Ci a ion: Sanchez-Duenas, L.;
Gomez, E.; La añaga, M.; Blanco, M.;
Goi andia, A.M.; A anzabe, E.;
Vilas-Vilela, J.L. A Re iew on
Sus ainable Inks o P in ed
Elec onics: Ma e ials o Conduc i e,
Dielec ic and Piezoelec ic
Sus ainable Inks. Ma e ials 2023,16,
3940. h ps://doi.o g/10.3390/
ma16113940
Academic Edi o : Pa ikshi Saha iya
Recei ed: 26 Ap il 2023
Re ised: 18 May 2023
Accep ed: 20 May 2023
Published: 24 May 2023
Copy igh : © 2023 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
This a icle is an open access a icle
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://
c ea i ecommons.o g/licenses/by/
4.0/).
ma e ials
Re iew
A Re iew on Sus ainable Inks o P in ed Elec onics: Ma e ials
o Conduc i e, Dielec ic and Piezoelec ic Sus ainable Inks
Lei e Sanchez-Duenas 1,* , Es ibaliz Gomez 1, Mikel La añaga 2, Mi en Blanco 1, Amaia M. Goi andia 1,
Es ibaliz A anzabe 1and JoséLuis Vilas-Vilela 3
1Su ace Chemis y & Nano echnologies Uni , Fundación Teknike , Inaki Goenaga 5, 20600 Eiba , Spain;
es ibaliz.gomez@ eknike .es (E.G.); mi en.blanco@ eknike .es (M.B.); amaia.ma inez@ eknike .es (A.M.G.);
es ibaliz.a anzabe@ eknike .es (E.A.)
2Elec onics and Communica ions Uni , Fundación Teknike , Inaki Goenaga 5, 20600 Eiba , Spain;
mikel.la anaga@ eknike .es
3Depa men o Physical Chemis y, Facul y o Science and Technology, Uni e si y o he Basque Coun y,
UPV/EHU, Ba io Sa iena s/n, 48940 Leioa, Spain; [email p o ec ed]
*Co espondence: lei e.sanchez@ eknike .es
Abs ac :
In he las decades, he demand o elec onics and, he e o e, elec onic was e, has inc eased.
To educe his elec onic was e and he impac o his sec o on he en i onmen , i is necessa y
o de elop biodeg adable sys ems using na u ally p oduced ma e ials wi h low impac on he
en i onmen o sys ems ha can deg ade in a ce ain pe iod. One way o manu ac u e hese ypes
o sys ems is by using p in ed elec onics because he inks and he subs a es used a e sus ainable.
P in ed elec onics in ol e di e en me hods o deposi ion, such as sc een p in ing o inkje p in ing.
Depending on he me hod o deposi ion selec ed, he de eloped inks should ha e di e en p ope ies,
such as iscosi y o solid con en . To p oduce sus ainable inks, i is necessa y o ensu e ha mos
o he ma e ials used in he o mula ion a e biobased, biodeg adable, o no conside ed c i ical aw
ma e ials. In his e iew, di e en inks o inkje p in ing o sc een p in ing ha a e conside ed
sus ainable, and he ma e ials ha can be used o o mula e hem, a e collec ed. P in ed elec onics
need inks wi h di e en unc ionali ies, which can be mainly classi ied in o h ee g oups: conduc i e,
dielec ic, o piezoelec ic inks. Ma e ials need o be selec ed depending on he ink’s inal pu pose.
Fo example, unc ional ma e ials such as ca bon o biobased sil e should be used o secu e he
conduc i i y o an ink, a ma e ial wi h dielec ic p ope ies could be used o de elop a dielec ic ink,
o ma e ials ha p esen piezoelec ic p ope ies could be mixed wi h di e en binde s o de elop a
piezoelec ic ink. A good combina ion o all he componen s selec ed mus be achie ed o ensu e he
p ope ea u es o each ink.
Keywo ds:
p in ed elec onics; sus ainable ma e ials o p in ed elec onic inks; conduc i e ink;
dielec ic ink; piezoelec ic ink; biobased ink; biodeg adable ink; sus ainable ink
1. In oduc ion
P in ed elec onics a e limi ed by pa ame e s such as hei cos , main enance, o he
p oduc ion o signi ican quan i ies o elec onic was e. In he las decades, due o he
inc easing demand o elec onics and he speed-up o p og ammed obsolescence, he e
has been an inc ease in he quan i y o p oduced elec onic was e (e-was e) [
1
]. Mo eo e ,
he ma e ials used o p oduce p in ed elec onics a e usually me als, such as gold, sil e ,
palladium, in, o coppe , because o hei chemical s abili y and non-deg adabili y. Using
me allic ma e ials in p in ed elec onics ensu es an app op ia e elec ical beha io , showing
elec ical conduc i i ies up o 10
7
S/m, while o he ma e ials, such as ca bon, p esen
elec ic conduc i i ies up o 10
5
S/m. Ne e heless, his ep esen s a high consump ion o
p ecious me als, as well as a p oblem o socie y, as some o hem a e conside ed c i ical
aw ma e ials (CRMs) [2]. The 2023 EU lis o CRMs is shown in Table 1[3].
Ma e ials 2023,16, 3940. h ps://doi.o g/10.3390/ma16113940 h ps://www.mdpi.com/jou nal/ma e ials
Ma e ials 2023,16, 3940 2 o 22
Table 1. 2023 Eu opean Union C i ical Raw Ma e ials.
2023 EU CRMs
An imony Bismu h Feldspa Helium Manganese Phospho us Tan alum
A senic Bo on Fluo spa Hea y Ra e Ea h Elemen s
(HREE) Na u al g aphi e Pla inum G oup Me als
(PGM) Ti anium me al
Bauxi e Cobal Gallium Ligh Ra e Ea h Elemen s
(LREE) Nickel Scandium Tungs en
Ba y e Coking coal Ge manium Li hium Niobium Silicon me al Vanadium
Be yllium Coppe Ha nium Magnesium Phospha e ock S on ium
The e o e, he e is a need o de elop mo e en i onmen ally iendly elec onic sys ems.
This need could be achie ed in wo di e en ways: by using sus ainable o biobased
ma e ials o by using biodeg adable and/o ecyclable ma e ials. Sus ainable o biobased
ma e ials come om a li ing ma e sou ce (biomass) and educe he en i onmen al impac .
Biodeg adable and/o ecycled ma e ials can deg ade pa ially o comple ely a e hei
se ice li e, as shown in Figu e 1[4].
Ma e ials 2023, 16, x FOR PEER REVIEW 2 o 23
p esen elec ic conduc i i ies up o 105 S/m. Ne e heless, his ep esen s a high
consump ion o p ecious me als, as well as a p oblem o socie y, as some o hem a e
conside ed c i ical aw ma e ials (CRMs) [2]. The 2023 EU lis o CRMs is shown in Table
1 [3].
Table 1. 2023 Eu opean Union C i ical Raw Ma e ials.
2023 EU CRMs
An imony
Bismu h
Feldspa
Helium
Manganese
Phospho us
Tan alum
A senic
Bo on
Fluo spa
Hea y Ra e Ea h Elemen s
(HREE)
Na u al g aphi e
Pla inum G oup Me als
(PGM)
Ti anium
me al
Bauxi e
Cobal
Gallium
Ligh Ra e Ea h Elemen s
(LREE)
Nickel
Scandium
Tungs en
Ba y e
Coking coal
Ge manium
Li hium
Niobium
Silicon me al
Vanadium
Be yllium
Coppe
Ha nium
Magnesium
Phospha e ock
S on ium
The e o e, he e is a need o de elop mo e en i onmen ally iendly elec onic
sys ems. This need could be achie ed in wo di e en ways: by using sus ainable o
biobased ma e ials o by using biodeg adable and/o ecyclable ma e ials. Sus ainable o
biobased ma e ials come om a li ing ma e sou ce (biomass) and educe he
en i onmen al impac . Biodeg adable and/o ecycled ma e ials can deg ade pa ially o
comple ely a e hei se ice li e, as shown in Figu e 1 [4].
Figu e 1. Scheme o he p ocesses o a disin eg able sys em.
Howe e , he de elopmen o biodeg adable elec onics (commonly called g een
elec onics) wi h high conduc i i y manu ac u ed using p in ing echnologies in
en i onmen al condi ions is cu en ly a challenge [5,6].
The e a e a ailable de ices wi h conduc i e lines (such as magnesium, zinc, o i on,
which a e deg adable in physiological en i onmen s, in ce ain condi ions o pH and
empe a u e). The conduc i e lines a e deposi ed in a biodeg adable subs a e (sodium
ca boxyme hyl cellulose, silk, e c.) and manu ac u ed using chemical o physical apo
deposi ion [7]. These de ices a e designed o deg ade in a wa e -based solu ion, making
hem inapp op ia e o applica ions in en i onmen s wi h conside able humidi y [6].
To conside p in ed elec onic de ices sus ainable, he subs a e and he ink
deposi ed on hem should also be sus ainable. Examples o a ious sus ainable subs a es
i i n
i in
i n
n i n
i
i
i
Figu e 1. Scheme o he p ocesses o a disin eg able sys em.
Howe e , he de elopmen o biodeg adable elec onics (commonly called g een elec-
onics) wi h high conduc i i y manu ac u ed using p in ing echnologies in en i onmen al
condi ions is cu en ly a challenge [5,6].
The e a e a ailable de ices wi h conduc i e lines (such as magnesium, zinc, o i on,
which a e deg adable in physiological en i onmen s, in ce ain condi ions o pH and
empe a u e). The conduc i e lines a e deposi ed in a biodeg adable subs a e (sodium
ca boxyme hyl cellulose, silk, e c.) and manu ac u ed using chemical o physical apo
deposi ion [
7
]. These de ices a e designed o deg ade in a wa e -based solu ion, making
hem inapp op ia e o applica ions in en i onmen s wi h conside able humidi y [6].
To conside p in ed elec onic de ices sus ainable, he subs a e and he ink deposi ed
on hem should also be sus ainable. Examples o a ious sus ainable subs a es a e ice
pape , biodeg adable polyme s such as PLA (polylac ic acid) o i s de i a es, silk, o
cellulose pape , among o he s [
7
,
8
]. Mo eo e , he used deposi ion echnique mus be
en i onmen ally iendly, making inkje p in ing a good deposi ion echnique because i
was es li le ma e ial [9].
Ma e ials 2023,16, 3940 3 o 22
Mos o he inks employed o p oduce elec onic de ices using inkje p in ing a e a
combina ion o o ganic polyme s and unc ional ma e ials, depending on he inal cha ac-
e is ics o he de ice [10].
The mos common classi ica ion o inks o p in ed elec onics is based on hei elec-
ical p ope ies. This classi ica ion includes conduc i e, semi-conduc i e, dielec ic, and
esis i e inks. Ne e heless, as his e iew ies o collec he ma e ials o he inks, he
selec ed classi ica ion ocuses on he ink’s composi ion. Taking his in o accoun , i is
possible o de ine ou main componen s o an ink, as shown in Figu e 2.
Ma e ials 2023, 16, x FOR PEER REVIEW 3 o 23
a e ice pape , biodeg adable polyme s such as PLA (polylac ic acid) o i s de i a es, silk,
o cellulose pape , among o he s [7,8]. Mo eo e , he used deposi ion echnique mus be
en i onmen ally iendly, making inkje p in ing a good deposi ion echnique because i
was es li le ma e ial [9].
Mos o he inks employed o p oduce elec onic de ices using inkje p in ing a e a
combina ion o o ganic polyme s and unc ional ma e ials, depending on he inal
cha ac e is ics o he de ice [10].
The mos common classi ica ion o inks o p in ed elec onics is based on hei
elec ical p ope ies. This classi ica ion includes conduc i e, semi-conduc i e, dielec ic,
and esis i e inks. Ne e heless, as his e iew ies o collec he ma e ials o he inks, he
i i i n n h ink’ i i n. T kin hi in coun , i is
possible o de ine ou main componen s o an ink, as shown in Figu e 2.
Figu e 2. Ink composi ion.
1. Func ional ma e ial
The unc ional ma e ial is esponsible o gi ing he ink i s p ope ies. Depending on
h ink’ in n i n i , i is possible o classi y inks in o h ee g oups: conduc i e
inks, dielec ic inks, and piezoelec ic inks. Conduc i e inks a e hose ha , a e being
p ocessed, ha e elec ical p ope ies. They a e composed o ma e ials such as me als ( o
example, sil e pa icles), ca bon de i a es, o conduc i e polyme s. Dielec ic inks a e
o mula ed o ha e he p ope y o isola ing he place whe e hey a e deposi ed. The used
ma e ials could be cellulose o silicon dioxide, among o he s. Piezoelec ic inks can
Figu e 2. Ink composi ion.
1. Func ional ma e ial
The unc ional ma e ial is esponsible o gi ing he ink i s p ope ies. Depending on
he ink’s inal unc ionali y, i is possible o classi y inks in o h ee g oups: conduc i e inks,
dielec ic inks, and piezoelec ic inks. Conduc i e inks a e hose ha , a e being p ocessed,
ha e elec ical p ope ies. They a e composed o ma e ials such as me als ( o example,
sil e pa icles), ca bon de i a es, o conduc i e polyme s. Dielec ic inks a e o mula ed o
ha e he p ope y o isola ing he place whe e hey a e deposi ed. The used ma e ials could
be cellulose o silicon dioxide, among o he s. Piezoelec ic inks can p oduce an elec ical
cu en when su e ing a sligh de o ma ion o being de o med when an elec ical cu en
is applied. Thei composi ion ma e ials mus exhibi his p ope y, such as he PVDF.
Ma e ials 2023,16, 3940 4 o 22
2. Sol en
The unc ional ma e ial is dispe sed in a sol en and hen mixed wi h he polyme ic esin.
3. Polyme ic esin o binde
The polyme ic esin ac s like he binde o he ink, being esponsible o he ink’s
mechanical p ope ies, i s s abili y, o a oiding pa icle agglome a ion.
4. Addi i es
The las componen o he inks is he addi i e, used o imp o e o secu e ce ain
p ope ies o he mix u e, such as con olling he su ace ension, i s heological p ope ies,
i s adhesion and we abili y, o p e en ing agglome a ion [11].
All hese elemen s o ink’s composi ion ha e o be used in he co ec p opo ion o
ensu e he unc ionali y o he ink and i s p ope ies. These p ope ies, such as he solid
con en o he ink, may a y depending on he deposi ion me hod used. Fo example, in
echnologies such as inkje p in ing, liquid inks a e needed. The iscosi y o hese ypes
o inks is lowe han pas es’ iscosi ies, which a e designed o be deposi ed by sc een
p in ing o simila me hods. To each he speci ic p ope ies o each ype o ink, he speci ic
componen s mus be adjus ed. The solid con en o each ink will a y depending on he
desi ed inal p ope ies as well as he unc ional ma e ial used. The iscosi y o he ink
ends o inc ease wi h inc easing solid con en . Inks wi h solid con en s a ying be ween
20% and 80% ha e been ound. The con en o addi i es in an ink is usually less han
5–10%, and he emaining con en is a combina ion o a polyme ic esin and he sol en
whe e he unc ional ma e ial is dispe sed [
12
–
14
]. Table 2shows he pe cen ages o each
componen o sc een p in ing and inkje p in ing inks.
Table 2. Ink composi ion.
Ma e ial Sc een P in ing Inkje P in ing Re s.
Func ional ma e ial 5–70% 10–30% [13,14]
Polyme ic esin/Binde 20–50% 5–30% [13,14]
Sol en 15–65% 60–90% [13,14]
Addi i es <5–10% <5–10% [13,14]
To ensu e he sus ainabili y o he o mula ed inks, i is impo an o selec he ma e ials
ha make up mos o he ink.
In his e iew, he di e en unc ional ma e ials ha can be used o de elop sus ainable
inks, as well as he sol en s and binde s used, a e collec ed. Addi i es, being a small
pe cen age o he whole ink, a e no analyzed.
To be conside ed sus ainable, he o mula ed ink mus ollow one o hese c i e ia:
1.
I is a biobased ink. Mos o i s compounds a e p oduced om a na u al o enewable
sou ce. Some examples a e ma e ials p oduced om biomass (p oduced om ag icul-
u al was e), ob ained om a biogenic p ocess, o p oduced using a sus ainable ou e,
among o he s. As wi h biobased plas ics, cu en ly, he e is no ule ha measu es he
sus ainabili y o an ink based on i s biobased con en [15].
2.
I is a biodeg adable ink. Mos o i s compounds deg ade pa ially o o ally in
a easonable pe iod. Fo polyme s, he deg ada ion is measu ed by he UNE-EN
ISO 14855 ule [
16
], which measu es he biodeg ada ion o he polyme in ambien
compos , o he UNE-EN ISO 14852 [
17
], measu ing he biodeg adabili y in aqueous
media, a 20–25 ◦C o 6 mon hs.
3.
I s compounds a e no conside ed c i ical aw ma e ials o ha m ul o he en i onmen .
The esul an ink could be also a combina ion o hese c i e ia, as shown in Figu e 3.
Each ma e ial used, i sus ainable, could be classi ied in a sec ion o Figu e 3. An aspec
o conside when de eloping sus ainable inks is he ab ica ion and ma e ial cos s. I he
de eloped ink is ex emely expensi e, i would no be compe i i e wi h comme cial ones.
Ma e ials 2023,16, 3940 5 o 22
Being oo expensi e p e en s hese ypes o inks om achie ing hei main objec i e: o
educe elec onic was e.
Ma e ials 2023, 16, x FOR PEER REVIEW 5 o 23
The esul an ink could be also a combina ion o hese c i e ia, as shown in Figu e 3.
Each ma e ial used, i sus ainable, could be classi ied in a sec ion o Figu e 3. An aspec o
conside when de eloping sus ainable inks is he ab ica ion and ma e ial cos s. I he
de eloped ink is ex emely expensi e, i would no be compe i i e wi h comme cial ones.
Being oo expensi e p e en s hese ypes o inks om achie ing hei main objec i e: o
educe elec onic was e.
Figu e 3. C i e ia o classi y a sus ainable ink.
2. Composi ion o Inks
2.1. Func ional Ma e ial
2.1.1. Conduc i e Inks
Conduc i e inks a e gene ally composed o a unc ional ma e ial (o i s p ecu so ),
which gi es he ink i s elec ical conduc i i y p ope ies. I he ink is composed o he
p ecu so , he me al pa icles a e p epa ed wi h bo om-up me hods, decomposing he
p ecu so ’s molecules he mally o h ough he educ ion o me al sal s eac ing wi h a
educ ion agen . Depending on he unc ional ma e ial used, conduc i e inks could be
classi ied in o h ee sepa a e g oups: (1) conduc i e inks o med o me allic pa icles, (2)
ca bon-based conduc i e inks, o (3) pa icle- ee conduc i e inks.
Mos conduc i e inks use me allic ma e ials, such as sil e o coppe , o achie e good
elec ic conduc i i y. Howe e , some o he used ma e ials a e conside ed c i ical aw
ma e ials o a e ha m ul o some species (such as sil e o subma ine li e), en ailing a isk
o he en i onmen [18].
Ca bon-based ma e ials a e he second amily o ma e ials used o gi e inks elec ical
conduc i i y. These ma e ials ha e demons a ed good conduc i i y and can come om
an inexhaus ible sou ce. Thei biodeg adabili y is secu ed in ce ain condi ions; o
example, ca bon nano ubes (CNTs) deg ada ion is p oduced by mac ophages [19]. These
allo opic o ms o ca bon (g aphi e, g aphene, CNTs, o ca bon black) can be used
Figu e 3. C i e ia o classi y a sus ainable ink.
2. Composi ion o Inks
2.1. Func ional Ma e ial
2.1.1. Conduc i e Inks
Conduc i e inks a e gene ally composed o a unc ional ma e ial (o i s p ecu so ),
which gi es he ink i s elec ical conduc i i y p ope ies. I he ink is composed o he
p ecu so , he me al pa icles a e p epa ed wi h bo om-up me hods, decomposing he
p ecu so ’s molecules he mally o h ough he educ ion o me al sal s eac ing wi h
a educ ion agen . Depending on he unc ional ma e ial used, conduc i e inks could
be classi ied in o h ee sepa a e g oups: (1) conduc i e inks o med o me allic pa icles,
(2) ca bon-based conduc i e inks, o (3) pa icle- ee conduc i e inks.
Mos conduc i e inks use me allic ma e ials, such as sil e o coppe , o achie e good
elec ic conduc i i y. Howe e , some o he used ma e ials a e conside ed c i ical aw
ma e ials o a e ha m ul o some species (such as sil e o subma ine li e), en ailing a isk
o he en i onmen [18].
Ca bon-based ma e ials a e he second amily o ma e ials used o gi e inks elec ical
conduc i i y. These ma e ials ha e demons a ed good conduc i i y and can come om an
inexhaus ible sou ce. Thei biodeg adabili y is secu ed in ce ain condi ions; o example,
ca bon nano ubes (CNTs) deg ada ion is p oduced by mac ophages [19]. These allo opic
o ms o ca bon (g aphi e, g aphene, CNTs, o ca bon black) can be used sepa a ely o by
combining hei p ope ies o ensu e good conduc i i y. Ca bon is an elemen p esen in
na u e ha can be ound in ossil o m, in he ai , o in he ocean, and can be p ocessed and
a e wa d ecycled and e u ned o na u e.
The hi d amily o inks con ains hose ha do no ha e pa icles, such as Poly(2,3-
dihyd o hieno-1,4-dioxin)-poly(s y enesul ona e) inks (PEDOT:PSS inks). This ma e ial is
Ma e ials 2023,16, 3940 6 o 22
no biodeg adable a all, bu , used in a small p opo ion, he inal ink could be conside ed
biodeg adable.
Depending on he selec ed ma e ial, he conduc i i y o he inal ink will a y. Table 3
con ains he alues o conduc i i y o a ious unc ional ma e ials.
Table 3. Conduc i i y o some unc ional ma e ials used in conduc i e inks.
Ma e ial Conduc i i y Re .
Sil e 6.8 ×107S/m [20]
Coppe 5.98 ×107S/m [20]
Gold 4.3 ×107S/m [20]
Aluminium 3.8 ×107S/m [20]
Magnesium 2.2 ×107S/m [21]
Wol amium 1.8 ×107S/m [22]
Zinc 1.7 ×107S/m [21]
Nickel 1.5 ×107S/m [20]
I on 1.04 ×107S/m [21]
Pla inum 9.5 ×106S/m [21]
Palladium 9.4 ×106S/m [21]
Tin 8.7 ×106S/m [21]
Ca bon 102–106S/m [23]
PEDOT: PSS 2×10−1–2.1 ×105S/m [23]
A s a egy o syn hesize biogenic sil e pa icles has been de eloped in ecen yea s.
This sin e ing me hod is low cos , causes less oxic was e, and consumes less ene gy
while a highe yield is ob ained. The p ocess is based on he abili y o ce ain o ganisms,
such as bac e ia, yeas s, o ungi, o al e he chemical na u e o me als and educe hem
in o nanopa icles [
24
]. Depending on he speci ic o ganism used and he en i onmen al
condi ions, he nanopa icles ob ained may ha e di e en physicochemical p ope ies [
24
].
Ne e heless, he a ainmen o sil e nanopa icles depends on nume ous ac o s,
such as he gene ic p ope ies o he o ganisms o he en i onmen al condi ions [
11
]. I
sin e ing sil e h ough his p ocess, we could conside i a biobased ma e ial [
25
]. E en so,
sil e is a p oblem o subma ine li e and is no biodeg adable, becoming a p oblem o he
en i onmen [18].
Ca bon can be p o ided om biomass p oduced om ag icul u al was e. This sou ce is
abundan , sus ainable, enewable, and ich in his ma e ial (up o 55% o biomass is ca bon).
I is necessa y o apply he mal ea men s o biomass o ob ain g aphi ic s uc u es [26].
O he ca bon sou ces a e ege able oils, chicken oil, o campho (C
10
H
16
O) a e being
co ec ly p ocessed [
27
–
30
]. Ca bon nanos uc u es could also be o med h ough he
he mal ea men o cellulose using a nickel sal du ing he p ocess [31].
I has been demons a ed ha g aphene could be ob ained om daily ma e ials such as
ood, was e, plas ics, o plan s, ea ing hem a ho empe a u es in an H
2
/Ag a mosphe e
o ob ain high-quali y g aphene laye s [30].
CNTs a e gene ally ob ained h ough he Chemical Vapo Deposi ion (CVD) p ocess.
The sus ainabili y o his p ocess can be imp o ed by using mo e sus ainable ca alys s o
enewable ca bon sou ces. Me als o me allic oxides ha can be ound in na u e, such as
la a o sand, could be used as ca alys s [
32
,
33
]. These sou ces a e no commonly conside ed
enewable bu a e abundan in na u e and low cos . Howe e , he use o hese ca alys s
does no lead o uni o mi y in he mo phology o he nanos uc u e o he CNTs [
30
]. CNTs
could also be p oduced using i on ex ac ed om plan s such as sesame seeds as o ganic
p ecu so s [
34
]. The ob ained CNTs p esen a uni o m size [
35
]. Pol e al. desc ibed a
p ocess o ob ain CNTs om polyme was e wi hou sol en s [
36
]. This p ocess is based
on a he mal dissocia ion in a closed sys em wi hin au ogenic p essu e and ca alys s. The
p ocedu e desc ibed could, in addi ion, sol e ano he cu en en i onmen al p oblem:
plas ic deg ada ion [30].
Ma e ials 2023,16, 3940 7 o 22
Focusing on he biodeg adabili y o ca bon-based ma e ials, he e a e s udies ha
demons a e he deg ada ion o CNTs. This deg ada ion occu s no only chemically wi h
s ong oxidan s o he mal ea men s in an oxygen a mosphe e bu also h ough enzyma ic
oxida ion wi h ho se adish pe oxidase. These s udies demons a e he o al deg ada ion o
he CNTs in
in i o
sys ems (using di e en animal issues, cells, o molecules) wi hou
cy o oxici y. Howe e , o
in i o
sys ems (e alua ing he deg ada ion o CNTs in li ing
o ganisms), he deg ada ion ha occu s is pa ial, and he e is s ill long- e m conce n abou
i s oxici y [19].
Taking all his in o accoun , we can classi y ca bon as biobased and a ma e ial ha
could be biodeg adable in ce ain condi ions.
O he me als ha a e conside ed o be biodeg adable, such as magnesium (Mg), zinc
(Zn), o i on (Fe), can be used o de elop conduc i e inks [
7
]. These a e co odible me als,
and hey deg ade ela i ely quickly. Mg and Zn a e mo e o en used due o hei lowe cos
and ease o p ocessing. Howe e , hey deg ade mo e quickly han Fe, making he las one
mo e sui able o applica ions wi h a longe li e ime [
7
]. Lee e al. de eloped bio eso bable
sys ems using Zn mic opa icles sin e ed elec ochemically. The subs a e used o deposi
hem was a bio eso bable polyme : a shee o polylac ic-co-glycolic acid (PLGA) [
37
].
Hwang e al. manu ac u ed biodeg adable elec onics using Mg, a biodeg adable polyme ,
among o he ma e ials [38].
The las ype o conduc i e inks ound in he ma ke a e hose composed o a con-
duc i e polyme , such as PEDOT:PSS. Thei conduc i i y is lowe han pa icle-based
ones, bu i could be in e es ing o s udy he possibili y o de eloping a conduc i e ink
based on a biopolyme . PEDOT:PSS is a conduc i e polyme exhibi ing biocompa ibili y,
elec ochemical p ope ies, good elec ic conduc i i y, and e sa ile p ocessing, and i is
comme cialized in wa e dispe sion. The e a e s udies demons a ing he biodeg adabili y
o mon mo illoni e/PEDOT:PSS composi es (MMT/PEDOT:PSS) a e being p ocessed
by speci ic supe wo ms [
39
]. PEDOT:PSS does no ul ill he biodeg adabili y ISO 14852
ule [
9
], which measu es i s deg ada ion in aqueous media. Ne e heless, Pie sch e al.
de eloped PEDOT:PSS biodeg adable elec odes acco ding o he ISO 14855 ule, which
measu es de deg ada ion o he comple e sys em in compos media [
9
]. This happens
because he quan i y o he PEDOT:PSS in he de ice is less han he non-deg adable quan-
i y accep ed by he ISO 14855 ule o conside i biodeg adable. The e o e, o conside a
PEDOT:PSS ink biodeg adable, i is necessa y o combine a low quan i y o PEDOT:PSS
wi h an app op ia e biodeg adable binde [
40
]. The main s a egy o using he PEDOT:PSS
as unc ional ma e ial is o mix i wi h a biodeg adable polyme , keeping conduc i i y. Con-
duc i i ies up o 4 7
×
10
−1
S/m ha e been eached in pa icle PEDOT sys ems dispe sed
in poly(L-lac ic acid) (PLLA) [
41
]. Man ione e al. de eloped di e en PEDOT:biopolyme
dispe sions, achie ing conduc i i ies up o 7
×
10
2
S/m. The di e en PEDOT:biopolyme
dispe sions and he conduc i i ies achie ed a e collec ed in Table 4[
42
]. These ypes o
inks a e pa icle- ee inks.
Table 4. PEDOT:Biopolyme dispe sions and achie ed conduc i i ies.
PEDOT:Biopolyme Conduc i i y (S/m) Re .
PEDOT:dex an sulpha e 7×102[42]
PEDOT:DNA 102[42]
PEDOT:hepa in 0.1–5 [42]
PEDOT:chond oi in Sulpha e 0.2–7.5 [42]
PEDOT:hyalu onic acid 0.3–7.1 [42]
PEDOT:sulpha ed cellulose 57.6 [42]
PEDOT:pec in <1 [42]
PEDOT:gua gum 2.8–12.9 [42]
Ma e ials 2023,16, 3940 8 o 22
2.1.2. Dielec ic Inks
Dielec ic inks a e insula o inks below a ce ain elec ic ol age, called he b eakdown
ol age. The e o e, dielec ic inks should be o mula ed wi h insula o ma e ials ha a e
biobased o biodeg adable and mixed wi h he app op ia e sol en s, binde s, and addi i es,
as shown in Figu e 2.
Dielec ic inks de ine hei isola ion capaci y depending on hei dielec ic cons an .
The dielec ic cons an o ma e ials usually used in elec onics is shown in Table 5.
Table 5. Dielec ic cons an s o ma e ials used in elec onics.
Ma e ial Dielec ic Cons an F equency Re .
Pape 2–4 106Hz [43]
Mica 3–6 103Hz [44]
Te lon 2 103Hz [45]
Rubbe 6.7 - [46]
Polyme s (gene al) ~2 103Hz [43]
High-Densi y Polye hylene (HDPE) 2.3 103Hz [43]
Low-Densi y Polye hylene (LDPE) 2.3 103Hz [43]
Polyp opylene (PP) 2.3 106Hz [43]
Myla 3.25 103Hz [43]
Kap on 3.9 103Hz [43]
Poly inyl chlo ide (PVC) 3.4 103Hz [47]
Glass (Py ex) 5 - [46]
Po celain 6–8 - [46]
He eina e , di e en biobased and/o biodeg adable ma e ials ha exhibi dielec ic
p ope ies and a e good candida es o o mula ing dielec ic inks a e collec ed.
The e a e biodeg adable ino ganic dielec ic ma e ials, such as silicon dioxide (SiO
2
),
magnesium oxide (MnO), o silicon ni ide (Si
3
N
4
), ha ha e been used as dielec ic
ma e ials due o hei dielec ic p ope ies [
7
]. SiO
2
has a dielec ic cons an o 3.9, while
he dielec ic cons an o Si
3
N
4
is 7.5 [
48
]. This ype o ma e ial could be dispe sed in a
binde o de elop a dielec ic ink o p in ed elec onics.
Se e al ypes o biodeg adable polyme s can be ound in na u e, as shown in Figu e 4a.
These polyme s exhibi dielec ic p ope ies. Due o hei low dielec ic loss and high
ol age b eakdown, na u al polyme s such as glucose, lac ose, o adenine, among o he s,
could be conside ed in he manu ac u e o biodeg adable and biocompa ible elec onic
de ices. Likewise, biodeg adable polyme s could be syn hesized, as shown in Figu e 4b.
As an ad an age, hese polyme s’ physicochemical p ope ies a e mo e con olled, in
compa ison o na u al ones, by con olling hei syn hesis condi ions. Polyme s such
as polylac ic acid (PLA), poly inyl alcohol (PVA), poly(dime hyl siloxane) (PDMS), and
polyu e hane (PU) ha e been used in dielec ic applica ions. The alues o he dielec ic
cons an o di e en biodeg adable polyme s a e collec ed in Table 6.
Cellulose is a na u al biopolyme ound abundan ly on Ea h and is ob ained om a
ege al sou ce (annually, be ween 10
11
and 10
12
ons o nanocellulose a e p oduced) [
49
].
The nanocelluloses a e cellulose-based ma e ials cha ac e ized by ha ing nanoscale di-
mensions aken om plan s, such as wood, coconu husk, sisal, algae, e c., o ob ained
om animals o bac e ia [
50
,
51
]. They can be classi ied in o h ee g oups: (1) cellulose
nanoc ys als (CNCs), ob ained chemically om plan s o animals, (2) cellulose nano ib ils
(CNFs), ob ained mechanically, also om plan s o animals, o (3) bac e ial nanocellulose
(BNCs), ob ained om bac e ia.
Ma e ials 2023,16, 3940 9 o 22
Ma e ials 2023, 16, x FOR PEER REVIEW 9 o 23
(a)
(b)
Figu e 4. (a) Na u al biodeg adable polyme s; (b) Syn he ic biodeg adable polyme s.
Cellulose is a na u al biopolyme ound abundan ly on Ea h and is ob ained om a
ege al sou ce (annually, be ween 1011 and 1012 ons o nanocellulose a e p oduced) [49].
The nanocelluloses a e cellulose-based ma e ials cha ac e ized by ha ing nanoscale
dimensions aken om plan s, such as wood, coconu husk, sisal, algae, e c., o ob ained
om animals o bac e ia [50,51]. They can be classi ied in o h ee g oups: (1) cellulose
nanoc ys als (CNCs), ob ained chemically om plan s o animals, (2) cellulose nano ib ils
(CNFs), ob ained mechanically, also om plan s o animals, o (3) bac e ial nanocellulose
(BNCs), ob ained om bac e ia.
Figu e 4. (a) Na u al biodeg adable polyme s; (b) Syn he ic biodeg adable polyme s.
Table 6. Dielec ic cons an s o po en ial biobased and/o biodeg adable ma e ials.
Ma e ial Dielec ic Cons an F equency Re .
Cellulose 3.9–7.5 103Hz [47]
Ke a in 8 3×106Hz [52]
Chi osan 5.5 103Hz [53]
S a ch 40–65 2.45 ×109Hz [54]
Silk Fib oin 6.1 3×105Hz [55]
Poly(glu amic acid) 130 103Hz [56]
Hyalu onic acid 60 9×109Hz [57]
Algina e 18.35 106Hz [58]
Dex an 45–60 1010 Hz [59]
Collagen 4.5 103Hz [60]
PLA 2.9 103Hz [61]
PVA 12 103Hz [47]
Polycap olac one 3 109Hz [59]
PDMS 2.6 103Hz [47]
PBS 17.5 - [62]
Ma e ials 2023,16, 3940 16 o 22
Table 8. Con .
Binde Ac i e
Elemen Sol en O he Ma e ials Tech. Sus ainabili y
Reason Re .
Aqueous
solu ion o
TEMPO
Sil e
Wa e -based:
Wa e /
Isop opyl
alcohol
CNCs NaB E hanol
Sc een
p in ing
Wa e as sol en and
cellulose as addi i e [50]
NaOCl NaOH AgNO3
NaBH4o Hyd azine
E hylene glycol
Dispe sing agen
(Dispe byk 2012 1–5 w .%)
Hyd oxyp opyl
me hylcellulose
Rheological addi i e
(Reobyk 7420)
HCl
-
PEDOT:
dex an
sul a e
Wa e -based - Inkje
p in ing
Biopolyme o inc ease
PEDOT conduc i i y and
wa e -based
[42]
-PEDOT:
DNA Wa e -based - Inkje
p in ing
Biopolyme o inc ease
PEDOT conduc i i y and
wa e -based
[42]
-PEDOT:
hepa in Wa e -based - Inkje
p in ing
Biopolyme o inc ease
PEDOT conduc i i y and
wa e -based
[42]
-
PEDOT:
chond oi in
sul a e
Wa e -based - Inkje
p in ing
Biopolyme o inc ease
PEDOT conduc i i y and
wa e -based
[42]
-
PEDOT:
hyalu onic
acid
Wa e -based - Inkje
p in ing
Biopolyme o inc ease
PEDOT conduc i i y and
wa e -based
[42]
-
PEDOT:
sul a ed
cellulose
Wa e -based - Inkje
p in ing
Biopolyme o inc ease
PEDOT conduc i i y and
wa e -based
[42]
-PEDOT:
pec in Wa e -based - Inkje
p in ing
Biopolyme o inc ease
PEDOT conduc i i y and
wa e -based
[42]
-PEDOT:
gua gum Wa e -based - Inkje
p in ing
Biopolyme o inc ease
PEDOT conduc i i y and
wa e -based
[42]
-Sodium
Algina e Wa e -based - Sc een
p in ing
Algina e as unc ional
ma e ial and wa e -based
[71]
- CNCs Wa e -based - Ae osol
p in ing
CNCs as ac i e ma e ial
and
wa e -based
[68]
3. Conclusions
In his e iew, ecen ad ances in he de elopmen o sus ainable inks (conduc i e,
dielec ic, and piezoelec ic) a e p esen ed. The sus ainabili y o he inks can be achie ed in
h ee ways: (1) using biobased ma e ials, (2) using biodeg adable ma e ials, o (3) a oiding
he use o c i ical aw ma e ials. The sus ainabili y o he ink is secu ed by selec ing
he app op ia e ma e ials and conside ing he quan i y o each ma e ial in he inks o be
conside ed biodeg adable. The classi ica ion o he di e en ma e ials collec ed o de elop
inks o elec onic applica ions is shown in Figu e 8.
Elec ic conduc i e inks need a unc ional ma e ial ha allows he elec ici y o pass
h ough he p in ed lines. Th ee ypes o ma e ials can be used: me allic ma e ials, ca bon-
based ma e ials, o polyme ic ma e ials.
Some me allic ma e ials, such as sil e o gold, despi e p esen ing he highes conduc-
i i ies (10
5
S/cm), a e conside ed c i ical aw ma e ials and, he e o e, a e inapp op ia e
o de eloping sus ainable inks. Howe e , he e a e s udies ha ha e de eloped a biobased
Ma e ials 2023,16, 3940 17 o 22
sil e , educing he impac on he en i onmen due o lowe ene gy consump ion in i s
syn hesis and less oxic p oduced was e.
Ma e ials 2023, 16, x FOR PEER REVIEW 17 o 23
Figu e 8. Classi ica ion o he ma e ials used o de elop sus ainable inks, conside ing he p ope
ma e ial and quan i y o each one o be conside ed biodeg adable.
Elec ic conduc i e inks need a unc ional ma e ial ha allows he elec ici y o pass
h ough he p in ed lines. Th ee ypes o ma e ials can be used: me allic ma e ials, ca bon-
based ma e ials, o polyme ic ma e ials.
Some me allic ma e ials, such as sil e o gold, despi e p esen ing he highes
conduc i i ies (105 S/cm), a e conside ed c i ical aw ma e ials and, he e o e, a e
inapp op ia e o de eloping sus ainable inks. Howe e , he e a e s udies ha ha e
de eloped a biobased sil e , educing he impac on he en i onmen due o lowe ene gy
consump ion in i s syn hesis and less oxic p oduced was e.
Ca bon-based ma e ials, such as g aphi e, g aphene, ca bon nano ubes, o ca bon
black, p esen conduc i i y (103 S/cm) and can be used o de elop sus ainable inks. Ca bon
can be ound in na u e in mul iple o ms and is biobased and biodeg adable. Taking all
his in o accoun , i is a good candida e o de eloping sus ainable inks o p in ed
elec onics.
O he conduc i e ma e ials ha can be used a e conduc i e polyme s, such as
PEDOT:PSS. Despi e he ac ha his ma e ial is nei he biobased no biodeg adable, an
ink wi h a low con en o i could be conside ed sus ainable and no ha m ul o he
en i onmen . Ano he s a egy o de elop a sus ainable ink based on a conduc i e
polyme is o dispe se he PEDOT in a biopolyme , eaching conduc i i ies o 101 S/cm.
In p in ed elec onics, dielec ic inks a e also needed o isola e pa s o he ci cui y
o o p o ec de ices om he na u al en i onmen , such as mois u e. I is also necessa y
o de elop sus ainable inks ha ensu e hese unc ionali ies. Thus, a unc ional ma e ial
wi h dielec ic p ope ies should be dispe sed in a binde , such as cellulose o na u al
esins, o na u al p o eins, such as ke a in. Di e en ma e ial dispe sions ha e been
h i
( n n , n )
ih n n
h
h
hi n
in
i ni i
T
i i n i n i i
( , , nin )
n h i
( A, A )
A
n
Figu e 8.
Classi ica ion o he ma e ials used o de elop sus ainable inks, conside ing he p ope
ma e ial and quan i y o each one o be conside ed biodeg adable.
Ca bon-based ma e ials, such as g aphi e, g aphene, ca bon nano ubes, o ca bon
black, p esen conduc i i y (10
3
S/cm) and can be used o de elop sus ainable inks. Ca bon
can be ound in na u e in mul iple o ms and is biobased and biodeg adable. Taking all his
in o accoun , i is a good candida e o de eloping sus ainable inks o p in ed elec onics.
O he conduc i e ma e ials ha can be used a e conduc i e polyme s, such as PE-
DOT:PSS. Despi e he ac ha his ma e ial is nei he biobased no biodeg adable, an ink
wi h a low con en o i could be conside ed sus ainable and no ha m ul o he en i onmen .
Ano he s a egy o de elop a sus ainable ink based on a conduc i e polyme is o dispe se
he PEDOT in a biopolyme , eaching conduc i i ies o 101S/cm.
In p in ed elec onics, dielec ic inks a e also needed o isola e pa s o he ci cui y o
o p o ec de ices om he na u al en i onmen , such as mois u e. I is also necessa y o
de elop sus ainable inks ha ensu e hese unc ionali ies. Thus, a unc ional ma e ial wi h
dielec ic p ope ies should be dispe sed in a binde , such as cellulose o na u al esins, o
na u al p o eins, such as ke a in. Di e en ma e ial dispe sions ha e been epo ed and
deposi ed on a subs a e using di e en echniques, p esen ing sui able p ope ies.
The las ype o sus ainable inks equi ed a e piezoelec ic inks. The e a e biobased
polyme s (na u al and syn he ic) ha p esen his p ope y in pa icula condi ions. Because
o his, hese ma e ials could be dispe sed in an app op ia e binde . A e he deposi ion,
he ma e ials mus be exposed o a poling p ocess o align all he dipoles o he ma e ial
in he co ec di ec ion o ob ain i s piezoelec ic p ope ies. Cu en ly, ew piezoelec ic
inks ha e been ound in he ma ke o epo ed in he li e a u e. By co ec ly selec ing he
Ma e ials 2023,16, 3940 18 o 22
ma e ials and o mula ing an ink, i could be possible o de elop a sus ainable piezoelec ic
ink o p in ed elec onics.
All hese unc ional ma e ials should be dispe sed in a sol en and mixed wi h a binde .
To be sus ainable, hese ma e ials should also be biobased o biodeg adable. Se e al ypes
o binde s and sol en s ha e been ound, such as wa e , PLA, cellulose dispe sions, o esins
coming om na u e, such as shellac o xan ana. The ma e ials used mus be compa ible
wi h he unc ional ma e ial selec ed, ensu ing conduc i i y and good beha io o be
p in ed using he me hod hey a e pu posed o.
Di e en sus ainable inks and dispe sions ha e been ound o sc een p in ing, ae osol
p in ing, and inkje p in ing. The sus ainabili y o hese inks and/o dispe sions depends
on di e en pa ame e s: he sus ainabili y o he unc ional ma e ial, he sus ainabili y o
he binde , and he sus ainabili y o he sol en .
To o mula e he di e en sus ainable inks, i is necessa y o e i y he compa ibili y
be ween he di e en ma e ials and o selec hem co ec ly. Mo eo e , i is necessa y o
de ine he deposi ion me hod and adjus he inal p ope ies o he o mula ed inks.
Cu en ly, he de elopmen o biobased o biodeg adable inks o p in ed elec onics
is a challenge. The inks mus ha e good beha io and p ope ies as well as compe i i e
cos compa ed wi h comme cial inks. S udies on de eloping his ype o ink a e being
conduc ed. Ano he challenge ha esea che s may ocus on is secu ing he ecyclabili y o
he ink and he global elec onics sys ems in o de o achie e sus ainabili y objec i es and
g een elec onics.
Au ho Con ibu ions:
Concep ualiza ion, E.A. and J.L.V.-V.; w i ing—o iginal d a p epa a ion,
L.S.-D.; w i ing— e iew and edi ing, E.G., M.L., M.B., A.M.G., E.A. and J.L.V.-V.; p ojec adminis a-
ion, E.G.; unding acquisi ion, E.G. All au ho s ha e ead and ag eed o he published e sion o
he manusc ip .
Funding:
This publica ion is suppo ed by he SUINK p ojec unded by he Eu opean Union’s
Ho izon Eu ope esea ch and inno a ion p og amme unde G an Ag eemen No. 101070112. Funded
by he Basque Go e nmen ELKARTEK2021 (KK-2021/00040) and ELKARTEK2023 KK-2023/0005.
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 :
No new da a we e c ea ed o analyzed in his s udy. Da a sha ing is
no applicable o his a icle.
Acknowledgmen s:
This publica ion is suppo ed by he SUINK p ojec unded by he Eu opean
Union’s Ho izon Eu ope esea ch and inno a ion p og amme unde G an Ag eemen No. 101070112.
Views and opinions exp essed a e, howe e , hose o he au ho (s) only and do no necessa ily e lec
hose o he Eu opean Union o he Eu opean Commission. Nei he he Eu opean Union no he
g an ing au ho i y can be held esponsible o hem. The au ho s would like o acknowledge he
Basque Go e nmen unding wi hin he ELKARTEK2021 (KK-2021/00040) and ELKARTEK2023
KK-2023/00056 P og ammes.
Con lic s o In e es : The au ho s decla e no con lic o in e es .
Re e ences
1.
Huang, X.; Liu, Y.; Hwang, S.W.; Kang, S.K.; Pa naik, D.; Co es, J.F.; Roge s, J.A. Biodeg adable Ma e ials o Mul ilaye T ansien
P in ed Ci cui Boa ds. Ad . Ma e . 2014,26, 7371–7377. [C ossRe ] [PubMed]
2.
Poulin, A.; Aeby, X.; Siquei a, G.; Nys öm, G. Ve sa ile ca bon-loaded shellac ink o disposable p in ed elec onics. Sci. Rep.
2021,11, 23784. [C ossRe ] [PubMed]
3.
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