlau, Espain) and mixed well for 20 min.
using magnetic stirrer. 4 gm from prepared strontium hydroxyapatite with 2 gm from chitosan (Shaanxi Co., China) and (2) gm from collagen type I ( Athena Co., USA) was added to the mixture and well mixed for 15 minutes. Ti-6Al-4V alloy samples (20 mm diameter) were grinded using SiC grinding paper grit 500.
The samples were cleaned in ultrasonic bath for 15 minutes twice in ethanol 96% and once in distilled water for 15 minutes. The samples was dipped in the mixture for 15 sec. and dried in the air, then it was dipped again in the mixture for 15 sec. to get another layer coating. The coated samples were dried in the oven at 90 ºC for 5 hr. in the oven under air atmosphere. The dipping procedure was repeated using solution consisting of (2 and 4) gm collagen individually in each time and 4 gm from prepared strontium hydroxyapatite with 2 gm from chitosan and the coated samples was dried in the oven at 90 ºC for 5 hr.
in the oven under air atmosphere. The treated samples were tested by XRD, FTIR, SEM and also corrosion test including open circuit potential (OCP), polarization curve (tafel) and electrochemical impedance spectroscopy EIS was done using electrochemical potentiostat (CS 350, China). The samples then were immersed in synthetic simulated body uid (SBF) solution prepared from chemicals listed in table (1) for two weeks. The samples were heat treated after immersing at 90 ºC for one hour in the oven then tested by, XRD, FTIR, SEM and corrosion test including open circuit potential (OCP), polarization curve (tafel) and electrochemical impedance spectroscopy EIS also done in order to evaluate the results.
Results And Discussion: The XRD pattern for samples coated with SrHA with different collagen quantity in Fig. (1) shows the presence of chitosan and collagen at (2θ = 18-21) with amorphous structure, the two components had very close (2θ) location. We observed also the change in intensity of collagen platue for higher value with grater quantity .The SrHA obviously was in many locations in the pattern, but we saw that the change in collagen quantity made the peaks of the SrHA with more intensity, i.e the higher quantity of collagen led the SrHA particles to agglomerate, also the collagen led to more particle adhesion on Ti-6Al-4V alloy surface make the SrHA more observed by XRD test. The Ti-6Al-4V peaks also observed in all XRD patterns.
The FTIR spectrum for collagen in Fig. (2) identi ed the amide A band position at 3484 cm −1 , and it associated with the N-H stretching vibration. The stretch of CH 2 , was found at 3071 cm −1 and 2971 cm −1 .
The amide I band of this collagen was at 1653 cm −1 . Amide II bands were found at 1524 cm −1 and 1448 cm −1 indicated N-H bending. Amide III bands were found at 1326 cm −1 indicated C-H stretching.
The FTIR spectrum for chitosan in Fig The SEM test images in Fig. (6) for Ti-6Al-4V alloy coated with SrHA-chitosan and different quantity of collagen showed clearly the agglomeration of SrHA and collagen with increasing the quantity of collagen.
After immersing in SBF for two weeks the XRD pattern in Fig. (7) showed an obvious peak of hydroxyapatite in all patterns re ected the biomimetic formation of hydroxyapatite from SBF. In the same time the collagen platue disappear because the new hydroxyapatite layer cover the old layer, the agglomeration in particles affect hydroxyapatite formation from SBF, i.e the more quantity collagen leads to more hydroxyapatite formation.
FTIR spectrum for Ti-6Al-4V coated with SrHA-chitosan -collagen after immersing in SBF for two weeks in Fig. ( The SEM test image in g (10) for images for Ti-6Al-4V alloy coated with SrHA-chitosan -collagen after immersing in SBF for two weeks showed the difference in morphology for hydroxyapatite formed from SBF compared with previous g (6).

Corrosion test:
The OCP test showed increasing in passivation with increasing collagen quantity as shown in Fig. (11) and table (2). The potential reach to -0.437 volt for sample coated with 6 gm collagen compared with -0.527 for uncoated sample. The polarization curve tafel showed decreasing in corrosion current and so on increasing in corrosion rate as shown in Fig. (12) and table (2). The corrosion rate reach to 1.806 × 10 −3 mmpy for the sample coated with 6 gm collagen compared with 1.344 × 10 −2 for uncoated one. This is an indication that the increasing in the collagen quantity positively affected the improvement of the corrosive characteristics. The polarization resistance can be calculated from relation below: Where Rp: polarization resistance, ba: anodic slop, bc: cathodic slop and icor.: corrosion current density (Amp/cm 2 ).
The protection e ciency can be calculated from relation below: The weight loss can be calculated from Faraday's Law as shown below:  We see the polarization resistance increase with increasing collagen quantity. Rp was 145.04 kΩ.cm 2 for sample coated with 6 gm collagen compared with 18.024 kΩ.cm 2 for uncoated one. Also the protection e ciency increased with increasing collagen quantity from 51.85% for sample coated with 2 gm collagen to 86.55% to that coated with 6 gm collagen. The weight loss decreased from 1.656 mdd (mg.dm −2 .day −1 ) for uncoated sample to 0.222 mdd to that coated with 6 gm collagen.  Fig. (14) and table (4) from 9.646 × 10 −3 mmpy for uncoated sample to 5.446 ×10 −4 mmpy for that coated with 6 gm collagen. The calculated corrosion parameters showed obviously increasing in polarization resistance Rp after immersing in SBF solution for two weeks. Rp increased from 29.315 kΩ.cm2 for uncoated sample to 651 kΩ.cm2 for that coated with 6 gm collagen. The protection e ciency increased from 35% for uncoated sample to 94.35% for that coated with 6 gm collagen. The weight loss decreased also much from 1.168 mdd for uncoated sample to 0.067 mdd for sample coated with 6 gm collagen as shown in table (5). The Nyquist and the Bode plots for Ti6Al4V samples coated with different collagen quantity shows a higher corrosion resistance with increasing collagen quantity from expansion in semicircles with increasing the collagen quantity. The samples with 6 gm collagen in the three Nyquist gures exhibited a greater radius for the capacitive loop indicating that they were provide a better corrosion protection as compared with the other samples as shown in Fig. (15 a and b). We observe from table (6) which represent equivalent circuit tting data for Ti6Al4V alloy base and coated samples that the coating resistance increased with increasing collagen quantity and reach to 4.985 kΩ.cm 2 for sample coated with 6 gm. That occurs because increasing in coating layer thickness and decreasing in porosity. The double layer resistance also increased obviously from 8.668 kΩ.cm 2 for uncoated sample to 18. 210kΩ.cm 2 for that coated with 6 gm collagen. That occurs because the collagen make good agglomeration to coating layer increasing with quantity make a great impediment for the ions to reach the surface and retarding the reaction occur on corroded surface (the corrosion reaction process) lead to decreasing the corrosion rate and improve the resistance to corrosion. After immersing the samples in (SBF) solution for two weeks the nyquist and bode plots shows more increasing in resistance, and the expansion in semicircles be higher than before as shown in Fig. (17 a  and b) which represent ) nyquist and bode plost for Ti6Al4V samples coated with different collagen quantity after immersing. We observe from table (7) which represent equivalent circuit tting data for Ti6Al4V alloy base and coated samples after immersing in (SBF) solution that the coating resistance increased with increasing collagen quantity and reach to 6.728 kΩ.cm 2 for sample coated with 6 gm compared with 3.186 kΩ.cm 2 for uncoated sample. That occurs because increasing in coating layer thickness and decreasing in porosity. The double layer resistance also increased obviously from 9.82kΩ.cm 2 for uncoated sample to 125.33 kΩ.cm 2 for that coated with 6 gm collagen. That occurs because the new hydroxyapatite layer formed biomematically from (SBF) solution reduce the pore size and make the reach of ions to the substrate surface very di cult, i.e more corrosion resistance.  Figure 1 Hierarchical structure of bone.

Figure 2
Structures of chitin, chitosan, and cellulose.