inkjet printed (bio)chemical sensing devices

REVIEW

Inkjet printed (bio)chemical sensing devices

Nobutoshi Komuro & Shunsuke Takaki & Koji Suzuki & Daniel Citterio

Received: 7 March 2013 /Revised: 19 April 2013 /Accepted: 23 April 2013 /Published online: 16 May 2013# Springer-Verlag Berlin Heidelberg 2013

Abstract Inkjet printing has evolved from an office print-ing application to become an important tool in industrialmass fabrication. In parallel, this technology is increasinglyused in research laboratories around the world for the fabri-cation of entire (bio)chemical sensing devices or singlefunctional elements of such devices. Regularly stated char-acteristics of inkjet printing making it attractive to replace analternative material deposition method are low cost, simplic-ity, high resolution, speed, reproducibility, flexibility, non-contact, and low amount of waste generated. With thisreview, we give an overview over areas of (bio)chemicalsensing device development profiting from inkjet printingapplications. A variety of printable functional sensor ele-ments are introduced by examples, and the advantages andchallenges of the inkjet method are pointed out. It is dem-onstrated that inkjet printing is already a routine tool for thefabrication of some (bio)chemical sensing devices, but alsothat novel applications are being continuously developed.Finally, some inherent limitations of the method and chal-lenges for the further exploitation of this technology arepointed out.

Keywords Inkjet printing . Screen printing . Electrodes .

Conducting polymers . Microfluidic paper-based analyticaldevices

AbbreviationsDBSA Dodecylbenzenesulfonic acidDOD Drop-on-demand

HRP Horseradish peroxidaseLbL Layer-by-layerLED Light emitting diodeLOD Limit of detectionODF OligodeoxyfluorosidePANI PolyanilinePEDOT-PSS Poly(3,4-ethylene

dioxythiophene)-poly(styrenesulfonate)SERS Surface enhanced Raman

scattering

Introduction

The application of inkjet printing technology is by far nolonger limited to the printing of text or graphical data ontopaper, films, or three-dimensional objects. The probablyearliest report on the application of inkjet printing for(bio)sensor fabrication was published in 1988 [1]. The au-thors of that work looked at inkjet technology to tackle thechallenge of selectively depositing an active sensing layeronto a small area of a field effect transistor. Over the pastyears, inkjet printing has evolved into a general industrialfabrication tool for depositing controlled small amounts ofliquids onto a user-selected, well-defined area. Inkjet print-ing technology has grown up to industrial-scale mass pro-duction, where it is, for example, nowadays used in thefabrication of color filters for light-emitting diodes andfull-color high-resolution flat panel displays [2]. The appli-cation of inkjet printing technology for material depositionand device fabrication has been regularly reviewed in thepast [36]. Promoted by the rapid expansion of applicationareas and the technical progress in printing equipment,inkjet printing technology has relatively early found its roleas a tool supporting the mass production of (bio)analyticaldevices. Probably among the earliest routine applications

Nobutoshi Komuro and Shunsuke Takaki have equally contributed tothis work.

N. Komuro : S. Takaki :K. Suzuki :D. Citterio (*)Department of Applied Chemistry, Keio University,3-14-1 Hiyoshi, Kohoku-ku,223-8522 Yokohama, Japane-mail: [email protected]

Anal Bioanal Chem (2013) 405:57855805DOI 10.1007/s00216-013-7013-z

was the use alternative to contact printing arrayers for thedeposition of DNA samples in the high throughput fabrica-tion of DNA analysis chips [7]. Since then, the prospects ofsimple laboratory scale prototyping and of mass fabricationat comparably low costs have remained driving forces forthe further expansion of inkjet printing technology into thefield of (bio)analytical chemistry, including the fabricationof (bio)chemical sensing devices.

In a previous review, Gonzalez-Macia et al. have givenan overview over advanced printing and deposition meth-odologies for the fabrication of biosensors and biodevices,where inkjet technology plays an essential role [8]. Delaneyet al. took a closer look at inkjet printing applied to proteindeposition [9], and Kukkola et al. published a short over-view of inkjet printed gas sensors [10]. With this criticalreview, we particularly address the (bio)chemical sensorcommunity by looking at the already widespread and furtherincreasing application of inkjet printing in the context of thefabrication of (bio)chemical sensing devices in general. Theterms sensor and sensing devices are used in a broadsense to include also analytical systems that are probably notregarded as sensors under a very strict definition. Based onselected, nonexhaustive examples, it is shown that inkjetprinting is already a routine tool for the fabrication of some(bio)chemical sensing devices, and that novel applicationsare being continuously developed. With the prospects ofsimple laboratory scale prototyping and low cost mass fab-rication in mind, discussion in this work is limited to theapplication of currently commonly available inkjet print-ing equipment. This is to be understood as covering avariety of inkjet printing devices reaching from simple con-sumer desktop printers to the more versatile material print-ing systems for research use.

In the fabrication of (bio)chemical sensing devices, inkjetprinting systems are in most situations used as one fabrica-tion tool among others to perform just a single specificmaterial deposition task in the multi-step process leadingto a complete sensing device. Very often this deposition stepconcerns the formation of an electrically conducting trace(e.g., electrode, electric contact) or the active sensing layer(e.g., polymer film, enzyme or antibody spot, colorimetricreagent, etc.). The first application field strongly profitsfrom developments achieved in printed electronics [11]. Inparticular, in the fabrication of metallic planar electrodes(Ag, Au) for electrochemical sensors, inkjet printing canalready be regarded as a routine technology. This is bestreflected by the fact that various ready-to-use inkjetprintable metal nanopaste inks are commercially avail-able. In more recent approaches, inkjet printing is ap-plied for the microfluidic patterning of paper sensorsubstrates. Finally, attempts are made to fabricate entire sens-ing devices (all inkjet printed devices) relying on inkjetprinting technology alone.

We try to provide some answers, in particular to thequestion why inkjet printing has been selected as the methodof choice among the available alternative technologies.

General comparison of inkjet printing and alternativetechnologies commonly applied in the fabricationof (bio)chemical sensing devices

The principal function performed by an inkjet printing systemused as a fabrication tool is the controlled deposition of smalldroplets of liquids (ink) onto a substrate (Fig. 1). After theevaporation of the solvent, the nonvolatile ink components areleft over as the deposited target material. When evaluating theusefulness of inkjet printing technology in the context of thefabrication of (bio)chemical sensing devices, this materialdeposition function of the inkjet printer can be comparedwith other methods fulfilling an equivalent purpose. Of par-ticular relevance are (1) drop casting (manual pipetting), (2)contact printing, (3) pneumatic dispensing, (4) screen printing,(5) spin coating, and (6) photolithography, among others.

From a merely process mechanics point of view, dropcasting, contact printing, and pneumatic dispensing are mostclosely related to inkjet printing technology. All methodstransfer a controlled amount of liquid from a reservoir (e.g.,pipette, ink tank, microplate, etc.) to user-selected spots on asubstrate. They all allow flexible spatial control of the materialdeposition process. However, when it comes to achievableresolution, there are significant differences. Both drop castingand pneumatic dispensing are inferior to inkjet printing, mainlybecause of the differences in dispensed volumes. The volumeof the liquid droplet determines to a large extent the resolutionof the deposited feature. Reproducible droplet formation in thepicoliter volume order is nowadays routinely possible withalmost any type of inkjet printing device (Fig. 2), whereas suchsmall droplets are not readily created with pipettes or pneu-matic dispensers. In the case of contact printing, similar reso-lution to inkjet printing is achieved [12], but disadvantages

Fig. 1 Ink droplets (10 pL nominal droplet size) deposited at 100 mdrop spacing onto a glass slide using a piezoelectric inkjet printing device

5786 N. Komuro et al.

include the risks of spot-to-spot cross-contamination and ofdamaging delicate substrates.

In (bio)chemical sensing device fabrication, screen printingis a widespread method to produce electrodes on planar sub-strates [13]. The target material is deposited onto the substrateby forcing the ink through the pores of the screen, whichdefines the outline of the printed feature. For the fabrication ofprinted electrodes or electrical contacts, screen printing isprobably the most serious competitor of inkjet printing,since it enables very fast and low cost production with largenumbers of replicates. In some cases, screen printing is ap-plied in combination with inkjet printing in order to profitfrom the advantages of both technologies. From a materialdeposition point of view, screen printing allows for the fastcreation of relatively thick films (20100 m). However, theachievable control over the layer thickness is low comparedwith inkjet printing. Furthermore, in terms of spatial control, aseparate screen is required for every print pattern, whichmakes the technology useful for the reproduction of largenumbers of identical layouts, but lacks the flexibility offeredby inkjet printing when it comes to the free choice of shapeand layout for device prototyping.

Spin coating is a fast and simple method for depositing thin,uniform layers of a material (e.g., a polymer film) on compa-rably large areas. However, it does not offer the possibility ofspatial control and, therefore, cannot compete with inkjetprinting in terms of resolution and flexibility. Furthermore, itis not very economical in material use, since a significantamount of the coating liquid does not remain on the substrate,but is wasted.

Inkjet printing and the alternative material depositionmethods (1)(4) listed above are referred to as additivedeposition methods, although inkjet printing can also beapplied in a subtractive way [14, 15]. With the exceptionof the solvent, only the amount of material required to obtain aspecific structural feature is deposited onto the substrate,avoiding the waste of potentially high-cost materials. Their

additive character makes those methods comparably econom-ical in terms ofmaterial use. In contrast, photolithography [16]is based on a subtractive approach, resulting in the creation ofwaste. While photolithographic methods are superior to inkjetprinting in terms of achievable resolution, their subtractivecharacter calls for a larger number of processing steps and theselective removal of pre-deposited material requires a specificmask for every pattern, resulting in lower flexibility.

Material deposition methods featuring even higher resolu-tion and patterning densities, such as for example soft lithogra-phy approaches [16, 17] and dip-pen nanolithography [16, 18],are not discussed here, although they are increasingly applied,in particular in the fabrication of bioanalytical devices.However, their processing throughput and their cost efficiencyare at present not competitive to inkjet printing.

Piezoelectric or thermal inkjet printing? Consumerdesktop or research-use material printer?

Drop-on-demand (DOD) piezoelectric and thermal inkjetprinting are the most widely used technologies in industrialprocess and consumer desktop printing, with the piezo-technology dominating the industrial area and the thermal-type the consumer office market [19]. Without providingfurther technical details, the following are the basic principlesof these two methods: In piezoelectric printing, applying avoltage to a piezoelectric element in contact with the printernozzle results in a volume displacement and the ejection of anink droplet. In thermal inkjet technology, the volume displace-ment is achieved by the creation of a gas bubble on a heatingelement inside the nozzle. Judged from the number of appli-cation examples (Table 1), there still is a significant preferencefor the piezoelectric technique when it comes to the fabrica-tion of (bio)chemical sensors. In particular, when workingwith protein containing printing inks, the fact of the ink beingexposed to a 300 C heat pulse seems to be detrimental toenzyme activity. However, studies confirming the suitabilityof thermal inkjet printing also for the printing of supposedlysensitive enzyme inks have been published [2023]. On theother hand, protein damage caused by shear forces occurringin piezoelectric printer nozzles has been reported [24]. Inmany situations, it is not necessarily the chosen printingtechnology but rather the influence of ink additives (e.g.,solvents, surfactants, viscosity modifiers, humectants), whichhave a negative impact on the printing of protein containinginks [25]. It is well known that enzyme denaturation occurs inthe presence of surfactants. Fortunately, the amount of surfac-tant required to reduce the surface tension of aqueous inks toan ideally printable range (30 mN/m) is usually relatively lowand does not significantly decrease enzyme activity, in partic-ular when using non-ionic surfactants (e.g., 0.1 wt% of TritonX-100). Conversely, evaluated on the example of horseradish

Fig. 2 Ink droplets (10 pL nominal droplet size) ejected from an arrayof nozzles with 21 m diameterdiameter; the distance between the bluelines is 100 m

Inkjet printed (bio)chemical sensing devices 5787

Tab

le1

(Bio)chemicalsensingdevicesfabricated

byinkjetprinting.Tableitemsaregroupedaccordingtothesensor

transductionmechanism

,which

isnotnecessarilythesameorderas

inthemaintext

Transductionmechanism

aSensing

applications

Inkjetprintedfeatures

Printerb

Type

Substrate

Ref

Electrochem

ical/

potentiometric,

amperometric

pH,glucose

AuandAgelectrodes

(nanoparticles)

Dim

atix

DMP-280

0Piezo

Coatedpaper

[31]

Electrochem

ical/

impedimetric

antiC-reactiveprotein

(CRP)antib

ody

Auelectrodes

(nanoparticles)

Dim

atix

DMP-280

0Piezo

Coatedpaper

[40]

Electrochem

ical/

amperometric

Interleukin-6

(cancerbiom

arker)

Auelectrodearray

(nanoparticles)

Dim

atix

DMP-280

0Piezo

Polyimide

[41]

O2

Auelectrodeprecursor

(nanoparticles)

Epson

R230

Piezo

Mixed

cellu

lose

ester

[42]

Aqueous

NH3

Polyanilin

enanoparticle

sensinglayer

Dim

atix

DMP-2811

Piezo

Carbonelectrodes

c

onPETfilm

[50]

Epinephrine,

norepinephrine,

dopamine

Ammonium

persulfateoxidant

forvaporphasepolymerization

ofpolyaniline

electrodes

Canon

PixmaIP1300

Therm

alPETfilm

[52]

Glucose,H2O2

PEDOT-PSSdfilm

Oliv

etti

Therm

alindium

tinoxideglass

oncellu

lose

acetate

[20,

21]

Glucose

oxidaseor

horseradish

peroxidase

enzyme

Triglycerides

Au/PEDOT-PSSnanocomposite

sensinglayer

Dim

atix

DMP-280

0Piezo

Carbonelectrodes

c[53]

H2O2,NADHe ,

Salbutamol

Graphene-PEDOT-PSS

nanocompositesensinglayer

Dim

atix

DMP-280

0Piezo

Carbonelectrodes

c[54,

55]

ChemicalO2demand

(COD)

TiO

2photoanode

(nanoparticles)

Epson

R290

Piezo

indium

tinoxideglassslides

[58]

Electrochem

ical/

chromoamperometric

Ascorbicacid

Conductingpolymer

layer

(polyanilin

e)Dim

atix

DMP-280

0Piezo

Screen-printedcarbon

electrodes

onfilterpaper

[104]

Electrochem

ical/

resistance

Alcohol

vapor

Conductingpolymer

carbon

nanotube

compositeelectrodes

HPDeskjet690C

Therm

alPETsheets

[45]

Organicvapors

Reduced

graphene

oxideelectrodes

HP4250

Therm

alPETtransparency

[46]

CO2

Pd-polymer

nanocompositeas

seed-layer

forelectrodeform

ation

Dim

atix

DMP-280

0Piezo

PETfilm

[56]

PEDOT-PSSsensinglayer

Ethanol,methanolvapor

PEDOT-PSSsensinglayer

HPDeskjet693C

Therm

alPolyester

film

[57]

Electrochem

ical/

conductometric

NH3gas

Polyanilin

enanoparticlesensinglayer

Epson

C46/C48

Piezo

Silv

erelectrodes

con

PETfilm

[49]

CO,H2,H2S,NO

WO3nanoparticlesensinglayer

Dim

atix

DMP-280

0Piezo

Si/S

iO2substrates

with

lithographically

defined

Ti/P

telectrodes

[10]

Optical/absorbance

Acetic

acid

vapor

Ethyl

cellu

lose

polymer

sensing

layerwith

pH-indicator

Dim

atix

DMP-2811

Piezo

LEDlens

surface

[59]

NH3gas

Plasticized

PMMAfsensingfilm

with

chromogenicindicator

Dim

atix

DMP-283

1Piezo

Polym

eropticalwaveguide

[60]

Optical/colorimetric

Organicsolventvapor,

temperature

Monom

erprecursorforform

ation

ofpolymericsensingfilm

HPDeskjetD2360

Therm

alCopypaper

[62,

63]


Tab

le1

(contin

ued)


aSensing

applications


Printerb

Type

Substrate

Ref

Phenolic

compounds

Layer-by-layerassembled

enzymatic

sensingfilm

Dim

atix

DMP-280

0Piezo

Filter

paper

[64]

Neurotoxins,

organophosphate

pesticides,bacteria

infood

samples

Enzym

e-dopedsol-gelsensing

layer,chromogenicreagents,

oxidizingagent

Dim

atix

DMP-280

0Piezo

Filter

paper,cardboardpaper

[6567]

Phosphate

Hydrophobicbarrierpattern

(reactionzones)

Canon

iP4700

Therm

alFilter

paper

[89]

Malariaantig

enDetectio

nantib

odylin

esScienion

Piezo

Nitrocellulose

strips

[93]

ATP,IgG

Detectio

nantib

odylin

es(antibodies

andDNAaptamerscoupledto

polymer

nanoparticles)

Dim

atix

DMP-280

0Piezo

Filter

paper

[105]

BSA,glucose

Outlin

eof

microfluidicpattern

gHPLaserJet1000

Therm

alFilter

paper

[106]

HRPactiv

ityHRPenzymeinside

microfluidicchannel

Canon

Pixmaip4500

Therm

alFilter

paper

[23]


(fluorescence-based)

Foodspoilage

andripening

Sensing

molecules

(olig

odeoxyfluorosides)

HPDeskjetF4280

Therm

alCottonpaper

[68]

Ricin

Carbohydratespots(galactose)

assensingmolecules

Epson

R280

Piezo

Chrom

atographypaper

[69]

Optical/fluorescence

pHPhotopolymerizablemonom

ermixture

includingpH

-indicator

Not-specifie

dresearch-typeprinter

Therm

alOpticalfibertip

[70]

C-reactiveprotein(CRP)

Fluorophore

labeledantib

ody

zone

inmicrofluidicchip

Autod

ropMD-P-705

-LPiezo

Microfluidicchip

(Si/polydimethylsilo

xane)

[71]

Carbohydrate-bindingproteins

BSAh-glycoconjugates

assensingspots

Scienion

S3Flexarrayer

Piezo

Siliconmicroring

resonators

[72]

Optical/SERS

Rhodamine6G

Substratehydrophobizatio

nEpson

Workforce

30Piezo

Chrom

atographypaper

[73]

Ram

anenhancer

layer

(Agnanoparticles)

Malathion,heroin,

cocain

Ram

anenhancer

layer

(Agnanoparticles)

Epson

Workforce

30Piezo

Chrom

atographypaper

[74]

Mechanical/mass

pH,DNA,gas

Alkanethiol

SAMs,DNA

oligom

ers,polymer

film

sAutod

ropMD-P-705

-LPiezo

Siliconcantilevers

[75]

Streptavidin

Protein

layer(biotin

ylated

BSA)

Dim

atix

DMP-283

1Piezo

Siliconcantilevers

[76]

NO2gas

Microsphere-tem

plated

BaC

O3film

sHew

lett-Packard

Therm

alQCM

[107]


pH,protein,

glucose

Microfluidicpattern

PicoJet-200

0Piezo

Filter

paper

[15]

Sensing

spotswith

chromogenicreagents,enzymes

pH,human

IgG,mouse

IgG

Microfluidicpattern

PicoJet-200

0Piezo

Filter

paper

[84]

Capture

antib

odies,

chromogenicreagents

Nitrite

ions

Microfluidicpattern

Canon

Pixmaip4500

Therm

alFilter

paper

[85,

86]


Tab

le1

(contin

ued)


aSensing

applications


Printerb

Type

Substrate

Ref

Sensing

spotswith

chromogenicreagents

pH,H2O2

Microfluidicpattern

Epson

PX-101

Piezo

Filter

paper

[87,

88]

Sensing

spotswith

pH-indicators


Heavy

metals

Enzym

e-dopedsol-gelsensing

layer,chromogenicreagents

Dim

atix

DMP-280

0Piezo

Filter

paper

[92]

Hydrophobicbarrier

Optical/electrochemi-

luminescence

DBAEi ,NADHe

Microfluidicpattern

Canon

Pixmaip4500

Therm

alFilter

paper

[90]


Glucose

Hydrophobicbarrierpattern

(reactionzones)c

Dim

atix

DMP-280

0Piezo

Various

papersubstrates

[91]

Electrochem

ical/resistance

pH

Optical/reflectometry

IgG

Microfluidicstructure

PicoS

potJet

DispensingSystem

Piezo

Hydrophobically

coated

glassslides

[108]

Photoniccrystals

Antibodyspots

Blockingsolutio

n

aBoldface

indicatesan

allinkjetprinted

approach,where

allmajor

sensor

fabricationstepswereachieved

with

aninkjetprinter.

bItalicfont

indicatestheapplicationof

aresearch-use

materialprinter,whereas

plainfont

indicatestheuseof

aconsum

erdesktopprinter.

cElectrodesarefabricated

byscreen-printing.

dPoly(3,4-ethylenedioxythiophene/polystyrene

sulfonicacid).

eNicotinam

ideadeninedinucleotid

e.fPoly(methylmethacrylate).

gThe

actualpattern

isdraw

nwith

awax-pen,follo

wingtheinkjetprintedpattern

outline.

hBovineserum

albumin.

i2-(D

ibutylam

ino)ethanol.


peroxidase (HRP), it was found that the presence of viscositymodifiers can lead to a significant decrease in enzyme activity,in particular when using high molecular weight additives. Inthe case of poly(ethylene glycol) addition, an increasinglynegative impact on HRP activity with increasing molecularweight of the poly(ethylene glycol) was observed. The use ofthe charged polymer sodium carboxymethyl cellulose wasshown to have the lowest impact on enzyme activity in thecase of HRP. This is due to the high viscosity modifyingpower of that material, making the application of smallamounts sufficient (e.g., 0.5 wt% of carboxymethyl celluloseare sufficient to achieve an ink viscosity of 5 cPs, comparedwith up to 80 wt% required to obtain the same viscosity in thecase of using ethylene glycol as additive).

As a consequence, the type of inkjet printing technologyapplied is not of primary concern. Both piezoelectric andthermal inkjet printing can be considered for the prototypingand mass production of (bio)chemical sensing devices. Withboth methods, the evaluation of the stability of a particularprotein/enzyme under inkjet printing conditions is required.Probably researchers will tend to stick with piezoelectricsystems, as long as those retain their dominance in theindustrial inkjet printing sector.

The question of what type of printer to use can beaddressed from several points of view. Factors to considerinclude cost, user-friendliness, and versatility. While themore sophisticated material printing systems targeting re-search applications offer full control over printing parame-ters in terms of hardware (e.g., choice of nozzle diameter,heatable nozzles and printing stages, visual confirmation ofink droplet formation, etc.) and software (e.g., control ofpiezo-pulse voltage and duration, etc.), consumer desktopprinters are basically black-box systems offering onlylimited user control via the original printer driver andgraphics software used for printing pattern creation (e.g.print quality selection, etc.). On the other hand, the simplic-ity of these devices allows for the adaptation to specificneeds, which is less possible with the high-cost materialprinters. In addition, all current consumer desktop printerscome equipped with at least four ink tanks (black, cyan,magenta, yellow), thus theoretically allowing the simulta-neous use of multiple inks. Although this feature is rarelyused in the printing of (bio)chemical sensing devices withconsumer desktop printers up to now, it is a potentially largeadvantage over many material printing systems, most ofwhich can handle only a single type of ink at a time. Astrong point in favor of research-use material printers is theirresistance to a variety of chemical substances, includingorganic solvents of high dissolving power. Among the ma-terial printing instruments available on the market, thoseoffering single-use exchangeable print heads and car-tridges might be advantageous when working with experi-mental printing inks that tend to clog printer nozzles,

especially by particle aggregation. Users of such difficultinks might feel more comfortable working with such asystem, compared with a high precision print head instru-ment costing several thousand dollars.

Many research groups rely on several types of inkjet print-ing systems (Fig. 3). In our own work, a desktop printer isgiven preference whenever applicable. One argument besidescost for favoring the use of consumer inkjet printers is the ideathat if a (bio)chemical sensing device can be fabricated on anunmodified consumer desktop printer, it is probably adaptablefor mass production on any type of currently applied inkjetprinting system. The consumer inkjet printer quasi providesthe proof-of-concept for the general printability of a sensingdevice.

(Bio)chemical sensing devices fabricated by inkjetprinting

In Table 1, a nonexhaustive list of application examples ofinkjet printing technology (both desktop and material printers)from the scientific literature dealing with (bio)chemical sens-ing devices is presented. Not all entries listed in the table arediscussed in detail. The table as well as the ensuing textsections have been organized based on the respective sensingtransduction mechanism, with overlaps occurring. As a result,it is readily visible that inkjet printing technology is nowadaysapplied to the development and fabrication of almost any typeof (bio)chemical sensing device.

Electrochemical sensors

Promoted by the progress in printed electronics in general,DOD inkjet printing technology is widely used to fabricateelectrodes, resistors, transistors and other electronic parts forelectrochemical sensing devices. The most regularly explicitlystated reasons for referring to inkjet printing in these applica-tions are the low cost, the simple process, the efficient use ofink and the compatibility with various substrates.

Electrode fabrication

Because of its simplicity and suitability for mass production,screen printing has become a major technology for the fabri-cation of planar electrodes, which form the basis for a varietyof electrochemical sensors [13]. Widely applied screen printedelectrode materials are carbon, silver and gold. However, be-sides of lacking flexibility by requiring a specific screen forevery electrode pattern, the loss of ink in particular in small-scale fabrication is a further disadvantage of this method.Therefore, DOD inkjet printing technology with its efficientuse of ink has become an attractive alternative for the fabrica-tion of electrodes, especially for research and prototyping


applications, where pattern flexibility is highly desired.Because metal electrode fabrication is the application of inkjetprinting technology closest to routine in (bio)chemical sensordevelopment, some work reported in this area is discussed inmore detail.

In particular, commercially available silver nanoparticlesuspensions (silver nanopaste) are already routinely usedfor Ag electrode printing in the fabrication of (bio)chemicalsensors, for example [2633]. The applied silver nanopasteinks are based on organic solvents and dispersing agents toprevent aggregation of the metal nanoparticles. Their print-ing requires the use of a research-type material printer. Inaddition, to gain sufficient conductivity of the printed metaltraces, an elevated temperature sintering process (>200 C)is necessary after printing to remove the solvent and theparticle stabilizers, which prevents the use of heat sensitivesubstrates. These limitations are the driving force for con-tinued research on alternative methods.

In a very simple, environmentally friendly approach,Bidoki et al. fabricated silver electrodes on paper, overheadprojector transparencies, and cotton, by subsequently printingaqueous solutions of ascorbic acid and silver nitrate with athermal desktop printer [34]. When the silver nitrate and theascorbic acid mix on the substrate, silver nitrate is reduced tometallic silver. Unreacted materials are removed by washingwith water, followed by heating (150 C, 20 s). An additionaladvantage of this approach, besides of being solvent-free,requiring no extended sintering, and being printable on adesktop printer, is that with the absence of suspended particlesin the ink, the risk of printer nozzle clogging is much less anissue. A serious drawback, however, is the low conductivity,which is only about 0.3% the value of bulk silver. For thisreason, researchers keep searching for methods to reduce thetemperature and time required for sintering of structuresprinted from silver nanoparticle inks, or to increase the con-ductivity of silver traces obtained by the printing of metal saltprecursors. An interesting example of the first approach is thework by Magdassi et al., where the sintering temperature foran inkjet deposited silver nanoparticle ink has been lowereddown to room temperature [35]. This has been achieved byprinting Ag nanoparticles (thermal desktop printer) stabilizedby a negatively charged polymer, poly(acrylic acid) sodium

salt, onto paper or plastic substrates precoated with a positive-ly charged polymer, poly(diallyldimethyl ammonium chlo-ride). The induced charge neutralization results in aspontaneous sintering of the silver nanoparticles at roomtemperature, leading to metallic silver traces with 20% theconductivity of bulk silver, a value otherwise only achievedby extended sintering at high temperatures. Perelaer et al. havedemonstrated a time reduced sintering method for electrodesprinted with a commercial silver nanopaste ink (piezoelectricresearch-type printer) resulting in 40% of bulk silver conduc-tivity [36]. The method relies on a combination of photonic-and microwave-induced sintering that can be completed inless than 15 s, making it suitable for a continuous roll-to-rollfabrication process. In another approach chosen by Tobjrk etal., conductivities corresponding to 10%20 % of that of bulksilver have been obtained by exposing electrodes printed witha commercial Ag nanopaste ink (piezoelectric research-typeprinter) for 15 s to an IR lamp [37]. This method was alsosuccessfully applied to electrodes printed from gold nanopar-ticle inks (piezoelectric research-type printer) [38]. The sameresearch group has finally combined inkjet printed gold work-ing and counter electrodes with an inkjet printed silver elec-trode, electrochemically converted into an Ag/AgCl referenceelectrode, to obtain an amperometric three-electrode papersensing device for glucose [31]. This low-cost inkjet printedthree-electrode paper chip platform, which can be obtainedthrough a roll-to-roll process, performs equally well to a morecostly conventional electrochemical sensor setup. Nie et al.have demonstrated the possibility to achieve highly conduc-tive silver lines on PET substrates (50% the conductivity ofbulk silver) from a particle free metal precursor solution(silver citrate) [39]. The approach is based on the thermalreduction of the inkjet deposited silver citrate (piezoelectricdesktop printer) in the presence of 1,2-diaminopropane. Theformation of a silver-amine complex results in a decreasedredox potential, allowing the thermal reduction of Ag+ atrelatively low temperatures (50 min at 230 C). By reducingthe heat treatment temperature to 150 C, which allows the useof more heat-sensitive substrates, still 10% of bulk silverconductivity can be achieved.

In many applications, the use of gold electrodes is prefer-able over silver electrodes because of the chemical inertness of

a b cc

Fig. 3 Different types of inkjet printing devices for laboratory use: (a) research-use material printer accommodating a multiple nozzle single-useprint head; (b) research-use inkjet dispenser with single nozzle print head; (c) piezoelectric consumer desktop printer


this metal. Commercially available or laboratory prepared Aunanopaste inks are generally composed of a high boilingorganic solvent and alkanethiol capped Au nanoparticles[38]. Therefore, they suffer from limitations similar to Agnanopaste inks. However, gold electrodes allow for very sim-ple surface modification with thiol compounds, making themparticularly useful for the immobilization of capture mole-cules, which has, for example, been applied in the develop-ment of inkjet printed electrochemical immunosensors(piezoelectric research-type printer) [40, 41]. The inks forthe printing of these electrodes contain 15 wt% of goldnanoparticles. The use of an organic solvent and of the cap-ping reagents is necessary to allow for the stable dispersion ofthe metal nanoparticles at such high concentrations. Hu et al.have realized an interesting solution to overcome this draw-back. For the fabrication of their paper-based solid-state elec-trochemical oxygen sensor, they printed a diluted aqueous Aunanoparticle ink from a piezoelectric desktop printer onto amixed cellulose ester membrane [42]. The amount of goldnanoparticles deposited by the 7-fold printing of the diluteaqueous ink is not sufficient to achieve electrically conductingmetal traces. However, the printed features act as nucleationsites for the following seeded growth of metallic gold uponimmersion of the substrate into a HAuCl4 containing platingsolution. After seven print cycles, followed by eight growthcycles, a conductivity of approximately 10% of that of bulkgold was achieved. With this method, the authors were able toproduce more than 200 electrode arrays in 30 min of printing,followed by 2 h of Au growth with no other equipment than acommon desktop inkjet printer (Fig. 4).

While the fabrication of metallic Ag electrodes (and to alesser degree also of Au and Cu electrodes) by inkjet print-ing is becoming a routine procedure, the situation is differ-ent for another important electrode material. For carbonelectrodes that very often form the basis of electrochemical(bio)sensors, screen printing continues to remain the fabri-cation technology of choice. The probably most impressivesuccess story of screen printed carbon electrodes is theirwidespread application as working electrodes in disposabletest strips of personal glucose monitoring devices for pa-tients suffering from diabetes [13, 43].

A few inkjet approaches with carbon nanotube-basedinks have been reported, since the printability of this mate-rial from aqueous dispersions has been first demonstrated[44]. One example is an inkjet printed (thermal desktopprinter) alcohol vapor sensor relying on conductivitychanges of carbon nanotube electrodes, stabilized with aconducting polymer, upon exposure to alcohols [45]. Theuse of a different carbon material, an aqueous dispersion ofreduced graphene oxide, has first been demonstrated for thefabrication of an inkjet printed (thermal desktop printer)electrochemical gas sensor on standard overhead projectorfilms [46]. Exposure to organic vapors results in changes of

the electrode conductivity. These are examples of applica-tions where the inkjet deposited electrode material simulta-neously acts as the active sensing layer.

Inkjet printing of active sensing layers for electrochemicalsensors

It has been shown in the previous section that inkjet printingcan be a great tool for the very simple and rapid fabricationof electrodes, in some cases requiring nothing more than anoff-the-shelf consumer desktop printer. Another importantapplication of inkjet printing technology is the coating ofelectrodes with an active sensing layer. Frequently, thatlayer consists of a film of a conducting polymer with thepurpose of fabricating an electrochemical gas sensorwhereby, in some cases, the polymer simultaneouslyacts as electrode and active sensing layer without therequirement of a separate underlying electrode substrate.Weng et al. have reviewed the printing of conducting poly-mers in general [47].

Widely inkjet printed conductive polymers includepolypyrrole, polyaniline (PANI), and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS).One problem encountered with PANI is the fact that it isinsoluble in water and even in many common organic sol-vents, which makes the straightforward formulation ofinkjet printable inks challenging. When the use of an off-the-shelf desktop printer is targeted, the inks have to be

Fig. 4 Inkjet printing of gold electrode arrays on mixed cellulose estermembranes for fabricating paper-based electrochemical O2 sensors: (1)inkjet printing of Au nanoparticle electrode precursor patterns, (2)growth of Au nanoparticle patterns into Au electrode arrays, (3) cuttinga paper-based Au electrode array (PGEA) from its ensembles, (4)electric connection and size control of a PGEA, and (5) addition ofBMIMPF6 ionic liquid electrolyte from the back side of a PGEA tofabricate the final O2 sensor. Reprinted with permission from Refer-ence [42] Hu C, Bai X, Wang Y, Jin W, Zhang X, Hu S (2012) Inkjetprinting of nanoporous gold electrode arrays on cellulose membranesfor high-sensitive paper-like electrochemical oxygen sensors usingionic liquid electrolytes. Anal Chem 84:37453750. Copyright(2012) American Chemical Society


based on water or mild organic solvents. One way to resolvethis defect is to use dodecylbenzenesulfonic acid (DBSA)-stabilized PANI nanoparticles for inkjet printing. The anion-ic dopant DBSA prevents the aggregation of PANI, which isthe main cause of printer nozzle clogging. It allows for thepreparation of aqueous dispersions with a relatively highpolymer content [48]. DBSA-PANI dispersions stabilizedagainst aggregation can be printed not only from state-of-the-art research printers but also from desktop printers.Crowley et al. reported sensors for aqueous or gas phaseammonia detection obtained by a combination of screenprinting of underlying silver or carbon electrodes and inkjetprinting of DBSA-PANI on a piezoelectric research-type ora piezoelectric desktop printer [49, 50]. The thickness of thedeposited PANI chemiresistor films significantly influencesthe gas sensor properties (e.g., response time, stability, re-producibility, etc.). A great advantage of the inkjet printingapproach is the flexible variation in the deposited total filmthickness by simply varying the number of applied printinglayers, which under the conditions used by the authors had athickness of 170 nm [51].

A different approach to a PANI pattern printed from athermal desktop inkjet printer has been reported for a biosen-sor targeting the detection of electrochemically active bio-molecules excreted from cells [52]. In this example, only anaqueous solution of an oxidant, ammonium persulfate, isinkjet patterned on the substrate. Doped PANI films are thencreated through vapor phase polymerization by exposing thepatterned substrate to aniline monomer/HCl at 70 C for 15min. With this method, dispersion stability of the printing inkis not an issue, since a simple aqueous solution is the onlyinkjet deposited material (Fig. 5).

Having advantages such as high conductivity and highoptical transparency, PEDOT-PSS is widely used in electro-chemical and optical sensing. The polymer is water-solubleand ready-to-print aqueous ink dispersions are commerciallyavailable. The research group of Setti and coworkers wasamong the first to use this conducting polymer for the fabri-cation of inkjet printed amperometric enzymatic biosensors ona modified thermal desktop inkjet printer [20, 21]. They inkjetprinted glucose oxidase and horseradish peroxidase solutionson conducting polymer layers obtained by inkjet deposition ofaqueous PEDOT-PSS dispersions on indium tin oxide glass.In an amperometric enzymatic biosensor for triglyceride de-tection, Phongphut et al. used a novel composite material ofAu nanoparticles and PEDOT-PSS inkjet printed on screenprinted carbon electrodes (SPCE), where the Au nanoparticlesenhanced the electrochemical response [53]. Another com-posite material enhancing the electrochemical activity of aPEDOT-PSS amperometric sensor has been presented byTuantranont and coworkers for the direct electrochemicalsensing of hydrogen peroxide (H2O2), nicotinamide adeninedinucleotide (NADH), and salbutamol, respectively [54, 55].

An electrochemically synthesized graphene-PEDOT-PSSnanocomposite was inkjet printed as an aqueous dispersionon top of screen printed carbon electrodes. Sensors preparedwith the composite material showed significantly higher elec-trochemical response compared to unmodified SPCE orPEDOT-PSS modified electrodes.

Tseng et al. made use of a piezoelectric research-typeinkjet printer for both the patterning of Au electrodes andthe coating with a PEDOT-PSS film for the development ofa CO2 gas sensor (Fig. 6) [56]. This work is one example forusing inkjet technology for both the electrode fabricationand deposition of the active sensing layer.

Mabrook et al. fabricated a chemiresistor for alcohol sens-ing by printing PEDOT-PSS on polyester films [57]. Theynoticed different response behavior depending on theconducting polymer film thickness, which was easily con-trolled by the number of printing layers. For thick films (4-5print layers, 500-700 nm thickness), reversible response toethanol vapor was found. On the other hand, thin films (singleprint layer, about 100 nm thickness) behaved as chemicalfuses, responding irreversibly above a certain alcohol vaporconcentration threshold. These differences can be explainedby the solubility of PEDOT-PSS in alcohols, resulting instructural rearrangements of the sensing films upon exposureto the analyte. Compared to a sensing layer obtained by dropcasting, a thin inkjet printed film has a more disordered andopened structure, since it is formed by single partiallyoverlapping droplets. The rapid penetration of alcohol vaporinto an open structure thin film results in the irreversiblebreakdown of electrical conductivity. This example demon-strates that different sensor response can be achieved from thedeposition of a single polymer film material at different den-sities and thicknesses.

The formation of controlled particle layers of semiconduct-ing metal oxides is of special interest for the development ofelectrochemical gas sensors. Kukkola et al. printed additive-free aqueous suspensions (piezoelectric research-type printer)of catalyst-modified WO3 nanoparticles for CO, H2, H2S andNO sensing [10]. Owing to the extremely efficient material useof the inkjet printing technology with almost no waste of inkcompared to other material deposition methods, the authorscalculated the material costs (only the WO3 nanomaterial) fortheir devices to be no more than 1 Euro per 1 million devices.They concluded that by combining inkjet deposition with lowcost substrates, the costs of a final sensor would be mostlydetermined by the measurement electronics, rather than thesensing materials.

Yang et al. have selected inkjet printing as a cost-effectivemass production technology to produce robust and highlyreproducible TiO2 photoanodes, applied to the measurementof glucose, phenol, potassium hydrogen phthalate, glutaricacid, malonic acid, and chemical oxygen demand [58]. Theywere particularly looking at a replacement for the dip coating


technique, which lacked in reproducibility. They used a pie-zoelectric consumer inkjet printer to deposit an aqueous col-loidal TiO2 particle (6 wt%) ink containing 1.8 wt% ofcarbowax as viscosity modifier onto indium tin oxide glass.The inkjet printed sensing layers were directly compared withthose deposited by the conventional dip coating method.Figure 7 shows a comparison of inkjet printed and dip coatedTiO2 electrodes. A more homogenous film formation is clear-ly confirmed for the inkjet printed electrode. As in other casesof depositing an active sensing layer by inkjet printing, filmthickness control was easily achieved by varying the numberof printed layers. The sensing results revealed that inkjetprinted TiO2 films have good fabrication reproducibility andthat the relative standard deviations for independently fabri-cated electrodes are smaller compared with commonly useddip coated sensors.

Optical sensors

Research on optical (bio)chemical sensors has been very in-tensive over the past decades and continues to increase. Theessential part of any optical sensor is the active sensing layerincorporating the signal transducing molecules. Major issueswith mass production of optical sensors are reproducible,speedy, and controlled deposition of the active sensing layer.Regarded as being rapid, reproducible, controllable (e.g., size,shape, position of deposited material), contamination-free, and

cost effective, inkjet printing technology is increasingly beingused as a deposition method for active layers of optical(bio)chemical sensors.

Absorbance/colorimetry-based sensors

Absorption spectrometry and colorimetry are among thesimplest optical detection methods. Inexpensive opticalcomponents such as light emitting diodes (LEDs) are now-adays widely available and applied to field-deployable op-tical (bio)chemical sensing devices. To fabricate sensitiveand reproducible devices, the uniformity of the sensing layerdeposited onto an LED is one of the most important factors.All of the deposition methods discussed earlier, includingdip coating, drop casting, spin coating, and screen printing,with their known deficiencies in terms of reproducibility,resolution, freedom of patterning and others, are regularlyreported for the fabrication of optical sensors. Inkjet tech-nology is regarded as a painless solution to overcome thesedeficiencies, and resulting sensors are assumed to havegreatly improved reproducibility and sensitivity. In the fol-lowing, a few selected examples will be discussed to verifythis assumption.

M. OToole et al. reported a sensor for gaseous aceticacid based on an ethyl cellulose sensing layer incorporatinga colorimetric pH-indicator [59]. They dissolved all sensingmaterials in an organic solvent (1-butanol) and inkjet printed

Fig. 5 Schematic diagram of an inkjet printing-assisted fabrication ofa polyaniline (PANI) pattern. Reprinted from Reference [52] Oh W-K,Kim S, Hwan Shin K, Jang Y, Choi M, Jang J (2013) Inkjet printed

polyaniline patterns for exocytosed molecule detection from live cells.Talanta 105:333339, Copyright (2013), with permission fromElsevier

Fig. 6 Schematic illustration of the fabrication process of a PEDOTPSS film modified electrode, where both the electrode pattern and theconducting polymer film are deposited by inkjet printing. Reprintedfrom Reference [56] Tseng CC, Chou YH, Hsieh TW, Wang MW, Shu

YY, Ger MD (2012) Interdigitated electrode fabricated by integrationof ink-jet printing with electroless plating and its application in gassensor. Colloid Surf A, 402:4552, Copyright (2012), with permissionfrom Elsevier.


(piezoelectric research-type printer) the solution directlyonto the lens surface of an LED light source. For compari-son, sensors were also prepared by drop casting 5 L of theprinting ink onto the LED by means of a micropipette. Inkjetdeposition allowed for the reproducible control of sensingfilm thickness by varying the number of printed 100 nmthick layers. The direct comparison of 10 repetitions ofinkjet printed and drop-casted sensing films (Fig. 8) clearlydemonstrates the superiority of inkjet printing in terms ofsensor fabrication reproducibility. The relative standard de-viations in the response signal of 10 independently deposit-ed sensing layers were 5.6% and 68.0% for the inkjet (sevenprint layers) and drop casting approach, respectively. Theseresults demonstrate an excellent improvement of fabricationreproducibility, especially when considering that sevenlayers had to be inkjet printed in order to achieve a sensorsignal intensity comparable to the drop casted devices.

Courbat et al. have applied inkjet printing (piezoelectricresearch-type printer) of an organic solution of sensingcomponents (pH-indicator, polymer matrix, plasticizer) ontoa low-cost polymer waveguide to develop an optical ammo-nia gas sensor [60]. The thickness of the printed film was140 nm. The limit of detection (LOD) of this sensor wascalculated as 104 ppb. The same group has earlier reportedan ammonia gas sensor fabricated by spin coating an organ-ic solution of identical sensing materials onto a planar glasswaveguide [61]. The LOD of the spin-coated sensor with159 nm film thickness was found to be 2 ppb. The authorshave demonstrated that inkjet printing is a reasonable alter-native to spin coating for the deposition of a polymericsensing layer onto a planar waveguide. Although a deterio-ration of the LOD has been observed, inkjet technology ismore suitable for mass-production by roll-to-roll processes

compared with spin coating. But this example also showsthat inkjet printing applied for the deposition of a sensingfilm does not always result in improvements of sensorperformance.

In a significant number of optical sensors, the active sensingcomponents are immobilized into an organic polymer matrix.Many of the applied polymer matrices are insoluble in water.Therefore, research-type printers able to withstand organicsolvents have to be used for their inkjet-based deposition. Inaddition, even when using a solvent-resistant printing system,intrinsic limitations of inkjet printable compositions regardingviscosity, density, and surface tension do apply. As a conse-quence, only inks with relatively low polymer concentrationscan be used, requiring repeated printing cycles to deposit asufficient amount of active sensing material. A large number ofprinting cycles is time-consuming and results in lower sensorfabrication reproducibility. As in the case of active sensinglayers for electrochemical sensors, there is a desire to depositpolymeric membranes from relatively concentrated printinginks based on aqueous media.

Yoon et al. demonstrated one example of overcoming thislimitation [62]. Polydiacetylenes have interesting properties asoptical sensing materials, because of their stimulus responsivecolor and fluorescence changes promoted by heat, mechanicalstress, environmental, chemical and biological interactions.Unfortunately, neither polydiacetylenes, nor their monomericprecursors are soluble in water. But the low cost commerciallyavailable 10,12-pentacosadiynoic acid monomer and its de-rivatives are known to form self-assembled vesicular struc-tures in aqueous environment, and were selected as inkjetprintable precursors. However, the vesicles tend to aggregate,which limits the concentration of the printing inks to about 5mM. The authors looked at the application of commerciallyavailable ionic and nonionic surfactants to stabilize the vesi-cles by co-assembly with the monomer molecules. With sim-ple 10,12-pentacosadiynoic acid and the nonionic surfactantBrij78, long-time stable aqueous printing ink suspensionswith up to 13 mM of monomer were obtained and printedon the surface of unmodified copy paper by a thermal desktopprinter. Conversion into a polymeric structure was achievedby UV irradiation with a handheld UV lamp during 3 min.Chemical sensors with colorimetric response to organic sol-vents (THF, hexane) and pH were demonstrated, amongothers. In a later report, the same research group has extendedthe system to develop microemulsion-based inks containing adiacetylene monomer, surfactant, co-surfactant, and organicsolvent [63]. This approach allowed the stabilization of or-ganic droplets containing a high concentration of diacetylenemonomer in aqueous solutions. The application to a paper-based colorimetric temperature sensor was demonstrated.

The conservation of enzyme activity is an important issuein the development of enzyme-based biosensors. The possi-ble influence of the selected printing technology (thermal or

Fig. 7 Comparison of an inkjet printed TiO2 electrode (left) and dip-coated electrode (right). Reprinted from Reference [58] Yang M; Li L.H.;Zhang S.Q.; Li G.Y.; Zhao H.J. (2010) Preparation, characteriza-tion, and sensing application of inkjet printed nanostructured TiO2photoanode. Sens Actuators B 147:622628, Copyright (2010), withpermission from Elsevier


piezoelectric) has already been discussed above. In thefollowing, two examples of enzyme-based inkjet printedbiosensor approaches with colorimetric signal detection arepresented. In all cases, the enzymes are chemicallyimmobilized onto the surface of paper substrates, and thesame piezoelectric research-type inkjet printer has beenapplied.

In the first example, Alkasir et al. reported a biosensor forthe colorimetric detection of phenolic compounds [64]. Thesensing layer was build up in a layer-by-layer (LbL) formatby sequential inkjet printing of aqueous chitosan, sodiumalginate, and tyrosinase enzyme solutions onto a filter papersubstrate. The sensor response relies on the color changecaused by the specific binding of quinone formed by enzy-matic conversion of the phenolic analyte to the immobilizedchitosan. The authors managed to demonstrate that inkjetprinting is a useful tool for the large-scale production ofLbL-based sensors on paper substrates, while retaining en-zyme activity and functionality. Although it was experimen-tally observed that the sensitivity (color intensity change persample concentration unit) of the inkjet printed sensors wasslightly lower than that of paper disks LbL coated by man-ual pipetting, the sensor performance can be regarded ascomparable. The authors of this study have not optimizedthe number of inkjet printed layers for each deposition step,while an optimization was performed for the manualpipetting approach. Therefore, it cannot be stated that theobserved reduction in sensitivity is directly related to theinkjet printing process (e.g., enzyme denaturation).

Hossain et al. have reported sol-gel-based colorimetricsensors on paper substrates for the detection of neurotoxinsand pesticides [65, 66]. Both systems rely on the inhibitionof acetylcholine esterase in the presence of the analyte,which is converted into a colorimetric signal by using chro-mogenic enzyme substrates. The authors put particular em-phasis on the permanent and stable immobilization of theenzyme on the paper surface [65]. In their work, the appliedenzyme is immobilized on the paper surface by sandwiching

between two layers of a sodium-silicate sol-gel materialdeposited on top of a poly(vinylamine) underlayer. Thethree materials are separately and sequentially inkjet printedon the paper substrate. No loss in enzyme activity wasobserved after storage of printed sensors at 4 C for 1month. The authors have optimized all three printing inks(sol-gel ink, enzyme ink and poly(vinylamine) ink) forinkjet printability by varying the amount of glycerol addedas viscosity control agent. Excellent printability wasachieved upon the addition of 30 wt% of the viscositymodifier to the enzyme ink. More than 95% of the enzymat-ic activity of acetylcholine esterase was maintained with suchink composition. This stands in contrast to an observed loss ofenzyme activity of 30%, reported by Di Risio et al. for anHRP-based enzyme ink with 30 wt% of glycerol as viscositymodifier [25]. This significant difference in impact of anidentical viscosity modifier on enzyme activity is anotherindication for the requirement to evaluate the stability of eachparticular enzyme under specific inkjet printing conditions, ashas already been mentioned further above.

The same sodium-silicate sol-gel material ink has alsoproven useful for the immobilization of other sensing re-agents, such as chromogenic enzyme substrates or an oxi-dizing agent, as has been demonstrated in paper-based inkjetprinted lateral-flow devices for the detection of bacterialcontamination of food samples [67]. All materials requiredfor the sensor have been deposited on filter paper by piezo-electric inkjet printing of aqueous media. Specific enzymespresent in lysed bacteria result in the development of acolored line in the case of contaminated samples.

Fluorescence-based sensors

For (bio)chemical sensors inkjet printed on paper substrates,colorimetry is preferred due to the extremely simple signaldetection (e.g., by the naked eye). However, there are reportsof fluorescence-based systems where, for example, a fluores-cence microscope is used as the signal detector. Kwon et al.

Fig. 8 Demonstration of superior fabrication reproducibility of inkjetdeposited sensing layers: plot obtained from multiple (n = 4) injectionsof gaseous acetic acid employing 10 (a) drop casted and (b) inkjetprinted colorimetric sensors. Reprinted from Reference [59] O'Toole

M, Shepherd R, Wallace GG, Diamond D. (2009) Inkjet printed LEDbased pH chemical sensor for gas sensing. Anal Chim Acta 652:308314, Copyright (2009), with permission from Elsevier


have realized the detection of food spoilage or the monitoringof fruit and vegetable ripening by headspace analysis with asensor array consisting of inkjet printed fluorescent labeledDNAs on paper [68]. DNA-like oligodeoxyfluorosides(ODFs), where DNA bases are replaced by fluorophores, havebeen deposited on a paper substrate as aqueous solutions by athermal desktop printer. The authors obtained uniform sensingspots of reproducible brightness by optimizing their printinginks in terms of viscosity to control the spreading behavior onthe substrate. They have also investigated a number of differ-ent substrates and concluded that brightener-free cotton paperwas the most suitable material. Only 25 pmol of ODF wasrequired per sensing test. The deposited ODFs directlyresponded by fluorescence signal increases or color shiftswhen exposed to volatiles released from the samples, allowingfor colorimetric data processing of fluorescence micrographs.As in other examples, the authors explain their selection of aninkjet printing approach in particular with its convenience,simplicity, effectiveness, and low costs.

In another inkjet-based paper sensor relying on colorimet-ric or fluorescence transduction, Yu et al. have developed amethod for the direct covalent immobilization of small mole-cules, proteins, and DNA used as sensing materials for bio-sensors [69]. In one application example, galactose isdeposited for covalent attachment on a divinyl sulfonepretreated filter paper by a piezoelectric desktop printer, andvisualized with fluorescently labeled ricin.

In the following, three examples of fluorescence-basedsystems are presented, where the spatial resolution and thedroplet formation reproducibility of the inkjet printing tech-nology are essential factors. However, it is needless to saythat in terms of manufacturing, there is no basic differencebetween absorbance/colorimetry based (bio)chemical sen-sors and those relying on fluorescence or other signal trans-duction, when it comes to the need for precisely depositing afunctional material on a sensor platform.

Already in 2006, Carter et al. have used a piezoelectricresearch-type inkjet setup for the deposition of a fluorescentpH-indicator dissolved in a photopolymerizable monomermixture in the form of multiple dots on the tip of a 500 mdiameter optical fiber [70]. The authors relied on the excellentspatial control and reproducibility of the method. The obtaineddots showed very small variations in diameter (92.2 2.2 m),height (35.0 1.0 m) and roundness. Up to seven dots wereindependently placed on a single fiber tip, without havingcontact with each other (Fig. 9).

Gervais et al. used a piezoelectric research-type printer toplace detection antibodies required in an fluorescence-basedimmunodiagnostic assay for C-reactive protein (CRP) ontoa microfluidic capillary device [71]. As in the previousexample, precision is the most essential reason for selectingan inkjet printing method. A total of 3.6 nL of detectionantibody solution in the form of 20 drops of 180 pL was

deposited into an area of the device that is only 100 mwide.

Kirk et al. applied a piezoelectric research-type printer forthe multiplexed deposition of bioactive reagents onto siliconmicro-ring resonator devices intended for label-free proteindetection [72]. The authors referred to inkjet printing to re-place the conventionally used fluidic masking techniques orpin-contact printing methods because they suffer from limitedresolution (low functional density), requirement of large re-agent volumes, or damage caused by contacting the wave-guide surface. The inkjet printer deposition accuracy wascalibrated by printing fluorescently labeled proteins, followedby fluorescence microscopic analysis of deposited spots. Bymodifying the printers software setting, a final spot precisionof 5 m was achieved. Besides ink and printer parameteroptimization towards reproducible droplet formation, the re-producible fixation of the substrate on the stage of the printeris an additional requirement when such high spatial resolutionis targeted. The authors used a precisely machined chip holderfor their experiments. In an impressive performance of rapidand efficient mass-production of densely functional micro-ring resonator arrays by inkjet printing, 10 sensor chips wereprinted, each containing six evenly spaced micro-rings (30m diameter), in a total of 9 s consuming less than 25 nL ofreagent. Finally, precise coating of individual micro-rings withdifferent protein conjugates without any cross-reactivity wasdemonstrated. With their results, the authors have proven thatinkjet printing can overcome the current limitations of siliconmicro-ring resonator devices in terms of fabrication through-put and functional density.

Fig. 9 Individual polymeric microdots (92 m diameter) incorporatingfluorescein as pH indicator deposited on the tip of a 500 m diameteroptical fiber bundle. Reprinted from Reference [70] Carter JC, Alvis RM,Brown SB, Langry KC, Wilson TS, McBride MT, Myrick ML, CoxWR,GroveME, Colston BW. (2006) Fabricating optical fiber imaging sensorsusing inkjet printing technology: a pH sensor proof-of-concept. BiosensBioelectron 21:13591364, Copyright (2006), with permission fromElsevier


Surface enhanced Raman scattering (SERS) sensors

Yu and White have applied a piezoelectric desktop printer todeposit aqueous Ag nanoparticle colloids as Raman scatteringenhancers onto paper substrates for a SERS-based sensorallowing the trace level detection of malathion, heroin, andcocaine [73, 74]. In contrast to the printing of electricallyconducting traces with Ag nanoparticles, the nanoparticle den-sity required for SERS substrates is significantly lower and,therefore, allows for the use of diluted water-based printinginks and standard desktop printers, although requiring multipleprinting cycles (5 or 10 cycles, respectively). During the dryingprocess, individual nanoparticles aggregate to form silver clus-ters. In contrast to conventional SERS substrates obtained bynanofabrication of surfaces with nanoscale metallic structures,inkjet printing on paper substrates is an extremely low-cost,simple, and rapid approach. All that is needed for substratefabrication is a common desktop printer and an aqueous Ag-nanoparticle ink, allowing sensor production at the time andpoint of use. This not only reduces the costs and time requiredto obtain the substrates but also allows overcoming the prob-lem of limited storage stability of Ag-based SERS substratesdue to oxidation. With the simple system developed by theauthors, which includes a lateral flow analyte concentrationstep, detection limits as low as 413 pg for malathion, 15 ng forcocaine, and 9 ng for heroin were achieved.

Sensors with mechanical (mass) transduction

As early as 2004, Bietsch et al. have made use of a piezoelec-tric inkjet material deposition system for the application ofvarious active sensing layers (alkanethiol SAM for pH and ionsensing, single-stranded DNA oligomers for the detection ofcomplementary DNA, polymer layers for water and ethanolvapor sensing) on an array of cantilevers [75]. The perfor-mance of the SAM coated sensors obtained by inkjet printingproved to be equivalent to a cantilever array conventionallymodified by insertion into an array of glass capillaries filledwith the coating liquid. The inkjet modified DNA or polymer-coated sensors also proved to be fully functional. It wasdemonstrated that inkjet printing can replace the capillarycoating technique, resulting in significantly faster sensor fab-rication, suitable for automation and large-scale application. Inaddition, only inkjet printing allowed for the selective single-sided coating of a cantilever without contaminating thebackside.

In a more recent paper, high spatial control of inkjet print-ing applied to the coating of cantilevers was nicely demon-strated (Fig. 10) and a method for further enhancement ofdeposited layer uniformity has been introduced [76]. In orderto prevent the contamination of the backside by spilling overof liquid deposited on the topside of a cantilever, droplet sizeand the number of droplets deposited per unit area have to be

kept below a certain value. The material inkjet printer usedreadily controls both parameters. However, the difficulty tocontrol random agglomeration of individually deposited drop-lets into larger pools on the cantilever surface results in non-uniform coverage of the substrate surface after drying. Ness etal. demonstrated that this disadvantage could be overcome byapplying a droplet swelling method after printing. Glycerolis widely used as a viscosity modifier in inkjet inks. In addi-tion, its hygroscopic nature makes it simultaneously work as ahumectant, preventing the rapid drying out of ink and theresulting nozzle clogging. When the cantilevers surface-coated with a glycerol containing protein ink are kept in acontrolled high-humidity environment for several hours afterprinting, inkjet deposited droplets increase in size by uptake ofwater, resulting in a uniform fluid coverage of the entiresubstrate surface and, after drying, in a more uniform anddense protein cover layer.

The first example presented in this section is again verytypical for the general tendency in using inkjet printing tech-nology, where low cost, speed and the possibility of massproduction are most important. The second example, howev-er, although probably not very competitive regarding speed offabrication, is a good demonstration that inkjet printing is notonly a tool for routine mass fabrication, but continues to bedeveloped in order to overcome its own limitations.

Microfluidic paper-based analytical devices (PADs)

Considering the historical origin of printing, the combinationof paper with a printing technology such as inkjet seems to bea very obvious approach. The previous sections have al-ready demonstrated that paper or paper-like materials areuseful and popular substrates for the deposition of active

Fig. 10 Highly precise deposition of biotinylated bovine serum albu-min on individual microcantilevers (45 m width, 300 m length) bypiezoelectric inkjet printing. Adapted from Reference [76] Ness SJ,Kim S, Woolley AT, Nordin GP (2012) Single-sided inkjetfunctionalization of silicon photonic microcantilevers. Sens ActuatorsB 161:8087, Copyright (2012), with permission from Elsevier


sensing layers by inkjet printing. Themicroporous structure ofpaper allows for the absorption of relatively large amounts ofliquids and for their wicking through the fiber network drivenby capillary forces. These specific characteristics of paperhave given rise to the recent trend to fabricate microfluidic(bio)analytical devices on paper substrates instead of the morecommon glass or polymer platforms, which has been initial-ized by Whitesides and coworkers in 2007 [77]. As with anytype of microfluidic analytical device, the fabrication ofPADs requires the two principal steps of substrate patterningand of sensing (bio)chemistry deposition. In the Whitesidesgroups original approach, microfluidic patterning wasachieved by photolithography, whereas chemical sensing re-agents have been deposited by manual spotting. The fieldof PADs and closely related topics has been reviewedseveral times [22, 7883]. Here, only a few examples limitedto the application of inkjet printing methods are introduced.

To the best of our knowledge, our research group has beenthe first to demonstrate an inkjet printing approach for thefabrication of PADs. While inkjet technology has been pre-viously used for the deposition of functional materials for(bio)chemical sensing, we have first demonstrated its applica-bility to the microfluidic patterning of paper substrates.Inspired by a report of Schubert and coworkers [14], we havechosen an approach known as inkjet etching and adapted itto a filter paper material to create microfluidic channels andareas for the deposition of (bio)sensing chemistries using apiezoelectric research-type inkjet printer [15]. In this method,the printing ink for substrate patterning simply consisted oftoluene, which was inkjet printed onto a filter paper pretreatedby soaking into a solution of hydrophobic poly(styrene)(Fig. 11a). Toluene ejected from the inkjet nozzle locallydissolves and displaces the polymer and re-exposes the orig-inally hydrophilic paper, leading to well-defined microfluidicchannels of 420 50 m width. In a second printing step,aqueous or ethanolic solutions of sensing materials for color-imetric determination of pH (mixture of four chromogenicpH-indicators), total protein (tetrabromophenol blue) and glu-cose (glucose oxidase, horseradish peroxidase, o-toluidine,ascorbic acid) were printed into specific detection areas usingthe same inkjet printer, resulting in a PAD as shown inFig. 11b. The application of 4.5 L of sample to a centralinlet area was sufficient for the simultaneous semiquantitativeanalysis of the three analytes after digital color analysis ofscanned images of the dried PADs.

We have also demonstrated the applicability of the sameinkjet printing approach for the fabrication of a combinedimmuno-chemical PAD, where two lateral-flow-type modelimmunoassays (mouse IgG, human IgG) and a simple chemicalsensor (pH) were integrated onto a single piece of filter paper[84]. Inkjet printing was used for the patterning of themicrofluidic structure and the deposition of recognition anti-bodies required for the immunochromatographic assay. The

decision to focus on inkjet printing technology in our work isstrongly driven by the possibility to perform both the patterningof the microfluidic structure and the deposition of sensingmaterials by the same method, which results in all inkjetprinted PADs.

Li et al. have first used a standard thermal desktop printer tofabricate all inkjet printed PADs [85, 86]. Hydrophobicbarriers defining the outline of a microfluidic structure weredirectly inkjet printed by using a heptane solution of the papersizing agent alkenyl ketene dimer (AKD), which is a commonmaterial for hydrophobization of cellulose applied in papermaking. After printing, patterned substrates were heated at100 C for 8 min, resulting in the covalent attachment of longalkyl chains (C16C20) onto cellulose fibers. In the secondinkjet printing process, the reagents for the colorimetric de-tection of nitrite anions according to the Griess reaction (aque-ous solution of citric acid, sulfanilamide and N-(1-naphthyl)-ethylenediamine) were deposited on the PAD. The advan-tages of this method compared with the previously describedapproach are that the paper substrate requires no pretreatmentand that heptane is a less aggressive solvent than toluene,allowing the use of a desktop printer. In addition, themicrofluidic channels and detection areas are not exposed toany solvents and polymers during the patterning process.

In the following, our research group has reported a differentall inkjet printed approach to PADs using a standardpiezoelectric desktop printer for all fabrication steps [87,88]. A UV-curable hydrophobic ink composition (octadecylacrylate, 1,10-decanediol diacrylate, benzyldimethylketal)free of volatile organic compounds has been applied for thedirect printing of hydrophobic barriers, and back cover layersof microfluidic patterns in filter paper. In a double-sided inkjetprinting process, the lines defining the microfluidic structureare first printed on the topside of the filter paper and cured by60 s exposure to UV light. Then, a uniform layer of the samehydrophobic ink is deposited on the backside of the paper andUV cured for 60 s, resulting in a back cover layer preventingaqueous samples from penetrating throughout the entire thick-ness of the paper device (Fig. 12a). Finally, the same inkjetprinter has been used to deposit reagents for chemical sensing(Fig. 12b), which has been demonstrated on the example of asimple pH sensor [87] and a hydrogen peroxide sensor [88].With this patterning method, microfluidic channels as narrowas 272 19 m have been achieved. This is, to the best of ourknowledge, the narrowest value reported for a standard print-ing method. For comparison, in their photolithography ap-proach requiring a photomask, the Whitesides group hasfabricated channels of widths down to 186 13 m [83].

All four examples of PADs presented above have beenobtained in an all inkjet printing approach. However, thereare also a number of reports on PADs, where inkjet print-ing is just used in one part of the entire sensor fabricationprocess.


Jayawardane et al. used the microfluidic patterning pro-cess developed by Li et al. described above [85] in thepreparation of a low cost PAD for the detection of reactivephosphate in water [89]. The sensing reagents for thephosphomolybdenum blue-based phosphate determinationwere deposited by manual pipetting.

Delaney et al. relied on the same alkenyl ketene dimer inkjetprinting method for microfluidic patterning to create PADSfor the electrochemiluminescence-based (ECL) sensing of2-(dibutylamino)-ethanol and nicotinamide adenine dinucleo-tide (NADH) [90]. The inkjet patterned paper sections wereloaded with a ruthenium(II) complex ECL-transducer by man-ual pipetting and laminated onto screen printed carbon elec-trodes to assemble the final sensing devices.

In a report on an all-printed glucose sensor for diagnostics,Mttnen et al. compared the performance of inkjet printing

and flexographic printing for the patterning of reaction zoneson paper substrates [91]. The material used for the creation ofhydrophobic barriers in paper was a commercially availablevinyl-substituted polydimethylsiloxane. While the ink was di-rectly applicable for flexographic printing, a dilution with xy-lene was required before inkjet printing with a piezoelectricresearch-type printer to reduce the viscosity. Comparable bar-rier properties were observed for both flexographically andinkjet printed films. However, in the case of flexographicprinting, six printed layers were necessary to form a hydropho-bic barrier throughout the thickness of the paper, while with theinkjet printing method, a single print cycle was sufficient.Despite of this difference, the authors concluded that flexo-graphic printing allows for faster patterning with no need todilute the commercial ink, whereas inkjet printing results inbetter print resolution. For the fabrication of their final all-

Fig. 11 (a) Schematic outline of the fabrication process of an allinkjet printed PAD featuring microfluidic channels connecting acentral sample inlet area with three different detection areas and areference area. Steps 2 (patterning) and 3 (chemical sensing reagentapplication) are performed on the same inkjet printing apparatus (thepen symbol indicates the use of the inkjet printer). (b) Optimized inkjet

printed PAD before sample application. The dotted squares indicatingthe sensing areas have been added for illustrative purposes only andform no part of the sensor pattern. Adapted with permission fromReference [15] Abe K, Suzuki K, Citterio D (2008) Inkjet printedmicrofluidic multianalyte chemical sensing paper. Anal Chem.80:69286934, Copyright (2008) American Chemical Society

Fig. 12 Schematic representation of an inkjet-based PAD fabricationprocess: (a) microfluidic patterning of a filter paper by a double-sidedprinting process (grey and black colors indicate the printed hydropho-bic features before and after UV curing, respectively) using a UVcurable ink consisting of a mixture of hydrophobic monomer andcrosslinker; (b) inkjet printing of sensing ink; all printing steps have

been performed on a standard piezoelectric desktop printer. Reprintedfrom Reference [88] RSC Advances, accepted manuscript, MaejimaK.; Tomikawa S.; Suzuki K.; Citterio D.; Inkjet printing: an integratedand green chemical approach to microfluidic paper-based analyticaldevices, Doi: 10.1039/C3RA40828K, Copyright (2013), Reproducedby permission of The Royal Society of Chemistry


printed colorimetric glucose sensor, the authors havechosen a combination of flexography for paper pattern-ing (polydimethylsiloxane) and screen printing for sens-ing chemistry deposition (glucose oxidase, poly(vinyl)alcohol, potassium iodide, molybdic acid).

Hossain and Brennan demonstrated a PAD for sensitivedetection of various heavy metals based on enzyme inhibition,where the enzyme, the chromogenic enzyme substrate, andcolorimetric metal ion indicators were inkjet printed (piezo-electric research-type printer) into flow channels and detectionareas of a filter paper, which has been microfluidically pat-terned using a wax printing device [92].

Fu et al. used a piezoelectric research-type inkjet system inthe patterning of detection antibodies on a two-dimensionalpaper-based (lateral-flow through nitrocellulose) immunosensingdevice for malaria antigen detection [93]. In this example, thefluidic pattern has been achieved by cutting out of a largersubstrate sheet (nitrocellulose) by a laser cutter.

Inkjet printing of living cells

Recently, the inkjet printing technology has also attractedattention as a tool for the fabrication of cellular patterns, withprobably the most fascinating perspective being the creation ofartificial organs [9497]. The use of thermal and piezoelectricprinting technology has been reported, including both conven-tional desktop printers and specialized 3-D bioprinters. Interms of analytical applications, inkjet patterning of cells in theform of microarrays on substrates is a possible approach torealize biosensors based on living cells, for example for appli-cation to drug screening [98]. Inkjet deposited cellular arrayshave also been used for single cell analysis by mass spectrom-etry [99]. The advantage of inkjet printing lies in the well-controlled non-contact deposition of cells to predeterminedlocations [100]. A profound discussion of these new develop-ments would go beyond the scope of this work. However, itshould be mentioned that as in the case of biomolecule (e.g.,enzymes) containing printing inks, the optimization of inkcompositions with the goal to obtain stable printing propertieswhile retaining the highest degree of cell viability is a key issue[101, 102]. The influence of jetting parameters (e.g., appliednozzle voltage) in piezoelectric printing on cell viability hasalso been investigated [103]. An additional challenge, althoughnot directly related to the inkjet printing technology, is theselection of substrates that prevent deposited cells from dryingand that allow delivery of the nutrients required to maintaincells alive.

Final remarks

Of course, inkjet printing is not a universally applicable tech-nology. One major limitation comes from the requirement for

the ink to fit a relatively narrow window of viscosity andsurface tension, both of which are dependent on the printer.It is not simply enough to prepare the material to be depositedin a liquid form such as a solution or dispersion, but theresulting composition must fulfill the viscosity and surfacetension requirements. This automatically results in limits ofthe concentration of functional materials in a printing ink,often leading to the requirement of multiple printing cyclesto achieve the deposition of sufficient amounts of materials.This requirement may then negatively influence sensing layerfabrication reproducibility, since small differences in amountsof repeatedly deposited materials are summing up. In addition,especially when industrial scale mass production or the soleuse of desktop printers is targeted, the amount of organicsolvents and volatile organic compounds should be reduced.This creates the challenge of developing inkjet printable liquidcompositions based on aqueous solvent systems alone. Inmany cases, it is necessary to use additives to control viscosityand surface tension, in particular with water as the ink solvent.Furthermore, inks based on nanoparticles generally requiredispersion stabilizing agents to prevent particle aggregationand printer nozzle clogging. A simple search in the patentliterature reveals numerous additives that are routinely used inthe formulation of standard color inkjet printing inks.However, it has to be considered that all nonvolatile additives(e.g., surfactants, etc.) will remain on the substrate after depo-sition by inkjet printing. While this does not pose a problemfor standard office printing, for the fabrication of sensingdevices or their components, the presence of materials otherthan the active functional sensing compounds is normally notdesired. It is therefore necessary to keep in mind a possiblenegative influence of ink additives.

One of the often-mentioned positive characteristics ofinkjet printing is the highly economical use of inks andthe low creation of waste. While this is true in princi-ple, it should be noted that many inkjet systems requirenozzle priming and/or nozzle purging cycles. During theseprocesses, the amount of wasted ink can be significant.Furthermore, often a minimal ink amount in the milliliterorder is required because of the dead volume between inkreservoirs and printer nozzles. These are not serious issues forlarge-scale manufacturing, but it becomes relevant duringsensor development and laboratory scale prototyping experi-ments. One example of overcoming this problem is the re-placement of the ink tanks of desktop printers withmicropipette tips to reduce the required minimal volume ofprinting ink from the milliliter order down to several tens ofmicroliters [69].

Despite of remaining challenges in particular related toprintable ink formulation, it has been shown by selectedexamples that inkjet printing technology is indeed applicablefor the fabrication of all types of (bio)chemical sensing de-vices. The motivation for using an inkjet printing system


might be different for each specific application. However, thefollowing list of keywords appears over and over againthroughout the literature and, therefore, best describes themost valuable characteristics of inkjet printing: simple, lowcost, high speed, precise, reproducible, flexible, non-contact,low material waste, and applicable to large scale production.In many cases, inkjet printing has been used to replace analternative material deposition method, and often improve-ments in fabrication reproducibility or sensor performancehave been reported. In other situations, no such improvementswere achieved, but the advantages gained in production speed,simplicity, and cost still favor the inkjet-based approach.There are also some characteristics of inkjet technology thatgo beyond that list of keywords. For example, the nature ofinkjet printing, where materials are deposited in the form ofsingle small-sized droplets in contrast to a continuous layer,allows not only to control the thickness of an applied layer(e.g., multiple print cycles), but also the density of the depos-ited structure (e.g., variations in drop spacing). The variationof these two parameters can, in some cases, result in sensinglayers with different physical and chemical properties, al-though an identical liquid has been deposited.

The number of (bio)chemical sensors fabricated in researchlaboratories with no other equipment but an off-the-shelfdesktop printer will certainly continue to increase. At the sametime, (bio)chemical sensing devices can be expected to be-come more widespread and economical, once their inkjet-based manufacturing approach is hopefully transferred toindustrial scale mass production.

References

1. Kimura J, Kawana Y, Kuriyama T (1988) An immobilized en-zyme membrane fabrication method using an ink jet nozzle.Biosensors 4:4152

2. Sirringhaus H, Kawase T, Friend RH, Shimoda T, Inbasekaran M,Wu W, Woo EP (2000) High-resolution inkjet printing of all-polymer transistor circuits. Science 290:2122126

3. Calvert P (2001) Inkjet printing for materials and devices. ChemMater 13:32993305

4. De Gans BJ, Duineveld PC, Schubert US (2004) Inkjet printingof polymers: State of the art and future developments. Adv Mater16:203213

5. Tekin E, Smith PJ, Schubert US (2008) Inkjet printing as adeposition and patterning tool for polymers and inorganic parti-cles. Soft Matter 4:703713

6. Singh M, Haveri

inkjet printed (bio)chemical sensing devices

Documents

inkjet technology

abstract inkjet printing

keywords inkjet printing

inkjet method

inkjet printingapplications

printing of text

printing equipment

screen printing