fluid-mediated parallel self-assembly of polymeric micro-capsules for liquid encapsulation and...

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This article can be cited before page numbers have been issued, to do this please use: L. Jacot-Descombes, C. Martin-Olmos,M. R. Gullo, V. J. Cadarso, G. Mermoud, L. G. Villanueva, M. Mastrangeli, A. Martinoli and J. Brugger, Soft Matter, 2013, DOI:10.1039/C3SM51923F.

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This journal is © The Royal Society of Chemistry [year] [journal], [year], [vol], 00–00 | 1

Fluid-mediated parallel self-assembly of polymeric micro-capsules for

liquid encapsulation and release

Loïc Jacot-Descombes*a, Cristina Martin-Olmos*

a, Maurizio Rosario Gullo

a, Victor Javier Cadarso

a,

Gregory Mermoudb, Luis Guillermo Villanueva

a, Massimo Mastrangeli

a,b, Alcherio Martinoli

b and

Jürgen Bruggera 5

Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX

DOI: 10.1039/b000000x

Fluid-mediated self-assembly is one of the most promising routes for assembling and packaging smart

microsystems in a scalable and cost-efficient way. In this work the pairwise fluidic self-assembly of 100

µm-sized SU-8 cylinders is studied with respect to two driving mechanisms: capillary forces at the liquid-10

air interface and hydrophobic effect while fully immersed in liquid. The pairwise self-assembly is

controlled by shape recognition and selective surface functionalization. Surface energy contrast is

introduced through oxygen plasma treatment and local deposition of a hydrophobic self-assembled

monolayer, respectively leading to face-selective hydrophilic and hydrophobic behavior. When in bulk

liquid, after less than a day face-wise self-assembly of more than 650 components is achieved with a yield 15

of up to 97% and with less than 1% of the cylinders assembled incorrectly. This technique is subsequently

adopted for self-assembling half-capsules into closed micro-capsules, thereby entrapping a liquid during

their self-assembly. The release of the liquid can subsequently be triggered in another medium, as

intended for applications involving e.g. chemical reactors, environmental engineering and drug release.

20

1. Introduction

The complexity of smart microsystems such as micro-electro-

mechanical-systems (MEMS) has been ever growing in recent

years, while being accompanied by a continuous decrease in their

size.1 For manufacturing devices of sizes well below the 25

millimeter range, assembly and packaging are key issues.2

Today’s methods based on robotic pick-and-place and flip-chip

approaches are not adequate to deal with millions of devices of

sub-millimetric sizes, as typically obtained by wafer-scale

processing. This is mainly due to sequentiality and adhesion 30

issues between the robotic gripper and the device.3 A more

appropriate manufacturing approach needs to be highly parallel,

cost-efficient and scalable, while remaining flexible and

controllable.4 Such an approach would benefit the assembly of

many types of microsystems,5 including notably liquid-filled 35

microcapsules. Robust and barely visible containers affording

controllable liquid release find a large variety of applications,6

such as distributed sensors, self-healing materials, fragrance

release and drug delivery. With specific reference to the latter,

microcapsules for the storage and further release of 40

pharmaceuticals are being developed.7, 8 Fully-integrated and

self-regulating devices might solve issues like secondary side

effects in non-targeted cell locations and stepped dosage

associated to current delivery methods 9, 10 However, integrating

microcapsules with smart (i.e. self-regulating) microsensors 45

requires fabricating, assembling and packaging them in a way

that is efficient and compatible with MEMS fabrication. An

established approach to fabricate such microcapsules consists in

the development of sophisticated containers,11 based on self-

folding 2D structures fabricated by lithography into various 50

polyhedra.12, 13 This technique is also envisioned for non-invasive

tracking by magnetic resonance of encapsulated cells for

application in cellular level therapy.14, 15 This approach, however,

does not easily allow the assembly of heterogeneous components,

barring its deployment into hybrid multi-functional devices. A 55

promising alternative to build hybrid MEMS devices is the

controlled self-assembly16 of micromachined components by

fluid-mediated self-assembly (FSA).17-19

State-of-the-art FSA is based on attractive interactions, e.g.

electrostatic, magnetic, capillary or hydrophobic, to position and 60

align specific units onto patterned substrates.20, 21 FSA is

particularly suited for objects with sizes below 1 mm, where

surface forces and stiction effects become dominant and accurate

assembly and placement rate is reduced.5 At a scale of few tens of

micrometers FSA permitted the highly-accurate integration onto 65

a target substrate of over 60’000 parts in 45 seconds with a yield

above 98%;22 i.e., a much higher rate than 3D robotic

microassembly.4 By combining forces of different nature and

ingenious strategies, complex and functional systems such as

microscale chains or LED arrays have been assembled.23-26 FSA 70

also works for larger scales, e.g. with centimeter-sized building

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Scheme 1 Synopsis of the FSA investigated in this work. a) More than 105 SU-8 micro microparts fabricated by photolithography. b) Microparts released.

c) Parts in liquid, FSA through orbital shaking and pair-wise self-assembled parts d) Assembled pairs; cylinders and capsules entrapping a colored ink. e)

Release of encapsulated ink.

5

blocks at liquid-liquid interface27 and with millimeter-sized ones

fully immersed in liquid.28 At liquid-air interface, dynamical

arrays of objects can also be controlled by magnetohydrodynamic

interactions.29-31 3D structures with specific shapes, such as 10

toroids or rounded tetrahedra could also be built by shape

recognition and selective surface coatings.32, 33 The control over

assemblies of 1-µm-sized polymeric particles depending on the

shape, roughness, electrostatic and depletion interactions has

been studied.34 Below the micrometer scale, 100 nm particles 15

with a selective hydrophobic versus hydrophilic surface behavior

could also be self-assembled into different structures being either

close to the water-air interface or completely immersed.35

Nevertheless, to our knowledge, liquid entrapment into

microcapsules by the self-assembly of half-capsules was not 20

investigated yet.

In this work, pairwise face-selective FSA of 100 µm-sized

polymeric components is investigated at liquid-air interface and

in bulk polar liquid through two different interaction

mechanisms: by capillary interaction, and by hydrophobic 25

effect,36 respectively. More than 105 components are fabricated

on a single 4’’ wafer using SU-8, a negative-tone photo-

structurable epoxy37 that has shown a lot of potential and

versatility for MEMS due to its good mechanical and insulating

properties 38 and for its ability to be functionalized 39, 40 Scheme 1 30

schematizes the microfabrication (Scheme 1a), release (Scheme

1b) and self-assembly in liquid during orbital shaking (Scheme

1c). Two kinds of components are investigated (Scheme 1d): i)

solid cylinders to study the control parameters of FSA, and ii)

half-capsules for liquid encapsulation through self-assembly. In 35

the first study, cylinders of different shapes, i.e. with both bases

flat or with one base rounded, are used to control the self-

assembly. In the second study, a face-selective hydrophobicity

contrast is created on the cylinders, and the time evolution of the

FSA in bulk liquid is studied. Finally, the proposed method is 40

applied to half-capsules in order to entrap liquid during their self-

assembly in pairs (Scheme 1d) and to release the liquid in another

media (Scheme 1e). For demonstration, water-based inks are used

as functional liquid.

2. Results 45

Transferring fabricated micro-cylinders from air into water

results typically in their segregation into two populations. A part

of them remains trapped at the water-air interface, whereas the

rest of them sink to the bottom of the beaker and are thus fully

submerged. At the water-air interface, long-ranged capillary 50

flotation forces are dominant.41 Conversely, in bulk water

flotation forces are not present, and the interactions among

microparts are mainly driven by the hydrophobic effect. In order

to investigate these two cases, a first set of FSA experiment has

been performed with microparts involving different shapes at the 55

liquid-air interface. The second case has been studied with fully

submerged microparts involving a face-selective treatment at the

bottom of the beaker. The SU-8 microparts with different shapes under investigation

are cylinders of 100 µm in diameter and in height with either both 60

bases flat (flat-flat) or with one base flat and one rounded (flat-

rounded). Their fabrication is described in details in the

experimental part (section 4). The second type of microparts

under investigation are flat-flat cylinders that have the same

dimensions but are treated with an O2 plasma to tune their surface 65

hydrophilicity and hydrophobicity in a face-selective way. The

third type is cylindrical flat-flat half-capsules, (i.e. featuring an

internal cavity on one side), involving a face-selective

hydrophobic surface treatment through local self-assembled

monolayer (SAM) deposition. 70

2. 1. Face-selective surface treatment of cylinders

In order to optically distinguish the orientation of the cylinders, a

two-layer SU-8 photolithography process using colored SU-8 is

applied. The bottom layer in contact with the silicon wafer is

blue-dyed SU-8 and the upper layer is colorless SU-8. The face-75

selective surface treatment of the flat-flat cylinders is achieved

with an O2 plasma treatment performed before their release from

the silicon wafer surface. Thereby, the plasma oxydizes all

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This journal is © The Royal Society of Chemistry [year] Journal Name, [year], [vol], 00–00 | 3

Fig. 1 Sketch of selective surface treatment and characterization of

bicolor SU-8 cylinders. a) Fabricated cylinders with one blue and one

colorless side. b) O2 plasma treatment. c) Release. d) Resulting

hydrophobicity contrast ∆θ1, compared to one on half-capsules, ∆θ2. e) 5

AFM line scans of the SU-8 surfaces (bottom surface is done on the

wafer).

exposed surfaces of SU-8, rendering them hydrophilic, except the

one shielded by the wafer (Fig. 1a to 1c). One hour after the 10

treatment, a water contact angle on the hydrophilic surface (θi) of

24° ± 2° and on the untreated surface (θn) of 74 ± 1° are

measured, as shown in Fig. 1d. The hydrophobicity contrast ∆θ1

= θn - θi is characterized over several days 42. Starting from 50° ±

2° one hour after the treatment, ∆θ1 amounts still to 35° ± 1° after 15

4 days, which is the time frame of the experiment (Fig. 1d). The

hydrophobicity contrast is not only a consequence of the surface

oxidation, but also of the induced roughness.43, 44 As expected

from,42 Atomic Force Microscope (AFM) scans show a 4x

increase of roughness from the original, spin-coated SU-8 to the 20

plasma-treated SU-8 (rms of 2.66 nm and 9.35 nm, respectively).

Fig. 2 Surface treatment of the half-capsules. a) Silicon wafer with the

two SU-8 layers after exposure and post exposure bake. b) SAM

deposited on SU-8. c) PGMEA developing the non-cross-linked SU-8 25

only. d) Resulting half-capsule with selective SAM deposition. e)

Released capsule and contact angle measurements on bare SU-8 surface

and on SAM-treated surface.

In addition, the untreated part of the cylinders (assumed to 30

conformally coat the Si substrate) has a surface roughness rms of

0.77 nm. Single AFM scans of roughness cross-sections are

shown in Fig. 1e. In order to avoid chemical wet etching of

silicon, which would affect the SU-8 surface properties, the

cylinders are mechanically released in an ultrasonic bath. 35

2. 2. Functionalization of half-capsules

The half-capsules are fabricated with the base hosting the cavity

on top. To render only the open base surface selectively more

hydrophobic than none treated SU-8, a Trichloro (1H, 1H, 2H,

2H-perfluorooctyl)silane SAM is applied just before developing 40

the SU-8, Fig. 2a and 2b. The covalent bonds are not affected by

the standard SU-8 developer (Propylene glycol monomethyl ether

acetate (PGMEA)). The non-cross-linked SU-8 is thus developed

through the SAM layer, allowing a lift-off-like functionalization

process (Fig. 2c) so that only the top base of the SU-8 half-45

capsules is covered by the hydrophobic SAM, Fig. 2d. In this

case the hydrophobicity contrast ∆θ2 between the SAM layer θo

and the untreated SU-8 (∆θ2 = θo – θn) is 41° ± 4° (see Fig. 2e),

with the respective contact angles being 115° ± 4° and 74° ± 1°.

Both SU-8 surface treatment options, i.e. O2 plasma and 50


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4 | Journal Name, [year], [vol], 00–00 This journal is © The Royal Society of Chemistry [year]

Fig. 3 a) Schemes of possible assemblies with flat-fat or flat-rounded

components. b) Histogram of chain length for self-assembled cylinders

with flat-fat and flat-rounded extremities. c) Optical images with

schematics after self-assembly of the cylinders. 5

silanization, yield similar ∆θ (∆θ1 ≈ ∆θ2), over the time frame of

the experiments, which is 4 days. On the other hand, ∆θ2 is more

constant in time.

2. 3. Pairwise self-assembly by shape recognition 10

We first investigate how microparts with different shapes self-

assemble into pairs at a liquid-air interface. In this case, the

floating microparts are under the influence of strong and long-

ranged capillary interaction, namely flotation forces. Flotation

forces are lateral capillary forces arising from the deformation of 15

the liquid-air interface caused by the floating objects41. The

shapes of the liquid-air meniscii are determined by the objects’

geometry, weight and wettability. Facing menisci induce

attractive (or repulsive) interactions if they have equal (or

opposite) profiles. Minimization of the interfacial and 20

gravitational energies drive the set of floating objects into

configurations of static equilibria27. The way the components

self-assemble can thus be controlled by geometrical design

besides surface chemical modification.32 To benefit from these

phenomena in our study, we used SU-8 cylindrical components 25

with flat-flat and flat-rounded bases. In order to individually

study the influence of the cylinder shape, their surfaces are not

treated and have a contact angle for water of θn, of 74 ± 1°. Under

orbital shaking, the cylinders form chains whose lengths depend

on the cylinder dimensions, the stability of the assemblies and the 30

orbital shaking rate. Focusing herein into one specific size and

orbital shaking rate, the flat-flat configuration can lead to most

stable assemblies as it is the one where components interact

through a surface. On the other hand, flat-rounded or rounded-

rounded configurations lead to less stable assemblies due to 35

point-based interaction only and thus reduced adhesion forces.

Thus, flat-flat components are expected to assemble into chains

whereas the flat-rounded components are expected to assemble

preferentially into pairs with the flat sides facing each other’s

(Fig. 3a). 40

The distributions of the chain lengths obtained for both flat-flat

and flat-rounded cylinders are shown in Fig. 3b. The results are

obtained after 8 hours of orbital shaking and complete

evaporation of the water. All samples are prepared with identical

water volume and number of cylinders. A pair is counted as an 45

assembly only if at least 75% of the facing diameters are

overlapping. The results show that, in the case of flat-flat

cylinders, the probability to reach a given chain is inversely

proportional to its length. In the case of flat-rounded cylinders,

89% of cylinders remain single, and 11% form pairs, without any 50

longer assemblies as shown in Fig. 3b. Optical images with

schematics of typical results for flat-flat and flat-rounded

assembled cylinders are shown in Fig. 3c.

More than twice as many flat-flat cylinder pairs as for flat-

rounded ones are counted. This is expected, as a flat-flat cylinder 55

has twice as many chances to self-assemble as a flat-rounded

cylinder. This is also the reason why the number of singles is

higher for flat-rounded than for flat-flat cylinders. As a summary,

by exploiting shape recognition of floating SU-8 cylinders, we

are able to self-assemble them selectively into pairs, but with a 60

relatively low yield.

2. 4. Pairwise self-assembly by face-selective hydrophobic effect

To increase the percentage of cylinders that assemble while

constraining self-assembly mostly into pairs, the process is then 65

performed with components fully immersed in polar liquid. The

long-ranged capillary interaction is hereby replaced by the short

range hydrophobic effect. 35 The fabricated components are

bicolor flat-flat cylinders with a face-selective hydrophobicity

contrast. Fig. 4a shows schematics and optical images before and 70

after the FSA in bulk liquid. The SU-8 microcylinders in a water-

filled Petri dish are agitated by orbital shaking. The cylinders

self-assemble into different combinations (Fig. 4b). With more

than 650 cylinders, the percentage of each combination of

cylinders (blue to blue, blue to colorless or colorless to colorless) 75

and its time evolution is studied over 4 days. The combinations

shown in Fig. 4b are: i) Correct assemblies defined as the number

of cylinders in contact only by the flat blue faces, ii) Aligned

assemblies, a sub-group of the correct assemblies, when at least

75% of their diameters are overlapping. iii) Wrong assemblies, 80

counting colorless to colorless and colorless to blue surface

assemblies. iv) Singles counting all cylinders which do not

assemble.

It can be seen in Fig. 4b, that after 20 minutes of shaking, both


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Fig. 4 a) Schematics of the fluid-mediated self-assembly (FSA) process and optical images; b) Statistics on FSA; configuration occurrence (percentage)

with legend on the right (see text for definition).

“Correct assemblies” and “Aligned assemblies” show constantly 5

increasing yield. After 15 hours the “Correct assemblies” reach a

stable yield above 97%, while the “Aligned assemblies” exhibits

a yield of 39%, which continues to increase to 59% after 4 days.

The stable yield of the “Correct assemblies” demonstrates that

once two cylinders meet with both hydrophobic surfaces in 10

contact, they form stable pairs. Furthermore, the constantly

increasing yield of the “Aligned assemblies” shows that there is a

relative motion and sliding between two correctly assembled

cylinders, fine tuning their alignment. Conversely, the “Wrong

assemblies” percentage decreases to below 1%, suggesting that 15

such combinations are not stable and can easily be separated by

the effects of the shaking. This proves that our proposed surface

treatment effectively enables the face-wise self-assembly of sub-

millimetric SU-8 microcomponents into pairs.

2. 5. Liquid encapsulation through self-assembling of 20

microcapsules

The face-wise self-assembly of microcomponents into pairs is

then applied to SU-8 half-capsules for liquid encapsulation. The

experiments are performed in the same way as described for the

cylinders. As a proof of principle, water-diluted colored inks 25

(green, yellow and blue) were used instead of pure water. The

half-capsules featuring the face-selective SAM are transferred

into Petri dishes and FSA is initiated with orbital shaking. Fig. 5a

shows a scheme and an optical image of the 100 µm-sized half-

capsules immersed in green ink before assembling. After 1 day of 30

FSA (Fig. 5b), 21% of the half-capsules have assembled pairwise

and face-wise into close capsules. Fig. 5b shows representative

examples of three self-assembled capsules in their respective

inks. The rest of the capsules are either not assembled or

assembled in the wrong orientation (closed bases assembled). For 35

the microcapsules that are fully immersed in water with diluted

ink, the hydrophobic effect is the main force keeping them

together. After draining the solution, the 200 µm-long capsules

entrapping about 1 nL of ink solution (0.5 nL for each half-

capsule) are transferred either onto a microscope glass slide or 40

into water (Fig. 5c). Once dried, the capillary forces exerted by

the entrapped ink keep the capsules together. A mechanical test

using tweezers also showed that by exerting a mechanical force

the capsules can be disassembled. In view of the application of

remote liquid unloading, we showed that the release of the 45

encapsulated ink can be achieved by transferring the capsules into

a less polar liquid, such as isopropanol (IPA), which reduces the

binding force. Fig. 5d shows few self-disassembled half-capsules

in IPA releasing encapsulated blue ink.

3. Discussions 50

Our results demonstrate the pairwise self-assembly of SU-8

micro-components by shape recognition and by face-selective

hydrophobic effect. The latter case has been applied for liquid

encapsulation and demonstrated as a proof-of-concept with

colored inks. The technique can be adopted for aqueous solutions 55

and polar liquids other than water, as well, and more generally for

other functional polar solutions under the condition that they do

not affect the assembly by interacting with the SU-8 surface. In

addition, no bubble has been observed inside the capsules. The

lift-off-like functionalization presented herein could be applied 60

with a different, compatible surface treatment, possibly allowing

other potential release mechanisms. Other photo-structurable

polymers having a contact angle with water significantly lower

than the one of the SAM-treated surface can be used in place of

SU-8 with similar fabrication process. Biodegradable, 65

biocompatible and functional polymers would represent a very

interesting possibility.


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Fig. 5 Functionalized half-capsules self-assembling into complete capsules entrapping colored ink. a) Unpaired half-capsules before FSA; b) Parallel FSA

experiments in blue, green and yellow inks c) Transferred capsules: With green and yellow inks into a drop of water (left), with blue ink onto a

microscope glass (right); d) Release of the encapsulated blue ink by self-disassembly of the capsules in Isopropanol.

A similar approach can be applied to a wide range of materials, 5

such as metals, silicon or oxides among others.

Herein, the focus has been set onto 100 µm-sized micro-capsules for several reasons: our capsules enable the encapsulation of nanoliter volumes, and can be perceived with bare eyes; furthermore, this size range allows the embedding of 10

functional microchips45 such as RFID chips onto the capsules. We argue that the range of capsule dimensions efficiently affordable by our self-assembly approach is mainly defined by the interplay between assembly and disassembly forces. The short-ranged hydrophobic effect driving the assembly of the 15

capsules competes with drag and inertial forces arising from orbital shaking which scale with the surface and the volume of the components, respectively. With millimetric capsule dimensions, the hydrophobic effect may not be sufficient to overcome disassembling forces. When considering downscaling, 20

the limit is set either by the resolution of the available design and patterning technique (e.g. around 10 µm for reproducible capsule fabrication when using standard photolithography) or by the minimum volume of liquid to be encapsulated. In general, as the dimensions of the components approach the spatial range of the 25

hydrophobic effect (e.g. a few hundreds of nanometers) the self-assembly process is expected to become more efficient.46 The self-assembly of 100 nm Ag cubes by hydrophobic interaction into ordered structures has been demonstrated.35

4. Experimental 30

4. 1. SU-8 bicolor cylinder fabrication

SU-8 bicolor (blue and transparent) cylinders are fabricated by

photolithography. Dimensions are 100 µm in diameter and in

height. A 4’’Si wafer is dehydrated and spin-coated (Sawatec

LMS200) with a 50 µm-thick blue colored SU-8 layer 35

(Gersteltec, CH) followed by a soft-bake (Sawatec LMS200). A

50 µm-thick colorless SU-8 layer (GM 1070 Gersteltec, CH) is

then equally spin-coated and soft-baked. Both layers are exposed

at once through a Chromium shadow mask (dose of 850 mJ cm-2

(Süss MA6/BA6)) to pattern the cylinders. After a post-exposure 40

bake (PEB) on a hot plate (Accu-Plate), the SU-8 is PGMEA and

rinsed with IPA. A hard bake process (Sawatec LMS200) is done

to give them more stability and optimize their aging 47.

Hydrophilic behavior of the SU-8 surfaces is induced by

exposure to an O2 plasma for 2’ (0.6 mbar, 100W; Diener 45

Electronic, Femto). The cylinders are released in ultrasonic bath

at 750 kHz in water for 30’. The release is enhanced by a prior

thermal stress between the SU-8 and the Silicon surface induced

by heating the wafer with the SU-8 cylinders at 200°C and fast

cooling by deeping in room-temperature water. To avoid surface 50

contamination, after release the cylinders are stored in water. The

water used throughout all the experiments is deionized (DI).

4. 2. Fabrication flat and rounded half-capsules

SU-8 half-capsules are fabricated with a 100 µm outer diameter,

50 µm inner diameter and 100 µm in height. For the capsules 55

with rounded bases, a 4’’ Si wafer is structured with spherical

holes obtained by isotropic silicon etching. A positive photoresist

(AZ ECI 3027) is deposited by spin-coating (Rite Track 88

Series), patterned by exposure (Süss MA150) and developed

(Rite Track 88 Series). The isotropic etch is subsequently 60

performed by fluorine plasma (Alcatel 601E). The photoresist

mask is then stripped and the wafer cleaned by O2 plasma at 1000

W (Tepla 300) for 7’. For both rounded and flat capsules, the

fabrication process starts with a thin aluminium layer deposition.

A 40 µm-thick layer of SU-8 (GM 1070, Gersteltec, CH) is 65

subsequently spin-coated (Sawatec LMS200), soft-baked

(Sawatec LMS200) and exposed (361 mJ cm-2, Süss MA6/BA6).

A second, 60 µm-thick SU-8 layer (GM 1070, Gersteltec, CH) is

then spin-coated and soft-backed (Sawatec LMS200). The layer

is exposed through a Chromium shadow mask with ring-shaped 70

holes (Süss MA6/BA6). Both SU-8 exposure doses are adapted

for the capsules with rounded bases. After PEB (Accu-Plate), the

SU-8 is developed in two PGMEA baths for 10’ and rinsed with

isopropanol. Hard bake process also helps to remove possible

cracks 47. The release is done by etching the aluminium layer in 75

water-diluted KOH (40 %) at 80 °C for a few minutes. SU-8 half-

capsules are rinsed with water and filtered subsequently through


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This journal is © The Royal Society of Chemistry [year] Journal Name, [year], [vol], 00–00 | 7

filters with 4-7 µm and 4-12 µm pores (Macherey & Nagel,

Whatman) up to 5 times.

4. 3. Selective surface treatment of half-capsules

For surface-treated half-capsules, a 7 W O2 plasma, at 0.6 mbar

for 30’’ is performed before developing the SU-8. Next, a SAM 5

layer (Trichloro (1H, 1H, 2H, 2H-perfluorooctyl)silane) (Sigma

Aldrich) is deposited in vapour phase at room temperature for 1

hour. The half-capsules are released in an ultrasonic bath.

4. 4. Self-assembly experiments

For capillary FSA, flat and rounded half-capsules are left at the 10

water surface of petri dishes containing 40 ml of water with

diameters of 70 mm, and orbitally shaken till full water

evaporation (about 8 hours). Same water volume and same

number of half-capsules is used in all experiments. Samples are

imaged with an optical microscope to quantify the FSA yields. 15

FSA of bicolor cylinders and half-capsules is performed by

submerging them in water at the bottom of 9 ml Petri dishes with

diameters of 30 mm. The beakers are covered with parafilm to

avoid water evaporation and so keeping the same water volume

during the entire experiments. Petri dishes with bicolor cylinders 20

are put on an orbital shaker for 4 days; only one day for the half-

capsules. Self-assembly yields are quantified by stopping the

shaking at prefixed times and counting the cylinder

configurations under an optical microscope. For the statistics

shown in Fig. 3, each counting involves 650 microcylinders. 25

Counting is done with top-view optical images, thus only side-

way oriented cylinders are counted (the majority of the

cylinders). The 21% of aligned microcapsules counted in section

2.5 are accepted only if at least 75% of the diameters are

overlapping, which is the minimum for liquid encapsulation. 30

5. Conclusion

Directional face-wise FSA in water of 100-µm-sized SU-8 bulk

and hollow cylinders into pairs is demonstrated. Two distinct

cases are presented: one occurring at the water surface, where

surface tension dominates, the other occurring underwater, where 35

hydrophobic effects do. In the first case, assembly control is

enabled via geometrical design and shape recognition. While

regular cylinders (both bases flat) assemble in chains of up to 4

units, cylinders with one rounded base assemble nearly

exclusively into pairs. In the second case, self-assembly is 40

favored via face-selective hydrophobicity contrast.

In addition, the pairwise self-assembly is applied to half-

capsules that encapsulate a functional liquid while self-

assembling. The hydrophobicity contrast is in this case

engineered through a lift-off-like process of a hydrophobic SAM 45

before developing the SU-8. After self-assembly and co-

encapsulation of liquid, changing medium allows to reduce the

adhesion between the half-capsules and thus to release the

entrapped liquid via self-disassembly. As a proof of concept,

water soluble colored inks are first enclosed inside the capsules 50

and later released in isopropanol.

This work presents a fluidic path for the assembly of two-

component polymeric microcapsules fabricated in SU-8 by

photolithography. It is compatible with MEMS technology and

scalable to massively parallel assembly. The presented novel 55

encapsulation method may open new routes for smart

microcapsules with potential applications for health care, drugs

delivery, food market, information technology, and local

distributed sensing for civil and environmental engineering.

Acknowledgements 60

L. Jacot-Descombes and Dr. C. Martin-Olmos contributed

equally to this work. We acknowledge Nano-Tera.ch and the

project “SELFSYS” for funding the work presented here. We

thank Ikjoo Byun and Prof. Beom Joon Kim of Tokyo University

(Japan) for fruitful discussions. We also thank Dr. Sebastien 65

Jiguet, from Gersteltec (CH) for providing us the blue colored

SU-8 as well as Prof. Philippe Renaud from the Laboratory of

Microsystems for sharing experimental tools and advices. We

finally express our gratitude to the Center of Micro-

Nanotechnology (CMi) of EPFL for the support during the 70

microfabrication.

Notes and references

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R. Gullo, Dr. L. G. Villanueva, Dr. M. Mastrangeli, Prof. J. Brugger,

Microsystems Laboratory, Ecole Polytechnique Fédérale de Lausanne 75

(EPFL), CH-1015 Lausanne (Switzerland), E-mails: loic.jacot-

[email protected], [email protected] b Dr. G. Mermoud, Dr. M. Mastrangeli, Prof. A. Martinoli , Distributed

Intelligent Systems and Algorithms (DISAL) Laboratory, ENAC, Ecole

Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne 80

(Switzerland), E-mail: [email protected]

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fluid-mediated parallel self-assembly of polymeric micro-capsules for liquid encapsulation and...

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