

Ci a ion: Delgado, G.F.; Pinho, A.C.;
Piedade, A.P. 3D P in ing o
Ca ilage Replacemen : A
P elimina y S udy o Explo e New
Polyme s. Polyme s 2022,14, 1044.
h ps://doi.o g/10.3390/
polym14051044
Academic Edi o : And ea Eh mann
Recei ed: 5 Feb ua y 2022
Accep ed: 2 Ma ch 2022
Published: 5 Ma ch 2022
Publishe ’s No e: MDPI s ays neu al
wi h ega d o ju isdic ional claims in
published maps and ins i u ional a il-
ia ions.
Copy igh : © 2022 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
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dis ibu ed unde he e ms and
condi ions o he C ea i e Commons
A ibu ion (CC BY) license (h ps://
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4.0/).
polyme s
A icle
3D P in ing o Ca ilage Replacemen : A P elimina y S udy o
Explo e New Polyme s
Gonçalo F. Delgado, Ana C. Pinho and Ana P. Piedade *
Depa men o Mechanical Enginee ing, CEMMPRE, Uni e si y o Coimb a, 3030-788 Coimb a, Po ugal;
[email p o ec ed] (G.F.D.); [email p o ec ed] (A.C.P.)
*Co espondence: [email p o ec ed]; Tel.: +351-239-790-700
Abs ac :
The use o addi i e manu ac u ing echnologies o biomedical applica ions mus begin
wi h he knowledge o he ma e ial o be used, by en isaging a e y speci ic applica ion a he han a
mo e gene al aim. In his wo k, he p elimina y s udy was ocused on conside ing he ca ilaginous
issue. This biological issue exhibi s di e en cha ac e is ics, such as hickness and mechanical
p ope ies, depending on i s speci ic unc ion in he body. Due o he lack o ascula iza ion, ca ilage
is a suppo ing connec i e issue wi h limi ed capaci y o eco e y and egene a ion. Fo his eason,
any app oach, whe he o epai / egene a e o as a o al eplacemen , needs o ul ill he adequa e
mechanical and chemical p ope ies o he su ounding na i e ca ilage o be success ul. This wo k
aims o explo e he possibili y o using new polyme s o ca ilage o al eplacemen app oaches wi h
polyme ic ma e ials p ocessed wi h he speci ic 3D p in ing echnique o used ilamen ab ica ion
(FFF). The ma e ials s udied we e Nylon
®
12 (PA12), al eady desc ibed o his pu pose, and LAY-
FOMM
®
60 (FOMM). FOMM has no been desc ibed in he li e a u e o biomedical pu poses.
The e o e, he chemical, he mal, swelling capaci y, and mechanical p ope ies o he ilamen s we e
ho oughly cha ac e ized o be e unde s and he s uc u e–p ope ies–applica ion ela ionships
o his new polyme . In addi ion, as he FFF echnology is empe a u e based, he p ope ies we e
also e alua ed in he p in ed specimens. Due o he en isaged applica ion, he specimens we e also
cha ac e ized in he we s a e. When compa ing he ob ained esul s wi h he p ope ies o na i e
ca ilage, i was possible o conclude ha : (i) PA12 exhibi s low swelling capaci y, while FOMM,
in i s d y and we o ms, has a highe swelling capaci y, close o ha o na i e ca ilage; (ii) he
mechanical p ope ies o he polyme ic ma e ials, especially PA12, a e highe han hose o na i e
ca ilage; and (iii) om he mechanical p ope ies e alua ed by ul a-mic o ha dness es s, he alues
o FOMM indica e ha his ma e ial could be a good al e na i e o ca ilage eplacemen in olde
pa ien s. This p elimina y s udy, essen ially de o ed o expanding he on ie s o he cu en s a e
o he a o new polyme ic ma e ials, p o ides aluable indica ions o u u e wo k a ge ing he
en isaged applica ions.
Keywo ds: ca ilage issue; 3D p in ing; Nylon®12; LAY-FOMM®60; mechanical p ope ies
1. In oduc ion
Ca ilaginous issue, o simply ca ilage, is a suppo ing connec i e issue composed
o collagen, p o eoglycan- ich ma ix, and a single cell ype, chond ocy es. This issue
di e s om o he human issues due o i s unique p ope ies, especially a lack o blood
essels and ne e cells [
1
]. In he human body, ca ilage o ma ion is based on a p ocess
called chond ogenesis ha , un o una ely, ails o enable he issue o na u ally sel - epai
a e inju y o degene a ion [2].
Damage in ca ilage can be induced by auma and some clinical pa hologies such as
os eoa h i is [
3
]. Al hough he degene a ion o ca ilage is mo e common in he elde ly
due o spo s ac i i y, younge pa ien s a e inc easingly being diagnosed [
3
]. Since his
Polyme s 2022,14, 1044. h ps://doi.o g/10.3390/polym14051044 h ps://www.mdpi.com/jou nal/polyme s
Polyme s 2022,14, 1044 2 o 19
condi ion s ongly in e e es wi h he pa ien s’ quali y o li e, an e ec i e solu ion o he
epai / eplacemen o ca ilaginous issue is needed.
As men ioned be o e, ca ilage plays an impo an ole in he human body, especially
conce ning suppo ing unc ions. The e o e, his issue can adap and bea mechanical
loading while being able o de o m and eco e o he o iginal olume, simila o a sponge
wi h wa e [
4
]. Such demanding equi emen s inc ease he di icul y when designing
de ices o i s subs i u ion.
Nowadays, he wo mos common app oaches o ca ilage eplacemen and epai
a e o al eplacemen , usually wi h cobal –ch ome (CoC ) o ul ahigh molecula weigh
polye hylene (UHMWPE)-based s uc u es, and he sca old implan a ion, in he case o
issue egene a ion app oach [
5
–
7
]. In he case o o al eplacemen , he majo challenges a e
ela ed o he ma e ials used and hei mechanical pe o mance, namely ensile s eng h and
Young modulus. E en hough CoC is conside ed as a biocompa ible and non-deg adable
ma e ial, i s s i ness leads o he mechanical shielding o he bone induced by mechanical
loading [
7
]. Al hough UHMWPE p esen s mechanical p ope ies simila o hose o na i e
ca ilage, i can be s uc u ally uns able unde loading, hinde ing i s mechanical and
ibological pe o mance [
6
]. Mo eo e , due o wea , me al ions can be eleased, leading o
geno oxic e ec s such as ca cinogenici y and DNA damage [8–10].
Fo epai o egene a ion pu poses, biomedical sca olds a e o en indica ed as an
ad an ageous solu ion as hey can p o ide a 3D amewo k o enable cell p oli e a ion, ma-
ix deposi ion, and consequen issue egene a ion [
11
]. F om wo k published in his a ea,
he mos common na u al ma e ials used in such de ices a e collagen, aga ose, chi osan,
hyalu onic acid, ib in, and algina e [
12
–
14
]. O he syn he ic polyme s, poly(e hylene
glycol) (PEG) [
15
,
16
], poly(lac ic acid) (PLA) [
17
,
18
], poly( inyl alcohol) (PVA) [
19
,
20
] and
polyu e hane (PU) [21,22] a e he mos desc ibed and he p oduce bes ou pu s.
Fo he design and a chi ec u e o he sca olds and biomedical de ices al eady e-
po ed o ca ilage egene a ion, s uc u es such as memb anes, hyd ogels, and nano ibe s
p oduced by phase in e sion [
23
], sol en -cas ing pa icle leaching [
24
], eeze–d ying [
25
],
and elec ospinning [
26
] a e among he app oaches p o iding he mos p omising esul s.
None heless, he ou comes a e s ill insu icien .
Addi i e manu ac u ing (AM) o 3D p in ing is a echnology ha enables he p epa a-
ion o ully cus omizable sca olds [
27
,
28
] wi h in ica e shapes ha can be designed using
compu e -aided design (CAD) o compu ed omog aphy (CT) da a [
29
–
31
]. Due o he ease
o p ocessing and geome ic eedom, his has been in es iga ed o ca ilaginous issue
egene a ion pu poses. Indeed, She e al. published a wo k desc ibing he p epa a ion o
a sca old p epa ed by 3D p in ing using wo di e en ma e ials [
32
]. The ou side was a
p in ed PCL hollow ing wi h a collagen sponge inside o mimic he ana omy o he na i e
achea o whi e abbi s.
In i o
es s p o ed he g ow h o acheal ca ilage wi hin he
sca old. The p oduc ion o a silk ib oin-based sca old wi h a 3D p in ed PCL mesh illing
was also epo ed elsewhe e [
33
]. 3D p in ed PCL/g aphene composi e sca olds we e also
epo ed showing imp o ed lub ici y and d ug- eleasing p ope ies [34].
In a di e en app oach, a sca old combining 3D p in ed polyca bona e–u e hane
(PCU) and UHMWPE was s udied o he pu poses o na i e lub ica ion mechanisms [
35
].
Un o una ely, su ace oughness and consequen high ic ion coe icien ha e jeopa dized
i s pe o mance. In o de o in es iga e he ole o inne sca old a chi ec u e, Jung e al.
de eloped a 3D-p in ed PU acheal sca old wi h mic oscale design, which p o ed o
be bene icial o cell in il a ion and biological in eg a ion o he de ice [
36
]. Despi e he
numbe o publica ions and s udies, each ma e ial and design app oach needs o be di ec ed
o a speci ic ype o ca ilage and local implan a ion, which impai s he ag eemen and
s anda diza ion o which ou e (ma e ial/p ocessing echnique) o ollow.
PA12 is a semic ys alline polyme wi h excellen impac esis ance a low empe a u es,
low wa e abso p ion, esis ance o s ess c acking, and a igue unde high- equency
cyclical loading condi ions, which is equen ly used in AM due o he easibili y o he
Polyme s 2022,14, 1044 3 o 19
p ocess [
37
,
38
]. In addi ion, i is commonly used o applica ions ela ed o he biomedical
ield, including o ca ilage [39]. The e o e, i was used as a con ol ma e ial.
FOMM is a new comme cially a ailable ma e ial, and cons i u es a mix u e o wo
polyme s, one o which is PVA. FOMM becomes lexible and po ous when imme sed in
wa e due o he emo al, by dissolu ion, o he PVA con en . This cha ac e is ic may be
o he main in e es when applying his polyme ic ma e ial o ca ilage eplacemen . As
ca ilage does no con ain blood essels o ne es, and is supplied wi h nu ien s h ough
he comp ession and lexion o he issue, i needs o ha e a po ous s uc u e o allow hese
in e ac ions. Pi a u e al. published a wo k in which hey use FOMM in an a emp o ma ch
he mechanical p ope ies o na i e an e io c ucia e ligamen s, wi h p omising esul s [
40
].
Ne e heless, he ci ed a icle is he only one conce ning he consul ed bibliog aphy ha
p esen s esea ch wi h FOMM ma e ial.
The p esen wo k desc ibes a p elimina y s udy explo ing he possibili y o using
PA12 and FOMM o he p epa a ion o s uc u es, by 3D p in ing, o ca ilage epai . To
he bes o ou knowledge, his is he i s ime ha such ma e ials ha e been p oposed o
his speci ic applica ion.
2. Ma e ials and Me hods
2.1. Ma e ials
In he p esen wo k, polyme ic ilamen s wi h a diame e o 1.75
±
0.03 mm we e
used. Nylon
®
12 (PA12) ilamen was supplied by DoWi e
®
(Seixal, Po ugal) and LAY-
FOMM
®
60 (FOMM) ilamen was acqui ed om Filamen 2p in
®
(Nig án, Spain). Fo
compa a i e pu poses, and p io o some cha ac e iza ion echniques, PA12 and FOMM
ilamen s and p in ed pa s we e imme sed in deionized wa e o ou days (wPA12 and
wFOMM, espec i ely).
2.2. P ocessing by 3D P in ing
All specimens we e p in ed using a FlashFo ge
TM
C ea o 3 3D p in e (Ílha o, Po u-
gal) wi h a dual ex ude , each wi h a 0.4 mm diame e nozzle. PA12 ilamen was p in ed
a 260
◦
C wi h a bed empe a u e o 110
◦
C, while FOMM was p in ed a 230
◦
C wi h a bed
empe a u e o 70
◦
C (Figu e 1). The p in ing pa ame e s o FOMM we e p e iously op i-
mized by a ying a se o pa ame e s ha included: p in ing empe a u e om
220–250 ◦C
,
bed empe a u e om 30–80
◦
C, and p in ing speed om 15 o 30 mm
·
s
−1
. Bo h ma e ials
we e p in ed a he same speed, 25 mm
·
s
−1
, wi h a 50% hexagonal in ill pa e n and 180
µ
m
laye hickness. Two bo om and uppe laye s (100% in ill wi h a linea pa e n) we e used
o suppo and acili a e he specimen p in ing. The geome y o he p in ed specimens
was chosen acco ding o he equi emen s o he cha ac e iza ion echnique, as discussed
in he ollowing sec ions.
Polyme s 2022, 14, 1044 3 o 19
equency cyclical loading condi ions, which is equen ly used in AM due o he
easibili y o he p ocess [37,38]. In addi ion, i is commonly used o applica ions ela ed
o he biomedical ield, including o ca ilage [39]. The e o e, i was used as a con ol
ma e ial.
FOMM is a new comme cially a ailable ma e ial, and cons i u es a mix u e o wo
polyme s, one o which is PVA. FOMM becomes lexible and po ous when imme sed in
wa e due o he emo al, by dissolu ion, o he PVA con en . This cha ac e is ic may be
o he main in e es when applying his polyme ic ma e ial o ca ilage eplacemen . As
ca ilage does no con ain blood essels o ne es, and is supplied wi h nu ien s h ough
he comp ession and lexion o he issue, i needs o ha e a po ous s uc u e o allow
hese in e ac ions. Pi a u e al. published a wo k in which hey use FOMM in an a emp
o ma ch he mechanical p ope ies o na i e an e io c ucia e ligamen s, wi h p omising
esul s [40]. Ne e heless, he ci ed a icle is he only one conce ning he consul ed
bibliog aphy ha p esen s esea ch wi h FOMM ma e ial.
The p esen wo k desc ibes a p elimina y s udy explo ing he possibili y o using
PA12 and FOMM o he p epa a ion o s uc u es, by 3D p in ing, o ca ilage epai . To
he bes o ou knowledge, his is he i s ime ha such ma e ials ha e been p oposed
o his speci ic applica ion.
2. Ma e ials and Me hods
2.1. Ma e ials
In he p esen wo k, polyme ic ilamen s wi h a diame e o 1.75 ± 0.03 mm we e
used. Nylon® 12 (PA12) ilamen was supplied by DoWi e® (Seixal, Po ugal) and LAY-
FOMM® 60 (FOMM) ilamen was acqui ed om Filamen 2p in ® (Nig án, Spain). Fo
compa a i e pu poses, and p io o some cha ac e iza ion echniques, PA12 and FOMM
ilamen s and p in ed pa s we e imme sed in deionized wa e o ou days (wPA12 and
wFOMM, espec i ely).
2.2. P ocessing by 3D P in ing
All specimens we e p in ed using a FlashFo geTM C ea o 3 3D p in e (Ílha o,
Po ugal) wi h a dual ex ude , each wi h a 0.4 mm diame e nozzle. PA12 ilamen was
p in ed a 260 °C wi h a bed empe a u e o 110 °C, while FOMM was p in ed a 230 °C
wi h a bed empe a u e o 70 °C (Figu e 1). The p in ing pa ame e s o FOMM we e
p e iously op imized by a ying a se o pa ame e s ha included: p in ing empe a u e
om 220–250 °C, bed empe a u e om 30–80 °C, and p in ing speed om 15 o 30 mm·s−1.
Bo h ma e ials we e p in ed a he same speed, 25 mm·s−1, wi h a 50% hexagonal in ill
pa e n and 180 μm laye hickness. Two bo om and uppe laye s (100% in ill wi h a
linea pa e n) we e used o suppo and acili a e he specimen p in ing. The geome y
o he p in ed specimens was chosen acco ding o he equi emen s o he cha ac e iza ion
echnique, as discussed in he ollowing sec ions.
Figu e 1. Mac og aph o FOMM p in ed es specimen o lexu al es s.
2.3. Cha ac e iza ion
2.3.1. Chemical Cha ac e iza ion
The in a ed (IR) spec a o he s udied ilamen s we e acqui ed wi h FTNIR/MIR
equipmen (Pe kinElme , F on ie model, Wal ham, MA, USA), equipped wi h an
Figu e 1. Mac og aph o FOMM p in ed es specimen o lexu al es s.
2.3. Cha ac e iza ion
2.3.1. Chemical Cha ac e iza ion
The in a ed (IR) spec a o he s udied ilamen s we e acqui ed wi h FTNIR/MIR
equipmen (Pe kinElme , F on ie model, Wal ham, MA, USA), equipped wi h an a enu-
a ed o al e lec ance (ATR), an FR-DTGS de ec o , and a KB beam spli e , a 20
◦
C. Fo
he da a acquisi ion, he esolu ion was 4 cm
−1
, a cons an o ce o 80 N, and 16 accumula-
ion in e e og ams. Pe kinElme also supplied he ATR module wi h a diamond/ZnSe
Polyme s 2022,14, 1044 4 o 19
c ys al. A e he da a collec ion, he spec ums we e analyzed h ough he SPECTRUM 10
STD so wa e.
2.3.2. The mal Cha ac e iza ion
The he mal s abili y o ilamen s and p in ed pa s was s udied using a TGA Q500
V20.13 equipmen by TA ins umen s (New Cas le, DE, USA), wi h a hea ing a e o
10 ◦C·min−1
, be ween 25–600
◦
C, wi h a ni ogen lux o 50 mL
·
min
−1
. The esul s
we e analyzed using he TA Ins umen s Uni e sal Analysis 2000 so wa e supplied by
he manu ac u e .
The he mal e en s o he s udied ilamen s and p in ed specimens we e assessed using
a DSC Q100 V9.9 equipmen by TA ins umen s, wi h a hea ing a e o 10
◦
C
·
min
−1
and a
50 mL
·
min
−1
cons an lux o ni ogen. The analysis o he esul s o he i s hea ing cycle
and he de e mina ion o he c ys alliza ion and en halpies (
∆Hcc
and
∆Hm
, espec i ely)
we e pe o med using TA Ins umen s Uni e sal Analysis 2000 so wa e, supplied by TA
Ins umen s. The pe cen age o c ys allini y (Xc) was calcula ed using Equa ion (1):
Xc(%)=∆Hm−∆Hcc
∆H∞
×100 (1)
whe e
∆H∞
is a cha ac e is ic alue o each ma e ial, co esponding o he mel ing en halpy
a ia ion conside ing 100% o c ys allini y [41].
The weigh o he samples used o bo h he mal cha ac e iza ion echniques was kep
cons an a 8 mg.
2.3.3. Mo phological Cha ac e iza ion
The scanning elec on mic oscopy (SEM) echnique was used o obse e he mo pho-
logical dissimila i ies be ween FOMM and wFOMM. The equipmen used o he ilamen
cha ac e iza ion was a ZEISS
®
Me lin 61–50 Mic oscope (Ca l Zeiss, Obe kochen, Ge -
many), Gemini 2, wi h an accele a ing ol age o 2 kV. Using a spu e ing echnique, all
samples we e coa ed wi h a 3 nm laye o gold. Samples we e coa ed o 60 s wi h he
help o EDWARDS EXC 120 spu e ing equipmen (C awley, UK), wi h a powe sou ce
Hu inge PFG 1500 DC (Schwaig bei Nu embe g, Ge many). The spu e ing condi ions
we e: powe , 0.11 kW; ol age, 1000 V; cu en , 1.83 A. The su ace and c oss-sec ion mo -
phologies o he p in ed specimens we e cha ac e ized using an FEI Quan a 400FEG ESEM
(FEI, Hillsbo o, OR, USA). Fo he c oss-sec ional obse a ion, he samples we e imme sed
o 90 s in liquid ni ogen. This allowed o a clean ac u e o he samples by mechanical
impac . The p in ed samples we e obse ed wi hou any me allic coa ing.
2.3.4. Swelling Capaci y
The wa e up ake o he ilamen s s udied a he p esen wo k was assessed by swelling
capaci y es s (SC). Fi e es samples o PA12, FOMM, and wFOMM ilamen s we e d ied
a 50
◦
C un il weigh equilib ium and hei ini ial weigh collec ed. Then, all samples we e
imme sed in 15 mL o ionized wa e a oom empe a u e o se en days. The weigh o
he samples was collec ed e e y 24 h o 48 h and he wa e was subs i u ed. The SC o he
ma e ials was de e mined h ough Equa ion (2):
SC (%)=WS−W0
WS
×100 (2)
whe e WS ep esen s he swollen weigh and W0is he ini ial d ied weigh [42].
2.3.5. Mechanical Cha ac e iza ion
The ensile s eng h o he s udied ma e ials was de e mined using a Shimadzu appa-
a us, mo e speci ically he Au og aph AGS-X model (Tokyo, Japan), wi h a 5 kN load cell
and a g ip speed o 5 mm
·
min
−1
. All ma e ials (PA12, wPA12, FOMM, and wFOMM) we e
Polyme s 2022,14, 1044 5 o 19
es ed a bo h ilamen (100 mm segmen s) and p in ed specimen (
100 mm ×20 mm ×2 mm
,
acco ding o ASTM D3039) con igu a ions. Figu e 2shows a ep esen a i e ilamen es .
Polyme s 2022, 14, 1044 5 o 19
The ensile s eng h o he s udied ma e ials was de e mined using a Shimadzu ap-
pa a us, mo e speci ically he Au og aph AGS-X model (Tokyo, Japan), wi h a 5 kN load
cell and a g ip speed o 5 mm·min−1. All ma e ials (PA12, wPA12, FOMM, and wFOMM)
we e es ed a bo h ilamen (100 mm segmen s) and p in ed specimen (100 mm × 20 mm
× 2 mm, acco ding o ASTM D3039) con igu a ions. Figu e 2 shows a ep esen a i e ila-
men es .
Figu e 2. Rep esen a i e mac og aph o ensile es o he FOMM ilamen s.
Fi e samples o each ma e ial and o m we e conside ed o he s udy. Fo all es ed
ma e ials, he dis ance be ween opposi e ends, span, was 50 mm, and he ob ained esul s
we e analyzed on T apezium X so wa e (Tokyo, Japan). The esul s we e displayed in
s ess–s ain cu es, om which he calcula ion o Young’s modulus (E) was pe o med,
acco ding o Equa ion (3) [43],
E=σ
ԑ (3)
whe e σ e e s o s ess and ԑ is he s ain.
Th ee-poin bending (3PB) es s de e mined he lexu al s eng h o he p in ed spec-
imens. Fi e samples o each p in ed ma e ial we e conside ed o he calcula ions. The
dimensions o he es ing specimens (60 mm × 10 mm × 2 mm) we e chosen acco ding o
he ASTM S anda d D790 ecommenda ions. Tes s we e conduc ed using an Au og aph
AGS-X equipmen om Shimadzu, wi h a 5 kN load cell and a displacemen a e o 2
mm·min−1. The lexu al s eng h (σ) was de e mined as he nominal s ess in he middle
span sec ion ob ained using he maximum load alue, acco ding o Equa ion (4),
σ

= 3PL
2bh (4)
whe e P e e s o he maximum load, h and b a e he hickness and he wid h o he spec-
imen, espec i ely, and L ep esen s he span leng h, which was kep cons an a 40 mm.
Flexu al modulus (E) was de e mined ollowing he linea elas ic bending beams heo y
ela ionship, which can be exp essed by Equa ion (5),
E

=∆PL
48∆uI (5)
whe e ∆P is he load ange, ∆µ is he lexu al displacemen ange, and I e e s o he mo-
men o ine ia. E was acqui ed by linea eg ession o he ob ained load–displacemen
cu es con empla ing he in e al in he linea segmen wi h a co ela ion ac o g ea e
han 95%.
Figu e 2. Rep esen a i e mac og aph o ensile es o he FOMM ilamen s.
Fi e samples o each ma e ial and o m we e conside ed o he s udy. Fo all es ed
ma e ials, he dis ance be ween opposi e ends, span, was 50 mm, and he ob ained esul s
we e analyzed on T apezium X so wa e (Tokyo, Japan). The esul s we e displayed in
s ess–s ain cu es, om which he calcula ion o Young’s modulus (E) was pe o med,
acco ding o Equa ion (3) [43],
E=σ
ε(3)
whe e σ e e s o s ess and εis he s ain.
Th ee-poin bending (3PB) es s de e mined he lexu al s eng h o he p in ed spec-
imens. Fi e samples o each p in ed ma e ial we e conside ed o he calcula ions. The
dimensions o he es ing specimens (60 mm
×
10 mm
×
2 mm) we e chosen acco ding
o he ASTM S anda d D790 ecommenda ions. Tes s we e conduc ed using an Au o-
g aph AGS-X equipmen om Shimadzu, wi h a 5 kN load cell and a displacemen a e o
2 mm·min−1
. The lexu al s eng h (
σ
) was de e mined as he nominal s ess in he middle
span sec ion ob ained using he maximum load alue, acco ding o Equa ion (4),
σ =3PL
2bh2(4)
whe e P e e s o he maximum load, h and ba e he hickness and he wid h o he
specimen, espec i ely, and L ep esen s he span leng h, which was kep cons an a
40 mm
. Flexu al modulus (
E
) was de e mined ollowing he linea elas ic bending beams
heo y ela ionship, which can be exp essed by Equa ion (5),
E =∆PL3
48∆uI (5)
whe e
∆P
is he load ange,
∆µ
is he lexu al displacemen ange, and I e e s o he
momen o ine ia.
E
was acqui ed by linea eg ession o he ob ained load–displacemen
cu es con empla ing he in e al in he linea segmen wi h a co ela ion ac o g ea e
han 95%.
Ul a-mic oha dness cha ac e iza ion esul s we e eco ded by Fische scope H100
equipmen (Sindel ingen, Ge many). Th ee p in ed specimens o PA12, wPA12, FOMM,
and wFOMM we e submi ed o 5 inden a ion uns in 2 di e en a eas. The es cycles

Polyme s 2022,14, 1044 6 o 19
consis ed o a load–hold–unload unc ion. The load a e was uned so ha each un
would las abou 60 s, wi h 30 s hold pe iod a he maximum load o he mal d i
co ec ion.
Six inden a ions
we e pe o med, in each un, a maximum load using
0.525 oo
inc emen s, and 19 measu emen s in 30 s we e made o access he c eep alue. The load
inc eased om 0.4 mN o 1000 mN.
3. Resul s and Discussion
3.1. Filamen Cha ac e iza ion
3.1.1. Chemical Composi ion
The polyme ic ilamen s we e used as ecei ed. As is usual, supplie s do no sha e
ac ual in o ma ion on se e al aspec s, including he pe cen age and ype o addi i es
mixed in he main polyme ic ma e ial. Fo his eason, he chemical composi ion o PA12
and FOMM ilamen s was e alua ed by FTIR. Poly( inyl alcohol) (PVA) and wFOMM
ilamen s we e also analyzed and compa ed wi h he o iginal FOMM spec um o con i m
he exis ence o PVA in he o iginal o mula ion and i s dissolu ion by imme sion in wa e ,
as s a ed by he supplie . Figu e 3displays he ob ained spec um o each es ed ilamen .
Figu e 3a con i ms he chemical s uc u e o PA12, as i displays simila i y wi h o he
spec a al eady epo ed in he li e a u e [
44
]. The p esence o he s e ching ib a ion o
N–H, CH
2
, and C=O a 3286 cm
−1
(a), 3000–2800 cm
−1
(b), and 1633 cm
−1
(c), espec i ely,
a e highligh ed; he o e lapping o he bands co esponding o he bending ib a ion
o C=O and he s e ching ib a ion o C–N a 1537 cm
−1
(d); and inally, he bending
ib a ion o CH
2
a 1447 cm
−1
(e) [
44
]. The e o e, i any addi i es ha e been added o PA12,
hey a e p esen in a esidual concen a ion ha will no a ec he chemical p ope ies o
he polyamide.
Fo he analysis o FOMM esul s, i mus be eminded ha he li e a u e lacks in o -
ma ion conce ning his ma e ial’s chemical composi ion. In addi ion, he supplie only
e e s o he p esence o PVA and does no p o ide any mo e de ails conce ning he o he
polyme . Fo his eason, a PVA ilamen spec um was o e lapped wi h FOMM o iden i y
he peaks e e ing o PVA. F om he compa ison he spec a o PVA and FOMM, i is
possible o iden i y he well-de ined PVA peaks loca ed be ween 3500–3000 cm
−1
( ) ela ed
o he s e ching ib a ions o he O-H g oup and he s e ching ib a ion o he C=O g oup
be ween 1750–1650 cm−1(c), e en hough hey a e sligh ly shi ed. These a ia ions we e
al eady expec ed since he mix u e o PVA wi h ano he polyme ic ma e ial, as epo ed
by Ali eza Kha azmi e al. ob ained a simila ou come when ZnS nanopa icles we e
inco po a ed in o PVA [45].
To iden i y he emaining FOMM peaks, Pi a u e al. p oposed ha FOMM is com-
posed o a mix u e o PVA and lexible he moplas ic polyu e hane (TPU) [40]. This is he
only published wo k ha analyzes he o he polyme p esen besides PVA, o he bes o
ou knowledge. Fo his eason, he ob ained wFOMM spec um (wi h no PVA due o
dissolu ion in wa e ) was compa ed wi h TPU spec a om he li e a u e. I is possible o
obse e he ypical bands associa ed wi h TPU, such as he s e ching ib a ion o he N–H
g oup a 3350–3250 cm
−1
(a), he band co esponding o CH
2
be ween 2950–2850 cm
−1
(b), he s e ching ib a ion o C=O a 1750–1650 cm
−1
(c), and he s e ching ib a ion o
C–N be ween 1260–1230 cm
−1
(g). Since only he N–H s e ching band is no common o
PVA, i is no possible o ully conclude, a his s age, ha TPU may be he o he polyme
mixed wi h PVA. Howe e , conside ing he li e a u e and he ob ained esul s, his is a
s ong possibili y.
Polyme s 2022,14, 1044 7 o 19
Polyme s 2022, 14, 1044 7 o 19
Figu e 3. FTIR spec a o he polyme ic ilamen s: (a) PA12, (b) FOMM, PVA and wFOMM. The
le e s (a–g) iden i y he cha ac e is ic bands discussed in he ex .
3.1.2. The mal Cha ac e iza ion
The mog a ime ic analysis (TGA) was used o assess he he mal s abili y o he
PA12 and FOMM ilamen s. The esul ing he mog a ime ic cu es a e plo ed in Figu e
4.
Figu e 3.
FTIR spec a o he polyme ic ilamen s: (
a
) PA12, (
b
) FOMM, PVA and wFOMM. The
le e s (a–g) iden i y he cha ac e is ic bands discussed in he ex .
3.1.2. The mal Cha ac e iza ion
The mog a ime ic analysis (TGA) was used o assess he he mal s abili y o he PA12
and FOMM ilamen s. The esul ing he mog a ime ic cu es a e plo ed in Figu e 4.
The he mal s abili y o ma e ials is especially impo an when p ocessing by 3D
p in ing since i is a empe a u e-based p ocess. Fo his eason, i is impo an o ensu e
ha ma e ials a e ex uded wi hou jeopa dizing hei in eg i y. F om he obse a ion o
Figu e 4a i is possible o conclude ha he decomposi ion o PA12 occu ed wi hin a single
s ep be ween 375
◦
C and 500
◦
C, which was an expec ed esul and in ag eemen wi h o he
esul s [46].
Polyme s 2022,14, 1044 8 o 19
Polyme s 2022, 14, 1044 8 o 19
Figu e 4. Weigh loss and de i a i e o weigh loss (DTG) he mog a ime ic cu es o (a) PA12
and (b) FOMM, as ecei ed.
The he mal s abili y o ma e ials is especially impo an when p ocessing by 3D
p in ing since i is a empe a u e-based p ocess. Fo his eason, i is impo an o ensu e
ha ma e ials a e ex uded wi hou jeopa dizing hei in eg i y. F om he obse a ion o
Figu e 4a i is possible o conclude ha he decomposi ion o PA12 occu ed wi hin a sin-
gle s ep be ween 375 °C and 500 °C, which was an expec ed esul and in ag eemen wi h
o he esul s [46].
In he case o FOMM, once again, no di ec compa isons can be es ablished wi h he
scien i ic li e a u e due o he lack o s udies o his polyme . None heless, he ob ained
he mog a ime ic cu es exhibi ed h ee weigh loss s ages: a ound 100 °C, be ween 250–
360 °C, and 360–475 °C. The i s s age (100 °C) is assigned o he loss o wa e . In u n, he
second and hi d s ages (250–340 °C and 340–450 °C) ma ch he decomposi ion s ages o
u e hane bonds and polyol chains, espec i ely, and a e usually ound in TPU decompo-
si ion p o iles [47]. One can hen assume ha TPU seems o be one o he coun e pa s ha
cons i u e he FOMM ilamen . Howe e , one canno exclude he in o ma ion conce ning
he chemical composi ion o FOMM p o ided by he supplie which indica es ha PVA
is pa o he composi ion o FOMM. Fo his eason, he p o ile ob ained o FOMM was
compa ed wi h pu e PVA decomposi ion p o iles ound in he li e a u e. He ein, h ee
decomposi ion s ages we e ound, and hei empe a u es also ma ched wi h FOMM.
Howe e , in he case o PVA, he second s age e e s o he decomposi ion o bound wa e ,
which is wa e ha is di ec ly bonded o he polyme ic s uc u e, no only abso bed on
he su ace, and he hi d s age is assigned o he decomposi ion and consequen ca bon-
iza ion o he PVA ne wo k [48].
Since TPU and PVA deg ada ion s ages o e lap and ma ch he FOMM p o ile, he
p esence o TPU in he composi ion o FOMM could no be wholly con i med by TGA
measu emen s.
The onse and peak empe a u es de e mined om he analysis o he displayed he -
mog ams a e p esen ed in Table 1.
Table 1. Re e ence empe a u es ob ained by TGA.
Filamen Ton (°C) T5% (°C) T10% (°C) Tp1 (°C) Tp2 (°C)
PA12 433.1 416.3 426.9 454.6 -
FOMM 296.5 298.7 314.4 333.5 403.1
Ton—Onse empe a u e; T5%—Tempe a u e o which co esponds 5% o weigh loss; T10%—Tem-
pe a u e o which co esponds 10% o weigh loss; Tp—peak empe a u e.
Figu e 4.
Weigh loss and de i a i e o weigh loss (DTG) he mog a ime ic cu es o (
a
) PA12 and
(b) FOMM, as ecei ed.
In he case o FOMM, once again, no di ec compa isons can be es ablished wi h he
scien i ic li e a u e due o he lack o s udies o his polyme . None heless, he ob ained
he mog a ime ic cu es exhibi ed h ee weigh loss s ages: a ound 100
◦
C, be ween
250–360
◦
C, and 360–475
◦
C. The i s s age (100
◦
C) is assigned o he loss o wa e . In u n,
he second and hi d s ages (250–340 ◦C and 340–450 ◦C) ma ch he decomposi ion s ages
o u e hane bonds and polyol chains, espec i ely, and a e usually ound in TPU decompo-
si ion p o iles [
47
]. One can hen assume ha TPU seems o be one o he coun e pa s ha
cons i u e he FOMM ilamen . Howe e , one canno exclude he in o ma ion conce ning
he chemical composi ion o FOMM p o ided by he supplie which indica es ha PVA
is pa o he composi ion o FOMM. Fo his eason, he p o ile ob ained o FOMM was
compa ed wi h pu e PVA decomposi ion p o iles ound in he li e a u e. He ein, h ee
decomposi ion s ages we e ound, and hei empe a u es also ma ched wi h FOMM. How-
e e , in he case o PVA, he second s age e e s o he decomposi ion o bound wa e , which
is wa e ha is di ec ly bonded o he polyme ic s uc u e, no only abso bed on he su ace,
and he hi d s age is assigned o he decomposi ion and consequen ca boniza ion o he
PVA ne wo k [48].
Since TPU and PVA deg ada ion s ages o e lap and ma ch he FOMM p o ile, he
p esence o TPU in he composi ion o FOMM could no be wholly con i med by TGA
measu emen s.
The onse and peak empe a u es de e mined om he analysis o he displayed
he mog ams a e p esen ed in Table 1.
Table 1. Re e ence empe a u es ob ained by TGA.
Filamen Ton (◦C) T5% (◦C) T10% (◦C) Tp1 (◦C) Tp2 (◦C)
PA12 433.1 416.3 426.9 454.6 -
FOMM 296.5 298.7 314.4 333.5 403.1
T
on
—Onse empe a u e; T
5%
—Tempe a u e o which co esponds 5% o weigh loss; T
10%
—Tempe a u e o
which co esponds 10% o weigh loss; Tp—peak empe a u e.
By compa ing he alues o he wo ilamen s, i is e iden ha PA12 has supe io
he mal s abili y and can wi hs and empe a u es close o 400
◦
C, as all decomposi ion
and onse empe a u es a e abo e his alue. The expe imen ally de e mined T
on
o PA12
was 433.1
◦
C; abo e his empe a u e, PA12 s a s o disin eg a e and does no main ain i s
s uc u al in eg i y. On he o he hand, he T
on
o FOMM occu s sligh ly be o e he ma e ial
loses 5% o i s mass.
The he mal e en s o he ilamen s we e s udied by DSC o co ec ly de ine he
p in ing pa ame e s acco ding o he he mal ansi ions o he ma e ials. The esul ing
Polyme s 2022,14, 1044 9 o 19
cu es a e plo ed in Figu e 5and he de e mined ansi ion empe a u es a e p esen ed in
Table 2.
Polyme s 2022, 14, 1044 9 o 19
By compa ing he alues o he wo ilamen s, i is e iden ha PA12 has supe io
he mal s abili y and can wi hs and empe a u es close o 400 °C, as all decomposi ion
and onse empe a u es a e abo e his alue. The expe imen ally de e mined Ton o PA12
was 433.1 °C; abo e his empe a u e, PA12 s a s o disin eg a e and does no main ain
i s s uc u al in eg i y. On he o he hand, he Ton o FOMM occu s sligh ly be o e he
ma e ial loses 5% o i s mass.
The he mal e en s o he ilamen s we e s udied by DSC o co ec ly de ine he p in -
ing pa ame e s acco ding o he he mal ansi ions o he ma e ials. The esul ing cu es
a e plo ed in Figu e 5 and he de e mined ansi ion empe a u es a e p esen ed in Table
2.
Figu e 5. Hea lux cu es ob ained o PA12 and FOMM ilamen s, as ecei ed.
Table 2. T ansi ion empe a u es ob ained by DSC o he ilamen s.
Filamen Tg1 (°C) Tg2 (°C) Tcc (°C) Tm (°C)
PA12 107.6 - 143.6 246.6
FOMM −42.9 82.4 - 155.9
The hea lux cu e o PA12 p esen s h ee di e en he mal e en s a speci ic em-
pe a u es, whe e he i s endo he mic eac ion, a 107.6 °C, co esponds o he glass an-
si ion empe a u e (Tg). Then, a 143.6 °C, he plo displays an exo he mic cu e which
indica es he cold c ys alliza ion (Tcc) o he polyme ic s uc u e. Finally, a 246.6 °C, he
ma e ial unde goes mel ing (Tm). The de e mined alues o ∆H and ∆H we e 3.25
J·g−1 and 19.06 J·g−1, espec i ely. Assuming ha PA12 ∆H is 209.3 J·g−1 [41], he calcu-
la ed alue o X was 6.7%.
Rega ding FOMM, he iden i ica ion o he he mal e en s was i s es ablished con-
side ing he known da a a ailable in he li e a u e o he same PVA sample used in he
p e ious FTIR analysis. Pu e PVA p esen s a single Tg close o a ound 80 °C [49], which
was also obse ed in he FOMM cu e, and hus ein o ced he possible p esence o PVA
in FOMM.
Then, he FOMM p o ile was compa ed wi h hea lux cu es o TPU a ailable in he
li e a u e o con i m i TPU was pa o FOMM composi ion. As epo ed elsewhe e [31],
TPU displays wo glass ansi ions, he i s nega i e and he second a ound 70–80 °C. In
addi ion, TPU has a mel ing empe a u e, Tm, be ween 150–160 °C [50], also obse ed in
he FOMM p o ile. Since hese h ee ansi ions can be obse ed in FOMM, i can be con-
cluded ha FOMM con ains PVA and TPU. DSC measu emen s we e c ucial o he se-
lec ion o p in ing empe a u es since hey should be highe han he Tm o he ma e ials
(246.6 °C o PA12 and 155.9 °C o FOMM).
3.1.3. Mo phological Cha ac e iza ion
Figu e 5. Hea lux cu es ob ained o PA12 and FOMM ilamen s, as ecei ed.
Table 2. T ansi ion empe a u es ob ained by DSC o he ilamen s.
Filamen Tg1 (◦C) Tg2 (◦C) Tcc (◦C) Tm(◦C)
PA12 107.6 - 143.6 246.6
FOMM −42.9 82.4 - 155.9
The hea lux cu e o PA12 p esen s h ee di e en he mal e en s a speci ic empe -
a u es, whe e he i s endo he mic eac ion, a 107.6
◦
C, co esponds o he glass ansi ion
empe a u e (T
g
). Then, a 143.6
◦
C, he plo displays an exo he mic cu e which indica es
he cold c ys alliza ion (T
cc
) o he polyme ic s uc u e. Finally, a 246.6
◦
C, he ma e ial
unde goes mel ing (T
m
). The de e mined alues o
∆Hcc
and
∆Hm
we e 3.25 J
·
g
−1
and
19.06 J
·
g
−1
, espec i ely. Assuming ha PA12
∆H∞
is 209.3 J
·
g
−1
[
41
], he calcula ed alue
o Xcwas 6.7%.
Rega ding FOMM, he iden i ica ion o he he mal e en s was i s es ablished con-
side ing he known da a a ailable in he li e a u e o he same PVA sample used in he
p e ious FTIR analysis. Pu e PVA p esen s a single T
g
close o a ound 80
◦
C [
49
], which
was also obse ed in he FOMM cu e, and hus ein o ced he possible p esence o PVA
in FOMM.
Then, he FOMM p o ile was compa ed wi h hea lux cu es o TPU a ailable in he
li e a u e o con i m i TPU was pa o FOMM composi ion. As epo ed elsewhe e [
31
],
TPU displays wo glass ansi ions, he i s nega i e and he second a ound 70–80
◦
C. In
addi ion, TPU has a mel ing empe a u e, T
m
, be ween 150–160
◦
C [
50
], also obse ed
in he FOMM p o ile. Since hese h ee ansi ions can be obse ed in FOMM, i can be
concluded ha FOMM con ains PVA and TPU. DSC measu emen s we e c ucial o he
selec ion o p in ing empe a u es since hey should be highe han he T
m
o he ma e ials
(246.6 ◦C o PA12 and 155.9 ◦C o FOMM).
3.1.3. Mo phological Cha ac e iza ion
The mo phology o FOMM and wFOMM ilamen s was obse ed by SEM, and he
ob ained mic og aphs a e displayed in Figu e 6.
Polyme s 2022,14, 1044 16 o 19
highe . On he con a y, FOMM esul s a e close o he human ib ca ilage e alua ed by
he same echnique. Howe e , i is impo an o no e ha he lis ed alues om na i e
ca ilage e e o male pa ien s be ween 63 and 86 yea s old [
60
]. Chond ocy es begin o
dissipa e om he supe icial egion wi h inc eased age, and accumula e in he deepe
laye s. As a esul , he hyd a ion dec eases, and he ma ix becomes s i e [
65
]. Thus, hese
alues should be lowe in younge pa ien s. Ne e heless, FOMM p esen s close esul s
o na i e ca ilage han PA12, indica ing ha his ma e ial could be a good al e na i e o
ca ilage eplacemen in olde pa ien s. Since he p e alence o ca ilage diseases is highe
in olde pa ien s, he impac o FOMM esul s is e en mo e ele an .
4. Conclusions
The p esen wo k aimed o p oduce 3D p in ed s uc u es ha could be used in he
biomedical ield, speci ically o ca ilage epai /subs i u ion. This wo k also in ended
o expand he on ie s o knowledge using a polyme ic ma e ial a ely epo ed in he
li e a u e, FOMM. The s udy also examined PA12. Bo h he d y and we o ms o each
ma e ials we e subjec ed o ilamen cha ac e iza ion (chemical, he mal, and mechanical),
con i ming he chemical composi ion o PA12 and, in he case o FOMM, calcula ing he
amoun o PVA in i s s uc u e (15%). In addi ion, i was concluded ha FOMM was
composed o PVA and TPU.
3PB es s conduc ed on d y and we specimens showed ha wFOMM samples had
compelling esul s simila o na i e ca ilage. The SC o FOMM and wFOMM p o ed o be
simila o na i e ca ilage. PA12, in u n, exhibi ed a poo swelling a e, which could be
help ul o ca ilage epai / egene a ion in a mul ima e ial app oach.
In he ul a-mic oha dness es , as expec ed, all es pieces had highe
E
compa ed
wi h he 3PB es . Howe e , wPA12 displayed a s i e beha io han d y PA12 owing o
he molecula in e ac ions be ween wa e and N–H g oups. On he o he hand, FOMM and
wFOMM p esen ed simila esul s o na i e ca ilage o olde pa ien s. This simila i y could
be bene icial as mos cases o ca ilage eplacemen occu in aged pa ien s. The ob ained
esul s p o ide p omising e idence ha 3D p in ed pa s may be pa o he u u e o
egene a i e medicine. Fu he mo e, his s udy highligh ed new esea ch pa hs, such as
designing mul ima e ial s uc u es wi h a PA12 co e and ou e shell in wFOMM. Fu u e
wo k should include he p epa a ion o such mul ima e ial s uc u es and hei
in i o
cha ac e iza ion, which will include p oka yo ic and euka yo ic cell es s.
Au ho Con ibu ions:
Concep ualiza ion, G.F.D., A.C.P. and A.P.P.; me hodology, A.C.P. and A.P.P.;
alida ion, A.C.P. and A.P.P.; o mal analysis G.F.D. and A.C.P.; in es iga ion, G.F.D. and A.C.P.;
esou ces, A.P.P.; w i ing—o iginal d a p epa a ion, G.F.D. and A.C.P.; w i ing— e iew and edi ing,
A.P.P.; isualiza ion, A.C.P.; supe ision, A.C.P. and A.P.P.; p ojec adminis a ion, A.P.P.; unding
acquisi ion, A.P.P. All au ho s ha e ead and ag eed o he published e sion o he manusc ip .
Funding:
The esea ch was de eloped wi hin he scope o he p ojec CEMMPRE, UIDB/00285/2020,
inanced by na ional unds h ough he FCT. This wo k was also pa ially suppo ed by Po ugal
2020 h ough he Eu opean Regional De elopmen Fund (FEDER), in he ame o Ope a ional
Compe i i eness and In e na ionaliza ion P og am (POCI), unde he scope o p ojec s POCI-01-0145-
FEDER-024533 and POCI-01-0145-FEDER-030767.
Ins i u ional Re iew Boa d S a emen : No applicable.
In o med Consen S a emen : No applicable.
Da a A ailabili y S a emen :
The da a p esen ed in his s udy a e a ailable on eques om he
co esponding au ho .
Con lic s o In e es : The au ho s decla e no con lic o in e es .

Polyme s 2022,14, 1044 17 o 19
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