Con en s lis s a ailable a ScienceDi ec
Heliyon
jou nal homepage: www.cell.com/heliyon
Resea ch a icle
De elopmen o a pho oacous ic acquisi ion sys em and hei
p oo -o -concep o hemoglobin de ec ion
B una Pinhei oa,b, Vânia Pin oa,b, Hugo Dinisa,b, Michael Belsley c,
Susana Ca a inoa,b, G aça Minasa,b, Paulo Sousaa,b, ,∗
aCen e o Mic oElec omechanical Sys ems (CMEMS), Uni e si y o Minho, Guima ães, 4800-058, Po ugal
bLABBELS — Associa e Labo a o y in Bio echnology and Bioenginee ing and Mic oelec omechanical Sys ems, Uni e si y o Minho, B aga,
4710-057, Po ugal
cCen e o Physics o Minho and Po o Uni e si ies (CF-UM-UP), Labo a o y o Ma e ials and Eme gen Technologies (LAPMET), Uni e si y o
Minho, B aga, 4710-057, Po ugal
A R T I C L E I N F O A B S T R A C T
Keywo ds:
Hemoglobin
Lase
O gan-on-a-chip
Pho oacous ic
Piezoelec ic
Ul asound
Recen ly, O gan-on-a-Chip (OoC) pla o ms ha e a isen as an inc easingly ele an expe imen al
ool o success ully eplica ing human physiology and disease. Howe e , he e is a lack o a
s anda d echnology o moni o he OoC pa ame e s, especially in a non-in asi e and label- ee
way. Pho oacous ic (PA) sys ems can be conside ed an al e na i e and accu a e assessmen me hod
o OoC pla o ms. PA sys ems combine an illumina ion sou ce o exci e he sample molecules,
wi h an ul asound senso o measu e he gene a ed ul asonic wa es, combining he ad an ages
o op ics and acous ic me hodologies o sa ely acqui e idimensional signals and images a a ious
dep hs.
This wo k is ocused on he design, implemen a ion and es o an acquisi ion elec onics ci cui ,
based on he PA p inciple, o hemoglobin (Hb) de ec ion, aiming owa ds a u u e in eg a ion
wi hin an OoC pla o m. Based on he measu ed equency esponse o comme cial piezoelec ic
ansduce s, an elec onics design comp ising a diffe en ial cha ge amplifie and a band-pass fil e
was de eloped. Expe imen ally i was e ified Hb de ec ion o concen a ions o Hb be ween
2.5 and 10 mg/mL in aqueous solu ions, oughly 48 imes lowe han he ypical in i o blood
concen a ions. This c ea es he possibili y o de eloping his echnique o moni o Hb a low
concen a ions in small olumes, which is highly app op ia e o OoC de ices.
1. In oduc ion
An OoC is a sys em ha con ains cells, issues o o gan models, na u al o a ificial, g own inside mic ofluidic channels. The
main pu pose o an OoC is he simula ion o he physiological en i onmen s o human issues and o gans, and one o i s g ea es
po en ial applica ions is o assess he sa e y o d ugs be o e hey en e clinical ials, inc easing he efficiency o pha maceu ical
de elopmen .
*Co esponding au ho a : Cen e o Mic oElec omechanical Sys ems (CMEMS), Uni e si y o Minho, Guima ães, 4800-058, Po ugal.
E-mail add ess: [email p o ec ed] (P. Sousa).
h ps://doi.o g/10.1016/j.heliyon.2024.e41083
Recei ed 9 Sep embe 2024; Recei ed in e ised o m 12 No embe 2024; Accep ed 8 Decembe 2024
Heliyon 11 (2025) e41083
A ailable online 12 Decembe 2024
2405-8440/© 2024 The Au ho s. Published by Else ie L d. This is an open access a icle unde he CC BY-NC-ND license
( h p://c ea i ecommons.o g/licenses/by-nc-nd/4.0/ ).
B. Pinhei o, V. Pin o, H. Dinis e al.
Fig. 1. Schema ic ep esen a ion o he pho oacous ic imaging p inciple. (a) Lase beam exci a ion (b) Inc ease o he local empe a u e (c) Rise and p opaga ion o
he p essu e wa e (d) Image econs uc ion om he p essu e wa es (adap ed om [10]).
The e o e, hese chips a e designed o con ol he mic oen i onmen o cells while main aining o gan/ issue unc ioning. By being
able o combine ad ances in issue enginee ing wi h nano and mic o ab ica ion, hese OoC pla o ms can be conside ed he nex -le el
sys ems o s udying bo h human pa hophysiology and he apeu ics effec s [1,2].
Hemoglobin is one o he molecules essen ial o moni o on hese pla o ms. Hb is an oxygen- anspo me allop o ein in he ed
blood cells (e y h ocy es) o almos all mammals, which plays a significan ole in oxygen-ca ying p ocesses and is also a ypical
bioma ke o ce ain diseases [3,4]. This biomolecule, ound in he ed blood cells o nea ly all mammals, is a me allop o ein
esponsible o ca ying oxygen and is a ypical bioma ke o specific illnesses. No mal le els o hemoglobin in heal hy issues
a e a ound 120 o 160 mg/mL in blood cells and below 0.04 mg/mL in human se um [3]. Specific diseases such as hea disease,
leukemia, anemia, and o he s can be closely linked o abno mal le els o hemoglobin in he blood [5]. Cu en ly, se e al me hods such
as elec ochemical assay, luminescence, ch oma og aphy, mass spec ome y and enzyme-linked immunoso ben assay, ha e been
de eloped o Hb moni o ing. Howe e , hese me hods a e expensi e and in ol e mul iple p econdi ioning s eps and a e no sui able
o in eg a ion in OoC pla o ms. In addi ion, hese me hods ha e been limi ed by he low sensi i i y and specifici y, equi ing
bulky and expensi e equipmen [6,7]. Thus, he de elopmen o low cos , non-in asi e, highly specific and label- ee de ec ion
me hodologies, able o be in eg a ed in o OoC pla o ms, is c ucial.
None heless, a pe sis en challenge wi h OoC pla o ms emains he absence o an in eg a ed and eliable moni o ing me hod,
c ucial o p o iding eal- ime in o ma ion on Hb dis ibu ion.
PA sys ems can be conside ed as an al e na i e and accu a e moni o ing me hod in OoC pla o ms. The measu emen o blood’s
Hb h ough pho oacous ic me hods has been clea ly alida ed in he li e a u e [8]. Hb has i s dominan op ical abso p ion coefficien
a wa eleng hs below 1000 nm, making i clea ly dis inguishable om o he molecules, such as wa e and lipids, and allowing i
o ob ain a pho oacous ic con as o his analy e. The specifici y o pho oacous ic sys ems is ob ained by he igh choice o he
exci a ion wa eleng h, assu ing maximum abso p ion peaks dis inc om o he biomolecules. Rega ding Hb, he 532 nm exci a ion
wa eleng h is o en conside ed o imaging Hb wi h high specifici y and a s ong pho oacous ic effec .
PA sys ems a e hyb id sys ems ha employ an illumina ion sou ce o exci e he sample, p oducing ul asonic wa es de ec ed by
an ul asound ansduce . This non-in asi e and label- ee biomedical imaging modali y le e ages he op imal ea u es o bo h op ics
and acous ics, enabling he sa e acquisi ion o one-dimensional signals and images ac oss a ious scales (including o ganelles, issues
and o gans), and wi h no need o ionizing adia ion. Consequen ly, i s p ima y benefi is he abili y o ga he in o ma ion om he
sample, as unc ional and molecula ela ed, wi hou dis up ing he egula g ow h o cells wi hin he OoC.
The PA effec occu s when a issue su ace is exposed o elec omagne ic wa es, in he in ensi y-modula ed con inuous o pulsed
o m [9]. Following his exposu e, he ligh pene a es he issue a ge up o a specific dep h, acco ding o i s wa eleng h. The
abso p ion o pho ons, ollowed by ch omopho es’ non adia i e elaxa ion, leads o a swi ise in he empe a u e, causing he
abso bing objec o unde go he moelas ic expansion. The ab up inc ease in p essu e causes he gene a ion and p opaga ion o
a sound wa e, de ec able by adi ional ul asound ansduce s (Fig. 1). Loca ing he sou ces o he p essu e wa es enables he
iden ifica ion o he specific posi ion o he abso bed ligh by he sample o issue, which p o ides c ucial molecula and unc ional
de ails o he sample.
This wo k p esen s he de elopmen o a fi s p o o ype o he PA de ec ion module (including senso s and ins umen a ion) and
i s expe imen al alida ion o hemoglobin de ec ion. I is a p oo -o -concep , as i aims o be he fi s s ep in he de elopmen o a
no el PA de ice, sui able o u u e in eg a ion in o an OoC.
Heliyon 11 (2025) e41083
2
B. Pinhei o, V. Pin o, H. Dinis e al.
Fo ha pu pose, a heo e ical, elec ical, and expe imen al cha ac e iza ion o a comme cial lead zi cona e i ana e (PZT) ans-
duce o he PA de ec ion will be conduc ed. Piezoelec ic ansduce s a e widely used in li e a u e o hei well-es ablished
ab ica ion echnology and scalable sensi i i y [11], making hem a simple and cheap op ion o ou p oo -o -concep .
2. Theo y
To p ope ly exci e he sample and efficien ly gene a e b oadband PA signals, he du a ion o he ligh pulse should be sho e
han he imes equi ed o he mal and s ess elaxa ion. The he mal elaxa ion ime, which desc ibes he he mal diffusion wi hin
a gi en a ea, can be app oxima ed by 𝜏𝑡ℎ =𝑑2
𝑐∕𝛼𝑡ℎ, whe e 𝛼𝑡ℎ ep esen s he he mal diffusi i y, while 𝑑𝑐deno es he hea ed a ea
cha ac e is ic dimension o he desi ed spa ial esolu ion. Addi ionally, he s ess elaxa ion ime cha ac e izes how he p essu e
wa es p opaga e h ough he domain, being defined by he 𝜏𝑠=𝑑𝑐∕𝑐, whe e 𝑐 ep esen s he speed o sound [9,12]. Gene ally,
pulsed lase s wi h nanosecond o picosecond peak du a ion a e used as ligh sou ces o pho oacous ic (PA) imaging o mee he
equi emen s o bo h he mal and s ess confinemen s [14].
Following exci a ion, he expansion o ac ional olume, which is gi en by 𝑑𝑉 ∕𝑉, can be exp essed as
𝑑𝑉
𝑉=−𝜅𝑝 +𝛽𝑇 (1)
whe e 𝜅 ep esen s he iso he mal comp essibili y o he medium, 𝛽 ep esen s he he mal coefficien o olume expansion, 𝑝 ep-
esen s p essu e while 𝑇deno es he empe a u e. Assuming ha bo h he mal and s ess induced expansion a e confined, i can be
assumed ha he ac ional olume expansion can be neglec ed, leading o an immedia e accumula ion o p essu e in he hea ed a ea.
The ini ial ise o p essu e, 𝑝0, is gi en by Equa ion (1)
𝑝0=𝛽𝑇
𝜅(2)
o desc ibed as
𝑝0=𝛽
𝜅
𝜂𝑡ℎ𝐴𝑒
𝜌𝐶𝑣
(3)
whe e 𝐴𝑒deno es he specified op ical abso p ion, 𝜂𝑡ℎ ela es o he op ical ene gy abso bed and con e ed in o hea (in pe cen age),
𝜌 ep esen s he densi y o he medium while 𝐶𝑣 ep esen s he specific hea capaci y a a cons an olume. The G uneisen coeffi-
cien , which is a dimensionless he modynamic cons an , defines he efficiency o he hea -p essu e con e sion, allowing o simpli y
Equa ion (3)
Γ= 𝛽
𝜅𝜌𝐶𝑣
(4)
By eplacing he G uneisen coefficien Γin Equa ion (3), i becomes
𝑝0=Γ𝜂𝑡ℎ𝐴𝑒(5)
In he specific condi ions o linea op ical abso p ion, i.e., when he local op ical fluence 𝐹and 𝐴𝑒a e p opo ional, 𝑝0can be
gi en by:
𝑝0=Γ𝜂𝑡ℎ𝜇𝑎𝐹(6)
whe e 𝜇𝑎is he coefficien o op ical abso p ion. P essu e wi hin he hea ed a ea immedia ely ises as a esul o he lase ’s swi
ene gy deli e y. This p essu e elease (as a consequence o he moelas ic expansion), leads o he p oduc ion o ul asound wa es,
when bo h he he mal linea i y and confinemen condi ions a e me .
The ollowing wa e equa ion go e ns he acous ic p essu e o he ul asonic wa e [9,13,15]
(▽2−1
𝑐2
𝜕2
𝜕𝑡2)𝑝=− 𝛽
𝐶𝑝
𝜕𝐻
𝜕𝑡
(7)
whe e, 𝐶𝑝 ep esen s specific hea capaci y, 𝐻is he hea ing unc ion (gi en by he hea deposi ed pe olume and ime uni s), and
is ela ed o 𝐴𝑒 h ough 𝐻=𝜂𝑡ℎ
𝜕𝐴𝑒
𝜕𝑡
[9,15].
Addi ionally, one o he s eps in his wo k was o cha ac e ize diffe en piezoelec ic ansduce s o unde s and hei beha io .
The e o e, one o he mos impo an pa ame e s unde s udy is he esonance equency o he piezoelec ic ansduce . The esonance
equency (𝑓𝑟) e e s o he na u al equency o ib a ion o oscilla ion o an objec , whe e he ampli ude o in ensi y is maximum
(and wi h lowe ene gy losses) [16]. This equency influences he efficiency o he mechanical wa e ansmission and is elian on
he ansduce ’s hickness, being gi en by
𝑓𝑟=𝑛(𝑐∕2𝑡𝑚)(8)
whe e 𝑐is he speed o sound in he medium, 𝑡𝑚is i s hickness, and 𝑛desc ibes an odd in ege ha ep esen s which odd mul iple
o a hal wa eleng h is being conside ed, whe e 𝑛=1 ep esen s he esonance equency o he ma e ial.
Heliyon 11 (2025) e41083
3
B. Pinhei o, V. Pin o, H. Dinis e al.
Fig. 2. Illus a ion o he de eloped PA sys em.
3. Me hods
3.1. Selec ion o he piezoelec ic ansduce
Fo p oceeding o he ansduce s’ cha ac e iza ion, wo comme cial piezoelec ic disks o lead zi cona e i ana e (PZT) wi h
app oxima ely 27 mm o diame e and 500 μm o hickness we e chosen [17].
3.2. Se up o he ansduce cha ac e iza ion
In o de o pe o m he elec ical cha ac e iza ion o he piezoelec ic ansduce s, a p e-calib a ed Vec o Ne wo k Analyze
(VNA) om Keysigh (E5071C) was used. Fo each equency in he kHz ange, om 10 kHz up o 5 MHz, he ansduce s’ e u n
loss (RL) was e alua ed.
Each o he PZT-5H ansduce s has wo me allic con ac s which we e coa ed wi h an ac ylic esin (Plas ik70 Kon ak Chemie) o
block he flow o cu en h ough he wa e medium whe e he ansduce s we e imme sed. Addi ionally, a pai o wi es assu es he
connec ion o he es ing equipmen s.
Expe imen al cha ac e iza ion was ca ied ou o in es iga e he co ela ion be ween he in ensi y o he emi ed and ecei ed
wa es, and o analyze how a ia ions in equency and he dis ance be ween he wo ansduce s affec he ecei ed ampli ude.
The e o e, wo piezoelec ic disks we e placed unde wa e in a glass con aine (10 × 8 × 7 cm), enabling a be e ansmission
o he p oduced mechanical ib a ions. Then, a signal gene a o om Mul icomp (MP750065) and a digi al oscilloscope (Rigol,
DS1102E) we e employed o gene a e he wa es and cap u e he emi ed and ecei ed wa es, espec i ely. Mo e de ail abou he
se up used o he elec ical and expe imen al cha ac e iza ion o he piezoelec ic ansduce s can be ound in [17].
3.3. Expe imen al se up o he PA sys em es
To alida e he PA sys em, se e al concen a ions o Hb solu ions (in wa e ) we e employed. The es s we e pe o med wi h human
Hb ob ained om Sigma-Ald ich (H7379, lyophilized powde ), since he main a ge o he applica ion is moni o ing human cells in
OoC pla o ms. Using his solu ion allows us o keep he low cos o he es s and ensu e ep oducibili y.
In o de o make PA signal acking easie , he oscilloscope was synch onized wi h a signal om a pho odiode ( igge ), indica ing
he ins an o he lase ’s ligh pulse emission. Subsequen ly, a goble con aining a hemoglobin solu ion was posi ioned in he lase ’s
pa h. The piezoelec ic disk, connec ed o he emaining de ec ion ci cui (de ailed in sec ion 4.1.2), was placed wi hin he solu ion
wi h an angle ha a oids he lase hi ing (app oxima ely 45º). Fig. 2illus a es he schema ic ep esen a ion o he sys em se up.
The es s we e hen conduc ed and he acous ic esponse was eco ded.
4. Pho oacous ic sys em design and implemen a ion
4.1. Sys em design
4.1.1. Illumina ion sou ce
To exci e he sample unde s udy, an illumina ion sou ce wi h a wa eleng h in he 400-600 nm ange (hemoglobin has maximum
abso p ion a 414, 541 and 576 nm wa eleng hs [18]), a na ow pulse wid h, and an ene gy le el o a ound mJ was equi ed o ou
PA sys em. Consequen ly, a solid-s a e lase was selec ed. The Nd:YAG lase (LPY604T-10, Li on Lase s) (Figs. 3(a) and (b)) emi ed
ligh pulses in o he PA sys em, wi h a wa eleng h o 532 nm, 10 Hz PRF, 20 ns pulse wid h, and 1.2 mJ o ene gy pe pulse, ocusing
on an a ea o 60 μm in diame e .
Heliyon 11 (2025) e41083
4
B. Pinhei o, V. Pin o, H. Dinis e al.
Fig. 3. Solid-s a e lase used as illumina ion sou ce wi h i s op ical pa h illus a ed om diffe en pe spec i es.
4.1.2. De ec ion module
A e selec ing he mos sui able piezoelec ic ansduce s o de ec ing he PA signals in he epo ed sys em, he design o he
elec onics de ec ion ci cui was pe o med.
The de ec ion module o he PA sys em uses a PZT-5H piezoelec ic ansduce wi h a 27 mm diame e and 500 μm hickness. I s
expe imen al esonance equencies we e a ound 94 kHz in he adial mode and 4 MHz in he TE mode (sec ion 5.1), cha ac e ized
using he se up p esen ed in sec ion 3.2.
This ansduce was chosen because i is widely used in simila sys ems [11,19–21] and i s a o able elec ical cha ac e iza ion,
showing good impedance ma ching and also a linea ela ionship be ween emi ed and ecei ed ampli udes in he expe imen al
cha ac e iza ion.
Following he piezoelec ic ansduce , a cha ge amplifie comp ising a wo-s age diffe en ial cha ge amplifie [18] was used.
The fi s s age was based on a modified Howland ci cui o con e diffe en ial-mode inpu o a single-ended signal while ejec ing
common-mode inpu . The second s age consis ed o a s anda d cu en in eg a o wi h high-pass fil e ing ou pu ed a single-ended
ol age. This se up offe s low noise and high sensi i i y, enhancing he signal o noise a io (SNR). Gain adjus men was achie ed by
egula ing esis o s 𝑅𝑎and 𝑅𝑏[22] (in his wo k, a gain o 46 dB was used). G ounded esis o s we e connec ed o each amplifie
inpu in he fi s s age o supply he necessa y bias cu en . This amplifie was implemen ed using he IC LT1259 (Analog De ices),
supplied by a 5V DC powe supply. Ma ching condi ions o p ope amplifie ope a ion a e de ailed in Equa ion (9) and Fig. 4(a).
⎧
⎪
⎨
⎪
⎩
𝑅1=𝑅
𝑅2=𝑅𝑎+𝑅𝑏=𝑅
(9)
wi h
⎧
⎪
⎨
⎪
⎩
𝑅𝑎=(1−𝑎)𝑅
𝑅𝑏=𝑎𝑅
0<𝑎≤1
Fo his cha ge amplifie , i s diffe en ial gain (𝐺𝐷) and sensi i i y (𝑆𝑄) a e gi en acco ding o Equa ions (10) and (11):
𝐺𝐷=2
𝑎
𝑠𝑅𝑓
1+𝑠𝑅𝑓𝐶𝑓
(10)
𝑆𝑄=2
𝑎
1
𝐶𝑓
(11)
I can be e ified ha educing he alue o 𝑎leads o an inc ease in bo h diffe en ial gain and sensi i i y. This can be achie ed
by inc easing 𝑅𝑎and dec easing 𝑅𝑏while dec easing 𝐶𝑓also boos s sensi i i y. The chosen ci cui alues esul ed in a low 𝑎 alue
(𝑎∼0.33). To mee low pole equency equi emen s, high 𝐶𝑓and 𝑅𝑓 alues a e ecommended, bu low noise and high sensi i i y
equi e a low 𝐶𝑓. Thus, a high 𝑅𝑓and low 𝐶𝑓we e chosen.
Secondly, a passi e Twin-T no ch fil e was implemen ed o emo e powe -line in e e ences, wi h a 50 Hz cu -off equency (𝑓𝑁)
as p ojec ed in Equa ion (12) (Fig. 4(a)).
𝑓𝑁=1
4𝜋𝑅𝐶 (12)
Finally, an ac i e band-pass fil e (Fig. 4(a)) was implemen ed wi h a lowe cu -off equency (𝑓𝐿) o 15.9 kHz and an uppe cu -
off equency (𝑓𝐻) o app oxima ely 5.3 MHz o a enua e e y low and e y high equencies. The TL084CN IC (Texas Ins umen s)
was chosen o he band-pass fil e , and his IC was supplied wi h DC ol ages o ±15 V.
Heliyon 11 (2025) e41083
5
B. Pinhei o, V. Pin o, H. Dinis e al.
Fig. 4. (a) Schema ic and (b) Bode plo o he de ec ion ci cui .
Bo h he uppe and lowe cu -off equencies we e calcula ed acco ding o he Equa ion (13):
𝑓𝑐=1
2𝜋𝑅𝐶 (13)
The comple e de ec ion ci cui is ep esen ed in Fig. 4(a).
Simula ion on LTspice so wa e analyzed he ci cui ’s Bode plo , as shown in Fig. 4(b). Ne e heless, i is essen ial o ecognize ha
his g aph ep esen s an es ima ion o he ci cui ’s ac ual pe o mance, since i was no es ed wi h he PZT ansduce . Howe e , he
ob ained cu e displays a significan dec ease nea 50 Hz, which aligns wi h he esponse o he no ch fil e . The inc ease con inues
un il i hi s a peak o 46 dB. I is impo an o poin ou ha a ound 13.4 kHz, he e is a educ ion o -3 dB om he maximum
gain, which co esponds closely o he lowe cu -off equency ha was se o he band-pass fil e . The uppe cu -off equency is
a ound 175.1 kHz, which is lowe han he designed cu -off equency o he band-pass fil e , which is a ibu ed mainly o he Gain
x Bandwid h p oduc .
O e all, he ci cui has a subs an ial gain o 46 dB and a bandwid h o a ound 161.7 kHz, mee ing he necessa y cha ac e is ics
o de ec ing PA signals om biomolecules ( equency a ound 100 kHz o hemoglobin), since e en i i is necessa y o de ec in he
Heliyon 11 (2025) e41083
6
B. Pinhei o, V. Pin o, H. Dinis e al.
Fig. 5. (a) Schema ic o he PCB (b) PCB connec ed o he PZT ansduce .
Table 1
Theo e ical esonance equencies calcula ed in di -
e en modes.
Mode Thickness Resonance F equency
TE 500 um 3.98 MHz
Radial 27 mm 73.3 kHz
ange o a ew MHz, he ci cui has enough gain. Fu he mo e, i should be no ed ha he gain can be easily adjus ed by a ying
𝑅𝑎
and 𝑅𝑏as equi ed, wi h he ci cui achie ing a maximum gain o app oxima ely 100 dB.
The de eloped ci cui design was implemen ed on a ab ica ed PCB o ob ain a mo e obus and compac ci cui . Fig. 5p esen s
he design (Fig. 5(a)) and he ab ica ed PCB connec ed o he PZT ansduce (Fig. 5(b)) o he de eloped ci cui .
5. Resul s and discussion
5.1. Piezoelec ic ansduce cha ac e iza ion
The ollowing subsec ions p esen he esul s ob ained om he measu emen s and cha ac e iza ion o he piezoelec ic PZT-5H
ansduce chosen o he PA sys em’s de ec ion module.
5.1.1. Theo e ical cha ac e iza ion
Fi s ly, i was calcula ed he heo e ical esonance equency o he PZT-5H disk ansduce , bo h in he hickness and adial
di ec ions. Fo his calculus, i was conside ed he hickness and adius o he ansduce , as well as he sound speed 3.98 × 103m/s
o he PZT-5H ma e ial [17,23].
Hence, Equa ion (8) was sol ed, ob aining he esonance equency alues p esen ed in Table 1. In he hickness ex ensional (TE)
mode, he hickness alue co esponds o he hickness o he ansduce , whe eas in he adial mode, he hickness alue equals he
ansduce ’s diame e . The p edic ed alues show ha , as expec ed, he TE esonan equency is conside ably supe io o he adial
one.
Heliyon 11 (2025) e41083
7
B. Pinhei o, V. Pin o, H. Dinis e al.
Fig. 6. Re u n loss a ia ion acco ding o equency o he PZT-5H ansduce om 10 kHz o 6 MHz.
Table 2
The ampli ude o he de ec ed wa e a ies acco ding o he am-
pli ude o he ansmi ed wa e.
Emi ing ansduce
ampli ude (V)
Recei ing ansduce
ampli ude (mV)
𝜎(mV)
0.100 19.867 0.377
0.200 34.533 1.050
0.500 79.733 0.377
1.000 154.667 0.943
2.000 301.333 1.886
3.000 452.000 0.000
4.000 593.333 1.886
5.000 752.000 0.000
As he ansduce s ib a e in a di ec ion pe pendicula o hei hickness, hey expand and con ac in ha di ec ion du ing o
each oscilla ion cycle. Oscilla ing in he TE mode causes g ea e de o ma ion and equi es less ene gy, ul ima ely leading o a highe
s iffness when compa ed o he adial mode. Because he adial mode equi es g ea e ene gy and has lowe s iffness, he esonance
equency is lowe in his o ien a ion han in he hickness o ien a ion [17].
5.1.2. Elec ical cha ac e iza ion
The elec ical cha ac e iza ion o he PZT-5H piezoelec ic ansduce was pe o med by measu ing he S-pa ame e s om 10 kHz
o 6 MHz, wi hou using an impedance ma ching ci cui , in o de o de e mine he e u n loss (RL), which indica es he eflec ed elec-
ical powe . Du ing he measu emen s, he ansduce was imme sed in wa e . Since wa e and PZT ha e simila acous ic impedance
alues (𝑍=1.48 × 106𝐾𝑔∕𝑚2𝑠and 𝑍=31×10
6𝐾𝑔∕𝑚2𝑠, espec i ely) compa ed o ai (𝑍= 400 𝐾𝑔∕𝑚2𝑠), his esul s in imp o ed
acous ic ma ching and educed eflec ion o acous ic wa es a he ansduce -medium in e ace, allowing o mo e efficien sound
wa e p opaga ion [17].
In Fig. 6, he RL spec um o he PZT-5H ansduce is p esen ed, ega ding o he equency.
The e o e, i was demons a ed ha he PZT-5H ansduce exhibi s significan ly lowe RL peak le els a 94 kHz (app oxima ely
-22 dB) and 4 MHz (a ound -0.5 dB), co esponding o he adial and TE esonance equencies, espec i ely. The alues closely
esemble he heo e ical alues p e iously discussed and ep esen he highes elec ical ansmission capabili y o he PZT ansduce .
Mo eo e , i can be no ed ha he ansduce exhibi s highe efficiency a he adial esonance equency as opposed o he TE
esonance equency, as he RL alue is much lowe a 94 kHz. I is also no ewo hy ha he peaks occu ing a e he adial esonance
equency ep esen he ha monics o he signal.
5.1.3. Expe imen al cha ac e iza ion
Sine wa es we e used du ing he expe imen al cha ac e iza ion o se e as inpu signals o he emi e ansduce . The ansduce s
we e e alua ed a he adial esonance equency, which is he equency a which he RL alue is he lowes . Ampli ude a ia ions
om 10 mV o 5 V we e applied o he emi ed wa e, wi h he ecei e placed 2 cm away. Table 2and Fig. 7illus a e he nea ly
linea connec ion be ween he emi ed and ecei ed wa e ampli udes. Fo ins ance, when he emi ed wa e has an ampli ude o 1V,
he ecei ed wa e’s ampli ude is a ound 160 mV.
Then, he ansduce s we e shi ed o de e mine how he wa e’s ampli ude changes wi h he dis ance be ween hem. Sine wa es
we e p oduced a ampli udes spanning om 100 mV o 5 V, a he adial esonance equency, and a dis ances o 1, 2, 4, and 6
cm. In Fig. 8and Table 3, i can be seen ha he in ensi y o he ecei ed wa e dec eases as he gap be ween ansduce s inc eases
(wi h a iable emi ing wa e ampli udes), wi h a mo e p onounced decline a sho e dis ances ha s abilizes as he dis ance om
he ansduce inc eases. Acco dingly, he ampli ude a ia ions based on dis ance we e deemed no significan due o he ela i ely
low a enua ion coefficien [17].
Heliyon 11 (2025) e41083
8
B. Pinhei o, V. Pin o, H. Dinis e al.
Fig. 7. The ampli ude o he ecei ed wa e a ies depending on he ampli ude o he emi ed wa e.
Table 3
Va ia ion o he ecei ed wa e ampli ude as a unc ion o he dis ance be ween ansduce s, o diffe en emi ing wa e ampli udes.
Dis ance
E. ampli ude 100 mV 𝜎(mV) 200 mV 𝜎(mV) 500 mV 𝜎(mV) 700 mV 𝜎(mV)
1 20.400 0.400 38.400 0.400 91.600 0.400 128.000 0.000
2 19.867 0.377 34.533 1.050 79.733 0.377 109.333 0.471
4 17.867 0.680 31.600 0.327 73.867 0.377 100.400 1.178
6 18.000 0.653 31.533 0.411 74.867 0.340 102.667 0.943
*E. ampli ude: Emi ing ampli ude.
Fig. 8. The ampli ude o he ecei ed wa e a ies depending on he dis ance sepa a ing he PZT disks, o a ious emi ed wa e ampli udes.
5.2. Hemoglobin pho oacous ic de ec ion
Fig. 9displays he mean o en measu emen s o he PA signals om a sample o dis illed wa e and o h ee hemoglobin concen-
a ions.
The ini ial peaks gene a ed o each sample a e p esen due o elec ical in e e ence when he lase is ac i a ed, as shown in
Fig. 9, i.e., e en when he lase is ocused on a dis illed wa e sample, he ini ial peak is also p esen , e en hough wa e does no
p oduce a PA esponse (a his wa eleng h). Thus, o all samples, his fi s peak was neglec ed when e alua ing he PA signal.
By examining he cu es o Figs. 9and 10, i was no ed ha mo e concen a ed hemoglobin solu ions yielded s onge esponses,
consis en wi h heo y. The e o e, a maximum ampli ude o app oxima ely 44 mV was ob ained o he 10 mg/mL solu ion. Fu he -
mo e, as shown in Fig. 10, i is obse ed a p ac ically linea ela ionship be ween ampli ude and hemoglobin concen a ion.
Heliyon 11 (2025) e41083
9
