Surface modification of ultrahigh molecular weight polyethylene by the poly(ethylene glycol)-grafted...

Surface modification of ultrahigh molecular weight polyethyleneby the poly(ethylene glycol)-grafted method and its effect on theadsorption of proteins and the adhesion of blood platelets

Bing Xia,1 Meiju Xie,2 Bangcheng Yang1

1Engineering Research Center for Biomaterials, Sichuan University, Chengdu, Sichuan 610064, China2Analytical and Testing Center, Sichuan University, Chengdu, Sichuan 610064, China

Received 8 February 2012; revised 25 May 2012; accepted 25 May 2012

Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/jbm.a.34301

Abstract: With the help of a silane coupling agent, poly(ethyl-

ene glycol) (PEG), a well-biocompatable agent, was grafted

onto the surface of ultrahigh molecular weight polyethylene

(UHMWPE) by ultraviolet initiation. Fourier transform infrared

spectroscopy and X-ray photoelectron spectroscopy analysis

proved the success of PEG grafting. Water contact angle

measurement showed that the modified UHMWPE was obvi-

ously improved in surface hydrophilicity and thermogravi-

metric analysis result showed that its thermostability did not

decline even it was pretreated by strong acids. Then, the pro-

tein adsorption of the modified UHMWPE was investigated

using three model proteins including bovine serum albumin,

lysozyme, and fibrinogen. Rabbit blood was used to study

the platelet adhesion on the surface of modified UHMWPE.

The results indicated that the quantity of protein adsorption

on the modified UHMWPE grafted PEG reduced apparently

for all the model proteins while there was some specific dif-

ferences or exceptions among them. It was ascribed to the

changed surface chemical composition, surface hydrophilicity

and surface topography after modification. The adhesive abil-

ity of blood platelets on the modified surface of UHMWPE

decreased after PEG grafting. Owing to the improved resist-

ance to fibrinogen adsorption and platelet adhesion, the

surface modification might endow the UHWMPE surface

better anticoagulation ability according to clotting mecha-

nism. VC 2012 Wiley Periodicals, Inc. J Biomed Mater Res Part A:

00A:000–000, 2012.

Key Words: PEG, UHMWPE, surface grafting modification,

protein adsorption, blood platelet adhesion

How to cite this article: Xia B, Xie M, Yang B. 2012. Surface modification of ultrahigh molecular weight polyethylene by thepoly(ethylene glycol)-grafted method and its effect on the adsorption of proteins and the adhesion of blood platelets. J BiomedMater Res Part A 2012:00A:000–000.

INTRODUCTION

Ultrahigh molecular weight polyethylene (UHMWPE) is aunique polymer with outstanding properties. The most notableincludes its chemical inertness, low friction coefficient, high ab-rasion resistance, and high impact strength.1 It has beentreated as one of the most versatile polymers, which can beused in different fields such as engineering, microelectronics,medicine, and biology. As an important biomaterial, UHMWPEis mainly used as the weight-bearing surface in total jointreplacements, vascular biomaterial, and bone scaffold.2,3

When UHMWPE was used as blood-contacting material, ithad also been challenged like other polymers for cardiovasculardevices by diverse and complex reactions of the blood compo-nents to the biomaterials, including plasma protein adsorptionand thrombogenesis.4,5 The blood compatibility of UHMWPEwas not satisfying owing to its inert and hydrophobic surface.6

It had been well demonstrated that blood coagulation on bio-material surfaces was related to protein adsorption thatoccurred within seconds following blood contact.7,8 The typesand amounts of adsorbed proteins were known to affect subse-quent platelet adhesion and activation, which also played amajor role in surface thrombogenesis. Especially, fibrinogenwas of particular importance due to its high concentration inblood and its high platelet affinity.5,9 And the protein adsorptionon the material surface depends on the surface chemistry, sur-face topography, surface energy (hydrophilicity/hydrophobic-ity) and charge, the mobility of the surface functional groups,and so forth.10 Therefore, surface modification often has to beperformed to improve the blood compatibility of UHMWPEbefore it is used in biomedical fields.

It is found that a few agents that are resistant to proteinadsorption and platelet adhesion can be used to coat

Correspondence to: M. Xie; e-mail: [email protected] or B. Yang; e-mail: [email protected]

Contract grant sponsor: National Natural Science Foundation of China; contract grant numbers: 31070848, 50961003, 30870615

Contract grant sponsor: National Basic Research Program of China (973 Program); contract grant number: 2012CB933603

Contract grant sponsor: Research Fund for the Doctoral Program of Higher Education of China; contract grant number: 20110181110064

Contract grant sponsor: Sichuan Youth Science Foundation of China; contract grant number: 09ZQ026-033

Contract grant sponsor: Open Research Fund of State Key Laboratory of Oral Disease, Sichuan University, China

VC 2012 WILEY PERIODICALS, INC. 1

biomaterial surfaces for improving the blood compatibility.The agents include biomolecules and synthetic polymerssuch as albumin,11 heparin,12 poly(ethylene glycol)(PEG),13,14 and other zwitterionic polymers.15 Among them,PEG has been extensively used, because it is nontoxic, hydro-philic, nonimmunogenic, and nonantigenic. PEG is a simplemolecule and has the structure as HOA(CH2CH2O)nAH,which is structural similar to water and characterized byhydroxyl groups at either end of the molecule. PEG coatingprovides a hydrophilic environment between the interface ofthe blood and materials,16 which is one of the main factorsresulting in reduced protein adsorption17 and thrombosis invessel repair.18 In addition, the steric exclusion effectresulted from the molecular conformation of PEG in aqueoussolution, where PEG exposes uncharged hydrophilic groups,shows very high surface mobility,19 and also enhances theresistance to protein adsorption.

There exist different approaches to introduce PEGgroups on different polymer surfaces, including physicaladsorption, covalent bonding such as grafting and chemicalcoupling. For the kind of substrate that is hydrophobic orabsent of reacting sites, it is usually surface activated beforeit reacts with PEG-containing molecules. For example, thepoly(dimethylsiloxane) (PDMS) surfaces were successfullyPEGylation through the adsorption of a graft copolymer,poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) fromaqueous solution. In that approach, the PDMS surface wastreated with oxygen plasma, followed by immersion intoaqueous solution containing PLL-g-PEG copolymers.20 Inanother report, PEG was grafted on the polyethylene (PE)powder by several steps by atmospheric glow dischargeirradiation, namely oxidation of the PE powder surfaces fol-lowed by the PEG adsorption and then the cross-linkingreaction of the adsorbed PEG.21 Plasma deposition was alsoused to produce tetraglyme coating, a cross-linked hydrogelcoating similar to PEG on UHMWPE.22 Apart from plasmatreatment, photochemical grafting of alkenes with specificfunctional groups is very attractive, because the reactionprovides a means for direct photopatterning the materialsurface. Through the reaction with the specific functionalgroups like amine groups, hydroxy groups, and carboxylgroups, some biomolecules and polymers for biomedical usesuch as DNA molecules23 and PEG chains24 could beattached to the substrate surface. Vinylsilane such as vinyl-trimethoxysilane (VTMS) and vinyltriethoxysilane is a kindof alkenes commonly used as coupling agent to link othercomplex molecules. The hydrolysis of the grafted silane andthe condensation of the resulted hydroxy groups can beused to prepare cross-linked PE.25 In fact, the resultedhydroxy groups also provide sites for other hydroxy-involved reactions. Vinylsilane can be grafted on the inertsurfaces of polyolefin initiated by heat, chemicals and light.Ultraviolet (UV)-initiated surface photografting, as a lightinitiated grafting method with relative low energy, is prom-ising due to the low cost of operation, the powerful effec-tiveness of functionalizing the substrate surface, the fastreaction rate, and relatively small influence on the bulkpolymer.26,27

In this study, we attempted to graft PEG on the surfaceof UHMWPE with the help of silane coupling agent. Thetotal process included the introduction of silane group[ACH2ACH2ASiA(OCH3)3] on UHMWPE by UV irradiationgrafting,27 hydrolyzation of ASiOCH3, condensation ofASiOHs and the hydroxyl ends of PEG combined with thecrosslinking of silanol groups. Then, the chemical composi-tion, surface structure, hydrophilicity, and thermal stabilityof the modified UHMWPE were characterized. The proteinadsorption and platelet adhesion on the PEG-grafted surfa-ces were also evaluated.

MATERIALS AND METHODS

MaterialsUHMWPE with an average molecular weight of 2.5 � 106 gmol�1 was supplied by Beijing Eastern Petrochemical Com-pany, Beijing, China. VTMS used as coupling agent was sup-plied by Nanjing XiangFei LiPai Organic Silicon Material,Nanjing, China. PEG (average molecular weight Mn ¼ 400)(PEG400) was supplied by Chengdu Kelong Chemical Rea-gent Works, Chengdu, China. The photoinitiator agent of di-phenyl ketone (BP) was supplied by Tianjin Bodi Chemical,which was purified by recrystallization in acetone beforeexperiment.

Molding and pretreating of UHMWPEUHMWPE powder dried in vacuum was first compressionmolded at approximately 200�C and 10 MPa for 10 min, fol-lowed by another 5 min at room temperature and 10 MPa,to get 1-mm thick sheets. Then, the sheets were cut intosmall ones with the size of 15 � 9 mm2. For activating thesurface of UHMWPE to make it easier to be grafted to, somesmall sheets were treated by the mix acid composed with65% HNO3 and 98% H2SO4 (v/v ¼ 1:3) at 70�C for 12 h inthe oven and washed by running water to remove the resid-ual acid. Consequently, there were two kinds of UHMWPEsamples, including the untreated UHMWPE and the acid-pretreated UHMWPE. Before UV-induced grafting, the twokinds of UHMWPE samples were washed with acetone toremove any possible impurities on the surface, then dried ina vacuum oven at 60�C to a constant weight.

Surface modification of UHMWPESurface modification was carried out on the preparedUHMWPE sheets after molding and pretreating. The wholeprocess of surface modification is schematically shown inFigure 1. First, the surface treating solution was preparedby dissolving 5%(w/v) initiator agent BP in ethanol at roomtemperature. Then, the UHMWPE samples were put into thedifferent cuvettes made of quartz with a cover and filled fullwith the surface treating solution. Followed, the cuvetteswere put into UV cross-linking device (CL1000, MYCRO) forUV-induced reaction at a photodensities of 900 mJ cm�2

with an irradiation time of 30 min. By above step, a largeamount of dormant semipinacol groups were introduced onthe substrate surface after BP abstracted hydrogen of theUHMWPE chains. Second, the surface treating solution inthe cuvettes was replaced by the mixture with 10% VTMS

2 XIA, XIE, AND YANG SURFACE MODIFICATION OF UHMWPE BY THE PEG-GRAFTED METHOD AND ITS EFFECT

and 90% ethanol (v/v), and the reaction continued in thesame conditions. By this step, the bonds between the sub-strate and the dormant end groups were re-activated,resulting in the surface radicals to react with VTMS. So far,silane groups were induced on the surfaces of UHMWPEsheets. The as-received samples were named PE/VTMS andAcid-PE/VTMS from the untreated UHMWPE and the acid-pretreated UHMWPE, respectively. Further experiment wasperformed to graft PEG400 to the surface of UHMWPEsheets. The PE/VTMS and Acid-PE/VTMS samples wereimmersed into the mixture of 10% PEG400 and 90% waterand grafted with PEG400 on the surfaces in the water-bathat 100�C for 12 h. In this course, the silane groups hydro-lyze and produce the SiAOH in boiling water,25 then, theesterification reaction between PEG and SiAOH occurred,19

combined with the crosslinking of the SiAOH groups. PEGchains were so chemically bonded on the UHMWPE surface,which would not soluble in aqueous solution while endowthe UHMWPE surface hydophilicity, attaining a long-term bi-ological function. The as-received samples were named PEg-PEG and Acid-PEgPEG from PE/VTMS and Acid-PE/VTMS,respectively.

Fourier transform infrared spectroscopy analysisFourier transform infrared spectroscopy (FTIR) was used tomonitor the changes of chemical structure in the surface ofUHMWPE after silane-grafting reaction, including the PE/VTMS and Acid-PE/VTMS, with UHMWPE as control. All ofthe UHMWPE sheets included the modified and the unmodi-fied were washed with an excess volume of acetoneto remove unreacted silane and residual impurities priorto FTIR measurement. The IR spectra were recorded usinga NEXUS 670 FTIR spectrometer (Thermo Nicolet Corpora-tion) in the range of 400–4000 cm�1 with a resolution of1 cm�1.

X-ray photoelectron spectroscopy analysisThe PEgPEG and Acid-PEgPEG samples were subjected toX-ray photoelectron spectroscopy (XPS) analysis to confirmthat PEG was successfully grafted on the surfaces. XPS anal-

yses were performed at a take-off angle of 90� (relative tothe surface plane) using a XSAM800 X-ray photoelectronspectrometer (KRATOS, UK). Spectra were acquired at achamber pressure of 10�9 mbar using a nonmonochromaticMg Ka source operating at 150 W. Spectra were referencedto the aliphatic hydrocarbon C1s signal at 284.8 eV and fit-ted with the XPSPEAK 4.1 software using the sum of a 20%Gaussian and 80% Lorentzian function.

Scanning electron microscopy and X-ray diffractionanalysisScanning electron microscopy (SEM; Hitach, S-4800, Japan)was used to examine the surface topography of theUHMWPE surfaces before and after surface modification toidentify whether the treatment caused morphologicalchanges. X-ray diffraction (XRD, DX1000, China) was appliedto get the crystallinity of the sample surface with the helpof MDI Jade5.0 software.

Thermogravimetric analysisA Thermal Analyzer (Netzsch Sat 449C, Germany) was usedto investigate the thermal stability of the PEG-graftedUHMWPE. The samples scraped from the sheet surfaceswere 5–10 mg, and all the measurements were carried outunder nitrogen atmosphere. Dynamic runs were carried outfrom 35 to 500�C at a heating rate of 10�C min�1.

Contact angle measurementsContact angle measurements of unmodified and modifiedsheets were undertaken by the sessile drop technique usinga contact angle goniometer (Kruss DSA200, German) underambient laboratory conditions. In brief, a 3 lL drop of dis-tilled water was added to the sheet surface, and the watercontact angle was measured within 30 s of contacting. Thecontact angle for at least three drops on three different sub-strates was measured for each type of sample.

Protein adsorption measurementsFor this study, bovine serum albumin (BSA; 66.43 kDa),lysozyme (14.6 kDa), and fibrinogen (340 kDa) were chosenas model proteins. Each of them was dissolved in phos-phate-buffered saline (PBS, pH 7.4) at 0.64 mg mL�1. Eachsheet of UHMWPE was incubated in a 2-mL centrifugal tubewith 1.6 mL of the above-prepared protein dilution at roomtemperature28 for 3 h. Then, the sample was washed threetimes in 15 mL of PBS to remove the protein that remainedon the surface of the material but did not adsorb. To desorbproteins from the sample surface, 1.6 mL of 2% (w/v) so-dium dodecyl sulfate was added to the 2-mL centrifugaltube with the sample and mixed vigorously for 30 s. It wasincubated at 37 �C for 1 h to desorb fully. Finally, the ab-sorbance of the protein solution with Micro BCA proteinassay reagent (Thermo Scientific) was measured at 570 nmby a microplate reader (Thermo Multiskan MK3, ThermoFisher Scientific). The protein concentration could be calcu-lated according to the functional relation between absorb-ance and concentration of standard samples offered byThermo Scientific.

FIGURE 1. The schematic reactions of the process of PEG graft on

the surface of UHMWPE.

JOURNAL OF BIOMEDICAL MATERIALS RESEARCH A | MONTH 2012 VOL 00A, ISSUE 00 3

ORIGINAL ARTICLE

Blood platelet adhesion measurementsTen milliliters of anticoagulant rabbit fresh blood was addedinto centrifuge tube and subsequently centrifuged at 800rpm for 10 min to obtain the top layer of platelet-richplasma (PRP). At room temperature (25�C), each of sheetsamples was put into a 2-mL centrifuge tube. PRP (1.6 mL)was added to the tube, maintaining contact for 30 min.Then, the samples were carefully rinsed in PBS (PH 7.4) toremove nonfirmly adsorbed platelet. After mixed with 2.5wt % glutaraldehyde solution for one night, the sampleswere washed with triple-distilled water several times. Theplatelet adsorbed on the surface was dehydrated with 30,40, 50, 60, 70, 80, 90, and 100% (v/v) ethanol/water solu-tion for 15 min of each in sequence. After drying in freezedrier, the resultant samples were observed with SEM in dif-ferent magnifications.

Platelet distribution was showed in SEM pictures magni-fied 1000 times. Three replicates were taken for each sam-ple, and six surface areas of each replicate were examined.The platelet density of every area was calculated, and theaverage value of 18 areas for each sample was regarded asits platelet density.

Statistical analysisA statistic method named t-test was used to handle the datain the contact angle measurements, protein adsorptionmeasurements, and blood platelet adhesion measurements.The above experiments were carried out in at least tripli-cate. All of the data results were expressed as means 6standard definition (SD). The statistical significance wasassessed by t-test, and the level of significance was chosenas p < 0.05. Sample marked with an asterisk (*) had signifi-cant difference with other samples within a figure. In thecase of three groups involved in statistic analysis, we choosetwo among the three groups to calculate the ANOVA by t-test in turn. Through three times of calculation, we couldget the results of ANOVA analysis of three groups.

RESULTS AND DISCUSSION

Grafting of PEG to UHMWPEA sequential photoinduced grafting process to introduce thesilane groups [ACH2ACH2ASiA (OCH3)3] onto the surfaceof the substrates including the untreated and the acid-treated UHMWPE was the first step to graft PEG. In the UVirradiating experiments with different energy densities, thereaction between silane and sample was found difficult tocarry out below the UV energy density of 900 mJ cm�2

through FTIR analysis. Thus, the photoenergy density of900 mJ cm�2 was adopted in the following experiments.Figure 2 indicates that the main feature absorption peaks ofCH, CH2, and CAC of UHMWPE29 were observed in all theIR spectra of UHMWPE, PE/VTMS, and Acid-PE/VTMS , buttwo new peaks around 1080 and 800 cm�1 generated inthe spectra of PE/VTMS and Acid-PE/VTMS, which werecorresponded to the absorption band of silane group.14,30

The results showed that the silane groups had been suc-cessfully introduced onto the surface of the samples. Com-paratively, more silane groups were grafted to the acid-pre-

treated sample than the unpretreated one because of thesurface activation of the acid pretreatment.

The condensation reaction between the hydroxyl groupsat the end of PEG and the SiAOHs that resulted from thehydrolysis of ASiOCH3 was considered to be the secondstep to lead to the chemically bonding of PEG to theUHMWPE surface. However, contributions associated withPEG in the range 1300–700 cm�1 overlapped with SiAOASiasymmetric stretching bands,31 it was difficult to make adistinction between the two feature peaks of PEG and silanegroup. Instead, XPS could give distinctive information aboutthem due to their different bonding energy, so XPS wasused to characterize the PEG grafting. High-resolution XPSof the Si2p signal at about 102.1 eV in Figure 3 providedevidence that Si element was added to the surface ofUHMWPE after modification.32 This also certified that thesilane groups were grafted successfully to the surface of thesamples. Figure 4 shows the XPS C1s spectra of UHMWPEsheets. Several kinds of carbon–oxygen bonds could be dis-tinguished by peak fitting. Among them, peaks appeared at286.1 and 287.9 eV could be assigned to CAO and C¼¼O,respectively,33 and peak at 284.8 eV could be regarded asthe combination of CAC and CASi, for the two peaks weretoo similar to be distinguished by software. Figure 4 alsoindicates lower CAO bond in untreated UHMWPE [Fig. 4(a)]than the PEG-grafted UHMWPE, no matter it was pretreatedby acid or not [Fig. 4(b,c)]. The statistical data in Table I,getting form the XPS spectra, also suggested that there wasa considerable increase, about 12 and 7%, respectively, inCAO bond for PEgPEG and Acid-PEgPEG samples comparedwith UHMWPE. The presence of CAO bond should beascribed to the PEG grafting. Although the quantity of CAObond on modified UHMWPE surface was much less than thetheoretical value of CAO bond in PEG, we thought that acertain amount of PEG was successfully grafted onUHMWPE. However, it was unexpected that less PEG wasgrafted to the acid pretreated UHMWPE than the untreated

FIGURE 2. FTIR spectra for the UHMWPE, the VTMS-grafted

UHMWPE (PE/VTMS), and the acid-pretreated UHMWPE grafted by

VTMS (Acid-PE/VTMS). [Color figure can be viewed in the online

issue, which is available at wileyonlinelibrary.com.]


UHMWPE, though more silane groups were introduced tothe acid pretreated UHMWPE, as shown in Figure 2. Itmight be due to cross-linking reaction between SiAOHsoccurred to a greater extent in the case of higher density ofsilane groups, suppressing the reaction between the SiAOHsand the hydroxyl end groups of PEG in certain degree.

Characterization of PEG-grafted UHMWPE sheetsThe surface morphology of UHMWPE, PEgPEG, and Acid-PEgPEG observed by SEM is shown in Figure 5. The SEMphotographs (10,000�) showed the changes of surface to-pography after surface modification. The surface of thetreated UHMWPE was denser and had many small grains.Because of the corrosion of the mixed acid, the Acid-PEgPEGhad a lot of tiny compartments and microgrooves.

Table II shows that the crystallization degrees ofUHMWPE, PEgPEG, and Acid-PEgPEG samples were about80–90%. Compared with the reported crystallinity ofUHMWPE, 39–75%,1 or �50% for most bulk UHMWPE cal-culated from differential scanning calorimetry (DSC) trace,34

the results seemed relatively high. They were measured byXRD analysis and calculated values by MDI Jade5.0 software.

In fact, they did not mean the real crystallinities of the sam-ples and were more meaningful in showing the crystallinitydifference resulted from modification. The results showedthat the surface modification had some effect on the crystal-linity of the material surface, but the crystallinity differencebetween UHMWPE and PEgPEG was not obvious. At thesame time, the results in Table II indicate that the PEgPEGhad higher crystallinity than the Acid-PEgPEG. A main rea-son might be that a higher degree of crosslinking wasresulted on the Acid-PEgPEG, which would reduce the chainmobility and suppress the rearrangement of UHMWPEchains in recrystallization during the hydrolyzation at tem-perature as high as 100�C.

The result of thermogravimetric analysis (TGA), asshown in Figure 6, displayed that the treated UHMWPE,including PEgPEG and Acid-PEgPEG, was more difficult todecompose at high temperature than the untreatedUHMWPE. It was explicable considering that cross-linkingreaction between SiAOHs occurred accompanied with thereaction of the SiAOHs and PEG end hydroxyl groups. In acertain content of silane, silane crosslinking on the surfaceof materials made the thermostability stronger.25 Compara-tively, PEgPEG exhibited stronger thermostability than Acid-PEgPEG. It might be ascribed to the surface pretreatment ofstrong acid before PEG grafting, which could cut the molec-ular chain, decrease molecular weight, and break the surfaceregular structure, resulting in the decrease of decompositiontemperature.

The hydrophilicity of material surface was an importantfactor to adsorb proteins on the basis of protein adsorptionprinciple. Water contact angle usually acted as a standard toassess the hydrophilicity of material. The results of watercontact angle in Figure 7 show that PEgPEG and Acid-PEg-PEG had mean contact angle of 63.4 and 53.6�, respectively,both were apparently lower than that of unmodifiedUHMWPE (82.0�), which indicated that the modifiedUHMWPE with PEG had better hydrophilicity than theuntreated material. Apart from the contribution of PEG,other polar groups on the surface resulted from the acidpretreatment, also enhanced the improvement of hydrophi-licity for the Acid-PEgPEG, which made it show the highesthydrophilicity. Although, it was reported that some coarsestructure in the surface could increase the contact angle,35

the Acid-PEgPEG with coarser surface on which there were

FIGURE 3. XPS spectra for the UHMWPE, the PEG-grafted UHMWPE

(PEgPEG), and the acid-pretreated UHMWPE grafted with PEG (Acid-

PEgPEG).

FIGURE 4. XPS C1s spectra of the materials: (a) UHMWPE, (b) PEgPEG, and (c) Acid-PEgPEG. [Color figure can be viewed in the online issue,

which is available at wileyonlinelibrary.com.]


ORIGINAL ARTICLE

lot of tiny microgrooves showed better hydrophilicity thanPEgPEG. It might imply that the polar groups played thedominant role in impacting on the surface hydrophilicity inthis case.

Analysis of the proteins adsorption on the surfaceof UHMWPEFigure 8 shows the protein adsorption ability of UHMWPEbefore and after PEG grafting. It indicated that the adsorbedquantities of the BSA protein [Fig. 8(a)] decreased on themodified UHMWPE compared with the untreated UHMWPE.The result was thought to be directly related to PEG thatwas unusually effective at excluding other polymers from itspresence in aqueous solution.36 Improvement of surfacehydrophilicity, the change of surface structure, and chemicalcomposition may also go against the adsorption of BSA.20,37

For lysozyme protein, the results in Figure 8(b) showthat the PEgPEG had better resistance to adsorb lysozymethan the untreated UHMWPE. But the Acid-PEgPEGadsorbed more lysozyme than the untreated materials. Oneexplanation of this phenomenon might be due to PEG chainsthat were able to provide the resistance to unspecificadsorption of protein for the materials. On the other hand,the adsorption behavior of lysozyme on the Acid-PEgPEGmight be influenced by the surface morphology of the mate-rial. There were many tiny micropores and microgrooves onthe surface of the Acid-PEgPEG due to the acid pretreat-ment, and lysozyme with the smallest size among the threemodel proteins might be likely to enter the micropores,

FIGURE 5. SEM micrographs of surfaces of (a) UHMWPE, (b) PEg-

PEG, and (c) Acid-PEgPEG.

TABLE II. Degrees of Crystallization on the Surfaces of

Different Treated UHMWPE Calculated by XRD Analysis

Materials Crystallinity (%)

UHMWPE 88.02PEgPEG 89.09Acid-PEgPEG 81.88

FIGURE 6. TGA curves of the untreated and modified UHMWPE sys-

tem. [Color figure can be viewed in the online issue, which is avail-

able at wileyonlinelibrary.com.]

TABLE I. Percentage of Carbon–Oxygen Bonds in the C Peak

of XPS C1s Spectra

C¼¼O (%) CAO (%) CAC, CASi (%)

UHMWPE 0.9 4.3 94.8PEgPEG 4.6 16.5 78.9Acid-PEgPEG 1.6 11.3 87.1


whereas the polymer chains with high molecular weightcould not enter. So a relatively high quantity of lysozymewas adsorbed on the surface of the Acid-PE gPEG.

For the adsorption of fibrinogen, the results in Figure8(c) show that the resistance to the fibrinogen adsorptionon the PEgPEG and the Acid-PEgPEG was improved, com-pared with that on the UHMWPE. It could also be ascribedto the contribution of PEG.38,39 But the PEgPEG and theAcid-PEgPEG had no significant difference in resistance toadsorb fibrinogen. It might be probable that the fibrinogenmolecules were too big to pass into the micropores andmicrogrooves on the surfaces of Acid-PEgPEG, so theadsorption ability of Acid-PEgPEG was nearly the same asthat of PEgPEG.

Analysis to the adhesion property of blood plateletson the surface of UHMWPESEM was used to analyze and assess the adhesion of bloodplatelets on the material surface in this experiment. Themorphologies and distribution of the adhered platelets areshown in Figure 9 with magnification of 1000� and 5000�.From platelet distribution densities, it was seen that themodified UHMWPE resisted strongly to the adhesion of pla-telets and the resistance of the Acid-PEgPEG was strongerthan that of PEgPEG. The PEG chains grafted on the surfacecould reduce the adhesion ability of blood platelets accord-ing to the hypothesis of maintaining the natural conforma-tion.40 The acid pretreatment may improve the resistanceability because of the micropore, microgroove, and roughersurface structures, which could make the effective contactarea and interaction force smaller between material surfaceand blood platelet.41 Most platelets on the surface hadgrown some pseudopodias, and they appeared to be acti-vated. Densities of the adhesive platelets on the samplesurfaces could be calculated by the statistical method basedon Figure 9. The calculating results are shown in Figure 10.It showed that the platelet density on PEG-grafted surfaceswas significantly lower than that on UMMWPE surface (p <

0.05, t test), and the platelet density on the Acid-PEgPEG

was significantly lower than that on PEgPEG (p < 0.05,t test). Only few platelets could be seen on the surface ofAcid-PEgPEG.

FIGURE 7. Water contact angle of different materials surface.

FIGURE 8. Quantity of the adsorbing protein on different materials

surface. Three model proteins were analyzed: (a) BSA, (b) lysozyme,

and (c) fibrinogen.


ORIGINAL ARTICLE

The results in Figures 9 and 10 indicate that the plateletdensity and the amount of fibrinogen adsorbed on the mate-rial surface were positively correlated. The results were in

good agreement with a number of previous investigations.Fibrinogen was part of the clotting cascade and unspecificadsorption of fibrinogen had an influence on the activationof platelets.5 After the surface grafting with PEG, the modi-fied UHMWPE had improved resistant ability to adsorbfibrinogen, so it showed lower platelets adhesion ability. Itsuggested that PEG-grafted UHMWPE might be relativelythromboresistant.

It was also reported that BSA adsorbed on the biomate-rials could reduce the platelet adhesion on the surfaces.42 Ifonly the effect of BSA was considered, lower platelet densityon UHMWPE surface should be seen in Figure 9, becauseUHMWPE could adsorb more BSA on its surface than PEg-PEG and Acid-PEgPEG (shown in Fig. 8). However, Figure 9shows opposite results. More platelets were adhered onUHMWPE than PEgPEG and Acid-PEgPEG for a given area. Itmight indicate that the role of BSA adsorbed on UHMWPEwas not sufficient in resisting platelet adhesion.

It was also very interesting to note the effect of lyso-zyme on the modified UHMWPE surfaces. It was reportedthat lysozyme could activate platelet,43 which implied thatthe less lysozyme adsorption was beneficial for resistingplatelet adhesion. The PEgPEG adsorbed less lysozyme thanUHMWPE, so it was beneficial for PEgPEG to show strongerresistance to platelet adhesion. For the Acid-PEgPEG,although it adsorbed the highest level of lysozyme, it stillhad the highest resistance to the platelet adhesion. The phe-nomenon could be explained considering that some of thelysozymes might enter into the micropores on the Acid-PEg-PEG surface and could not interact with the platelet.

The results of this study indicated that the PEG graftedon the surface of UHMWPE had a great effect on reducingthe platelet adhesion. It can be explained from thedecreased protein adsorption resulted from the PEG graft-ing. In addition to this, it should be noted that not only theprotein adsorption and surface morphology discussed ear-lier but also the PEG-grafted density, the shapes of PEGchain, and other chemical factors played parts in platelet ad-hesion.44,45 Their effects on the platelet adhesion would beinvestigated in our following work.

FIGURE 9. SEM images (1000�) showing platelet adhesion on (a)

UHMWPE, (b) PEgPEG, and (c) Acid-PEgPEG surfaces. The insets (5000�)

in each image is the magnification showing the adhered platelet.

FIGURE 10. Platelet adhesion from PRP to the untreated and PEG-

grafted surfaces. Adsorption time 15 min, Data were mean 6 SD, n ¼ 6.


CONCLUSIONS

A modified UHMWPE grafted with PEG was developed by amethod involved VTMS grafting induced by ultraviolet irra-diation, hydrolyzation of ASiOCH3, condensation ofASiOHs, and PEG hydroxyl ends combined with the cross-linking of silanol groups. The surface chemical composi-tion, surface hydrophilicity, and surface morphology ofUHMWPE changed after modification. The modifiedUHMWPE grafted with PEG showed good hydrophilicityand thermostability. The acid pretreatment to UHMWPEcould activate the substrate surface to promote the silanegrafting and meanwhile produce a coarse surface. PEGgrafted on the surface of UHMWPE had great influence onthe protein adsorption on the material. Consequently, itcould affect the platelet adhesion on the material. It is pos-sible to optimize the biocompatibility of UHMWPE by thiskind of surface modification.

ACKNOWLEDGMENTS

The FTIR and XPS analysis were assisted by the Analytical andTesting Center, Sichuan University. The authors thank Profes-sor Jiang Bo in Engineering Research Center for Biomaterialsin Sichuan University for his kindly providing the UV-irradia-tion device.

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