Letter pubs.acs.org/macroletters
pH-Induced Softening of Polyelectrolyte Microcapsules without Apparent Swelling Miju Kim,†,‡ Junsang Doh,*,‡,§ and Daeyeon Lee*,† †
Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States Department of Mechanical Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 790-784, Republic of Korea § School of Interdisciplinary Bioscience and Bioengineering (I-Bio), Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 790-784, Republic of Korea ‡
S Supporting Information *
ABSTRACT: Polyelectrolyte microcapsules represent a versatile platform to encapsulate and release active ingredients. Understanding the effect of environmental conditions on the mechanical properties of microcapsules is critically important for enabling their applications under various settings. In this report, we investigate the effect of solution pH on the mechanical properties of polyelectrolyte microcapsules made of two weak polyelectrolytes (poly(acrylic acid) and branched poly(ethylenimine)), formed via recently introduced nanoscale interfacial complexation in emulsion (NICE). Interestingly, the stiffness of the NICE microcapsule shell is reduced significantly (by 2 orders of magnitude) when the solution pH is raised from 2 to 6 even though there is little change in the size of microcapsules. These seemingly counterintuitive results suggest that the molecular structure of NICE microcapsules may be different from those of conventional polyelectrolyte microcapsules. The possibility of tuning the shell stiffness without inducing significant changes in the microcapsule size could offer unique advantages in practical applications.
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molecules and nanomaterials,6 making it a complementary method to well-established microcapsule preparation approaches such as layer-by-layer (LbL) assembly.7 Inspired by the pH-induced disassembly of NICE microcapsules, we hypothesize that the mechanical properties of the NICE microcapsule shell depend strongly on the solution pH. We examine the changes in the size and the mechanical properties of the NICE microcapsules under various pH conditions. We use the osmotic pressure-induced buckling method to monitor the changes in the elasticity of the NICE microcapsule. We show that the stiffness of the NICE shell made of two weak polyelectrolytes, poly(acrylic acid) (PAA) and branched poly(ethylenimine) (bPEI), depends strongly on the solution pH, whereas there are negligible changes in the size of microcapsules under different pH conditions. These seemingly counterintuitive observations between the lack of microcapsule swelling and a significant reduction in the shell stiffness potentially point to some subtle differences between the molecular structures of NICE microcapsules compared to conventional polyelectrolyte microcapsules formed via LbL assembly.
icrocapsules made of complexes of oppositely charged polyelectrolytes possess stimuli-responsive properties, making them useful vehicles for applications in encapsulation and triggered release of active ingredients as well as recapitulation of cellular functions such as compartmentalized biochemical reactions.1−4 To enable the utilization of polyelectrolyte microcapsules in various settings, it is critical to understand the mechanical properties of the shell under different conditions. For example, microcapsules that are orally ingested experience significant changes in the acidity of their external environment as they transported through the gastrointestinal tract, which likely changes their mechanical properties and in turn their deformability and robustness.5 We have recently demonstrated a one-step approach for generating multifunctional polyelectrolyte microcapsules using nanoscale interfacial complexation in emulsion (NICE) (Figure 1). This method relies on the interfacial complexation of two polymers at the inner water−oil interface of water-in-oil-inwater (W/O/W) double emulsions. By creating highly uniform double emulsions using a microfluidic device, it is possible to achieve high encapsulation efficiency and simultaneously enable one-step formation of polyelectrolyte microcapsules. NICE microcapsules can be induced to undergo disassembly and release encapsulated species by changes in the pH and ionic strength of the solution. Moreover, the NICE method enables the functionalization of microcapsules with hydrophobic © 2016 American Chemical Society
Received: February 15, 2016 Accepted: March 25, 2016 Published: March 30, 2016 487
DOI: 10.1021/acsmacrolett.6b00124 ACS Macro Lett. 2016, 5, 487−492
Letter
ACS Macro Letters
Figure 1. (A) Schematic illustration for the formation of NICE microcapsules from water-in-oil-in-water (W/O/W) double emulsions. (B) Schematic illustration showing complete dewetting of a NICE microcapsule from an oil droplet. (C) Optical microscopy image of W/O/W double emulsions generated using a glass capillary microfluidic device. (D) Optical microscopy image of NICE microcapsules completely separated from oil droplets (pink). The oil droplets contain 0.1 wt % Nile red.
amount of osmotic pressure is applied to the microcapsule surface the microcapsule should undergo buckling. A theoretical model based on continuum mechanics has been developed previously to describe the relationship between the critical pressure difference Pc, at which a buckling-based indentation of a thin spherical shell occurs, the elastic modulus of the shell E, the shell thickness δ, and the capsule radius R
Polyelectrolyte microcapsules made of poly(acrylic acid) (PAA) and branched poly(ethylenimine) (bPEI) are prepared by using the NICE method (Figure 1). Upon complexation of PAA and bPEI at the inner oil−water interface of the microfluidic W/O/W double emulsion, polyelectrolyte microcapsules separate from the oil phase of the double emulsion via spontaneous dewetting (Figure 1C,D). Our prior report showed that the shell thickness of the NICE microcapsule depends on the amount of polyelectrolytes present in the emulsion because all of the polyelectrolytes participate in interfacial complexation and thus end up in the microcapsule shell.6 It was found to be critical to collect W/O/W double emulsions in an acidic condition (pH < 3) to induce reliable formation of NICE microcapsules. Once generated, these (PAA/bPEI) NICE microcapsules exhibited triggered disassembly at pH 7.0, making them potentially useful for triggered delivery of actives. Based on this observation, it is highly plausible that the mechanical properties of these NICE microcapsules strongly depend on the solution pH. Prior studies indeed have shown that the physical properties of LbL films made of PAA and bPEI depend strongly on the solution pH.8,9 We use osmotic-pressure-induced buckling to characterize the shell elasticity of NICE microcapsules (Figure 2).10,11 In this method, a known concentration of an osmolyte is added to the medium outside the microcapsule, setting osmotic pressure difference between the interior and exterior of the shell. If the elastic restoring force is not able to compensate the arising hydrostatic pressure on the shell, water transports out of the microcapsule and induces buckling. We are especially encouraged by our failure to use micropipette aspiration to characterize the mechanical properties of the NICE microcapsule. The NICE microcapsules comprising PAA and bPEI undergo buckling-based collapse when it is subjected to aspiration, indicating that the shell is quite stiff (Supporting Information, Figure S1); thus we speculate that if a sufficient
Pc =
⎛ δ ⎞2 ⎜ ⎟ 3(1 − σ 2) ⎝ R ⎠ 2E
(1)
Since uniform hydrostatic pressure is applied on a large numbers of capsules, highly reliable analyses of the capsule mechanics are possible. Moreover, the technique does not require any expensive or sophisticated instruments or setups to characterize the shell stiffness. For these reasons, this technique has been used for the mechanical characterization of polyelectrolyte microcapsules.1,10,12−15 To apply different osmotic pressures on the NICE microcapsule shell, we prepare solutions containing different concentrations of polyethylene glycol (PEG, Mw = 4000 g/ mol). PEG is an ideal osmolyte because of its inertness and high solubility in water. Also our prior work showed that dextran with molecular weight of 4000 does not transport through the (PAA/bPEI) NICE microcapsule shell; thus, it is highly unlikely that PEG with Mw = 4000 can permeate through the NICE microcapsule shell during the time scale of our experiments (≪1 h). A calibration curve is generated to relate the osmotic pressure of PEG solution to its concentration (Figure S2). (PAA/bPEI) NICE microcapsules do indeed undergo buckling-induced collapse as the concentration of PEG in the exterior phase is increased. At low concentrations of PEG (
