Persulfate initiated ultra-low cross-linked poly(N-isopropylacrylamide) microgels possess an unusual inverted cross-linking structure

Virtanen, Otto L. J.; Mourran, Ahmed; Pinard, P.; Richtering, Walter

doi:10.1039/c6sm00140h

Cited by 73 publications

(99 citation statements)

References 49 publications

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“…[14, 15] For pNIPAM-A (7 mol% BIS), the average Young’s modulus was determined to be 201 ± 10 MPa from sampling 30 individual particles. This result is consistent with a previous nanoindentation result reported by Tagit et al .…”

Section: Resultsmentioning

confidence: 99%

“…for low BIS (mol% < 2.0%) incorporation and guides the interpretation of our nanoindentation results. [15]…”

Section: Resultsmentioning

confidence: 99%

“…Both KPS and BIS act to crosslink NIPAM, however, the regions of crosslinking are distinct, with KPS introducing crosslinks at the particle periphery while BIS crosslinking is predominantly located near the particle core. [15] As demonstrated by Virtanen et al , the proportional balance between these two crosslinking agents yields pNIPAM microgels with distinct microgel interiors ranging from inverted (higher surface crosslinking) to fuzzy (higher core crosslinking). [15] These separate microgel architectures then permits the opportunity to contrast BLS (whole particle) and AFM (surface sensitive) for deeper insight into the composition dependence of microgel mechanics.…”

Section: Introducitonmentioning

confidence: 99%

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Core and surface microgel mechanics are differentially sensitive to alternative crosslinking concentrations

et al. 2017

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Microgel mechanics are central to the swelling of stimuli-responsive materials and furthermore have recently emerged as a novel design space for tuning the uptake of nanotherapeutics. Despite this importance, the techniques available to assess mechanics, at the sub-micron scale, remain limited. In this report, all mechanical moduli for a series of air-dried, polystyrene-co-poly(N-isopropylacrylamide) (pS-co-NIPAM) microgels of varying composition in monomer and crosslinker (N,N′-methylene-bisacrylamide (BIS)) mol% have been determined using Brillouin light scattering (BLS) and AFM nanoindentation. These techniques sample the material through distinct means and provide complementary nanomechanical data. An initial demonstration of this combined approach is used to evaluate size-dependent nanomechanics in pS particles of varying diameter. For the pS-co-NIPAM series, our BLS results demonstrate an increase in Young’s (E) and shear moduli with increasing NIPAM and/or BIS mol%, while the Poisson’s ratio decreased. The same rank order in E was observed from AFM and the two techniques correlate well. However, at low BIS crosslinking, an inverted particle structure persists and small increases in BIS yield a higher increase in E from AFM relative to BLS, consistent with a higher density at the particle surface. At higher BIS incorporation, the microgel reverts to a typical, dense-core structure and further increasing BIS yields changes to core-particle mechanics reflected in BLS. Lastly, at 75 mol% NIPAM, the microgels displayed a broad volume phase transition and increased crosslinking resulted in a minor, yet unexpected, increase in swelling ratio. This complementary approach offers new insight into nanomechanics critical for microgel design and application.

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Section: Resultsmentioning

confidence: 99%

“…for low BIS (mol% < 2.0%) incorporation and guides the interpretation of our nanoindentation results. [15]…”

Section: Resultsmentioning

confidence: 99%

Section: Introducitonmentioning

confidence: 99%

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Core and surface microgel mechanics are differentially sensitive to alternative crosslinking concentrations

et al. 2017

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“…PNIPAm microgels are synthesized in the presence of an exogenous cross-linker, such as N,N′-Methylenebis(acrylamide) (BIS); however, recent studies have demonstrated that pNIPAm microgels can be synthesized in the absence of an exogenous crosslinker, resulting in the formation of ultra-low cross-linked (ULC) particles 1–4 . Microgel mechanics are easily modulated through alterations in the degree of intra-particle crosslinking.…”

Section: Introductionmentioning

confidence: 99%

Study of Poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAM) Microgel Particle Induced Deformations of Tissue-Mimicking Phantom by Ultrasound Stimulation

Joshi¹,

Nandi

Chester

et al. 2018

Langmuir

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Poly (N-isopropylacrilamide) (pNIPAm) microgels (microgels) are colloidal particles that have been used extensively for biomedical applications. Typically, these particles are synthesized in the presence of an exogenous cross-linker, such as N, N′-Methylenebis (acrylamide) (BIS); however, recent studies have demonstrated that pNIPAm microgels can be synthesized in the absence of an exogenous crosslinker, resulting in the formation of ultra-low crosslinked (ULC) particles, which are highly deformable. Microgel deformability has been linked in certain cases to enhanced bioactivity when ULC microgels are used for the creation of biomimetic particles. We hypothesized that ultrasound stimulation of microgels would enhance particle deformation and that the degree of enhancement would negatively correlate with the degree of particle crosslinking. Here, we demonstrate in tissue-mimicking phantoms that using ultrasound insonification causes deformations of ULC microgel particles. Furthermore, the amount of deformation depends on the ultrasound excitation frequency and amplitude, and on the concentration of ULC microgel particles. We observed that the amplitude of deformation increases with increasing ULC microgel particle concentration up to 2.5 mg / 100 ml, but concentrations higher than 2.5 mg / 100 ml result in reduced amount of deformation. In addition, we observed that the amplitude of deformation was significantly higher at 1 MHz insonification frequency. We also report that increasing the degree of microgel crosslinking reduces the magnitude of the deformation and increases the optimal concentration required to achieve the largest amount of deformation. Stimulated ULC microgel particle deformation has numerous potential biomedical applications, including enhancement of localized drug delivery and biomimetic activity. These results demonstrate the potential of ultrasound stimulation for such applications.

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“…investigated the structure of ULC pNiPAm microgels by static light scattering. [57] Therein, the particles were prepared via non-stirred precipitation polymerization using KPS as initiator. The scattering experiments of these particles suggest an inverted cross-linking structure having the highest polymer density close to the particle surface.…”

Section: Resultsmentioning

confidence: 99%

Oligo(ethylene glycol)-sidechain microgels prepared in absence of cross-linking agent: Polymerization, characterization and variation of particle deformability

Welsch

Lyon

2017

PLoS ONE

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We present a systematic study of self-cross-linked microgels formed by precipitation polymerization of oligo ethylene glycol methacrylates. The cross-linking density of these microgels and, thus, the network flexibility can be easily tuned through the modulation of the reaction temperature during polymerization. Microgels prepared in absence of any difunctional monomer, i.e. cross-linker, show enhanced deformability and particle spreading on solid surfaces as compared to microgels cross-linked with varying amounts of poly(ethylene glycol diacrylate) (PEG-DA) in addition to self-crosslinking. Particles prepared at low reaction temperatures exhibit the highest degree of spreading due to the lightly cross-linked and flexible polymer network. Moreover, AFM force spectroscopy studies suggest that cross-linker-free microgels constitute of a more homogeneous polymer network than PEG-DA cross-linked particles and have elastic moduli at the particle apex that are ~5 times smaller than the moduli of 5 mol-% PEG-DA cross-linked microgels. Resistive pulse sensing experiments demonstrate that microgels prepared at 75 and 80°C without PEG-DA are able to deform significantly to pass through nanopores that are smaller than the microgel size. Additionally, we found that polymer network flexibility of microgels is a useful tool to control the formation of particle dewetting patterns. This offers a promising new avenue for build-up of 2D self-assembled particle structures with patterned chemical and mechanical properties.

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Persulfate initiated ultra-low cross-linked poly(N-isopropylacrylamide) microgels possess an unusual inverted cross-linking structure

Cited by 73 publications

References 49 publications

Core and surface microgel mechanics are differentially sensitive to alternative crosslinking concentrations

Core and surface microgel mechanics are differentially sensitive to alternative crosslinking concentrations

Study of Poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAM) Microgel Particle Induced Deformations of Tissue-Mimicking Phantom by Ultrasound Stimulation

Oligo(ethylene glycol)-sidechain microgels prepared in absence of cross-linking agent: Polymerization, characterization and variation of particle deformability

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