Vol.:(0123456789)
T ibology Le e s (2025) 73:15
h ps://doi.o g/10.1007/s11249-024-01950-5
ORIGINAL PAPER
E ec o Su ace Tex u ing onF ic ion andLub ica ion o Ti6Al4V
Bioma e ials o Join Implan s
Ma úšRanuša1· LukášOdehnal1· OndřejKuče a1· Da idNečas1· Ma inHa l1· I anKřupka1· Ma inV bka1
Recei ed: 6 Augus 2024 / Accep ed: 4 Decembe 2024 / Published online: 19 Decembe 2024
© The Au ho (s) 2024
Abs ac
The numbe o endop os he ic implan s o bo h la ge and small join s is inc easing a a s eady a e, he eby c ea ing a g owing
demand o du able p oduc s ha closely eplica e he unc ionali y o human join s. No wi hs anding he a o emen ioned
ad ancemen s, challenges pe aining o implan ixa ion and ibological su aces pe sis . The ad en o p og essi e echnolo-
gies, such as h ee-dimensional p in ing, o e s a p omising a enue o add essing hese challenges in implan design and
su ace enginee ing. The Ti6Al4V and CoC Mo alloys, enowned o hei biocompa ibili y and osseoin eg a ion p ope ies,
ep esen p omising p in able ma e ials, al hough hey a e suscep ible o wea on a icula ing su aces. In o de o mi iga e
he e ec s o ab asion, i is essen ial o implemen su ace ea men s o acili a e he o ma ion o a obus lub ica ing
ilm. This esea ch in es iga es he po en ial o ex u ing and elec ochemical polishing o enhance p o ein agg ega ion in
he con ac a ea. The s udy employs a ecip oca ing simula o and colo ime ic in e e ome y o obse e he con ac a ea
and measu e he coe icien o ic ion (CoF) o modi ied su aces. The indings demons a e ha ex u ed su aces and he
combina ion o elec ochemical polishing esul in an inc ease in he hickness o he p o ein lub ica ion ilm, which may
po en ially educe wea . These ou comes sugges he po en ial o he u iliza ion o Ti6Al4V alloy implan s wi h ewe ele-
men s manu ac u ed by addi i e echnology.
* Ma úš Ranuša
[email p o ec ed]
1 Bio ibology Resea ch G oup, Facul y o Mechanical
Enginee ing, B no Uni e si y o Technology, Technická
2896/2, 61669B no, CzechRepublic
T ibology Le e s (2025) 73:1515 Page 2 o 18
G aphical Abs ac
Glass pla e
Ligh sou ce
Ti6Al4V alloy
&
CoC Mo alloy
Ma e ials and Me hods
Film hickness &Coe icien o ic ion
Keywo ds Ti6Al4V· Mic o- ex u e· Implan · Op ical in e e ome y· F ic ion· Film hickness
1 In oduc ion
Join a h oplas y is a common su ge y wi h an inc easing
incidence. The ma e ials used in a h oplas y ha e a high
s anda d, wi h a good biocompa ibili y, and osseoin eg a ion
abili y. The opic o join wea emains a signi ican a ea o
esea ch, wi h he de elopmen o new ma e ials leading o
a educ ion in he amoun o loose ma e ial. [1]. Addi i e
Manu ac u ing (AM) ha e ad anced his ield by cus om-
izing he implan s o be e com o and a longe se ice
li e [2]. Ne e heless, he ad en o addi i e echnologies
has also in oduced a numbe o new challenges. One such
challenge is he iden i ica ion o he op imal manu ac u ing
pa ame e s o he p oduc ion o s uc u es sui able o osse-
oin eg a ion o he imp o emen o he ibological p ope -
ies o a icula ing su aces [3].
The longe i y o implan s is a ec ed by he ibological
beha io o he a icula ing su aces, and in e ac ion wi h
he coun e body su ace. A numbe o s udies in his ield
demons a e he in e play be ween indi idual su ace ma e-
ials and he complexi y o he molecula componen s o
he syno ial luid, which change wi h pa ien age [4]. The
basic ma e ials ha combine he ad an ages o biocompa -
ibili y and esis ance, while allowing he use o 3D p in -
ing by selec i e lase mel ing (SLM), a e ep esen ed by he
CoC Mo and Ti6Al4V alloys [5].
The possibili y o 3D p in ing o bo h ma e ials o e s he
ad an ages o cus omiza ion and os eoin eg a ion, bu b ings
new challenges a he same ime. One o he challenges is
he beha io o he p in ed a icula ing su ace compa ed
o con en ional p oduc ion. One o he di e ences is he
ensile s eng h. The SLM-p oduced CoC Mo alloys exhibi
a highe ul ima e ensile s eng h and ha dness compa ed o
hose made ia in es men cas ing. Fo ins ance, he SLM
samples can achie e yield s eng hs a ound 1.4 GPa, bu a
he same ime, hey end o ha e a highe wea a e compa ed
o he cas samples, which exhibi lowe wea a es due o
hei dense mic os uc u e [6, 7]. Howe e , in bo h cases,
he wea esis ance is ela i ely high, and bo h p oduc ion
echnologies exhibi su icien mechanical p ope ies and
co osion esis ance [8]. Fu he mo e, he o ma ion o a
passi e oxide ilm on he me al su ace in he human body
also con ibu es o a be e wea esis ance o he CoC Mo
implan s [9]. This oxide laye is mainly composed o cobal ,
T ibology Le e s (2025) 73:15 Page 3 o 18 15
ch omium, and molybdenum oxides. Among hese, C 2O3
inhibi s bo h he anodic and ca hodic eac ions. I ac s as a
physical ba ie , limi ing he anspo o ca ions and anions
o he me al su ace, and se es as an elec onic ba ie o
elec ons [10, 11]. In he case o he Ti6Al4V alloys, he
con en ional me hods p oduce coa se g ains, which can
hinde disloca ion mo emen , leading o a lowe s eng h
compa ed o he 3D-p in ed su aces. The 3D-p in ed
Ti6Al4V alloys can achie e s eng hs up o 1492.89MPa,
al hough wi h a educed elonga ion (5.76%) due o mic o-
s uc u al de ec s [12]. Ti6Al4V is equen ly used in onco-
logical implan s due o i s mechanical p ope ies, which a e
simila o hose o a human bone. This minimizes he s ess
shielding and p omo es he osseoin eg a ion [13]. Howe e ,
he alloy is suscep ible o a high ab asion on he a icula ing
su aces. This issue can be po en ially esol ed by a ious
su ace modi ica ions, such as a coa ing o mic o- ex u ing.
Bo h coa ing and mic o- ex u ing can enhance he mechani-
cal p ope ies and he du abili y o he implan s. Howe e ,
he coa ing echniques may su e om an ins abili y o he
coa ing laye . I is, he e o e, a challenging ask o de e mine
he mos app op ia e manu ac u ing echnique o su ace
ea men and, a he same ime, o design he op imal ex u e
geome y o mul i-le el ex u e wi h espec o he kinema -
ics o he selec ed join . Mic o- ex u ing has he po en ial o
modi y he beha io o he lub ica ion laye , he eby enhanc-
ing he sus ainabili y o he pe o mance, while allowing
he use o di e en ma e ials [14, 15]. Modi ied ma e ials
equen ly exhibi no only enhanced du abili y bu also he
p esence o highly c oss-linked polye hylenes, which a e
o en doped wi h a ange o subs ances, including he E
i amin. This doping p ocess augmen s he ma e ial's ic-
ional esis ance while simul aneously s abilizing he ee
adicals wi hin he s uc u al ma ix. This dual e ec esul s
in a no able enhancemen in he ma e ial's esis ance o he
oxida ion and delamina ion a he su ace [16, 17].
The ex u ing o su aces has been demons a ed o exe
a bene icial in luence on he ibological p ope ies, esul -
ing in an augmen ed hyd odynamic p essu e and diminished
su ace wea . Howe e , he e icacy o his app oach emains
a opic o con en ion, pa icula ly in he con ex o non-con-
o mal con ac s [18]. In con as , o he con o mal con ac s,
i esul s in he sepa a ion o he con ac su aces by a hicke
laye o p o ein-con aining lub ican , which al e s he lub i-
ca ion egime and educes he wea . Simul aneously, he ex-
u es se e as lub ican ese oi s, acili a ing he desi ed
sepa a ion o he a icula ing su aces [19, 20]. Addi ion-
ally, he ex u es can cap u e and emo e wea pa icles om
he con ac a ea. Asep ic wea pa icles a e p oduced as a
esul o he ab asi e wea o he ma e ials, p ima ily in he
bounda y and mixed lub ica ion egimes. Ca e ully chosen
ex u es can e icien ly emo e he pa icles om he con ac
a ea o e ain hem wi hin he ex u e, he eby enabling he
a icula ing su aces o main ain a smoo h su ace opog a-
phy o longe pe iods in i o [21, 22].
Se e al s udies ha e in es iga ed he e ec o he mic o-
ex u ing on he coe icien o ic ion [23–25]. Some o
hese s udies sugges an inc ease in ic ion, which is mainly
caused by he beha io o he p o ein componen s o he syn-
o ial luid as i passes h ough he con ac . This beha io is
due o he shea s ess o he agg ega ed p o eins adhe ing o
he su ace (γ-globulin) o o he u he laye ing o he albu-
min wi h al eady a lowe shea . Ne e heless, i is impo an
o supplemen he gi en conclusions wi h kinema ic condi-
ions, which ha e a signi ican in luence on he beha io o
he lub ica ing laye and he design o he ex u e [26–28].
The po en ial applica ions o mic o- ex u ing ex end o
a ange o implan s, wi h he possibili y o i becoming a
s anda d ea men o all a icula ing su aces in he u u e.
Howe e , he cu en ocus is mainly on small join eplace-
men s, whe e he numbe o su ge ies is s eadily inc easing
yea on yea [29]. De elopmen s in his ield o en p o ide
solu ions ha do no e lec cu en ends in endop os he ics
and eplace hem wi h p o en p ocedu es ha a e o en a he
expense o pa ien com o . A clea example is he me a a -
sophalangeal join , which is o en a ec ed by hallux algus
and hallux igidus. In such cases, wo ea men op ions a e
simul aneously o e ed o he pa ien . One is a h odesis,
which esul s in he loss o join unc ionali y. The o he
op ion is a mobile eplacemen o he a ec ed join . Despi e
a numbe o disad an ages, a h odesis is he mos common
solu ion due o i s simplici y and eliabili y. This is because
i does no con ain in e locking mo ing pa s [30]. These
s a is ics also indica e he necessi y o u he de elopmen
in he ield o unc ional MTP eplacemen s in o de o make
hem he su geons' p e e ed solu ion due o hei eliabili y.
Mic o- ex u ing is la gely dependen on he load and he
a e o mo emen o he in e ac ing su aces. As o load-
ing, he con ac con o mi y, con ac p essu e and ma e ial
a e impo an . Mic o- ex u ing on ha d ma e ials such as
he CoC Mo o Ti6Al4V is especially p omising o he
small join implan s, whe e he a icula ing su aces a e
less exposed o con ac p essu e and anges o mo ion. A
s udy by She e e al. [31] desc ibed he kinema ics o he
me a a sophalangeal join o pa ien s wi h a join disabil-
i y. The a e age o al ange o mo ion in he sagi al plane
was 111°, wi h app oxima ely 76° o do sal lexion and 34°
o plan a lexion. The implan a emp s o main ain a ull
ange o mo ion, which has a signi ican impac on a pe -
son's s abili y and p ope oo unc ion. Zhang e al. [32]
conduc ed a nume ical s udy on he p essu e dis ibu ion.
They ound ha inc ease in he con ac p essu e o pa ien s
a ec ed by he hallux algus disease, which also led o a
highe isk o he join damage. The p essu e o he no mal
join is 1.53MPa and a ec ed join 2.21MPa. In he case o
he Mo gan e al. esea ch [33], he con ac p essu es we e
T ibology Le e s (2025) 73:1515 Page 4 o 18
highe . In he case o he cada e es s, he alues we e in
excess o 30MPa, wi h nume ical simula ions showing al-
ues as low as 10MPa in he 200–230MPa load ange. The
alues we e signi ican ly highe o join implan s, depend-
ing on he ma e ial used. The p essu e on he implan was
ecalcula ed in ela ion o he geome y o he pai , based on
he p edic ions om he heal hy join [25, 34].
The CoC Mo alloy shows a good ab asion esis ance.
Howe e , since i s mechanical p ope ies di e signi ican ly
om hose o a human bone, i is suscep ible o iboco o-
sion and asep ic loosening. Wang e al. [35] compa ed he
CoC Mo alloys wi h he Ti alloys, speci ically in e ms o
he iboco osion. The s udy concluded ha he Ti alloy
was be e al e na i e o he CoC Mo alloy, due o i s lowe
wea and po en ial o he heal h haza dous ions, such as
Co(III) and C (VI). The Ti6Al4V exhibi s a high wea a e
[36, 37], which can be esol ed by su ace modi ica ions o
modi ica ions ha c ea e a lub ica ing ilm o sepa a e he
join su aces.
1.1 P oduc ion andImpac o  heMic o‑ ex u es
Mic o- ex u es a e equen ly used in he a h oplas y o
la ge join s as hey o e a be e su ace wea esis ance and
lub ica ion compa ed o he smoo h su aces [36, 38–40].
Fo small eplacemen s, he ex u ing has an e en g ea e
po en ial due o lowe con ac p essu es.
The mos used ex u e shape is a ci cula o ounded
dimple o ien ed in he slip di ec ion. Con e sely, al e na-
i e ex u e shapes ha e been iden i ied ha demons a e he
po en ial o educe he coe icien o ic ion (CoF) e en u -
he . Howe e , he ques ion o he e iciency o he p oduc-
ion o hese shapes and hei applicabili y in he con ex o
mo e complex kinema ics and in combina ion wi h di e en
ma e ials emains unanswe ed [41].
The basic ex u e pa ame e is he a io o dep h (hp) o
diame e (dp), which is c i ical o he wea pe o mance.
Co ec adjus men o his a io (ε) can esul in a educ-
ion in he CoF o up o 30% due o locally a ying hyd o-
dynamic p essu es [42]. In gene al, small dep h ex u es
(a ound 1μm) can inc ease he CoF and wea a low loads
due o insu icien hyd odynamic p essu e gene a ion. How-
e e , his s a emen does no apply o he use o a p o ein
lub ica ing ilm due o he o ma ion o p o ein clus e s du -
ing he con ac passage, and he e o e, an inc ease in he
CoF does no necessa ily imply wo se esul s in e ms o he
long- e m wea . In he syno ial join en i onmen , deepe
ex u es (1–15μm) appea op imal as hey bo h inc ease
he hyd odynamic p essu e and ac as lub ican ese oi s
[43]. Despi e he posi i e e ec o he ex u e on he lub ic-
i y, a nega i e e ec is possible as well, especially i he
load exceeds op imum h esholds and he lub ica ion limi is
eached [44]. Ano he c i ical pa ame e is he combina ion
o he ε a io wi h he ex u e, su ace co e age densi y (Sp),
and shape, which de e mine he e ec i eness in he di e en
lub ica ion egimes du ing he implan cycle [45, 46].
I is expec ed ha he ic ion educ ion will be mo e
p onounced o shallow dimples (3–10μm) ha ing smalle
diame e s (100–200μm) [47]. The e ec o su ace ex u es
on ic ion will be u he d i en mainly by he pa ame e s
o he ex u es. Depending on he aspec a io ε, he ex-
u es will ei he p o ide an enhanced hyd odynamic e ec
(ε < 0.1) o se e as a lub ican ese oi (ε > 0.1). Se e al
s udies sugges ha an op imal a io alue is 0.1 o lowe ,
depending on he ma e ial used [48]. Howe e , he conclu-
sions ega ding he co e age densi y a e no clea . Some
s udies p o ed ha ibological p ope ies can imp o e wi h
an inc easing ex u e densi y, while o he s showed he oppo-
si e end, wi h imp o emen occu ing wi h a dec easing
densi y [49]. Qiu e al. [50] conduc ed expe imen s on he
con o mal con ac sys em unde he bounda y lub ica ion
condi ions. They in es iga ed h ee ex u e densi ies o 26%,
41%, and 58% a an ε a io o 0.1. The esul s showed ha
he lowes ic ion coe icien was achie ed a 58%. Li e al.
[51] in es iga ed he e ec o h ee di e en densi ies o
hemisphe ical pi s (5%, 13%, and 35%) a he ε a io o 0.01.
They ound ha he lowes ic ion coe icien was achie ed
wi h a densi y o 13%. Simila ly, Raeymaeke s e al. [52]
ound ha densi ies o app oxima ely 15% and ε a ios in
he ange o 0.1 o 0.3 p oduced he bes esul s. Zhang
e al. [53] p oposed a pa elloid ex u e p oduced by a pulsed
lase abla ion. Pin-on-pla e es s demons a ed a long- e m
dec ease in he coe icien o ic ion, esul ing in a educ ion
o he wea a e in he hyd odynamic lub ica ion egime.
The lub ica ion p ope ies a e in luenced no only by
he densi y o he mic o- ex u e co e age bu also by he
a angemen o he ex u e. The ex u es a e a anged
acco ding o he di ec ion o mo emen , wi h longi udinal,
ans e se, o oblique o ien a ion. The ex u es a e a anged
mainly in a squa e, iangula (hexagonal), o a andom pa -
e n. Choudhu y e al. [54] in es iga ed he dis ibu ion o
he mic o- ex u es in squa e, ci cula , and iangula a ays
on he hip eplacemen s. They concluded ha he squa e
a angemen p o ided he bes ibological p ope ies.
Acco ding o B aun e al. [55], he use o he ci cula ex-
u es o a sui able size in a iangula a angemen can lead
o a educ ion in he ic ion o up o 80%. Schneide e al.
[48] concluded ha a pi ing aspec a io o 10% esul s in a
educ ion in ic ion o 0.1. A iangula pa e n is supe io
o a squa e a angemen , p o ided ha he ex u e design is
o high quali y.
The ex u ing echnology has a majo in luence on he
o e all unc ion o he ex u e. Among mic o-machining
illing me hod [56], lase machining is he mos common
echnique. Howe e , i has a d awback: i o ms sha p co -
ne s ha ac as s ess concen a o s. This leads o wo-body
T ibology Le e s (2025) 73:15 Page 5 o 18 15
ab asi e wea , whe e he sha p ims cause mic o-sc a ches
on he coun e -su ace, inc easing he coe icien o ic ion.
To mi iga e his issue, e o s a e made o ound he ims and
educe hei nega i e impac . This ex discusses di e en
me hods o he su ace modi ica ion o ough su ace p epa-
a ion in i anium implan s. The me hods a e based on he
mechanical, he mal, chemical, elec ochemical, and lase
echniques. I should be no ed ha hese me hods do no
only emo e he ims caused by he lase machining bu also
modi y he o e all su ace [57, 58]. O e he pas decade,
he pa en ed DLy e echnology has ga ne ed a signi ican
a en ion. Unlike he con en ional echnologies, he DLy e
only smoo hs he peaks o he oughness, no he alleys,
h ough a selec i e su ace smoo hing. The DLy e ope a es
on he p inciple o an ion anspo by ee solids, which is a
combina ion o an elec ical low and a pa icle mo emen
h ough an elec oly ic medium [59–61].
1.2 Aim o  heS udy
Addi i e me hods oge he wi h he mic o- ex u ing and he
DLy e echnology ha e he po en ial o in oduce a cus om-
ized, on-o de manu ac u ing o he implan s. Howe e , i
is c ucial o in es iga e close he beha io o he speci ic
alloys ha a e sui able o addi i e p oduc ion, such as he
CoC Mo and Ti6Al4V alloys, in join implan simula ions.
This s udy in es iga es he beha io o syno ial luid in
he con ac egion o ex u ed specimens made o Ti6Al4V
alloy using colo ime ic in e e ome y and ic ion coe -
icien . Colo ime ic in e e ome y was p o en o b ing an
essen ial insigh in o assessing lub ica ion mechanisms in
hip eplacemen s be o e [62]. Subsequen ly, he esul s a e
compa ed wi h hose o a con en ional CoC Mo alloy. In
addi ion, a en ion is paid o he DLy e me hod, which has
he po en ial o modi y he inal su ace in e ms o local
i egula i ies.
2 Ma e ials andMe hods
2.1 Appa a us
The analysis o he lub ica ion ilm o ma ion and he ic-
ion was conduc ed using a ecip oca ing mo ion simu-
la o wi h a pin-on-pla e con igu a ion. This allowed o
simul aneous obse a ion o he con ac and insi u ic ion
measu emen s. Figu e1 displays he design o he de ice,
which was modi ied om he one used by Čípek e al. o
ca ilage analysis [63, 64]. The de ice was equipped wi h a
Fig. 1 Schema ic o he measu ing appa a us in a pin-on-pla e con igu a ion

T ibology Le e s (2025) 73:1515 Page 6 o 18
colo ime ic in e e ome y appa a us o obse e he hick-
ness o he lub ican ilm. The simula o ba h was hea ed
o a empe a u e o 37°C o simula e he human body
en i onmen . A glass pla e was moun ed on a mo able
ca iage, which pe o med a ecip oca ing mo ion, while
he specimen emained s a iona y. The o ma ion o he
lub ica ion ilm was obse ed using an op ical imaging
sys em ha included a mic oscope, a halogen ligh sou ce,
a CMOS digi al high-speed came a (Phan om 710), and
a PC. When he me al pin and he glass pla e came in o a
con ac and we e illumina ed, he colo New on ings we e
obse ed. Ha l e al. used a hin ilm colo ime ic in e -
e ome y o e alua e he ilm hickness [65]. In ou s udy,
he con ac su ace o he glass pla e was coa ed wi h a
semi- e lec i e ch omium laye o inc ease he con as o
he in e e ence inges. The ilm hickness e alua ion was
based on h ee s eps:
(1) The calib a ion cu es we e ob ained om an in e -
e og am o a ligh ly loaded s a ic con ac , which was
hen ma ched wi h he measu ed con ac p o ile. This
p o ided in o ma ion abou he ela ionship be ween
he colo and he ilm hickness.
(2) In e e og ams o a ully loaded con ac du ing he
ansla ion o he ca iage we e cap u ed using a high-
speed came a (Phan om V710, Vision Resea ch, USA).
(3) The hickness a any a bi a y loca ion o he con ac
could be de e mined by ma ching he cap u ed in e -
e og ams wi h he calib a ion cu es.
2.2 Samples
A sample o an op ical glass BK270 wi h dimensions o
155 × 44 × 4mm was used as a pla e. One side o he glass
was coa ed wi h a semi- e lec i e ch omium laye and he
o he wi h an an i- e lec i e laye . The expe imen al pins
we e made o wo main ma e ials used in he implan ology:
he medical g ade cas CoC Mo alloy (ASTM-F75) and he
Ti6Al4V alloy (ISO 5832-3).
The CoC Mo pin was manu ac u ed om a cold-d awn
ba , cu and u ned unde he same cu ing condi ions as he
con en ional join eplacemen implan s. The adius o he
head o he pin was R100 and he diame e o he pin was
9.7mm. The ball su ace was u he polished o he equi ed
minimum oughness Rq o 0.012 ± 0.005μm. The Ti6Al4V
pin was manu ac u ed om a 10-mm-diame e cold-d awn
ba and hen machined and polished o achie e he same
geome y and su ace inish as he CoC Mo pin.
The su ace geome y o he samples was e alua ed
be o e he expe imen s. An op ical p o ilome e based on
phase-shi ing in e e ome y (B uke , Con ou GT-X8) was
used o analyze he su ace oughness (Rq) in he con ac
a ea wi h dimensions o 1 × 1.2mm co e ing a heo e ically
calcula ed ci cula con ac a ea wi h adii o 0.06mm.
The dis ibu ion o he mic o- ex u es was chosen o be
iangula , wi h he basic elemen o his la ice being an
equila e al iangle o side leng h 42.7µm. The shape o he
mic o- ex u e i sel was ci cula , while he bo om o he
ex u e was la as a as possible (Fig.2). This ex u e shape
Fig. 2 Dis ibu ion and geome y o he ex u es on he su ace o he samples
T ibology Le e s (2025) 73:15 Page 7 o 18 15
was chosen because i is easy o ab ica e wi h a picosecond
lase while main aining he geome ic epea abili y a each
pi . The ci cula shape also gi es us he ad an age o he
consis ency o he ilm beha io e en wi h changes in he
di ec ion o he mo ion, which is expec ed due o i s use in
an a icula ing implan . A schema ic o he a angemen and
shape o he ex u e is shown in Fig.2. Based on he esul s
o he li e a u e sea ch, he mos e ec i e co e age densi y
Sp o 15% was chosen in e ms o he lub ican ilm o ma-
ion, conside ing he numbe o ex u es in con ac wi h he
ma e ials a ull load. The condi ion was he pa icipa ion o
a leas 5 ex u es in he con ac a ea, which is ensu ed in he
gi en con igu a ion. The chosen a iable pa ame e is he
a io ε. In o de o a y he alue in he ange o 0.01–0.2,
which is gi en in he li e a u e as he mos e ec i e alue
o he gi en co e age, he only a iable pa ame e le is
he ex u e dep h dp. Fo his eason, 5 ex u e pa ame e s
we e chosen (dp 0.4–6μm). The dis ance be ween he cen -
e s o he dimples was 74μm, and he adii o he dimples
we e 15μm.
The ex u es we e c ea ed using a lase mic omachining
wi h a picosecond lase [Pe la 100 (Hilase)] o minimize
he he mal impac on he su ace. The lase ope a ed a a
wa eleng h o 1030nm, a pulse leng h o 1ps, a epe i ion
a e o 60kHz, and a maximum pulse ene gy o 1mJ. The
lase beam was guided h ough a ha monic equency a enu-
a o and an op ical combine . The lase beam was guided
h ough a sys em o mi o s o an In elliscan 14 (Scanlab)
scanning head, which was placed on a sliding z-axis s age.
The ex u es we e ab ica ed a an a e age powe o 72 mW,
mo ing he lase beam along a spi al ajec o y a speeds up
o 400mm/s. Ten samples we e c ea ed o each ma e ial,
wi h each ex u e (0.4–6 dep hs). The DLy e me hod was
used o emo e he sha p ims ha we e p oduced du ing
he p ocess on he ex u e. The CoC Mo pins we e polished
using CoC DLy e MIX MSA-S H FOR S100 elec oly e,
while Ti DLy e MIX MSA PLUS-S elec oly e was used o
polishing i anium samples.
2.3 Lub ican
The model solu ion comp ised bo ine se um albumin (BSA,
Sigma Ald ich A7030, Da ms ad , Ge many), bo ine se um
y-globulin (BSG, Sigma Ald ich G5009, Da ms ad , Ge -
many), hyalu onic acid wi h a molecula weigh o 1000kDa
(HA), and phospholipids. The cons i uen s we e added o
a phospha e-bu e ed saline (PBS). Concen a ions co e-
sponded o he composi ion o he syno ial luid o pa ien s
a e a o al join a h oplas y (albumin 26.3mg/ml, y-glob-
ulin 8.2mg/ml, Phospholipids 0.35mg/ml, Hyalu onic acid
0.82mg/ml) [66]. The lub ican s we e hawed p io o he
es ing and s o ed in a e ige a o o ensu e a comple e p o-
ein dissolu ion in he PBS. Each expe imen used a o al
olume o 14ml o lub ican and was conduc ed unde ully
looded condi ions o a oid con ac s a a ion.
2.4 Expe imen al Design
The kinema ic condi ions o he expe imen we e de i ed
om he kinema ics o he i s me a a sophalangeal (MTP)
join . Acco ding o Du an e al.'s s udy [67], he MTP join
unde goes a 65° o a ion. The angula alues o he ini ial
and he inal declina ion di e acco ding o he physiology,
gai ype, and join damage o each pe son. Howe e , an
angle o 65° ep esen s he mean alue o an a e age pe son
[39, 40]. Fo he MTP join implan s, a head and a socke
wi h a adius o R100 we e used. The a c pa h leng h was
calcula ed as he p oduc o he adius and angle in adians,
esul ing in an app oxima e leng h o 11.3mm. Howe e ,
due o he ibome e condi ions, he leng h was inc eased o
20mm. Each cycle in ol ed a back-and- o h mo ion, esul -
ing in a o al cycle leng h o 40mm. The equency o he
mo emen was 0.5Hz, esul ing in a speed o 20mm/s o
a 40mm pa h. The kinema ic condi ions o he expe imen
a e summa ized in Table1. The load o 0.5 N was applied
o bo h mic o- ex u es. Fo he CoC Mo compa ed o glass,
he con ac a ea diame e was 180μm and he con ac p es-
su e was 29.6MPa. Fo he Ti6Al4V compa ed o glass, he
con ac a ea diame e was 191μm and he con ac p essu e
was 26.3MPa. The con ac a ea diame e and p essu e we e
calcula ed using he con ac He z heo y.
The o e all design o he expe imen was de ised wi h
he objec i e o obse ing phenomena in he con ac egion,
o gain some undamen al insigh s and compa isons. In his
con ex , se e al simpli ica ions we e made o he in i o
condi ions in compa ison o he eal in i o condi ions in
he MTP join . The p ima y limi a ion is he non-con o mal
su ace and he associa ed inc ease in con ac p essu e o a
highe ange o alues ha occu a he join s. Concu en ly,
he al e a ion in con ac p essu e esul s in he eplacemen
o he a icula ing pai wi h a anspa en glass wi h a lowe
Young's modulus han he Ti6Al4V alloy. These di e ences
can a ec he hickness o he lub ica ing ilm, which can
ul ima ely ha e a nega i e e ec on he o e all wea . The e-
o e, hese limi a ions mus be conside ed in any wea e alu-
a ion and long- e m wea es s [68, 69].
Fo each ex u e, including he e e ence wi hou ex u e
(six samples), a single measu emen was conduc ed, com-
p ising h ee consecu i e cycles. A schema ic o he expe i-
men is p esen ed in Fig.3.
2.5 Da a P ocessing
The ibological beha io o he lub ican and he implan
ma e ials was e alua ed based on wo main pa ame e s:
he coe icien o ic ion (CoF) and he ilm hickness.
T ibology Le e s (2025) 73:1515 Page 8 o 18
The coe icien o ic ion was measu ed in h ee con-
secu i e cycles, which we e hen combined in o a single
g aph. The discon inui y o he measu emen s in o indi-
idual cycles was necessa y due o he memo y capaci y
o he high-speed came a. The sampling equency was
50Hz, esul ing in he ic ion coe icien alues ob ained
o ime pe iods o 0.02s. Fil e ing and da a p ocessing
we e conduc ed using MATLAB so wa e. The da a we e
il e ed based on a 5% de ia ion om he mean eloc-
i y o emo e he ou lie s and he da a om he mo ion
change sec ions whe e he null eloci y was eached. The
ilm hickness was quan i ied using he colo ime ic in e -
e ome y. The undamen al p inciple o his me hod is
he op ical eco ding o he con ac a ea, cap u ed a a
a e o 100 ames/s. The ames we e ex ac ed om he
scanned da a a egula in e als. The glass pa o he
simula o was mo ed a a cons an speed along he pin.
This esul ed in 20 ames o e alua ion a 20 cycles. The
in e e og ams we e e alua ed using a cus om so wa e
[65]. In each o he 20 ames, h ee e e ence loca ions in
he con ac a ea we e selec ed, and he a e age hickness
o he lub ica ing ilm was de e mined. The inal alue o
one cycle was calcula ed as an a i hme ic mean o hese
h ee alues. Fo each measu emen , a o al o 20 a e age
hicknesses we e ob ained, esul ing in 60 alues in h ee
cycles o each ma e ial pai es ed.
3 Resul s andDiscussion
The esul s p esen ed conce n he mic o- ex u ed su ace
and i s in luence on he coe icien o ic ion, wi h he
objec i e o desc ibing he lub ica ion egime using he
obse ed ilm hickness in he con ac a ea. The main
con ibu ion is he use o Ti6Al4V alloy, which has he
po en ial o u u e use in he ab ica ion o indi idualized
implan s using addi i e manu ac u ing. The esea ch is
mainly conce ned wi h he desc ip ion o he ibologi-
cal p ocesses in he con ac egion, wi h he objec i e o
p epa ing he basis o u he necessa y su ace ea -
men s imp o ing wea esis ance. The CoC Mo alloy was
employed as a e e ence, enabling esul s compa ison.
The a icula ing su ace was hen augmen ed wi h a ange
o mic o- ex u es, which we e subsequen ly modi ied by
elec ochemical machining. This allowed o he in es iga-
ion o he impac o hese modi ica ions on he lub ica-
ion p ocesses in he con ac a ea. The a ionale behind
modi ying he su ace in his manne is o iden i y op imal
condi ions o he o ma ion o a ilm capable o sepa a ing
he a icula ing su aces, he eby educing he quan i y o
loose ab asi e pa icles.
Table 1 Expe imen al condi ions (A) sample cha ac e is ics, (B) kinema ic condi ions
(A)
Samples Ma e ial Semi- inished
p oduc
Young’s modulus
(GPa)
Poisson’s a ios Con ac p essu e
pin on pla e
(MPa)
Pin ( adii 100mm) Ti6Al4V alloy Ba 114 0.34 26.3
CoC Mo alloy Ba 230 0.28 29.6
Pla e Bo osilica e glass B270 62 0.22 –
Albumin γ-Globulin Phospholipids Hyalu onic acid
26.3 8.2 0.35 0.82
(B)
Load (N) Veloci y (mm/s) S oke (mm) To al dis ance (mm) Numbe o cycles Du a ion (s) Tempe a u e (°C)
0.5 20 20 2400 60 120 37
Fig. 3 Expe imen al se -up
T ibology Le e s (2025) 73:15 Page 9 o 18 15
3.1 Su ace Topog aphy
In he ini ial s age o he s udy, he a icula ing su aces
o he samples we e e alua ed wi hou ex u e, and subse-
quen ly, a e ex u e p oduc ion and elec ochemical ea -
men wi h DLy e. The ini ial su ace was polished o achie e
a oughness alue below 10nm (wi hou ex u e), which
mee s he ISO 7206-2 s anda d o a icula ing su aces o
me allic implan s [70]. The su ace i sel was also e alua ed
in e ms o i s o e all geome y, wi h espec o he pos-
sible in luence on he con ac a ea and, hus he con ac
p essu e o be induced [71]. O e all, a de ia ion o up o
10% o he equi ed nominal adius R100 was accep ed. This
de ia ion was based on he accu acy o he measu emen s
hemsel es and he manu ac u ing capabili ies. I was also
conside ed ha he specimens we e manu ac u ed using he
same me hod as commonly used implan s, which gua an-
eed he same manu ac u ing de ia ions and su ace quali y
achie ed in endop os he ics. Howe e , an issue a ose in he
e alua ion o he o e all su ace wa iness, whe e i egula i-
ies we e mo e equen , which in luenced he shape o he
con ac a ea. Based on hese condi ions, he specimens ha
demons a ed he mos a o able esul s in he analysis o he
con ac a ea we e selec ed. The samples we e hen ex u ed
wi h i e di e en ex u e dep hs o he same dis ibu ion and
diame e using a picosecond lase . Lase su ace ex u ing
p ocess has a he mal abla ion p ocess, due o hese high
empe a u es a e encoun e ed a ound he dimple and cause
mic os uc u al changes, esidual he mal s esses, im o -
ma ion in he lase -i adia ed zone and subs a e [72]. As he
dep h o he ex u e inc eased, he im o dimples ela i e o
he e e ence su ace also inc eased. This was caused by he
g ea e ma e ial emo al and ma e ial mel ing a he im o
he ex u e. Some la ge ims (270–380nm) a ound he dim-
ples (2–6µm) we e obse ed o he Ti6Al4V alloy (Fig.4C,
D). Fo he CoC Mo, he ims we e signi ican ly lowe , he
maximum im heigh s we e up o 270nm. To elimina e
hese inequali ies, he elec ochemical me hod DLy e has
p o en and led o a ela i e dec ease in he ims o bo h
ma e ials. Howe e , despi e he emo al o he ims, his
me hod p o ed o be unsui able o he CoC Mo alloy. E en
a sho exposu e wi h he DLy e me hod (0.4s) esul ed
in he emo al o ims, a he same ime i led o damage
o he es o he su ace, se e ely damaging he obse ed
a ea and making u he e alua ion o he esul s impos-
sible. This phenomenon can be obse ed in he plo o o al
oughness (Fig.4A, B), whe e no able disc epancies we e
obse ed in he CoC Mo alloy. Se e al s udies ound ha
Co–C –Mo alloys ha e a dend i ic su ace s uc u e wi h
a 1 o 4nm hick oxide laye , p o iding a co osion ba ie
in he human body. Co osion ba ie can a ec p ocesses
du ing elec ochemical machining, which p e en s us om
c ea ing an ideal geome y wi h a uni o m con ac su ace.
Fo his eason, he esul s om colo ime ic in e e ome y
Fig. 4 A e age su ace ough-
ness wi h masked ex u e: A
Ti6Al4V, B CoC Mo, im
heigh o ex u es o Ti6Al4V:
C be o e DLy e, D a e DLy e
T ibology Le e s (2025) 73:1515 Page 16 o 18
and analyzed he expe imen s and w o e he o iginal d a o he man-
usc ip . D. Nečas, M. Ha l and I. Křupka supe ised he s udy. M.
V bka supe ised and inanced he s udy.
Funding This esea ch was ca ied ou unde he P ojec “F ic-
ion and lub ica ion o small join implan s p oduced by 3D me al
p in ing addi i e echnology” unded by he Czech Science Foun-
da ion, No. 22-02154S and by he P ojec “Mechanical Enginee -
ing o Biological and Bio-inspi ed Sys ems,” unded as P ojec No.
CZ.02.01.01/00/22_008/0004634 by P og amme Johannes Amos
Commenius, Call Excellen Resea ch, adminis e ed by he Minis y
o Educa ion, Spo s and You h. Also, hanks o P oSpon, spol. s . o.
company o he p epa a ion o he samples.
Da a A ailabili y The da a ha suppo he indings o his s udy a e
openly a ailable in eposi o y Zenodo a h p:// doi. o g/h ps:// doi. o g/
10. 5281/ zenodo. 13235 495.
Decla a ions
Con lic o in e es The au ho s decla e no con lic o in e es .
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