Preparation and Timed Release Properties of Self-Rupturing Gels

Abstract

Swelling of polymeric hydrogels is sensitive to their cross-link densities. Here, we exploit this principle to prepare self-rupturing gels which are based on a commonly-used, nontoxic, and inexpensive polyelectrolyte, poly(acrylic acid), and are prepared through a simple and low-cost polymerization-based technique. The self-rupture of these covalently cross-linked gels is achieved by preparing them to have highly nonuniform cross-link densities. This heterogeneity in cross-linking leads to highly nonuniform swelling, which generates stresses that are high enough to induce gel rupture. The time required for this rupture to occur depends on the difference in the cross-link densities between the adjoining gel regions, gel size, order in which the variably cross-linked gel portions are synthesized, and on the ambient pH and ionic strength. Furthermore, when these self-rupturing gels are prepared to have liquid-filled (capsule-like) morphologies, they can act as timed/delayed release devices. The self-rupture of these capsules provides a burst payload release after a preprogrammed delay, which is on the timescale of days and can be easily tuned by varying the rupture time, i.e., by varying either the cross-link nonuniformity or the pH and ionic strength of the release media.

Swelling-Activated, Soft Mechanochemistry in Polymer Materials

“…(B) Nonuniform swelling of regions of different cross-link densities results in complete rupture at the interface. (Reprinted with permission from ref . Copyright 2016 American Chemical Society.)…”

Gel rupture during dynamic swelling

“…This was shown to be a reasonable assumption for polyacrylamide gels in water having spherical shapes. 8 However, later studies observed phenomena such as surface instabilities 31 and self-rupturing 9 in response to swelling. Therefore, the assumption of internal stresses approaching zero and the appropriateness of theoretical regimes to account for geometrical constraints, mechanical loads, and non-idealities or heterogeneity associated with polymer networks used in practice has come into question.…”

“…While the research literature is rich with examples of biomaterials developed from PEG, 2,[4][5][6][7] there are still challenges associated with this platform: there remains a need to better understand the physical behavior and transient dynamics associated with swelling and how it contributes to observed phenomena such as surface instabilities and rupturing. [8][9][10][11][12][13][14] To address these limitations, here we employ high-speed imaging of rupture events during hydrogel swelling to determine what factors lead to catastrophic failure.…”

Gel rupture in a dynamic environment

“…While the research literature is rich with examples of biomaterials developed from PEG, [2,[4][5][6][7] there are still challenges associated with this platform: there remains a need to better understand the physical behavior and transient dynamics associated with swelling and how it contributes to observed phenomena such as surface instabilities and rupturing. [8][9][10][11][12][13][14] To address these limitations, here we employ high-speed imaging of rupture events during hydrogel swelling to determine what factors lead to catastrophic failure.…”

“…This was shown to be a reasonable assumption for polyacrylamide gels in water having spherical shapes. [8] However, since this analysis more recent studies have revealed surface instabilities [28] and self-rupturing [9] behavior in response to swelling, and thus the appropriateness of these theoretical regimes to account for geometrical constraints, mechanical loads, and non-idealities or heterogeneity associated with polymer networks used in practice has come into question. [29] Phenomena such as in situ surface structuring and self-rupturing are interesting and relevant for interfacial design and drug delivery applications.…”