Unraveling the Mechanisms of Ultrasound-Induced Mechanical Degradation of Microgels: Effects of Mechanoresponsive Crosslinks, Softness, and Core-Shell Architecture

TL;DR

This study addresses how ultrasound cavitation mechanically degrades soft microgels, focusing on mechanoresponsive disulfide crosslinks and asymmetric core-shell architectures. A multi-technique approach (DLS, SLS, SAXS, AFM, Raman spectroscopy, and inline calorimetry) is used to map degradation pathways in conventional BIS/BAC microgels and asymmetrically crosslinked core-shell variants, revealing that swelling degree and network topology jointly govern susceptibility. Degradation proceeds through erosion of the periphery, near-bubble rupture, and fragmentation into fragments, with core-shell microgels showing non-selective, whole-particle degradation rather than targeted weakening. The findings advance the design of ultrasound-responsive drug carriers and deepen understanding of mesoscopic fracture under cavitation, with implications for controlled release and microplastic formation.

Abstract

Ultrasound-induced degradation of soft polymeric colloids, like microgels, as well as a controlled drug release enabled by mechanoresponsive bonds, has recently attracted considerable attention. However, most examples in the literature focus primarily on the applications rather than examining the underlying mechanisms of the structural changes occurring in microgels due to cavitation - changes that are crucial for developing effective drug delivery systems. In this work, we provide a comprehensive view on how microgel structure governs the susceptibility to rupture and mass loss upon cavitation, investigating both conventional microgels containing mechanoresponsive disulfide bonds and more complex asymmetrically crosslinked core-shell microgels. By combining dynamic and static light scattering, small-angle X-ray scattering, and atomic force microscopy, we demonstrate that an interplay between mechanoresponsive crosslinks and the swelling degree determines the microgels susceptibility to ultrasound-induced damage. Our findings indicate that local stress from cavitation bubbles varies strongly within the microgel dispersion. The majority of microgels undergo gradual erosion at their periphery, resulting in smaller yet structurally intact particles over time, observable by light scattering and AFM. In contrast, microgels closer to a cavitation bubble can experience partial rupture or completely disintegrate, producing smaller, more polydisperse fragments, which contributes substantially to the overall mass loss observed. In the core-shell microgels with different crosslinkers in the core and shell, degradation occurs nearly uniformly across both regions, instead of selectively targeting the weaker part. These observations highlight the complexity of the degradation dynamics as well as the similarity to processes seen in linear polymers and bulk hydrogels.

Figures (7)

• Figure 1: (A,D) Normalized hydrodynamic radii of the conventional microgels (A) and core-shell microgels (D) in the swollen state ($T=20\;^{\circ}$C), $R_{\textrm{h}}/R_{\textrm{h},0}$, after different ultrasonication times $t$. (B,E) Swelling ratio $\alpha$ of the conventional microgels (B) and core-shell microgels (E) after different ultrasonication times $t$. (C,F) Normalized volume of the conventional microgels (C) and core-shell microgels (F) in the collapsed state ($T=40\;^{\circ}$C), $V_{\textrm{coll}}/V_{\textrm{coll,0}}$, after different ultrasonication times $t$.
• Figure 2: (A) Schematic of the different parameters describing microgel size and internal structure: the hydrodynamic radius $R_{\textrm{h}}$, the radius of gyration $R_{\textrm{g}}$, the total radius $R$ and radius of the core $R_{\textrm{c}}$ from the form factor fits. (B) Guinier plots of SLS intensity, $\ln{I(q)}$vs. scattering vector $q$, for BAC-1 microgels after 0 min, 5 min, 10 min, 20 min, 30 min and 60 min ultrasonication (from bottom to top). Black lines correspond to the fits with Equation \ref{['eq:Guinier']}. (C) The $R_{\textrm{g}}/R_{\textrm{h}}$ ratios for microgels vs. ultrasonication time $t$. The dashed line correspond to the value for hard spheres, $R_{\textrm{g}}/R_{\textrm{h}}=0.778$. (D) SAXS intensities $I(q)$vs. scattering vector $q$ for BAC-1 microgels after 0 min, 5 min, 10 min, 20 min, 30 min and 60 min ultrasonication (from bottom to top). Black lines correspond to the fits using the fuzzy-sphere model. (E) Normalized radii of the microgels $R/R_0$ obtained from the SAXS fits vs.$t$. (F) Radial profiles of relative polymer density, $\phi(r)$, for BAC-1 microgels, obtained from the SAXS fits. (G) Polydispersities of the microgels $p$, obtained from the SAXS fits vs.$t$.
• Figure 3: (A) SLS intensities $I(q)$vs.$q$ for CS-A microgels after 0 min, 5 min, 10 min, 20 min, 30 min and 60 min ultrasonication (from bottom to top). Black lines correspond to the fits using the fuzzy core-shell model. (B) SLS intensities $I(q)$vs.$q$ for CS-B microgels after 0 min, 5 min, 10 min, 20 min, 30 min and 60 min ultrasonication (from bottom to top). Black lines correspond to the fits using the fuzzy core-shell model. (C) Normalized radii of the core-shell microgels $R/R_0$ obtained from the SLS fits vs.$t$. (D) Radial profiles of relative polymer density for CS-A microgels, $\phi(r)$, obtained from the SLS fits. (E) Radial profiles of relative polymer density for CS-B microgels, $\phi(r)$, obtained from the SLS fits.(F) Polydispersities of the core-shell microgels, $p$, obtained from the SLS fits vs.$t$.
• Figure 4: Examples of AFM images of BAC-1 microgels (A-D), CS-A microgels (E,F), and CS-B microgels (G,H) after different ultrasonication times (indicated in the figure). Upper images in each panel are height images, and lower images are phase images. Scale bars are 2 $\mu$m. Inset in panel A (phase image) shows an enlarged microgel and its contact radius $R_{\textrm{cont}}$ (white dashed circle). Green squares in panel B highlight ruptured microgels.
• Figure 5: (A-D) Averaged height profiles of BAC-1 microgels, BIS-5 microgels, CS-A microgels and CS-B microgels (left to right) before and after extensive ultrasonication ($t$ indicated in the legend). (E) Mean maximum height of the microgels, $H_{\textrm{max}}$, normalized by the pristine value, $H_{\textrm{max},0}$, vs. ultrasonication time $t$. (F) Mean contact radius of the microgels, $R_{\textrm{cont}}$, normalized by the pristine value, $R_{\textrm{cont},0}$, vs.$t$. (G) Mean shape parameter of the adsorbed microgels, $S=H_{\textrm{max}}/2R_{\textrm{cont}}$, vs.$t$. (H) Normalized number densities of the microgels in suspension, $n/n_0$, vs.$t$. Shaded areas in all panels correspond to standard deviation.