THE INFLUENCE OF TEXTURED SURFACES ON

THE LUBRICATION OF ARTIFICIAL JOINT PROSTHESES

Eduardo de la Guerra Ochoa, David del Sordo Carrancio, Javier Echávarri Otero,

Enrique Chacón Tanarro, Andrés Díaz Lantada and Pilar Lafont Morgado

Grupo de Investigación en Ingeniería de Máquinas-E.T.S.I. Industriales, Universidad Politécnica de Madrid,

c/ José Gutiérrez Abascal 2, 28006, Madrid, Spain

Keywords: Tribology, Textured Surface, Artificial Prosthesis, Photolithography, Chemical Etching.

Abstract: The development of artificial joint prostheses is an especially relevant advance linked to the combined use

of medical and engineering sciences. In ideal conditions a joint prosthesis should last the whole patient’s

life. For a younger patient, that is normally linked to using enhanced designs capable of reducing friction

and wear rate, thus increasing patient’s comfort and prosthesis service life. Present study concentrates on

validating the use of micro-textured surfaces for improving friction and minimizing wear rate by means of

increasing the (elasto) hydrodynamic lubrication range. Micro-textured surfaces have been obtained by UV-

photolithography upon photosensible films and subsequent chemical etching of the uncoated surface zones.

A ball-plane contact tribometer has been used to assess the friction coefficient of the different micro-

textured surfaces in order to validate our approach. Significant reductions of friction coefficient have been

obtained thanks to the micro-textures, what provides useful information for computer-aided design &

manufacturing processes linked to the development of innovative and efficient biomimetic prostheses.

1 INTRODUCTION

Synovial joints allow the relative movement

between bones of the human body. They operate in a

similar way as bearings, reducing friction and wear

in the contact. Typical examples of such joints are

that of the hip, knees, shoulders and phalanges of the

fingers. Figure 1 shows a schematic illustration

(Bergmann, 2010).

Figure 1: Outline of a synovial joint.

As shown in Figure 1, the two bones, with

relative movement, are separated by a cartilage and a

synovial fluid that lubricates the contact. In order to

avoid the loss of lubricant, a fibrous capsule covers

the joint. By means of this configuration, friction

coefficients attained in the joints vary from 0.005 to

0.025, depending on the age and physical conditions

of the patient (Gohar, 2008).

As time goes by, likewise in case of lesion or

disease, joints can present inflammations or wear,

i.e. arthritis. Therefore, characteristic consequences

may arise: movement constraint, joint swelling,

trembling, pain, progressive strength loss and

deformity of the body part affected (Dumbleton,

1981). These symptoms can become very annoying

and dramatically reduce life-quality of the sufferer.

When neither rehabilitation nor medication can

mitigate the arthritis symptoms, damaged joint is

generally replaced by an artificial prosthesis. Most

artificial joint prostheses present two main parts,

each of them placed in one of the two bones of the

joint.

Relative movement is allowed between them,

sometimes including an additional intermediate part

for improving contact phenomena. The types of

contacting materials used for prostheses are very

diverse: ceramic, polymeric, metallic and their

291

Ochoa E., Del Sordo Carrancio D., Echávarri Otero J., Chacón Tanarro E., Díaz Lantada A. and Lafont Morgado P..

THE INFLUENCE OF TEXTURED SURFACES ON THE LUBRICATION OF ARTIFICIAL JOINT PROSTHESES.

DOI: 10.5220/0003908902910294

In Proceedings of the International Conference on Biomedical Electronics and Devices (BIODEVICES-2012), pages 291-294

ISBN: 978-989-8425-91-1

 2012 SCITEPRESS (Science and Technology Publications, Lda.)

combinations (Pinchuk, 2006). The most usual

selection is a quasi-spherical metallic element

(stainless steel, Co-Cr-Mo, Co-Ni-Cr-Mo or Ti

alloys, in many cases with ceramic coatings) housed

in a plastic element (mainly ultra-high-molecular-

weight polyethylene or UHMWPE). Prostheses must

operate under very variable conditions (from static

conditions to very high sliding velocities) and in

such a delicate environment like the human body.

Therefore, good friction behavior and wear

resistance are required with the aim of avoiding the

need to substitute them, i.e. life of the prostheses is

expected to be longer than that of the patient.

2 LUBRICATION IN

PROSTHESES AND JOINTS

In mechanical systems three main lubrication

regimes are distinguished: boundary, mixed and

elasto(hydrodynamic)-EHL. The friction coefficient

and wear under each regime are very different, as

shown in Stribeck’s curve (Figure 2).

Figure 2: Different lubrication regimes in Stribeck’s curve.

The differences among regimes are given by the

specific film-thickness parameter (λ), which

provides the relation between the lubricant film-

thickness (h) and the combined roughness of both

surfaces (σ), see Eq. 1. When λ>3 a complete

separation between surfaces is attained and therefore

EHL regime prevails. When 1<λ<3 the separation is

partial and mixed lubrication is assumed. Finally,

when λ<1, the contact works under boundary

lubrication regime (Stachowiak, 2005).

(1)

In most working conditions, joints operate under

mixed lubrication, where the separation between

contacting surfaces is incomplete. Thus, low friction

coefficients are obtained due to the composition of

synovial fluid: including a liberation and

accumulation of glycoproteins and hyaluronic acid

within the cartilage interstices when the joint is

submitted to pressure (Gohar, 2008).

In the case of artificial joint prostheses, the

accumulation of synovial fluid is much more

complex, as they are manufactured with very low

surface roughness (10 – 50 nm), what stands for a

specular finish on both contact surfaces. Such

extremely low roughness promotes a positive increase

of lubricant film thickness for low sliding velocities.

However a polished surface cannot effectively

retain lubricant and important adhesion and wear

problems appear at rest or at the beginning of

movement, as the contact is almost dry in such

conditions.

3 SURFACE

MICRO-TEXTURIZATION

3.1 Expected Benefits

of Micro-texturization

The main objective for introducing micro-textures

on the contact surfaces of artificial joint prostheses

is to fulfill the need of retaining synovial fluid at

very low velocities. At the same time low surface

roughness is maintained, what allows for a complete

lubrication regime in such conditions.

Hence a lower friction coefficient is expected for

the whole functioning range and wear rate of

prostheses can be minimized and debris particle

formation can also be importantly reduced

(Kennedy, 2000). In addition the presence of micro-

textures somehow imitates nature and promotes

tissue formation and biointegration of the prostheses

(Díaz Lantada, 2010).

Figure 3: Improvement in Stribeck curve thanks to the use

of micro-textured surfaces.

Coefficient of friction, μ

λ=h/σ

EHL

Mixed

Boundary

High

wear rate

Contact

between

surfaces

Very-low wear rate

Complete separation

between surfaces

Low wear rate

Partially separation

between surfaces

h/σ

EHL

Mixed

Boundary

Textured surface enlarge the EHL zone

reducing friction and wear

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The expected effects upon Stribeck curve (as

shown in Figure 2) are schematically described in

Figure 3, showing a beneficial reduction of friction

thanks to the presence of micro-textures (Petterson,

2003, Wakuda, 2003).

3.2 Micro-texture Design

Computer-aided design (CAD) tools have been used

for the design of micro-texturization patterns, so that

the characteristics of the textures could be controlled

and modified in an easy way. A texturization pattern

based on the repetition of circular features, with

diameter (φ) and surface density (d) as main control

parameters (see Eq. 2), has been used for our

validation..

(%) ·100

πφ

(2)

Figure 4 provides an example of one of the

micro-textured surfaces obtained by following the

manufacture process explained further on. Main

parameters are also included (in this case φ = 400

μm and l = 1.12 mm).

Figure 4: Final textured surface.

Feature diameter was selected using as reference

the hertzian contact radius (a), Eq. 3, for a ball-plane

contact (Echavarri, 2011), as a ball-plane contact

tribometer was applied for analyzing the influence of

micro-texturization on friction coefficient (see trials

and results section).

The contact radius depends on the load (W), on

the equivalent contact radius (R*) and on the

reduced Young modulus (E´). In this study three

different diameters have been used, one below, one

similar and one above estimated contact diameter:

100, 200 and 400 μm. Surface densities of 10% and

25% have been studied in combination with the

aforementioned radii.

2'2

WWR

(3)

3.3 Micro-texture Manufacture

In this work we have used copper discs as substrate

material for studying the effect of micro-

texturization on friction coefficient. Micro rapid

prototyping technologies, in this case a combination

of UV photolithography and chemical etching, have

allowed us to obtain the micro-textures. In this

preliminary validation we have used copper as

substrate material due to its easier processability and

the need of a lower etching time.

For the manufacture of the micro-textures we

have followed several steps including:

 Initial preparation of the copper discs by

washing out the possible surface oxides in

ultrasonic cube for around 30 minutes and

subsequent drying.

 Coating of the discs using Dupont Riston PM-

100 photoresin.

 Exposure of the photoresin to UV light by

means of a SF-100 equipment from Intelligent

Micro Patterning LLC. The process is known

as mask-less photolithography, as the use of

programmable light filters prevents from using

a physical mask.

 Development, using a Na

0.85% w.

solution, for eliminating the uncured

photoresin in those pattern zones that are

going to be chemically etched

 Chemical etching introducing the disc in a

FeCl

40% w. solution for attacking the

uncoated pattern zones, hence obtaining the

micro-texture.

 Stripping or elimination of the remaining

photoresisn.

 Washing out debris and drying.

 Final dimensional verification.

Final result of the manufacturing process is

included in Figure 4, in which 400 μm has been used

as diameter of the circular features and a surface

density of 10% has been applied. The geometry of

the micro-holes obtained is almost semi-spherical

and, as results explained below show, they act as

reservoirs of lubricant and promote friction

reduction.

1.12mm

Φ=400μm

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4 TRIALS AND RESULTS

Once the textures of the different probes were

obtained, several trials have been carried out in a

Mini Traction Machine from PCS Instruments

(Lafont, 2009), simulating a ball-disc contact, using

a steel ball and the micro-texturized copper discs.

Figure 5 shows the evolution of friction

coefficient as a function of mean velocity (u

), for

different diameters of the circular features of the

micro-textured patterns. A smooth disc without

micro-texturization has been used as reference.

Figure 5: Friction reduction by using textured surfaces.

The friction coefficient reduction is specially

noteworthty in the mixed lubrication regime

<10

mm/s). In addition smaller textures promote

greater reductions of friction, specially for lower

contact velocities.

5 CONCLUSIONS

AND APPLICATIONS

Our results show that the use of very simple micro-

textures helps to improve surface contact behavior

and reduce friction coefficient in the whole mixed

lubrication regime, although additional in vivo

validation needs to be further researched.

Applying this methodology to the contact

surfaces of artificial joint prostheses can help to

obtain low surface roughnesses with micro-textures

acting as reservoirs of lubricant, thus minimizing

friction and improving wear behavior and prostheses

service life. Such textures have also potential

benefits regarding cell growth and tissue formation

besides the prostheses. Future studies will be linked

to the use of end materials and geometries adapted to

those of the different artificial joint prostheses

designs. Industrialization of the proposed process

can be achieved by means of CAD-CAM

technologies, in combination with advanced additive

manufacturing or laser micro-mechanization.

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0.02

0.025

0.03

0.035

0.04

(mm/s)

Effect of textured diameter. W=20N

R=50

R=100

R=200

Smooth

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