Stimuli-Responsive Core–Shell Microcapsules with Tunable Rates of Release by Using a Depolymerizable Poly(phthalaldehyde) Membrane

Abstract

Flow-focusing microfluidic techniques were used to provide access to core−shell microcapsules in which the shell is composed of end-capped poly(phthalaldehydes) that depolymerize completely from head-to-tail in response to fluoride. Microcapsules made from these depolymerizable polymers provide an amplified response to the applied chemical signal, where the rate of the response can be tuned both by varying the length of the polymer and the thickness of the shell wall. ■ INTRODUCTIONStimuli-responsive polymeric core−shell microcapsules are promising materials for controlled release applications. 1−8 Capsules of this type typically release their contents either through induced chemical changes in the polymers that compose the shell or via bulk structural changes to the shell wall. 1 These traditional triggered-release strategies provide responses that are nonamplified: i.e., a single membrane−signal interaction produces one small structural change in the shell wall, whereas release occurs only after many signals have reacted with and altered the shell wall. In contrast, a new release strategy has emerged where shell walls are formed from polymers that depolymerize continuously and completely from head-to-tail when an appropriate signal is detected by the polymer. 1,9 This depolymerization reaction provides an amplified response that, in theory, increases the sensitivity of the capsules as well as their rate of response once the appropriate signal is detected. Despite these favorable attributes, however, only one example of a stimuli-responsive core−shell polymeric microcapsule made from a head-to-tail depolymerizable polymer has been demonstrated to date, 10 with additional related examples demonstrated in responsive nanocapsules and micelles. 11,12 Further development of this depolymerizable shell wall concept has been hindered by the limited number of polymers that are capable of depolymerizing from head-to-tail in response to a specific signal, as well as by substantial challenges in fabricating core−shell microcapsules using polymers that are primed to depolymerize.In this article, we describe the use of flow-focusing microfluidic strategies for fabricating these types of depolymerizable core−shell microcapsules. 13−15 This technique is highly reproducible, readily forms capsules that contain aqueous interiors, is capable of generating capsules that can be suspended in an aqueous solution that differs from the solution within the capsule, and does not require synthetic manipulation of the polymer for incorporation into the shell wall. Moreover, the fabrication technique is exceptionally mild and thus enables the formation of core−shell microcapsules using sensitive depolymerizable polymers such as poly(phthalaldehyde) (PPHA) (Figure 1). 16,17 Herein we demonstrate these concepts by fabricating model stimuli-responsive core−shell microcapsules using PPHA that contains a fluoride-responsive endcap. The end-cap controls the stability and reactivity of the PPHA polymer and thus enables release of the ...

Coaxial Electrospinning Formation of Complex Polymer Fibers and their Applications

“…Depolymerized SIP was also abruptly released indicating a quick sheath depolymerization . Since this report demonstrated the concept of SIP core‐sheath nanofibers, possibly other SIP materials triggered by UV light, enzyme, or specific ions can be adopted for novel application developments.…”

Amplified responses in materials using linear polymers that depolymerize from end‐to‐end when exposed to specific stimuli

“…These polymers include poly(benzyl carbamates) [Figure (a)], poly(benzyl ethers) [Figure (b)], and polymers that depolymerize via alternating intramolecular cyclization/quinone methide elimination reactions [Figure (c)] . The two other classes offer unique mechanisms of depolymerization from one another as well as from the first three classes of polymers: they depolymerize via intramolecular cyclization reactions [Figure (d)] or acetal chemistry [Figure (e)] . There is vast chemical space yet to be explored.…”

“…In fact, several proof‐of‐concept studies are already beginning to reveal these capabilities. For example, CD r polymers have been used to make (i) stimuli‐responsive, non‐mechanical pumps; (ii) micellar aggregates, polymersomes, and micro‐ and nanocapsules for controlled release applications; and (iii) shape‐changing and vanishing plastics. They also have been used as signal amplification reagents in the context of point‐of‐care diagnostics .…”

Photo‐Triggered Microcapsules

“…One of the most challenging tasks, and the ultimate purposes of developing delivery systems, is to modulate the release of encapsulated substances. Strategies such as heat treatment, ionic strength, magnetic field, and light‐induced morphology change have been used to alter the shell density, integrity, and to influence capsule permeability . Capsules made of photo‐stimuli responsive materials are a subject of great interest, as they are capable of changing their micro‐/nano‐structures by the use of external light as a remote control trigger .…”

“…Strategies such as heat treatment, ionic strength, magnetic field, and light-induced morphology change have been used to alter the shell density, integrity, and to influence capsule permeability. [3][4][5][6] Capsules made of photostimuli responsive materials are a subject of great interest, as they are capable of changing their micro-/nano-structures by the use of external light as a remote control trigger. [7,8] The use of sunlight for example could impact the shell structure causing the release of the cargo material without requirement of direct contact or interactions.…”