Surface & Coatings Technology 205 (2010) 2495–2502

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Surface & Coatings Technology

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Diamond like carbon films as a protective surface on PMMA for biomedical applications

Rishi Sharma a,⁎, Alok Kr. Pandey a, Neelima Sharma b, D. Sasmal b, P.K. Barhai a

a Plasma Thin Film Laboratory, Department of Applied Physics, Birla Institute of Technology, Mesra, Ranchi 835215, Indiab Department of Pharmaceutical Sciences, Birla Institute of Technology, Mesra, Ranchi 835215, India

⁎ Corresponding author. Tel.: +91 651 2275522; fax:E-mail address: rishisharmabit@hotmail.com (R. Sha

0257-8972/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.surfcoat.2010.09.054

a b s t r a c t

a r t i c l e i n f o

Article history:Received 16 April 2010Accepted in revised form 27 September 2010Available online 1 October 2010

Keywords:DLCPMMABiocompatibility

Diamond like carbon films are deposited on PMMA using the Radio Frequency Plasma Enhanced ChemicalVapour Deposition technique with the variation of RF power at a constant pressure of 5×10−2 mbar.Acetylene diluted with argon is used as a precursor for the deposition of DLC films. Deposited films arecharacterized using Raman, FTIR, optical contact angle technique and AFM. Optical contact angle is measuredwith water and blood of mice. It has been found that DLC coated PMMA is more hydrophobic compared to anuncoated surface. Samples are soaked with simulated body fluid for 30 days for the assessment ofbiocompatibility. Surface morphology of the samples before and after interaction with simulated body fluidhas been studied by using AFM. The haemolysis test has also been carried out for the haemocompatibilityassessment.

+91 651 2275401.rma).

l rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Biomaterials are becoming increasingly important in the develop-ment of biomedical devices and implants. This area of research hasbeen expanding rapidly over the last 50 years. A large number ofartificial medical devices such as hip joints, heart valves, dental roots,intraocular lenses. etc. are implanted into the human body every day.However, very few surfaces are truly biocompatible. It is the surface ofa biomaterial, which directly comes in contact with living tissues andbody fluid when the biomaterial is placed inside the body. Therefore,the initial response of the living tissues with the biomaterial dependson its surface properties. Polymethylmethacrylate (PMMA) has beenthe first choice polymer for biomedical industries because it has manydesirable mechanical, optical and electrical properties. PMMA hasbeen widely used in the fabrication of artificial dentures, bones andophthalmic intraocular lenses [1–5]. However, due to its direct contactwith blood and tissues inside the body, PMMAdegradeswith time dueto its poor surface properties. The long term failure of the PMMAprostheses is now believed to be directly or indirectly due to its poorsurface properties like low contact angle, lower hardness, poor wearresistance and vulnerability. The degradation of biomaterial can resultin reactions of the sensitive tissues with the prostheses, infection,thinning and cellular events. Therefore, surface modification is a keyprocess to improve the surface properties of biomaterials as well thedevices for biomedical applications [6]. A hard and wear resistant

coating which is also biocompatible, can reduce these problems anddramatically extend prostheses life [7–13]. The diamond-like carbon(DLC) film has been considered as a suitable material for the surfacemodification of PMMA because of its unique properties such as highhardness, low coefficient of friction, chemical inertness, high electricalresistance and high optical transparency [7–17]. Many experimentshave demonstrated that DLC has no toxicity towards various livingcells and no inflammatory response or loss of cell integrity [7–13].Thus, DLC coated PMMA is a better option for artificial completedentures, artificial bones, ophthalmic intraocular lenses and varioussurgical implants. The DLC coating on PMMA will reduce the fracturerisk, release of particles (hydroxyapatite) from PMMA cements anddeterioration of mechanical properties of PMMA. DLC will also help inreducing the necrosis, inflammatory reactions and cytotoxicity in thesurrounding tissues due to PMMA. The life span of PMMA prostheseswill also increase by the DLC coating. Extensive research has beencarried out on the biocompatible study of PMMA and DLC coatings,separately [2–5,12,13,18–20]. There are very few reports on thedetailed study of biocompatibility of DLC coated PMMA [8,9].

In the present work, DLC films are deposited on PMMA substratesusing the Radio Frequency Plasma Enhanced Chemical VapourDeposition (RF-PECVD) technique and biocompatibility of DLC coatedPMMA is assessed. A new approach by using AFM is used for the studyof biocompability. AFM micrographs (1 μm×1 μm) are used for theanalysis before and after their interaction with simulated body fluid(SBF). Formation of different structures in DLC films is explained usingsubplantation model and correlated with the degradation of surfacesafter treating with SBF. The optical contact angle (OCA) of the sampleswith water and blood of mice are also studied. The haemocompat-ibility of DLC coated PMMA is assessed by the haemolysis test.

Table 1Deposition parameters of DLC films.

Working pressure: 5×10−2

Deposition time: 60 min

RF power in watt Sample name

10 DLC 120 DLC 230 DLC 340 DLC 450 DLC 560 DLC 670 DLC 780 DLC 890 DLC 9

Table 2Composition of simulated body fluid (500 ml).

Name of chemical Quantity

NaCl 6.55 gmNaHCO3 2.27 gmKCl 0.373 gmNa2HPO4.2H2O 0.178 gmMgCl2.6H2O 0.305 gm37 wt.% HCl 5 mlCaCl2 0.278 gmNaSO4 0.071 gmTris buffer 6.055 gmDeionised water 500 ml

1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 320096.0

96.5

97.0

97.5

98.0

98.5

99.0

99.5

100.0

Tra

nsm

itta

nce

(%

)

DLC 1DLC 2DLC 3DLC 4DLC 5DLC 6DLC 7DLC 8DLC 9

2700 2750 2800 2850 2900 2950 3000 3050 310097.75

98.00

98.25

98.50

98.75

99.00

99.25

99.50

99.75

100.00

DLC 9

DLC 8

DLC 7

DLC 6

DLC 5

DLC 4

DLC 3DLC 2

Tra

nsm

itta

nce

(%

)DLC 1DLC 2DLC 3DLC 4DLC 5DLC 6DLC 7DLC 8DLC 9

DLC 1

99.75

100.00

DLC 9

DLC 7

Wave Number (cm-1)

Wave Number (cm-1)

(a)

(b)

(c)

2496 R. Sharma et al. / Surface & Coatings Technology 205 (2010) 2495–2502

2. Experimental setup

Deposition of the DLC films is carried out in a RF-PECVD systemusing a gas mixture of Ar/C2H2 on PMMA. For deposition of highquality, smooth and uniform DLC films with reproducible properties,the deposition must be performed in a high purity environment. Thus,experiments are carried out with 99.99% pure argon and acetylene.Prior to deposition, the chamber is evacuated to ~5×10−6 mbar usingthe combination of turbo and rotary pumps. The operating pressure ofthe system is maintained at ~5×10−2 mbar by introducing processgases. A RF generator of 600 W maximum power and 13.56 MHzfrequency is used for the generation of plasma. RF power is deliveredthrough matching network to a circular power electrode of diameter110 mm. Before deposition, the PMMA substrate (10 mm×10 mm) iscleaned with deionised water in ultrasonic vibrator for 30 minutes.Substrates are also cleaned with Ar+ ion sputtering before deposition.

1000 1200 1400 1600 1800 2000 22000

1000

2000

3000

4000

5000

6000

7000

Inte

nsi

ty (

a.u

.)

Wave Number (cm-1)

DLC 1DLC 2DLC 3DLC 4DLC 5DLC 6DLC 7DLC 8DLC 9

DLC 9

DLC 8

DLC 7

DLC 6

DLC 5

DLC 4

DLC 3

DLC 2

DLC 1

Fig. 1. Raman spectra of DLC films.

1200 1300 1400 1500 1600 1700 1800 1900 200098.25

98.50

98.75

99.00

99.25

99.50

DLC 8

DLC 6

DLC 5

DLC 4

DLC 3DLC 2

16401450

1245

1515

1730

Tra

nsm

itta

nce

(%

)

DLC 1DLC 2DLC 3DLC 4DLC 5DLC 6DLC 7DLC 8DLC 9

DLC 1

Wave Number (cm-1)

Fig. 2. a–c. FTIR spectra of DLC films.

Deposition parameters of different DLC films are shown in Table 1.Deposition is carried out with the variation of RF power (10 to 90 W)at a constant pressure of 5×10−2 mbar for 60 min deposition time.

0 20 40 60 80 100

150

200

250

300

350

400

450

Th

ickn

ess

(nm

)

RF-Power (Watt)

Fig. 3. Plot between thickness and RF power.

4.0

2497R. Sharma et al. / Surface & Coatings Technology 205 (2010) 2495–2502

Raman spectroscopy (LABRAM, Horiba-Jobin Yvon, softwareLabSpec 5.) of deposited films is carried out with the laser ofwavelength 618 nm. All observations are recorded at 100× magnifi-cation and 10 s exposure time. FTIR spectroscopic studies are carriedout in Attenuated Total Reflection (ATR) mode with SHIMADZU IRprestige-21 (Japan) for the analysis of different bonds present in theDLC films. The thickness of the films is determined by usingellipsometry (Nano-View Inc. Korea). The contact angle is measuredby a contact angle setup (Data physics, Germany). Analysis of thesurface morphology of the samples is done by AFM (NT-MDT Solverpro 47 AFM, USA) in lateral force (LF) mode. In this mode, lateraldeflections (twisting) of the cantilever that arise from forces on thecantilever parallel to the plane of the sample surface is measured. LFmode is useful for imaging variations in the surface friction that canarise from inhomogeneity in the surface material and also forobtaining edge-enhanced images of any surface. This force ismeasured in terms of the current. Images produced in LF mode haveX,Y axis in the dimension of length (μm or nm). Z axis is in terms ofcurrent (nA or pA) which is not shown in figures.

A pull off adhesion test is carried out using a universal testingmachine (Instron-USA, Model 3366, Series IV). The uncoated side ofPMMA samples (10 mm×10 mm) is glued to a stainless steel (SS) rodof 20 mm dia and the coated side of the samples is glued to a SS rod ofdia 4 mm. In all the cases, the glue used is a standard two component

0 20 40 60 80 10030

40

50

60

70

80

90

100

Co

nta

ct A

ng

le (˚)

RF-Power (Watt)

with bloodwith water

Fig. 4. Plot between optical contact angle and RF power.

epoxy (Araldite® from Huntsman Advanced Materials (India) Pvt.Ltd.). The weakest bond is in between the coated side and SS rod of4 mm dia (as area is small). When load is applied, at all the times thebond between the coated surface and small SS rod breaks [21–23].

An in vitro biocompatibility test of DLC coated PMMA is carried outunder sterile conditions. Samples are soaked in the SBF solution insterile polystyrene bottles (using laminar flow) and kept for 30 daysat room temperature. Samples are taken out of the SBF solution after30 days and cleaned by keeping under running deionised water for2 min to remove the SBF solution which remains on the surface of thesample in liquid form. Prior and after the soaking in SBF, samples areanalyzed by AFM to observe the changes in surface morphology. Theratio of the solution volume to the surface area of the specimen is keptat 0.1 ml/mm2 [24]. The composition of the simulated body fluid isgiven in Table 2 [13]. All the constituents (NaCl, NaHCO3, KCl,K2HPO4.3H2O, MgCl2.6H2O, CaCl2 and Na2SO4) are dissolved into400 ml milli Q water i.e. deionised water at 37 °C by continuousmagnetic stirring and buffered at pH 7.4 with tris (hydroxymethyl)amminomethane ((CH2OH)3CNH3) and hydrochloric acid (HCl).Finally, the volume is made up to 500 ml. The solution is filteredusing 0.22 μm vacuum filter and stored at 4 °C until use.

For the haemolysis test, DLC coated PMMA is taken in a standardtest tube containing N-saline (0.9% NaCl) and incubated at 37 °C for30 min for providing temperature equilibrium. 0.2 ml of anticoagu-lated and diluted blood (N-saline : blood; 8 : 10) is then added to thetest tube, mixed gently and incubated for 60 min. The optical density(OD) of the incubated solution is measured in an UV spectrometer at545 nm wavelength. Sodium carbonate (1%) is used as a positivecontrol and N-saline is used as negative control. The haemolysispercentage is calculated using the equation [25]

Haemolysisð%Þ ¼ODðPMMASamplesÞ�ODðnegativeÞODðpositiveÞ�ODðnegativeÞ

× 100:

The accepted norm is that if the haemolysis percentage is less than10 the test material is taken as haemocompatible and if it is less than 5the material is highly haemocompatible.

3. Results and discussion

TheRaman spectra ofDLCfilms are shown in Fig. 1. TheRaman spectraof DLC films show one broad peak centered approximately at 1520 cm−1

Uncoat

ed P

MMA

DLC 1

DLC 2

DLC 3

DLC 4

DLC 5

DLC 6

DLC 7

DLC 8

DLC 91.0

1.5

2.0

2.5

3.0

3.5

Ad

hes

ion

Str

eng

th (

MP

a)

Fig. 5. Adhesion strength of DLC films.

2498 R. Sharma et al. / Surface & Coatings Technology 205 (2010) 2495–2502

(corresponding to the G line associated with the optically allowed E2gzone center mode of crystalline graphite) and a weak peak centered at1390 cm−1 (corresponding to theD line associatedwithdisorder-allowedzone-edgemodes of graphite) [17,26]. This confirms the formation of DLC

Fig. 6. AFM images of samples

films during deposition. The FTIR spectra of DLC films are shown inFig. 2a to c. The peaks related to C–H and C–C are present in the rangeof 1200 cm−1 to 3200 cm−1 as shown in Fig. 2a. Peaks near 2900 cm−1

which correspond to the C–H stretchingmode are shown in Fig. 2b. Peaks

before interaction with SBF.

Fig. 6 (continued).

2499R. Sharma et al. / Surface & Coatings Technology 205 (2010) 2495–2502

below2000 cm−1 are shown in Fig. 2c. Peaks near 1450 cm−1 correspondto sp3 bonded CH2, peaks at 1245 cm−1 and peaks in between 1515 cm−1

and1640 cm−1 arebecauseof sp2/sp3bondedC–C,peaksnear 1730 cm−1

are because of the ester carbonyl group (NC=O) present in PMMA[17,27–29]. As the FTIR spectra are recorded in the ambient temperatureand pressure, therefore, a prominent peak of CO2 at 2300 cm−1 is presentin all the spectra. FTIR spectra also confirm the formation of DLC filmsduring deposition.

The plot between RF power and thickness is shown in Fig. 3. Theaverage thickness is found to be near 400 nm. The thickness of DLC 1,deposited at 10 W RF power has been found to be very low (Fig. 3).This may be because of low power. The plot between optical contactangle (with water and blood of Swiss albino mice) and RF power isshown in Fig. 4. The plot reveals that the contact angle for DLC coatedPMMAwithwater lies between 75° and 91° which is higher comparedto uncoated PMMA (45°). The contact angle of DLC coated PMMAwithblood of mice is more interesting and practically more useable forbiomaterials. Uncoated PMMA has a low contact angle (38.9°)compared to DLC coated PMMA. The contact angle of DLC coatedPMMA smeared with blood of mice varies from 49.7° to 70.4° (DLC 9and DLC 4, respectively). There is no more variation of the contactangle (withwater) with the variation of RF power. Significant changeshave been observed in the case of blood. It shows low contact angles

53.1°, 54.7°, 55.7° and 49.7°, corresponding to 30 W, 60 W, 70 W and90 W RF power, respectively. The rest of the samples have a goodcontact angle with blood. It is difficult to correlate the variation of thecontact angle with RF power but it can be concluded that DLC coatedPMMA is more hydrophobic compared to uncoated PMMA. Fig. 5shows the adhesion strength of different DLC samples. UncoatedPMMA shows an adhesion strength of 3.35 MPa. All the coatedsamples show adhesion strength in between 2.96 and 3.25 MPa,which is almost similar to the strength of the uncoated sample. Hence,DLC films are significantly adhered to the PMMA surfaces.

The AFM images (Figs. 6 and 7) of the samples before and afterinteraction with SBF depict the biocompatible nature of DLC coatedPMMA. SBF contains different compounds (NaCl, NaHCO3, KCl,K2HPO4.3H2O, MgCl2.6H2O, CaCl2 and Na2SO4), therefore, as thesurface comes in contact with SBF, it starts to corrode. The effect ofthis corrosion is clearly observed in uncoated PMMA (Figs. 6j and 7j).Different cracks formed on the PMMA surface indicate the highcorrosion of the surface. It is clear from AFM images (Figs. 6 and 7)that the DLC coated surface shows good protection against the SBFsolution compared to uncoated PMMA. However, few DLC films(Fig. 6a and g–i) have not maintained their structure after interactionwith SBF (Figs. 7a and g–i) while others (Figs. 6b–f) have maintainedtheir surface structures (Figs. 7b–f). AFM images of DLC coated PMMA

2500 R. Sharma et al. / Surface & Coatings Technology 205 (2010) 2495–2502

(before soaking with SBF, Fig. 6a–i) show the formation of two typesof structures: (1) The clusters with sharp grain boundaries (Figs. 6b–f)and (2) the clusters having no clear grain boundaries (Fig. 6a and g–i).

The formation of two types of structures can be understood fromthe subplantation model [30–33]. According to this model, ions

Fig.7. AFM images of samples

having energy higher than the penetration threshold (in the presentstudy, films deposited at 70 W to 90 W RF power) penetrate thesurface and enter the subsurface interstitial sites. This increases thelocal density. Penetration occurs by direct entry or knock-ondisplacement of the surface atoms. High energy ions penetrate deeper

after interaction with SBF.

Fig. 7 (continued).

2501R. Sharma et al. / Surface & Coatings Technology 205 (2010) 2495–2502

into the film. A small fraction of this energy is used to penetrate thesurface, and the rest is dissipated in atom displacements. The ion mustdissipate the rest of this energy ultimately as phonons (heat). Thisprocess allows the excess density to relax to zero and causes a loss of sp3

bonding at a higher energy. Ions of low energy (in the present study, thefilm deposits at 10W RF power) will not have enough energy topenetrate the surface, so theywill just stick to the surface and remain inits lower energy state i.e. sp2 state. At an optimum ion energy (in thepresent study, films deposited at 20–60W RF power), the surfacepenetration is maximum but the relaxation is minimum i.e. high sp3

bonded C–C content formed during the deposition. In the present study,deposition is carried out with the variation of RF power. Otherparameters like pressure and gas composition are kept constant(Table 1). Hence, variation of the surface structure is because ofvariation of RF power. According to the subplantation model, it can bepredicted that films at low and high RF power form structures whichmayhavemore sp2 content compared to others. It is also clear fromAFMimages that samples having sharp grain boundaries maintain theirstructures (Fig. 7b–f) compared to other samples (Fig. 7a and g–i) afterinteraction with SBF. Hence, films with sharp grain boundaries mayhave high concentration of sp3 bonds.

It can be argued that less sp3 content is present at the grain boundariesbecause the cluster part of thefilms shows good resistance against SBF (asshown by AFM images Figs. 6 and 7) and maintains the structure afterinteraction with SBF. Therefore, it may be that SBF tries to interact with

sharp grain boundaries. Only sharp grain boundaries are found to havesmall changes in their structures (Figs. 6b–f and 7b–f). Contrary to this,DLC films having no sharp grain boundaries (Fig. 6a and g–i) allow SBF tointeract with the surface and may cause more changes in the surfacestructure (Fig. 7a and g–i). Hence, it may be inferred that formation ofsurface structures with cluster and sharp grain boundaries in the films(Fig. 6b–f) due to the optimum ion energy (RF power 20–60W) isresponsible for a good protective surface on PMMA. This supports theexplanation on the basis of the subplantation model.

The results of the haemolysis test (Fig. 8) show that uncoatedPMMA is found to be incompatible (haemolysis %NN10) in blood,whereas all DLC coated PMMA samples are found to be highlyhaemocompatible. The pattern of the haemolysis test is similar to theSBF test, as the samples deposited at very low and high RF power havehigh haemolysis % compared to the samples deposited in mid powerrange.

4. Conclusion

The present study shows that DLC is a biocompatible material notonly at the bulk level but also at the nano level. Raman and FTIRspectra confirm the deposition of DLC films on PMMA. It is found thatDLC coated PMMA has the high contact angle (water and blood ofmice). Hence, DLC coatings increase the hydrophobicity of the surface.DLC coated PMMA samples show good resistance against SBF

DLC 1

DLC 2

DLC 3

DLC 4

DLC 5

DLC 6

DLC 7

DLC 8

DLC 9

Uncoat

ed P

MMA

Positive

Contro

l

Negat

ive C

ontrol

-10123456

60

70

80

90

100

110

Hae

mo

lysi

s %

Fig. 8. Percentage haemolysis of DLC coated PMMA.

2502 R. Sharma et al. / Surface & Coatings Technology 205 (2010) 2495–2502

compared to uncoated PMMA. Significant changes are found inuncoated PMMA after soaking with SBF for 30 days. Formation ofdifferent surface structures can be understood with the help of thesubplantation model, according to which sharp boundaries are foundto occur at ion energy 20–60 W. It can be concluded that formation ofthe cluster with sharp grain boundaries is responsible for the goodprotection against SBF. The haemolysis test shows that uncoatedPMMA is not haemocompatible and all DLC films are highlyhaemocompatible. Results show that percentage haemolysis of DLCcoated PMMA is similar to standard saline.

Acknowledgements

The authors gratefully acknowledge the support from DST, NewDelhi and BMBF, Germany for the grant of a project under Indo-GermanBilateral Cooperation in Science andTechnology. Authors are thankful toProf. Volker Buck, Department of Physics, University of Duisburg-Essen,

Germany for providing Raman characterization facility. Authors arethankful to Dr. Gautam Sarkhel, Department of Polymer Engineering,BIT, Mesra, Ranchi for his guidance in pull off adhesion test.

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