Strong Microcapsules with Permeable Porous Shells Made through

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Strong microcapsules with permeable porous shells made through phase separation in double emulsions Eve Loiseau, Fabian Niedermair, Gerhard Albrecht, Marion Andrea Frey, Alina Hauser, Patrick Alberto Rühs, and André R. Studart Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04408 • Publication Date (Web): 14 Feb 2017 Downloaded from http://pubs.acs.org on February 20, 2017

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Strong microcapsules with permeable porous shells made through phase separation in double emulsions Eve Loiseau, 1 Fabian Niedermair, 2 Gerhard Albrecht, 2 Marion Frey, 1 Alina Hauser, 1 Patrick A. Rühs, 1 André R. Studart 1 1

Complex Materials, Department of Materials, ETH Zurich, 8093 Zurich,

Switzerland 2

BASF Construction Solutions GmbH, Trostberg, Germany

Abstract Microcapsules for controlled chemical release and uptake are important in many industrial applications but are often difficult to produce with the desired combination of high mechanical strength and high shell permeability. Using water-oil-water double emulsions made in microfluidic devices as templates, we developed a processing route to obtain mechanically robust microcapsules exhibiting a porous shell structure with controlled permeability. The porous shell consists of a network of interconnected polymer particles that are formed upon phase separation within the oil phase of the double emulsion. Porosity is generated by an inert diluent incorporated in the oil phase. The use of undecanol and butanol as inert diluents allows for the preparation of microcapsules covering a wide range of shell porosity and force at break values. We found that the amount and chemical nature of the diluent influence the shell porous structure by changing the mechanism of phase separation that occurs during polymerization. In a proof-of-concept experiment, we demonstrate that the mechanically robust microcapsules prepared through this simple approach can be utilized for the ondemand release of small molecules using a pH change as exemplary chemical trigger.

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Introduction

Porous hollow spheres and microcapsules are suitable in filtration systems for analytical assays, as encapsulation vehicle for controlled storage and release of active components 1, as efficient catalytic systems

2, 3

remove specific components from gases or solutions

or as adsorbents to

4-7

. In comparison to

spheres with homogeneous porosity, porous hollow capsules enable the storage of larger quantities of material since its solid-free core leads to a very high pore volume. The composition of the solid material forming the shell of such porous hollow spheres and capsules may include all the main classes of materials: organics, inorganics, metals, and composites 8, 9. The permeability of capsules is key for their use in all the aforementioned applications and is determined by the porosity and the sizes of pores within the shell. Capsules with porous or permeable shells are usually produced through self-assembly of particles or macromolecules on the surface of a solid or liquid template.

10-12

Macromolecules can be deposited in a layer-by-layer fashion on

the surface of a templating solid particle to generate microcapsules with permeable shells upon dissolution of the particle core.

10, 13-18

In another

approach, capsules with a shell constituted by particles, also known as colloidosomes, can be made through the interfacial assembly of colloidal particles around templating droplets.

11, 12, 19

In this case, the permeability of the

shell is dominated by small pores in the form of interstitial holes between the particles and assembly defects

12, 20

. Particles have also been used as sacrificial

templates to create shells with pores approaching dimensions on the order of one tenth of the capsule size

21-23

. In both cases, the resulting microcapsules

have poor mechanical properties, making them unsuitable for industrial applications involving high pressures and mechanical forces. In contrast to self-assembly strategies at solid-liquid or liquid-liquid interfaces, the preparation of microcapsules from double emulsions does not depend on the interfacial adsorption of particles and molecules to form the shell. Instead, the shell is made of the particles and molecules that have been


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incorporated into the middle phase of the water-oil-water (w/o/w) double emulsion, thus opening new possibilities for the design of the capsule shell material and structure. Because other non-equilibrium states are reachable during controlled emulsification under laminar flow conditions, additional design freedom is possible when the double emulsion templates are made using microfluidic techniques. This approach has enabled for example the preparation of microcapsules exhibiting thick polymer shells with tunable mechanical properties, 26

24, 25

microcompartments with magnetically-triggered bursting shells,

colloidosomes with thick walls using particles with a variety of chemical

compositions

27-30

and microcompartments displaying organic-inorganic hybrid

shells with hydrophobically controlled gate response.

31

However, microcapsules

with permeable porous shells, such as colloidosomes, remain relatively weak, since particles in the shell are typically held together solely by attractive van der Waals forces or by the entanglement of surface anchored macromolecules. A well-established procedure to prepare porous polymer spheres with reasonable mechanical stability involves the suspension polymerization of droplets of a monomer solution in water.

32

Porous spheres with tunable pore

sizes for chromatography applications have been widely produced following this route.

33

Typically, an oil phase containing the monomers, an inert diluent

(solvent) and an initiator is first emulsified in water with the help of a surfactant to generate oil-in-water emulsions. The inert diluent is deliberately chosen to be completely miscible with the monomers but cannot totally dissolve the macromolecules generated upon polymerization. During polymerization of the oil droplets, the oligomers become insoluble in the diluent and phase separate, forming a network of polymer particles. The inert diluent that initially wets the polymer network can be removed later to obtain beads with homogeneous porosity throughout the cross-section. Because the resulting porosity derives from the presence of the inert diluent, this is also referred to as the porogen. Glycidyl methacrylate (GMA) and ethylene glycol dimethacrylate (EGDMA) are well-known monomers that can lead to porous beads with controlled porosity upon polymerization in the presence of inert diluents. Indeed, poly-GMA-co-


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EGDMA systems were intensively studied in the 1980’s

34-40

media for chromatography due to the controlled porosity

and are popular

32

. Such controlled

porosity obtained by polymerization-induced phase separation can be used to form microcapsules with nanoporosity (100-300 nm) and thin shells (1 µm and below) 41, 42 for the selective release and uptake of molecules and/or particles. Here we exploit polymerization-induced phase separation within the oil phase of w/o/w double emulsions as a means to obtain microcapsules with a large hollow core and a permeable thick shell (15 µm) of improved mechanical properties relative to state-of-the-art systems. A glass capillary microfluidic device is used to produce monodisperse w/o/w double emulsions that serve as template for the formation of capsules. In comparison to conventional emulsification techniques, this method permits accurate control over the dimension of the generated droplets, which enables the formation of the thick shells required to create strong microcapsules. The shell thickness can be tuned by varying the inner and middle flow rates.

25, 43

By changing the shell thickness,

the pore volume and the mechanical properties of the microcapsule can potentially be controlled without changing the pore size. Microfluidics is also advantageous in terms of encapsulation efficiency, since cargo molecules and particles can be directly incorporated in the inner aqueous droplets during the emulsification process.

28, 43, 44

As inert diluents, we selected the two linear

alcohols undecanol and butanol, which had already been successfully used for the preparation of homogeneous porous particles with microfluidics

45, 46

. The

influence of the composition of the oil mixture on the porous microstructure of the shell and on the mechanical properties of the capsule is systematically studied. We also investigate the diffusion of a dye through the capsule walls to estimate the permeability of the porous shell. As a proof of concept, permeable capsules loaded with aluminum oxide nanoparticles are finally produced to generate particle-filled colloidosomes for reversible capture and release of model cargo molecules.


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Materials and Method

Device fabrication Glass capillary devices were built according to Utada et al

43

. Two glass

capillaries were tapered using a P-97 Flaming/Brown micropipet puller (Sutter Instrument, USA). The tips of the emitters were adjusted using a MF-830 microforge (Narishige, Japan) and then hydrophobized using a solution of 5wt% octadecyltrimethoxysilane (ODTMS, Acros Organics, Belgium) and 0.5% nbutylamine (Sigma-Aldrich, USA) in toluene. The final dimensions of the collectors were adjusted by manual polishing. Capsules were produced using the same device with the dimensions of 200 µm for the inner diameter of the collector, 34 µm and 52 µm for the inner and outer diameter of the emitter, and with a distance of 75 µm between the tips.

Preparation of fluids An 2wt% aqueous solution of poly(vinyl alcohol) (PVA, Mw 31000-50000 g/mol, Sigma-Aldrich, USA) was used as outer phase. Distilled water was used as inner phase of all capsules, except those loaded with Al2O3 nanoparticles for the final controlled release experiments. The oil phase consists of a mixture that includes glycidyl

methacrylate

(GMA,

Sigma-Aldrich,

USA)

and

ethylene

glycol

dimethacrylate (EGDMA, Acros Organics, Belgium) in a ratio 4:6, 4.5 wt% of 2hydroxy-2-methylpropriophenone (HMPP, Sigma-Aldrich, USA) and various amounts of the inert diluent (Supporting Table S1 and S2). Butanol (Merck, Germany) and undecanol (ABCR, Germany) were used as inert diluents.

Microcapsules To form the double emulsion templates the three fluids were pumped using a PHD 2000 syringe pumps (Harvard Apparatus, USA) at controlled flow rates into the microfluidic device. The formation of the double emulsion was investigated using an inverted microscope (DMIL LED, Leica, Germany) equipped with a high-


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speed camera (Fastcam Mini UX-100, Photron, Japan). The flow rates of the inner fluid / middle fluid / outer fluid were fixed at values of approximately 2500 / 5000 / 40000 µL/hr. The double emulsions were collected and immediately exposed to a UV light from an Omnicure 1000 (APM Technica, Switzerland), which initiated the polymerization of the capsules. After synthesis the capsules were rinsed three times with water. The inert diluent was removed from the capsules shell by immerging them in ethanol for two days.

Microscopy Capsules were visualized using an inverted microscope (DMIL, Leica, germany) in bright field mode. The transparency of the shell was improved by matching the optical index of the polymer with the surrounding fluids. For this purpose, capsules were first immersed in ethanol for 12 hrs and then in benzyl alcohol for 12 hrs and finally observed with a fluorescence microscope with dichroic contrast (DM6000B, Leica, Germany). Capsules imaged in a scanning electron microscope (LEO 1530, Carl-Zeiss SMT AG, Germany) were dried in air at room temperature, then in a vacuum oven at 120°C (VT6060, Thermo Scientific, USA) and finally covered by a 20 nm layer of platinum using a SCD 050 sputter coater (Bal-Tec, Liechtenstein). A minimum of 30 particles were counted for each type of composition (see supporting Table S3).

Compression tests of single capsules Compression measurements were performed using a custom-made device inspired by the tool originally developed by Keller et al elsewhere

47

and described

25

. Before each measurement the capsule size and shell thickness

were measured and controlled. For testing, one capsule was placed on the top of a load cell while a piezoelectric element compressed the capsule at a constant speed of 1 µm/s. The force and displacement values were recorded and the deformation of the capsules was observed during the test using an optical macroscope (Z16 APO, Leica, Germany). The stiffness was calculated from the


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linear regime of the compression curve. The capsule diameter, force at break, and stiffness are displayed in Supporting Table S4.

Permeability measurements To assess the permeability of the porous shell, capsules were first transferred to benzyl alcohol. A few drops of a solution of 0.1 wt % Nile Red (Sigma, USA) in benzyl alcohol were then added and the diffusion of the dye inside the capsules was followed using a confocal microscope (ZEISS LSM 510-Screening, Germany) using an excitation light at 561 nm, an emission light over 575 nm and a slice thickness of 20 µm.

Particle-filled microcapsules Microcapsules with on-demand release characteristics were prepared by entrapping alumina nanoparticles into their hollow core, following the concept originally proposed by Sander et al.

28

An aqueous suspension of 1 wt% Al2O3

nanoparticles (50 nm) at pH 4 was prepared by dispersing the nanoparticles in a HCl solution (Merck, Germany) using a sonicator (Vibra-Cell VCX 130, Sonics & Materials, US). We used 50 nm Al2O3 particles to avoid clogging in the microfluidic device. The resulting diluted suspension was utilized as inner aqueous phase during emulsification. For the middle phase, a GMA/EGDMA mixture containing 30% of undecanol was employed. After UV polymerization, the capsules were rinsed with water and immersed in ethanol for 24 hours in order to remove the undecanol. This was followed by a second rinsing step with water and immersion in a solution of calcein sodium salt (ABCR, Germany) in HCl at pH 4 for 24 hrs. Calcein was used as a model cargo molecule that adsorbs on the surface of Al2O3 nanoparticles at low pHs and can be easily detected by fluorescence microscopy. Non-absorbed calcein was removed by regularly rinsing the capsules with a solution of HCl at pH 4 for 24 hrs. After this time period the continuous phase appears transparent. The release of calcein was triggered by quickly rinsing the capsules with a solution of KOH (Brenntag, Switzerland) at pH 10. This causes the calcein molecules to desorb from the


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Al2O3 nanoparticles. The fluorescence of the continuous phase was measured using a fluorometer with an excitation light at 488 nm and an emission light at 515 nm. Measurements were always performed after complete sedimentation of the capsules to the bottom of the probing cuvette. In order to improve the diffusion of the dye within the outer aqueous phase, the cuvette was agitated between individual measurements. Capsules loaded with calcein and initially rinsed in HCl solution were immersed at pH 4 or at pH 10 for 1 hr before fluorescence imaging using a blue light source equipped with a short pass filter ( 500 nm, FEL500, Thorlabs, USA).

Results and discussion Water-oil-water (w/o/w) double emulsions with well-defined monodisperse morphology are produced in the glass capillary device shown in Figure 1A. In the microfluidic emulsification process, an inner water-based fluid (1) is injected through a hydrophobized capillary and drips into a UV-curable mixture of monomers (2) forming a water-in-oil droplet. Such droplet is sheared by an outer water phase flowing in the opposite direction (3), thus leading to the formation of water-oil-water droplets (4). The flow rates of the fluids can be fine tuned to control the dimensions of the inner and outer droplets. The resulting double emulsions are converted into monodisperse microcapsules with macroporous shells upon UV polymerization (Figure 1B). Using a mixture of acrylate monomers and an inert diluent as middle oil phase, the porosity in the capsule shell is generated through phase separation of the growing polymer followed by removal of the diluent in a simple washing step (Figure 1C-F). Capsules with a high content of inert diluent and highly porous shells scatter light and therefore are hard to observe using transmission and reflection optical microscopy (Figure 1B inset). To enable visualization, the capsules were transferred into benzyl alcohol, whose refractive index ஽ (݊ଶହ =1.5396) is close to that of the polymer in the shell (n ≈ 1.57). Imaging of


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such capsules was eventually possible using an optical microscope in Differential Interference Contrast mode (Figure 1B).

Figure 1: Preparation of microcapsules with macroporous shell using a capillary microfluidic device. (A) Double emulsification process through flow-focusing of three fluid phases at the entry point of the capillary collector. Red arrows 1, 2 and 3 indicate the flow of the inner, middle and outer phases, respectively. Arrow 4 shows double emulsions moving towards the collector outlet. (B) Phase contrast microscopy images of w/o/w microcapsules immersed in benzyl alcohol. The inset displays bright field images in water. Scale bar, 200 µm. (C) Scheme illustrating the formation of the porous structure during polymerization of the monomers within the middle oil phase. The oil phase comprises a mixture of (D) glycidyl methacrylate (GMA), (E) ethylene glycol dimethacrylate and (F) undecanol.


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The internal porosity of the microcapsule shell can be made open and accessible to the continuous phase using linear alcohols like butanol and undecanol as inert diluents. The inner structure and surfaces of the microcapsule shell were investigated by scanning electron microscopy (SEM) (Figure 2A,B). Capsules produced with more than 20 wt % of undecanol alone exhibit porous surfaces on both sides of the capsules shell (Figure 2B). However, for diluent concentrations below 20 wt%, a polymer skin is formed on the outer surface of the shell (not shown here). We hypothesize that the absence of a skin on the surface of capsules made with high amounts of undecanol results from the adsorption of such molecules at the oil-water interface during emulsification (Figure 2C). Such adsorption displaces the more amphiphilic monomer glycidyl methacrylate from the interface, preventing the adsorbed monomers to polymerize into a skin on the surface of the droplet.


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Figure 2: Microstructure of capsules with open shell surfaces. (A) Surface morphology and (B) cross-section of a microcapsule produced using 30wt% of the inert diluent undecanol. (C) As the diluent is amphiphilic, it is expected to adsorb at the oil-water interface of the double emulsion, preventing the formation of a polymer skin on the outer droplet surface.

The microstructure of the capsule porous shell is strongly influenced by the amount of inert diluent. SEM images of shell cross-sections (Figure 3 A,B,C and supporting Figure S1 for images at the same magnification) reveal a major morphology difference between capsules prepared with low and high contents of undecanol. For a low undecanol concentration of 20wt%, the polymer phase forms a continuous solid network into which small pores below 100nm are embedded. This

microstructure

is

typically

observed

“microsyneresis” during phase separation.

35

in

systems

that

undergo

In such phenomenon, the diluent is


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expelled from the continuous polymer gel in the form of dispersed small liquid droplets during the polymerization reaction (Figure 3). After removal of the diluent phase, a continuous polymer phase containing small interspersed pores is obtained, which agrees well with the microstructure shown in Figure 3A. By contrast, high undecanol concentrations result in a shell microstructure consisting of much larger polymer particles that are interconnected to form a percolating open porous network. In this case, particles sizes approach 1µm and the polymer particles are more easily distinguishable. Microstructures exhibiting such features are obtained when “macrosyneresis” occurs during phase separation.

35

As opposed to “microsyneresis”, the liquid in this case constitutes

the continuous phase and the polymer gel form the dispersed phase that deswells and is unconstrained to continuously grow into larger particles. Removal of the diluent phase eventually leads to large pores that are comparable in size to the polymer particles. The particle size and resulting pore sizes within the capsule shell also depend on the chemical nature of the linear alcohol used as diluent, more specifically its solubility parameter. As stated by Dubinsky

48

, the difference

between the solubility parameters of the polymer and of the diluent (∆δ) is the parameter that determines the size of the polymer particles (see Table 2). A solubility parameter of 20.5 MPa1/2 is observed for undecanol, which is relatively far from the value of 24 MPa1/2 estimated for the polymer poly-GMA-co-EGDMA that is formed upon illumination of the monomer mixture. This ensures that phase separation takes place during the polymerization reaction, as confirmed by the SEM images shown in Figure 3A,B,C. Interestingly, the solubility parameter increases when the number of carbons in the linear alcohol decreases. As an example, butanol (4 carbons) shows a δ value of 23.1 MPa1/2, which is quite close to that of poly-GMA-co-EGDMA. As a result, we observe that the use of butanol alone as diluent does not lead to a porous structure but a homogeneous polymer shell after conversion of the double emulsions into capsules. Since long and short linear alcohols can be continuously mixed in different proportions, using mixtures of such molecules allows us to deliberately tune the


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solubility parameter difference value, ∆δ, and thus the phase separation process. For the same total diluent content, we observe that an increasing amount of butanol to an undecanol-monomer mixture leads to a reduction of the particle size formed within the shell (Figure 3D and Figure S2). However, if the undecanol/butanol ratio is too low, the solubility parameter difference is not high enough to generate shells with a porous structure. At 20 wt% total diluent concentration at a 50/50 ratio, no porous capsule formation could be observed. The smaller polymer particle sizes obtained when undecanol is partially replaced with butanol can be explained by the accompanying reduction in ∆δ (Figure 3E). Lowering of the ∆δ value changes the mechanism of nanoparticle formation from χ-syneresis to ν-syneresis.

35

In ν-syneresis, the better solubility

of the gel segments in the monomer-diluent liquid phase reduces the extent of phase separation and thus the tendency of nuclei to agglomerate into larger particles. Overall, phase separation within the oil phase of the w/o/w emulsion templates enables the preparation of microcapsules with shells comprising a network of polymer particles with tunable open porosity, particle and pore sizes. Varying the chemical nature of the diluent allows us to change the polymer particles dimensions within the range 40nm – 1 µm, while independently controlling the total pore volume between 20 and 40vol%.


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Figure 3: Effect of the concentration of inert diluent (porogen) on the size of the particles (beads) formed within the microcapsule shell upon polymerization. (A, B and C) Cross section SEM of the capsule shell containing 20wt%, 30wt% and 40% of undecanol. (D) Influence of inert diluent composition and concentration on the diameter of the polymer particles inside the shell. (E) Dependence of the particle diameter on the solubility parameter mismatch ∆δ. The ∆δ values for the solvent mixtures were approximated using a simple rule of mixtures. Table 2: Solubility parameters of the monomers and inert diluents used in the oil phase of the double emulsions. Porogen Solubility parameters (MPa1/2) GMA 18.2 45 EGDMA 18.2 45 Poly-GMA-co-EGDMA 24 45 Undecanol 20.5 49 Butanol 23.1 49

Microcapsules whose shells are structured via polymerization-induced phase separation are unique in comparison to previously reported colloidosomes 27-30

and hollow spheres because they combine high shell permeability with high

mechanical strength. To assess the mechanical properties of the microcapsules prepared in this study, we performed single capsule compression tests using a custom-built


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mechanical setup (Figure 4). The mechanical response of the microcapsules reflects the microstructure of their corresponding shells. The relatively dense, nanoporous shells obtained with small concentrations of linear alcohols (20wt% undecanol) lead to stiff and strong elastic capsules (Figure 4B). By contrast, shells consisting of a more porous network of polymer particles, formed with higher alcohol contents (30wt% undecanol), are significantly weaker, more compliant, and break at much lower deformations. When compared to microcapsules with dense polymer shells (Figure 4A), our results indicate a strong decrease in mechanical properties associated with the introduction of pores. For instance, the maximum load decreases by, respectively, one and two orders of magnitude (62 mN and 4.8 mN) for capsules with 20 and 30wt% undecanol in comparison to those made without undecanol (684 mN) (Figure 4 A,B). Likewise, undecanol concentrations of 20 and 30% reduces the stiffness of the capsule by one and two orders of magnitude respectively, in comparison to samples containing no porogen (Figure 4C). Despite the considerable decrease in mechanical properties upon the addition of larger amounts of diluent, the force at break of the capsules prepared with as much as 40% undecanol is still higher than that of colloidosomes of similar dimension, which according to our own measurements cannot withstand a compressive force of 1 mN.


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Figure 4: Mechanical properties of capsules with different types and concentrations of inert diluent. (A,B) Load versus displacement curves for capsules containing 0, 20, and 30 wt% of undecanol. (C) Stiffness and (D) force at break as a function of amount and composition of inert diluent. The mechanical properties of capsules with highly porous shells (high diluent contents) can be significantly improved by tuning the chemical nature of the diluent using, for example, mixtures of undecanol and butanol. As discussed earlier, the addition of butanol to undecanol decreases the solubility mismatch (∆δ) between the diluent phase and the newly formed polymer. This in turn decreases the size of the polymer particles forming the open porous network within the shell (Figure 3C). Such decrease in particle size was found to increase


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the stiffness and the load bearing capacity of the microcapsules, as shown in Figure 4C,D. Taking capsules made with a 50/50 weight ratio mixture of butanol/undecanol and a total diluent content of 30% as an example, we observe that the stiffness and force at break increases by a factor of 8.6 and 15.6 in comparison to microcapsules prepared with undecanol alone. SEM images of broken capsules show mostly intact polymer particles within the fractured shell. Therefore, we hypothesize that the capsules deformation and failure mainly occurs at the junction and contact points between the polymer particles. This is analogous to the deformation mechanisms observed in colloidal and granular systems, for which smaller particles increase the load bearing capacity and stiffness of the network by increasing the density of contact points. Thus, the higher density of load-bearing contact points is a possible explanation for the improved mechanical properties of microcapsules prepared with butanol-undecanol mixtures. Further mechanical improvements can be obtained by varying the inner and middle flow rate to form thicker shells 25, 43

.

Figure 5. Transmission (middle) and fluorescence (right) optical microscopy images depicting the permeable nature of the capsule shell (schematic) against the fluorescent dye Nile Red when immersed in benzyl alcohol. Capsules were made with a 75/25 mixture of undecanol/butanol and were imaged 3 min after immersion. In addition to the mechanical measurements, microcapsules prepared through polymerization-induced phase separation were also characterized in terms of the permeability of the porous shell using confocal microscopy (Figure


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5). For that purpose, dry capsules were first immersed in a solution of benzyl alcohol containing Nile red as fluorescent molecular probe. The fluorescent dye quickly diffused into the capsules, increasing strongly the fluorescent signal within the first minute after immersion. Fluorescence remained unchanged after 3 minutes. Such high permeability proves the presence of an open structure in the shell. The permeable nature of the microcapsule shell offers multiple possibilities for the selective release and uptake of molecules and/or particles. Here, we exploited the size-selective permeability of the shell to implement a triggered, reversible release mechanism, originally shown for colloidosomes by Sander et al.

28

In this mechanism, metal oxide nanoparticles entrapped within

the microcapsule serve as a platform for the reversible adsorption and desorption of the cargo molecules of interest (Figure 6). Adsorption and desorption events can be externally triggered by a change in pH or ionic strength of the continuous phase. This makes it possible to achieve controllable, on-demand release (Figure 6 A).


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Figure 6 Porous microcapsules loaded with Al2O3 nanoparticles to enable triggered, multiple release of cargo molecules. (A) Schematic illustrating the loading and release of the model cargo calcein using porous polymer capsules containing Al2O3 nanoparticles. (B) Cross-section SEM of the capsule shell, indicating the polymer particles that constitute the shell (yellow arrow) and the entrapped Al2O3 nanoparticles on the inner surface of the shell (red arrow). (C) The release of calcein is triggered by a pH change and is quantified by fluorescence spectroscopy measurements. The inset shows fluorescence images of Al2O3 loaded microcapsules after 1 hr at pH 4 and pH 10.


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The enhanced mechanical properties of our porous shells compared to those of previously reported colloidosomes enable the preparation of particlefilled microcapsules that are both mechanically robust and suitable for multiple triggered release. To demonstrate such triggered release feature, hydrophilic aluminum oxide nanoparticles were entrapped inside our porous capsules by suspending them in the inner aqueous phase during the double emulsification process. Because of their hydrophilic nature, the alumina nanoparticles remain in the emulsion inner phase and do not adsorb at the oil-water interface. Capsules were prepared using 30% of undecanol as diluent phase. The diluent content and the size of the Al2O3 nanoparticles were chosen to form a porous permeable shell while keeping the pores small enough to ensure entrapment of the oxide particles. Indeed, SEM imaging confirmed the entrapment of Al2O3 particles within the inner wall of the capsule’s shell (Figure 6B and Figure S3). To achieve triggered release, capsules were first loaded with the model fluorescent dye calcein by immersing them in a calcein salt solution at pH 4. At this pH, the calcein is negatively charged, thus it binds strongly to the positivelycharged aluminum oxide nanoparticles entrapped inside the capsule. After removing the unbound calcein through extensive washing, no significant leakage of calcein from the capsules is observed at pH 4, as indicated by the fluorescence data shown in Figure 6C. The uniform fluorescence inside the capsule indicates that the calcein-coated alumina nanoparticles are distributed evenly within the inner aqueous phase, confirming that the nanoparticles do not adsorb at the oil-water interface. When the pH is increased to 10, the aluminum oxide nanoparticles becomes negatively charged, thus leading to the desorption of calcein. This results in a quick and massive increase in the fluorescence signal within just a few minutes, demonstrating the on-demand release capability of the particle-filled porous microcapsules. If desired, the capsules containing bare aluminum oxide particles can be recovered by simply washing away the desorbed calcein at high pH. As long as the nanoparticles are large enough to


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remain trapped inside the capsule, loading and unloading of the capsules with cargo molecules can be repeated multiple times.

Conclusion We show that polymer capsules with permeable porous shells can be successfully produced in microfluidic devices by using water-oil-water double emulsions containing a reactive oil phase that undergoes phase separation during light-induced polymerization. Phase separation is achieved by introducing an inert diluent in the oil phase that solubilizes the initial monomers but later causes the precipitation of macromolecules generated upon polymerization. The polymerization-induced phase separation of such monomer/diluent mixture leads to the formation of a network of polymer particles, whose size is determined by the amount and composition of the oil phase. The use of different concentrations of undecanol and butanol as diluents enables independent control of porosity and size of the particles within the capsule shell. The morphology of the shell porous structure strongly influences the mechanical properties of the resulting microcapsules. Capsules with high porosity and large polymer particles obtained using high diluent concentrations exhibit a low density of particle interconnects, which ultimately leads to low stiffness and force at break. On the contrary, capsules prepared with low diluent concentrations exhibit shells with a densely interconnected particle network that results in higher stiffness and force at break up to 200 mN. Such breaking force is orders of magnitude higher than that achievable by previously reported porous microcapsules. Thus, the proposed method leads to microcapsules that are both mechanically robust and amenable to the transport of small molecules that can permeate through their porous shell. Using aluminum oxide nanoparticles trapped inside the capsule to reversibly capture or release a model cargo, we showed that these microcapsules can be used for the on-demand multiple release of small molecules. Altogether, our ability to control the shell porous structure and the high mechanical strength of


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these capsules make them attractive for industrial applications in agriculture, civil engineering, and material science. Acknowledgements We acknowledge BASF Construction Chemicals for the funding. We thank the Multifunctional Materials group, particularly Dr. Alessandro Lauria, for the fluorescence spectroscopy, the ScopeM center for the confocal microscope and Dr. Elena Tervoort-Gorokhova for the precious help and advice, and Yves Blickenstorfer for the carrying out part of the experiments. Supporting Information: The supporting information is available free of charge on the ACS publication website. Table S1-S2 middle phase formulations; Table S3-S4 particle size statistics and capsule mechanics; SEM of undecanol (Figure S1) and undecanol/butanol mixtures (Figure S2); SEM cross section of capsule with Al2O3 nanoparticles (Figure S3).

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Table of content:


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Double emulsion 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Phase separation Langmuir

Porous Microcapsule

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1) Washing

UV

Active cargo 2) Loading

water oil water


Strong Microcapsules with Permeable Porous Shells Made through

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