Stoichiometry, crystallinity, and nano-scale surface morphology of the graded

calcium phosphate-based bio-ceramic interlayer on Ti-Al-V

J.D.Long,K.Ostrikov,andS.Xu

Advanced Materials and Nanostructures Laboratory, Natural

Sciences, Nanyang Technological University, 637616 Singapore

V. Ligatchev

School of Electrical and Electronic Engineering,

Nanyang Technological University, Nanyang Avenue,

Singapore 639798, Singapore

ABSTRACT

A plasma-assisted concurrent Rf sputtering technique for fabrication of biocompatible,

functionally graded CaP-based interlayer on Ti-6Al-4V orthopedic alloy is reported. Each

layer in the coating is designed to meet a specific functionality. The adherent to the

metal layer features elevated content of Ti and supports excellent ceramic-metal interfa-

cial stability. The middle layer features nanocrystalline structure and mimics natural bone

apatites. The technique allows one to reproduce Ca/P ratios intrinsic to major natural cal-

cium phosphates. Surface morphology of the outer, a few to few tens of nanometers

thick, layer, has been tailored to fit the requirements for the bio-molecule/protein attach-

ment factors. Various material and surface characterization techniques confirm that the

optimal surface morphology of the outer layer is achieved for the process conditions

yielding nanocrystalline structure of the middle layer. Preliminary cell culturing tests

confirm the link between the tailored nano-scale surface morphology, parameters of the

middle nanostructured layer, and overall biocompatibility of the coating.

INTRODUCTION

Calcium phosphate-based bio-ceramics have recently attracted a great deal of in-

terest as functional surface coatings in dental and orthopedic implants because of excel-

lent bio-activity, bio-compatibility, chemical and mechanical properties [1-5]. In particu-

lar, hydroxyapatite (HA, Ca

(PO

)

(OH)

, Ca/P=1.67) coatings reveal inspiring clinical

advantages in promoting efficient implant fixation and implant-to-bone adhesion shortly

after the implantation, as well as faster bone remodeling due to enhanced bi-directional

growth and formation of a bonding interlayer between bone and implant [1]. Further-

more, calcium phosphates with apatite-like structure are the major constituents of the

bone mineral phase, are compatible with various soft and muscular tissue types, and can

efficiently sustain protein attachment and growth [1]. Clinical applications of CaP-based

bio-ceramics for improved fixation between bone and implant pose a number of chal-

lenges for tailoring the coating quality specifications. The key quality factors include

stoichiometry, crystallinity, microstructure, metal-implant interfacial stability and several

others [6]. In addition, a successful technique for fabrication of a viable biocompatible

coating would intrinsically imply a certain degree of replication of biological apatites,

featuring nano-crystalline structures in bone and dentin materials. Above all, surface

morphology with nano-scale features and excellent island uniformity, appears to be a

critical factor in promoting bio-molecule/protein - surface interactions.

Thus, fabrication of a graded bioceramic-implant structure capable of simultaneously sat-

isfying the basic requirements for the interfacial stability at the coating-implant interface,

controllable stoichiometry and crystallinity in the bulk, and the optimized, from the point

of view of sustained protein attachment and growth, nano-scaled surface morphology, is a

critical problem in the biomaterials research. In particular, a large number of the existing

techniques for CaP-based films (including HA) deposition suffer from poor coating-metal

implant interfacial bonding strength, excessive amorphosity or larger, than in natural apa-

tites, crystal size in the bulk, as well as irregular surface morphology features typically in

the micrometer range.

EXPERIMENTAL DETAILS

Here, we report on a new and efficient technique for synthesis of a graded CaP-based

biocompatible interlayer on orthopedic alloy Ti-6Al-4V, consistently satisfying all the

above requirements. The essential part of the method is a concurrent low-temperature

plasma-assisted Rf magnetron sputtering (PA-RFMS) of crystalline HA and metallic tita-

nium targets in low-pressure discharges of reactive gas mixtures of argon and water va-

por sustained in PSAC/PA-RFMS facility. The titanium target has purposely been intro-

duced to create a titanium-rich layer adjacent and stronger adhering to the implant-

simulating metallic sample. The discharges were sustained in the range of Rf powers of

= 300 - 700 W applied to a water-chilled Rf magnetron electrode with several rows of

concentrically positioned permanent magnets with specific polarities. The working pres-

sure p

was typically maintained in the range of 10 to 70 mTorr. In this pressure and

power range, large DC sheath potentials near the HA/Ti target surface, promoting high

sputtering yields, and eventually high film deposition rates, can be achieved. An electri-

cally floating substrate heater powered by an external temperature controller supports the

Ti-6Al-4V samples, with the surface being coated facing downwards, approximately 6

cm above the HA/Ti sputtering target. In the experiments, the substrates were negatively

biased with V

= 25 - 200 V. Chemical composition and elemental bonding states in the

interlayer were studied by VG ESCALAB 220i-XL spectrometer (XPS). The crystal

structure was characterized using SIEMENS D5005 X-Ray diffractometer (XRD) in a

lock coupled (θ − 2θ) mode with an incident x-ray wavelength of 1.540A (Cu Kα line).

Cross-sectional structure of the functionally graded structure was examined with the

Field Emission Scanning Electron Microscopy (FESEM). Nano-scaled features of the

surface morphology were studied by the Atomic Force Microscope in a contact mode

(AFM). Bio-ceramic-metal interfacial bonding strength was assessed via a Micro-scratch

teste. Further details of the PSAC/PA-RFMS sputtering facility, routine steps in pre-

deposition substrate treatment, as well as film characterization instruments and tech-

niques, can be found elsewhere [7,8].

F3.42.2

RESULTS AND DISCUSSION

XPS layer-by layer analysis of the composition and chemical bonding states reveals clear

presence of Ca, P, O, and Ti in the film, the concentration of the latter being gradually

increased towards the bottom layer of the coating. In addition, varying the DC substrate

bias V

and deposition pressure p

, one can efficiently control the elemental composition

and Ca/P ratio (Fig.1), which is a key factor defining the phases of apatite. In particular, a

near-stoichiometric HA (Ca/P ~ 1.67) can simply be achieved by applying DC bias of

= -100 V at p

=70 mTorr. The elemental presence in the bulk of the coating is meas-

ured to be approximately Ca (27 %), P (15 %), O (50 %), and Ti (5 %). Furthermore,

most of calcium and phosphorus in the film enter calcium phosphate phase, whereas tita-

nium is predominantly a part of CaTiO

and TiO

. Formation of the phosphate phases is

further supported by the results of FTIR analysis (Perkin Elmer Spectrum I spectrometer)

showing the characteristic absorption peaks of PO

. Thus, the results of the XPS and

FTIR analysis confirm that stoichiometry, elemental composition, and bonding states in

the coating in question can indeed be tailored by varying the substrate bias, and under

certain conditions the near-stiochiometric phase of HA can be synthesized.

Figure 1. Ca/P ratio versus DC bias voltage

TheXRDspectrumofthefilmpreparedatP

=700 W and p

=70 mTorr is presented in

Fig.2, suggesting preferential crystal growth along (130) or (132) planes depending on

deposition parameters. Furthermore, the diffraction pattern suggests presence of mono-

clinic crystalline tetracalcium phosphate Ca

(TTCP) structure. It is remarkable that

the diffraction pattern is strongly affected by variation of the substrate bias and working

pressure. An amorphous phase is observed to be characteristic to weakly biased films. A

close examination of the broadening nature of the diffraction peaks reveals that the films

prepared at higher negative bias feature clear nanocrystal structures (see, for example,

(132) plane), and the (132) direction becomes a preferential crystal growth plane at sub-

strate biases exceeding 150 V. It is also worth noting that a high crystallinity of the coat-

ing can be achieved at lower working pressures and moderate substrate biases.

50 100 150 200

Ca/P Ratio

Bias Votage (V)

F3.42.3

Cross-sectional structure of the graded CaP-based film on Ti-6Al-4V has been analyzed

using FE SEM. The total thickness of the CaP-based interlayer appears to be approxi-

mately 200 nm, and it contains three major areas namely, adjacent to the implant alloy,

bulk area, and outer surface area (the latter to be in contact with bone tissues). Further-

more, cross-sectional SEM imaging (not shown here) suggests about formation of a func-

tionally-graded titanium-embedded CaP-based graded coating on Ti-6Al-4V alloy.

Figure 2. Typical XRD spectra from Ca-P-Ti thin films at different DC substrate bias.

Nano-scaled surface morphology of graded CaP-based crystalline interlayers was ana-

lyzed with the Atomic Force Microscopy. Fig.3 demonstrates remarkable changes in sur-

face morphology with variation of the substrate bias. Indeed, V

affects homogeneity of

distribution and architecture (e.g. sharpness) of the elements of morphology as well as

inter-element spacing and surface areas in the "humps" and "dips". We note that homo-

geneous distribution of very much similar by shape and size nano-sized morphology ele-

ments over the micro-scaled surface area is ideal for promoting biomolecule/protein at-

tachment and growth. Furthermore, sharp "humps" and smooth "dips" are especially de-

sirable for this purpose, which has recently been confirmed by cell culturing experiments

on ordered arrays of sharp pillars. The inspiring conclusion that can be drawn from the

AFM analysis is that all the above quality factors of the surface morphology of the outer

functional layer are met for the films prepared at V

= -100 V (Fig.3a). The latter sug-

gests direct link between the origin of nano-crystalline structure reflected in Fig.2, and

self-organization of the outer functional layer, which can be inferred as a possible reason

for preferred nano-crystalline self-assembly of natural apatites in various bone embodi-

ments. The AFM data thus confirm a challenging opportunity of controlling the nano-

scale surface morphology of the outer layer of the graded structure by varying the process

parameters.

Meanwhile, the entire coating strongly adheres to the Ti-6Al-4V alloy, which has con-

vincingly been confirmed by the interfacial bonding strength measurement with the Mi-

20 30 40 50 60

(260)

(303)

(143)

(141)

substrate

(132)

(130)

25 V

100 V

200 V

Intensity (a.u.)

2 θ (degrees)

F3.42.4

croscratch test. The critical load that the coating can withstand ranges from 2.0 N to 5.7

N. It is notable that the best interfacial strength is achieved for moderately-biased and

higher pressure-grown coatings with well-resolved nano-features. We remark that higher

gas pressures favor enhanced sputtering of the titanium target, thus eventually improved

doping of the bottom layer by titanium/titania in the atomic/molecular or nanocluster

states.

(a) (b)

Figure 3. AFM images of the outer nano-sized layer of Ca-P-Ti coating deposited at

–100 V (a) and –50 V (b) DC bias.

It is notable that preliminary in-vitro cell culturing tests on the graded CaP-based inter-

layer suggest promising biocompatibility of the coating. The relevant imaging reveals

well-preserved cyto-sceletal structure of COS7 monkey kidney cell series after up to 48

hours after cell culturing. The work is continuing into in-vivo assessments of these bioac-

tive HA films.

CONCLUSION

In summary, the plasma-assisted concurrent Rf sputtering in low-temperature, low-

pressure reactive environment offers a great deal of control of the major coating quality

parameters, including functional grading, stoichiometry, phase purity, chemical composi-

tion, crystallinity, crystal size, interfacial bonding strength, as well as of tailoring the

nano-scale features of the surface morphology of the outer thin (few nm) layer with spe-

cific bio-active functionality. Another unique features of this technique include, but are

not limited to, low-temperature, no need for post-treatment, easy deposition process con-

trol, etc. From practical considerations, the technique proposed is indeed promising to

mimic a number of the basic parameters of natural bone minerals via tailoring certain fea-

tures at nano-scales, such as nano-crystalline structure and surface morphology. Success-

ful cell culturing tests have demonstrated excellent in-vitro biocompatibility, which

sounds favorably in view of future in-vivo studies. Finally, the results obtained have fur-

ther supported the usefulness of nano-scale approach for assembly of functionally graded

F3.42.5

coatings and give us a hope that such an approach will soon be widely adopted for a

wider class of nano- and bio-materials.

ACKNOWLEDGMENTS

This work was supported in part by the Agency for Science, Technology, and Research of

Singapore (Project No. 012 101 00247). The authors are grateful to J. H. Lu for his assis-

tance in in-vitro analyses of the films. Fruitful discussions with N. Voelcker, S. Kumar, I.

Brown, and C. H. Diong are also gratefully acknowledged.

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