Study on the tribological properties of pHEMA hydrogels for use in artificial articular cartilage

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Wear 265 (2008) 269–277

Study on the tribological properties of pHEMA hydrogels foruse in artificial articular cartilage

V.P. Bavaresco a, C.A.C. Zavaglia b, M.C. Reis a, J.R. Gomes c,∗a CTC, State University of Campinas (UNICAMP), Campinas, SP, Brazil

b Department of Materials Engineering, College of Mechanical Engineering, State University of Campinas (UNICAMP),Cx. P. 6122, 13081-970 Campinas, SP, Brazil

c Department of Mechanical Engineering, CT2M, University of Minho, 4800-058 Guimaraes, Portugal

Received 29 June 2006; received in revised form 3 September 2007; accepted 22 October 2007Available online 11 December 2007

bstract

The tribological properties of synthetic hydrogels based on poly (2-hydroxyethyl methacrylate) (pHEMA) were studied in a pin-on-disc equipmentsing stainless steel 316 L as disc counterface lubricated by distilled water. This work establishes the correlation between the crosslinking density,hemical changes and tribological properties of the pHEMA/poly (methyl methacrylate-co-acrylic acid) (75:25) blend using 10% (w/w) crosslinkinggent and pHEMA/n-vinyl pirrolidone (10% (w/w)) blend with 0, 5% and 10% (w/w) of crosslinking agent. The tribological parameters investigatedere the sliding speed (0.16 ≤ v ≤ 0.5 ms−1) and the contact pressure (2.4 ≤ p ≤ 5.5 MPa). The friction coefficient was continuously evaluateduring each test and the wear rate was quantified by weight loss. The results showed that the hydrogel crosslink density and hydration aremportant factors to determine the wear behavior of hydrogels. The friction coefficient decreased with the increasing of the sliding speed from.16 to 0.50 ms−1 (0.01 ≤ m ≤ 0.09 for v = 0.16 ms−1 and 0.01 ≤ m ≤ 0.06 for v = 0.50 ms−1). The wear rate ranged from ≈10−6 to 10−5 gm−1,epending on the interactions between crosslinking density of hydrogels, contact pressure and sliding speed. The dominant wear mechanisms
ere identified by Scanning Electron Microscopy. The most compliant hydrogels (0% (w/w) of crosslinking agent) presented adhesive wear as
he main wear mechanism. As the crosslinking density of hydrogels increased, the capacity of absorption of water was reduced and the dominantear mechanism became abrasion.2007 Elsevier B.V. All rights reserved.

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eywords: Hydrogels; Biomaterials; Biotribology; Friction; Wear

. Introduction

There has been an increased interest in using hydrogelss synthetic compliant materials for articulating surfaces ofeplacement joints [1–11]. The idea is to create a more com-liant form of synthetic articular cartilage than the ultra higholecular weight polyethylene (UHMWPE), thus reducing

ontact stresses and encouraging fluid film lubrication mech-nisms. Synthetic hydrogels based on poly (2-hydroxyethylethacrylate) (pHEMA) have been investigated for use as syn-

hetic compliant materials due to their superior biocompatibility,igh permeability to small molecules (i.e. tissue metabolites),ydrophilic properties, soft consistency and low friction coef-

∗ Corresponding author. Tel.: +351 253 510232; fax: +351 253 516007.E-mail address: [email protected] (J.R. Gomes).

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043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2007.10.009

cient [12–18]. The physical and mechanical properties ofydrogels can be controlled by different methods of polymeriza-ion and copolymerization, the crosslinked density, the amountf water which they absorb and the superficial characteristics10,12,14,16,19,20].

Several studies were developed on the possible use of hydro-els as compliant materials. Liu [21] studied the influence ofoad on the friction coefficient of poly (acrylic acid) hydrogel inir and water using a ball-on-plate friction tester. Yasuda et al.22] studied of wear properties of four double-network hydro-els using pin-on-flat wear testing, whereas Freeman et al. [23]tudied the friction and wear of the pHEMA hydrogel usingn oscillating contact device. These studies indicate that load,
ydrogel crosslink density and hydration are important factors toetermine the wear behavior of hydrogels. The average frictionoefficient ranged from 0.05 to 1.70, while hydrogel wear variedy a factor of over 60 [23]. Other studies on the use of hydrogels
mailto:[email protected]

dx.doi.org/10.1016/j.wear.2007.10.009

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bs(pHEMA/NVP) and pHEMA/poly (MMA-co-AA). The poroussubstrates were immersed in a mixture of monomer, initiator andcrosslinking agent getting a layer of hydrogel of ≈1 mm thick-ness as exemplified in the schematic representation of Fig. 1(a)

70 V.P. Bavaresco et al. /

s compliant materials tend to focus on friction. In these studies,t was verified that the friction properties of hydrogels dependsn the water content of it, namely, the higher the water content,he lower the friction coefficient [24,25].

This work describes the tribological properties of a seriesf hydrogels blends based on pHEMA, where the moleculartructure varied by using the reinforcing hydrophobic polymer-vinyl pirrolidone (NVP) and by the incorporation of the polymethyl methacrylate-co-acrylic acid) copolymer. The effectsf load, sliding speed, equilibrium water content (EWC) andardness of the hydrogels was investigated.

. Experimental

.1. Materials

Indentation creep test, water swelling (EWC) and wear testssing a pin-on-disc equipment were used to characterize theechanical and tribological properties of hydrogels. The hydro-

els blends were obtained from a mixture of 2-hydroxyethylethacrylate (HEMA) (Aldrich) with 10% (w/w) of linear rein-

orcing polymer – n-vinyl pirrolidone (NVP), benzoil peroxide0.5% (w/w)) as an initiator of polymerization, and trimethy-ol propane trimethacrylate (TMPTMMA) (Retilox) 0, 5% and0% (w/w) as a crosslinking agent. The pHEMA/poly (methylethacrylate-co-acrylic acid) (pHEMA/poly (MMA-co-AA))as prepared using the poly (methyl methacrylate-co-acrylic

cid) (75:25). The poly (MMA-co-AA) (10% (w/w)) was dis-olved in the HEMA monomer after which benzoil peroxide0.5% (w/w)) and TMPTMMA (10% (w/w)) crosslinking agentere added to the solution under agitation before the poly-erization. Both hydrogels blends were prepared by thermal

olymerization (70–85 ◦C) by 4 h.

.2. Indentation creep test

The indentation creep test was performed using a Materialest System (MTS – Teststar II). The samples were immersed inistilled water and indented using a spherical tip with a diameterf 3.2 mm. A load of 4.935 N (0.5 kgf) was applied on the sampleor 180 s. The indentation height (h) was measured through timet).

The indentation creep modulus was calculated using the equa-ion described by Kempson [26] (Eq. (1)).

= 9 · 104p

16√

r

[1 − exp(−0.42 · e/a)

h

]3/2

(1)

: creep modulus (kgf m−2); p: load (kgf); r: indenter radiuscm); e: sample thickness (cm); h: indentation height (cm); a =

(2rh − h2) (cm)

.3. Equilibrium water content (EWC)

For the equilibrium water content (EWC) determination,ried samples of the hydrogels were immersed in deion-zed/distilled water at room temperature and the EWC was

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265 (2008) 269–277

alculated by weight difference, usually expressed by Eq. (2).

WC = (weight of wet gel − weight of dried gel)

weight of wet gel× 100 (2)

The swollen gel weight was measured allowing the hydrogelso reach equilibrium state in distilled water, which was indicatedy their constant final weight.

.4. Preparation of hydrogel coatings

Pin samples for friction and wear tests were preparedy using solid porous substrates of UHMWPE (cylindricalhape – ∅ 5 mm), coated with pHEMA/n-vinyl pirrolidone

ig. 1. Pin sample made by solid porous substrate of UHMWPE coated withydrogel: schematic representation (a); SEM micrograph of the longitudinalection (b) and SEM micrograph of the initial contact surface (c).

V.P. Bavaresco et al. / Wear

Table 1Indentation creep modulus (E) and equilibrium water content (EWC) for thepHEMA/NVP (10% w/w) with 0, 5 and 10% (w/w) of crosslinking agent (CA),and for the pHEMA/poly (MMA-co-AA) with 10% (w/w) of CA

Hydrogel blend – CA (%) Indentation creepmodulus, E (MPa)

EWC(%)

pHEMA/NVP – 0 4.0 38pHEMA/NVP – 5 9.7 30pp

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HEMA/NVP – 10 15.0 25HEMA/poly (MMA-co-AA) – 10 7.0 32

nd illustrated in the longitudinal section of the pin presentedn Fig. 1(b). The system went through thermal polymerizationor 4 h (70–85 ◦C). After that, it was placed in water for resid-al monomer and initiator removal. Thus, the hydrogels wereormed on the surface and inside the pores of the substrates,ssuring its mechanical fixation. The initial contact surface of theydrogels was characterized by a smooth appearance (Fig. 1(c)).

.5. Tribological tests

The tribological characterization was performed at room tem-erature in pin-on-disc equipment, Plint TE67, using stainlessteel 316 L as disc counterface. The sliding occurred in distilled

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ig. 2. Friction coefficient versus contact pressure (MPa) for pHEMA/NVP (10% w/w= 0.16 ms−1 (a); v = 0.33 ms−1 (b) and v = 0.50 ms−1 (c).

265 (2008) 269–277 271

ater, with different sliding speeds (0.16, 0.33 and 0.50 ms−1),nd variable contact pressure (2.4 ≤ p ≤ 5.5 MPa). The contacturfaces of the discs were polished with silicon carbide paperollowed by finishing with 1 �m diamond paste. The final sur-ace roughness of the discs, as measured by profilometry, was.03 �m Ra. The discs were washed, first with detergent andater and then with absolute ethyl alcohol. After that, the sol-ent was evaporated in hot air flow. Prior to sliding, the pinemained during 30 min, immersed in distilled water in ordero attain the equilibrium water content. Under these conditions,he hydrogels became highly compliant and due to water absorp-ion the layer of hydrogel increased to a thickness of ≈2 mm.herefore, the hydrogels were able to perfectly adapt to thepposing flat polished surface of the stainless steel disc. Theriction coefficient was evaluated during each test and the wearate of the pins was quantified by weight loss. A lubricant-ontaining vessel was used, where a control static pin wasmmersed to estimate the weight gain due to absorption of dis-illed water.

The dominant wear mechanisms on the hydrogel contact
urfaces were identified by Scanning Electron Microscopy –EM (JEOL JXA 860A). Previous to SEM characterization,
he hydrogel samples were fixed in a metallic support and goldputtered using Sputter Coater BAL-TEC SCD equipment.

) and pHEMA/poly (MMA-co-AA) hydrogels tested at different sliding speeds:

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. Results and discussion

The hardness of the hydrogels was modified by the effect ofhe crosslinking agent and by the type of reinforcing polymers.able 1 shows the indentation creep modulus (E) and the equilib-ium water content (EWC) measured for the pHEMA/NVP (10%/w) with 0, 5% and 10% (w/w) of crosslinking agent (CA), and

or the pHEMA/poly (MMA-co-AA) with 10% (w/w) of CA.The increase of the crosslinking agent concentration caused

n increase in the value of the indentation creep modulus and aecrease in the equilibrium water content of the pHEMA/NVPydrogels. There was also an increase of the crosslinking densityn the polymeric network, causing a decrease in its flexibil-ty. Consequently, the hardness of the material was improvednd, in that way, the absorption of water was hindered. TheHEMA/(poly (MMA-co-AA) blend presented a smaller valueor the indentation creep modulus than the values presented byhe pHEMA/NVP with 5% and 10% (w/w) of crosslinking agentnd a similar value (≈32%) to the pHEMA/NVP hydrogel with% (w/w) of CA.

The effect of the changes of reinforcing polymer and the
rosslinking density of the pHEMA/NVP (10% w/w) andHEMA/poly (MMA-co-AA) (75:25) hydrogels in the frictionoefficient (μ) and wear rate for different values of contact
Iic

ig. 3. Wear rate versus contact pressure (MPa) for pHEMA/NVP (10% w/w) an= 0.16 ms−1 (a); v = 0.33 ms−1 (b) and v = 0.50 ms−1 (c).

265 (2008) 269–277

ressure (p = 2.4; p = 4.0 and p = 5.5 MPa) and sliding speedv = 0.16 ms−1, v = 0.33 ms−1 and v = 0.50 ms−1) is shownn Figs. 2 and 3, respectively.

From Figs. 2 and 3, it can be observed that regardless ofest conditions and of the indentation creep modulus (E), theHEMA/(poly (MMA-co-AA) hydrogels presented better tri-ological properties, with low friction values (μ ≈ 0.01) andear rate in the order of 10−6 gm−1. These friction values tend

o the results reported in the literature for tests conducted withynovial joints, where friction coefficient values as low as 0.003ere found [27].Gong et al. [19,28] showed that the tribological behavior of

olymeric hydrogels in contact with solid surfaces is complexue to their structure and specific properties, and that the frictionorce will depend on the chemical and physical characteristicsf the interaction (attractive or repulsive) at the contact sitesetween the mating surfaces, as well as on the contact pressurend sliding speed. Furthermore, the frictional force substan-ially changes with the change in the chemical structure of theydrogel, such as the charge crosslinking density, the solventontent and the chemical properties of the solid counterface.
t is also known that the incorporation of acid groups (COO-),n this case, coming from the acrylic acid, increases the con-entration of negative load at the surface, which hinders the
d pHEMA/poly (MMA-co-AA) hydrogels tested at different sliding speeds:

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oss of water of the polymeric network changing their super-cial properties [29]. It is believed that this superficial changeenerating a repulsive effect at the interface between the matingurfaces promotes the best response of the pHEMA/poly (MMA-o-AA) hydrogels in terms of both friction coefficient and wearate. On the other hand, it is observed that the tribological
ehavior of blends synthesized with NVP showed a signifi-ant and complex dependence on the sliding speed and contactressure. When the pHEMA/NVP hydrogels are submitted toow sliding speed (v = 0.16 ms−1) the hydrogels synthesized
ig. 4. SEM worn surfaces of the pHEMA/NVP (10% w/w) hydrogels synthe-ized with: 0% (w/w) of crosslinking agent (CA) (a); 5% (w/w) of CA (b) and0% (w/w) of CA (c) (v = 0.16 ms−1; p = 2.4 MPa).

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265 (2008) 269–277 273

ith 0% (w/w) of CA presented better tribological propertiesFigs. 2(a) and 3(a)), as they are more compliant materials, witharger capacity to absorb the normal load. The increase of CAoncentration causes a structural change on the hydrogels, whichncreases their wear rate (Fig. 3(a)) by lack of compliance. Forntermediate sliding speed (v = 0.33 ms−1) the pHEMA/NVP
ydrogel with 5% (w/w) of CA presented lower friction for thentermediate pressure (p = 2.4 MPa), whereas the other two com-ositions (pHEMA/NVP with 0% and 10% (w/w) CA) were

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haracterized by a decreasing of the friction coefficient andear rate with the increasing of pressure (Figs. 2(b) and 3(b)).

n addition, for these sliding conditions, the more compliantydrogel (pHEMA/NVP – 0% (w/w) CA) presented the higherear rates, although combined with considerable low friction

oefficient values. This frictional response of the compliantydrogel can be explained by its high absorbed water con-ent, contributing to the increase of the lubricating effect duringliding [30].


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265 (2008) 269–277

For the higher sliding speed (0.50 ms−1) the friction valuesecreased with the increasing of the contact pressure for allf the compositions of pHEMA/NVP hydrogels. It is believedhat in the low sliding speed conditions the hydrodynamic effectas insufficient to generate an effective lubricant film between

he mating surfaces and the direct contact was kept during theliding motion, whereas the sliding conditions corresponding to= 0.50 ms−1 promotes the establishment of a hydrodynamic

ubrication regime [22].


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The type and nature of the lubricant directly influences thestablishment of the lubrication regime. Therefore, it is expectedhat the tribological response of the tested hydrogels could beffected by the presence of a synovial fluid instead of distilledater. Previous studies suggested that on the outer surface of

he articular cartilage a lipid layer exists with improves theear resistance and promotes boundary lubrication conditions

hat prevent direct cartilage to cartilage contact [31,32]. In theresent study, the effects of a hydrodynamic lubrication regime,

ig. 8. SEM worn surfaces of the pHEMA/poly (MMA-co-AA) (75:25)lend synthesized with 10% (w/w) of CA (v = 0.16 ms−1) p = 2.4 MPa (a);= 4.0 MPa (b) and p = 5.5 MPa (c).

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265 (2008) 269–277 275

ogether with the increasing of the crosslinking density, explainhe reduction of the wear rate of more resistant hydrogels (5%nd 10% (w/w) CA) (Fig. 3(c)), although with a small increasef the corresponding friction values (Fig. 2(c)). Freeman et al.23] verified that the normal applied load, hydrogel crosslinkensity and hydration present significant influence on the wear
f polyHEMA hydrogels, with high loads and hydration lev-ls producing higher wear while high crosslink density led toower wear. Several combinations of test conditions (load andubrication), hydrogel crosslinking and hydrogel hydration were
ig. 9. SEM worn surfaces of the pHEMA/poly (MMA-co-AA) (75:25)lend synthesized with 10% (w/w) of CA (v = 0.50 ms−1) p = 2.4 MPa (a);= 4.0 MPa (b) and p = 5.5 MPa (c).

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ested by Freeman et al. The average friction coefficient of theydrogels ranged from 0.05 to 1.70, while the wear values var-ed by a factor of over 60, denoting a complex dependence forhe tribological parameters [23].

The experimental results obtained in the present study showhat the processes involving the friction and wear of hydro-els are complex and are determined by both the properties ofydrogels (crosslinking density, reinforcing polymer, hardness,quilibrium water content) and by the parameters of the tribosys-em (contact pressure, lubrication, sliding speed) [19,23,27].

The morphological wear features of the pHEMA/NVP (10%/w) hydrogels synthesized with 0%, 5% and 10% (w/w) CA

fter testing at v = 0.16 ms−1 are shown in Figs. 4 and 5, respec-ively for p = 2.4 MPa and p = 5.5 MPa. Figs. 6 and 7 show theorn surface morphology of the same hydrogels after testing

t high sliding speed (v = 0.50 ms−1) for p = 2.4 MPa (Fig. 6)nd p = 5.5 MPa (Fig. 7). It is observed that the less crosslinkedydrogel (0% (w/w) CA) is characterized by high plastic defor-ation (Figs. 4(a), 5(a), 6(a) and 7(a)), denoting adhesion as

he dominant wear mechanism, independently of the slidingpeed and contact pressure, although no transfer of polymericaterial was detected on the mating steel surface. How-

ver, with the increase of hardness, for hydrogels crosslinkedith 5% and 10% (w/w), abrasive wear starts to be impor-

ant and plowing grooves are evidenced at the worn surfacesFigs. 4(b) and (c), 5(b) and (c), 6(b) and (c) and 7(b) and (c).t is important to note that the extensive cracks that can be seenn Figs. 4(c) and 5(c) were originated during the coating pro-ess with gold for SEM characterization after the tribologicalest.

Figs. 8 and 9 present the morphological wear featuresf the pHEMA/poly (MMA-co-AA) hydrogels synthe-ized with 10% (w/w) CA after testing at v = 0.16 ms−1

Fig. 8) and v = 0.50 ms−1 (Fig. 9) under p = 2.4 MPaFigs. 8(a) and 9(a)), p = 4.0 MPa (Figs. 8(b) and 9(b)) and= 5.5 MPa (Figs. 8(c) and 9(c)). In all test conditions the worn

urface of hydrogels synthesized with MMA-co-AA revealedeatures of cracked surfaces together with some abrasion marks.orroborating these observations for pHEMA/poly (MMA-co-A) hydrogels, no significant changes were found for theeasured friction and wear values at different test conditions

Figs. 2 and 3).

. Conclusions

In this work, the complex effect of reinforcing polymer androsslinking density on the tribological properties of pHEMAased hydrogels sliding against stainless steel 316 L in distilledater was studied. The blends synthesized with NVP testednder conditions that favoured the occurrence of hydrodynamicffect (v = 0.50 ms−1) presented better tribological response forore resistant hydrogels, with smaller capacity of water absorp-

ion. However, the results denoted a significant and complex
ariation on the tribological properties of hydrogels with slidingpeed and pressure, making difficult to infer about the tribolog-cal response of these materials operating as artificial articularartilage.
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For all test conditions, the wear rate of hydrogels was in theange of 10−6 to 10−5 gm−1 and the friction coefficient valuesere relatively low (0.01 ≤ μ ≤ 0.03) and comparable with the

eported values for other polymeric biomaterials developed toe substitutes of articular cartilage.

It was verified that although the pHEMA/poly (MMA-co-A) hydrogel is characterized by smaller indentation creepodulus, this material exhibited the better tribological proper-

ies (μ ≈ 0.01 and wear rate in the order of 10−6 gm−1), whichncreases the interest in this hydrogel for in vivo studies.

For all the compositions and independently of the test condi-ions, the most compliant hydrogels (0% (w/w) CA) presenteddhesive wear as the main wear mechanism. As the crosslink-ng density of hydrogels increased, the capacity of absorption ofater was reduced and the dominant wear mechanism became

brasion.

cknowledgements

This work was supported by FAPESP (Foundation for thessistance to Research of the State of Sao Paulo), University ofinho, Portugal, and State University of Campinas, Brazil.

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Documents

Study on the tribological properties of pHEMA hydrogels for use in artificial articular cartilage