Inlet protein aggregation: a new mechanism for lubricating film formation with model synovial fluids.

2.6 MB·English

by  Fan, J

696
Inlet protein aggregation: a new mechanism for
lubricating film formation with model synovial fluids
JFan1,CWMyant1*,RUnderwood2,PMCann1,andAHart2
1TribologyGroup,DepartmentofMechanicalEngineering,ImperialCollegeLondon,London,UK
2LondonRetrievalCentre,CharingCrossHospital,ImperialCollegeLondon,London,UK
Themanuscriptwasreceivedon1February2010andwasacceptedafterrevisionforpublicationon19January2011
DOI: 10.1177/0954411911401306
Abstract: This paperreports a fundamental study oflubricant filmformation with model syno-
vial fluid components (proteins) and bovine serum (BS). The objective was to investigate the
roleofproteinsinthelubricationprocess.Filmthicknesswasmeasuredbyopticalinterferome-
try in a ball-on-disc device (mean speed range of2–60 mm/s). A commercial cobalt–chromium
(CoCrMo) metal femoral head was used as the stationary component. The results for BS
showed complex time-dependent behaviour, which was not representative of a simple fluid.
After a few minutes sliding BS formed a thin adherent film of 10–20 nm, which was attributed
to protein absorbanceat the surface.This layerwas augmented by a hydrodynamic film, which
oftenincreasedatslowspeeds.Attheendofthetestdepositedsurfacelayersof20–50 nmwere
measured. Imaging of the contact showed that at slow speeds an apparent ‘phase boundary’
formed in the inlet just in front ofthe Hertzianzone.This was associated with the formationof
a reservoir of high-viscosity material that periodically moved through the contact forming
a much thicker film. The study shows that proteins play an important role in the film-forming
processandcurrentlubricationmodelsdonotcapturethesemechanisms.
Keywords: artificial hipjoint, synovialfluid,boundary lubrication, CoCrMo alloy
1 INTRODUCTION the reduction of wear is still an important goal for
tribological research. If new low-wear materials and
Despite significant advances in recent years, surface surface coatings for artificial joints are to be devel-
wear of artificial hip implants remains a serious oped, it is important to understand the fundamen-
concern. For metal-on-polymer (MOP) joints the tal lubrication mechanisms and the influence of
formation of polyethylene wear particles can lead to synovial fluid (SF) chemistry. The present study
osteolysis and bone loss resulting in loosening of explores the role ofSF proteins.
the implant [1]. As a result some surgeons advise Implant wear is essentially controlled by the
that these joints are not employed for younger or properties of the contacting surfaces (metallurgy,
more active patients. Thus to increase implant lon- surface roughness, hardness) and the nature of the
gevity second-generation metal-on-metal (MOM) SF lubricating film that is formed during the gait
joints have been employed. Although these give cycle. The relevant lubricating properties are
much lower surface wear they do result in the the film thickness and chemical composition; both
formation of metallic nanoparticles, which give are determined by the prevalent film formation
increased metal ion levels in the blood [2] and are mechanism occurring during the gait cycle. At
associated with periprosthetic lesions [3]. Clearly present there are two broad schools of thought
concerning these mechanisms: boundary lubrica-
*Correspondingauthor:TribologyGroup,Departmentof tion [4, 5], where film formation is determined by
MechanicalEngineering,ImperialCollegeLondon,Exhibition the chemistry at the surface; and fluid film lubrica-
Road,LondonSW72AZ,UK. tion, which obeys classical elastohydrodynamic
email:[email protected] theory [6].
Proc.IMechEVol.225PartH:J.EngineeringinMedicine
Inletproteinaggregation 697
5 50 Recent work [14] in the authors’ research pro-
gramme has shown that BS and protein solutions
ht 4 40 form abnormally thick films at low sliding speeds.
g
ei M Such behaviour has important implications for sur-
w e
e/body 3 30 an spee fTahcee oprriogtiencstioonf tahtislofwilmspbeeedh,avstioarutr-uapn,dorthaet srtoalnecoef.
c d
Vertical for 21 2100 [mm/s] tfohreTaShFirsapnsrgtouetdeoyifnBsreSapraoenrttdhsepfirtloomtpeiitcnhsoicfskotnhlueestipsornmesse.eanTsthuesrteuemxdpyee.nrtis-
mental method differs from the earlier publication
0 0 [14] as a commercial cobalt–chromium (CoCrMo)
0.0 0.2 0.4 0.6 0.8 1.0 metal femoral head was used as one of the rub-
Time [secs]
bing surfaces. This change provides a significant
Fig.1 Representationofstandardhipgaitcycle improvement of the authors’ laboratory simulation
of the problem.
The kinematics of hip joint articulation is com-
plex as the contact undergoes transient loading and 2 RESEARCH PROGRAMME
speeds over the gait cycle [6]. A simplified example
isshowninFig. 1.Thejointwilltypicallyexperience The research programme was designed to study the
two loading peaks per cycle (~1 Hz), with linear fundamental mechanisms of SF lubrication, and in
speeds of 2–50 mm/s. The fluctuating speed and particular the role of proteins in film formation. BS
load conditions will probably result in the predomi- and protein solutions (albumin and g-globulin)
nant film formation mechanism (boundary to fluid were used as the test fluids. The research pro-
film) varying over the gait cycle. Central elastohy- gramme includedthe following:
drodynamic (EHL) film thickness is typically pre-
dicted to be in the range of 30–70 nm over the gait
(a) film thickness measurements and in-contact
cycle [6]for SF.
imaging to study lubricating film formation in
SF is a viscoelastic shear-thinning fluid that is
a sliding contact;
composed of a complex mixture of large and
(b) optical examination of the metal surface before
surface-active molecules including proteins, hya-
and after film thickness testing;
luronic acid, and phospholipids [7, 8]. SF is non-
(c) measurement of surface roughness changes
Newtonian, but at physiological shear rates the
using opticalprofilometry;
effective viscosity (for film thickness calculations) is
(d) Fourier transform infrared (FTIR) spectro-
usually assumed to be constant (for example
scopic analysis of surface deposits close to the
0.0025 Pa s [6]). The chemical and physical proper-
rubbed area on the femoral head.
ties of SF are affected by disease (osteoarthritis and
rheumatoid arthritis) and trauma. This can result in
a significant decrease in the viscosity and changes An optical interferometric method was used to
in the protein content and pH [8,9]. measure film thickness for a commercial CoCrMo
It is widely recognized that proteins play an alloy head rubbing against a glass disc. All measure-
important role the lubrication process. Healthy SF ments were made at 37 (cid:2)C.
contains albumin and globulin at concentrations of The protein and BS test fluids were made using
0.7–1.8 g/dl and 0.05–0.29 g/dl, respectively [9]. In deionized water (DIW). For the present study it was
periprosthetic SF the protein content increases [8]. decided not to simulate physiological pH as the
However, it is more common to use bovine serum ionic salts used in buffer solutions are expected to
(BS), typically diluted with water to a concentration contribute to surface film formation, thus obscuring
of 25 per cent w/w, as model SF in hip simulator the protein lubricating mechanisms. Protein solu-
tests [10–12]. Wear studies, in both hip simulators tions were made with DIW; however, the poor solu-
and pin-on-disc tests, have shown a correlation bility of g-globulin in DIW meant that only
between the protein content and wear [10, 11]. a limited concentration range could be studied.
Examination of metal implant surfaces from both in This approach will be extended in later work where
vitro and in vivo function has shown that proteins film formation under different pH conditions and
aredeposited onthe surfaces [12,13]andareimpli- in the presence of ionic salts (physiological and
cated in boundaryfilm formation [10]. buffer) will be examined.
Proc.IMechEVol.225PartH:J.EngineeringinMedicine
698 JFan,CWMyant,RUnderwood,PMCann,andAHart
Table1 Measuredviscosities,pHlevels,andprotein
concentrationsfortestsolutions
Composition Viscosity pHat19(cid:2)C Totalprotein
(Pas) concentration
at103s21 (g/dl)
100%w/wBS 0.026 7.60 6.45
50%w/wBS 0.022 7.54 3.23
25%w/wBS 0.021 7.34 1.62
Albuminsolution – 6.91 1
g-Globulinsolution – 7.28 0.18
Albumin/g-globulin – 7.18 1.18
solution
3 EXPERIMENTALPROCEDURE
3.1 Test fluids
Two groups of test fluids were used.
1. Bovine serum: bovine calf serum (BS; Sigma
Aldrich 12133C, protein concentration 5.8–7.1
g/dl). BS concentrations of 100, 50, and 25 per
cent w/w were tested.
2. Protein solutions: single protein solutions were
prepared containing albumin and g-globulin at
aconcentrationwithintherangeforhealthySF[9]. Fig. 2 Film thickness measurement device: (a) sche-
maticdiagram;(b)photographoffemoralhead
mountedintestdevice
All solutions were prepared with deionized water
(Sigma-Aldrich S-37531-356) to the appropriate w/
w concentration. The solutions were stored in
Instruments) using thin-film optical interferometry
a fridge (5(cid:2)C) immediately after preparation and
[16]. A diagram of the test device is shown in
used within 1 week; consequently it was not neces-
Fig. 2(a). The underside of the glass disc was coated
sary to use antibacterial agents. Test solution prop-
with a thin (10 nm) chromium coating overlaid by
erties are summarized in Table 1; measured
a thicker silica layer (500 nm). The coatings provide
viscosity, pH level, and protein concentrations are
the necessary reflection conditions for the interfero-
given for all test lubricants. Approximate protein
metric method. The technique measures central
concentrations are given for BS solutions as quoted
filmthicknessintherange1–1000 nmwitharesolu-
by the supplier.
tion of 61 nm. A personal computer controlled the
The test fluid rheology was measured using
temperature, load, and sliding speeds. Film thick-
acone-on-platerheometer(AR2000;TAInstruments)
with a hard-anodized aluminium peltier cone. ness testconditions are summarized in Table 2.
A resurfacing femoral head (CoCrMo alloy,
Steady-state flow tests were carried out where shear
38 mm diameter, non-heat-treated) was used as the
stress was measured as a function of shear rate. The
experimental protocol used was similar to that fol- stationary counterface. This was held in the test
lowed by Oates et al. [15]. The fluids were pre- device and loaded against the underside of the
shearedat60 s21for5mintoeliminateanyprevious rotating glass disc, which was driven by an electric
shear history. The shear rate was then increased motor. The head was held in a supporting mount
from 0.002 to 1000 s21 and viscosity measurements (see Fig. 2(b)) and could be rotated so that different
taken over the range. Each measurement was parts of the head could be tested. A new sample
repeated twice. The effective viscosity measured at areawasusedfor each test.
103s21wasusedtocalculatefilmthickness. The glass disc and femoral head were cleaned in
1% w/w didodecyl sulfonate solution in an ultra-
sonic bath, rinsed repeatedly in distilled water, and
3.2 Film thickness measurements
then washed in acetone before air-drying and
Lubricating film thickness at the centre of the con- assembly. The test device was preheated to 37(cid:2)C
tact was measured in a ball-on-disc device (PCS and the entire assembly was allowed to reach
Proc.IMechEVol.225PartH:J.EngineeringinMedicine
Inletproteinaggregation 699
Table2 Filmthicknesstestconditions where U is the entrainment speed; h is the viscosity
of the lubricant; R# is the reduced radius, which for
Parameter Value
this case is the radius of the femoral head; W is the
Testconditions applied load; and E# is the reduced Young’s modu-
Contactpressure 100MPa
Speedrange 2–60mm/s lus, which is given by the formula
Temperature 37(cid:2)C
Specimens
2 1(cid:2)n2 1(cid:2)n2
Upperwindow Glassdisc = 1 + 2 (2)
Lowerspecimen CoCrMoresurfacingfemoral E0 E E
1 2
head,38mmdiameter
where n is the Poisson’s ratio and E is the Young’s
modulus for the femoral head and the glass disc
thermal equilibrium before the test was started. denoted as1and 2, respectively.
Test fluids were pumped into the contact zone to In some tests a charge-coupled device (CCD)
camera was mounted on the microscope and
avoid contamination with hydrocarbons from the
images captured from the contact zone during the
shaft bearings and seals. The lubricant reservoir was
test. Images were taken at the start, at different
held in a burette outside the test chamber. Silicone
speeds, and at the end ofthe test sequence.
tubing was used to feed the fluid into the exit
region, which ensured that an extensive meniscus
wasmaintainedaroundthe contact.Italsoprovided 3.3 Examination of worn surfaces
a thick fluid film layer in the exit so that the effects
The femoral head surface was examined before and
of evaporation were minimized as the disc rotated.
after testing using an optical microscope and opti-
The silicone tubing was held within the heated test
cal interferometric surface profilometer (Wyko
chamber to ensure that the test fluid was at the cor-
NT1100). The femoral head was rinsed with DIW to
rect temperature. A thermocouple positioned at the
removethebulkfluidpriortoexamination.Inalim-
exit of the silicone tubing was used to monitor the
ited number of cases an FTIR spectrometer (Perkin
fluid feed temperature.
Elmer Spectrum One) coupled to an FTIR micro-
Film thickness was measured as a function of
scope (Perkin Elmer Multiscope) was used to ana-
mean speed over the range of 2–60 mm/s, which
lyse surface deposits (100 mm sample area) near the
corresponds to the range of linear speeds occurring
wear scar on the femoral head. IR reflection–
during hip joint articulation. Film thickness read-
adsorption spectroscopy (IRRAS) provides informa-
ings were taken periodically over the entire speed
tion on the chemical composition and distribution
range. Tests were carried out by slowly increasing
of lubricant films on metal surfaces [18]. FTIR spec-
the disc speed in steps from 2 to 60 mm/s (results
troscopy has been used extensively to characterize
labelled ‘UP1’) then decreasing (results labelled
the composition and molecular conformation of
‘DOWN1’). The speed increase/decrease curves
protein films [19].
were then repeated (‘UP2’/‘DOWN2’). At the end of It was also interesting to compare the laboratory
each test, the residual film thickness was measured tests with those from explanted joints. These were
under static loading (5 N); this provided an indica- received from the London Implant Retrieval Centre
tion of the formation of deposited or adsorbed (LIRC). The LIRC was established in 2008 by one of
layers at the surface. These layers are able to main- the authors (A.H.) to analysis causes of failure in
tain surface-separation under loading. explanted MOM hips. As part of that work, wear
The load was held constant during the test at 5 N; and surface roughness measurements are made
this is not representative of a real hip joint which from the explanted metal components[20].
experiences transient conditions during the gait
cycle.Howeveritdoesprovideameasurementofthe
film thickness under the most severe loading (i.e. 4 RESULTS
heelstrike)andpresumablyhighwearcondition.
The BS results were compared with theoretical 4.1 Film thickness measurements
values plotted in the graphs. The film predictions
Typical film thickness measurements for BS con-
were calculated from the isoviscous film formula
centrations of 25, 50, and 100 per cent w/w are pre-
from Hooke [17]asfollows
sented in Figs 3 to 5. Central film thickness is
plotted against mean speed and four different
(Uh)0:6R00:67 (1) curves are shown for each fluid. Film thickness
h=4:18
W0:13E00:47 results for the protein solutions are given in Figs 6
Proc.IMechEVol.225PartH:J.EngineeringinMedicine
700 JFan,CWMyant,RUnderwood,PMCann,andAHart
50
UP1
DOWN1
UP2
DOWN2
40 Theory
m)
n
(
c
h
s, 30
s
e
n
k
c
hi
m t
20
al fil
ntr
e
C
10
0
0 10 20 30 40 50 60
Entrainment speed, U (mm/s)
Fig.3 FilmthicknessresultsforBSconcentrationof25%w/w,withcalculatedresult[17]plotted
asadottedline
50 UP1
DOWN1
45 UP2
DOWN2
40 Theory
m)
n 35
(
c
h
s, 30
s
e
n
k 25
c
hi
m t 20
al fil
ntr 15
e
C
10
5
0
0 10 20 30 40 50 60
Entrainment speed, U (mm/s)
Fig.4 FilmthicknessresultsforBSconcentrationof50%w/w,withcalculatedresult[17]plotted
asadottedline
and 7. At the end of the tests the residual film was obeyed classical EHL rules and increased with
measured under static loaded conditions. These speed. The predicted film thickness is also plotted
results are summarized in Table 3. for each of the BS concentrations. However subse-
The film results for the BS solutions are complex quent curves showed very different behaviour.
and time dependent. Initial speed curves (UP1) Because of the complexity of the results only one
Proc.IMechEVol.225PartH:J.EngineeringinMedicine
Inletproteinaggregation 701
50 UP1
DOWN1
45
UP2
DOWN2
40
Theory
m)
n 35
(
c
h
s, 30
s
e
n
k 25
c
hi
m t 20
al fil
ntr 15
e
C
10
5
0
0 10 20 30 40 50 60
Entrainment speed, U (mm/s)
Fig. 5 Film thickness results for BS concentration of 100% w/w, with calculated result [17] plot-
tedasadottedline
test is shown for each fluid. However the tests were 3. A residual surface film is deposited within the
repeated at least twice and the pattern was the contact region. This is in the rangeof 20–50 nm.
same each time; results representative of this
behaviour are shown. Figure 6 shows representative results for the
The results in Fig. 3 are typical of the speed–time single protein solutions: albumin (1 g/dl) and g-
behaviour observed with BS solutions. In the initial globulin (0.18 g/dl). The results for albumin show
curve (UP1) film thickness increases with speed to the same characteristic curves as the BS solutions.
amaximumof~20 nmat60 mm/s.Decreasingspeed The final curve is in the range 40–60 nm. The g-
givesaverydifferentresult;thefilmthicknessremains globulin solution, despite the much lower concen-
fairly constant (15–19 nm) down to 5 mm/s when it tration, gives slightly thicker films with a final curve
starts to increase again. Subsequent speed increase/ of 65–80 nm.
decrease curves are similar except the overall film The mixed protein results are shown in Fig. 7.
thicknessisnowintherange20–50 nm.Attheendof The pattern of film formation is very similar to the
the test, the residual film (film thickness measured at albumin test. A final film in the range of 50–80 nm
0 mm/s,5 Nload)istypicallyintherange20–40 nm. is obtained. The residual film thicknesses measured
The results for BS concentrations of 50 and at the end of the tests are summarized in Table 3.
100 per cent w/w are shown in Figs 4 and 5. Again The overall range for these measurements is
very similar behaviour is seen; the initial film thick- 20–60 nm.
ness increases with speed followed by growth of the
film with increasing time or rubbing. These results
4.2 In-contact imaging
can be summarized as follows.
In-contact images (field of view 1.2 mm30.96 mm)
1. The initial film thickness increased with speed are shown in Fig. 8 for 25 per cent w/w BS at differ-
to amaximum of 16–20 nm at ~60 mm/s. ent speeds (under 5 N load) as follows: 0 mm/s
2. The overall film thickness increases with each (start),5 mm/s, and 20 mm/s.
subsequent speed curve. The final results are Figure 8(a) shows the loaded contact at the start
typically in the range 25–50 nm. The thicker of the test. The blue region is the Hertzian contact
films are usually seen for the lower BS concen- zone (blue coloration is due to the silica spacer
tration. The thickest films are usually recorded layer). Outside the Hertzian region the femoral head
atthe lowest speeds. surface appears corrugated – this texturing is
Proc.IMechEVol.225PartH:J.EngineeringinMedicine
702 JFan,CWMyant,RUnderwood,PMCann,andAHart
(a)
100
UP1
90 DOWN1
UP2
80
m) DOWN2
n
( 70
c
h
ss, 60
e
n
k
c 50
hi
m t
al fil 40
ntr 30
e
C
20
10
0
0 10 20 30 40 50 60
Entrainment speed, U (mm/s)
(b)
100
90
80
m)
n
( 70
c
h
s, 60
s
e
n
k 50
c
hi
m t 40
al fil 30 UP1
ntr DOWN1
Ce 20
UP2
10 DOWN2
0
0 10 20 30 40 50 60
Entrainment speed, U (mm/s)
Fig.6 Filmthicknessresultsforsingleproteins:(a)albumin,1 g/dl;(b)g-globulin,0.18 g/dl
discussed in section 4.4. As the disc starts to rotate and end of the test (after cleaning). An example is
a film builds up rapidly in the contact zone; this is shown in Fig. 9; these images were taken before
seen in localized regions (Fig. 8(b)) particularly at (Fig. 9(a)) and after (Fig. 9(b)) the same test with
low speeds. At higher speed (Fig. 8(c)) this film 25 per cent w/w BS.
tends to break down. Figure 9(a) shows the surface texture of the new
femoral head; it is characterized by ‘block’ features
typical of as-cast CoCrMo alloys. Examination of
4.3 Examination of the femoral head contact
the wear surface at the end of the test showed loss
region
of the carbides and the formation of a wear scar.
Images of the femoral head contact region without These block structures were lost rapidly at the start
the glass disc present werealsorecorded at the start of the test and this was accompanied by the
Proc.IMechEVol.225PartH:J.EngineeringinMedicine
Inletproteinaggregation 703
100 Up 1
90 Down 1
Up 2
m) 80
n Down 2
c ( 70
h
ss, 60
e
n
k 50
c
hi
m t 40
al fil 30 Fig. 9 Optical images from the femoral head before
entr 20 and after testing (field of view 1.2 mm 3
C 0.96 mm)
10
0
0 10 20 30 40 50 60
attributed to third body wear [21] due to abrasion
Entrainment speed, U (mm/s)
by carbide particles embedding in the surface of the
Fig. 7 Film thickness results for mixed albumin/ glass disc. No damage was observed to the silica
g-globulinproteinsolution
coating during the test, which lasted for less than 15
min.
Table3 Summaryofresidualfilmthicknessmeasure- Figure 10 shows line profiles taken from inside
ments(loaded,staticcontactatendoftest) and adjacent to the wear scar. Loss of the surface
block structures occurs within the contact zone.
Lubricant Residualfilmthickness(nm6range)
The blockstructuresonthe unusedsurfacearetypi-
BS,25%w/w 35.7562.5 cally 80–100 nm high with typical surface dimen-
BS,50%w/w 19.463.5
BS,100%w/w 3460.00 sions of40 mm315 mm.
Albumin,1g/dl 52.95624.05 Optical examination of the femoral head at the
g-Globulin,0.18g/dl 29.55614.00
end of the test showed deposited surface films in
Albumin/g-globulin,2:1 60.00620.00
and around the contact patch. These deposits often
remained after light rinsing and it was possible to
analyse their chemical composition using IRRAS.
Figure 11 shows a spectrum taken from the surface
deposits on the inlet side of the wear scar (similar
to those shown in Fig. 9(b)). The test fluid was
25 per cent w/w BS solution. The spectrum shows
intense absorbance bands at ~1650 and 1535 cm21
that are characteristic of protein amide I (n ) and
C=O
II (n , d ) band vibrations. The IR reflection
C–N N–H
spectra confirm the deposits are proteins; however,
the analysis can also provide insights as to the
molecular conformation. The amide I/amide II
absorbance ratio and the frequency of the amide 1
band have been used as indicators of protein con-
formation [19]. The relatively low frequency of the
amide I band suggests the ‘presence of packed
hydrophobic helical domains’ [19]. The use of IR
reflection spectra to examine the conformation and
composition of protein boundary films will be
Fig. 8 Images from the contact zone during sliding at
explored in a later paper.
different speeds for 25% w/w BS (field of view
1.2 mm30.96 mm)
5 DISCUSSION
formation of a wear scar, which can be seen in
5.1 Film formationmechanisms
Fig. 9(b). This was an extremely surprising result as
the head was rubbing against a glass disc covered The film thickness results demonstrated complex,
by a chromium/silica layer. Scratches were also time-dependent behaviour of the BS and protein
observed in the direction of sliding, these were solutions. At the start film thickness increased with
Proc.IMechEVol.225PartH:J.EngineeringinMedicine
704 JFan,CWMyant,RUnderwood,PMCann,andAHart
0.14
0.12
0.1 Outside
0.08
m) 0.06
μ
( 0.04
h
ht, 0.02
g
ei
H 0
-0.02
-0.04
-0.06 Inside
-0.08
0 50 100 150 200
Position along scan, X (μm)
Fig. 10 Interferometric roughness images of femoral head surface texture outside and inside the
wearscar
Fig.11 FTIRmicro-reflectionspectrafromthewearscardeposits,BSsolution
speed as expected from an EHL model [17]. typeofbehaviourobservedhere.Theresultssuggest
Subsequent speed curves showed significant devia- that protein aggregation and deposition cause com-
tion from the fluid film prediction. At present EHL plex film formation mechanisms. Film thickness
theories (to predict film thickness) are based on tended to increase particularly at low speeds; this
simple continuum fluids and do not account for the often resulted in an almost speed-independent film
Proc.IMechEVol.225PartH:J.EngineeringinMedicine
Inletproteinaggregation 705
over the speed range. The same film-building agglomerate can be large; for example, Santos et al.
behaviour was obtained with all test fluids, suggest- [22] reported hydrodynamic radii of 3 nm for native
ing it is characteristic of protein-containing solu- species and 110–160 nm for aggregates.
tions. Film formation at lower speeds is often The present results indicate that a ‘high-viscosity’
associated with the formation of a phase boundary layer augmented the residual film. At low speeds BS
in the inlet region. Clearly the film thickness distri- often forms much thicker films than expected,
bution shown in Figs 8(b) and (c) does not resem- although there is considerable fluctuation in the
ble a simple fluid film (see Fig. 2(a) for a classical results. This behaviour appears to be associated
elastohydrodynamic film pattern). with the formation of a new phase boundary in the
By analysing the film thickness–speed behaviour inlet, which is indicated by a change in refractive
it is possible to infer properties of the films and index (step change in interference contours). The
gain an insight into the formation mechanisms. new phase is attributed to a local concentration of
After completion of the sliding tests under static protein molecules forming a gel-like deposit. This
loaded conditions (U = 0, W = 5 N), the residual material passes through the contact in a non-
film between the surfaces was measured. Overall uniform manner causing the film thickness to fluc-
the residual films were in the range of 20–60 nm, tuate and is responsible for the scatter in the
although the thicker films were often observed to results. The new phase builds up locally in the inlet
decrease with prolonged loading. Earlier work [14] at slow speeds to form a reservoir of high-viscosity
also showed deposition of solid films in the contact; material. As the speed increases this tends to break
although these were thinner in the range of 10– down and as a result the film thickness often drops
20 nm. However these measurements were taken at higher speeds. The local concentration and depo-
after two speed sweeps, while in the current tests sition of protein molecules is thought to be due to
four speed sweeps were used. The nature of the shear-induced aggregation in the inlet region [22,
residual film formed in these tests was also differ- 25]. The residual film thickness will include this
ent. In the earlier work the film appeared to be ‘inlet agglomeration’ component (possibly as an
‘solid’ and did not decrease significantly with time entrapment) as the measurements were made at
under load. In the current study the authors the end of the slow speed cycle. It is unclear why
observed that for some measurements the thickness this mechanism breaks down at higher speeds; pos-
did drop slightly with time, suggesting a component sibly the protein agglomerations are swept around
of the film was high-viscosity material rather than the contact and do not contributeto film formation.
a solid adherent layer. This effect will be studied in However, as the MOM head/cup contact is highly
more detail in a subsequent paper. This observation conformal with a semi-contact width of ~5 mm
supports the earlier conclusion that the film is (50 mm diameter head, 100 mm clearance, 3500 N),
biphasic [14], comprising a thin adherent film over- in this case it is unlikely that protein agglomera-
laid by a much thicker, high-viscosity, loosely- tions can escape entrainment quite so easily.
bound component. The thin films appeared to be The main conclusion from this work is that the
securely bound to the surfaces and were not lubrication mechanism of protein solutions in slid-
removed by repeated loading. These are likely to be ing contacts is complex and not described by
composed of protein molecules adsorbed at the simplefluidconsiderations.Filmformationisdomi-
metal or glass surfaces to form multi-molecular natedbysurfacedeposition andshear-flowaggrega-
layers. The adsorption of protein molecules from tion of protein molecules. A solid film of 10–20 nm
flowing solutions at interfaces has been the subject is formed initially which the authors believe is due
of considerable research because it is of importance to adsorbance of protein molecules at the rubbing
in many industrial areas such as the food industry surface; this is augmented by a thicker, hydrody-
[22, 23]. A number of different models have been namically generated film of high-viscosity material.
proposed for the initial monolayer formation, Protein molecules are aggregated in the inlet shear
including reversible/irreversible Langmuir adsor- field and come out of solution to form gel-like
bance, protein exchange reactions, and molecular deposits in the inlet; this material adheres to the
conformational changes (see Introduction to refer- metal surface and periodically passes through the
ence [22]). The formation of a precursor monolayer contact forming amuchthickerhydrodynamic film.
might then be followed by multilayer formation [22, The formation of ‘gel-like’ deposits has been
24]. Proteins are also known to aggregate in solu- reportedinearlierstudies[1,26].Intheirstudyofthe
tion [22], under shear [22], at surfaces [21], and wear of ultrahigh-molecular-weight polyethylene,
after heat treatment [22, 23], and these structures Wang et al. [1] suggested that these deposited films
may be involved in the multilayer formation. The provided surface protection and soluble proteins did
Proc.IMechEVol.225PartH:J.EngineeringinMedicine