Unique AFM unravels microgel mysteries


Rebecca Pool

Tuesday, June 4, 2019 - 13:45
AFM analyses indicates microgel structure depends on production method. [Nishizawa et al, Angewandte Chemie International Edition, 2019]
Japan-based researchers have used temperature-controlled high-speed atomic force microscopy to image previously unexplained behaviour in hydrogel microspheres.
Constructed by Dr Takayuki Uchihashi of Nagoya University to investigate proteins, the AFM is said to be the only such instrument in the world. 
Researchers from Daisuke Suzuki Laboratory, Graduate School of Textile Science & Technology and the Research Initiative for Supra-Materials of Shinshu University, are the first to use the instrument to study microgels.
The structure of microgels and the thermoresponsive properties of core-shell structures, have been studied extensively using electron microscopy, fluorescence microscopy, atomic force microscopy and super-resolution microscopy.
However, using their temperature-controlled high-speed atomic force microscope, Yuichiro Nishizawa and colleagues could also observe and record particles within non-thermoresponsive inhomogeneous decanano-sizes spherical domains.
As Nishizawa states: "Our research indicated, hydrogel microspheres have heterogeneous structure in almost every case.”
“Moreover, the heterogeneous nanostructure would have an impact on the physical and chemical properties of water swollen microgels and would lead to a gap between theory and result,” adds the researcher. “We believe that our findings can contribute to the understanding of these gaps."
Temperature-controlled high-speed atomic force microscopy of microgels synthesized by different polymerization techniques: (left) precipitation polymerization, (centre) inverse miniemulsion polymerization below the VPTT, and (right) inverse miniemulsion polymetization above the VPTT. [Nishizawa et al, Angewandte Chemie International Edition, 2019]
The Shinshu University team first studied poly(N‐isopropyl acrylamide) (pNIPAm)‐based microgels synthesized by precipitation polymerisation.
Analyses revealed that this gel had the core-shell structure, as well as non-thermoresponsive spherical domains.
Then, using inverse miniemulsion polymerization techniques, they produced two more types of microgels previously thought to all be the same.
Microgels made by inverse miniemulsion polymerization below the volume phase transition temperature produced a gel that was uniformly homogenous, and did not have the non-thermoresponsive domain or classic core-shell structure.
Meanwhile, inverse miniemulsion polymerization above the VPTT produced an inhomogeonous gel with no core-shell structure, but with the nano- to submicron-sized non-thermoresponsive domains.
In this way, the researchers showed the production method greatly affects the structure and behaviour of microgels, with clear implications for the manufacture of microgel glass/crystal and other medical materials.
The researchers now intend to continue studying hydrogel microspheres, with Nishizawa saying: “Ultimately, we want to develop new types of microspheres which improve people's standard of living."
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