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Inkjet printing for biosensor fabrication: Combining chemistry and technology for advanced manufacturing

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Inkjet printing is emerging at the forefront of biosensor fabrication technologies. Parallel advances in both ink chemistry and printers have led to a biosensor manufacturing approach that is simple, rapid, flexible, high resolution, low cost, efficient for mass production, and extends the capabilities of devices beyond other manufacturing technologies. Here we review for the first time the factors behind successful inkjet biosensor fabrication, including printers, inks, patterning methods, and matrix types. We discuss technical considerations that are important when moving beyond theoretical knowledge to practical implementation. We also highlight significant advances in biosensor functionality that have been realised through inkjet printing. Finally, we consider future possibilities for biosensors enabled by this novel combination of chemistry and technology.
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ISSN 1473-0197
Lab on a Chip
Miniaturisation for chemistry, physics, biology, materials science and bioengineering
CRITICAL REVIEW
Joanne Macdonald et al.
Inkjet printing for biosensor fabrication: combining chemistry and
technology for advanced manufacturing
Volume 15 Number 12 21 June 2015 Pages 2525–2712
Lab on a Chip
CRITICAL REVIEW
Cite this: Lab Chip,2015,15,2538
Received 26th February 2015,
Accepted 24th April 2015
DOI: 10.1039/c5lc00235d
www.rsc.org/loc
Inkjet printing for biosensor fabrication:
combining chemistry and technology for
advanced manufacturing
Jia Li,
a
Fabrice Rossignol
b
and Joanne Macdonald*
ac
Inkjet printing is emerging at the forefront of biosensor fabrication technologies. Parallel advances in both
ink chemistry and printers have led to a biosensor manufacturing approach that is simple, rapid, flexible,
high resolution, low cost, efficient for mass production, and extends the capabilities of devices beyond
other manufacturing technologies. Here we review for the first time the factors behind successful inkjet
biosensor fabrication, including printers, inks, patterning methods, and matrix types. We discuss technical
considerations that are important when moving beyond theoretical knowledge to practical
implementation. We also highlight significant advances in biosensor functionality that have been realised
through inkjet printing. Finally, we consider future possibilities for biosensors enabled by this novel
combination of chemistry and technology.
Introduction
Inkjet printing is a non-impact printing technology that
deposits ink in the familiar patterned array known as the dot
matrix. It is based on digitally controlled ejection of fluid
drops from a small aperture directly to a pre-specified posi-
tion.
1
The original idea of an inkjet is attributed to Lord Ray-
leigh in 1878, who proposed a liquid jet of constant radius
able to fall vertically under gravity.
2
As the liquid length
increases and reaches a critical value, the jet loses its cylin-
drical shape and decomposes into a stream of droplets
(Fig. 1). Lord Rayleigh's idea was applied first to a recording
device in 1930, but it was not until Rune Elmqvist patented
the first commercial inkjet recorder
3
in 1951 that the idea
was more widely adopted.
2538 |Lab Chip,2015,15,25382558 This journal is © The Royal Society of Chemistry 2015
Jia Li
Jia Li did her undergraduate
degree in the University of Syd-
ney (Australia), working on the
structure and activity relation-
ship of carborane phosphonium
salts for the Boron Neutron Cap-
ture Therapy (BNCT). She then
went to England for a research
project about onco-protein
protein interactions at the Uni-
versity of Leeds. She is now
doing a PHD at the University of
the Sunshine Coast under Dr.
Joanne Macdonald's supervision.
Her PHD project involves developing rapid and novel virus detec-
tion biosensors towards point-of-care.
Fabrice Rossignol
Dr Fabrice Rossignol is a senior
researcher at the French
National Research Council
(CNRS) working in the labora-
tory of Science of Ceramic Pro-
cesses and of Surface Treatments
(SPCTS) in Limoges, France.
Since 2007, he has been the
team leader for Ceramic Pro-
cesses at SPCTS. His team con-
ducts integrated researches rang-
ing from powder synthesis to the
fabrication of prototype objects
with improved or new properties
using various shaping and consolidation techniques. His personal
research interest is focused on the shaping of nanostructured
materials and the development of additive manufacturing method-
ologies. Biosensors are among the application fields covered.
a
Inflammation and Healing Research Cluster, Genecology Research Centre, School
of Science and Engineering, University of the Sunshine Coast, Maroochydore, QLD,
Australia. E-mail: jmacdon1@usc.edu.au; Tel: +61 7 5456 5944
b
ENSCI, SPCTS, CNRS UMR 7315, University of Limoges, Limoges Cedex, France
c
Division of Experimental Therapeutics, Columbia University, New York, NY, USA.
E-mail: jm2236@columbia.edu
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In the 1960s, Sweet from Stanford University developed the
first continuous inkjet.
4
This type of inkjet applied a pressure
wave pattern onto the ink stream that could be broken into
droplets of uniform size and spacing. Later, extensive devel-
opment by IBM allowed the application of this continuous
inkjet technology to computer printers.
5
In the mid-1970s,
Zoltan invented drop-on-demand (DOD) piezoelectric inkjet
printing.
6
It differed from the continuous inkjet by ejecting
ink droplets only when needed. Modern-style inkjet printers
were first displayed in 1979, by Canon and Hewlett-Packard
(HP) who developed two similar types of inkjet printers.
These used pressure generated from the growth and collaps-
ing of water droplets to control ejection. The former was
called a bubble jet
7
and the latter was named as Thinkjet.
8
While the best-known application for inkjet printing is
photos and graphics, in 1985 the use of inkjet printers to
print other functional materials was first described.
10
Nowa-
days, the uses of inkjet printing are diverse and include
large-scale high-end products such as organic thin-film tran-
sistors,
11
single-crystal films for microelectronics,
12
light-
emitting diodes,
13
and solar cells.
14
Indeed, inkjet printing is
no longer limited to two dimensional (2D) structures, but is
emerging for 3D structures, as exemplified by ceramic scaf-
folds,
15
drug formulations,
16
and live tissues.
17
One particularly interesting and highly beneficial applica-
tion of inkjet printing is the fabrication of sensors and bio-
sensors, and this is the focus of our review. While these sen-
sors can be fabricated using other technologies, a topic
reviewed by Gonzalez-Macia et al.,
18
the use of inkjet printing
provides distinct advantages over these other fabrication
technologies. The method is extremely versatile for develop-
ment of prototype biosensors, and enables rapid manufactur-
ing via high pattern precision and resolution. The applica-
tions of such technology are numerous, and have been
reviewed by Gonzalez-Macia et al.
18
and Komuro et al.
19
Also,
Di Risio and Yan
20
have provided a detailed review on dis-
pensing biomolecules on bioactive paper. Here, we focus on
the manufacturing technology itself, which is critical for
understanding the limitations and advantages when applied
to biosensor fabrication.
Our review comprehensively describes the major types of
inkjet printers, the basic ink components, the functional
molecules that can be incorporated into ink formulations,
and the relationship between ink formulation, ink rheologi-
cal properties and their compatibility with print heads. In
addition, we address strategies for inkjet patterning of reac-
tion chambers, and discuss critical parameters for this tech-
nique, as well as post-production processes. Our review
serves as a detailed primer for general chemists interested in
modern fabrication technologies for biosensing and other
applications. We also highlight some outstanding functional-
ities developed using inkjet printing that open new frontiers
for biosensor technologies. We conclude by critically consid-
ering future perspectives and developments for inkjet print-
ing and biosensor development.
Inkjet printers
The machines to precisely form ink droplets have been the
subject of intense commercial development.
7,8
This has led
to two main modes of operation, continuous and drop-on-
demand. Further subdivisions for each operation mode are
listed in Fig. 2.
Fig. 1 Lord Rayleigh's idea of instability of jets. From ref. 9. Copyright
2001 Society of Photo Optical Instrumentation Engineers.
Joanne Macdonald
Dr. Joanne Macdonald's research
focuses on the molecular engi-
neering of advanced devices,
including a DNA automaton able
to play tic-tac-toe against a
human opponent, and a molecu-
lar calculator able to add and
multiply small numbers. She is
jointly appointed as a Senior
Lecturer in Molecular Engineer-
ing at the University of the Sun-
shine Coast in Australia, and
Assistant Professor in Clinical
Medical Sciences at Columbia
University in New York, USA. She received her PhD in Virology in
2003 from the University of Queensland, Australia. Fig. 2 Categories of inkjet printers.
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Continuous inkjet printer
In a continuous inkjet printer, the creation of ink droplets is
constant, and this is controlled by a high-pressure pump
vibrating the nozzle with a piezoelectric crystal. The gener-
ated ink droplets are selectively charged via signals from the
printer. Charged droplets are deflected into a gutter for
recirculation, while the uncharged droplets are ejected onto
the matrix to form an image (Fig. 3).
1
The droplets generated
are usually twice the size in diameter than the printing ori-
fice; 150 μm is the typical drop size, but it can be as small as
20 μm.
9
The advantage of the continuous inkjet printer is
that it performs printing at high speed, and thus is very use-
ful in an industrial environment. The printing nozzle is not
easily clogged, as the ink droplets are generated continuously
and volatile inks are used to allow rapid drying. However, the
resolution can be reduced due to the high-speed of printing.
Other disadvantages of the continuous inkjet printer include:
(1) inks are restricted to those that can be charged; and (2)
the printer is relatively expensive because of the requirement
for drop selection, a recycling system, and a generally high
maintenance cost.
In terms of the drop deflection method, the continuous
inkjet printer can be classified into four systems (Fig. 2):
binary deflection, multiple deflection, hertz, and microdot.
For the binary deflection system, the uncharged droplets are
printed onto the matrix and a single nozzle can only print at
one dot position (Fig. 3A).
1,21
In contrast, in the multiple
deflection system, the charged droplets are deflected onto
the matrix; this allows multiple dot positions per nozzle
(Fig. 3B).
1,22
By virtue of this multiple deposition, the multi-
ple deflection system prints faster than the binary deflection
system. The hertz system is a modified version of the binary
deflection inkjet system with an improved color printing
capability. The number of ink droplets deposited is con-
trolled through the volume of ink in each pixel in the hertz
system. Consequently, the color density can be manipulated
to achieve the desired gray tone.
23,24
The microdot ejects
large and small diameter droplets from the nozzle, however,
only the droplets with small diameter are entered into the
electrical field for selective ejection.
25
This is remarkable,
because small ink droplets can be produced without reducing
the diameter of the inkjet nozzle.
25
Drop-on-demand inkjet printer
In contrast to the continuous inkjet printer, the drop-on-
demand (DOD) inkjet printer ejects the ink only when it is
required. The DOD eliminates the complex droplet charging,
deflection and recycling system required for the continuous
inkjet printer, and allows smaller drop size generation and
higher placement accuracy. The ejected drop size approxi-
mates the diameter of the orifice, and less than 20 μm drop-
lets can be achieved.
9,26
The DOD printer relies on a pressure
pulse created to form ink droplets. The method used to gen-
erate this pressure pulse defines the primary subclasses of
the DOD printer, namely thermal,
24,27
piezoelectric,
6,28,29
acoustic,
30,31
electrostatic,
32
electrohydrodynamic (EHD),
3335
and valve
36
methods. The first two are dominant in modern
inkjet printing, EHD is becoming prominent, and the others
are still in the developmental stage.
Thermal inkjet printer
The development of the thermal inkjet printer was inspired
by the natural process of water boiling to form water bub-
bles.
7
In this technology, the ink in the ink chamber is rap-
idly heated up to a high temperature (350 to 400 °C) to vapor-
ize. The vaporization promptly creates a bubble at the surface
of a heater (resistor), causing a pressure pulse to push the
ink droplets out through the nozzle. As the ink droplets are
ejected, the vapor bubble collapses, which generates a force
to refill the ink (Fig. 4A).
37
The entire procedure is fast, tak-
ing less than 10 microseconds. Depending on the location
between the nozzle and the heater, the thermal DOD printer
can adopt either roof-shooteror side-shooterconfigura-
tions. The roof-shooter(Fig. 4B) has the nozzle located on
Fig. 3 Continuous inkjet printer systems. Ink droplets are constantly
ejected, and the ejection is pumped by a piezoelectric crystal. The ink
droplets are selectively charged via the printing signals. The charged
droplets are deflected into a gutter for recirculation when passing
through an electric field, while the uncharged droplets are ejected
onto the matrix to form an image. A: In a binary deflection system, the
uncharged droplets are printed onto the matrix and a single nozzle
can only print at one dot position; B: in a multiple deflection system,
the charged droplets are deflected onto the matrix. Multiple dots are
deposited per nozzle.
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the top of the heater, while the nozzle is located nearby the
heater in the side-shooter(Fig. 4C).
1
The thermal inkjet
printer offers high nozzle density and generates small ink
volume (150 to 200 picoliters, pl, 10
12
of a liter), however,
the ink chemistry is limited to vaporizable and thermally sta-
ble inks.
Piezoelectric inkjet printer
Unlike the thermal inkjet printer, that uses the expanding
and collapsing of ink bubbles by heating to control the ejec-
tion, the piezoelectric inkjet printer applies a piezo-ceramic
plate to create ink droplets. A thin diaphragm is bonded to
the piezo-ceramic plate to prevent unintended interactions
between the inks and the plate. The piezo-ceramic plate
deforms in response to an electric impulse. This generates a
pressure wave that causes the ink to be ejected out from the
nozzle. On the removal of the electric pulse, the ink is
replenished as the piezo-ceramic plate returns to its normal
shape.
1,37
Since the ink is not heated to a high temperature
(as in the thermal inkjet printer), the piezoelectric inkjet
printer accepts a wider range of inks. The print head has a
longer life since it is not subject to heat damage. The drop
volume is around 150 pl, which is comparable to the thermal
inkjet printer.
38
However, the cost of the print head and the
associated software (that directs the head to apply certain
droplets of ink per dot) is considerable.
The piezoelectric inkjet printer can be classified into
squeeze, bend, push, and shear mode based on the distortion
of the piezo-ceramic plate. The squeeze mode (Fig. 5A) com-
prises a radially polarized piezo-ceramic tube surrounding
the nozzle.
1
In both the bend (Fig. 5B) and push modes
(Fig. 5C), the directions of the electric field and piezo-
ceramic plate deformation are in parallel.
29,39
Whereas in the
shear mode these two directions are perpendicular to each
other (Fig. 5D).
1
In all the modes, the voltage strength, the
pulse duration and the orifice diameter influence the size of
the ink droplets.
Other DOD inkjet printers
Other DOD inkjet printers are advances on the above technol-
ogies. For instance, instead of applying the electric pulse to
deform a piezo-ceramic plate, the electrostatic inkjet printer
(Fig. 6A) dispenses the ink droplets by directly modulating
the electric field.
40
When the stream of ink passes through
the electric field, the field imparts charges on the ink drop-
lets. The jetting position of ink on the matrix can be con-
trolled by varying the electrical potential applied on the plate.
This allows deposition of droplets much smaller than the ori-
fice diameter, producing finer droplets than the piezoelectric
inkjet printer.
41
The electrohydrodynamic (EHD) inkjet printer (Fig. 6B) is
an advance of the electrostatic inkjet printer in that printing
is no longer controlled only by the print head but is synergis-
tically manipulated via both the nozzle and a translation
stage, both of which are connected to a voltage. The electric
field generated between them creates an electrohydrodynamic
phenomenon that induces ink ejection to the matrix.
35
A
unique feature of the EHD inkjet printer is that the droplets
ejected are from outside rather than from within the nozzle.
42
Fig. 4 Drop-on-demand thermal inkjet printer. A: Mechanism. Ink is
rapidly heated to a high temperature to vaporize which creates a
bubble at the surface of a heater causing a pressure pulse that exudes
ink droplets through the nozzle. The vapor bubble collapses, as the ink
droplets are ejected, thereby generating a force to refill the ink; B: thermal
roof-shooterconfiguration; C: thermal side-shooterconfiguration.
Fig. 5 Drop-on-demand piezoelectric inkjet printer. The piezoelectric
inkjet printer applies a piezo-ceramic plate to create ink droplets. A
thin diaphragm is bonded to the piezo-ceramic plate to prevent
unintended interactions between the inks and the plate. The piezo-
ceramic plate deforms in response to an electric impulse to generate a
pressure wave to eject the ink. The ink is replenished on the removal
of the electric pulse. At the same time, the piezo-ceramic plate returns
to its normal shape. A: Squeeze mode; B: bend mode; C: push mode;
D: shear mode.
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To do this, the EHD inkjet printer forms a droplet that is
attached to the nozzle, the droplet then concentrates and jets
out by overcoming the surface tension under electric field
modulation.
42
This dual-controlled mechanism and special jet-
ting mechanism enable the EHD inkjet printer to achieve
nano-sized spots.
34
However, the ink application range for the
electrostatic inkjet printer and the EHD inkjet printer is much
narrower, since only conductive inks can be applied.
The valve inkjet printer (Fig. 6C) uses solenoid valves to
control ink ejection. Resolution in this system is poor com-
pared to the thermal, piezoelectric, electrostatic, and EHD
inkjet printers, with a minimum drop size of 500 μm.
43,44
In
comparison to all the other DOD inkjet printers, the acoustic
inkjet printer (Fig. 6D) does not have a nozzle. This inkjet
printer adopts a high-frequency transducer to the back of an
acoustic lens, which launches acoustic waves through the
lens. By focusing the acoustic energy from the waves, this
induces a pressure wave to expel the ink from the surface of
the ink chamber.
45
The advantage of not having an orifice is
the elimination of nozzle clogging, a common problem in the
thermal and piezoelectric inkjet printers.
Inkjet printing inks
The unique chemistry of individual inks is indispensible to
the inkjet printing system. This is because the ink properties
not only dictate the printing quality but also determine the
characteristics of the drop ejection and the reliability of the
printing system.
1
The inks contain base and colorants,
together with some additives, as well as components that
endow biosensor functionality (e.g. signalling molecules),
which are described in the subsequent sections. The base
acts as the liquid carrier of the colorants, which enables
them to bind to the matrix after printing.
46
Both the base
and the colorants have subdivisions and will be described in
details.
Base
The base is usually divided into four groups: aqueous-based,
non-aqueous-based, phase-change and reactive. The main
component of the liquid in the aqueous-based base is the
ink. The non-aqueous-based base contains several organic
solvents as well as the ink.
1,46
The drying mechanism for
both the aqueous-based and non-aqueous-based bases is that
the ink evaporates and penetrates to the porous matrix simul-
taneously.
1
However, the image quality can be poor, because
the ink tends to diffuse. Rather than being liquid at the
ambient temperature, the phase-change base is solid at room
temperature but melts to liquid while jetting, and solidifies
immediately once it reaches the matrix (a typical example is
wax-like ink). Conventional phase-change inks consist of a
wax containing ink binder, melting point between 90 °Cto
150 °C, tackifiers, adhesion promoters, and additives.
1,53,54
This property can overcome the image quality issue from the
other two bases, as the phase-change base does not diffuse
on the matrix. The aqueous, non-aqueous and phase-change
bases can produce acceptable images when ejected upon a
porous or coated matrix, however, they do not perform well
upon non-porous matrices due to poor adhesion. The reactive
base solves this problem by remaining as stable liquid until
it is cured by ultraviolet (UV) irradiation. The curing triggers
the liquid to undergo polymerization, leading to more com-
pact adhesion to the matrix.
46
Thus similar to the phase-
change base, the reactive base has less dispersion of the ink.
However, the ink coverage on the matrix is correspondingly
low, resulting in a higher ink consumption. Despite this, the
reactive base is non-volatile and does not contain organic sol-
vents, which is less toxic for the environment.
55
Colorants
The colors produced in inkjet printing are due to the addi-
tion of colorants in the ink. The colorants are either dyes
(organic or polymeric) or pigments.
1
The dyes are soluble
and exist as individual molecules in the ink, while the pig-
ments are insoluble and tend to form clusters. This chemical
property distinguishes the dyes and pigments in color perfor-
mance. The dye-containing ink is more stable than the
pigment-containing ink, because the pigments sometimes
aggregate to each other.
46
The aggregation will influence the
ink flow and supply systems and even clog the print head. To
address these problems, additives (e.g. surfactants, disper-
sants, polymers) are used to stabilize the pigments via
electrostatic and/or steric mechanisms. For example, the
anionic surfactant sodium dodecyl sulfate (SDS) is usually
mixed with carbon black (hydrophobic). The imparted nega-
tive charge from the SDS prevents the carbon black pigment
from aggregating, thus stabilizing the ink.
46
Although
pigment-containing ink is relatively unstable, it is advanta-
geous in terms of light fastness and humidity stability. This
is because clustering properties of the pigments provide
greater resistance towards these environmental impacts.
Additives
Apart from base and colorants, additives are other important
ink constituents. Additives stabilize or adjust the property of
Fig. 6 Other drop-on-demand inkjet printers. A: Electrostatic inkjet
printer; B: electrohydrodynamic inkjet printer; C: valve inkjet printer;
D: acoustic inkjet printer.
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the ink for printing and storage. The main types of additives
include surfactants, viscosity modifiers, dispersants, humec-
tants, biocides, and chelating agents.
To achieve good printing quality, ink viscosity and surface
tension are the two most important rheological parameters
(Fig. 7).
56,57
Low viscosity ink disperses quickly on the matrix
leading to poor image quality. Viscosity modifiers, such as
glycerol, ethylene glycol, poly(vinyl alcohol), and sodium
carboxymethyl cellulose can be used to regulate the viscos-
ity.
40,58,59
High surface tension can prevent jetting out from
the print head and cause it to clog, while low surface tension
causes incorrect ink release, so that it streams out of nozzle
or form unstable drops. For this reason, surfactants are usu-
ally applied to modulate surface tension (e.g. anionic surfac-
tants such as sodium dodecyl sulfate; cationic surfactants
such as cetyl trimethyl ammonium bromide; or non-ionic
surfactants such as Triton X-100; zwitterionic surfactant,
betaine).
60
Other reagents can alter multiple properties, for
example, ethanol (a co-solvent) can lower the surface tension
while facilitating the sample solution wettability, and it also
solubilizes insoluble pigments.
6163
Dispersants contain two classes: surfactants and polymers.
They stabilize the pigments from aggregation by imparting
charges via electrostatic and/or steric hindrance.
46
Ink formu-
lation with good dispersion stability is critical for conductive
inks (see conductive molecules section) and affects the func-
tion of the resulting biosensor. Woo et al.
64
reported that sil-
ver ink with stable dispersion exhibited low electrical resistiv-
ity due to decreased inter-particle junctions. Humectants (e.g.
glycerol or glycerine) are mainly used to control or limit the
evaporation of the inks. They act as hygroscopic agents dur-
ing printing or in the idle position of printer to prevent clog-
ging of print heads.
34,65,66
In order to control biological
growth, biocides are also included in the ink formulation.
Similarly, chelating agents (e.g. ethylenediaminetetraacetic
acid, EDTA) also have antimicrobial growth properties and
can chelate unwanted trace metals from mixtures of dyes in
the ink formulation.
40,58,67
Importantly, the choice of additives must include consid-
erations of compatibility. For example, ethanol may not be
compatible with some biomolecules such as proteins. In this
case, sugars (e.g. glucose, sucrose) can prevent biomolecules
from denaturing and dehydrating during printing, as the
sugar can form a rigid crystal to support the 3D structure of
the proteins.
68
Alternatively, a carrier protein such as bovine
serum albumin (BSA) can be included to stabilize proteins
and minimize any non-specific adsorptions onto the ink
chamber.
69
Critical parameters for ink printability
The final ink formulation must be a collection of carefully
titrated compatible components with properties that are (1)
in lieu with the printable rheological properties of the
printer, and (2) suitable to the chosen print head, leading to
formation of stable droplets with good jettability. For (1) the
theoretical printability can be calculated from three physical
constants, namely the Reynolds number, Weber number, and
Ohnesorge number (Table 1). In particular, the Zparameter
(the inverse of the Ohnesorge number) is commonly used to
indicate printability, where a Zvalue between 1 and 10 is
expected to generate stable drop formation.
52
In addition, (2)
is paramount, as the print head greatly affects printing qual-
ity (outside of interactions between the ink and the printing
matrix). Incompatibilities can lead to an error in deposition
precision, such as satellite droplets problem (Fig. 8). Several
reports suggest including polymers in the ink to minimize
the satellite droplet formation,
7073
since polymers maintain
attachment in the falling droplet.
72
Similarly, if the surface
tension of the ink formulation is low, the ink can spread as a
thin layer on the nozzle plate, causing faceplate wetting. Sub-
sequent solidification after evaporation affects the trajectory
of droplets or even inhibits jetting,
46
but can be prevented by
addition of a surfactant to increase ink surface tension.
Another type of nozzle clogging can occur after idle use,
known as the First drop problem.
74
Here, evaporation at
the nozzle causes local changes in the ink composition and
rheological properties,
66
and addition of a humectant or the
usage of less volatile solvents can mitigate this problem.
Additional parameters to consider include the diameter of
the nozzle, jetting voltage, stand-off distance, and humidity.
The nozzle diameter controls the drop size deposited, and
thus the printing resolution. The jetting voltage impacts the
speed of the droplet firing; its adjustment can be manipu-
lated according to the rheological properties of the ink. The
stand-off distance (the distance between the nozzle and the
matrix) affects the formation of the satellite droplets (Fig. 8),
and printing accuracy and resolution.
75
Finally, a suitable
and constant relative humidity maintains the stability and
Fig. 7 Relationship between ink formulation, ink rheological
properties and their compatibility with the print heads.
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activity of the molecules (e.g. biomolecules and polymers),
and a stable printing process (e.g. ink evaporation).
76
In summary, the ink formulation, ink rheological proper-
ties and their compatibility with the print heads are closely
inter-related. Modulation of these parameters is a trial and
error process and is critical for successful inkjet printing.
Inkjet printed biosensors
The purpose of a biosensor is to detect an analyte, and inkjet
printed versions of these devices are useful in various fields
ranging from industrial to clinical.
78,79
To achieve analyte
detection, a biosensor critically requires (1) sensing mole-
cules that interact with the analyte and (2) a transducer
(physicochemical detector) to transform the interaction into
a measurable signal (Fig. 9). In addition, contemporary bio-
sensors can also contain a series of interconnected zones for
sophisticated interactions between components. Inkjet print-
ing provides a rapid, inexpensive, and convenient method to
deposit some or all of these components with high precision.
Partially printed devices only use the inkjet printer as one
application in the sensor fabrication process,
8082
however,
the multiple cartridges inherent in colored inkjet printers
provide the opportunity to rapidly and precisely deposit mul-
tiple components at once, and fully inkjet printed devices
have been reported
63,8385
(also see Table 2).
Inkjet deposition of sensing and transducing molecules
The sensing molecules used in biosensors can be enzymes,
antibodies, or proteins, hormones, nucleic acids, and even
micro-organisms or whole cells. They can also include syn-
thetic components with biomimetic properties. Some of the
considerations for incorporating biomolecules or conductive
molecules as ink components are described in the next
sections.
Transduction of the analyte/sensing molecule interaction
can occur via any of the wide variety of systems available for
biosensors, such as electrochemical, optical, potentiometric,
Table 1 Typical fluidic parameters for ink drop ejection from the thermal and piezoelectric inkjet print heads
Dimensionless
groupings of physical
constants Equations Parameters and units Ranges Ref.
Reynolds number
Re
Lρ= density of the fluid (kg m
3
)
ν= velocity (m s
1
)
L= characteristic linear dimension (travelled length of the fluid) (m)
η= dynamic viscosity of the fluid (Pa s or N s m
2
or kg (m s)
1
)
γ= surface tension (N m
1
)
50500 47,
48
Weber number
We vL
2
20300 48,
49
Ohnesorge number
Oh We

Re
L
NA 50
Z
Z1
Oh
10 >Z>1 for
stable drop
formation
51,
52
NA = not applicable.
Fig. 8 Satellite droplets of inkjet printing. A: A high-speed photo-
graphic image showing three drops ejected from a DOD printer at dif-
ferent stages of drop formation. From left to right: the drop forms
from a single ejected liquid column that rapidly forms a leading droplet
followed by a ligament. The tail breaks up into a trail of satellite drop-
lets behind the leading droplet. ©IOP Publishing. Reproduced with
permission from ref. 77. All rights reserved. B: Satellite droplets forma-
tion with Zvalue of 3.57; C: satellite droplets formation with Zvalue of
17.32. The satellite droplets retract to the leading droplet in B (when Z
value lies between 1 and 10) but not in C (when Zvalue lies outside of
the range of 1 and 10). Reprinted with permission from ref. 75. Copy-
right 2009 American Chemical Society.
Fig. 9 Mechanism and components of biosensing systems.
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Table 2 Examples of fully or partially inkjet printed biosensors, and the details of their fabrication
Fully or partial
inkjet printed
biosensorIJs)
Inkjet
patterning
strategy Inkjet printer(s) Ink for patterning Ink for sensing
Types of inkjet
printed biosensorIJs) Ref.
Partial (for
patterning)
Multi-layers
surface inkjet
patterning
Thermal Canon inkjet
printer (i905D)
NA Biological ink
(1.7 mg ml
1
of
HRP in 0.1 M
phosphate buffer,
pH 6.5, with 1.5 mM
EDTA as antimicrobial
agent and 10% (w/v)
glycerol as stabilizer);
electronic ink (dilute
20 ml of 1.3 wt.%
PEDOT/PSS dispersion
in distilled water, to
a final volume of 50 ml)
Hydrogen peroxide
biosensor
112
Partial (for
patterning)
Multi-layers
surface inkjet
patterning
Thermal Canon inkjet
printer (i905D)
NA Biological ink (0.6 or
6mgml
1
of GOD in
0.1 M phosphate buffer,
pH 6.5, with 1.5 mM
EDTA as antimicrobial
agent and 10% (w/v)
glycerol as stabilizer);
electronic ink (20 ml
1.3 wt.% PEDOT/PSS
dispersion in distilled
water, to a final volume
of 50 ml)
Glucose biosensor 58
Partial (for
reagents
dispensing)
NA Piezoelectric inkjet
(Dimatix)
NA 20 wt% CuO nanoparticles
(58 nm) in a mixed
solvent of deionized water,
ethanol, isopropyl alcohol,
and ethylene glycol in the
ratio of 50: 20: 5 : 5 vol%
Glucose sensor 80
Fully Multi-layers
surface
(entrapment)
inkjet
patterning
Piezoelectric Dimatix
Materials Printer 2800
(DMP 2800)
A polypyrrole (PPy)
bottom layer; ethyl
cellulose (EC, 0.5% w/v
in butanol solution); a
nonconductive dielectric
layer (Electrodag 452 SS
BLUE) worked as
insulation layer
PPy/HRP (2.5 mg HRP in
1 ml PPy dispersion);
PPy/GOD (5 mg GOD in
1 ml of PPy)
Glucose biosensor 115
Partial (for
patterning)
Direct inkjet
patterning of
hydrophilic
ink upon
hydrophobic
surface
Piezoelectric Dimatix
inkjet printer (Fujifilm
2831 series)
Silver nanoparticles NA Glucose sensor 153
Fully Indirect
reaction
channel or
zone inkjet
patterning
(Piezoelectric)
PicoJet-2000 device from
Microjet (Shiojiri,
Nagano, Japan)
Toluene (to dissolve the
polystyrene treated filter
paper)
0.2 mg ml
1
human IgG
in water (control line);
1.22 mg ml
1
anti-human
IgG in water (test line)
Lateral flow
biosensor
83
Fully Multi-layers
surface
(entrapment)
inkjet
patterning
Piezoelectric inkjet
printer (Model
DMP-2800, Fujifilm
Dimatix, Inc, Japan)
Polyvinylamine (PVAm)
underlayer directly onto
the paper surface; silica
sol intermediate layer;
silica sol overlayer
A buffered enzyme
solution that contained
acetylcholinesterase
(AChE, 50 U ml
1
) and
5,5-dithiobis-IJ2-nitrobenzoic
acid) (DTNB) (500 μM)
Lateral flow dipstick 117
Fully Multi-layers
surface
(entrapment)
inkjet
patterning
Piezoelectric inkjet
printer (Model
DMP-2800, Fujifilm
Dimatix, Inc, Japan)
PVAm layer,
intermedium and
overlayer of solgel silica
AChE (in the sensing
region) and indophenyl
acetate (IPA) (in the
substrate region)
sandwiched between
the two layers of
solgel silica
Lateral flow dipstick 118
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Table 2 (continued)
Fully or partial
inkjet printed
biosensorIJs)
Inkjet
patterning
strategy Inkjet printer(s) Ink for patterning Ink for sensing
Types of inkjet
printed biosensorIJs) Ref.
Fully Multi-layers
surface
(entrapment)
inkjet
patterning
Piezoelectric inkjet
printer (Model
DMP-2800, Fujifilm
Dimatix, Inc, Japan)
Solgel silica to entrap
enzymes in the substrate
and sensing zones.
Methyltrimethoxysilane
(MTMS) for hydrophobic
barrier (HB zone)
Chlorophenol red
β-galactopyranoside
(CPRG) and
β-galactosidase (β-GAL) in
the substrate and sensing
zones, respectively
Bi-directional lateral
flow dipstick
114
Fully Direct
non-fencing
reaction
channel or
zone inkjet
patterning
Piezoelectric EPSON
Workforce 30 inkjet
printer
10% hexadecenyl
succinic anhydride (ASA)
in hexanol
Silver nanoparticles (with
glycerol in a volume ratio
of 2 : 5 glycerol/colloid
solution)
Surface enhanced
raman spectroscopy
(SERS) lateral flow
sensor
169
Partial (for
reagents
dispensing)
NA Piezoelectric EPSON
Workforce 30 inkjet
printer
NA Gold nanoparticles (GNP,
glycerol and ethanol in a
volume ratio of 5 : 4 : 1)
SERS lateral flow
sensor
170
Partial (for
reagents
dispensing)
NA Piezoelectric EPSON
Workforce 30 inkjet
printer
NA Silver nanoparticles (with
40% glycerol and 10%
ethanol)
SERS lateral flow
sensor
171
Partial (for
reagents
dispensing)
NA Piezoelectric inkjet
printer (Scienion, Berlin,
Germany)
NA A murine antibody to
Plasmodium falciparum
histidine rich protein 2
(PfHRP2) at 1 mg ml
1
at
the test line; an anti-mouse
antibody at 0.1 mg ml
1
at the control line
2D paper network
lateral flow
biosensor
172
Partial (for
reagents
dispensing)
NA Piezoelectri spotter
(SciFLEXARRAYER S3,
Scienion AG)
NA Mouse monoclonal
anti-PfHRP2 IgM (0.375 μl,
1mgml
1
) at the test line;
ImmunoPure Antitoby
goat anti-mouse IgG (0.375
μl, 0.5 mg ml
1
) at the
control line; 2 μl of each
enhancersolution,
activatorsolution, and
initiatorsolution on the
third inlet
2D paper network
lateral flow
biosensor
108
Partial (for
patterning)
Direct
non-fencing
reaction
channel or
zone inkjet
patterning
Thermal CanonTMiP4700
inkjet printer
4% Alkenyl ketene dimer
(AKD)
NA 2D and 3D sensors
by stacking
151
Partial (for
patterning)
Direct
non-fencing
reaction
channel or
zone inkjet
patterning
Thermal CanonTMiP4700
inkjet printer
4% AKD in n-heptane NA 2D and 3D sensors
by stacking
149
Partial (for
patterning)
Direct
non-fencing
reaction
channel or
zone inkjet
patterning
Thermal CanonTMiP4700
inkjet printer
4% AKD in n-heptane NA 2D and 3D sensors
by folding
148
Partial (for
patterning)
Direct
non-fencing
reaction
channel or
zone inkjet
patterning
Thermal CanonTMiP4700
inkjet printer
4% AKD in n-heptane NA 2D and 3D sensors
by folding
150
Fully Direct
fencing
reaction
channel or
Piezoelectric EPSON
PX-101 inkjet printer
(Seiko Epson, Suwa,
Japan)
Non-volatile UV-curable
ink (59.5% octadecyl
acrylate, 25.5% 1,10-
decanediol diacrylate,
H
2
O
2
sensing ink (1.0 ml
of 2.8 mg l
1
HRP in
citrate-phosphate buffer
(pH 7.0), 2.0 ml of 7.5 mM
Single-analyte
chemical sensor
84
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Table 2 (continued)
Fully or partial
inkjet printed
biosensorIJs)
Inkjet
patterning
strategy Inkjet printer(s) Ink for patterning Ink for sensing
Types of inkjet
printed biosensorIJs) Ref.
zone inkjet
patterning
and 15% Irgacure 651) 3,3,5,5-tetramethylbenzidine
(TMB) in 2-propanol, and
1.0 g of glycerin to control
the viscosity)
Fully Direct
fencing
reaction
channel or
zone inkjet
patterning
Piezoelectric EPSON
PX-105 inkjet printer
(Epson, Suwa, Japan) for
patterning microfluidic
structures; Piezoelectric
Dimatix DMP 2831
(Dimatix Fujifilm Inc.,
Santa Clara, USA) for
depositing reagents
Octadecyl acrylate and
1,10-decanediol
diacrylate UV-curable ink
1 mM TbCl
3
solution with
15 vol% ethylene glycol
(sensing areas); 25 mM
NaHCO
3
(sampling areas)
Single-analyte
chemical sensor
85
Fully Indirect
reaction
channel or
zone inkjet
patterning
(Piezoelectric)
PicoJet-2000 device from
Microjet (Shiojiri,
Nagano, Japan)
Toluene (to dissolve the
polystyrene treated filter
paper)
pH-responsive ink;
protein-sensitive ink;
glucose-sensitive ink
Multi-analyte
chemical biosensor
63
Fully Direct
non-fencing
reaction
channel or
zone inkjet
patterning
Thermal Canon inkjet
printer (Pixma ip4500)
AKD in 2% IJw/v) heptane Anti-A, clone 10090;
anti-B, clone 10091; and
anti-D, clone 20093 were
introduced into the
hydrophilic patterns
A,B, and /
Blood typing
biosensor
145
Partial (for
patterning)
Direct
non-fencing
reaction
channel or
zone inkjet
patterning
Thermal Canon inkjet
printer (Pixma ip4500)
AKD in n-heptane NA Blood typing
biosensor
146
Fully Direct
non-fencing
reaction
channel or
zone inkjet
patterning
Thermal Canon inkjet
printer (Pixma ip4500)
AKD in heptane No
2
as indicator and
alkaline phosphatase
Microfluidic
analytical sensor
147
Partial (for
patterning)
Direct
non-fencing
reaction
channel or
zone inkjet
patterning
Thermal Canon inkjet
printer (Pixma ip4500)
AKD in n-heptane NA Microfluidic
analytical sensor
81
Partial (for
patterning)
Direct
non-fencing
reaction
channel or
zone inkjet
patterning
Thermal Canon inkjet
printer (Pixma ip4500)
AKD in 2% IJw/v) heptane NA Microfluidic
analytical sensor
82
Parital (for
reagent
dispensing)
NA Piezoelectric DMP-2831
material printer (Fujifilm
Dimatix, Santa Clara, CA)
NA Two types of
dye-encapsulating polymer
nanoparticle emulsions
(pBzMA or pIJDEGMMA-co-
MMA)) in mixing ratio
(100 : 0, 80 : 20, 60 : 40, 40 :
60, 20 : 80, and 0 : 100); the
final ink composition
corresponds to a polymer
solid content of 10 mg ml
1
in H
2
O with 10% IJv/v) of
ethylene glycol
Strip paper sensor 173
Partial (for
reagents
dispensing)
NA Thermal Canon inkjet
printer (Pixma ip4500)
NA Albumin-FITC (FITC =
fluorescein isothiocyanate)
in buffer with 1.0 mg ml
1
HRP
Paper biosensor 174
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amperometric, calorimetric, enthalpimetric, impedance
spectroscopy (IS), piezoelectric detection, surface plasmon
resonance (SPR), surface enhanced Raman spectroscopy
(SERS), scanning probe microscopy (SPM), or quartz crystal
microbalance (QCM), etc. (Fig. 9).
86
The choice of the trans-
duction mechanism depends on the application field, but the
most widely used measurements for inkjet printed biosensors
have historically been electrochemical and optical, yet
optical-based biosensors are advantageous of being more
point-of-care inclined.
An example electrochemical biosensor is the glucose oxi-
dase sensor described in the whole surface patterning section
below (see Fig. 10A), and further examples are provided in
Table 2 (#25). Electrochemical techniques can be ampero-
metric, conductometric, voltammetric or potentiometric. The
most versatile technique is voltammetry, since it allows both
current and potential measurements for a short response of
the system, with possibilities for multi-component detection.
87
Optical techniques collect photon measurements of the
analyte/sensing molecule interaction, rather than electrons.
These can be based on luminescence, fluorescence or color
change, and can be measured by absorbance, or reflectance
of fluorescence emissions in the UV, visible, or near-infrared
(NIR) regions of the light spectrum.
88
Colorimetric measure-
ments are most popular for point-of-care biosensors, and
examples constructed using inkjet printing are provided in
the Inkjet patterning sections below (Fig. 10B14A).
Biomolecules
Examples of inkjet printed enzymes include the glucose oxi-
dase biosensor (Fig. 10A) and β-galactopyranoside/β-galactosi-
dase in a bi-directional lateral flow dipstick (Fig. 10B). In
addition, several examples of inkjet printed antibodies are
described in the inkjet patterning sections below (Fig. 11B,
12A, 14A). For inkjet printing of these and other biomole-
cules, the manufacturing technique and the intrinsic molecu-
lar properties are important considerations during formula-
tion of the inkjet ink, as are the quantity of material available
and its stability.
A major issue for these biomolecules during inkjet print-
ing is their non-specific adsorption onto the ink chamber.
This quantity loss can have an impact on the printing quality
and cost. While antibodies, hormones, proteins, and enzymes
can be expressed, the time it takes to accumulate large quan-
tities of these molecules is much longer than with nucleic
acids and cells which can be amplified relatively easily.
Delehanty and Ligler
69
found that the addition of BSA could
minimize protein loss in the ink tank due to adsorption.
Interestingly, the BSA additive also optimized the spot
uniformity.
Another major issue for biomolecules is the maintaining
of their stability during printing. Most biomolecules, except
certain nucleic acids, tolerate heat in the range between 40 °C
to 80 °C.
8992
Therefore, the piezoelectric inkjet printer com-
pares favorably with the thermal inkjet printer, since the pie-
zoelectric printer does not employ heating mechanisms for
ejection. However, the practical reality of these expected con-
ventions needs to be assessed on a case-by-case basis.
Viravaidya-Pasuwat et al.
93
modified a commercial thermal
inkjet printer (Cannon IP 2700) to print BSA (together with a
red dye for visualization) onto a nitrocellulose slide. The BSA
could be repeatedly printed as a 1 mm spot 20 times on one
nitrocellulose slide without clogging. Roda et al.
94
also
showed that there was no denaturation of horseradish
Table 2 (continued)
Fully or partial
inkjet printed
biosensorIJs)
Inkjet
patterning
strategy Inkjet printer(s) Ink for patterning Ink for sensing
Types of inkjet
printed biosensorIJs) Ref.
Partial (for
reagents
dispensing)
NA Thermal inkjet office
printer (HP Deskjet
D2360)
NA 4BCMU emulsion ink
(4% 4BCMU, 23%
1,2,4-trimethylbenzene,
10% SDS, 37% 1-propanol,
26% water)
Paper sensor 175
Partial (for
reagents
dispensing)
NA Thermal inkjet office
printer (HP Deskjet
D2360)
NA Diacetylene, bisurea, and
olitoethylene oxide
Paper sensor 176
Partial (for
reagents
dispensing)
NA (Piezoelectric) MicroFab
JetLab4 system (MicroFab
Technologies Inc.)
NA TiO
2
nanoparticels and
Ag nanoparticles
Printed-paper-based
memory devices
177
Fig. 10 Inkjet printer fabricated biosensors with surface patterning. A:
Schematic diagram of total inkjet printed GOD and/or HRP biosensor
using whole surface patterning strategy. B: Inkjet fabricated bi-
directional lateral flow dipstick using whole surface patterning strategy
via entrapment (T: top, B: bottom). From ref. 114. Copyright 2012
Springer-Verlag.
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peroxidase (HRP) printed by a thermal inkjet printer (DeskJet
600). Whereas, Lonini et al.
95
found that the polyclonal rabbit
anti-human IgG labeled with HRP caused clogging when
printed by a thermal inkjet printer (HP Deskjet 5740) but the
same protein could be printed by piezoelectric inkjet printing
with no problems. Similarly, Setti et al.
40
measured a 15% loss
of β-galactosidase activity after printing by a thermal inkjet
printer (Olivetti Tecnost).
In addition to the thermal stress, another problem is the
printing shear rate. For current DOD inkjet printers, the
shear rate ranges from 2 ×10
4
to 2 ×10
6
s
1
, which may
potentially destroy proteins.
96
Another stress, pointed out by
Nishioka et al.
97
is the compression rate, a force exerted on
the ejecting ink to improve resolution and to enhance pene-
tration of the ink into the paper. They found that even an
extremely low compression rate (2.56 ×10
4
μm
3
μs
1
) caused
damage to peroxidase. However, the addition of sugar (treha-
lose or glucose) significantly reduced the damage.
In summary, the non-specific adsorption and all the above
mentioned tumultuous disturbances can affect inkjet printing
of these biomolecules, yet using suitable additives (see addi-
tives section) can reduce or even eliminate these disturbances.
Conductive molecules
Conductive molecules used for inkjet printing of biosensors
include nanoparticles, organometallic compounds, and con-
ductive polymers (e.g. PEDOT/PSS in Fig. 10A; PVAm in
Fig. 10B; and CuO, Si, Ag, or TiO
2
nanoparticles, Table 2 # 3,
1012, 31).
98
Other novel nanomaterials (e.g. carbon nano-
tubes and graphene) are also emerging as popular molecules
for inkjet fabrication of biosensors.
99103
In comparison to
biomolecules, quantity is less of an issue during ink formula-
tion than stability. In particular, the conductive molecules
Fig. 12 Inkjet fabricated biosensors with direct channel or zone
patterning. A: Total inkjet fabricated blood typing biosensors. (iv)
Fabrication and testing procedures of the text-reporting blood-typing
devices. (i) Anti-A and anti-B are introduced into the corresponding
letters. An equal-volume mixture of anti-A and anti-B is introduced
into ×, and anti-D is introduced into I. (ii) Letter Oand symbol
are printed over ×and , respectively, using a non-bioactive and
water-insoluble ink. (iii) A blood sample is introduced in the device for
blood typing test. (iv) The blood typing result is displayed after washing
with saline solution. (v) The actual tests of all eight ABO rhesus blood
types. From ref. 145. Copyright 2012 WILEY-VCH Verlag GmbH & Co.
KGaA, Weinheim. B: More complicated pattern designs using the direct
approach by inkjet patterning (i) multi-lined pattern and (ii) Chinese
paper cut pattern of a dragon. From ref. 147. Copyright 2010 Elsevier
B.V. C: Inkjet printed biosensor that contains switches or valves. (ac) A
design of a simple paper-based microfluidic reactor consisting of two
sample dosing sites, two valves, and one central reaction site; (df) a
paper-based microfluidic reactor based on this design was tested using
acidbase neutralization reaction. (d) Phenolphthalein indicator solu-
tion was deposited onto the central reaction zone. NaOH and HCl
solutions were added into reagent zones A and B, respectively. (e)
NaOH solution was introduced into the reaction zone to trigger color
change. (f) HCl solution was introduced later into the reaction zone via
valve B to neutralize NaOH in the reaction zone. From ref. 81. Copy-
right 2010, Springer Science+Business Media B.V. D: 3D biosensor by
folding the inkjet patterned paper. (i) Schematic diagrams of the pro-
posed 2D biosensor. (ii) Schematic diagrams and real images of the
proposed 3D biosensors. From ref. 149. Copyright 2014 American
Chemical Society. E: 3D biosensor by stacking the inkjet patterned
paper. The paper was inkjet patterned while leaving the hydrophilic
matrix unprinted. The hydrophilic zones were impregnated with PBS
buffer; one of the hydrophilic zone was deposited with PIM (sensing
chemical for detecting Cu
2+
). These paper were stacked to form a 3D
biosensor. From ref. 151. Copyright 2013 Elsevier B.V.
Fig. 11 Inkjet fabricated biosensors with indirect channel or zone
patterning. A: Schematic representation of the fabrication process of
the inkjet printed multi-analyte biosensor. From ref. 63. Copyright
2008 American Chemical Society. B: (i) Schematic representation of
the fabrication process of the inkjet printed unassembled lateral flow
biosensor. (ii) Reaction scheme of the sandwich assay. From ref. 83.
Copyright 2008 Springer-Verlag.
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need to exist at a uniform dispersion state to prevent agglom-
eration. Stabilization of these molecules relies on the addi-
tion of additives (e.g. dispersants and polymers, see additives
section). Polymers are also used as additives to prevent satel-
lite droplets (see critical parameters for ink printability sec-
tion), and so the concentration of conductive polymer should
be adjusted to avoid generating long strands of liquid. In
addition, an inert environment is required for inkjet printing
of conductive molecules, as these molecules are highly sus-
ceptible to ambient humidity and reactivity with oxygen.
Moreover, the ejection velocity needs to be well controlled, as
it affects the distribution of the dispensing conductive
molecules.
104
Advantage of using inkjet printing of sensing molecules
In comparison to the other dispensing methods (e.g. screen
printing and microspotting), the dispensing of sensing mole-
cules using inkjet printing is advantageous because: (1) print-
ing is straightforward and inexpensive via cheap desktop
printers; this is especially true for carbon nanotube deposi-
tion, where the conventional chemical vapour deposition
method requires complex processes and sophisticated tech-
niques;
105
(2) there is a low risk of contamination, since
inkjet printing is a non-contact method; (3) minimal mate-
rials are wasted, particularly for DOD where the ink is only
ejected as required; (4) multiple sensing molecules can be
printed using a single device,
106
since inkjet printers usually
have multiple ink cartridges and printing nozzles; (5) printers
enable precise spatial control without any cross interference
on a matrix;
107,108
and (6) printing enables gradient creation,
whereby different densities of inks can be placed in desired
regions (also see inkjet patterning on superhydrophobic
matrices section).
109111
Inkjet patterning
Unlike the dispensing of the sensing molecules, inkjet pat-
terning is about dispensing certain reagents on a matrix to
create a reaction surface, or to define liquid pathways as
channels or zones for the sensing event(s). Inkjet patterning
inherits all the merits of inkjet dispensing for sensing mole-
cules, and in addition permits new functionalities, which is a
benefit for simplicity and miniaturization of biosensors for
point-of-care diagnostics.
Whole surface patterning
Whole surface patterning involves the inkjet printing a
mono- or multiple-layers of material to cover the entire sur-
face of a matrix to serve as a reaction phase. In some biosen-
sors, the reaction surface that contains biomolecules is on the
top layer. Setti et al.
58
fabricated an amperometric glucose
biosensor prototype by a thermal inkjet printer. The thermal
inkjet printer was applied to deposit two types of inks electronic
ink and biological ink. The electronic ink is composed of con-
ductive polymer ijpolyIJ3,4-ethylenedioxythiophene/polystyrene
sulfonic acid), PEDOT/PSS], which was printed onto an
indium-tin-oxide (ITO) coated glass surface (Fig. 10A). The bio-
logical ink that contained glucose oxidase (GOD) with buffer
and additives was then deposited onto the polymeric film. The
printed polymer provided a good electron transfer surface for
the redox reaction between the glucose and the GOD with no
loss of enzyme activity.
58
By applying the same procedures,
Setti et al.
112
fabricated an HRP-based amperometric hydrogen
peroxide biosensor, which showed the same success but with
a higher inkjet printing resolution (1500 ×1500 dot per inch,
dpi). Based on Setti and co-worker's work, Yun et al.
113
fabri-
cated a prototype of a bienzymatic (GOD and HRP glucose)
biosensor using a piezoelectric inkjet printer (Dimatix Mate-
rials Printer DMP-2800). The bienzymatic glucose biosensor
displayed a fast response time (<3 seconds). Also, their result
showed that the inkjet printed conductive surface improved
the current response approximately 1.5 times.
Entrapment
Although printing the biomolecule-containing surface as the
top layer demonstrates effective enzymatic or electrochemical
reactions, the shelf-life of this patterning is relatively short
(<1 or 2 weeks) due to the instability of the biomolecules. A
commonly-used surface patterning strategy is to entrap the
sensing reagents between the other reagent layers, which
retains the stability and activity of the biomolecules. Weng
et al.
115
applied this strategy in printing a similar HRP-based
amperometric biosensor to that of Setti.
112
The polypyrrole
(PPy)/enzyme layer was sandwiched between a bottom PPy
layer and a top ethyl cellulose (EC) layer. However, the
resulting sensitivity was 0.5 times lower (0.25 μAM
1
cm
2
)
than Setti's device (0.544 μAM
1
cm
2
).
The same patterning strategy was also conducive to lateral
flow devices. Hossain and co-workers
114
built a bi-directional
lateral flow dipstick using a piezoelectric inkjet printer
(Fujifilm Dimatix 2800 N) for bacterial detection. The inkjet
printer printed four layers: a capture polyvinylamine (PVAm)
layer (on top of the matrix), a lower solgel-derived silica
layer and a top silica layer between which embedded inkjet
deposited sensing solutions (Fig. 10B). The dipstick assay
was sensitive and could detect less than 10 cfu mL
1
of E. coli
without culturing if immune-magnetic separation (a sample
pre-concentration step) was used. The inkjet printing
entrapped sensing reagents were stable, with no reagents
denaturation occurring for at least two months at room tem-
perature (25 °C). Wang et al.
116
subsequently explored the
function of each printing layer. They determined that the bot-
tom layer isolated the entrapped sensing molecules from the
cationic PVAm layer, preventing the potential inhibition of
the sensing molecules, while the top layer protected the sens-
ing molecules from proteolysis. The inkjet printed solgel
(top and bottom layers) confined the sensing molecules in
their initial place after lateral flow detection. A transmission
electron microscopy (TEM) analysis indicated that the inkjet
printed solgel material formed a thin film (with thickness of
35 ± 15 nm) that promoted rapid substrate transport and
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enhanced mechanical stability.
116
Such lateral flow dipsticks
could also be adapted to detect other β-glucuronidase and
β-galactosidase producing bacteria or to other intracellular
enzyme markers by simply varying the reaction reagents.
114
The same research group also successfully demonstrated the
detection of neurotoxins and pesticides on lateral flow
devices by using the same entrapment strategy.
117,118
Inkjet patterning as channels or zones
In contrast to whole surfacing patterning, inkjet printing
can also be used to patterning reaction channels or zones.
That is, rather than printing a homogenous surface, selective
sections of the surface are printed with inks of varying
properties, to define liquid passageways and sensing areas.
A range of methods for patterning channels or zones
have been reported including photolithography using
photoresistors,
119125
plotting with polyIJdimethylsiloxane)
(PDMS),
126
cutting,
127129
wax printing,
130137
wax dipping
138
or wax screen-printing,
139,140
plasma treatment,
81,141
marker
pens,
142
flexographic printing,
143
and laser treatment.
144
Each
of these methods has advantages and disadvantages. For
instance, the physical barrier generated by photolithography
lacks flexibility and is not resistant to bending and folding
damage.
126
Although plotting with PDMS overcomes the flexi-
bility issue, the topology of channels formed is not
straight.
141
Similarly, while the plasma treatment improved
the topological issue, it requires fabrication of masks for
each channel pattern. Likewise, the flexographic printing
requires two prints of polystyrene solution and different
printing plates for each printing, but has issues with resis-
tance to the low surface tensions of fluid.
143
Patterning using
cutting or a marker pen is simple, rapid, and low cost, but
the border of defined channels is not smooth. The printing
of wax results in low resolutions. In particular, the wax tends
to spread out when it melts, and thin reaction channels are
hard to achieve. Finally, laser treatments can generate high
resolution, but the channels patterned do not allow lateral
flow of fluidsthis requires extra coating (e.g. hydrophilic sil-
ica microparticles) to properly channel liquid flow.
144
In contrast, an inkjet printed channel or zone can be pro-
duced rapidly and reproducibly, with high resolution and
flexibility of design that is efficient for mass production. In
general, inkjet patterning of reaction channels or zones on a
porous matrix (usually hydrophilic) aligns with the principle
of enclosing the hydrophilic portion of the matrix (e.g. cellu-
lose membrane) by hydrophobic barriers. This can be
achieved indirectly and directly.
Indirect inkjet patterning as channels or zones
The indirect approach to channel or zone patterning is to
fully hydrophobize the matrix at the first step, and then apply
a hydrophobic solvent to locally dissolve the hydrophobic
cover, thus re-exposing the hydrophilic matrix as channels or
zones in desired patterns. Abe et al.
63
built a multi-analyte
chemical biosensor for the detection of pH, total protein
(human serum albumin, HSA), and glucose simultaneously
using a piezoelectric inkjet printer (PicoJet-2000). They
applied toluene to the pre-printed hydrophobic cover (poly-
styrene treated filter paper) to generate reaction zones. When
the sample solution was applied to the central loading area,
it moved evenly toward the sensing areas (Fig. 11A). The
hydrophobic ink penetrated deep through the porous matrix,
forming a strong barrier that guided liquid smoothly without
incurring leaking to the other channels.
63
Their second fabri-
cation was a lateral flow device based on a sandwich assay
(involving antibody-antibody interactions, Fig. 11B(ii)).
83
The
lateral flow device required several consecutive sensing
reagent deposition areas, necessitating a change in the print-
ing order of the hydrophilic channels. In this case, the con-
trol and the test regions, along with the sensing reagents in
these two regions were printed first, and then followed by
implementing the other channels and the rest of the sensing
reagents (Fig. 11B(i)). This was to prevent the spreading of
the sensing reagents in the control and the test regions out
to the other flow channels.
Direct inkjet patterning as channels or zones
Since the matrix for fabricating the biosensors is usually
porous and hydrophilic in nature (e.g. a cellulose mem-
brane), an alternative way of patterning channels is to
directly print the hydrophobic portion while leaving the
hydrophilic matrix unprinted as reaction channels or zones.
Using this direct approach, Li et al.
145
manufactured a practi-
cal and economic paper-based blood typing device that was
capable of detecting all eight blood types (A+, A, B+, B, O+,
O, AB+, and AB, Fig. 12A(v)). The alkenyl ketene dimer (in
n-heptane) hydrophobized the unprinted hydrophilic regions
as letters or symbols to represent the blood type. The detec-
tion was based on haemagglutination reactions, where a col-
ored blood type pattern would display if there were interac-
tions between the antigens in the blood sample and the
corresponding antibodies within the letters and symbols after
a saline washing step (Fig. 12A(iv)).
145
Recently, Li et al.
146
also successfully showed the detection of secondary human
blood groups using the same patterning approach. Even more
complex designed patterns can also be patterned through the
direct patterning approach. Fig. 12B shows some of the more
complicated graphic designs by Li et al.
147
The patterning via
such approach is flexible, because the hydrophobic pattern
can be simply designed by drawing software (e.g. Microsoft
PowerPoint), and then executed by an inkjet printer.
Li et al.
81
demonstrated that even switches or valves could
be built into paper-based biosensors using directly patterned
paper (Fig. 12C). The inkjet printed hydrophobic areas were
resistant enough to confine the solution within the hydro-
philic channel, yet allowed the solution to flow smoothly
through the channel to the adjacent reaction zone when the
switch was pressed down.
The direct patterning strategy also permits 3D paper-based
biosensor development. For example, by patterning the whole
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reaction channels or zones into one piece of paper, a 3D bio-
sensor was generated by folding the paper so that the corre-
sponding channels or zones overlapped (Fig. 12D).
148,149
The
testing reagent was applied to a reaction zone, and flowed
through to folded areas that overlapped with the original
zone.
148,149
In another example, the reaction channels or
zones were patterned on different pieces of paper, which
were then stacked together to form an overlapping reaction
chamber (Fig. 12E).
150,151
In both these 3D biosensor designs,
the inkjet printer precisely deposited the reagent(s) into
desired positions on the matrix. The patterned hydrophobic
region not only served as a rigid structure support, but also
hydrophobized to direct the flow of the hydrophilic solutions.
One problem with the above patterning approach is that the
hydrophobic portion usually occupies a much larger area com-
pared to the hydrophilic channels, resulting in excessive ink
usage. To reduce the ink requirements, an alternative method
is to only print a hydrophobic border to enclose the entire sen-
sor area. Yamada et al. demonstrated the feasibility of this
approach by printing a UV-curable ink to enclose a sampling
area and a sensing area that was connected via a thin channel
on the topside, with a fully covered backing.
85
Subsequently,
they printed a top cover to conceal the hydrophilic channels
(Fig. 13) and this created a functional tunnel that protected the
reaction sections of the device from the environment.
84,152
However, this fencing patterning is not as frequently used com-
pared to printing the whole hydrophobic area, possibly because
the non-fencing patterning provides a more solid barrier and
rigid structure support for the biosensor, especially when the
matrix of the biosensor is not rigid (e.g. paper-based).
Inkjet patterning on non-porous matrices
As opposed to the patterning strategy upon porous (usually
hydrophilic) surfaces, patterning upon non-porous surfaces
requires different considerations. In particular, hydrophilic
ink can be printed onto non-porous hydrophobic surfaces. In
this case, the printed hydrophilic ink directly forms a pattern
(channel or zone) on the hydrophobic matrix through adhe-
sion (as opposed to forming a channel between hydrophobic
ink and hydrophilic matrix as described above). However,
poor adhesion can cause difficulties, resulting in the need to
modify the surface with certain reagents, which not only
improves the adhesion but also other properties (e.g. wetta-
bility, hydrophobicity). Wu et al.
153
fabricated a glucose bio-
sensor by inkjet printing silver nanoparticles upon a PDMS
modified surface (by IJ3-mercaptopropyl)trimethoxysilane,
MPTMS). The nanoparticles showed good adhesion to the
modified PDMS surface via a soaking test (the pattern was
immersed into water for 2 h), a blowing test (the pattern was
blown with an air stream of 0.5 MPa), and an ultrasonication
test (the pattern was placed into a water beaker and
ultrasonicated). A comparison was also made to the plasma
treatment patterning strategy. Although the plasma-treated
PDMS allowed silver nanoparticle droplets to form more
coherent patterns, the pattern failed the soaking test because
of the extremely weak adhesion between the plasma-treated
PDMS surface and the silver nanoparticle-based ink.
153
Another problem for inkjet patterning upon a non-porous
matrix is that the adjacent ink droplets tend to coalesce,
thereby affect the quality of patterning. Optimization of the
ink drop spacing can help solve this coalescence
problem.
153155
Wu et al. found that a drop spacing greater
than 38 μm did not cause coalescence of the silver nano-
particles on the MPTMS modified PDMS surface.
153
Instead
of changing the ink base (to be a reactive one, see the section
on ink bases) to solve the coalescence problem, they applied
a discontinuous overlapping printing strategy, which was to
stagger the printing of the ink droplets. Once the first round
of printing droplets dried, a second round followed to
deposit the ink droplets next to the dried ones, and this con-
tinued for subsequent additions. At the end, all the ink drop-
lets would overlap without interfering with each other to
obtain a uniform pattern.
153
Belgardt et al. also stated that
the isolated droplets (without coalescence) printed on non-
porous matrix improved edge sharpness, and more continu-
ous distribution of the effective wettability by the hydrophilic
ink patterned channels was obtained.
155
Inkjet patterning on superhydrophobic matrices
A major advantage of inkjet patterning with hydrophilic ink
upon a hydrophobic non-porous matrix is that fewer assay
reagents are lost in the transporting channels before they
reach the detection zones.
109
This is because the liquid does
not adhere to the non-porous matrix rather than absorbing
into a hydrophilic porous matrix as it travels along the chan-
nels. Thus, liquid wettability is more controlled with inkjet
patterned hydrophobic non-porous matrix.
An example of controlled wettability is inkjet patterning
using superhydorphobic matrices. This surface has a contact
Fig. 13 Inkjet fabricated biosensors with direct channel or zone
patterning by hydrophobic fencing. A: Schematic representation of the
fabrication process of the inkjet printed peroxide biosensor (i)
patterning of the filter paper by a double-sided printing process (grey
and black colors indicate the printed hydrophobic features before and
after UV curing, respectively). (ii) Inkjet printing of peroxide sensing ink.
(iii) Color-scanned images of the inkjet printed peroxide biosensor with
a bottom cover by top, cross-sectional, and bottom views. B: Inkjet
printed peroxide biosensor with both a top and a bottom cover.
Reproduced from ref. 84 with permission from the Royal Society of
Chemistry.
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angle of >150°(compared to hydrophobic matrices, where
the contact angle is ~90°). On a superhydrophobic matrix, liq-
uid tends to roll off and aggregates into droplets, analogously
to the effect of water on a lotus leaf. Such phenomena are
advantageous for reagent transfer, mixing or sample splitting
on a biosensor. A proof-of-concept using inkjet patterning
was first tested by Balu et al.,
156
where hydrophilic black
phaser ink was deposited as dot and line patterns on super-
hydrophobic handsheets. The water droplets (colored with
food dye for visualization) showed efficient transferring,
mixing, and splitting along the black phaser ink patterned
channels simply by slightly tilting the patterned matrix. Stem-
ming from Balu et al.'s prototype, several research groups
have focused on improving the droplet mobility by control-
ling the droplet hysteresis level in a detection zone before it
wets the next detection area.
109111
This improvement was
achieved by inkjet printing different hydrophilic ink densities
in the desired detection zones respectively. The resultant bio-
sensor allowed more focused sensing and required less
reagent volumes.
Critical parameters for inkjet patterning and post-inkjet fab-
rication of biosensors
As seen in the above sections, there is a great depth and vari-
ety of patterning that can be achieved using inkjet printing.
When developing for new applications, there are some key
parameters to consider both for patterning and also for post-
testing once the inkjet fabrication of biosensors is completed.
For most chemists, the choice of surface (e.g. hydrophilic,
hydrophobic, porous, and non-porous) is paramount, and
highly dependent on the final application. However, this sur-
face also critically determines the type of patterning strategy,
which may require pre-treatment or modification of the sur-
face to permit a better printing quality or to adopt a certain
bio-activity for the final biosensor. Thus bonding of the ink
to the surface becomes the next consideration (see the sec-
tion on ink properties), which also drives the choice of
printer (as described in the section on printer choices). The
complex interactions between the matrix, the ink, and the
printer allow achievement of the required resolution.
After considering the surface, ink, and printer interplay,
the next step is to choose a suitable reagent for patterning
the channels or zones. On a porous matrix, the choice of
hydrophobizing reagent for patterning relates to its mecha-
nism of grafting, and is divided into three categories: (1)
physical blocking of the pores in paper (e.g. using photoresist
or PDMS); (2) physical deposition of a hydrophobizing agent
(e.g. polystyrene or wax); and (3) chemical modification of
porous matrix (e.g. alkyl ketene dimers = AKD and alkenyl
succinic anhydride = ASA, both are cellulose reactive
agents).
157
The hydrophobization through the chemical mod-
ification is more stable than that of the physical ones,
because the modification cannot be removed by organic solvent
extraction.
158
For example, wax does not tolerate strong acids or
bases, and is not compatible with organic solvents.
130,132
Therefore, a reagent tolerability test for the inkjet patterned
barriers should be conducted before performing the subse-
quent sensing test. While wax and AKD are the two com-
monly used hydrophobizing reagents for inkjet patterning,
other newly developed hydrophobizing reagents have
displayed superior performances. These include methyl-
silsesquioxane (MSQ),
159
silicon resin,
160
and Teflon.
161
All
these new hydrophobizing reagents were tolerant to organic
solvents (e.g. glycerol, toluene, piperidine, trifluoroacetic acid =
TFA), lower surface energy solvents (e.g. dimethyl sulfoxide =
DMSO), and surfactant solutions (e.g. SDS, Triton X-100,
CTAB = hexadecyltrimethylammonium bromide).
159161
Patterning on non-porous surfaces hinges on the adhesion
of the ink to the matrix and droplet coalescence issues. The
adhesion can be strengthened by including additives in the
ink formulation and/or modifying the matrix with suitable
reagentIJs).
153
Droplet coalescence can be solved via optimiza-
tion of the minimum drop spacing, or adopting a staggered
printing manner (mentioned in inkjet patterning on non-
porous matrices section). It is noteworthy that sintering
maybe necessary after inkjet patterning. This is especially
true for conductive inks, as they require sintering (a thermal
treatment) to form the connections between neighbouring
molecules, which improves electrical conductivity and
mechanical adhesion. Some hydrophobizing reagents (e.g.
AKD and silicon resin) also showed better performance after
sintering.
160,162
Finally, other parameters that improve inkjet patterning
include optimizing the number of printing cycles to allow
maximum ink coverage; double-sided printing (top and bot-
tom) to allow full hydrophobization on porous matrix via ink
penetration and coalescence;
163
and optimizing reagent print-
ing order during multiple layered printing to preserve opti-
mal activity of the biomolecules.
164,165
Highlights of functionality features by
inkjet printing
Beyond the added convenience of biosensor fabrication via
inkjet printing, the technology is providing a transformative
platform to reach new frontiers in biosensor manufacturing.
While described in detail via a manufacturing perspective in
the preceding sections, some of the most significant
advances in regards to new functionality achieved by inkjet
printing of biosensors include:
Switches and valves
81
(Fig. 12C). Inkjet printed hydropho-
bic barriers are flexible and resistant enough to regulate liquid
flow between different reaction zones. This is a good imitation
of the sophisticated Lab-on-chip(or microfluidics) system
but with a lower cost and simpler fabrication.
Three-dimensional biosensor via folding or stacking
148151
(Fig. 12D and E). Inkjet-patterned shapes can be overlapped to
guide the directions of liquid flow into different detection
zones. The resulting folded or stacked 3D biosensors adopt a
flow-through detection; this eases the liquid movement via
gravitational forces and minimizes the device size.
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Controlled sequential mixing
108
(Fig. 14A). Inkjet printed
neighboring reagents can be deposited to not interfere with
each other when stored on the device in dried form. Mixing of
these reagents then occurs when liquid is added and moves
along the printed regions. This alleviates the requirements for
pre-mixing reagents prior to assay commencement.
Erasable enzyme-enabled 2D code
166
(Fig. 14B). Gradient
printing by varying the ink deposition density allows develop-
ment of an enzyme 2D code. This code appears shortly after
initiation of the reaction, formed by different enzyme reac-
tion rates between blackand whiteareas (which repre-
sent inkjet printing of two different gradients of enzyme and
substrate mixture at a different ratios). The signal disappears
when the enzyme substrate is depleted.
Microchannel built within a liquid matrix
167
(Fig. 14C).
Ink deposited with a lower density than the liquid matrix can
coalesce to form desired shapes as microchannels. This
transforms microfluidics fabrication by reducing cost and
simplifying fabrication.
Conclusions
Due to the simple, rapid, flexible, high resolution, low cost,
and efficient properties for mass production, inkjet printing
is becoming a routine tool for biosensor fabrication as this
printing technology matures.
19
It has been nearly 30 years
since the first report of an inkjet printed biosensor.
168
Since
then the diversity in terms of ink, matrix, and patterning
methods has dramatically increased. Inkjet printing is now a
functional printing method, and a competitive fabrication
tool for biosensor manufacturing, especially for point-of-care
diagnostic biosensors. The variety of inkjet printers available
enable both fabrication of smart biosensors (e.g. electronics)
that require high-resolution deposition (e.g. EHD inkjet
printer), and fabrication of common biosensors (e.g. glucose
biosensor) in an efficient way and at a low cost. Numerous
partially inkjet printed biosensors have been reported, how-
ever, fully inkjet printed biosensors have also been realized,
which enables rapid mass production because the whole fab-
rication process is realized within a single machine.
The potential of inkjet printing in this burgeoning field,
however, is still largely untapped, and we anticipate that even
more possibilities and novelties are highly likely to emerge.
For instance, developing automated processes to test and
adjust inks (such as rheological measurements) would greatly
aid the inkjet printing process to develop a step further and
increase the efficiency of biosensor fabrication. Miniaturiza-
tion of the inkjet printer to be a portable machine is another
issue, so that the printing process can even be conducted in
field. Perhaps even more critical, however, is to integrate
inkjet printing with other analytical processes for advanced
and fully-automated analytical systems. The potential of
inkjet printing should not be underestimated the technol-
ogy is being applied to a variety of fields, and its develop-
ment is becoming more versatile as multi-disciplinary knowl-
edge combine.
Acknowledgements
We thank John Bartlett and David McMillan for support and
advice, and Richard Burns for careful reading of this manu-
script. This work was funded by the Queensland Government,
Department of Science, Information Technology, Innovation
and the Arts (DSITIA, Australia).
Fig. 14 Some functional advances in biosensors that were realised by
inkjet printing. A: Sequential mixing of signal enhancement solutions in
a 2D paper network biosensor. Three signal enhancement solutions
were inkjet printed next to each other. Mixing only occurs when
solution is applied, and in turns, wicks along the printed regions. From
ref. 108. Copyright 2014 American Chemical Society. B: An erasable
enzyme-enabled 2D code. The coding was achieved by inkjet printing
two different gradients of enzyme and substrate mixture (at a different
ratios). The decoding was initiated by spraying the paper with a water/
solvent mixture (50% ethanol). The enzyme 2D code appeared at maxi-
mum intensity after 12 minutes, and disappeared after 150 minutes
when the printed enzyme substrate was depleted. From ref. 166.
Copyright 2014 Royal Society of Chemistry. C: Inkjet fabrication of a
microfluidic reactor on a liquid template involved the following steps:
(i) Pouring a PDMS prepolymer mixture into a container; (ii) printing a
Y-shape pattern on the surface of the prepolymer mixture (inset: rela-
tive position sketch of the pattern and prepolymer mixture surface);
(iii) standing for a few seconds to allow the pattern to be wrapped
spontaneously; (iv) after heating the prepolymer mixture with the liquid
template, the prepolymer mixture thermally cures and the liquid tem-
plate evaporates, leaving the microchannel in the PDMS matrix; (v)
peeling off the fabricated PDMS matrix containing the microchannel;
(vi) the typical microfluidic reactor in the PDMS matrix. From ref. 167.
Copyright 2015 Royal Society of Chemistry.
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... The parameters must be adjusted so that the value of Z lies between the range of 1 and 10 for reliable jetting, without forming satellite droplets or demanding high driving voltage for droplet ejection that eventually damages the cartridge head. [49,50] Generally, to secure compatibility with Dimatix cartridge head, inks should contain particles with sizes below 200 nm and viscosities between 5 and 40 mPa s to prevent any clogging effect. [51] A stable ink, without sedimentation, eliminates the appearance of aggregations that will potentially harm the cartridge and decrease ink shelf life. ...
... which was within the acceptable range for efficient jetting. [49] The pH for both cases was around 4 (AE0.3), that was optimal for continuous jetting, without causing any damage to the cartridge and nozzles. ...
Article
Full-text available
This study aims to contribute to the burgeoning field of brain‐inspired computing by expanding it beyond conventional fabrication methods. Here we encounter the obstacles toward effective inkjet printing process and explore the electrical characteristics, providing new insights into reliability aspects of fully printed Ag/a‐TiO2/Ag electronic synapses. The versatility of the approach is further enhanced by the highly stable in‐house developed a‐TiO2 ink, exhibiting optimal shelf life of five months and repeatable jetting, producing layers with nanoscale thickness resolution. Most importantly, device electrical characterization reveals synaptic dynamics leading to activity‐dependent conductance state retention and adaptation characteristics, implying inherent learning capabilities. The synaptic dynamics are attained by solely adjusting the duty cycle of the applied pulsed voltage trigger, while keeping amplitude and polarity fixed, a method readily compatible with realistic applications. Furthermore, I‐V analysis demonstrates a dynamic range dependence on a‐TiO2 layer thickness, and conduction mechanism that is akin to the conventionally developed electronic TiO2 synapses. The developed devices provide a time‐ & cost‐effective ecologically benign alternative toward biomimetic signal processing for future flexible neural networks. This article is protected by copyright. All rights reserved.
... Another solution is the addition of minor amounts of solvents with a high boiling point and a low surface tension to slow evaporation at the contact angle [48]. This technology seems to be quite promising to design electrochemical sensors [49][50][51], photodetectors, electronics [49,[51][52][53], and alternative energy applications [50,54]. The advantages usually include a compatibility with biological materials, low cost, programmed control, relatively high spatial resolution down to 20-50 µ m, relatively high speed, and performance. ...
... Another solution is the addition of minor amounts of solvents with a high boiling point and a low surface tension to slow evaporation at the contact angle [48]. This technology seems to be quite promising to design electrochemical sensors [49][50][51], photodetectors, electronics [49,[51][52][53], and alternative energy applications [50,54]. The advantages usually include a compatibility with biological materials, low cost, programmed control, relatively high spatial resolution down to 20-50 µ m, relatively high speed, and performance. ...
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... Inkjet printing (IJP) has been widely used in the deposition of coatings and patterning in various applications, ranging from ceramic components [51,52] to electronics [53,54] and biotechnologies [55,56]. The deposition process is automated in IJP with a precise control of ink droplet volume and location, which ensures a consistent and homogenous deposition over the substrates. ...
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