Printed electronic switch on flexible substrates using printed microcapsules

Printed electronic switch on flexible substrates using printedmicrocapsules

A. Tessa ten Cate • Cristina H. Gaspar • Heini L. K. Virtanen •

Ralph S. A. Stevens • Robin B. J. Koldeweij • Juuso T. Olkkonen •

Corne H. A. Rentrop • Maria H. Smolander

Received: 26 December 2013 / Accepted: 22 April 2014 / Published online: 24 May 2014

� Springer Science+Business Media New York 2014

Abstract Printed electronics, the manufacturing of elec-

tronic components on large, flexible, and low-cost sub-

strates by printing techniques, can facilitate widespread,

very low-cost electronics for consumer applications and

disposable devices. New technologies are needed to create

functional components in this field. This paper introduces a

new method to create an all-additive printed switch on

flexible substrate materials, such as polymer foils and paper

substrates. The active layer of the switch component con-

sists of neutral polyaniline (PANI), which can be doped by

acid to induce a shift from a non-conductive to a conductive

oxidation state. Monodisperse core–shell microcapsules

containing an acidic aqueous core liquid were produced by

a novel inkjet-based encapsulation technology. It was

shown that unfavorable water evaporation from the micro-

capsules could be reduced by the addition of calcium

chloride to the core liquid. A switch component was pre-

pared, consisting of inkjet-printed interdigitated silver

electrodes, PANI active layer and printed microcapsules. If

an external pressure was applied, for instance with a finger,

then the switch component changed its state from non-

conductive to conductive with a simultaneous distinct color

change. The results clearly demonstrate the feasibility of the

presented approach to create either a visual or electronic

signal for use in printed electronic applications.

Introduction

Printed electronics involves the manufacturing of elec-

tronic devices, actuators [1, 2], and sensors [3–5] on vari-

ous substrate materials, using traditional printing methods.

Printed sensors on different flexible materials have been

developed in recent years, for different purposes, such as

sensing of different gases [6, 7]. Printed electronics allows

miniaturization and adds functionality to ultra-thin elec-

trical circuits printed on flexible substrates with different

printing techniques, such as screen, flexo, or inkjet printing

[8], instead of the conventional microfabrication tech-

niques, such as lithography or thin-film deposition. The

functional inks used for printing are deposited on the

substrate, creating active or passive devices. Inkjet is a

non-contact digital printing method (drop-on-demand),

creating a functional pattern by delivering ink droplets to

the substrate surface. Current improvements have led to

extremely small drop sizes (\1 pL) and thus, these patterns

can be very small and compact, leading to higher resolution

A. T. ten Cate (&) � R. S. A. Stevens � R. B. J. Koldeweij �C. H. A. Rentrop

Netherlands Organisation for Applied Scientific Research TNO,

P.O. Box 6235, 5600 HE Eindhoven, The Netherlands

e-mail: [email protected]

R. S. A. Stevens


R. B. J. Koldeweij


C. H. A. Rentrop


C. H. Gaspar � H. L. K. Virtanen � J. T. Olkkonen �M. H. Smolander

VTT Technical Research Centre of Finland, P.O. Box 1000,

02044 Espoo, Finland


H. L. K. Virtanen


J. T. Olkkonen


M. H. Smolander


123

J Mater Sci (2014) 49:5831–5837

DOI 10.1007/s10853-014-8271-7

[9]. Furthermore, it enables cost-efficient mass manufac-

turing of electrodes and other functional materials on large,

flexible, and low-cost substrates like plastic [10, 11], paper

[12–14], and fabrics [15], replacing glass and silicon sub-

strates traditionally used for electronic applications [16].

In future, innovative and disposable products will

emerge from printed electronics, in various fields such as

point-of-care applications, diagnosis, power sources, bio-

sensors, and smart/interactive packaging. Thus, printed

electronics opens up new markets and opportunities, where

conventional silicon products are not able to penetrate,

mainly because printed electronics is a more cost efficient

as well as a more environmentally friendly manufacturing

method for disposable devices, when compared with con-

ventional microfabrication techniques. For example, prin-

ted electronics is expected to facilitate widespread, very

low-cost electronics for applications such as flexible dis-

plays, smart labels, decorative, and animated posters, and

active clothing that do not require high performance [17].

Microcapsules can be used to create functional compo-

nents for printed electronics, for instance in electrophoretic

displays [18], to enable protection of labile sensing moie-

ties by a stable shell [19], or to produce thermally or

mechanically triggered activation. One of the most well-

known and oldest applications for the use of microcapsules

in mechanically activated functionality on paper substrates

is carbon-less copy paper [20]. To obtain mechanically

triggered release, the use of core–shell microcapsules is

most suitable. Different technologies for the production of

core–shell microcapsules exist; however, these are often

batch processes and in many cases, polydisperse micro-

capsules are obtained. Inkjet-based technologies, using

either a concentric nozzle [21] or an encapsulating liquid

film to create a core–shell morphology [22, 23], can pro-

duce well-defined monodisperse microcapsules.

This paper introduces a new way to create an all-addi-

tive printed switch on flexible substrate materials, includ-

ing polymer foil and paper. The active layer is composed of

polyaniline (PANI) [24], a conductive polymer that can be

found in three oxidation states [25]. The neutral state, often

referred to as the emeraldine base, is non-conductive. If the

material is doped by acid, then the resulting emeraldine salt

form of PANI is electrically conductive [26]. Actuation of

the switch is based on the use of microcapsules that release

acid upon mechanical activation. Release of the acid is

followed by a change in conductivity, owing to the doping

of PANI. Inkjet printing of acidic core liquids through a

liquid film of shell material allows the preparation of well-

defined core–shell microcapsules that, ideally, could be

printed directly onto the substrate material. The inkjet

printing of the conductive paths for the electrical circuit

enables a fast, low cost, and easy way to mass-manufacture

switch components on different flexible materials.

Materials and methods

Materials

Citric acid monohydrate, calcium chloride hexahydrate,

stearic acid, carnauba wax (No. 1 yellow), and PANI

emeraldine base (average Mw *20,000) were obtained

from Sigma-Aldrich. Panipol-w (PANI ink) (Panstat W1-

00-041.RD\10 % of PANI sat) was obtained from Panipol

Ltd. DGP-40LT-15C silver nanoparticles colloidal ink,

with *30 wt%, was obtained from Advanced Nano Pro-

ducts Co. Ltd. Suntronic U5603 ink (20 wt% dispersed

silver nanoparticles (30–50 nm), stabilized with poly(vi-

nylpyrrolidone) in ethylene glycol) was obtained from Sun

Chemical. For preparing the switch component, polyeth-

ylene terephthalate (PET) (Melinex 238, 125 lm thick)

was used as polymer foil substrate material, and

p_e:smart� paper type 3 (Felix Schoeller) was used as

paper substrate material.

Preparation of microcapsules

Printed microcapsules are produced using an experimental

set-up described previously [23] and schematically shown

in Fig. 1. In brief, the set-up consists of a heated printing

reservoir connected to a high pressure pump; a heated print

head with a 50-lm-diameter nozzle; a heated reservoir for

liquid shell material, connected to a pump system; and a

splash-plate type nozzle, in which a jet of fluid shell

material impinges on a splash plate, resulting in a thin, fluid

film of shell material.

Three different core fluids were used (Table 1): citric

acid in deionized water (CAc), citric acid and calcium

chloride in deionized water (CAc/CC), and pure deionized

water as reference (H2O). The shell material, consisting of

a mixture of carnauba wax (25 wt%) and stearic acid

(75 wt%), was processed at 105 �C and at a flow rate of

120 ml min-1 to produce a thin liquid film. Core fluids

Fig. 1 Preparation of core–shell microcapsules by inkjet printing

through a liquid film of shell material

5832 J Mater Sci (2014) 49:5831–5837

123

were inkjet printed at 40 �C, at a flow rate of 0.7 ml min-1

and a frequency of 20 kHz, producing core droplets of

104 lm that were impacted on the liquid film of shell

material.

Characterization of microcapsules

Microcapsule diameter and polydispersity were evaluated

by optical microscopy, using a Zeiss AxioImager M1m,

equipped with Epiplan objectives and an AxioCam MRc 5

camera, and AxioVision software.

Core–shell ratios (wt%) of the freshly prepared micro-

capsules were determined gravimetrically, by comparing

the mass of the obtained microcapsules with the mass of

the printed core liquids. Using the obtained results, average

shell thicknesses were calculated based on core and shell

volume, taking into account differences in density of the

core liquids.

To evaluate storage stability, accurately weighted

amounts of microcapsules (0.7–0.9 g) were distributed

over large petri dishes to maximize direct contact of the

microcapsules with the surrounding atmosphere, and were

kept at room temperature (21–22 �C) and moderate

humidity (30–70 % R.H.) for 4 weeks. At regular time

intervals, the remaining mass of the microcapsules was

measured. Residual core mass was calculated by correcting

for the wt% shell material, as previously determined.

Preparation of the switch component on polymer foil

Interdigitated electrodes were printed on PET substrate

using a multi-nozzle piezoelectric inkjet printer, Dimatix

2831 (Fujifilm Dimatix Inc.), using 10 pL cartridges and

DGP-40LT-15C silver nanoparticles ink. The used drop

spacing was 40 lm. The ink spreading was reduced by

keeping the substrate temperature at 60 �C during the

printing. The printed silver patterns were sintered at 150 �C

for 60 min. The layout of the used interdigital electrode

structure is shown in Fig. 2. The electrode fingers are

9 mm long and 1 mm wide. The gap between two adjacent

fingers is 450 lm.

Dedoped PANI ink was prepared mixing 1 mL of 2 wt%

NaOH water solution and 5 mL of Panipol-w. The dedoped

PANI ink was applied on PET foil on top of the inkjet-

printed silver electrodes by spin-coating (500 rpm, for

20 s). The coated film was dried on a hot plate at 100 �C

for 15 min. Finally, to complete the switch structure,

10 mg of microcapsules were placed on top of the PANI

layer.

Preparation of the switch component on paper substrate

A 1 wt% solution of PANI emeraldine base in N-methyl-2-

pyrrolidone (NMP) was dropcast onto p_e:smart� paper.

For each sample, 20 ll of solution was used to create a

PANI layer of about 1 cm2. Subsequently, the samples

were dried at 120 �C for 1 h. On top of the PANI layer,

interdigitated silver electrodes (spacing 0.75 mm) were

applied by inkjet printing, using Suntronic U5603 ink and

an Epson Stylus Photo P50 printer (5760 9 1440 dpi, 1.5

picoliter ink per droplet). Two subsequent layers of silver

ink were printed and afterward cured at 120 �C for 1 h.

Electrical measurements

The resistance of the switch component on polymer foil was

measured as a function of time by Keithley 3706 digital

multimeter. Sampling period was set to 5 s and the mea-

surement was started with a switch in the insulating state,

and then the switch was activated by breaking approxi-

mately 10 mg of the microcapsules with a finger, due to

which core liquid was released from the microcapsules onto

the PANI layer. Resistance was measured for 10 min from

the switch activation.

For the microcapsule-based switch on paper, resistance

was measured with a custom-made set-up in which the sam-

ples were connected in parallel with a second resistor of

3.8 MX. The total resistance of the electrical circuit was

measured with a Beckhoff EL 3692 module attached to a

computer set-up and monitored in time every 5 s. The resis-

tance of the samples (Rsample) can be calculated using Eq. 1:

Rsample ¼1

1Rmeas� 1

R2

; ð1Þ

in which Rmeas is the measured resistance and R2 is the

resistance of the additional second resistor (3.8 MX).

Results and discussion

The printed switch consists of three functional parts (as

schematically shown in Fig. 3): printed microcapsules

containing acidic fluid, dedoped PANI layer that can be

doped by acid to become conductive, and printed silver

electrodes to form an electrical circuit.

Table 1 Composition and properties of the core fluids

CAc core

fluid

CAc/CC

core fluid

H2O core

fluid

Citric acid monohydrate 10 wt% 8 wt% –

Calcium chloride hexahydrate – 44 wt% –

Deionized water 90 wt% 48 wt% 100 wt%

Density (g cm-3, room T) 1.03 1.21 1.00

J Mater Sci (2014) 49:5831–5837 5833

123

Printing of core–shell microcapsules

Based on preliminary tests with dedoped PANI layers,

aqueous citric acid solution (CAc) was determined to be a

suitable and non-toxic reagent to dope PANI to a con-

ductive form [27]. Therefore, it was selected as core fluid

for the microcapsules. To ensure the functionality of the

capsules, it was important to keep the core fluid liquid

inside the microcapsules in order to release the acid onto

the PANI layer only after applying mechanical pressure.

Due to the small wall thickness of the envisioned micro-

capsules, their permeability to water vapor was expected to

be substantial, even if a shell material with high water

barrier properties would be used. Therefore, calcium

chloride, a deliquescent salt, was also added to the core

liquid to keep water inside the microcapsules.

For the shell material, combinations of different waxes

and fatty acids were considered. An important criterion for

the microencapsulation process was the melting point of

the shell material, which should be around 70–80 �C to

allow processing in the molten state in combination with

aqueous core liquids and ensure thermal stability of the

obtained microcapsules at room temperature. In addition,

the material should have good film-forming properties and

allow the formation of a non-brittle shell. Carnauba wax

was an interesting candidate for the shell material, as it had

a suitable melting temperature (82–86 �C), in combination

with good film-forming and mechanical properties.

However, melt viscosity of the pure carnauba wax was

found to be too high to be able to be processed into a thin

liquid film. Therefore, combinations of stearic acid, which

has a low melt viscosity, and carnauba wax were evaluated,

and a mixture of 75 wt% stearic acid and 25 wt% carnauba

was selected as shell material. This material had a melting

temperature of 74–79 �C and a melt viscosity of 10 mPa s

at 100 �C.

Core–shell microcapsules were prepared by inkjet print-

ing of the aqueous core fluids through a thin film of molten

shell material, followed by cooling the obtained two-com-

ponent droplets to room temperature to obtain solid particles.

Three compositions of core fluids were used (Table 1): CAc,

citric acid solution with calcium chloride (CAc/CC), and

deionized water as reference (H2O). The obtained micro-

capsules were characterized by optical microscopy (Fig. 4).

The microcapsules were found to be monodisperse in size,

with a capsule diameter of approximately 110 lm, which is

in agreement with the core droplet diameter of 104 lm

produced by the inkjet printing process. Many of the mi-

crocapsules contained a small air bubble entrapped during

the encapsulation process. Core–shell ratios of the micro-

capsules were determined gravimetrically. The amount of

shell material was found to vary between 6 and 8 wt% for the

different samples, corresponding to an average shell thick-

ness of 1.3–1.8 lm (Table 2).

To test the functionality of the microcapsules for

mechanically activated acid release, microcapsules were

Fig. 2 Schematic (left) and

optical picture of the inkjet-

printed electrodes on PET

substrate (right) and magnified

microscope image of the Ag

lines and dimensions

Fig. 3 Schematic of the printed

switch

5834 J Mater Sci (2014) 49:5831–5837

123

deposited on pH indicator paper and pressed. A clear color

shift was observed in the pH indicator for the CAc mi-

crocapsules and CAc/CC microcapsules, while no color

change was found for reference microcapsules containing

deionized water.

Storage stability of microcapsules

In order to obtain a functional switch component, it is

important that the acidic core of the microcapsules remains

liquid until the capsules are broken, in order to allow

release and spreading of the core fluid onto the PANI layer.

The storage stability of the capsules was measured by

keeping the capsules in an open container at moderate

humidity (between 30 and 70 % R.H.) and measuring the

weight loss as a result of water evaporation in time. The

results, representing residual mass of the capsule core (i.e.,

capsule mass corrected for shell mass), are presented in

Fig. 5. For the capsules containing only water, the core

fluid was completely evaporated after 1 week of storage in

an open container. In the presence of citric acid, evapora-

tion was slightly reduced. Loss of water from the core fluid

was much further slowed down and reduced for capsules

containing calcium chloride. After 2 weeks, a plateau was

reached in the weight loss indicating no further evaporation

of the core fluid. Visual inspection showed that the core of

the capsules remained liquid. Additional weight loss was

not observed after further storage, indicating that equilib-

rium was reached as a result of the deliquescent nature of

the calcium chloride.

The increase in storage stability was confirmed by a series

of tests in which capsules were deposited on pH paper and

pressed. For freshly prepared CAc and CAc/CC microcap-

sules, this resulted in a clear color change, demonstrating the

release of acidic liquid onto the pH indicator layer. The same

result was obtained for CAc/CC microcapsules which were

stored in an open container for 7 days before testing. How-

ever, no pH change was observed for CAc microcapsules

stored in an open container for 7 days and subsequently

deposited on pH paper and pressed. This shows that the

addition of calcium chloride to the core liquid is a suitable

way to prevent complete drying of the microcapsules.

Resistance of the switch component on polymer foil

To prepare the switch component, interdigitated electrodes

were inkjet printed directly on PET foil and sintered.

Optical microscope images were taken to ensure the quality

of the printed lines (Fig. 2). The dimensions of the printed

structures were close to the aimed dimensions. Subse-

quently, dedoped PANI ink was spin-coated on top of the

electrodes in a uniform way, without the presence of pin-

holes, in order to ensure a good surface coverage of the

electrodes. It is possible to identify the state of PANI

doping by visual inspection. After spin-coating, the PANI

layer was blue, indicating the insulating form, which turns

to green after doping.

To characterize the fabricated switch components on

polymer foil, either CA or CAc/CC microcapsules were

Fig. 4 Core–shell microcapsules containing CAc

Fig. 5 Evaporation of water from microcapsules during storage in

open container

Table 2 Composition and shell thickness of the different types of

microcapsules

Microcapsule type Wt% shell

material (wt%)

Average shell

thickness (lm)

CAc capsules 6.3 ± 0.5 1.3 ± 0.1

CAc/CC capsules 7.2 ± 0.5 1.8 ± 0.1

H2O capsules 8.1 ± 0.5 1.7 ± 0.1

J Mater Sci (2014) 49:5831–5837 5835

123

deposited on top of the PANI layer of the switch and then

pressed with a finger. The resistance values of the switch

after 100 s from the activation are shown in Table 3.

Before the activation, the switch resistance was over

100 MX, which means that the conductivity difference

between the ON and OFF states is over five orders of

magnitude. Also, a clear color change from dark blue to

yellowish green was observable in the PANI layer with

both microcapsule types. Switch resistance with the CAc/

CC microcapsules is slightly higher than with the CAc

capsules as CaCl2 reduces the doping effect of citric acid.

However, the slight increase in resistance due to CaCl2 is

only a minor effect as compared to its benefit of keeping

moisture inside the capsules.

The obtained switch resistance in the ON state is relatively

high but it could be minimized by optimizing the electrode

geometry and the thickness of the PANI layer. In the inter-

digitated electrodes, the gap between two adjacent fingers

should be as small as possible. However, the fabrication of

the electrodes gets more challenging, the smaller the gap.

When the interdigitated electrodes are below the PANI layer,

the PANI layer thickness should be properly optimized. If the

PANI layer is too thick, then there is not enough acid to dope

the entire PANI layer and the resulting resistance will be

high, as there will be an insulating residual PANI layer left on

top of the electrodes preventing the formation of the con-

ductive path. On the other hand, if the PANI layer is too thin,

then its resistance due its small thickness is not the lowest

possible. In addition, the PANI layer should be pinhole free

as holes reduce the conductivity.

Resistance of the switch component on paper substrate

Similarly, resistance of the paper samples was measured,

while microcapsules were deposited on top of the inter-

digitated electrode switch and subsequently pressed. The

results are shown in Fig. 6. Before the microcapsules were

deposited onto the PANI layer, the samples were non-

conductive; a resistance of 3.8 MX was measured, which

represents the resistance of the second resistor placed in

parallel with the sample, meaning that the resistance of the

samples is much higher than that. When deposited CAc or

CAc/CC microcapsules were pressed, the measured resis-

tance rapidly dropped to 11 kX or 4 kX, respectively

(values measured after 15 s). This shows that the principle

of the pressure-activated switch is feasible using micro-

capsules on paper substrates. In time, the resistance slowly

increased again, reaching a resistance of 28 kX (CAc/CC

capsules) or 60 kX (CAc capsules) after 5 min. As a ref-

erence, capsules containing deionized water were also

tested. When these capsules were pressed, the resistance

decreased to 49 kX (measured after 15 s) and in a few

minutes increased again to the maximum level, after which

no additional decrease was observed. This seems to indi-

cate that the wetting of the PANI layer or of the paper itself

improves conductivity, which again decreases when the

samples dry.

Conclusions

Well-defined, monodisperse core–shell microcapsules

containing aqueous solutions of citric acid or citric acid and

calcium chloride in a wax-based shell with a diameter of

110 lm and a shell thickness of 1.3–1.8 lm could be pre-

pared by inkjet printing through a liquid film of molten shell

material. The presence of calcium chloride in the core fluid

reduced water evaporation from the microcapsules, thus

keeping the core fluid in a liquid state and increasing the

functional lifetime of the microcapsules. When depositing

the microcapsules on a PANI layer in contact with inter-

digitated silver electrodes and applying mechanical pres-

sure, a clear shift from a non-conductive to a conductive

state was observed, demonstrating the proof-of-principle of

the pressure-sensitive conductivity switch. Apart from

inducing a shift in conductivity, the printed microcapsules

can also be used to create a visual signal as the acid release

from the microcapsules creates a color change, either on a

Fig. 6 Measured resistance Rmeas (X) of the paper-based switch using

microcapsules containing CAc (dashed line), CAc/CC (dash-dot line)

or deionized water (solid line); the dotted line denotes the pressing of

the capsules

Table 3 Resistance (kX) values, measured 100 s after breaking the

microcapsules

Sample Resistance (kX)

CAc microcapsules 2 ± 0

CAc/CC microcapsules 5 ± 2

The data represent mean ± standard deviation (n = 3)

5836 J Mater Sci (2014) 49:5831–5837

123

PANI layer that is being doped, or on another active layer

containing a colorimetric pH indicator. The results pre-

sented in this paper demonstrate the feasibility of creating

an all-additive printed switch on flexible substrate materi-

als, such as polymer foils or paper substrates, which can be

used as functional component in printed electronics.

Acknowledgements The present work was supported by the RO-

PAS project, European Community Framework Programme 7, Grant

Agreement No.: 263078, and by the FlexSMELL project, part of a

Marie Curie Initial Training Network (ITN), from European Com-

munity Framework Programme 7, Grant Agreement No.: 238454. The

authors thank Jorgen Sweelssen and Milan Saalmink for experimental

support at TNO.

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